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T2- and diffusion-weighted magnetic resonance imaging of a focal ischemic lesion in rat brain
Abstract
We sought to evaluate the application of T2-weighted and diffusion-weighted magnetic resonance imaging techniques in the study of a focal ischemic lesion in the rat brain.
Unilateral cortical infarcts were induced using the photosensitive dye rose bengal and 560 nm light irradiation. Magnetic resonance images were recorded from a total of 11 rats at selected intervals from 1.5 hours to several days after induction of the lesion. Parallel experiments were performed in which Evans blue dye was injected into the lesioned animals either immediately after lesion induction (n = 11) or 1 hour before the animals were killed (n = 11). The second procedure was designed to show regions of blood-brain barrier permeability to plasma proteins at the time of sacrifice, whereas the first procedure showed the accumulation and subsequent dispersion of plasma protein following disruption of the blood-brain barrier.
Regions of the cortex highlighted by the T2-weighted images corresponded well to the pattern of dye staining seen from the first procedure while the diffusion-weighted images showed visual correspondence with the staining pattern obtained using the second procedure.
These results illustrate the complementary use of T2-weighted and diffusion-weighted magnetic resonance imaging in discerning the pathophysiology of developing lesions.
... Conventional technique, such as magnetic resonance imaging (MRI) and Diffusive tensor imaging (DTI) [14][15][16] have been applied in vivo imaging of brain tissues to characterize the biochemical and anatomical alterations post-stroke [17][18][19]. Although these techniques provide important data, they yield spatial resolution of about 250 µm which is not sufficient to study biochemical and biomolecular changes at cellular and sub-cellular levels. ...
... Although these techniques provide important data, they yield spatial resolution of about 250 µm which is not sufficient to study biochemical and biomolecular changes at cellular and sub-cellular levels. Routine histology [14,18,19] and immunohistochemistry (IHC) [14] also have been used to examine the biochemical and morphological changes in the brain tissues [5,[17][18][19]. Whereas, histology and IHC give high spatial resolution they are disrupting the tissue morphology and only limited to certain biochemical markers. ...
... Although these techniques provide important data, they yield spatial resolution of about 250 µm which is not sufficient to study biochemical and biomolecular changes at cellular and sub-cellular levels. Routine histology [14,18,19] and immunohistochemistry (IHC) [14] also have been used to examine the biochemical and morphological changes in the brain tissues [5,[17][18][19]. Whereas, histology and IHC give high spatial resolution they are disrupting the tissue morphology and only limited to certain biochemical markers. ...
Objective:
Brain damage, long-term disability and death are the dreadful consequences of ischemic stroke. It causes imbalance in the biochemical constituents that distorts the brain dynamics. Understanding the sub-cellular alterations associated with the stroke will contribute to deeper molecular understanding of brain plasticity and recovery. Current routine approaches examining lipid and protein biochemical changes post stoke can be difficult. Fourier Transform Infrared (FTIR) imaging spectroscopy can play a vital role in detecting these molecular alterations on a sub-cellular level due to its high spatial resolution, accuracy and sensitivity. This study investigates the biochemical and molecular changes in peri-infract zone (PIZ) (contiguous area not completely damaged by stroke) and ipsi-lesional white matter (WM) (right below the stroke and PIZ regions) nine weeks post photothrombotic ischemic stroke in rats.
Materials and methods:
FTIR imaging spectroscopy and transmission electron microscopy (TEM) techniques were applied to investigate brain tissue samples while hematoxylin and eosin (H&E) stained images of adjacent sections were prepared for comparison and examination the morphological changes post stroke.
Results:
TEM results revealed shearing of myelin sheaths and loss of cell membrane, structure and integrity after ischemic stroke. FTIR results showed that ipsi-lesional PIZ and WM experienced reduction in total protein and total lipid content compared to contra-lesional hemisphere. The lipid/protein ratio reduced in PIZ and adjacent WM indicated lipid peroxidation, which results in lipid chain fragmentation and an increase in olefinic content. Protein structural change is observed in PIZ due to the shift from random coli and α-helical structures to β-sheet conformation.
Conclusion:
FTIR imaging bio-spectroscopy provide novel biochemical information at sub-cellular levels that be difficult to be obtained by routine approaches. The results suggest that successful therapeutic strategy that is based on administration of anti-oxidant therapy, which could reduce and prevent neurotoxicity by scavenging the lipid peroxidation products. This approach will mitigate tissue damage in chronic ischemic period. FTIR imaging bio-spectroscopy can be used as a powerful tool and offer new approach in stroke and neurodegenerative diseases research.
... A model to study ischemic stroke is the photo-thrombotic model of focal ischemia, where photosensitive dye is excited by fluorescence causing a blood clot (Pevsner et al., 2001). Conventional techniques, such as magnetic resonance imaging (MRI) (Van Bruggen et al., 1992; Abbreviations: CC, corpus callosum; CL-GM, contra-lesioned gray matter; CL-WM, contra-lesioned white matter; FTIR, Fourier transform Infrared; GM, gray matter; IHC, immunohistochemistry; LA-ICPMS, laser-ablation inductivelycoupled-plasma mass-spectrometry; L-WM, lesioned white matter; PL-GM, perilesioned gray matter; PS-GM, primary stroke lesion gray matter; ROI, regions of interest; TC-GM, time control gray matter; WM, white matter. Pevsner et al., 2001), auto-radiography (Dietrich et al., 1986), routine histology (Van Bruggen et al., 1992;Pevsner et al., 2001;Kuroiwa et al., 2009), and IHC (Van Bruggen et al., 1992) have been applied to characterize the biochemical and anatomical alterations after stroke in many studies (Dietrich et al., 1986;Pevsner et al., 2001;Kuroiwa et al., 2009). ...
... Conventional techniques, such as magnetic resonance imaging (MRI) (Van Bruggen et al., 1992; Abbreviations: CC, corpus callosum; CL-GM, contra-lesioned gray matter; CL-WM, contra-lesioned white matter; FTIR, Fourier transform Infrared; GM, gray matter; IHC, immunohistochemistry; LA-ICPMS, laser-ablation inductivelycoupled-plasma mass-spectrometry; L-WM, lesioned white matter; PL-GM, perilesioned gray matter; PS-GM, primary stroke lesion gray matter; ROI, regions of interest; TC-GM, time control gray matter; WM, white matter. Pevsner et al., 2001), auto-radiography (Dietrich et al., 1986), routine histology (Van Bruggen et al., 1992;Pevsner et al., 2001;Kuroiwa et al., 2009), and IHC (Van Bruggen et al., 1992) have been applied to characterize the biochemical and anatomical alterations after stroke in many studies (Dietrich et al., 1986;Pevsner et al., 2001;Kuroiwa et al., 2009). These techniques provide important data, but for example, MRI is not sufficient to study bio-chemical and bio-molecular changes at cellular and sub-cellular levels. ...
... Conventional techniques, such as magnetic resonance imaging (MRI) (Van Bruggen et al., 1992; Abbreviations: CC, corpus callosum; CL-GM, contra-lesioned gray matter; CL-WM, contra-lesioned white matter; FTIR, Fourier transform Infrared; GM, gray matter; IHC, immunohistochemistry; LA-ICPMS, laser-ablation inductivelycoupled-plasma mass-spectrometry; L-WM, lesioned white matter; PL-GM, perilesioned gray matter; PS-GM, primary stroke lesion gray matter; ROI, regions of interest; TC-GM, time control gray matter; WM, white matter. Pevsner et al., 2001), auto-radiography (Dietrich et al., 1986), routine histology (Van Bruggen et al., 1992;Pevsner et al., 2001;Kuroiwa et al., 2009), and IHC (Van Bruggen et al., 1992) have been applied to characterize the biochemical and anatomical alterations after stroke in many studies (Dietrich et al., 1986;Pevsner et al., 2001;Kuroiwa et al., 2009). These techniques provide important data, but for example, MRI is not sufficient to study bio-chemical and bio-molecular changes at cellular and sub-cellular levels. ...
Objective: Stroke is the main cause of adult disability in the world, leaving more than half of the patients dependent on daily assistance. Understanding the post-stroke biochemical and molecular changes are critical for patient survival and stroke management. The aim of this work was to investigate the photo-thrombotic ischemic stroke in male rats with particular focus on biochemical and elemental changes in the primary stroke lesion in the somatosensory cortex and surrounding areas, including the corpus callosum.
Materials and Methods: FT-IR imaging spectroscopy and LA-ICPMS techniques examined stroke brain samples, which were compared with standard immunohistochemistry studies.
Results: The FTIR results revealed that in the lesioned gray matter the relative distribution of lipid, lipid acyl and protein contents decreased significantly. Also at this locus, there was a significant increase in aggregated protein as detected by high-levels Aβ1-42. Areas close to the stroke focus experienced decrease in the lipid and lipid acyl contents associated with an increase in lipid ester, olefin, and methyl bio-contents with a novel finding of Aβ1-42 in the PL-GM and L-WM. Elemental analyses realized major changes in the different brain structures that may underscore functionality.
Conclusion: In conclusion, FTIR bio-spectroscopy is a non-destructive, rapid, and a refined technique to characterize oxidative stress markers associated with lipid degradation and protein denaturation not characterized by routine approaches. This technique may expedite research into stroke and offer new approaches for neurodegenerative disorders. The results suggest that a good therapeutic strategy should include a mechanism that provides protective effect from brain swelling (edema) and neurotoxicity by scavenging the lipid peroxidation end products.
... Injection and illumination conditions were chosen to potentially produce minor strokes by selecting conditions that could enhance early platelet disaggregation and thrombus clearance as has been reported to be visualized directly using confocal microscopy (Zhang et al., 2005). Thus the concentration of Rose Bengal (Sigma-Aldrich, St Louis, MO) administered IV (prepared as 10 mg/ml in sterile distilled water and filtered through an 80 m filter) was low (10 mg/kg, 1 ml/kg) compared to many previous studies using 15-50 mg/kg (Jolkkonen et al., 2007;Pierpaoli et al., 1993;Van Bruggen et al., 1992;Verlooy et al., 1993). The pharmacokinetics of Rose Bengal was used to help select the duration of illumination (Fig. 1A). ...
... Thus, for the duration of illumination, a rather short period of five minutes was selected compared to the 15-30 min periods of illumination used in previous studies (e.g. Jolkkonen et al., 2007;Moon et al., 2009;Pevsner et al., 2001;Pierpaoli et al., 1993;Schroeter et al., 2001;Van Bruggen et al., 1992;Watson et al., 1985). ...
... Photothrombosis has traditionally been used to produce consistent rather large infarcts within the cortex and similar to the present study, MR imaging changes of increased T 2 and Dw within the infarct lesions have been reported previously (Pierpaoli et al., 1993;Van Bruggen et al., 1992). Similar to the present results, a reduction in T 2 from maximal values at 1-2d post insult has been reported to occur during resolution of edema within the lesion by others (Jolkkonen et al., 2007;Schroeter et al., 2001). ...
Background:
Transient ischemic attack, minor stroke and stroke recurrence need improved treatment but lack animal models for research. The aim was to modify photothrombosis methods thereby producing both a minor stroke (with adjacent mild damage) or a minor recurrent stroke.
New method:
A minor stroke, as detected using magnetic resonance imaging and histology, was produced using a low intensity beam of white light with a bright centre, a low dose of Rose Bengal and a short 5min illumination of thinned skull. A recurrent minor stroke was produced by repeating the procedure two days later except the cortical mask was positioned 1.5mm posteriorly.
Results:
The minor photothrombosis procedure produced a small superficial infarct surrounded by a region of scattered necrosis detected histologically. Marked hyperintensities in Diffusion weighted and T2 images identified the infarct. Peri-infarct regions with modest T2 increases corresponded to regions of scattered cell death. A recurrent minor photothrombosis produced additional damage in regions with overlapping mild injury.
Comparison with existing methods:
Previous photothrombosis methods usually produce large cortical infarcts with little penumbra. The current method produces small infarcts with diffuse mild peri-infarct ischemic injury that can be diagnosed using T2 imaging.
Conclusions:
The modified photothrombotic procedure will produce a minor stroke consisting of a small infarct in a region with marked diffusion and T2 hyperintensities and a peri-infarct region of selective necrosis with modest T2 changes. Minor recurrent stroke is readily produced but imaging is key for assessing size and location of each insult.
... The high metabolic demands of neurons make them particularly vulnerable to ischemic insult. In addition, cells killed or irreversibly damaged by the ischemic insult can release factors, including water (i.e., edema), which may spread into and harm nearby tissue that was not initially damaged by the stroke (Lipton, 1999;Witte and Stoll, 1997;van Bruggen et al., 1992;Lee et al., 1996). This process can be observed as delayed or ongoing cell death following the ischemic insult (Lipton, 1999;Witte and Stoll, 1997;Lee et al., 1996). ...
... Previous studies have characterized the anatomical and biochemical alterations occurring in the photothrombotic stroke model using conventional techniques, such as routine histology, (van Bruggen et al., 1992;Kuroiwa et al., 2009;Pevsner et al., 2001) immunohistochemistry (van Bruggen et al., 1992), magnetic resonance imaging (MRI) (van Bruggen et al., 1992;Pevsner et al., 2001), or auto-radiography (Dietrich et al., 1986). Despite the valuable information garnered with these methods, they suffer from a lack of spatial resolution or provide only a limited view of the biochemical "picture". ...
... Previous studies have characterized the anatomical and biochemical alterations occurring in the photothrombotic stroke model using conventional techniques, such as routine histology, (van Bruggen et al., 1992;Kuroiwa et al., 2009;Pevsner et al., 2001) immunohistochemistry (van Bruggen et al., 1992), magnetic resonance imaging (MRI) (van Bruggen et al., 1992;Pevsner et al., 2001), or auto-radiography (Dietrich et al., 1986). Despite the valuable information garnered with these methods, they suffer from a lack of spatial resolution or provide only a limited view of the biochemical "picture". ...
... However, the situation has changed in the last decade. Neuronal stem cells with the ability to produce new neurones and glial cells remain in adulthood in some parts of the brain (Temple 2001), and stem cells have been successfully transplanted into animal models of ischemia or injury (Björklund et al. 2003) (Isacson 2003) (Silani and Leigh 2003). This opens up new strategies for recovery and repair. ...
... Useful methods have been developed for culturing different types of stem cells and transplanting them into animal models of injury (Björklund and Lindvall 2000) (Björklund et al. 2003) (Isacson 2003). It has been shown that neuronal stem cells persist throughout life in many mammals, including humans (Temple 2001), and that progenitor cells can differentiate into neural tissue in the adult brain (Gage 2002) (Taupin and Gage 2002) (van Praag et al. 2002). After brain ischemia in animals, hippocampal pyramidal neurones have been shown to be recruited from endogenous neural progenitors (Nakatomi et al. 2002). ...
... give information about proton motion and are affected by the inhomogeneities in the local magnetic field within the tissue. Damaged tissue, where hindering protein structures are disrupted, provides greater freedom for proton motion and thus increased T 2 values (van Bruggen et al. 1992) (van der Toorn et al. 1996). ...
... Furthermore, the severity of the ischemic damage was quantified by computation of regional changes in Apparent Diffusion Coefficient (ADC). We compared our findings with those from earlier MRI experiments on the photochemical model (Van Bruggen et al., 1991; Pierpaoli et al., 1993; Lee et al., 1996), including our own (Lanens et al. 1993Lanens et al. , 1995). Part of this work was presented as an abstract and poster at the Fourth Scientific Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine, New York 27/04–3/05/1996, p. 503. A. Van der Linden, M. Verhoye, M. de Ryck. ...
... Within the first 4 hrs after photochemical induction, there is a virtually simultaneous onset of DWI and T 2 hyperintensities, the latter in areas of extravasation and blood–brain barrier breakdown. (Van Bruggen et al., 1991; Lanens et al. 1993 Lanens et al. , 1995). This agrees well with the early, virtually simultaneous onset of neurotoxicity and edema/blood–brain-barrier (BBB) breakdown in the photochemical model (Dietrich et al., 1987; Grome et al., 1988). ...
... Up to 24 hrs, the ischemic damage becomes more dense in the core and progressively expands towards the periphery to grow into an even larger irreversible necrotic area, consuming the peripheral zone, an area that can be considered penumbral because of the mixture of normal and ischemic-type cells (Dietrich et al., 1986), moderate DWI hyperintensity (Pierpaoli et al., 1993), and moderately reduced ADC (Pierpaoli et al., 1993). This peri-infarct zone is also characterized by reduced cerebral blood flow (CBF) (Lanens et al., 1995), post-contrast T 1 -weighted evidence of blood–brain barrier breakdown (Lanens et al. 1993Lanens et al. , 1995 ), and T 2 -weighted evidence of marked extravasation and vasogenic edema (Van Bruggen et al., 1991; Lanens et al. 1993 Lanens et al. , 1995) Therefore, lubeluzole-induced attenuation of the reduction of ADC in the peri-infarct zone may indicate functional improvement of penumbral tissue, possibly upgrading it to the status of reversible damage. ...
We studied the neuroprotective effect of lubeluzole, a NOS (nitric oxide synthase) pathway modulator, on the development of ischemic damage within the first six hours after a photochemically induced neocortical infarct in rats using diffusion-weighted MRI and Apparent Diffusion Coefficient (ADC) maps. A unilateral photochemical infarct was induced in the hindlimb sensorimotor neocortex of Wistar rats. One hour after infarction, rats received either vehicle (n=10) or lubeluzole (n=11; a 0.31 mg/kg i.v. bolus followed by a one-hour 0.31 mg/kg i.v. infusion). During the first six hours after infarct induction, multislice T2- and Diffusion-Weighted magnetic resonance images (MRI) were obtained to measure percent change of volume of ischemic damage, whereas regional ADC maps were used to measure time-dependent density of ischemic damage. Lubeluzole reduced the percent increase of volume of ischemic damage relative to baseline (at 1 h after infarct induction just before drug treatment), by 18% at 5 and 6 hrs after infarct induction. Lubeluzole attenuated the ADC decreases in the peripheral rim of the infarct, but left the ADC values in the core unaffected. In conclusion, the neuroprotectant lubeluzole attenuates growth of ischemic damage as well as its density in the periphery of a photochemically induced neocortical infarct in rats.
... This nding likely represents vasogenic edema in the acute phase (day 0), also visible on T2W. In the subacute stage (day 2) and chronic stage (day 7), hyper-diffusivity was noted in the penumbra and the lesion, likely re ecting the further loss of neurons and necrosis in the lesion, allowing for more free movement of water within the tissue [27,28] The increased diffusivity in the penumbra over time was also found in a study by Hilger et al. (2004) using the ring stroke model of photothrombosis [29]. However, the relative change in signal intensity was higher in our study. ...
... The high EB extravasation on Day 0 was not surprising, as photothrombosis is known to induce a high degree of vasogenic edema at the acute stage of stroke [38], and vasogenic edema is linked with the occurrence of BBB disruption. [27,30,35]. The decrease in EB leakage on Day 1 and Day 2 can be attributed to the reduction in oxygen radicals that trigger in ammation in the hyperacute stage and cause vasogenic edema [39]. ...
Background: Pediatric stroke is a significant cause of childhood mortality and morbidity. The clinical research in this field bears certain limitations that do not exist in the pre-clinical setting. In pre-clinical research, experimental models of ischemic stroke show differences in lesion evolution and blood-brain barrier (BBB) permeability between adult and neonatal rats. However, little is known about these factors in the juvenile stage.
Aims: To characterize the evolution of the lesion, penumbra and degree of BBB permeability in a photothrombotic ring model of juvenile stroke.
Methods: The design is a mixed longitudinal and cross-sectional study. In 14 Sprague Dawley juvenile rats (weight 130-189 g), lesion, penumbra volume and blood-brain barrier (BBB) leakage were measured longitudinally on days 0, 2, and 7 following photothrombotic stroke. Magnetic resonance imaging (MRI) techniques were conducted to measure lesion and penumbra volumes (T2-weighted imaging [T2] and water restriction (diffusion-weighted imaging [DWI]) and BBB leaking (with dynamic contrast-enhanced imaging [DCE]). Histology was performed to confirm stroke (n=9) with Triphenyltetrazolium chloride staining (TTC); (n=3) for Haemotoxylin and Eosin (H&E) staining; and (n=9) Evans Blue (EB) staining to assess BBB permeability.
Results: We found the penumbra volume to be larger and better delineated on MRI and histology in the acute compared to the subacute and chronic stages, and the lesion to be smaller in volume, increasing over time following same time trajectory. The BBB was most compromised at the hyperacute stage (day 0) and decreasingly, yet persistently, disrupted to day 7.
Conclusions: Our in vivo and ex vivo findings provide insight into the evolution of stroke and could serve as a study model to test blood-brain barrier stabilization agents in the pediatric setting.
... The infarct size can be controlled by changing the concentration of Rose Bengal and the duration of light illumination. 258,[262][263][264][265][266][267][268][269] Poor responses to tPA-mediated thrombolytic therapy, which is likely due to a low fibrin content in the platelet-rich clots, was improved by increasing the fibrin content by mixing Rose Bengal (50 mg/kg) with thrombin (80 U/kg). 270 Lu et al. 255 showed that photochemical thrombotic stroke could be induced in conscious animals. ...
Despite recent advances in recanalization therapy, mechanical thrombectomy will never be a treatment for every ischemic stroke because access to mechanical thrombectomy is still limited in many countries. Moreover, many ischemic strokes are caused by occlusion of cerebral arteries that cannot be reached by intra-arterial catheters. Reperfusion using thrombolytic agents will therefore remain an important therapy for hyperacute ischemic stroke. However, thrombolytic drugs have shown limited efficacy and notable hemorrhagic complication rates, leaving room for improvement. A comprehensive understanding of basic and clinical research pipelines as well as the current status of thrombolytic therapy will help facilitate the development of new thrombolytics. Compared with alteplase, an ideal thrombolytic agent is expected to provide faster reperfusion in more patients; prevent re-occlusions; have higher fibrin specificity for selective activation of clot-bound plasminogen to decrease bleeding complications; be retained in the blood for a longer time to minimize dosage and allow administration as a single bolus; be more resistant to inhibitors; and be less antigenic for repetitive usage. Here, we review the currently available thrombolytics, strategies for the development of new clot-dissolving substances, and the assessment of thrombolytic efficacies in vitro and in vivo .
... Early investigations of ADC changes during ischaemia used isotropic diffusion weighting i.e. single axis diffusion sensitisation (25,39,216,221,222,245). If diffusion of water in the CNS is isotropic delineation of the ischaemic lesion is independent of the applied sensitising gradient and unidirectional PGSE techniques would be adequate. ...
This thesis examines the relationship between certain magnetic resonance image (MRI) parameters and the properties of brain tissue studied in vitro. The importance of this derives from the fact that various pathological conditions of brain tissue in vivo can be identified using MRI protocols, but the relationship to underlying physiological changes is easier to study in vitro. The anoxia resistant turtle cerebellum is free of haemodynamic effects and responds to anisosmotic challenge as an osmometer. Apparatus was developed to maintain this preparation within a MRI magnet. Hypotonic solutions, causing cell swelling, led to decreases in apparent diffusion coefficient (ADC) and increases in the MR relaxation time T2; cell shrinking was associated with increases in ADC and decreases in T2. These effects were compared with simple models of the dynamics of tissue compartments, diffusion and relaxation. The ADC data fitted a two compartment model, with fast exchange between extracellular (EC) and intracellular (IC) space. The relaxation data are not well fitted in this way and seem to involve interaction between relaxation and compartment volumes. Directional differences of ADC were observed in relation to tissue micro- and macrostructure. The MRI measurements were weakly anisotropic, although with the available techniques it was not possible to assign the effects in a clear manner to the known oriented and layered arrays of cerebellar cells. Techniques were applied to separate the signals from EC and IC compartments, using (i) an EC contrast agent (Gd-DTPA), and (ii) imaging techniques in which MR sequence parameters influence the visibility of water in a compartment. A lower ADC observed in the presence of the contrast agent is consistent with the conclusions from other data that ADCIC<ADCEC although interpretation of the data may be complicated by intercompartmental exchange and susceptibility effects due to the contrast agent.
... Vasogenic edema examined by T 2 WI is strongly correlated with BBB disruption after stroke [37,38]. This was also observed in present study. ...
Photochemically induced cerebral ischemia is an easy-manipulated, reproducible, relatively noninvasive, and lesion controllable model for translational study of ischemic stroke. In order to longitudinally investigate the characterization of the model, magnetic resonance imaging, 18F-2-deoxy-glucose positron emission tomography, fluorescence, and bioluminescence imaging system were performed in correlation with triphenyl tetrazolium chloride (TTC), hematoxylin-eosin staining, and immunohistochemistry examinations of glial fibrillary acidic protein, CD68, NeuN, von willebrand factor, and α-smooth muscle actin in the infarct zone. The results suggested that the number of inflammatory cells, astrocytes, and neovascularization significantly elevated in peri-infarct region from day 7 and a belt of macrophage/microglial and astrocytes was formed surrounding infarct lesion at day 14. Both vasogenic and cytotoxic edema, as well as blood brain-barrier leakage, occurred since day 1 after stroke induction and gradually attenuated with time. Numerous cells other than neuronal cells infiltrated into infarct lesion, which resulted in no visible TTC negative regional existence at day 14. Furthermore, recovery of cerebral blood flow and glucose utilization in peri-infarct zone were noted and more remarkably than that in infarct core following the stroke progression. In conclusion, these characterizations may be highly beneficial to the development of therapeutic strategies for ischemic stroke.
... The highest level of platelet aggregation occurs at the pial surface since it is exposed to the most direct light. It is possible to monitor the progression of the lesion via nuclear magnetic resonance imaging, and multiple MRI studies have plotted the time course of photothrombotic lesions under different parameters [68,70,88,113,123]. injected into the medial saphenous vein or the tail vein. ...
... Digital images of brain slices were analyzed with ImageJ software for calculating infarct volume as a percentage of whole brain as reported [16]. T2-weighted magnetic resonance imaging (MRI) was also used to evaluate cerebral infarction [18]. Images were collected 24 h after onset of reperfusion. ...
Remote ischemic preconditioning protects against ischemic organ damage by giving short periods of subcritical ischemia to a remote organ. We tested the hypothesis that remote ischemic conditioning can attenuate cerebral stroke in a rat middle cerebral artery occlusion (MCAO) model by microparticles (MPs).
MPs were extracted from healthy rats that underwent hindlimb ischemia-reperfusion preconditioning (RIPC), and were transfused into rats that had undergone MCAO without RIPC. The transfusion resulted in an increase in platelet-derived MPs in blood and reduction in infarction area, confirmed by both 2-3-5-triphenyltetrazolium chloride staining and magnetic resonance imaging, albeit to a lesser degree than RIPC itself. Behavioral tests (modified Neurological Severity Score [mNSS]) were calculated to judge the behavioral change. However, no significant difference was observed after MP transfusion in 24 h or the following consecutive 9 days.
RIPC induces an increase in MPs, and platelet-derived MPs may confer at least part of the remote protective effect against cerebral ischemic-reperfusion injury.
... 106 MRI of photothrombotic stroke shows early increases in T2 signal at the same time as decreased apparent diffusion of water, indicating the simultaneous development of substantial vasogenic edema at the same time as ischemic infarction. 116,117 In acute brain lesions, T2 signal is a function of extracellular water. This means that photothrombosis induces simultaneous vasogenic (extracellular) edema and cytotoxic (intracellular) edema, i.e., leaky vessels and swollen cells at the same time. ...
Rodent stroke models provide the experimental backbone for the in vivo determination of the mechanisms of cell death and neural repair, and for the initial testing of neu-roprotective compounds. Less than 10 rodent models of focal stroke are routinely used in experimental study. These vary widely in their ability to model the human disease, and in their application to the study of cell death or neural repair. Many rodent focal stroke models produce large infarcts that more closely resemble malignant and fatal human infarction than the average sized human stroke. This review focuses on the mechanisms of ischemic damage in rat and mouse stroke models, the relative size of stroke generated in each model, and the purpose with which focal stroke models are applied to the study of ischemic cell death and to neural repair after stroke.
... Importantly, histological evaluation at this late stage of lesion development showed iron deposits merely intracellular, but not in the vessel lumina. It is well established that PT lesions induce early and persistent breakdown of the BBB within the lesion and transiently for 24 h within the entire, morphologically intact ipsilateral cortex [23,24]. Although PFC and SPIO are likely to pass through a broken BBB, the timely and locally restricted patterns of 19 F (this study) and SPIO enhancement [14,15] are not compatible with passive accumulation of these contrast agents within the lesion as seen with the conventional MR contrast agent Gadolinium (Gd)-DTPA [15]. ...
(19)F magnetic resonance imaging (MRI) was recently introduced as a promising technique for in vivo cell tracking. In the present study we compared (19)F MRI with iron-enhanced MRI in mice with photothrombosis (PT) at 7 Tesla. PT represents a model of focal cerebral ischemia exhibiting acute vessel occlusion and delayed neuroinflammation.
Perfluorocarbons (PFC) or superparamagnetic iron oxide particles (SPIO) were injected intravenously at different time points after photothrombotic infarction. While administration of PFC directly after PT induction led to a strong (19)F signal throughout the entire lesion, two hours delayed application resulted in a rim-like (19)F signal at the outer edge of the lesion. These findings closely resembled the distribution of signal loss on T2-weighted MRI seen after SPIO injection reflecting intravascular accumulation of iron particles trapped in vessel thrombi as confirmed histologically. By sequential administration of two chemically shifted PFC compounds 0 and 2 hours after illumination the different spatial distribution of the (19)F markers (infarct core/rim) could be visualized in the same animal. When PFC were applied at day 6 the fluorine marker was only detected after long acquisition times ex vivo. SPIO-enhanced MRI showed slight signal loss in vivo which was much more prominent ex vivo indicative for neuroinflammation at this late lesion stage.
Our study shows that vessel occlusion can be followed in vivo by (19)F and SPIO-enhanced high-field MRI while in vivo imaging of neuroinflammation remains challenging. The timing of contrast agent application was the major determinant of the underlying processes depicted by both imaging techniques. Importantly, sequential application of different PFC compounds allowed depiction of ongoing vessel occlusion from the core to the margin of the ischemic lesions in a single MRI measurement.
... Many pathological changes, such as tumors, edema and hemorrhage, are associated with a change in water content. Therefore, T2-weighted MRI has been extensively applied to delineate lesions in both clinical and experimental diagnostic studies, 30,31 because both T1 and T2 have been reported to correlate with tissue water content. In particular, the T2 change is shown to be larger than the T1 change. ...
Stroke is a major cause of mortality and morbidity in hypertensive patients. This study investigated the effects of nifedipine, an L-type voltage-gated Ca(2+) channel blocker, on ischemic lesion volume after focal cerebral ischemia and reperfusion in rats. Rats were subjected to 1 h of transient middle cerebral artery occlusion (MCAO). At 2 days after MCAO, the rats were randomized into two groups that were fed either a normal control diet (n=10) or a nifedipine (0.001%) containing diet (n=11) for 2 weeks. Nifedipine treatment significantly reduced ischemic lesion volume (116.5 ± 10.8 vs. 80.0 ± 8.2 mm(3), P < 0.05) without affecting body weight or blood pressure. It also decreased thiobarbituric-reactive substances, an index of lipid peroxide, (2.6 ± 0.4 vs. 1.7 ± 0.1 μmol g(-1) tissue, P < 0.05) and increased glutathione peroxidase (54.9 ± 4.7 vs. 70.9 ± 6.4 U g(-1) protein, P < 0.05) and glutathione reductase activities (32.4 ± 1.4 vs. 39.9 ± 2.7 U g(-1) protein, P < 0.05) in the mitochondria from the ischemic hemispheres. These results suggest that nifedipine treatment can reduce ischemic lesion volume after focal cerebral ischemia, possibly because of the decrease in oxidative stress with an increase in antioxidant activities within the ischemic area.
Following a stroke, the ability to discriminate between areas of non-recoverable tissue and potentially salvageable tissue remains a goal of diagnostic imaging. There have been a number of recent developments both in nuclear medicine and in magnetic resonance imaging (MRI) techniques, which may have potential to achieve this goal. This thesis describes the use of these techniques for the evaluation of focal pathophysiology in a rat model of model cerebral artery occlusion (MCAO). Methods were described to use autoradiographic markers to study cerebral blood flow (99mTc-hexamethylpropylene amine oxime) and cerebral hypoxia (125I-iodoazomycin arabinoside) simultaneously following MCAO in the rat. The uptake of these tracers was compared to MRI measurements of the apparent diffusion coefficient of water (ADC) and to histology. It was shown that the combined use of 125I-IAZA or ADC imaging with perfusion imaging may allow differentiation between areas of oligaemic misery perfusion, ischaemic misery perfusion and ischaemic lesion core. MRI was used to examine the effects of cerebral tissue anisotropy on calculation of the spatial distribution of ischaemia in the rat using ADC measurements. Demonstration of grey matter anisotropy led to the use of rotationally invariant ADC measurements which were shown to improve stroke lesion delineation. To investigate the acute changes in NMR parameters of diffusion, perfusion, T1 and T2 a remote controlled MCAO model in the rat was refined for a vertical 8.5T high field magnet. Combined perfusion and diffusion data distinguish between a "moderately affected area", with reduced perfusion but normal diffusion; and a "severely affected area", in which both perfusion and diffusion were significantly reduced. Two novel MRI observations were reported, namely a decrease in T2 and an increase in T1, both within the first few minutes of ischaemia. A rat model of oligaemic misery perfusion was developed in which a large region of homogeneously reduced blood flow was produced without cytotoxic oedema forming. Early NMR changes in CBF, T1 and T2 were noted without reduction in the ADC value. This approach may provide a model of penumbral flow, which may be of value in studying and evaluating neuroprotection.
Background
Neonatal hypoxic-ischemic encephalopathy is brain injury caused by decreased perfusion and oxygen delivery that most commonly occurs in the context of delivery complications such as umbilical cord compression or placental abruption. Imaging is a key component for guiding treatment and prediction of prognosis, and the most sensitive clinical imaging modality for the brain injury patterns seen in hypoxic-ischemic encephalopathy is magnetic resonance imaging.
Objective
The goal of this review is to compare magnetic resonance imaging findings demonstrated in the available animal models of hypoxic-ischemic encephalopathy to those found in preterm (≤ 36 weeks) and term (>36 weeks) human neonates with hypoxic-ischemic encephalopathy, with special attention to the strengths and weaknesses of each model.
Methods
A structured literature search was performed independently by two authors and the results of the searches were compiled. Animal model, human brain age equivalency, mechanism of injury, and area of brain injury were recorded for comparison to imaging findings in preterm and term human neonates with hypoxic-ischemic encephalopathy.
Conclusion
Numerous animal models have been developed to better elicit the expected findings that occur after HIE by allowing investigators to control many of the clinical variables that result in injury. Although modeling the same disease process, magnetic resonance imaging findings in the animal models vary with the species and methods used to induce hypoxia and ischemia. The further development of animal models of HIE should include a focus on comparing imaging findings, and not just pathologic findings, to human studies.
Stroke is a major cause of disability in adults and children. Recently, we have developed an adult rat model of minor stroke containing a peri-infarct region with a modest T2 increase and mild ischemic damage. We hypothesized that a neonatal minor stroke with mild peri-ischemic changes could also be produced, but with potential ontogenic differences. Using our minor photothrombosis method, we produced a range of severities of ischemic lesions (mini, minor, moderate and severe) within magnetic resonance imaging (MRI) slices of adult and neonatal rats. In both age groups, the lesion region showed a marked increase in T2 and diffusion-weighted intensity and decrease in apparent diffusion coefficient (ADC), corresponding to a cortical infarct detected using fluorojade and hematoxylin and eosin staining. Perilesional regions showed modest increases in T2 and ADC in adults, but not neonates, and this corresponded to scattered cell death, but not necessarily extravasation of plasma protein, i.e. blood-brain barrier disruption. Mini and minor insults in neonates generally showed homogeneous and rather modest changes in T2 and ADC. MR perfusion maps demonstrated a penumbral area of greater hypoperfusion in adults compared with neonates. Together, the results indicate that, in neonatal cortex, a similar severity of photothrombosis occurs throughout the area of photoactivation, whereas, in adult brain, spontaneous clot lysis and/or partial thrombosis occurs adjacent to permanently occluded vessels. Thus, by comparing differing severities of photothrombotic ischemia in neonates and adults, ontogenic differences were detectable using MRI, with mature brain having a greater penumbral region. Mild ischemic injury and scattered cell death in both neonates and adults could be identified by a modest increase in T2 and decrease in ADC. A better understanding of the effects of development on ischemic responses and associated MRI changes will provide a basis for the improved diagnosis of mild or minor ischemic insults relevant to pediatric and adult stroke.
Imaging energy metabolites as markers of the energy shuttle between glia and neurons following ischemia is an ongoing challenge. Traditional microscopies in combination with histochemistry reveal glycogen accumulation within glia following ischemia, indicating an altered metabolic profile. Although semi-quantitative histochemical glycogen analysis is possible, the method suffers from typical confounding factors common to histochemistry, such as variation in reagent penetration and binding. In addition, histochemical detection of glycogen does not reveal information on the metabolic fate of glycogen (i.e., lactate production). Therefore, validation of a direct semi-quantitative method to simultaneously image both brain glycogen and lactate in the same tissue section would benefit this research field. In this study, we demonstrate the first application of FTIR spectroscopy for simultaneous direct spectroscopic imaging of brain glycogen and lactate, in situ within ex vivo tissue sections. Serial tissue sections were analysed with GFAP immuno-histochemistry to provide a comparison between the glycogen and lactate distribution revealed by FTIR and the glial distribution revealed by GFAP immuno-histochemistry. The distribution of glycogen revealed by FTIR spectroscopic imaging has been further compared with histochemical detection of glycogen on the adjacent tissue sections. This approach was then applied to study spatio-temporal disturbances in metabolism, relative to glia and neuronal populations, following cerebral ischemia in a murine model of stroke.
The first in vivo study of brain activation by nuclear magnetic resonance (NMR) methods was done early in the development of in vivo NMR techniques to help validate them (Prichard, et al, 1983). Observations on status epilepticus induced by bicuculline in rabbits showed that 31P magnetic resonance spectroscopy (MRS) could reliably demonstrate in vivo the reduction of phosphocreatine and elevation of inorganic phosphate that were known to occur in that condition, as shown in Figure 1.
PURPOSE: To investigate whether ischemic infarct can be staged by evaluating signal intensities on diffusionweighted (DWI) and turbo spin echo T2-weighted images(T2WI).
MATERIALS AND METHODS: DWI and T2WI of 27 patientswith ischemic infarct were retrospectively evaluated. Infarcts were divided into five stages depending on time ofonset : hyperacute within 12 hours, acute between 12 hours and 3 days, subacute between 3 and 10 days, earlychronic between 10 and 30 days, and late chronic after 30 days. Signal intensities of these lesions compared withnormal brain on DWI and T2WI were visually evaluated and divided into six patterns. Pattern 1 included high signalon DWI and iso-signal on T2WI, pattern 2 showed high signal on both DWI and T2WI with higher contrast on DWI,pattern 3 showed the same high signal on both images; pattern 4 revealed high signal on both images with highercontrast on T2WI, pattern 5 showed iso-signal on DWI and high signal on T2WI, and pattern 6 revealed low signal onDWI and high signal on T2WI. These five clinical stages and six MRI patterns were correlated in each patient.
RESULTS: Six cases were hyperacute, six were acute, eight were subacute, three were early chronic, and four werelate chronic. At the hyperacute stage, five cases showed pattern 1 (83%) and one case, pattern 2. At the acutestage, all six cases showed pattern 2. At the subacute subcute stage, seven cases showed pattern 2(87.5%) and onecase pattern 4. At the early chronic stage, two cases showed pattern 2(66.7%) and one case, pattern 5. At the latechronic stage, all cases showed pattern 6.
CONCLUSION: DWI is useful for the detection of early ischemic infarct,and stages of ischemic infarcts can be estimated by evaluating signal intensities on DWI and T2WI.
One of the most important issues for the therapy of cerebral stroke is the early detection of the ischaemic site and the identification of the penumbra surrounding the core of the lesion. In this context, we have studied the potential of a superparamagnetic contrast agent on a rat model of Middle Cerebral Artery occlusion (MCA-o). T2-weighted NMR images acquired after injection showed the ischaemic lesion as soon as one hour following the MCA-o. The use of the contrast agent revealed, on the diffusion-weighted images, a cortical zone where the signal intensity was reduced in comparison with the images before injection. This cortical zone may correspond to the penumbra, as also suggested by perfusion autoradiography.
T1 and magnetization transfer at a field strength of 7 Tesla were used to discriminate between water accumulation and protein mobilization in tissue undergoing infarction. Twelve rats subjected to acute stroke via intralumenal suture occlusion of the middle cerebral artery, and 19 controls, were studied. In MRI studies to 6 hr post-ictus, serial data acquisition allowed the measurement of cerebral blood flow (CBF), apparent diffusion coefficient of water (ADCw), equilibrium magnetization (M0) and T1, and equilibrium magnetization and T1 under an off-resonance partial saturation of the macromolecular pool (Msat and T1sat). Using these parameters, the apparent forward transfer rate of magnetization between the free water proton pool and the macromolecular proton pool, kfa, was calculated. Regions of interest (ROIs) were chosen using depressed areas in maps of the ADCw. T1 measurements in bovine serum albumin at 7T were not affected by the mobility of the macromolecular pool (P > 0.2), but magnetization transfer between free water and protein depended strongly on the mobility of the macromolecular pool (P < 0.001). For 6 hr after ictus, kfa uniformly and strongly decreased in the region of the infarct (P < 0.0001). Ratios (ischemic/non-ischemic) of parameters M0, Msat, T1, and T1sat all uniformly and strongly increased in the infarct. The ratio T1/T1sat in the region of infarction showed that a progressive accumulation of free water in the region of interest was the major (>80%) contribution to the decrease in kfa. There also existed a small contribution due to changes at the water-macromolecular interface, possibly due to proteolysis (P = 0.005). Magn Reson Med 41:696–705, 1999.
Background and purpose:
This study was performed to document the progression of ischemic brain damage after middle cerebral artery occlusion in the rat using magnetic resonance imaging and histopathologic methods.
Methods:
Cerebral ischemia was induced through permanent tandem occlusion of ipsilateral middle cerebral and common carotid arteries. The evolution of magnetic resonance imaging and histopathologic parameter changes was studied, both short term (1.5 to 8 hours) and long term (24 to 168 hours), in five specific brain regions within the middle cerebral artery territory.
Results:
Significant changes in proton nuclear magnetic resonance spin-lattice and spin-spin relaxation times and the "apparent" diffusion coefficient of water could be detected within hours after the onset of permanent focal cerebral ischemia, whereas significant alterations in proton spin-density ratios were not apparent until approximately 48 hours. Histological changes were evident within 12 hours, with a significant loss of neurons seen in the most severely damaged regions at 7 days. Diffusion-weighted imaging was the most sensitive technique for visualizing acute ischemic alterations. The water diffusion coefficient was the only magnetic resonance imaging parameter studied to indicate significant alterations within the first 4 hours after arterial occlusion in all five brain regions.
Conclusions:
The degree of change for a particular magnetic resonance imaging parameter appeared to be related to the location and extent of neuronal injury, with the most dramatic changes occurring within the areas displaying the most severe histological damage. These results indicate that complete specification of all brain regions affected by ischemic brain injury may require a combination of imaging strategies applied over a period of days and suggest the possibility of using magnetic resonance imaging to distinguish between permanent and reversible cell damage.
Ischaemia is a condition in which blood flow either drops to zero or proceeds at severely decreased levels that cannot supply sufficient oxidizable substrates to maintain energy metabolism in vivo. Brain, a highly oxidative organ, is particularly susceptible to ischaemia. Ischaemia leads to loss of consciousness in seconds and, if prolonged, permanent tissue damage is inevitable. Ischaemia primarily results in a collapse of cerebral energy state, followed by a series of subtle changes in anaerobic metabolism, ion and water homeostasis that eventually initiate destructive internal and external processes in brain tissue. (31)P and (1)H NMR spectroscopy were initially used to evaluate anaerobic metabolism in brain. However, since the early 1990s (1)H Magnetic Resonance Imaging (MRI), exploiting the nuclear magnetism of tissue water, has become the key method for assessment of ischaemic brain tissue. This article summarises multi-parametric (1)H MRI work that has exploited diffusion, relaxation and magnetisation transfer as 'contrasts' to image ischaemic brain in preclinical models for the first few hours, with a view to assessing evolution of ischaemia and tissue viability in a non-invasive manner.
Nuclear magnetic resonance (NMR) methods for the noninvasive, chemically specific investigation of living tissue have proliferated in number and advanced greatly in power since the first NMR images of anatomy appeared in the 1970s. By 1994, about 50 different NMR measurements had become available for the study of normal and pathological brain. These include aspects of biochemistry, angiography, perfusion, activation- sensitive metabolic rates for glucose and oxygen, monitoring of function through activation-induced changes in blood flow and water diffusion, and normal and pathological anatomy with submillimeter resolution. At least as many more measurements of biomedical importance are under development. Neuroscience research and management of neurological illness will be profoundly affected by NMR methods as they mature and become routine.
Advances in magnetic resonance imaging (MRI) have moved the technology beyond its application solely as a diagnostic test to become a tool for addressing questions of in vivo pathophysiology and higher cortical function in humans. Diffusion-weighted MRI measures the apparent rate of translational movement of water molecules through brain parenchyma. This measurement can be used to determine axonal orientation within white matter, to define regions of tissue edema, and to permit early identification of ischemic neuronal injury related to impairment of Na+-K +-ATPase activity in experimental and human stroke. Changes in various aspects of cerebral perfusion—blood volume, blood flow, and hemoglobin oxygen saturation—can be mea sured with MRI, and altered cerebrovascular circulation and regional brain activation can thereby be inves tigated. Echo planar imaging is a method of ultrafast data acquisition with MRI—individual images are ac quired on the order of 100 msec. Echo planar imaging makes diffusion and perfusion measurements more practicable for diverse applications and allows for the study of temporal characteristics of regional brain responses to stimuli. Diffusion and perfusion MRI, generally termed functional MRI, are tools for studying in vivo brain physiology with MRI and are being applied to a broad range of questions in neuroscience. The Neuroscientist 1:221-235, 1995
The relaxation properties of water and metabolites were measured in rat brain following the occlusion of the middle cerebral artery (MCA) with localized 1H MRS. The PRESS sequence was employed to select volumes of 39 m̈L in the ischemic and the contralateral hemisphere. T1 and T2 relaxation times and peak intensities of water, choline containing compounds (Cho), creatine and phosphocreatine (Cre) and N-acetyl aspartate (NAA) in both hemispheres were determined at 3-6 h, 1 day and 3 or 4 days after occlusion. Lactate in the ischemic hemisphere was also quantified. The relaxation properties and peak intensities of NAA, Cre and Cho remained unchanged in the ischemic volume during the first 3-6 h of ischemia as compared to the contralateral volume. Water T1 was slightly increased in the ischemic volume. After 24 h the T1 and T2 of water and Cre and the T1 of Cho had increased significantly in the ischemic volume, while the peak intensities of Cho, Cre and NAA were reduced. It appears therefore that tissue changes which occur in the early phase of ischemia have no significant effects on the relaxation behaviour of the metabolites. However, ischemic brain damage affects the relaxation behaviour and concentration of the metabolites and water at later stages.
Diffusion- and T2-weighted MRI were used to evaluate changes in brain water characteristics following closed-head injury in rats. Images were collected within the first 2 h and at 24 h and 7 days following the traumatic event and then compared with histology. The ratios between the apparent diffusion coefficients (ADCs) of the traumatized tissues and normal brain tissues were significantly different from unity and were found to be 0.79 ± 0.25 (p < 0.01), 0.49 ± 0.33 (p < 0.0002), and 3.47 ± 1.36 (p < 10−6) at 1–2 h, 24 h, and 1 week after the trauma, respectively. In severe trauma, areas of hyperintensity which were not apparent on the T2-weighted images could be detected on the diffusion-weighted images within 1–2 h after the trauma. At 24 h following the traumatic event, large areas of hyperintensity are observed in both types of images. One week following the trauma, the ADCs of the traumatized tissues (1.84 ± 0.69 × 10−5 cm2/s) are much larger than those of normal brain (0.57 ± 0.19 × 10−5 cm2/s) and approach the value of free water. At 7 days, the areas of hyperintensity in the T2-weighted images seem to underestimate the injured areas found by histology. At this time point a good correlation is obtained between the areas of hypointensity observed on the diffusion-weighted images and the infarct areas obtained by histology (r = 0.88).
Global forebrain ischemia in the Mongolian gerbil is a common animal model for use in stroke research. We produced lesions of graded severity in gerbil brains (after prescreening by MR imaging) by performing 6 minute bilateral carotid artery occlusions while monitoring pericranial temperature with a temporalis muscle thermocouple probe and maintaining the temperature at 32°C 36°C. or 40°C. Lesion severity was scored 4 days after occlusion from findings on spin-echo images acquired at 7 T and from histologic scores. Statistically signiflcant correlation was observed between the MR imaging score and brain temperature and between the MR imaging score and the area of the CA region of the hippocampus measured by histology. In addition, because prescreening with MR imaging revealed abnormalities in the hippocampus of some of the animals, and these animals were rejected from the study. the statistical significance of the result could be strengthened.
The principal barrier to clinical application of diffusion-weighted MR imaging is the severe image degradation caused by patient motion. One way to compensate for motion effects is the use of a “navigator echo” phase correction scheme. In this work, a modification of this technique is introduced, in which the phase correction step is performed in the frequency domain (i.e., after the readout Fourier transform). This significantly improves the robustness of the navigator echo approach and, when combined with cardiac gating, allows diagnostic quality diffusion-weighted images of the brain to be routinely obtained on standard clinical scanner hardware. The technique was evaluated in phantom studies and in 23 humans (3 normal volunteers and 20 patients). Diffusion anisotropy and apparent diffusion coefficient maps were generated from the image data and showed decreased apparent diffusion in acute stroke lesions and, in several cases, increased apparent diffusion in chronic stroke lesions.
A reproducible technique has been developed to create reversible middle cerebral artery (MCA) stenosis in the cat. Under pentobarbital anesthesia, the right eye was enucleated, MCA dissected and a suture looped around the vessel. A short segment of thicker suture was set horizontally on the MCA. The looped suture was then tied and the segment suture removed, resulting in partial stenosis. The knot could be easily removed, restoring blood flow.
Perfusion and diffusion‐sensitive magnetic resonance (MR) images were obtained over the next 6 hours to document cerebral blood flow abnormalities and developing ischemic changes, respectively. Laser‐Doppler was performed to further document reduced flow through the ipsilateral hemisphere following MCA stenosis. Histopathologic sections were assessed for ischemic damage.
Our results show this to be a reliable model of MCA stenosis and reperfusion, which is suitable for the evaluation of acute cerebral perfusion abnormalities resulting from arterial stenosis. © 1994 Wiley‐Liss, Inc.
Diffusion-weighted single voxel experiments conducted at b-values up to 1 × 104 smm−2 yielded biexponential signal attenuation curves for both normal and ischemic brain. The relative fractions of the rapidly and slowly decaying components (f1, f2)are f1 = 0.80 ± 0.02, f2 = 0.17 ± 0.02 in healthy adult rat brain and f1 = 0.90 ± 0.02, f2 = 0.11 ± 0.01 in normal neonatal rat brain, whereas the corresponding values for the postmortem situation are f1 = 0.69 ± 0.02, f2 = 0.33 ± 0.02. It is demonstrated that the changes in f1 and f2 occur simultaneously to those in the extracellular and intracellular space fractions (fex, fin) during: (i) cell swelling after total circulatory arrest, and (ii) the recovery from N-methyl-D-aspartate induced excitotoxic brain edema evoked by MK-801, as measured by changes in the electrical impedance. Possible reasons for the discrepancy between the estimated magnitude components and the physiological values are presented and evaluated. Implications of the biexponential signal attenuation curves for diffusion-weighted imaging experiments are discussed.
The development of the Rose Bengal model of cerebral infarction in the rat is monitored using rapid NMR imaging techniques. The lesion volume,T
1,T
2, and the apparent diffusion coefficient (ADC) are measured in the time interval between 24 and 168 h postinduction. VolumetricT
1-weighted 3D imaging of the lesions produced by this model is shown to have a standard error of about 5% over the first 3 days postinduction, rising to about 15% at 1 week. Both theT
1 andT
2 contrasts are found to be more durable than those in ADC and are, hence, more suitable for volumetric monitoring over this time period. An elevation in ADC is also observed in tissue outside the lesion and this is interpreted as being caused by the transport of edema away from the lesion.
The apparent diffusion coefficient (ADC) of rat brain was determined for the cortex [(77123)10–6 m2/s] and caudate-putamen (CP) [(60125)10–6 m2/s]. Using the ultrafast imaging technique U-FLARE changes in ADC were followed with a 2.4-min temporal resolution after the induction of total circulatory arrest by intravenous KC1 injection. For both tissue types, a biphasic decrease of ADC was observed. The initial fast phase led to an ADC decrease by (274)% (cortex) and (293)% (CP) within 5 min, whereas the slow continuous decrease of the second phase resulted in (683)% (cortex) and (663)% (CP) of control after 18 min. The similar relative reduction in ADC for the cortex and the CP meant that an effective distinction between both tissue types persisted after the cessation of systemic and cerebral blood flow.
The study investigated the correlation between infarction areas and behavioural deficits in middle cerebral artery occlusion (MCAO) and photothrombosis stroke models. In the MCAO model, a 0.38 mm-diameter silicone-coated thread was introduced through the left external carotid artery and advanced 18 mm via the internal carotid artery to the origin of middle cerebral artery of male Sprague-Dawley rats weighing 300-350 g. The thread was removed for reperfusion after occlusion for 0.5, 1 or 2h. In the photothrombosis model, after a midline incision on the scalp, a focused light (10,000 lux, 6 mm-diameter) was delivered 1mm anterior to the bregma and 3mm left of the midline for 5, 10 or 20 min. During the first 2 min of irradiation, Rose Bengal dye (30 mg/kg) was injected intravenously. Twenty four hours post-surgery, the animals were subjected to neurological scoring and behavioural performances, and were sacrificed for macroscopic and microscopic examinations of brain injury. Total infarction volumes in the MCAO model rats increased in an occlusion time-dependent manner, while the infarction volumes in photothrombosis model rats plateaued relatively quickly with no time-dependent increase. The MCAO model displayed neurological scores and behavioural deficits that correlated well with infarction volumes, while relatively poor correlation between infarction volume and neurobehavioural abnormalities was evident in the photothrombosis model. The results indicate the suitability of the MCAO model for studies on preventive or therapeutic compounds related to functional recovery, although the photothrombosis model might be useful to generate focused lesions leading to the location-related behavioural changes.
Magnetic susceptibility contrast-enhanced and diffusion-weighted echo planar magnetic resonance (MR) imaging was performed using a cat model of acute regional cerebral ischemia induced by partial stenosis of the right middle cerebral artery (MCA). The imaging data were correlated with triphenyltetrazolium chloride (TTC)-stained histopathologic coronal brain sections to determine the prognostic efficacy of high-speed MR imaging techniques in differentiating mild, moderate, and severe cerebral hypoperfusion. Brains of animals without cortical injury on TTC staining were found to have a reduction in peak contrast enhancement of 32 +/- 6% (mean +/- SD) below control values with no significant change in the apparent diffusion coefficient (ADC), determined from the diffusion-weighted MR images. In cases where moderate ischemic injury was observed in the TTC-stained sections, a 10-20% drop in the ADC was found over the 6-h study period, accompanied by a much wider variation in peak contrast enhancement. Finally, where TTC staining showed severe ischemic brain damage, a 40-50% drop in ADC and a reduction in peak contrast enhancement effect of > 95% were observed as early as 1 h following MCA stenosis. The significant correlation between imaging observations and histologically confirmed cerebral ischemia indicates that magnetic susceptibility contrast-enhanced echo planar MR imaging is sensitive to slight reductions in cerebral perfusion that fall below the threshold for reliably detectable ischemia-induced alterations in ADC. First-pass perfusion-sensitive imaging may thus be diagnostically useful in differentiating severely hypoperfused permanently injured tissue from the mildly hypoperfused ischemic penumbra.
High speed magnetic resonance imaging (MRI) and short diffusion times are used to investigate the appearance of restricted diffusion in three different models of cerebral infarction. The models are: the middle cerebral artery occlusion (MCAO) model in the rat, the carotid occlusion model in the gerbil, and the Rose Bengal microvascular occlusion model in the rat. All three were investigated for 16 b-values equally spaced between 10 and 1510 s/mm2 using two distinct experiments. In the ct (constant time) experiment, the diffusion time was held constant at 11.7 ms while the b-value was varied with the gradient strength. In the cg (constant gradient) experiment, the gradient strength was held constant and the b-value increased by varying the diffusion time from 4.4 to 11.7 ms. A monoexponential decay of the signal intensity with b-value in the ct experiment accompanied by nonmonoexponential (NME) decay in the cg experiment is indicative of restricted diffusion. As this phenomenon is detectable only at short diffusion times, it cannot be due to restriction by impermeable membranes, and we have thus termed this apparent restriction. For the MCAO model and the carotid occlusion model, apparent restriction was found both inside the infarct territory and in some regions outside it. No definite evidence for restriction was found for the Rose Bengal model, which was, however, only studied from 24 h post-insult.
Apparent diffusion coefficients (ADCs) of tissue water were determined in chronic brain lesions of a rat stroke model, the stroke-prone spontaneously hypertensive rat, and compared with histology. ADCs increased in the order normal < edema < gliosis < cyst. The differences between individual groups were statistically significant. The increase in ADC is thought to mainly reflect a relative increase in the extracellular space in brain tissue. ADC may be a new parameter for tissue characterization.
In experimental stroke research one of the key questions is whether animals models truly reflect human disease. One important reason for modelling stroke is to produce tissue changes that can be used to validate the correlation between the results of advanced radiological imaging and pathology. The aim of this study was to compare two minimally invasive rat models of focal ischaemic stroke as to their ability to simulate clinical stroke reproducibly. In one model a focal infarct was induced using the photochemical Rose Bengal technique. The second model was based on transvascular occlusion of the middle cerebral artery using a cervical approach. In all animals we performed MRI at different times with two different paramagnetic contrast agents. The time course of blood-brain-barrier disruption in the Rose Bengal model differed entirely from that in human brain ischaemia. The experimental stroke showed marked contrast enhancement in the first hour after the onset of ischaemia. On the other hand, the MRI changes in the suture occlusion model were very similar to the changes observed in human brain ischaemia: no early disruption of the blood-brain-barrier and increased T2-signal 4-6 h after the onset of stroke. We hope that the suture occlusion model will be used for research not only by basic and clinical scientists but also by radiologists, who, using modern imaging methods, may greatly influence the diagnosis as well as the management of this devastating disease.
The purpose of the present study was to quantify the early changes which occur on diffusion-weighted imaging following focal cerebral ischaemia. We have developed a method for the remote occlusion of the middle cerebral artery in the rat, allowing early changes to be monitored and images to be acquired before and after ischaemia under identical conditions. Sequential diffusion-weighted images were acquired before and for up to 6 h following the occlusion. The data show that a hyperintense area appears soon after occlusion which expands during the first 2 h. It has previously been shown that the hyperintensity observed after ischaemia is linked to events associated with energy failure. Our findings are in good agreement with previous pathophysiological studies of focal ischaemia, which suggest a growing of the energy depleted area. In conclusion, our data extend the usefulness of diffusion-weighted imaging to the study of the early spatial evolution of the most compromised area after focal ischaemia. Diffusion-weighted imaging may constitute a very powerful tool to investigate pharmacological therapies which may interact with this evolution, both in experimental and clinical studies.
Rats treated with 250 mg/kg/day vigabatrin showed lesions detected by magnetic resonance imaging (MRI) in the cerebellar white matter in vivo. No lesions were seen in any control animal. As well as these visually apparent lesions, quantitative T2 relaxation time measurements showed a 12 ms increase in cerebellar white matter from 66 +/- 4 ms (SD, n = 5) to 78 +/- 2 ms (SD, n = 7). This region, as expected from previous studies, showed microvacuolation on post-mortem pathology. Additionally, significant increases in T2 relaxation times of 4-9 ms were found in the cerebral cortex, thalamus and hippocampus. Microvacuolation was not detected by post-mortem histopathology in the cerebral cortex or hippocampus, however, immunohistochemical staining for glial fibrillary acidic protein and for macrophages (ED1) showed reactive astrocytes (gliosis) and in more severe cases, microglial proliferation in these regions; such changes were also seen in association with the microvacuoles. No T2 increase was found in the cerebellar grey matter or olfactory bulbs. MRI techniques, including T2 relaxometry, are therefore sensitive for detecting vigabatrin-induced changes, including reactive astrocytosis, microglial proliferation and vacuolation in the rat brain. These results suggest that quantitative MRI should be a useful method for evaluating whether vigabatrin has neuropathological effects when given to patients.
Diffusion-weighted magnetic resonance (MR) images from rats during acute cerebral ischemia induced by middle cerebral artery occlusion were analyzed for correspondence with changes in brain water, cation concentrations, and Na+,K(+)-ATPase activity measured in vitro after 30 or 60 min of ischemia. In the ischemic hemisphere, signal intensity was increased at 30 min (p < 0.05 vs contralateral hemisphere) and further increased at 60 min. Na+,K(+)-ATPase activity was 34% lower in ischemic cortex and 40% lower in ischemic basal ganglia after 30 min (p < 0.05), but water content and Na+ and K+ concentrations were not significantly different between hemispheres. After 60 min, water content and Na+ concentration were increased, and both Na+,K(+)-ATPase activity and K+ concentration were decreased in the ischemic hemisphere (p < 0.05). These findings are consistent with the hypothesis that the early onset of signal hyperintensity in diffusion-weighted MR images may reflect cellular edema associated with impaired membrane pump function. Early in vivo detection and localization of potentially reversible ischemic cerebral edema may have important research and clinical applications.
DWI, diffusion-weighted imaging; EPI, echoplanar imaging; MRA, magnetic resonance angiography; MRI, magnetic resonance imaging; MRS, magnetic resonance spectroscopy; NMR, nuclear magnetic resonance; PET, positron emission tomography.
This study was performed to document the progression of ischemic brain damage after middle cerebral artery occlusion in the rat using magnetic resonance imaging and histopathologic methods.
Cerebral ischemia was induced through permanent tandem occlusion of ipsilateral middle cerebral and common carotid arteries. The evolution of magnetic resonance imaging and histopathologic parameter changes was studied, both short term (1.5 to 8 hours) and long term (24 to 168 hours), in five specific brain regions within the middle cerebral artery territory.
Significant changes in proton nuclear magnetic resonance spin-lattice and spin-spin relaxation times and the "apparent" diffusion coefficient of water could be detected within hours after the onset of permanent focal cerebral ischemia, whereas significant alterations in proton spin-density ratios were not apparent until approximately 48 hours. Histological changes were evident within 12 hours, with a significant loss of neurons seen in the most severely damaged regions at 7 days. Diffusion-weighted imaging was the most sensitive technique for visualizing acute ischemic alterations. The water diffusion coefficient was the only magnetic resonance imaging parameter studied to indicate significant alterations within the first 4 hours after arterial occlusion in all five brain regions.
The degree of change for a particular magnetic resonance imaging parameter appeared to be related to the location and extent of neuronal injury, with the most dramatic changes occurring within the areas displaying the most severe histological damage. These results indicate that complete specification of all brain regions affected by ischemic brain injury may require a combination of imaging strategies applied over a period of days and suggest the possibility of using magnetic resonance imaging to distinguish between permanent and reversible cell damage.
Diffusion magnetic resonance imaging (MRI) can quantitatively detect focal ischemic injury within minutes of onset, and perfusion MRI can evaluate the brain's microcirculation. N-Methyl-D-aspartate (NMDA) antagonists and reperfusion can reduce lesion size in stroke models. We used diffusion and perfusion MRI to evaluate the in vivo effects of a noncompetitive NMDA antagonist, CNS 1102, in a temporary ischemia model.
Sixteen Sprague-Dawley rats underwent suture occlusion of the middle cerebral artery. Fifteen minutes after occlusion, animals were randomly assigned to treatment with CNS 1102 (n = 10) or placebo (n = 6), receiving a bolus of 1.13 mg/kg at that time and an infusion of 0.785 mg.kg-1.h-1 for the next 165 minutes. The placebo group received a saline bolus and infusion. Diffusion MRI studies by a spin-echo technique were initiated 30 minutes after occlusion and repeated every 30 minutes for the next 3 hours. Perfusion MRI studies were obtained using echo-planar imaging after injection of superparamagnetic iron oxide particles, immediately before and 15 minutes after withdrawal of the occluder at 3 hours after middle cerebral artery occlusion. At 24 hours, the animals were clinically evaluated (scale of 0 to 5) and electively killed, and the brain was stained with triphenyltetrazolium chloride to evaluate infarct size.
Diffusion imaging demonstrated markedly reduced ischemic lesion area in the CNS 1102 group during occlusion--10.5 +/- 7.3% (mean +/- SEM) of the ischemic hemisphere (optic chiasm slice) at 30 minutes after occlusion versus 50.0 +/- 2.7% of the hemisphere in controls (P < .02). With reperfusion after 3 hours of temporary ischemia, diffusion imaging documented an additional 29% reduction of the ischemic lesion area in the CNS 1102-treated group (P < .01) compared with the prereperfusion ischemic lesion area, with no change in the placebo group. During occlusion, perfusion imaging demonstrated a relative signal intensity decline of 31.5 +/- 7.7% in controls and 83.4 +/- 7.6% in the CNS 1102 group (P < .005), indicating better perfusion in the latter group. After removal of the occluder, perfusion improved in both groups and was not significantly different. Post mortem infarct volume was 53.8 +/- 20.0 mm3 in the CNS 1102 group and 216.8 +/- 16.1 mm3 in the controls (P < .0001). Clinical outcome at 24 hours was 1.1 +/- 0.4 in the CNS 1102 group and 4.0 +/- 0.5 (scale of 0 to 5) in the controls (P < .005).
This study demonstrates that CNS 1102 reduces early postischemic injury as documented by diffusion MRI and improves perfusion as documented by perfusion MRI and that reperfusion confers additional reduction of ischemic lesion size.
Triethyl tin(TET)-induced cerebral oedema has been studied in cats by magnetic resonance imaging (MRI), and the findings correlated with the histology and fine structure of the cerebrum following perfusion-fixation. MRI is a sensitive technique for detecting cerebral oedema, and the distribution and severity of the changes correlate closely with the morphological abnormalities. The relaxation times, T1 and T2 increase progressively as the oedema develops, and the proportional increase in T2 is approximately twice that in T1. Analysis of the magnetisation decay curves reveals slowly-relaxing and rapidly-relaxing components which probably correspond to oedema fluid and intracellular water respectively. The image appearances taken in conjunction with relaxation data provide a basis for determining the nature of the oedema in vivo.
Correlations between T1 and T2 relaxation times and water and electrolyte content in the normal and ischemic rat and gerbil brains were studied by means of both nuclear magnetic resonance (NMR) spectroscopic and imaging methods. In the spectroscopic experiment on excised rat brains, T1 was linearly dependent on tissue water content and T2 was prolonged in edematous tissue to a greater extent than expected by an increase in water content, showing that T2 possesses a greater sensitivity for edema identification and localization. Changes in Na+ and K+ content of the tissue mattered little in the prolongation of relaxation times. Serial NMR imaging of gerbil brains insulted with permanent hemispheric ischemia offered early lesion detection in T1- and especially T2-weighted images (detection as soon as 30 min after insult). The progressive nature of lesions was also imaged. Calculated T1 and T2 relaxation times in regions of interest correlated excellently with tissue water content (r = 0.892 and 0.744 for T1 and T2, respectively). As a result, detection of cerebral ischemia utilizing NMR imaging was strongly dependent on a change in tissue water content. The different nature of T1 and T2 relaxation times was also observed.
This chapter discusses the exchange of labeled proteins and electrolytes between circulating blood and edematous brain tissue. In the chapter, various types of cerebral edemas were produced experimentally. The tissues were then examined uniformly by conventional histological methods and by analyzing their water and electrolyte content. The dynamic exchange of constituents between blood and brain tissue was studied with trypan blue, radioactive iodinated serum albumin, 14C-labeled amino acids, and 24Na, 42K and 32P. The temporal and spatial aspects of the exchange of labeled substances were determined and correlated to the chemical composition of the tissue, including its regional alterations. The experimental prototype of focal and traumatic edema was produced by freezing the dura protected cortex. The resulting swelling was essentially localized to the white matter and was accompanied by hypertrophic changes of the astrocytes. The experiments reveal that this swelling is caused by a protein-rich serum exudate that originates in the cortical lesion. The process starts by an increased permeability of the involved cortical capillaries and continues by a gradual spread of the exudate into the underlying white matter. Microscopic radioautography revealed even distribution of albumin without any predilection to any particular cell. There is no evidence of primarily increased permeability in the edematous white matter itself.
The authors present the results of an investigation of the vasogenic type of brain edema using cold injury in cats as a model. Their findings indicate that bulk flow and not diffusion should be considered the main mechanism for the spread of edema through the white matter. This conclusion is based on: 1) comparison of the distances actually traveled by various substances during edema spread with those calculated theoretically for migration of the substances by diffusion; 2) coincidence in the speed of movement by two substances (sucrose and albumin) with widely different diffusion coefficients; 3) measurement of interstitial fluid pressure (IFP) at various distances from the lesion showing the presence of increased IFP in the lesion area and decreasing pressures along the edema pathway toward the normal tissue; and 4) the fact that spreading of edema can be significantly impeded by inducing before the cold lesion an intracellular type of brain edema that reduces the size of the extracellular space (ECS) and increases the resistance to flow of edema fluid. The pressure-volume curve of the brain ECS, as derived from determinations of IFP and tissue water content, indicates that initial steep slope in IFP probably represents the high resistance to fluid mobility through the small diameter extracellular channels and the counteracting resistance of the intermingled structures of brain parenchyma to be separated. Once the IFP exceeds these opposing forces, the ECS dilates, fluid mobility increases, and the edema front advances.
We present T2-weighted and diffusion-weighted images of kainic acid lesions in the rat brain. Our observations show improved image contrast between edematous lesions and unaffected tissue using diffusion-weighted imaging. Furthermore, we show that the anisotropic intensity changes associated with this sequence can be used to highlight white matter tracts and to provide information concerning their orientation in the rat brain.
We studied the effect of focal cerebral ischemia on the "state" of brain water using proton nuclear magnetic resonance imaging. Focal cerebral ischemia was induced in five halothane-anesthetized rats via tandem occlusion of the left common carotid artery and the left middle cerebral artery. The proton transverse relaxation time, the proton density, and the water diffusion coefficient were measured at various times from the same region of brain tissue from 1.5 to 168 hours after occlusion. Early measurements indicated significant changes in the transverse relaxation time (p = 0.004) and water diffusion coefficient (p = 0.002) of ischemic brain tissue compared with a homologous region from the contralateral hemisphere. However, the transverse relaxation time, proton density, and water diffusion coefficient in ischemic brain tissue showed different temporal evolutions over the study period. Diffusion coefficient weighting was superior to relaxation time and proton density weighting for the visualization of early cerebral ischemia. Our data suggest that nuclear magnetic resonance imaging is sensitive in detecting changes in proton-associated parameters during early cerebral ischemia and confirm significant changes (p less than or equal to 0.01) in the temporal evolution of transverse relaxation times, proton densities, and diffusion coefficients following middle cerebral artery occlusion.
The sensitivity of diffusion-weighted MRI was compared to that of T2-weighted MRI following temporary middle cerebral artery occlusion (MCA-O) for 33 min followed by 4 h of reperfusion in rats. Diffusion-weighted spin-echo images using strong gradients (b value of 1413 s/mm2) demonstrated a significant increase in signal intensity in ischemic regions as early as 14 min after onset of ischemia in comparison to the normal, contralateral hemisphere (p less than 0.05). This hyperintensity returned to baseline levels during reperfusion. T2-weighted images showed no evidence of brain injury during the temporary occlusion. In three rats subjected to permanent MCA-O, diffusion-weighted MRI demonstrated an increased signal intensity on the first image following occlusion and continued to increase during the 4-h observation period. T2-weighted images failed to demonstrate significant injury until approximately 2 h after MCA-O. Signal intensity ratios of ischemic to normal tissues were greater in the diffusion-weighted images than in the T2-weighted MR images at all time points (p less than 0.05). Close anatomical correlation was found between the early and sustained increase in diffusion-weighted MRI signal intensity and localization of infarcts seen on post-mortem histopathology.
The recently established single-shot technique of echo-planar imaging of intravoxel incoherent motion (IVIM) for determining and imaging the variations of microscopic motions of water has been applied to studies of water perfusion in phantoms and to in vivo studies of diffusion and perfusion in cat and human brains. The phantom results demonstrate that perfusion levels comparable with those found in vivo have easily observable and reproducible effects on signal amplitude that are consistent with previous IVIM theory. Reliable measurements of the diffusion coefficient in various types of brain tissue have been obtained. The results for white matter are consistent with the existence of anisotropic diffusion in oriented bundles of myelinated nerve fibers. The results for gray matter can be fitted to the IVIM theory and suggest a value of up to 14% for the fraction of the signal contributed by randomly perfusing fluid in normal cerebral cortex.
Quantitative measurements of perfusion and molecular diffusion were made in human white matter in two orientations of the motion-sensitization gradient to document anisotropy of these parameters. Measurements were localized to a 10 X 10-mm tissue column oriented in an anterior-to-posterior direction in the left cerebral hemisphere just above the body of the left ventricle. This region was selected because of the relatively high directionality of white matter fibers. In this study of five healthy volunteers, strong diffusion anisotropy was observed in all cases. Twofold or greater anisotropy was commonly observed, with the higher diffusion value associated with motion sensitivity along the fiber directions. By combining data from both gradient orientations in all cases, diffusion values of solid tissue ranged from 0.38 X 10(-3) mm2/sec to 1.12 X 10(-3) mm2/sec, and measured perfusion fractions were in the range of 2%-5% (excluding areas highly contaminated by cerebrospinal fluid). Little or no perfusion-fraction anisotropy was observed; however, perfusion measurements were limited by noise. Data were collected without cardiac gating by using a technique that offers good immunity to bulk tissue motion artifacts.
A pulsed magnetic field gradient spin echo technique was used to study the brain of two volunteers and eight patients. The pulsed gradients were applied both perpendicular and parallel to the image slice. Striking changes in signal intensity were demonstrated in white matter depending on the direction in which pulsed gradients were applied. These effects enabled specific white matter tracts to be identified depending on the direction of their fibres. Abnormalities were also demonstrated in these tracts in patients with a variety of diseases, including cases where only minor abnormalities were seen with conventional, highly T2-weighted sequences. The effects were attributed to anisotropically restricted diffusion within white matter. The technique may have application in a wide range of neurological disease and result in better localisation of lesions and improved detection of disease.
Diffusion-weighted MR images were compared with T2-weighted MR images and correlated with 1H spin-echo and 31P MR spectroscopy for 6-8 h following a unilateral middle cerebral and bilateral carotid artery occlusion in eight cats. Diffusion-weighted images using strong gradient strengths (b values of 1413 s/mm2) displayed a significant relative hyperintensity in ischemic regions as early as 45 min after onset of ischemia whereas T2-weighted spin-echo images failed to clearly demonstrate brain injury up to 2-3 h postocclusion. Signal intensity ratios (SIR) of ischemic to normal tissues were greater in the diffusion-weighted images at all times than in either TE 80 or TE 160 ms T2-weighted MR images. Diffusion- and T2-weighted SIR did not correlate for the first 1-2 h postocclusion. Good correlation was found between diffusion-weighted SIR and ischemic disturbances of energy metabolism as detected by 31P and 1H MR spectroscopy. Diffusion-weighted hyperintensity in ischemic tissues may be temperature-related, due to rapid accumulation of diffusion-restricted water in the intracellular space (cytotoxic edema) resulting from the breakdown of the transmembrane pump and/or to microscopic brain pulsations.
The evolution of acute cerebral ischemia was documented by magnetic resonance (MR) imaging in 13 mongrel cats with occlusion of the middle cerebral artery through a transorbital approach. The animals were imaged under anesthesia at intervals from 30 min to 10 days after production of the lesion. An MR imager operating at 0.35 T was used with multislice, multi-spin-echo technique (TR = 500-2000 msec; TE = 28, 56 msec). The animals were sacrificed after imaging for pathologic correlation. Infarcts beyond 4 hr of age were visualized in all subjects. The earliest infarct was seen at 30 min (two cats) as an area of high signal intensity on T2-weighted images. In three other cats, however, 3-hr-old infarcts were not detectable. In one animal, a hemorrhage within a 1-week-old area of infarction was not characterized by MR imaging but was identified on CT scanning. The mass effect of the infarction appeared greatest at 2-4 days after infarction. The basal ganglia showed ischemic effects to best advantage. MR imaging offers previously unavailable sensitivity for the early noninvasive detection of cerebral ischemia in vivo.
A circuit is described for the electrical isolation of double coil probes where separate transmitter and receiver coils are both tuned to the same frequency and are coupled by mutual inductance. There are only passive components in the probe, and so no additional control signals are required for switching between transmit and receive conditions. An analysis of the operation of the circuit is included together with details of component selection and setting-up procedures, thus facilitating the implementation of the design for a variety of applications in both imaging and spectroscopy.
This study was carried out with a recently developed model of focal cerebral ischemia in the rat based on the photochemical induction of thrombotic stroke using the dye Rose Bengal. We examined the change in the volume of the lesion and brain water content, in separate groups of rats, at different times (1, 4, 24, 72, and 168 h) after the induction of the ischemic lesion. The volume of ischemic damage increased rapidly between 1 and 24 h after the ischemic insult and decreased between 24 and 168 h. The lesion at 168 h was significantly larger than that following 1 h of ischemia and similar to that obtained at 4 h, suggesting that the maximum extent of tissue damage (without the involvement of significant edema) was reached within the first 4 h in this model. The enlargement of the lesion after 4 h correlated closely with changes in brain water content.
Alterations in the blood-brain barrier to proteins, and regional water and electrolyte content were documented in a rat model of photochemically induced small-vessel thrombosis leading to infarction. Horseradish peroxidase (HRP) or Evans blue was given immediately following a 2-min photochemical sensitization period. At 5 min following irradiation, multifocal sites of peroxidase extravasation were noted within the irradiated area. Ultrastructural examination revealed endothelial cells filled with HRP which in some cases extended into the basal lamina and extracellular spaces. At 15 min, protein leakage was more pronounced within the irradiated zone and reaction product was also apparent within the subarachnoid and perivascular spaces of brain regions remote from the site of irradiation. Widespread staining on the surface of the irradiated hemisphere was apparent in rats perfused 8 h following Evans blue infusion. Water content increased significantly by 15 min within the irradiated zone but not in brain regions remote from this site. Although vasogenic edema is an early event in this stroke model, increases in water content are restricted to the irreversibly damaged site. In contrast, protein tracer escaping from microvessels coursing within the irradiated zone was widely distributed. These findings implicate endothelial barrier dysfunction in the genesis of tissue injury in this model. Morphological evidence for the capability of macromolecules to escape from a site of evolving infarction and to migrate to distances remote from the area of primary microvascular damage is also discussed.
Cerebral ischemia leading to infarction was produced in rats by intravascular thrombosis induced by a photochemical reaction between systemically injected rose bengal and green light (560 nm) transmitted through the intact skull for a 2-min period. At 2 or 15 min following photochemical sensitization, animals were perfusion-fixed for scanning (SEM) and transmission (TEM) electron microscopic analyses of the cerebral vasculature. At 2 minutes, ultrastructural examination of cortical regions destined to undergo infarction revealed numerous platelet aggregates within both pial and intraparenchymal vessels. Platelets close to the endothelial walls were routinely degranulated with pseudopodia. Endothelial cells were frequently swollen and contained dilated mitochondria and granular endoplasmic reticulum. The endothelial luminal membrane structure was shown by high-power TEM to be focally damaged. If brain temperature was reduced by 4 degrees C during the photochemical sensitization period, the platelet response was inhibited without interfering with other ultrastructural changes. These results are consistent with the hypothesis that photochemically induced endothelial alterations stimulate platelet activation and implicate abnormal endothelial function as a primary event in the pathogenesis of photochemically induced cerebral infarction.
Many aspects of the use of high-resolution nuclear magnetic resonance (NMR) imaging in the examination of brain edema have not been fully explored. These include the quantitation of edema fluid, the ability to distinguish between various types of edema, and the extent to which tissue changes other than a change in water content can affect NMR relaxation times. The authors have compared NMR relaxation times obtained by both in vivo magnetic resonance imaging (MRI) and in vitro NMR spectroscopy of brain-tissue samples from young adult rats with cold lesions, fluid-percussion injury, hypoxic-ischemic injury, bacterial cerebritis, and cerebral tumor. Changes in relaxation times were compared with changes in brain water content, cerebral blood volume, and the results of histological examination.
In general, both in vivo and in vitro longitudinal relaxation times (T 1 ) and transverse relaxation times (T 2 ) were prolonged in the injured hemispheres of all experimental groups. Water content of tissue from the injured hemispheres was increased in all groups. A linear correlation between T 2 (but not T 1 ) and water content was found. Changes in the values of T 1 and T 2 could be used to distinguish tumor from cold-injured tissue. Cerebral blood volume was reduced in the injured hemispheres and correlated inversely with prolongation of T 1 and T 2 . The results of this study suggest that, in a clinical setting, prolongation of T 2 is a better indicator of increased water content than prolongation of T 1 , yet quantitation of cerebral edema based solely upon prolongation of in vivo or in vitro T 1 and T 2 should be undertaken with caution.
Coupling between large transmit and surface-receive coils is eliminated using a lambda/4 cable, terminated with crossed diodes to ground, inserted between the receive coil and ground, and by using a cable of length a little less than lambda/2 inserted between the transmit match capacitance and the normal crossed diodes in the transmit input line.
The present study was undertaken to analyze the roles of brain cations and of the blood-brain barrier (BBB) to albumin in the development of ischemic brain edema. Using the rat middle cerebral artery (MCA) occlusion model, changes in the brain water, sodium, and potassium contents were followed for a period of seven days. The permeability of the BBB to proteins was also followed by 125I-albumin transfer from the blood into the brain. A significant edema developed as early as three hours after MCA occlusion. This progressed rapidly to reach a maximum on the third day, gradually regressing thereafter. The increase in the brain water contents showed a parallel time course to the increase in the sodium and decrease in the potassium contents. A significant increase in the BBB permeability to albumin occurred 72 hours after MCA occlusion. However, there was no correlation between the brain water content and BBB permeability to albumin in the hemispheres studied 72 hours after MCA occlusion. The correlation between the brain water and sodium contents was not clear during the first six hours, but became highly significant thereafter. The data suggest that an increase in the BBB permeability to sodium occurred 12-48 hours after MCA occlusion, which, together with an antecedent intracellular shift of sodium, resulted in a massive influx of water and sodium into the brain. The BBB permeability change to sodium, not to proteins, seems to play a predominant role in the pathogenesis underlying ischemic brain edema.
We have used a photochemical reaction in vivo to induce reproducible thrombosis leading to cerebral infarction in rats. After the intravenous injection of rose bengal, a potent photosensitizing dye, an ischemic lesion was formed by irradiating the left parietal convexity of the exposed skull for 20 minutes with green light (560 nm) from a filtered xenon arc lamp. Animals were allowed to survive from 30 minutes to 15 days after irradiation. Early microscopic alterations within the irradiated zone included the formation of thrombotic plugs and adjacent red blood cell stasis within pial and parenchymal vessels. Scanning electron microscopy revealed frequent platelet aggregates adhering to the vascular endothelium, often resulting in vascular occlusion. Carbon-black brain perfusion demonstrated that occlusion of vascular channels progressed after irradiation and was complete within 4 hours. Histopathological examination at 1, 5, and 15 days revealed that the associated infarct evolved reproducibly through several characteristic stages, including a phase of massive macrophage infiltration. Although cerebral infarction in this model is initiated by thrombosis of small blood vessels, the fact that the main pathological features of stroke are consistently reproduced should permit its use in assessing treatment regimens. Further, the capability of producing infarction in preselected cortical regions may facilitate the study of behavioral, functional, and structural consequences of acute and chronic stroke.
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- Me Moseley
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- Mf Wendland
- Weinstein
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- Bd Watson
- Wd Dietrich
- R Busto
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