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Energy spectrum of Lu 176 from a piece of LSO placed on a 1 diameter, 1 long NaI(Tl) detector. The 511-keV energy peak is shown as a reference.
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The ECAT high-resolution research tomograph (HRRT) is a three-dimensional (3-D)-only dedicated brain positron emission tomograph with LSO and GSO scintillators. In this paper, the system has been looked at as an example of issues that need to be addressed when evaluating LSO-based system following the recent NEMA NU 2-2001 protocols. The LSO scinti...
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... of 2.6% of the Lu in LSO and creates approximately 250 c/s per cubic centimeter of LSO, using the information from the full energy spectrum. In addi- tion to singles and randoms coincidence count rates created by the , a certain number of true coincidence counts may be present due to the cascading gamma rays. Fig. 1 shows the decay of into and Fig. 2 shows the energy spectrum ac- quired with a NaI(Tl) ( diameter long) detector from a 4 4 LSO crystal placed on the ...
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As part of the EXPLORER total-body positron emission tomography (PET) project, we have designed and built a high-resolution, high-sensitivity PET/CT scanner, which is expected to have excellent performance for companion animal whole body and human brain imaging. The PET component has a ring diameter of 52 cm and an axial field of view of 48.3 cm. T...
Citations
... Right: gamma energy spectrum of 176 Lu. The picture comes from [132]. ...
The following thesis presents research which constitutes the first steps towards the construction of a novel SiFi-CC detector intended for real-time monitoring of proton therapy. The detector construction will be based on inorganic scintillating fibers and silicon photomultipliers. The scope of the presented thesis includes the design optimization of the components of the proposed detector, as well as the construction, characterization, and tests of a prototype. The design optimization comprised an extensive systematic comparison of chosen inorganic scintillating materials, different types of scintillator surface modifications (wrappings and coatings), and different types of interface materials ensuring optical contact between the scintillators and the photodetector. The propagation of scintillating light in all investigated samples was described using two models: the exponential light attenuation model (ELA), and the exponential light attenuation model with light reflection (ELAR). The two models yielded the corresponding methods for energy and position reconstruction. Furthermore, the samples were investigated for energy and position resolution, light collection, and timing properties. Based on the results obtained from the optimization study, the detector prototype was constructed. Prototype tests were performed with two different photodetectors and data acquisition systems. The performance of the prototype was evaluated using the same metrics as in the case of single-fiber measurements. The best results were obtained in measurements with Philips Digital Photon Counting photosensor and the Hyperion platform, yielding a position resolution of 33.38 mm and an energy resolution of 7.73 %. The results obtained are satisfactory and sufficient for the successful operation of the proposed SiFi-CC detector.
... Indeed, if the detection of a 511 keV photon is confounded with a scattered photon of energy less than the lower energy threshold, pile-up may lead to an energy signal that lies within the energy window. Therefore, the detection probability for this scattered photon and the measured scatter fraction increases as activity increases [27,28]. ...
Background:
This article compares the physical performance of the 4-ring digital Discovery MI (DMI) and PMT-based Discovery MI-DR (DMI-DR) PET/CT systems. Physical performance was assessed according to the NEMA NU 2-2012 standards. Performance measures included spatial resolution, image quality, scatter fraction and count rate performance, and sensitivity. Energy and timing resolutions were also measured. Published DMI and DMI-DR performance studies from other centers are reviewed and compared.
Results:
4-ring DMI spatial resolution at 1-cm radial offset in the radial, tangential and axial directions was 4.62, 4.18 and 4.57 mm, respectively, compared with the DMI-DR system values of 4.58, 4.52, and 5.31 mm. Measured sensitivity was 13.3 kcps/MBq at the center of the FOV and 13.4 kcps/MBq 10 cm off-center for the SiPM-based DMI system. DMI-DR system sensitivity was 6.3 kcps/MBq at the center of the FOV and 6.8 kcps/MBq at 10 cm off-center. DMI measured noise equivalent count rate peak was 175.6 kcps at 20.1 kBq/ml; DMI-DR was 146.7 kcps at 31.7 kBq/ml. Scatter fraction was 40.5% and 36.6%, respectively. DMI image contrast recovery (CR) values ranged from 73.2% (10 mm sphere) to 91.0% (37 mm sphere); DMI-DR, values ranged from 68.4% to 91.4%. DMI background variability (BV) was 1.8%-6.5%; DMI-DR was 2.3%-9.1%. The Q.Clear algorithm improved image quality, increasing CR and decreasing BV in both systems. The photopeak energy resolution was 9.63% and 12.19% for DMI and DMI-DR, respectively. The time-of-flight (TOF) resolution was 377.26 ps and 552.71 ps, respectively. Compared with measurements in other centers, results were similar and showed an absolute mean relative deviation of 6% for DMI and 7% for DMI-DR overall performance results.
Conclusions:
Performance measures were higher for the 4-ring DMI than the DMI-DR system. The biggest advantages of the 4-ring DMI vs DMI-DR are improved sensitivity and count rate performance. This should allow a better image signal-to-noise ratio (SNR) for the same acquisition times or, similar SNR with lower acquisition times or injected activity. In its 3-ring configuration, the DMI showed worse performance results than the PMT-based system in terms of count rate scatter fraction and image quality (for similar axial FOV).
... In addition, the coincidence method can minimize the amount of background gamma photons from the surrounding regions or deeper parts of the body. We previously developed a prototype coincidence detector by using lutetium oxyorthosilicate (LSO) scintillators [10], and we showed the possibility of using this new type instrument, although it had some problems to be solved such as background counts due to the beta-gamma coincidence from natural radioactivity of LSOs111213, low sensitivity with small scintillator size, and the light loss with the use of the optical fibers [10]. This time, we developed a new version of the tweezers-type coincidence detector in which many of the drawbacks were resolved. ...
A Gd(2)SiO(5) (GSO) tweezers-type coincidence detector was developed and tested for tumor detection in procedures such as (18)F-fluorodeoxyglucose (FDG)-guided surgery. The detector consists of a pair of GSO scintillators, a pair of metal-packaged small-sized photomultiplier tubes (PMTs), and a coincidence circuit. Because the GSO scintillators are located on the tips of tweezers, a target organ such as a lymph node or the colon can be easily positioned between them. The size of a single GSO was 8 × 14 × 14 mm. The results show that the energy resolution was 30 % full-width at half-maximum (FWHM) and the timing resolution was 6 ns FWHM for 511-keV gamma photons. The point-spread function perpendicular to the detector was 4.5 mm FWHM, and the point-spread function parallel to the detector was 7.5 mm FWHM. The absolute sensitivity of the coincidence detector was 0.6 % at the center of the detector when the two GSOs were 5 mm apart. Background counts due to the accidental and scatter coincidence were 2 cps up to 48 MBq from the positron source contained in a 20-cm-diameter, 20-cm-high cylindrical phantom. From these results, we conclude that the proposed tweezers-type coincidence detector is useful for tumor detection by the use of FDG, such as that in radio-guided surgery.
... The system scatter and true counts were directly obtained from the ROOT 22 output, and the scatter fraction values were calculated from these outputs without needing to follow lengthy NEMA procedure. In the case of the Inveon PET scanner, the intrinsic radioactivity from its LSO crystals 23 [83] was not incorporated in the simulations since that correction was already assumed. ...
Positron emission tomography (PET) and single photon emission tomography (SPECT) are two nuclear emission-imaging modalities that rely on the detection of high-energy photons emitted from radiotracers administered to the subject. The majority of these photons are attenuated (absorbed or scattered) in the body, resulting in count losses or deviations from true detection, which in turn degrades the accuracy of images. In clinical emission tomography, sophisticated correction methods are often required employing additional x-ray CT or radionuclide transmission scans. Having proven their potential in both clinical and research areas, both PET and SPECT are being adapted for small animal imaging. However, despite the growing interest in small animal emission tomography, little scientific information exists about the accuracy of these correction methods on smaller size objects, and what level of correction is required. The purpose of this work is to determine the role of attenuation and scatter corrections as a function of object size through simulations. The simulations were performed using Interactive Data Language (IDL) and a Monte Carlo based package, Geant4 application for emission tomography (GATE). In IDL simulations, PET and SPECT data acquisition were modeled in the presence of attenuation. A mathematical emission and attenuation phantom approximating a thorax slice and slices from real PET/CT data were scaled to 5 different sizes (i.e., human, dog, rabbit, rat and mouse). The simulated emission data collected from these objects were reconstructed. The reconstructed images, with and without attenuation correction, were compared to the ideal (i.e., non-attenuated) reconstruction. Next, using GATE, scatter fraction values (the ratio of the scatter counts to the total counts) of PET and SPECT scanners were measured for various sizes of NEMA (cylindrical phantoms representing small animals and human), MOBY (realistic mouse/rat model) and XCAT (realistic human model) digital phantoms. In addition, PET projection files for different sizes of MOBY phantoms were reconstructed in 6 different conditions including attenuation and scatter corrections. Selected regions were analyzed for these different reconstruction conditions and object sizes. Finally, real mouse data from the real version of the same small animal PET scanner we modeled in our simulations were analyzed for similar reconstruction conditions. Both our IDL and GATE simulations showed that, for small animal PET and SPECT, even the smallest size objects (~2 cm diameter) showed ~15% error when both attenuation and scatter were not corrected. However, a simple attenuation correction using a uniform attenuation map and object boundary obtained from emission data significantly reduces this error (~1% for smallest size and ~6% for largest size, in non-lung regions). In addition, we did not observe any significant improvement between the uses of uniform or actual attenuation map (e.g., only ~0.5% for largest size in PET studies). The scatter correction was not significant for smaller size objects, but became increasingly important for larger sizes objects. These results suggest that for all mouse sizes and most rat sizes, uniform attenuation correction can be performed using emission data only. For smaller sizes up to ~ 4 cm, scatter correction is not required even in lung regions. For larger sizes if accurate quantization needed, additional transmission scan may be required to estimate an accurate attenuation map for both attenuation and scatter corrections.
... This allowed PS-NECR-versus-time curves to be extrapolated for many possible injected activities. Second, the validity of the new method was qualified in a patient study, in which 6 cancer patients were each given between 3 and 6 injections of 15 O-H 2 O at injected activities that varied in magnitude but were similar in all other regards. Such a validation is rarely performed because it requires multiple scans of the same subject over a range of injected activities, which is not practical with 18 F-or 11 C-labeled compounds. ...
... Such a validation is rarely performed because it requires multiple scans of the same subject over a range of injected activities, which is not practical with 18 F-or 11 C-labeled compounds. Finally, we estimated the injected activities that were optimal for measurements of perfusion (F) and the volume of distribution of water in tissue (V T ), as derived from dynamic 15 O-H 2 O scans of the body using a lutetium oxyorthosilicate (LSO)-based 3-dimensional (3D) PET/CT camera. A preliminary report of this work has been presented previously (11). ...
... The NECR is dependent on both the distribution and the magnitude of radioactivity within an emitting object. After a bolus injection of 15 O-H 2 O into a patient, significant changes in both the distribution and the magnitude of radioactivity occur with time. The latter is due to the radioactive decay of 15 O (half-life, 2 min) during the course of the scan (6 min). ...
The magnitude of the injected activity (A(0)) has a direct impact on the statistical quality of PET images. This study aimed to develop a generalized method for maximizing the statistical quality of dynamic PET images by optimizing A(0).
Patient-specific noise-equivalent counts (PS-NECs) were used as a metric of the statistical quality of each time frame of a dynamic PET image. Previous methodology developed to extrapolate the NEC as a function of A(0) was extended to dynamic PET, enabling the NEC to be extrapolated as a function of both A(0) and the time after injection. This method allowed A(0) to be optimized after a single scan (at a single A(0)), by maximizing the NEC within the time interval for which the parameter estimation is most sensitive. The extrapolation method was validated by a series of (15)O-H(2)O scans of the body acquired in 3-dimensional mode. Each patient (n = 6) underwent between 3 and 6 scans at 1 bed position. The injected activities were varied over a wide range (140-840 MBq). Noise-equivalent counting rate (NECR) versus A(0) curves and the optimal injected activities were calculated from each injection.
PS-NECR versus A(0) curves as extrapolated from different injected activities were consistent (coefficient of variation, typically <5%). The optimal injected activities for an individual, as derived from these curves, were also consistent (maximum coefficient of variation, 4.3%). For abdominal (n = 4) and chest (n = 1) scans, we found optimal injected activities of (15)O-H(2)O in the range of 220-350 MBq for estimating blood perfusion (F) and 660-1,070 MBq for estimating the volume of distribution (V(T)). Higher optimal injected activities were found in the case of a pelvic scan (n = 1; 570 MBq for F and 1,530 MBq for V(T)).
PS-NECs are a valid and generic method for optimizing the injected activity in PET, allowing scanning protocols to be improved after the collection of an initial, single dynamic dataset. This generic method can be used to estimate the optimal injected activity, which is specific to the patient, tracer, PET scanner, and body region being scanned.
... Therefore if the energy window of the PET scanner is chosen large enough to include at least one of the three previously mentioned gamma energy peaks, random coincidences between the gamma photons of the true annihilation events and the gamma photons induced by 176 Lu decay can take place, affecting prompt coincidences when source distributions of very low activity concentration are scanned and possibly resulting in significant artifacts at the final reconstructed image. Consequently, the detectability of point sources within a uniform background activity region can be also adversely affected in the presence of LSO.345 Our purpose in this study is to assess the MDA value of the clinical LSO-based Siemens Biograph 6 PET scanner by using a validated model of the scanner and simulating various point-like signal activity distributions, lower than 5nCi/mm 3 , that are surrounded by uniform background activity regions for different acquisition lengths. ...
Recent studies in the field of molecular imaging have demonstrated the need for PET probes capable of imaging very weak activity distributions. Over this range of applications the sensitivity and the energy resolution of a PET system can be critical, as it can possibly affect the minimum amount of activity which can be reliably detected. Clinical PET systems, as opposed to small animal systems, are less sensitive and, therefore, imaging of very low activity sources could be challenging. Moreover the presence of LSO crystal detectors can further raise the minimum detection threshold due to the intrinsic radioactivity of the <sup>176</sup>Lu contained in the LSO compound. Our aim is to examine the feasibility of using an LSO-based clinical PET scanner for imaging activity distributions of 4nCi/mm<sup>3</sup> or less. In this study the parameter of minimum detectable activity (MDA) has been used for the quantification of the detection threshold of a system. A series of acquisitions was simulated using the Monte Carlo simulation software package of GATE. Existing validated GATE models of the clinical Siemens Biograph 6 PET/CT and pre-clinical microPET Focus 220 scanners have been applied to quantify and compare the effect of the LSO background and the energy window on the MDA performance of the systems. Four square regions, each with a unique signal-to-background activity concentration ratio (SBR), were scanned simultaneously. The intrinsic LSO background spectrum and the total energy spectrum, as well as their relative positions and intensities were estimated. The simulated data were histogrammed on various time frames, which were later reconstructed using a filtered back-projection algorithm. Detectability in every image was quantified using a modified Currie equation to associate an MDA value with a specific region and frame length. In the case of Biograph an MDA of 4nCi/mm<sup>3</sup> can be reliably detected for frame lengths longer than 5 min and in regions where -
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the SBR was higher than 4. When higher contrast regions are imaged, detection can be achieved even for frame lengths down to 1 min. The previous analysis was repeated by using a GATE model of a hypothetical BGO-based clinical PET scanner. The results between the two scanner models were compared with each other as well as with those of a previous MDA study on a pre-clinical microPET Focus 220 scanner.
... When a source with activity on the order of tens of nCi is imaged, then the standard deviation of background originating from the scintillator crystals can significantly raise the detection limit set by Eq.(1). Consequently the ability to detect point sources within a uniform background activity region is also adversely affected.345 In this study we are interested in evaluating the counting performance of the small animal LSO-based microPET Focus 220 scanner, using the MDA parameter, when point-like activity concentrations of less than 5nCi/mm 3 are placed within a uniform background activity region for different acquisition lengths. ...
Novel molecular imaging applications increasingly involve studies where very low amount of activity is present. This operation point can be challenging in terms of image quality especially when the intrinsic detector activity from scintillators such as LSO is considered. LSO crystals contain <sup>176</sup>Lu which emits beta<sup>-</sup> particles followed by gamma photons, resulting in the detection of true and random events. This background activity has been shown to contribute a significant percentage of the total detected true events, when a very weak activity distribution is imaged. This can affect the weakest signal that can be detected by an LSO PET scanner which determines its "detection limit" and is evaluated by the parameter of minimum detectable activity or MDA. A series of acquisitions was performed in order to study the effect of the energy window to the intrinsic true and randoms rate. The experiments were also simulated with GATE, and the results were validated by comparing them with the experiment. Four square regions each with a unique signal to background activity concentration ratio were used. The background activity level was held constant between the different regions, while the activity of the point sources varied in 4 selected levels. The signal to background ratio was calculated separately for each region. The energy spectrum of the intrinsic background activity and its contribution to the total energy spectrum both for singles and coincidences was estimated through the GATE simulation. We histogrammed both the measured and simulated data on various time frames, which we reconstructed using the filtered backprojection algorithm. Every image was quantified based on the Currie equation in order to associate an MDA value for each of the 4 point sources as a function of the frame length. In the case of the microPET Focus 220 a total amount of 4nCi/mm<sup>3</sup> can be reliably detected for frame lengths longer than 5 min and at regions where the-
signal to background activity concentration ratio is higher than 4. In the case of higher contrast regions detection can be achieved even for frame lengths down to 1 min.
... When low activity levels are being imaged, there is a very limited number of true counts that can be acquired so that one would consider using a wider energy window to increase the sensitivity (7). However, the choice of energy window at low activity levels is complicated for PET scanners that use lutetium-based scintillators such as lutetium oxyorthosilicate (LSO) because of the intrinsic radioactivity present in the scintillator (8)(9)(10). The lutetium used in LSO contains 2.6% 176 Lu, which decays by b 2decay (mean b 2 -energy of 420 keV). ...
Lutetium oxyorthosilicate (LSO)- or lutetium-yttrium oxyorthosilicate (LYSO)-based PET scanners have intrinsic radioactivity in the scintillator crystals due to the presence of (176)Lu, which decays by beta-emission followed by one or more prompt gamma-ray emissions. This leads to intrinsic true counts that can influence the image when scanning low levels of activity. An evaluation of the effects of this intrinsic activity for low levels of activity and different energy windows is performed on an LSO-based small-animal PET scanner.
Intrinsic count rate and sensitivity were measured for a range of lower-level discriminators (LLDs) ranging from 100 to 750 keV. The noise equivalent count rate (NECR) as a function of LLD for activity levels from 100 Bq to 100 kBq was estimated using a combination of measurement and previously published data for this scanner. Phantom imaging was performed using three (68)Ge sources of strength 55, 220, and 940 Bq and LLD levels of 250, 350, and 400 keV. The images were assessed using a contrast-to-noise ratio (CNR) analysis and by comparing the observed ratio of source activities to the true ratio value.
The intrinsic true count rate is reduced from 940 counts per second (cps) for a 250- to 750-keV energy window to <2 cps for a 400- to 750-keV window. There is a corresponding 2-fold drop in sensitivity for detected true events for external positron sources for these 2 energy windows. The NECR versus LLD curves showed a highly peaked shape, with the optimum LLD being approximately 425 keV. The phantom image results were dominated by the intrinsic true counts when an energy window of 250-750 keV was used. The intrinsic true counts were almost completely removed by raising the LLD to 400 keV. The CNR for each of the sources was higher for the narrow energy window and the 55 Bq could be easily visualized in images acquired with LLD levels of 350 and 400 keV but not when the 250-keV LLD was used. Images acquired with an LLD of 400 keV and reconstructed with 2-dimensional filtered backprojection were the most quantitatively accurate.
It is possible to visualize sources of <1 kBq in LSO-based animal PET systems by raising the LLD to 400 keV to exclude the majority of the counts due to the intrinsic activity present in the LSO.
... This protocol also prescribes that for the scatter fraction data shall be used with randoms rate at less than 1.0% of the trues rate. This, however, could not be realized due to the natural activity of LSO (Eriksson et al 2005, Huber et al 2002, Vaska and Alexoff 2003. Therefore, in accordance with the suggestion of Watson et al (Brambilla et al 2005, Watson et al 2004, the scatter fraction as a function of activity in the FOV was measured using prompts minus delays sinograms. ...
The ECAT high resolution research tomograph (HRRT) is a dedicated brain and small animal PET scanner, with design features that enable high image spatial resolution combined with high sensitivity. The HRRT is the first commercially available scanner that utilizes a double layer of LSO/LYSO crystals to achieve photon detection with depth-of-interaction information. In this study, the performance of the commercial LSO/LYSO HRRT was characterized, using the NEMA protocol as a guideline. Besides measurement of spatial resolution, energy resolution, sensitivity, scatter fraction, count rate performance, correction for attenuation and scatter, hot spot recovery and image quality, a clinical evaluation was performed by means of a HR+/HRRT human brain comparison study. Point source resolution varied across the field of view from approximately 2.3 to 3.2 mm (FWHM) in the transaxial direction and from 2.5 to 3.4 mm in the axial direction. Absolute line-source sensitivity ranged from 2.5 to 3.3% and the NEMA-2001 scatter fraction equalled 45%. Maximum NECR was 45 kcps and 148 kcps according to the NEMA-2001 and 1994 protocols, respectively. Attenuation and scatter correction led to a volume uniformity of 6.3% and a system uniformity of 3.1%. Reconstructed values deviated up to 15 and 8% in regions with high and low densities, respectively, which can possibly be assigned to inaccuracies in scatter estimation. Hot spot recovery ranged from 60 to 94% for spheres with diameters of 1 to 2.2 cm. A high quantitative agreement was met between HR+ and HRRT clinical data. In conclusion, the ECAT HRRT has excellent resolution and sensitivity properties, which is a crucial advantage in many research studies.
... The impact of the background on the performance indicators previously obtained for the breast exam of patient C has been also evaluated. The NEC rate was re-calculated taking into account the true–β γ and random coincidences from LYSO:Ce as [22]: ...
Positron Emission Mammography (PEM) with 18F-Fluorodeoxyglucose (18F-FDG) is a functional imaging technique for breast cancer detection. The development of dedicated imaging systems with high sensitivity and spatial resolution are crucial for early breast cancer diagnosis and an efficient therapy. Clear-PEM is a dual planar scanner designed for high-resolution breast cancer imaging under development by the Portuguese PET Mammography consortium within the Crystal Clear Collaboration. It brings together a favorable combination of high-density scintillator crystals coupled to compact photodetectors, arranged in a double readout scheme capable of providing depth-of-interaction information.
A Monte Carlo study of the Clear-PEM system counting rates is presented in this paper. Hypothetical breast exam scenarios were simulated to estimate the single event rates, true and random coincidence rates. A realistic description of the patient and detector geometry, radiation environment, physics and instrumentation factors was adopted in this work. Special attention was given to the 18F-FDG accumulation in the patient torso organs which, for the Clear-PEM scanner, represent significant activity outside the field-of-view (FOV) contributing to an increase of singles, randoms and scattered coincidences affecting the overall system performance. The potential benefits of patient shielding to minimize the influence of the out-of-field background was explored. The influence of LYSO:Ce crystal intrinsic natural activity due to the presence of the 176Lu isotope on the counting rate performance of the proposed scanner, was also investigated.
