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Abstract
A Monte Carlo program has been developed that incorporates a voxel phantom of an adult patient in a model of the complete X-ray imaging system, including the anti-scatter grid and screen–film receptor. This allows the realistic estimation of patient dose and the corresponding image (optical density map) for a wide range of equipment configurations. This paper focuses on the application of the program to lumbar spine anteroposterior and lateral screen–film examinations. The program has been applied to study the variation of physical image quality measures and effective dose for changing system parameters such as tube voltage, grid design and screen–film system speed. These variations form the basis for optimization of these system parameters. In our approach to optimization, the best systems are those that can match (or come close to) the calculated image quality measure of systems preferred in a recent European clinical trial, but with lower patient dose. The largest dose savings found were 21% for a 400 speed class system with a grid having a strip density of 40 cm⁻¹ and a grid ratio of 16. A further dose saving of 13% was possible when a 600 speed class system was employed. The best systems found from the optimization correspond to those recommended by the European Commission guidelines on image quality criteria for diagnostic radiographic images.
... These parameters will influence the optimal working point of the system. A rigorous optimization procedure should include as many of these elements as possible and should obviously aim to establish the required image quality while keeping the dose to the patient as low as reasonably achievable [5][6][7][8]. ...
Purpose
To evaluate image quality of chest radiography for a number of systems in Belgium, using a contrast-detail (c-d) test object and Visual Grading Analysis (VGA) of an anthropomorphic phantom.
Methods
The study comprised 22 chest imaging systems in Belgium. C-d data were measured using Leeds TO20 test object, imaged using poly(methyl methacrylate) (PMMA) thicknesses of 9, 13 and 16 cm. Images of the Lungman phantom, with additional tissue-equivalent chest plates to represent different patient sizes, were then acquired. Perceived image quality was evaluated using VGA by three radiologists. Images were acquired at a patient equivalent position with system-specific exposure settings for Posterior-Anterior chest protocol. Incident air kerma (IAK) was measured using a solid-state dosemeter.
Results
C-d results showed large differences between the systems. Total number of visible discs ranged from 38 to 83 (for 9 cm PMMA) with a consistent average drop of 10% as PMMA thickness was systematically increased. However, no correlation was found between number of visible discs and IAK. Perceived image quality scored by the readers from the Lungman images decreased with increasing phantom thickness, however no correlation of VGA score with IAK was seen. Moderate correlation was found between the VGA score of one of the readers and the TO20 results, and no correlation for the rest.
Conclusions
The spread in dose and image quality measures was high and no correlation was seen between either image quality measure and IAK, suggesting the need for optimization. A more powerful tool is required for task-based optimization in chest radiography.
... However, study focusing on fluoroscopy has not been found yet. Previous researches were concerned about the impact of scattered radiation on patient dose [10,11,14,16,17] and radiographic image quality [14,18,19] , and the physical properties of radiation [12,20] , but there has not been study on the occupational dose of radiology department staff. Besides, the results obtained in general X-ray study cannot represent validly in fluoroscopy because the properties of these two modalities are different. ...
Objective:
This study developed a three-dimensional visualization method for presenting the geometric pattern of the
relative dose generated by Monte Carlo (MC) simulation and validated the simulated dose values against the physical measurement.
Methods:
In fluoroscopy and interventional radiology examinations, relatively high level of occupational dose is
delivered to healthcare workers due to prolonged radiation exposure in imaging procedures. As the spatial difference in dose is never negligible, the scatter radiation distribution under fluoroscopic exposures is thus worth investigating. MC simulation is a sophisticated statistical method, which has been widely applied for modeling scatter radiation in general X-ray examination room. However, the application and validation of MC simulation under fluoroscopy setting have not been found yet. In this study, a stack of tissue equivalent slabs were used as the object under fluoroscopic irradiation. EGSnrc-based DOSXYZnrc code was applied to simulate the scatter dose distribution and the physical measurement of radiation dose was performed using an ionization chamber radiation detector.
Results:
At 19 representative locations taking into account the work area and radiosensitive organs of healthcare workers, the trend compliance of the simulated with the measured dose values was examined using the correlation analysis. A significant monotonic association between the MC simulation and physical measurement of dose values was identified (rs=0.822 and P<.01). At the same horizontal distance from the irradiation axis, it was observed that the air dose was relatively higher at the level of gonad region than at the eye level.
Conclusions:
The proposed visualization approach illustrates a three dimensional dose simulation in local display density,
which arouses the awareness about prolonged radiation exposure in clinical environment. The reliability and validity of the simulation were examined.
... This interpretation is supported by Monte Carlo calculations simulating the corresponding exposure parameters [21] . The radiation contrast of the studied anatomical details is reduced by 30% when the tube voltage increases from 70 kV to 90 kV according to the Monte Carlo model calculations [22]. The model calculations also show a reduction of the SNR of these details (by 30–40%) at 90 kV compared with 70 kV in the original images which may additionally contribute to the lower ICS at 90 kV. ...
The "European Guidelines on Quality Criteria for Diagnostic Radiographic Images" do not address the choice of the film characteristic (H&D) curve, which is an important parameter for the description of a radiographic screen–film system. The image contrast of clinical lumbar spine and chest radiographs was altered by digital image processing techniques, simulating images with different H&D curves, both steeper and flatter than the original. The manipulated images were printed on film for evaluation. Seven experienced radiologists evaluated the clinical image quality by analysing the fulfilment of the European Image Criteria (ICS) and by visual grading analysis (VGA) of in total 224 lumbar spine and 360 chest images. A parallel study of the effect of the H&D curve has also been made using a theoretical model. The contrast (OD) of relevant anatomical details was calculated, using a Monte Carlo simulation-model of the complete imaging system including a 3D voxel phantom of a patient. Correlations between the calculated contrast and the radiologists' assessment by VGA were sought. The results of the radiologists' assessment show that the quality in selected regions of lumbar spine and chest images can be significantly improved by the use of films with a steeper H&D curve compared with the standard latitude film. Significant (p<0.05) correlations were found between the VGA results and the calculations of the contrast of transverse processes and trabecular details in the lumbar spine vertebrae, and with the contrast of blood vessels in the retrocardiac area of the chest.
... The calculations were made using a Monte Carlo computer program written originally for modelling chest and lumbar spine screen-film radiography (Sandborg et al 2001b, McVey et al 2003. The imaging system was modelled by simulating photon transport from the x-ray tube and through the patient, anti-scatter device, chest support stand and finally into the image receptor. ...
A Monte Carlo based computer model of the x-ray imaging system was used to investigate how various image quality parameters of interest in chest PA radiography and the effective dose E vary with tube voltage (90-150 kV), additional copper filtration (0-0.5 mm), anti-scatter method (grid ratios 8-16 and air gap lengths 20-40 cm) and patient thickness (20-28 cm) in a computed radiography (CR) system. Calculated quantities were normalized to a fixed value of air kerma (5.0 microGy) at the automatic exposure control chambers. Soft-tissue nodules were positioned at different locations in the anatomy and calcifications in the apical region. The signal-to-noise ratio, SNR, of the nodules and the nodule contrast relative to the contrast of bone (C/C(B)) as well as relative to the dynamic range in the image (C(rel)) were used as image quality measures. In all anatomical regions, except in the densest regions in the thickest patients, the air gap technique provides higher SNR and contrast ratios than the grid technique and at a lower effective dose E. Choice of tube voltage depends on whether quantum noise (SNR) or the contrast ratios are most relevant for the diagnostic task. SNR increases with decreasing tube voltage while C/C(B) increases with increasing tube voltage.
... This interpretation is supported by Monte Carlo calculations simulating the corresponding exposure parameters [21] . The radiation contrast of the studied anatomical details is reduced by 30% when the tube voltage increases from 70 kV to 90 kV according to the Monte Carlo model calculations [22]. The model calculations also show a reduction of the SNR of these details (by 30–40%) at 90 kV compared with 70 kV in the original images which may additionally contribute to the lower ICS at 90 kV. ...
The "European Guidelines on Quality Criteria for Diagnostic Radiographic Images" do not address the choice of the film characteristic (H&D) curve, which is an important parameter for the description of a radiographic screen-film system. The image contrast of clinical lumbar spine and chest radiographs was altered by digital image processing techniques, simulating images with different H&D curves, both steeper and flatter than the original. The manipulated images were printed on film for evaluation. Seven experienced radiologists evaluated the clinical image quality by analysing the fulfilment of the European Image Criteria (ICS) and by visual grading analysis (VGA) of in total 224 lumbar spine and 360 chest images. A parallel study of the effect of the H&D curve has also been made using a theoretical model. The contrast (DeltaOD) of relevant anatomical details was calculated, using a Monte Carlo simulation-model of the complete imaging system including a 3D voxel phantom of a patient. Correlations between the calculated contrast and the radiologists' assessment by VGA were sought. The results of the radiologists' assessment show that the quality in selected regions of lumbar spine and chest images can be significantly improved by the use of films with a steeper H&D curve compared with the standard latitude film. Significant (p<0.05) correlations were found between the VGA results and the calculations of the contrast of transverse processes and trabecular details in the lumbar spine vertebrae, and with the contrast of blood vessels in the retrocardiac area of the chest.
Evaluation of patient radiation dose after the implementation of a high kV technique during a cerebral angiographic procedure is an important issue. This study aimed to determine and compare the patient radiation dose of intracranial aneurysm patients undergoing cerebral angiography using the conventional and high kV techniques in a retrospective study and a phantom study. A total of 122 cases (61 cases with conventional technique and 61 cases with high kV technique) of intracranial aneurysm patients, who underwent cerebral angiographic procedure and met the inclusion criteria, were recruited. The radiation dose and the angiographic exposure parameters were reviewed retrospectively. The radiation dose in the phantom study was conducted using nanoDotTM optically stimulating luminescence (OSLD), which were placed on the scalp of the head phantom, the back of the neck, and the phantom skin at the position of the eyes. The standard cerebral angiographic procedure using the conventional and high kV techniques was performed following the standard protocol. The results showed that the high kV technique significantly reduced patient radiation dose and phantom skin dose. This study confirms that the implementation of a high kV technique in routine cerebral angiography for aneurysm diagnosis provides an effective reduction in radiation dose. Further investigation of radiation dose in other interventional neuroradiology procedures, particularly embolization procedure, should be performed.
This chapter gives a review for both conventional X-ray and computed tomography (CT) scan imaging modalities and their medical applications. The chapter presents a brief history on the discovery of X-ray, X-ray imaging, and computed tomography scan. The linear projection for the generation of the sinogram (the detector’s signals versus the rotational angle) and the filtered backprojection for image reconstruction are discussed. Computer simulations for linear and fan beams X -ray are also presented. The chapter discusses some medical applications of both the conventional X-ray and CT scan imaging.
The clinical assessment of spinal deformities often involves the assessment of posture and back shape together with the associated mobility of the spine, pelvis and rib cage. Currently, there is a wide range of posture and back shape assessment tools available for clinical use. The choice varies from conventional approach to advanced structured light methods. The advanced methods like ultrasound, 3D radiography and inertial sensors are not easily accessible to most clinicians, as they are either expensive, require specialist training or are complex and/or difficult to use. Thus, simple conventional methods like eyeballing, photography and the plumb line are still used within clinical practice today. The primary aim of this article is to give an overview of different tactile and non-tactile measurement systems that have been developed for the measurement of posture and whole-body analysis.
Diagnostic reference levels (DRLs) is a veritable tool for dose optimisation and patient protection in diagnostic radiology. However, it is essential to have information on the local situation especially in a large hospital with several units or a cluster of healthcare centres within a geographical region with several X-ray units. In the present study, entrance surface doses (ESDs) were measured in twelve (12) healthcare centres consisting of 15 radiological units using thermoluminescent dosimeters (TLDs). Seven radiological procedures such as; chest PA, abdomen AP, pelvis AP, lumbar spine AP, skull AP, knee AP, and hand AP frequently carried out in Nigeria were included in the study, and their local diagnostic reference levels (LDRLs) were determined. The values of the determined LDRLs were compared with established NDRLs in UK, US, Slovenia, Italy and Brazil. The LDRLs determined in the two groups (healthcare centres) studied ranged from 1.78 to 3.01, 2.71 to 2.84, 2.11 to 3.79, 3.93 to 8.79, 1.06 to 1.73 and 1.10 to 1.44 mGy for chest PA, pelvis AP, lumbar spine AP, skull AP, knee AP and hand AP respectively. Large variations were found among the X-ray units studied even within the same centre. Entrance surface doses obtained in pelvis AP and lumbar spine AP in both GROUP A and were found to be lower than the NRPB-HPA 2010 review for UK, while in all other five examinations, value of the measured entrance surface dose (ESD) are higher than the doses reported in the UK review. The relative higher doses found in the study are attributable to higher tube load (mAs) used and indicative of the need for dose optimisation in Nigerian radiological practice.
2008 by the Chinese Medical Association.
PurposeTo obtain baseline data for implementing optimisation protocols in Nigeria, a study of the patient entrance surface doses (ESDs) and image quality for lumbar radiography in the two major hospitals in Calabar, Nigeria has been carried out.MethodA total of 74 patients cutting across the two hospitals were monitored during lumbar X-ray examination for entrance surface doses (ESDs) using Lithium Fluoride Thermoluminescent dosimeters (LiF-TLDs). Image quality of ensuing radiographs was studied by three experienced Radiologists using the European Commission (EC) criteria for lumbar radiography.ResultsMean patient entrance dose for the studied population was about 10.5 mGy (AP) and 23.1 mGy for the lateral projection. Third quartile ESD values suggest that up to 75% of the exposed population in lumbar radiography may be receiving doses above the EC reference level. Image quality scores against a reference image were marginally above average. There was no correlation between dose and image quality per radiograph. Results confirm the need for optimisation of procedure in the area.Conclusion
Procedural changes are suggested in order to lower the patient doses and improve on the image quality of radiographs. These results are to be used as a reference for future review after the optimisation protocol is put in place.
The principles of justification and optimisation, and the establishment and use of Diagnostic Reference Levels are core tenets of the European Medical Exposures Directive [Council Directive 97/43], and ensuing legislation across Europe. In Ireland, the European Medical Exposures Directive [Council Directive 97/43] was enacted into national law in Statutory Instrument 478 of 2002. This series of three review articles discusses the status of justification and optimisation of X-ray examinations nationally, and progress with the establishment of Irish Diagnostic Reference Levels.The current article will outline the ICRP recommendations which preceded the European Medical Exposures Directive, describe how the Directive was transposed into Irish Statute, and review the literature associated with some of the justification concepts arising from the Statute in order to elucidate the prevailing practice regarding justification in Ireland. Three levels of justification are identified: general, generic and individual. The relationship between referral guidelines and generic justification is explored, and practical issues in the implementation of individual justification are considered with reference to the role of the radiographer.Two further articles in the series will adopt a similar approach in discussing optimisation, and the establishment and use of Diagnostic Reference Levels.
Our purpose in this study was to establish a selection standard for anti-scatter grids for a direct conversion flat-panel detector (FPD) system. As indices for grid evaluation, we calculated the selectivity, Bucky factor, and the signal-to-noise ratio improvement factor (SIF) by measuring rates of scatter transmission, primary transmission, and total transmission (based on the digitally displayed measurement values of the FPD system), using 4 acrylic phantoms of different thicknesses. The results showed that the SIF was less than 1.0 when the phantom thickness was 5 cm. When the phantom thickness was 25 cm and the grid ratio was 16:1, the SIF was 1.505 and 1.518 (maximum value) at 90 and 120 kV, respectively. Compared with the grid ratio of 12:1, the SIF at the grid ratio 16:1 was improved by 6.1% at 90 kV, and by 7.0% at 120 kV. In a direct-conversion FPD system, the grid ratio of 16:1 is considered adequate for eliminating the scattered-radiation effect when much scattered radiation is present, such as with a thick imaged object or a high X-ray tube voltage.
Radiography using film has been an established method for imaging the internal organs of the body for over 100 years. Surveys carried out during the 1980s identified a wide range in patient doses showing that there was scope for dosage reduction in many hospitals. This paper discusses factors that need to be considered in optimising the performance of radiographic equipment. The most important factor is choice of the screen/film combination, and the preparation of automatic exposure control devices to suit its characteristics. Tube potential determines the photon energies in the X-ray beam, with the selection involving a compromise between image contrast and the dose to the patient. Allied to this is the choice of anti-scatter grid, as a high grid ratio effectively removes the larger component of scatter when using higher tube potentials. However, a high grid ratio attenuates the X-ray beam more heavily. Decisions about grids and use of low attenuation components are particularly important for paediatric radiography, which uses lower energy X-ray beams. Another factor which can reduce patient dose is the use of copper filtration to remove more low-energy X-rays. Regular surveys of patient dose and comparisons with diagnostic reference levels that provide a guide representing good practice enable units for which doses are higher to be identified. Causes can then be investigated and changes implemented to address any shortfalls. Application of these methods has led to a gradual reduction in doses in many countries.
The use of digital radiography (DR) systems offers a number of advantages over film-screen detectors. One potential disadvantage, however, is that some fixed DR systems do not allow the user to change the antiscatter grid to suit the imaging task. Instead, the user must choose the grid at the time of purchase. Six grids, which are offered as installation options for one commercial fixed-room DR system, were experimentally evaluated, using a range of scatter conditions and tube voltages. In addition, three grids, which are available with a portable DR system in which the user can change the grid to suit the imaging task, were also evaluated. The grids were compared using the primary transmission, scatter fraction, and calculated signal-to-noise improvement factor (SIF). It was found that the grids with low atomic number interspace and cover material had an SIF up to 15% higher than did the grids with aluminium interspace and cover material; the grid with a grid ratio of 12:1 had the highest SIF for all tube voltages and scatter conditions tested here. This 12:1 grid probably represents a good general-purpose non-removable grid in DR.
The aim of this work was to calculate distributions of scatter-to-primary ratios (epsilon(s)/epsilon(p)) and signal-to-noise ratios per pixel (SNRp) in chest images. Such distributions may provide useful information on how physical image quality (contrast, SNR) is distributed over the posterior/anterior (PA) chest image. A Monte Carlo computer program was used for the calculations, including a model of both the patient (voxel phantom) and the imaging system (X-ray tube, anti-scatter grid and image detector). The calculations were performed for three PA thicknesses 20, 24 and 28 cm. For a 24 cm patient, the epsilon(s)/epsilon(p) varies between 0.5 in the lung to 2.5 behind the spine and heart. The corresponding variation of the SNRp is a factor of 3, with the highest values in the lung. Increasing the patient thickness from 20 to 28 cm increases the epsilon(s)/epsilon(p) by a factor of 2.2 behind the spine and heart.
The aim of this work was to study the influence of patient thickness, tube voltage and image detector on patient dose, contrast and ideal observer signal-to-noise ratio (SNR(I)), for pathological details positioned at different regions in the image in posterior-anterior (PA) chest radiology. A Monte Carlo computational model was used to compute measures of physical image quality (contrast, SNR(I)) and patient effective dose, E. Two metastasis-like details positioned in the central right lung and right lung near the spine, respectively, were studied. The tube voltage was varied between 100 and 150 kV and the patient thickness between 20 and 28 cm. Both, a computed radiography (CR) system and a direct radiography (DR) system, were investigated. The DR system provides both lower doses and better image quality compared with the CR system. The SNR(I)2/E is approximately 2.9 times higher for the DR system compared with the CR system.
The purpose of the study was to evaluate the image quality at different tube potential (kV) settings using anteroposterior lumbar spine radiography as a model. An Alderson phantom was used with a flat-panel detector. The tube potential varied between 48 and 125 kV while the tube charge (mAs) was adjusted to keep an effective dose of 0.11 mSv. Image quality was assessed with a visual grading analysis and with a CDRAD contrast-detail phantom together with a computer program. The VGA showed inferior image quality for the higher kV settings, > or =96 kVwith similar results for the contrast-detail phantom. When keeping the effective dose fixed, it seems beneficial to reduce kV to get the best image quality despite the fact that the mAs is not as high as with automatic exposure. However, this cannot be done with automatic exposure, which is set for a constant detector dose.
Determining the amount of scatter inside and outside a diagnostic X-ray room is important for evaluating the dose to staff and the public. The amount of scatter is affected by many physical factors including beam quality and field size. However, there is little published data on patient scatter and there are large differences between the available data sets. Hence, a Monte Carlo code was developed to allow a systematic study of the factors affecting patient scatter. A voxel phantom was used to provide a realistic model of the patient. The variation of scatter with different phantom types was investigated to show the effect of patient inhomogeneities and obliquities. The effect of altering tube voltage, filtration, voltage ripple, field size and position on patient scatter was studied. A larger than expected variation in the patient scatter was observed with increasing field area due to the proximity of the field borders with the patient obliquities. The effect of the tube voltage ripple on the patient scatter was also calculated. This showed that there would be little effect on the scatter levels within X-ray rooms if ageing X-ray generators, which produce substantial voltage ripple, were replaced by X-ray tubes with modern medium frequency generators. Recommendations are made on the choice of published scatter data for X-ray room design.
The increasing use of small animals in basic research has spurred interest in new imaging methodologies. Digital subtraction angiography (DSA) offers a particularly appealing approach to functional imaging in the small animal. This study examines the optimal x-ray, molybdenum (Mo) or tungsten (W) target sources, and technique to produce the highest quality small animal functional subtraction angiograms in terms of contrast and signal-difference-to-noise ratio squared (SdNR2). Two limiting conditions were considered-normalization with respect to dose and normalization against tube loading. Image contrast and SdNR2 were simulated using an established x-ray model. DSA images of live rats were taken at two representative tube potentials for the W and Mo sources. Results show that for small animal DSA, the Mo source provides better contrast. However, with digital detectors, SdNR2 is the more relevant figure of merit. The W source operated at kVps >60 achieved a higher SdNR2. The highest SdNR2 was obtained at voltages above 90 kVp. However, operation at the higher potential results in significantly greater dose and tube load and reduced contrast quantization. A reasonable tradeoff can be achieved at tube potentials at the beginning of the performance plateau, around 70 kVp, where the relative gain in SdNR2 is the greatest.
The aim of this work was to study the dependence of image quality in digital chest and pelvis radiography on tube voltage, and to explore correlations between clinical and physical measures of image quality. The effect on image quality of tube voltage in these two examinations was assessed using two methods. The first method relies on radiologists' observations of images of an anthropomorphic phantom, and the second method was based on computer modeling of the imaging system using an anthropomorphic voxel phantom. The tube voltage was varied within a broad range (50-150 kV), including those values typically used with screen-film radiography. The tube charge was altered so that the same effective dose was achieved for each projection. Two x-ray units were employed using a computed radiography (CR) image detector with standard tube filtration and antiscatter device. Clinical image quality was assessed by a group of radiologists using a visual grading analysis (VGA) technique based on the revised CEC image criteria. Physical image quality was derived from a Monte Carlo computer model in terms of the signal-to-noise ratio, SNR, of anatomical structures corresponding to the image criteria. Both the VGAS (visual grading analysis score) and SNR decrease with increasing tube voltage in both chest PA and pelvis AP examinations, indicating superior performance if lower tube voltages are employed. Hence, a positive correlation between clinical and physical measures of image quality was found. The pros and cons of using lower tube voltages with CR digital radiography than typically used in analog screen-film radiography are discussed, as well as the relevance of using VGAS and quantum-noise SNR as measures of image quality in pelvis and chest radiography.
A Monte Carlo simulation has been used to estimate the dose and contrast advantages of replacing radiographic cassette fronts fabricated from aluminium with cassette fronts fabricated from low atomic number material (carbon fibre). The simulation used a realistic imaging geometry and calculations were made both with and without an anti-scatter grid. Account was taken of the scatter generated in the cassette front and the effect of beam hardening on primary contrast. Dose and contrast were evaluated for a range of cassette front thicknesses and tube potentials (60-150 kV) as well as for four examinations representative of situations with varying amounts of scatter. The results with an anti-scatter grid show a clear dose and contrast advantage in all cases when an aluminium cassette front is replaced with a low attenuation cassette front. The contrast advantage is dependent upon the examination and is generally greater for imaging bony structures than for imaging soft tissue. If a 1.74 mm aluminium cassette front is compared with a 1.1 mm carbon fibre cassette front, then the dose advantages are 16%, 9%, 8% and 6% and the contrast advantages are 10%, 7%, 4% and 5% for the AP paediatric pelvis examination at 60 kV, the anteroposterior (AP) lumbar spine examination at 80 kV, the lateral lumbar spine examination at 100 kV and the posteroanterior (PA) chest examination at 150 kV, respectively. The results without an anti-scatter grid show an increased dose advantage when a low attenuation cassette front is used, but the contrast advantage is small and in some situations negative.
In this study we have investigated the image quality of lumbar spine radiographs taken after recording technical and physical parameters. Two technical parameters were altered, tube voltage (70 kV and 90 kV for the anteroposterior (AP) projection and 77 kV and 95 kV for the lateral projection) and sensitivity of the film-screen system (sensitivity class 400 and 600). In total, 85 images were included in the study. Entrance surface dose (ESD) was measured using thermoluminescent dosemeters. The mean value of ESD for the different technique groups varied between 1.9 mGy (90 kV, sensitivity class 400) and 4.6 mGy (70 kV, sensitivity class 400) for the AP projection, and between 6.4 mGy (95 kV, sensitivity class 600) and 20.4 mGy (70 kV, sensitivity class 400) for the lateral projection. Image criteria given in the "European Guidelines on Quality Criteria for Radiographic Images" were used to assess image quality. Two evaluation methods have been employed. A straightforward scoring of fulfilled image criteria, and visual grading analysis using the structures defined in the image criteria. The latter method provided a sharper distinction between groups of images taken using different radiographic techniques. The average number of fulfilled image criteria for the AP projections varied between 0.74 (90 kV, sensitivity class 400) and 0.87 (70 kV, sensitivity class 400). For the lateral projection this number varied between 0.79 (95 kV, sensitivity class 600) and 0.84 (77 kV, sensitivity class 600). This study shows that image criteria are useful tools in clinical studies of image quality.
The ability to predict clinical image quality from physical measures is useful for optimization in diagnostic radiology. In this work, clinical and physical assessments of image quality are compared and correlations between the two are derived. Clinical assessment has been made by a group of expert radiologists who evaluated fulfillment of the European image criteria for chest and lumbar spine radiography using two scoring methods: image criteria score (ICS) and visual grading analysis score (VGAS). Physical image quality measures were calculated using a Monte Carlo simulation model of the complete imaging system. This model includes a voxelized male anatomy and was used to calculate contrast and signal-to-noise ratio of various important anatomical details and measures of dynamic range. Correlations between the physical image quality measures on the one hand and the ICS and VGAS on the other were sought. 16 chest and 4 lumbar spine imaging system configurations were compared in frontal projection. A statistically significant correlation with clinical image quality was found in chest posteroanterior radiography for the contrast of blood vessels in the retrocardiac area and a measure of useful dynamic range. In lumbar spine anteroposterior radiography, a similar significant correlation with clinical image quality was found between the contrast and signal-to-noise ratio of the trabecular structures in the L1-L5 vertebrae. The significant correlation shows that clinical image quality can, at least in some cases, be predicted from appropriate measures of physical image quality.
The ability to predict image quality from known physical and technical parameters is a prerequisite for making successful dose optimisation. In this study, imaging systems have been simulated using a Monte Carlo model of the imaging systems. The model includes a voxelised human anatomy and quantifies image quality in terms of contrast and signal-to-noise ratio for 5-6 anatomical details included in the anatomy. The imaging systems used in clinical trials were simulated and the ranking of the systems by the model and radiologists compared. The model and the results of the trial for chest PA both show that using a high maximum optical density was significantly better than using a low one. The model predicts that a good system is characterised by a large dynamic range and a high contrast of the blood vessels in the retrocardiac area. The ranking by the radiologists and the model agreed for the lumbar spine AP.
Manual segmentation of 129 x‐ray CT transverse slices of a living male human has been done and a computerized 3‐dimensional volume array modeling all major internal structures of the body has been created. Each voxel of the volume contains a index number designating it as belonging to a given organ or internal structure. The original x‐ray CTimages were reconstructed in a 512×512 matrix with a resolution of 1 mm in the x,y plane. The z‐axis resolution is 1 cm from neck to midthigh and 0.5 cm from neck to crown of the head. This volume array represents a high resolution model of the human anatomy and can serve as a voxel‐based anthropomorphic phantom suitable for many computer‐based modeling and simulation calculations.
A Monte Carlo computer program has been developed to model X ray imaging systems realistically using an adult voxel phantom. Image quality is quantified in terms of contrast and signal-to-noise ratio (SNR) for six important anatomical details chosen in accordance with recent European guidelines. The program has been used to study and optimise lumbar spine (LS) radiographic systems. As an example, the effect of tube potential on contrast, SNR and effective dose for the LS AP examination is demonstrated. Optimisation involves comparison of performance with that of a reference system. Configurations (tube potential, grid and screen-film) have been found which approximately match the reference image quality, but at lower effective dose. Various configurations give good performance. Some result in significant dose reduction. For LS AP 400 and 600 speed-class systems, dose savings of 21% and 34% are achievable. The model is thus a powerful tool for system optimisation.
A Monte Carlo computer model of X ray imaging systems has been developed which uses a voxel phantom to simulate the patient. Image details were selected in accordance with the European quality criteria document. Contrast and signal-to-noise ratio for these details were calculated to estimate image quality. Effective dose was computed to enable optimisation. The program was validated with measurements on phantoms, patients and digitised patient images. It was demonstrated that the computational model of the imaging system provides predictions of entrance dose and contrast that lie within the range of values measured on patients. To illustrate the importance of using a realistic model of the patient, scatter-to-primary ratios, S/P, in a chest PA examination were calculated. It was found that the S/P varied by a factor of 10 in the image and that a grid was slightly more efficient than an air gap in removing the scatter behind the heart.
A Monte Carlo program has been developed to model X-ray imaging systems. It incorporates an adult voxel phantom and includes anti-scatter grid, radiographic screen and film. The program can calculate contrast and noise for a series of anatomical details. The use of measured H and D curves allows the absolute calculation of the patient entrance air kerma for a given film optical density (or vice versa). Effective dose can also be estimated. In an initial validation, the program was used to predict the optical density for exposures with plastic slabs of various thicknesses. The agreement between measurement and calculation was on average within 5%. In a second validation, a comparison was made between computer simulations and measurements for chest and lumbar spine patient radiographs. The predictions of entrance air kerma mostly fell within the range of measured values (e.g. chest PA calculated 0.15 mGy, measured 0.12 - 0.17 mGy). Good agreement was also obtained for the calculated and measured contrasts for selected anatomical details and acceptable agreement for dynamic range. It is concluded that the program provides a realistic model of the patient and imaging system. It can thus form the basis of a detailed study and optimization of X-ray imaging systems.
A theory is presented concerning the spatial light-energy fluctuation in a photographic emulsion placed in intimate contact with a fluorescent screen that is excited by x rays. For simplicity, the fluorescent screen is assumed to be irradiated with monoenergetic x rays. Formulas have been developed for the average light energy received by the emulsion, the autocorrelation function, and the Wiener spectral density associated with the fluctuation of light energy about the average value.For a linear, time-invariant electrical network, the transfer function can be determined by means of a white-noise input. Theoretical considerations indicate that an analogous method is not valid for fluorescent-screen-film systems. This point is illustrated only analytically for a homogeneous fluorescent screen which consists of a thin, but large, macrocrystal of calcium tungstate.The recording of the fluctuations of light energy as variations of optical density by the photographic film is discussed quantitatively.
When the signal in an x‐ray image system is formed by integrating the scintillation pulses rather than by counting them, the signal‐to‐noise ratio is reduced by a factor which depends on the shape of the pulse‐height distribution. The signal‐to‐noise ratio cannot be related directly to either quantum absorption or energy absorption, and a new quantity called noise‐equivalent absorption is defined which bears a simple relationship to the signal‐to‐noise ratio. Quantum, energy, and noise‐equivalent absorption are calculated as a function of thickness and x‐ray energy for CsI, Gd 2 O 2 S, LaOBr, Zn 0.6 Cd 0.4 S, and CaWO 4 .
Images acquired on modern digital volumetric clinical imaging instruments can be used to create 3-dimensional surfaces of the human anatomy for use in computer databases. Such a volume array based software phantom delineates internal organs with millimeter resolution, and lends itself to fully 3-dimensional Monte Carlo simulations. Our simulation models 45 internal human organs (each with an associated radioisotope concentration and attenuation coefficient), calculates gamma radiation histories through these structures, and accepts gamma events onto a collimated planar camera. Variance reduction techniques are applied to decrease the time required to compute a given number of events at the detector. Stratification and two implementations of forced detection variance reduction techniques are compared to ‘brute force’ calculations for their efficiency speed-ups in this heterogeneous geometry. Simulated clinical images of the liver are shown.
We have developed a theoretical model to predict the modulation transfer function (MTF), the shape of the x-ray quantum noise power spectrum (NPS), and the spatial-frequency-dependent detective quantum efficiency (DQE) of an x-ray phosphor screen. The transfer of energy through the screen is modelled as a series of cascaded stochastic processes assuming that the screen consists of many thin phosphor layers. In this way, the model is able to account for the possibility of secondary-quantum noise and the difference in shape between MTF2 and the x-ray quantum NPS. Modelling a Kodak Min-R screen we were able to predict both the number of light quanta emitted per absorbed x-ray and MTF(f) to better than +/- 5%, and the scintillation efficiency to within 10% of experimentally measured values. The shape of the x-ray quantum NPS is predicted to within +/- 5% for spatial frequencies less than about 6 mm-1 and to within +/- 20% for higher frequencies.
We have studied image quality in fluoroscopy, as related to the detectability of low-contrast iodine or acrylic (PMMA) details added to a homogeneous 20 cm thick PMMA phantom, by experimental measurements of the signal-to-noise ratio (SNR) and by Monte Carlo calculation. The agreement between the measured and calculated SNR at equal absorbed dose in the phantom showed that the imaging performance of x-ray image intensifier (XRII) based fluoroscopic systems is well understood and can be mainly accounted for by x-ray attenuation in the phantom and the detail, and by the interaction statistics of primary and secondary (scattered) x-ray quanta in the input phosphor of the XRII. The electronic noise sources in the video chain had only a small effect on the detectability of the details studied here. The optimal x-ray tube potential was 50-60 kV for detecting the low-contrast iodine detail in the phantom, and 70-100 kV for detecting the thin PMMA detail. For the task of detecting the iodine detail the use of a fibre-interspaced antiscatter grid improved the dose-to-information conversion efficiency of the imaging system by a factor of 2.2 as compared to imaging without the grid, and additional filtering of the x-ray beam by 0.25 mm Cu increased the efficiency by a factor of 1.6. Monte Carlo results were further used to estimate the potential of increasing the dose-to-information conversion efficiency by imaging system design changes. For the detection task of a static, low-contrast, low-spatial-frequency iodine contrast material detail embedded in a 20 cm thick soft-tissue phantom, the greatest contributions for further improvement could be achieved by improved antiscatter devices, x-ray spectrum modification, and by decreasing the absorption in the material layers in front of the CsI phosphor of the XRII. Contrary to this, no significant efficiency increase could be obtained by increasing the CsI phosphor coating thickness from the present value of 180 mg cm-2, or by changes in the video chain characteristics. The maximum potential of efficiency improvement is a factor of 6.3 when compared to the reference fluoroscopy system operated at 60 kV with 2.7 mm Al primary beam filtration, and a factor of 3.9 when compared to the reference system at 50 kV with the primary beam filtration added by 0.25 mm Cu.
A Monte Carlo computer program has been developed for the simulation of X-ray photon transport in diagnostic X-ray examinations. The simulation takes account of the incident photon energy spectrum and includes a phantom (representing the patient), an anti-scatter grid and an image receptor. The primary objective for developing the program was to study and optimise the design of anti-scatter grids. The program estimates image quality in terms of contrast and signal-to-noise ratio, and radiation risk in terms of mean absorbed dose in the patient. It therefore serves as a tool for the optimisation of the radiographic procedure. A description is given of the program and the variance-reduction techniques used. The computational method was validated by comparison with measurements and other Monte Carlo simulations.
The large dose values found for lumbar spine examinations in a centre participating in the European quality criteria trial have been investigated within a 5 year quality control programme. Actions focused mainly on optimizing the focus-to-film distance, tube potential (kV), film optical density and X-ray beam filtration. These actions lead to overall dose reductions of up to 75% in lumbo-sacral joint examinations and prove the need for a regular survey of patient skin doses.
In this study, interobserver and intraobserver variations in the interpretation of plain radiographs of the lumbosacral spine were evaluated. Three radiologists independently interpreted the radiographs from 200 consecutive outpatients, aged 13-93 years, mostly referred from general practitioners. Interobserver agreement was best for vertebral fractures, osteopenia, spondylolisthesis at L5-S1, lumbosacral junctional vertebra, reduced disc height at L4-S1 and osteophytes at L2-S1 (kappa 0.61-0.95), and poorest for spina bifida of S1, degenerative spondylolisthesis and facet joint arthrosis at T12-L4, sacroiliac joint arthrosis, narrow central spinal canal, film quality, and for decisions concerning evaluation of facet joints and spinal canal (kappa < 0.34). For several diagnoses, the number of abnormal findings differed significantly between observers (p < 0.05, McNemar's test), indicating different diagnostic thresholds. Intraobserver agreement in 36 reevaluated patients was fair to excellent for almost all variables (kappa > 0.46). Although some diagnoses related to low back pain were quite consistently evaluated, the substantial disagreement on many findings should alert clinicians and radiologists against overestimating the validity and usefulness of the examinations. To improve diagnostic consistency, it is important to reduce variation caused by different thresholds for abnormality.
A Monte Carlo computational model of a fluoroscopic imaging chain was used for deriving optimal technique factors for paediatric fluoroscopy. The optimal technique was defined as the one that minimizes the absorbed dose (or dose rate) in the patient with a constraint of constant image quality. Image quality was assessed for the task of detecting a detail in the image of a patient-simulating phantom, and was expressed in terms of the ideal observer's signal-to-noise ratio (SNR) for static images and in terms of the accumulating rate of the square of SNR for dynamic imaging. The entrance air kerma (or air kerma rate) and the mean absorbed dose (or dose rate) in the phantom quantified radiation detriment. The calculations were made for homogeneous phantoms simulating newborn, 3-, 10- and 15-year-old patients, barium and iodine contrast material details, several x-ray spectra, and for imaging with or without an antiscatter grid. The image receptor was modelled as a CsI x-ray image intensifier (XRII). For the task of detecting low- or moderate-contrast iodine details, the optimal spectrum can be obtained by using an x-ray tube potential near 50 kV and filtering the x-ray beam heavily. The optimal tube potential is near 60 kV for low- or moderate-contrast barium details, and 80-100 kV for high-contrast details. The low-potential spectra above require a high tube load, but this should be acceptable in paediatric fluoroscopy. A reasonable choice of filtration is the use of an additional 0.25 mm Cu, or a suitable K-edge filter. No increase in the optimal tube potential was found as phantom thickness increased. With the constraint of constant low-contrast detail detectability, the mean absorbed doses obtained with the above spectra are approximately 50% lower than those obtained with the reference conditions of 70 kV and 2.7 mm Al filter. For the smallest patient and x-ray field size, not using a grid was slightly more dose-efficient than using a grid, but when the patient size and field size were increased a fibre interspaced grid resulted in lower doses than imaging without a grid. For a 15-year-old patient the mean absorbed doses were up to 40% lower with this grid than without the grid.
A computer program has been developed to model chest radiography. It incorporates a voxel phantom of an adult and includes antiscatter grid, radiographic screen, and film. Image quality is quantified by calculating the contrast (deltaOD) and the ideal observer signal-to-noise ratio (SNR(I)) for a number of relevant anatomical details at various positions in the anatomy. Detector noise and system unsharpness are modeled and their influence on image quality is considered. A measure of useful dynamic range is computed and defined as the fraction of the image that is reproduced at an optical density such that the film gradient exceeds a preset value. The effective dose is used as a measure of the radiation risk for the patient. A novel approach to patient dose and image quality optimization has been developed and implemented. It is based on a reference system acknowledged to yield acceptable image quality in a clinical trial. Two optimizations schemes have been studied, the first including the contrast of vessels as measure of image quality and the second scheme using also the signal-to-noise ratio of calcifications. Both schemes make use of our measure of useful dynamic range as a key quantity. A large variety of imaging conditions was simulated by varying the tube voltage, antiscatter device, screen-film system, and maximum optical density in the computed image. It was found that the optical density is crucial in screen-film chest radiography. Significant dose savings (30%-50%) can be accomplished without sacrificing image quality by using low-atomic-number grids with a low grid ratio or an air gap and more sensitive screen-film system. Dose-efficient configurations proposed by the model agree well with the example of good radiographic technique suggested by the European Commission.