Graham McVey's research while affiliated with The Royal Marsden NHS Foundation Trust and other places

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.

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 anthropo-morphic 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 novel approach to patient dose and image quality optimization was developed and implemented for chest and lumbar spine radiography. A Monte Carlo model of the imaging chain, including an anthropomorphic voxel-phantom to simulate the patient, was utilized. Detector noise and system unsharpness were modeled and their influence on image quality considered. Image quality was quantified by the contrast ((Delta) OD) and the ideal observer signal-to-noise (SNR) for a number of relevant image details at various positions in the anatomy and measures of dynamic range (DR). Among systems evaluated in a clinical trial, a reference system, acknowledged to yield acceptable image quality, was selected. A large variety of other imaging conditions were simulated and compared to the reference system. Some of the simulated systems were found to give as good imaging performance but at substantially reduced patient doses: 35% and 50% reduction in the lumbar spine AP and the chest PA view, respectively. The model was also used to define a single-valued 'figure-of- merit,' the physical image quality score, PIQS, with the aim to make possible ranking of the imaging systems. By comparing the ranking according to PIQS with radiologists' ranking it was possible to analyze the features in the images which are clinically important.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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 novel approach to patient dose and image quality optimization was developed and implemented for chest and lumbar spine radiography. A Monte Carlo model of the imaging chain, including an anthropomorphic voxel-phantom to simulate the patient, was utilized. Detector noise and system unsharpness were modeled and their influence on image quality considered. Image quality was quantified by the contrast ((Delta) OD) and the ideal observer signal-to-noise (SNR) for a number of relevant image details at various positions in the anatomy and measures of dynamic range (DR). Among systems evaluated in a clinical trial, a reference system, acknowledged to yield acceptable image quality, was selected. A large variety of other imaging conditions were simulated and compared to the reference system. Some of the simulated systems were found to give as good imaging performance but at substantially reduced patient doses: 35% and 50% reduction in the lumbar spine AP and the chest PA view, respectively. The model was also used to define a single-valued 'figure-of- merit,' the physical image quality score, PIQS, with the aim to make possible ranking of the imaging systems. By comparing the ranking according to PIQS with radiologists' ranking it was possible to analyze the features in the images which are clinically important.