Graham McVey

Schemes for the optimization of chest radiography using a computer model of the patient and x-ray imaging system.

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.

Calibration and validation of a voxel phantom for use in the Monte Carlo modeling and optimization of x-ray imaging systems

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.

Demonstration of correlations between physical and clinical image quality measures in chest and lumbar spine radiography

Clinical and physical assessments of image quality are compared and the correlation between the two derived. Clinical assessment has been made by a group of expert radiologists who evaluated the fulfillment of the European Image Criteria for chest and lumbar spine radiography; yielding the so-called Image Criteria Score, ICS. Physical measures of image quality were calculated using a Monte Carlo model of the complete imaging system. This model includes a voxelised male anatomy and calculates contrast and signal-to-noise ratio of various anatomical details and a measure of useful dynamic range. Correlations between the ICS and the physical image quality measures were sought. Four lumbar spine and 16 chest imaging systems were evaluated and simulated with the model. The most useful physical quantities for chest radiography were the dynamic range and contrast of blood vessels in the retro-cardiac area. In lumbar spine, it was the signal-to-noise ratio of trabecular structures. The significant correlation is encouraging and shows that clinical image quality can be predicted provided the imaging conditions are well known and that relevant measures of physical image quality are used to assess the quality of the image.