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Trends in image quality in high magnification digital specimen cabinet radiography

Regional Medical Physics Department, Newcastle General Hospital, Westgate Road, Newcastle NE4 6BE, UK

Advances in microfocus X-ray tube design together with the availability of high resolution charge coupled device (CCD) detectors have led to the introduction of high magnification digital specimen cabinets for the examination of tissue samples. This paper explores the effect that the high magnification geometry permitted by such units has upon image quality in terms of phase contrast edge enhancement, spatial resolution and the appearance of test phantom images. Phase contrast effects and spatial resolution were studied using a previously established method (using edge profiles) and by computing the system spatial frequency response at various geometries. It was demonstrated that the magnitude of the phase contrast enhancement effect reaches a stable maximum at a magnification of x 4. It has also been shown that a continual increase in both the spatial resolution together with an improved signal to noise ratio occurs up to the maximum permissible magnification geometry, with effects of focal spot blur being negligible. In practice, the limited size of the digital detector and the difficulty of object alignment can constrain the use of the very high magnification option.