Physical and Technical Considerations of Computerized Fluoroscopy Difference Imaging

Considerations for the optimum design and use of a computerized fluoroscopy apparatus for performing time dependent image subtraction are presented. The advantages of logarithmic processing are presented. Assuming such processing, the interrelationship of achievable signal to noise, dynamic range and the minimum number of grey levels needed to digitize each image is discussed, and a formula relating these three quantities is derived. Image quality limits imposed by noise sources not associated with the detected x-ray fluence are discussed and a criterion for choosing a maximum x-ray fluence which will not waste patient dose is presented. The limits to spatial resolution achievable with conventional image intensifiers are discussed and it is shown that the maximum one dimensional spatial resolution in the object plane is achieved when the magnification of the x-ray system is such that the image of the x-ray focal spot projected through a point in the object plane onto the detector plane just covers the width of two pixels. The effect of the detection of scattered radiation on the difference image is discussed and it is shown that a conventional scatter reduction grid will improve image quality only if the ratio of the number of detected scattered photons to the number of detected primary photons is greater than 0.8 when no grid is used.