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Computer simulation study of blood vessel imaging in cerebral angiography: question of isoplanatism

The line spread function (LSF) and modulation transfer function of geometric unsharpness have been used to evaluate magnification radiography. The application of these concepts requires the magnification process to be linear and isoplanatic. However, because of the finite object thickness and the angulation of the X-ray tube target, it is not obvious that the magnification process is isoplanatic. In this investigation the X-ray images of blood vessels filled with a contrast medium were computed by two different methods. The first involves convoluting the input X-ray pattern of a blood vessel with the LSF of the X-ray tube focal spot. This corresponds to an 'isoplanatic model'. In the other method the X-ray intensity passing through the blood vessel from the X-ray tube target to the image plane is calculated by a ray tracing technique. This corresponds to the 'actual case'. Comparison of results from these two methods shows that for geometric conditions usually encountered in cerebral angiography the magnification process can be regarded as isoplanatic. A theoretical analysis of image formation in magnification radiography was discussed.