Digital subtraction angiography (DSA) is a valuable and frequently used diagnostic technique which allows the evaluation of vascular anatomy and pathology. In order to improve the utility and diagnostic performance of DSA, a filter wheel system is presented which can provide mechanically simple, rapidly adaptable, and clinically practical radiographic equalization to DSA images. The filter wheel equalization system consists of a series of computer-driven filter wheels, mounted between the x-ray tube and collimator. Each wheel has an annular area of attenuator material which intersects the x- ray beam and carries a rotationally varying compensation pattern on it. Rotation of the filter wheel modulates the attenuator pattern in the x-ray field under computer control, in a manner designed to increase the x-ray exposure to underexposed areas in the image. A system with eight wheels has 1019 possible equalization patterns. The compensation patterns are removed in the subtracted DSA images. Computer simulation techniques were used to evaluate various design scenarios for the filter wheel system, using a data base of 191 clinical DSA mask images. For a practical implementation of the filter wheel system, an improvement in the signal to noise ratio of 23%, averaged over the image, was found.
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