FILTER WHEEL EQUALIZATION IN DSA - SIMULATION RESULTS

Digital subtraction angiography (DSA) is a valuable and frequently use d diagnostic technique which allows the evaluation of vascular anatomy and pathology. In order to improve the utility and diagnostic perform ance of DSA, a filter wheel system is presented which can provide mech anically simple, rapidly adaptable, and clinically practical radiograp hic equalization to DSA images. The filter wheel equalization system c onsists of a series of computer-driven filter wheels, mounted between the x-ray tube and collimator. Each wheel has an annular area of atten uator material which intersects the x-ray beam and carries a rotationa lly varying compensation pattern on it. Rotation of the filter wheel m odulates the attenuator pattern in the x-ray field under computer cont rol, in a manner designed to increase the x-ray exposure to underexpos ed areas in the image. A system with eight wheels has 10(19) possible equalization patterns. The compensation patterns are removed in the su btracted DSA images. Computer simulation techniques were used to evalu ate 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 rati o of 23%, averaged over the image, was found.