A COMPUTER-ASSISTED METHOD FOR 3D SUBTRAC TION ANGIOGRAPHY BASED ON SPIRAL CT

The objective of this study was to develop a method for 3D subtraction CT angiography and to optimize the visualization after semiautomatic segmentation. Ten patients with aneurysms of the abdominal aorta were examined using spiral CT. To reconstruct the vessels, as well as adjac ent organs such as the liver and kidneys, one image data volume was ac quired before and after injection of the contrast agent. The CT scans were obtained with a Siemens Somatom Plus 4. To improve the results of automatic segmentation, as well as visualization by maximum intensity projection (i.e. removal of bony structures), subtraction of both ima ge volumes is necessary. However, small translation shifts disturb the subtraction process and produce artificial contours. To calculate the disparities along the three coordinate axes of two corresponding imag e volumes, a cepstrum filter is applied to a pair of image volumes. Af ter detection of the disparities, which manifest as bright spots, the real shift of the two subsignals can be calculated. Translation of the corresponding image volume pairs to their correct positions improves the subtraction process. In all cases the size of the aneurysm and the abdominal organs could be better segmented and visualized. Applicatio n of the cepstrum filter and subtraction of the image volumes before a nd after contrast medium injection completely removes the bony structu res in the image data and results in superior visualization results.