Sequential subtraction for improved display of cerebrovascular occlusions.

The subtraction method, first developed by Ziedses des Plantes [1] in 1953, eliminates overlying bone shadows from angiographic films, enhancing the display of vascular anatomy. To completely eliminate all bone details, however, a second order subtraction is sometimes necessary; this requires the superimposition of two mask films over the angiographic film to make the final subtraction print [2]. One modified subtraction technique, called sequential subtraction, involves making a mask film of each film in an angiographic sequence and superimposing that mask film onto the next film in the series. This process allows analysis of the filling and c learance phases of different vascular territories and demonstrates an anatomic relation between arteries and veins [3]. We have modified slightly the sequential subtraction technique developed by Rivoir and Huber [3]. With cerebrovascular occ lusion , the early arterial phase of the cerebral angiogram usually shows the proximal end of the occlusion, while the distal part is defined during a late phase. In our modified technique, a mask film is made of the early arterial phase to show the proximal part of the occlusion (fig. 1 A). By superimposing and then subtracting this image from a