Treatment of subarachnoid aneurysms with endovascular techniques (e.g. placement of Guglielmi coils) is currently limited by the inability to visualize the neck of the aneurysm after the initial coils have been placed. Coils projecting into parent vessels may cause thrombosis, while incomplete filling leads to regrowth of the aneurysm. Since the procedure is performed using a gantry-mounted x-ray image intensifier (XRII), we have used this system to obtain 2- dimensional (2-D) images over approximately 200 degrees and reconstructed a 3-D image of the embolizing coil, residual aneurysm, and the parent vessel. The required data can be acquired, reconstructed and presented to the neuroradiologist during the interventional procedure. We have characterized an existing clinical C-arm radiographic system, and have developed correction procedures which provide a consistent and adequate data set for standard CT reconstruction using convolution-backprojection techniques. The angle of acquisition for each image is recorded using a hub-mounted angle encoder accurate to within plus or minus 0.3 degrees. We have characterized the XRII distortion over the rotation, and can correct images acquired at any known angle to within plus or minus 0.06 pixels. We have also measured the motion of the center of rotation (reproducible to within plus or minus 0.13 pixels) and correct for the displacement using an image shift and interpolation algorithm. Finally, we have investigated the effects on CT reconstruction of variable dilutions of contrast agent during the cardiac cycle, and have shown the contrast-to-noise ratio (CNR), in the absence of photon noise, to be 28. This is larger than the CNR which we achieve due to photon noise alone.