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In interactive, image-guided surgery (IIGS) we use stacked slice tomographic image sets as three dimensional maps of the patient's anatomy. Such sets can provide exquisite information on bony anatomy (computed tomography), soft tissue structure and lesion definition (magnetic resonance) or function (positron emission tomography). However, none of these tomographic sets clearly show the location and extent of vascular structures, which may be of critical importance to the surgical process. Vascular information can be obtained from conventional x- ray angiography (XRA), a form of projection imaging, or from tomographic scans sensitive to flow such as magnetic resonance angiography (MRA) or computed tomography angiography (CTA). Projection images show the extent and intersection of vessels at high resolution but lose the three-dimensional relationship of the vessels during image formation. The high resolution available from conventional angiograms makes them attractive for the localization of small aneurysms and other vascular anomalies, but the projection nature of the image makes displaying surgical position difficult. We have developed techniques for interactively displaying surgical position on both topographically derived vascular sets such s CTA and MRA and on projection sets such as XRA. In the XRA images,homologous points are determined on the film and on the patient. Since we use extrinsic, bone-implanted fiducial markers for our surgical guidance system, these markers can serve as most, if not all, of our homologous points and can be localized very precisely on the film and in 3-space. A homogeneous transform matrix (HTM) is constructed to provide a best 'first-guess' of position. The HTM is decomposed into nine spatial parameters which represent the XRA process and those parameters are optimized using Powell's Conjugate Direction Method. In the tomographic vascular images, the issue is not one of registration but one of display. We presently track probe position on tomographic slices but the information inherent in vascular tomograms is masked by a raster-slice display; the vessel's path and extent are difficult to discern. Vascular path and extent are best displayed as a rotoscope of projection images created from the raster-slice set. By 'rotating' a series of projection images the three- dimensional geometry is discerned by motion parallax.
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