A three-film technique for reconstruction of radioactive seed implants.

Computer dose calculations for interstitial implants of radioactive seeds require a knowledge of the spatial coordinates of each seed in the implant. These coordinates are usually constructed from the seed images on either an orthogonal or a stereo pair of radiographs. Such a procedure, however, requires that each seed can be identified on each film. We have developed a computer algorithm which computes the locations of the seeds from a set of two stereo and one AP radiographs, even when seed identities on all films are indeterminable. This is done by means of a ray tracing technique. Under ideal conditions the projections from the three films uniquely define all seed locations. Under clinical conditions, however, where digitizing uncertainties and patient movement are inevitable, the algorithm must determine the most likely set of seed locations from the larger set of all possible seed locations. The criteria for making this selection, as well as clinical examples, are presented.