Combined use of transverse and scout computed tomography scans to localize radioactive seeds in an interstitial brachytherapy implant.

Various techniques have been developed to localize radioactive sources in brachytherapy implants. The most common methods include the orthogonal film method, the stereo-shift film method, and recently, direct localization from a series of contiguous CT transverse images. The major advantage of the CT method is that it provides the seed locations relative to anatomic structures. However, it is often the case that accurate identification and localization of the sources become difficult because of partial source artifacts in more than one transverse cut and other artifacts on CT images. A new algorithm has been developed to combine the advantages of using a pair of orthogonal scout views with the advantages of using a stack of transverse cuts. In the new algorithm, a common reference point is used to correlate CT transverse images and two orthogonal scout CT scans (AP and lateral). The radioactive sources are localized on CT transverse images. At the same time, the sources are displayed automatically on the two CT scout scans. In this way, the individual sources can be clearly distinguished and ambiguities arising from partial source artifacts are resolved immediately. Because of the finite slice thickness of transverse cuts, the longitudinal coordinates are more accurately obtained from the scout views. Therefore, the longitudinal coordinates of seeds localized on the transverse cuts are adjusted so that they match the position of the seeds on scout views. The algorithm has been tested on clinical cases and has proved to be a time saving and accurate method.