An automated technique for SPECT marker-based image registration in radionuclide therapy

An automated technique for mark-er-based image registration in radionuclide therapy is described. This technique is based on localization of the centr oids of external markers and on establishing correspondence between the ind ividual markers of the two studies to be registered. Localization of the ce ntroids of markers relies on segmenting the markers using iterative thresho lding. Thresholding is locally adaptive in order to account for study-depen dent conditions (e.g. crossover between adjacent markers and markers with v arying radioactive concentrations). Following marker segmentation, the cent roids of the markers are computed based on an intensity-weighted method. Fi nally, prior to the least-squares fit registration, the markers of the two sets are matched to achieve one-to-one correspondence. The technique was ap plied to both simulated and patient studies resulting in mean residual thre e-dimensional registration errors (+/- 1 SD) of 1.7 +/- 0.1 mm and 3.5 +/- 0.3 mm respectively. The technique was compared with a semi-automated appro ach and no significant difference was found between the mean residual three -dimensional registration errors (t = 0.28 1 p = 0.8). This automated marke r-based image registration technique provides robust and accurate registrat ion. Although it was developed as part of a programme to generate three-dim ensional dose distributions for radionuclide therapy, it could be useful fo r other clinical applications.