Registration of serial SPECT/CT images for three-dimensional dosimetry in radionuclide therapy

For radionuclide therapy, individual patient pharmacokinetics can be measured in three dimensions by sequential SPECT imaging. Accurate registration of the time series of images is central for voxel-based calculations of the residence time and absorbed dose. In this work, rigid and non-rigid methods are evaluated for registration of 6-7 SPECT/CT images acquired over a week, in anatomical regions from the head-and-neck region down to the pelvis. A method for calculation of the absorbed dose, including a voxel mass determination from the CT images, is also described. Registration of the SPECT/CT images is based on a CT-derived spatial transformation. Evaluation is focused on the CT registration accuracy, and on its impact on values of residence time and absorbed dose. According to the CT evaluation, the non-rigid method produces a more accurate registration than the rigid one. For images of the residence time and absorbed dose, registration produces a sharpening of the images. For volumes-of-interest, the differences between rigid and non-rigid results are generally small. However, the non-rigid method is more consistent for regions where non-rigid patient movements are likely, such as in the head-neck-shoulder region.