CT and MRI derived source localization error in a custom prostate phantom using automated image coregistration

Dosimetric evaluation of completed brachytherapy implant procedures is crucial in developing proper technique. Additionally, accurate dosimetry may be useful in predicting the success of an implant. Accurate definition of the prostate gland and localization of the implanted radioactive sources are critical to attain meaningful dosimetric data. MRI is recognized as a superior imaging modality in delineating the prostate gland. More importantly, MRI can be used for source localization in postimplant prostates. However, the MRI derived source localization error bears further investigation. We present a useful tool in determining the source localization error as well as permitting the fusion, or coregistration, of selected data from multiple imaging modalities. We constructed a custom prostate phantom of hydrocolloid material precisely implanted with I-125 seeds. We obtained CT, the accepted modality, and MRI scans of the phantom. Subsequently, we developed an automated algorithm that employs a sequential translation of data sets to initially maximize coregistration and minimize error between data sets. This was followed by a noniterative solution for the necessary rotation transformation matrix using the Orthogonal Procrustes Solution. We applied this algorithm to CT and MRI scans of the custom phantom. CT derived source locations had source localization errors of 1.59 mm +/- 0.64. MRI derived source locations produced similar results (1.67 mm +/- 0.76). These errors may be attributed to the image digitization process.