MRI signal intensity based B-spline nonrigid registration for pre- and intraoperative imaging during prostate brachytherapy.

MRI-GUIDED BRACHYTHERAPY is a method to treat localized prostate cancer (1–3). It enables treatment planning on site through its delineation of prostate substructures and surrounding tissue, especially when using an endorectal (ER) coil. D’Amico et al and Landis et al demonstrated the feasibility of this approach by performing 248 cases of MRI-guided prostate brachytherapy using a 0.5 Tesla (T) open-configuration MRI scanner, and comparing the clinical outcomes with other forms of brachytherapy (4,5). Their study reported that MR-guided prostate brachy-therapy presented fewer complications and a better quality of life than when using ultrasound for image guidance. One of the drawbacks of the MR-guided procedure, however, is that the open-configuration 0.5T scanner had significantly lower field strength and less gradient performance than standard high-field scanner cylindrical magnets, resulting in a lower signal-to-noise ratio and low contrast and resolution. Image registration between preoperative MRI obtained in a high-field scanner and intraoperative images used for guidance in the open magnet can help contribute information from the diagnostic scan into the procedure (6). Once registration is performed at the beginning of the procedure, the fused image is useful to better comprehend the prostate substructures and suspicious foci that would otherwise be more difficult to detect using intraoperative MRI alone. Hirose et al found that a significant prostate shape change occurred between preoperative 1.5T ER coil images and intraoperative 0.5T imaging due to postural change from a supine to lithotomy position and the absence of an ER coil at 0.5T. In particular, in the preoperative images, the prostate gland appeared to have a smaller anterior–posterior dimension and wider transverse dimension than in the intraoperative images. Furthermore, the shape change of the peripheral zone was greater than that of the central gland between pre- and intraoperative images (7). Nonrigid registration, as opposed to rigid registration is, therefore, necessary to compensate for the deformation of the prostate between pre- and intra-operative imaging. Bharatha et al reported nonrigid registration of ER coil-based 1.5T preoperative MRI with 0.5T intraoperative images using segmented data sets. The results from this study indicated that the method was feasible and significantly improved the accuracy of registration when compared to its rigid counterpart (8,9). The method, however, requires manual segmentation of the prostate as a preprocessing step, which can introduce intra- and interobserver variability. Furthermore, the manual segmentation step considerably prolongs the registration process, which impacts the procedure time of the brachytherapy intervention. Intensity-based nonrigid registration which does not require manual segmentation would allow the mapping of high-resolution images to the intraoperative ones in less time. In response to this need, Wang et al used such an approach to effectively track anatomical variations within CT images (10). While intensity-based registration was documented in this study, the method was tested in phantom trials and only a few clinical data sets, and was not tested for MR images. Our objective is to test and validate the feasibility of intensity-based nonrigid registration between preoperative MRI and intraoperative MRI obtained during MRI-guided brachytherapy of the prostate. Specifically, we tested the accuracy of the intensity-based registration method using a 1.5T preoperative MRI taken with an ER coil and a 0.5T intraoperative MRI taken without the ER coil.