In this paper, a deformable registration method is proposed that enables automatic
alignment of preoperative PET/CT to intraoperative ultrasound in order to achieve
PET-determined focal prostate brachytherapy. Novel PET imaging agents such as prostate
specific membrane antigen (PSMA) enables highly accurate identification of intra/extra-prostatic
tumors. Incorporation of PSMA PET into the standard transrectal ultrasound (TRUS)-guided
prostate brachytherapy will enable focal therapy, thus minimizing radiation toxicities.
Our registration method requires PET/CT and TRUS volume as well as prostate segmentations.
These input volumes are first rigidly registered by maximizing spatial overlap between
the segmented prostate volumes, followed by the deformable registration. To achieve
anatomically accurate deformable registration, we extract anatomical landmarks from
both prostate boundary and inside the gland. Landmarks are extracted along the base-apex
axes using two approaches: equiangular and equidistance. Three-dimensional thin-plate
spline (TPS)-based deformable registration is then performed using the extracted landmarks
as control points. Finally, the PET/CT images are deformed to the TRUS space by using
the computed TPS transformation. The proposed method was validated on 10 prostate
cancer patient datasets in which we registered post-implant CT to end-of-implantation
TRUS. We computed target registration errors (TREs) by comparing the implanted seed
positions (transformed CT seeds vs. intraoperatively identified TRUS seeds). The average
TREs of the proposed method are 1.98 +/- 1.22 mm (mean standard deviation) and 1.97
+/- 1.24 mm for equiangular and equidistance landmark extraction methods, respectively,
which is better than or comparable to existing state-of-the-art methods while being
computationally more efficient with an average computation time less than 40 seconds.
