SU-D-213CD-05: Identifying Prostate Brachytherapy Seeds at MRI: A Study in Phantom

Conventionally, post-implant CT scans are used for identification of prostate brachytherapy implant seed locations. The dosimetric quality of the seed implant in our institution is evaluated based on CT/MRI fusion and contouring of prostate and rectal wall on MRI. Post-implant evaluation of prostate brachytherapy using MRI alone is generally not feasible due to the uncertainty associated with seed localization despite its excellent anatomical delineation. The fusion of CT and MRI has some variability and may be time consuming. The goal of our current work was to use SWI phase images for identification of prostate brachytherapy seeds. Using MRI alone to identify seeds will eliminate the need for CT scan of the patient post-implant and eliminate the variability of the CT/MRI fusion.

A prostate gel phantom containing five inactive brachy seeds (Advantage I-125™, Biocompatibles, Oxford, CT) each has longitudinal cross section area of 3.6 mm2. It was assessed using CT, and MRI. Imaging was done using a GE Signa 3T HD MRI system (GE Heathcare, Millwaukee, WI). Imaging parameters for SWI were: 512×384 (zero filled to 512×512), FOV=10 cm, ASSET factor=2, TE/TR=20/42 ms, FA=15°, RBW= 80 Hz/pixel, spatial resolution=0.3 × 0.3 × 2.0 mm.

Brachytherapy seed, as confirmed on CT images, were easily identified in the phantoms on the filtered SWI phase images. The mean area for the 5 seeds, as measured on CT and SWI filtered phase images, was 3.5±0.5 mm² and 3.8±0.6 mm² , respectively. There appeared to be linear relationship in seed area as determined by SWI filtered phase compared to CT (R2=0.8).

With the improved resolution, SNR and proper filtering on high field MRI systems, SWI phase images can be used to identify prostate brachytherapy seeds on conventional MRI without using CT.