Room: Exhibit Hall | Forum 2
Purpose: The use of electromagnetic Calypso transponders has demonstrated efficacy for real-time tracking of intra-fraction motion in prostate cancer radiotherapy. MRI, increasingly important in the care of prostate cancer patients, suffers from artifacts when metallic materials are present, making pre- and post-treatment imaging difficult. Previous studies have characterized these artifacts for the 14-gauge (14G) version of the transponders. This study investigates artifact reduction for the newer and smaller 17G transponders using clinically available metal artifact-reduction techniques.
Methods: 17G Calypso beacons were imaged in a water phantom on a Philips Ingenia 1.5 T scanner. Baseline T1-weighted gradient echo (T1-GRE) coronal scans were performed. To assess the quality of clinical artifact-reduction techniques, T1-weighted turbo spin echo (T1-TSE) scans were performed, including (1) 3D high-bandwidth, (2) 2D MARS (Metal Artifact Reduction Sequences), which incorporates high-bandwidth and view-angle tilting (VAT), and (3) 3D SEMAC (Slice Encoding for Metal Artifact Correction). The extent of the artifact was quantified by measuring the artifact range parallel and radial to the axis of the beacon.
Results: The baseline T1-GRE scan artifact measured 3.6x2.5 cm and included dark signal-loss regions, bright signal pile-up regions, and geometric distortion near the beacon. The artifact was reduced using the artifact-reduction techniques, with artifact range measurements of 2.7x2.2, 2.5x1.7, and 2.4x1.8 cm for high-bandwidth, MARS, and SEMAC, respectively. Scan times were nearly four times longer when using SEMAC compared to the high-bandwidth technique, for only marginal reduction in artifact.
Conclusion: While artifact-reduction sequences reduced the total artifact size, no sequence was able to reduce them for the 17G beacons to a level deemed acceptable for use in treatment plan delineation. Scan times were prohibitively long for SEMAC. More advanced techniques are required to reduce artifacts further in order for the MR images have utility for planning and diagnostic follow up.
Funding Support, Disclosures, and Conflict of Interest: Varian provided Calypso beacons for the study, and one of the authors is an employee at Philips Healthcare. No financial compensation was obtained for this investigation.