SU‐F‐J‐163: In Vivo Quantification of Sequence Parameter Effect On Geometric Distortion Caused by Implanted Titanium Brachytherapy Applicator

Abstract

Purpose:The use of MR to plan and evaluate brachytherapy treatment for cervical cancer is increasing given the availability of MR conditional or safe applicators and MRI's proven superiority to CT for characterizing soft tissue lesions. The titanium applicators, however, cause geometric distortions or imaging artifacts, which reduce the utility of MRI for dosimetry. We sought to quantify the observed volume of the same applicator on a previously optimized T2 sequence in comparison to the conventional T2 sequence and CT obtained for brachytherapy planning.Methods:Prior work with testing in phantoms showed that increases in readout bandwidth yielded reductions in artifact area and distortion measurements even with voxel increases. Following IRB approval, nine patients with titanium tandem & ovoid applicator (Varian Medical Systems) in place were scanned with a standard periprocedural protocol which included sagittal T2 fast spin echo (FSE) acquisition (res 0.98×0.78×4.0 mm3; BW 200Hz). An additional T2‐weighted FSE sequence (res 0.98×0.98×3–4 mm3; BW500Hz) with increased readout bandwidth, readout voxel size, and echo train length was added to the protocol. Volume measurements of the applicator (from tip to cervical stop) were hand‐segmented in Velocity AI 3.1 (Velocity Medical Solutions) for the two T2 FSE sequences and a planning CT obtained shortly after MRI. Differences were analyzed using a paired t‐test.Results:Average apparent volumes of the applicator on standard T2 sequence, decreased bandwidth T2 sequence and CT were 5.922±1.283 cm3, 4.544±1.524 cm3, and 2.304±0.509 cm3 respectively.Conclusion:Apparent volumes of a brachytherapy applicator can be compared in vivo. The modified sequence results in decreased apparent size of the cervical applicator. Both MR sequence volumes were larger than the planning CT, which was expected. Future work will focus on the diagnostic quality of the new sequence and quantifying any geometric shifts after CT to MRI registration based on anatomical landmarks.