Impact of abdominal compression on MR-guided radiation therapy.

Purpose: Abdominal compression (AC) has been shown to reduce liver and thoracic lesion motion during respiration, however implications for gated MR-guided radiation therapy have not been explored. This work evaluates changes observed due to using an MR-compatible AC system on an MRLINAC. Methods: The AC system was comprised of two inflatable compression bladders designed to maintain pressure on the diaphragm. Five male volunteers were consented to an IRB study and underwent serial MR acquisitions on a 0.35 T MR-LINAC. Fitted bladder assemblies were placed beneath the xiphoid process and inflated to calibrated pressures. Four pressures were applied (no pressure to highest tolerable pressure). At each pressure setting, three 25 s 0.15 cm×0.30 cm resolution true fast imaging and steady precession (TrueFISP) MRIs were acquired for free breathing (FB), end-inhale (EI) and end-exhale (EE). Livers were delineated at each pressure and respiration- induced liver motion was calculated via centroid difference analysis of EE and EI contours. Three reviewers performed image grading to compare artifacts with/without AC on FB datasets (four-point scale, 1 = significant artifacts, 4 = none/very few artifacts). External anatomy displacement and subject comfort were assessed. Results: Contoured liver vector excursion decreased by 60% (3.2 cm to ∼1.7 cm) from no pressure to the third pressure. However, a 15% average increase was observed from third to fourth pressures (1.71-1.98 cm). FB image quality improved with increased pressure from 1 (significant artifacts) to 3 (minor artifacts). Average external anatomy displacement due to AC was 3.71 ± 0.41 cm (3.23-4.56 cm). Subject discomfort levels increased with pressure from 1 (no discomfort) to 3 (rather uncomfortable). Conclusion: The MR-compatible AC system decreased liver excursion while improving FB image quality, which may yield options for patients who cannot comply with breath-holding. AC displaced external anatomy which must be accounted for in treatment planning. Assessing AC\u27s impact on dosimetry and gating efficiency is warranted