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Accelerated 3D BSSFP Imaging for Treatment Planning On a Low-Field MRI-Guided Radiotherapy System

Y Gao

Y Gao*, Z Zhou , F Han , M Cao , N Shaverdian , J Hegde , B Bista , M Steinberg , P Lee , A Raldow , D Low , P Hu , Y Yang , UCLA School of Medicine, Los Angeles, CA


TU-FG-FS2-3 (Tuesday, August 1, 2017) 1:45 PM - 3:45 PM Room: Four Seasons 2

Purpose: To introduce a compressed sensing and parallel imaging combined technique to reduce the acquisition time of planning MR for MR-guided radiotherapy(MRgRT) systems.

Methods: A variable-density Poisson-Disk(VDPD) under-sampling acquisition along with compressed sensing reconstruction(L1-ESPIRiT) technique was developed and compared with the current planning MR protocol, which uses an optimized balanced steady-state free-precession(bSSFP) sequence with 7.5-fold(7.5x) acceleration achieved by GRAPPA and partial Fourier. Image quality of GRAPPA and VDPD with 7.5x and 15x acceleration were compared with fully sampled images on an ACR phantom. Two volunteers were recruited to evaluate whether 15x acceleration and high-resolution imaging can be achieved in-vivo. Ten patients with abdominal tumors were included where two breath-hold images were acquired using GRAPPA-7.5x(25s) and VDPD-15x(12.5s). Three readers scored the two approaches in terms of quality for organ/tumor delineation. The GTV and two kidneys were contoured by two radiation oncologists. Centroid location difference, percent volume difference, and DICE coefficient between contours drawn using the two techniques were calculated. All studies were performed on a 0.35T MRgRT system.

Results: The VDPD-15x approach had lower noise than the GRAPPA-7.5x technique(nRMSE 0.036 vs. 0.044). In both phantoms and volunteers, noise amplification was apparent when the acceleration was increased from 7.5x to 15x using GRAPPA acquisition, but minimal using the VDPD approach. The GRAPPA-7.5x approach failed to generate sufficient quality under 1mm isotropic resolution whereas VDPD-15x provided acceptable quality. In the patient study, two GRAPPA-25s breath-holds failed, while all VDPD-12.5s breath-holds were successfully acquired. Good agreements between scoring and contouring statistics with no statistically significant difference were observed. Volume differences were around 7.6%, 2.1% and 2.8%, and DICE indices were greater than 0.84, 0.92 and 0.90 for the GTV, and two kidneys, respectively.

Conclusion: The proposed technique reduced the acquisition time by 50% and provided equivalent or even higher quality than the standard protocol.

Funding Support, Disclosures, and Conflict of Interest: The authors acknowledge research support from ViewRay, Inc.

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