The integration of MR-Simulator in RT workflow has sparked interest in the development of MRI-guided RT treatment protocols. The purpose of this study is to optimize an isotropic 3D TSE (SPACE) MR sequence for daily positional verification using MR-MR based imaging guidance in pelvic RT. T2-weighted (T2W) pelvic images of 10 male and 10 female healthy volunteers (age 23-45) were acquired in a 1.5T MRI-Simulator with a dedicated 18-channel phased-array body coil. Images were scanned in RT treatment position immobilized in a customized vacuum pillow system. Rapid imaging was achieved with partial Fourier techniques and with adjustments to ETL, NEX and iPAT, to generate a total of four axial 3D verification image sequences (MR-X) with a short acquisition time. Signal-to-noise ratio (SNR), TR, TE, BW, NEX and voxel size remained constant (Table 1). To avoid positional deviation, a high-resolution axial T2W 3D SPACE sequence (MR-Refx) was scanned as a reference (TR/TE: 1500/149ms, 1.17mm isotropic voxel) prior to the acquisition of each MR-X. Using four radiation therapist observers (10±5 years of experience), the impact of image quality of the four verification image sequences was investigated by employing both a qualitative analysis using a 7-point scale evaluation, based on delineation of organ outline and motion artifact, and a quantitative approach using a 6-DoF offline manual registration. The results were analysed using paired t-tests.Abstract 3656; Table 1MR-AMR-BMR-CMR-DTE (Echo Time)/TR (Repetition Time) (ms)147/1500147/1500147/1500147/1500Echo train length (ETL)217200217200Voxel size (mm)1.3x1.3x1.31.3x1.3x1.31.3x1.3x1.31.3x1.3x1.3BW (Bandwidth) (Hz/pixel)814814814814Parallel imaging factor (iPAT PE)3434Parallel imaging factor (iPAT 3D)3221Number of signals acquired (NEX)1111Partial Fourier (slice direction)OffOffOff6/8Acquisition time (min)1:151:301:562:41 Open table in a new tab Quantitatively, MR-A, with the shortest acquisition time, had a significantly larger shift error (p=0.007) in the Z translation direction with an average difference of 0.165mm compared with MR-RefA. MR-D, with the longest scan time, had a significantly larger yaw rotation (p=0.014) with a mean difference of 0.13o compared with MR-RefD. Qualitatively, the observers favored MR-D and MR-B sequences. MR-D and MR-B were not significantly different in image quality scores compared with their references. MR-B sequence had a high score, comparable to MR-RefB with bladder outline and visualization (p=0.212) and bladder motion artifact (p=0.124). MR-B is a promising imaging sequence. Not only was image quality not impeded to affect accurate registration but good visuals of the OARs and overall little artifact and noise were also reported. In this study, four MR sequences optimized for pelvic positional verification have been tested and the recommended image protocol, MR-B, has been identified based on image quality, registration accuracy and acquisition time.