(Tuesday, 7/31/2018) 3:45 PM - 4:15 PM
Room: Exhibit Hall | Forum 2
Purpose: Standard ACR MRI accreditation test based on 2D sequence was designed mainly for diagnostic MRI application. In addition to 2D sequence, 3D sequences are valuable for many radiotherapy (RT) applications due to its isotropic spatial resolution, but its quality assurance (QA) test cannot be fully fulfilled by the ACR MRI test. We proposed a ACR phantom test based on 3D MR sequence to accommodate MR simulation for radiotherapy.
Methods: The ACR MRI phantom was positioned on flat couch-top and aligned with calibrated external three point laser. Sixteen sets of phantom images were acquired on a 1.5T MR-simulator using two 8-channel array coils wrapped around the phantom with T1 weighted 2D spin echo (SE) sequence (sagittal localizer (TE/TR=20/200ms, 1 slice, voxel size = 0.94x0.94x20 mm³); axial scan(TE/TR=20/500ms,11 slices, voxel size=0.94x0.94x5mm³) and T1 weighted 3D SE sequence (TE/TR=21/500 ms, 176 slices, voxel size=1x1x1mm3) over a three-month period. Standard tests (geometric accuracy, image intensity and percent-signal ghosting along phase encoding direction) were conducted for both sequences referring to the ACR guidelines. Additional measurements of laser positional shift and percent-signal ghosting in 2nd phase encoding direction (slice encoding direction) were conducted on 3D sequence.
Results: All standard tests passed the ACR recommended criteria using the 2D sequence. Comparing 2D and 3D analysis approach, geometric accuracy (end-to-end length=147.5±0.3mm(2D) and 147.1mm±0.2(3D); diameter=190.6±0.2mm(2D) and 190.5±0.2mm(3D)), and percent ghosting (0.0027±0.001(2D) and 0.022±0.002(3D)) were within criteria. Higher image intensity uniformity was obtained using 2D approach than 3D approach (90.6±1.1%(2D) and 86.8±1.4%(3D)). For additional tests in 3D protocol, the percent signal ghosting in 2nd phase encoding direction was 0.011±0.001 and the laser positional shift was -0.12±0.49mm.
Conclusion: A 3D ACR phantom test is developed to better accommodate the special requirement of MR use in RT. It is of value for regular QA test in MR simulation for radiotherapy.
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