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B0-Inhomogeneity Induced Geometric Distortion in the Liver: Measurement and Consistency

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O Wong

O Wong*, J Yuan , Y Zhou, S Yu , K Cheung , Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong


SU-I-GPD-J-59 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Geometric accuracy is one major concern when applying MRI for radiotherapy. Subject specific B₀-inhomogeneity (Δ₀0) variation is one source of MR geometric distortion. In this study, we aimed to investigate the magnitude of the ΔB0-induced MR image geometric distortion in the liver and the consistency of this distortion measurement when MR images were acquired at different orientations in healthy volunteers.

Methods: Liver imaging was conducted in 10 healthy volunteers using a 1.5T MR-simulator. Coronal and sagittal images at inhalation breath-hold status were acquired using a dual-echo GE sequence (TR=70-137ms, TE1/TE2=4.76/9.52ms, pixel size = 2-5mm2, slice thickness = 8-10mm, frequency encoding (FE) direction = superior-inferior, receiver bandwidth=260 or 1000Hz/pix). After calculating the ΔB₀ map using the MR phase-subtraction technique, geometric distortion map along the superior-inferior direction was subsequently calculated. For each image sets, ROIs at the right liver lobe proximal (RLp) and distal (RLd) to the lung, and left liver lobe (LL) were drawn. A Wilcoxon signed-rank test was performed to compare the geometric distortion at different scan orientations and at different liver regions.

Results: For both scan orientations and all ROIs, the maximum ΔB0-induced geometric distortion was smaller than 1mm. Comparing with the RLp, significantly smaller mean distortion (mm) was observed in LL(RLp vs. LL=0.38±0.33 vs. 0.12±0.12, p≤0.01(coronal); 0.35±0.31 vs. 0.14±0.20, p≤0.01(sagittal)) for both scan orientations. In terms of distortion measurement consistency, insignificant mean distortion (mm) between coronal and sagittal orientation was observed for three liver ROIs (Coronal vs. Sagittal=0.38±0.33 vs. 0.35±0.31, p=0.24(RLp); 0.13±0.13 vs. 0.11±0.12, p=0.17(RLd); 0.12±0.12 vs. 0.14±0.20, p=0.80(LL)).

Conclusion: ΔB₀-induced MR image geometric distortion in liver was generally small (less than 1mm), so might be negligible when compared to the respiratory motion induced displacement for radiotherapy applications. ΔB0-induced distortion measurement was consistent and independent of image acquisition orientation.

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