Purpose: To find out a suitable sequence in the MR-only workflow, the variability of contouring pelvic OAR on different MRI sequences from inter- and intra-observer was analyzed.
Methods: A total of 29 patients with pelvic tumors had three sequences (T1WI, T1dixonc and T2WI) scanned from MR. Two radiation oncologists as observers performed manual outlines of the bladder, anal canal, rectum, and left and right femoral heads on all images. Inter- and intra-observer variability was analyzed by DSC, 95% HD, and volume assessment index, and stability of organ delineation between sequences was observed. Furthermore, we construct a single-sequence automatic segmentation model to observe the variability of the three sequences in automatic segmentation.
Results: Analysis of inter-observer differences found that T1WI was more dominant than the other two sequences on the investigated organs. The bladder delineated on the T1WI sequence achieved the highest DSC (0.944) and the smallest 95% HD (2.967mm) of all OARs and all sequences. In the intra-observer variability analysis, the correlation between the contour volumes delineated on T1WI and T2WI was the strongest, and the ICC (intraclass correlation coefficient) values of all organs except the anal canal (0.614) exceeded 0.940. All OARs had the highest DSC and the lowest 95% between T1WI and T2WI. In particular, the right femoral head had the highest DSC (T1WI-T2WI: 0.959) and the lowest 95% HD (T1WI-T2WI: 1.181mm). On the DSC value between automatic segmentation and the contour is manually delineated by two observers, T2WI is higher than other sequences. And on 95% HD, T2WI is mostly lower than the other two sequences in value.
Conclusion: For organ contouring on MRI, most organs have good delineation stability on T1WI and T2WI. When considering MRI to delineate organs, T1WI can be used as the preferred sequence, and T2WI can be used as the complementary sequence.