Evaluation of automatic contour propagation in T2‐weighted 4DMRI for normal‐tissue motion assessment using internal organ‐at‐risk volume (IRV)

PURPOSE: The purpose of this study was to evaluate the quality of automatically propagated contours of organs at risk (OARs) based on respiratory-correlated navigator-triggered four-dimensional magnetic resonance imaging (RC-4DMRI) for calculation of internal organ-at-risk volume (IRV) to account for intra-fractional OAR motion. METHODS AND MATERIALS: T2-weighted RC-4DMRI images were of 10 volunteers acquired and reconstructed using an internal navigator-echo surrogate and concurrent external bellows under an IRB-approved protocol. Four major OARs (lungs, heart, liver, and stomach) were delineated in the 10-phase 4DMRI. Two manual-contour sets were delineated by two clinical personnel and two automatic-contour sets were propagated using free-form deformable image registration. The OAR volume variation within the 10-phase cycle was assessed and the IRV was calculated as the union of all OAR contours. The OAR contour similarity between the navigator-triggered and bellows-rebinned 4DMRI was compared. A total of 2400 contours were compared to the most probable ground truth with a 95% confidence level (S95) in similarity, sensitivity, and specificity using the simultaneous truth and performance level estimation (STAPLE) algorithm. RESULTS: Visual inspection of automatically propagated contours finds that approximately 5-10% require manual correction. The similarity, sensitivity, and specificity between manual and automatic contours are indistinguishable (P > 0.05). The Jaccard similarity indexes are 0.92 ± 0.02 (lungs), 0.89 ± 0.03 (heart), 0.92 ± 0.02 (liver), and 0.83 ± 0.04 (stomach). Volume variations within the breathing cycle are small for the heart (2.6 ± 1.5%), liver (1.2 ± 0.6%), and stomach (2.6 ± 0.8%), whereas the IRV is much larger than the OAR volume by: 20.3 ± 8.6% (heart), 24.0 ± 8.6% (liver), and 47.6 ± 20.2% (stomach). The Jaccard index is higher in navigator-triggered than bellows-rebinned 4DMRI by 4% (P < 0.05), due to the higher image quality of navigator-based 4DMRI. CONCLUSION: Automatic and manual OAR contours from Navigator-triggered 4DMRI are not statistically distinguishable. The navigator-triggered 4DMRI image provides higher contour quality than bellows-rebinned 4DMRI. The IRVs are 20-50% larger than OAR volumes and should be considered in dose estimation.