Impact of MRI Sequence Resolution for Target Volume Definition in Stereotactic Radiosurgery
Huang Y, Liang E, Schaff EM, Zhao B, Snyder K, Wen N, Chetty IJ, Shah MM, and Siddiqui S. Impact of MRI Sequence Resolution for Target Volume Definition in Stereotactic Radiosurgery. Int J Radiat Oncol Biol Phys 2021; 111(3):e127-e128.
Int J Radiat Oncol Biol Phys
Purpose/Objective(s): MRI is a standard image modality in cranial radiosurgery (SRS) for defining the target volume. There is disparity with different MRI sequences however due to differences in acquisition resolution, which can directly impact accuracy of target segmentation. Here we acquired MRI using different sequences on phantoms to evaluate the effect on volume delineation in the context of cranial SRS.
Materials/Methods: Four different T1-weight MR pulse sequences were included: (1) SE5mm: axial and coronal Spin Echo (SE) 2D acquisition with 0.4 × 0.4 mm2 in-plane resolution and 5 mm cross-plane; (1) SE3mm: axial and coronal SE with 0.5 × 0.5 mm2 in-plane and 3 mm cross-plane; (3) TFE: gradient echo 3D acquisition with 0.92 × 0.92 mm2 in axial plane and 1.25 mm cross-plane; (4) BRAVO: gradient echo 3D with 0.4 × 0.4 mm2 in axial plane and 0.5 mm cross-plane. Four phantoms with different shape and volume (0.54 to 25 cm3) were imaged, resulting in 16 sets of MRIs. Four radiation oncologists provided contours on individual sets of MRIs. All observer contours were compared with ground truth, which was defined on CT image according to the absolute dimensions of the phantom structures. Dice coefficients (DSC), Hausdorff distance (MaxHD, MeanHD, σHD) as well as ratio between reconstructed and true volume (Ratiovol) were evaluated between observer and ground truth contours. A two-sided signed-rank test was performed to determine whether differences were statistically significant.
Results: The comparison to ground truth as a function of MR sequence is presented in the Table. All indices improved as the MR resolution increased from SE5mm to BRAVO. All differences, with the exception of Ratiovol between SE3mm and TFE, were statistically significant (P < 0.01). Inter-observer variation which must be considered was likely due to: (1) differences in user window/level preferences and related impact on visualization of target boundaries; (2) discrepancies in visualization at the superior/inferior aspect of the target. The first factor leads to systematic over- or under-estimation of the target volume. The second factor affects only SE5mm and SE3mm and leads to over- or under-estimation at superior/inferior aspect of the target. Both factors have ramification in clinical SRS treatment planning.
Conclusion: Significant improvement in target definition was observed as the MR image resolution improved. Results imply that the highest resolution 3D MR sequences should be used to minimize potential errors in target definition, and multi-slice 2D sequence should be avoided.