Simulating Magnetic Resonance Imaging Susceptibility-Induced Geometric Distortions for Cranial Stereotactic Radiotherapy Planning
Abstract
Purpose
Magnetic resonance (MR) imaging is routinely used to contour targets for stereotactic radiotherapy planning. However, MR images can suffer from susceptibility-induced geometric distortions, especially near air-tissue interfaces. The objective of this work is to quantify and evaluate the dosimetric impact of susceptibility-induced MR geometric distortions in cranial targets for stereotactic radiotherapy cases near air-tissue interfaces.
Methods
19 stereotactic radiotherapy plans with targets close to air-tissue interfaces were identified: 9 brain metastases, 6 acoustic neuromas, and 4 pituitary adenomas. These targets were contoured using MR (T1-weighted gadolinium contrast MPRAGE, 1.5 T field strength, 4 mT/m readout gradient) and thus subject to susceptibility distortions. To estimate the susceptibility distortions, a co-registered computed tomography scan was segmented into air, bone, and soft-tissue regions that were assigned magnetic susceptibility values. A fast Fourier transform convolution algorithm was used to compute local magnetic field perturbations and the resulting spatial shifts. Inverse shifts were applied to the target structures to estimate the undistorted geometries. The resulting dosimetric differences to the target and non-target tissues were then evaluated.
Results
Maximum shifts within the targets ranged from 0.06-2.48 mm, with an average (median) mean shift of 0.27 mm (0.30 mm). Target volume changes were generally <5%. Across all cases, 84% of all changes in target minimum, median, mean, and maximum doses were within ±1%, with the full range of differences ranging from -9.23% to 2.95%. Changes in target coverage (volume receiving 90%, 95%, 98%, or 100% dose) ranged from -10.34% to 4.90%. Change in maximum dose to healthy tissue outside the GTV ranged from -0.91% to 1.87%.
Conclusion
At 1.5 T and ≥4 mT/m readout gradient, susceptibility-induced distortions produce sub-millimeter target shifts near air cavities and generally result in minimal dosimetric impact in stereotactic cranial radiotherapy.