Dosimetric Impact of MRI Distortions: A Study on Head and Neck Cancers

Abstract

The dosimetric impact of distortions on 21 head and neck cancer magnetic resonanceeonly radiation therapy treatment plans was studied. Distorted computed tomography (CT) images were obtained by deforming patient CT scans with displacement fields from residual system and patientinduced susceptibility effects. The feasibility of magnetic resonanceeonly radiation therapy was demonstrated by a dose difference of less than 2% within the target volume for all dosimetric parameters evaluated between distorted CT and CT treatment plans. Purpose: To evaluate the effect of magnetic resonance (MR) imaging (MRI) geometric distortions on head and neck radiation therapy treatment planning (RTP) for an MRI-only RTP. We also assessed the potential benefits of patient-specific shimming to reduce the magnitude of MR distortions for a 3-T scanner. Methods and Materials: Using an in-house Matlab algorithm, shimming within entire imaging volumes and user-defined regions of interest were simulated. We deformed 21 patient computed tomography (CT) images with MR distortion fields (gradient nonlinearity and patient-induced susceptibility effects) to create distorted CT (dCT) images using bandwidths of 122 and 488 Hz/mm at 3 T. Field parameters from volumetric modulated arc therapy plans initially optimized on dCT data sets were transferred to CT data to compute a new plan. Both plans were compared to determine the impact of distortions on dose distributions. Results: Shimming across entire patient volumes decreased the percentage of voxels with distortions of more than 2 mm from 15.4% to 2.0%. Using the user-defined region of interest (ROI) shimming strategy, (here the Planning target volume (PTV) was the chosen ROI volume) led to increased geometric for volumes outside the PTV, as such voxels within the spinal cord with geometric shifts above 2 mm increased from 11.5% to 32.3%. The worst phantom-measured residual system distortions after 3dimensional gradient nonlinearity correction within a radial distance of 200 mm from the isocenter was 2.17 mm. For all patients, voxels with distortion shifts of more than 2 mm resulting from patient-induced susceptibility effects were 15.4% and 0.0% using bandwidths of 122 Hz/mm and 488 Hz/mm at 3 T. Dose differences between dCT and

Geometric distortion characterization and correction for the 1.0 T Australian MRI‐linac system using an inverse electromagnetic method

“…The geometric inaccuracy of the target will potentially decrease the tumor control probability or overlap regions with the adjacent healthy structures, which could be detrimental for patients in terms of normal tissue complications. The proposed method reduced the geometric displacements to <2 mm, and studies showed that a 2‐mm error could cause ≤5% dosimetry uncertainty, which might be considered acceptable for the delivery of the absorbed dose . In this work, pelvis images were acquired from a medium‐size male.…”

“…Although images showed that most of the anatomy of the pelvis was inside of the 30‐cm DSV, the accuracy improvement outside of the 30‐cm DSV was particularly important for the dosimetry implementation where the body contour had to be geometrically accurate. Moreover, for some large patients, a large FOV was required to acquire images in anatomic regions, which could be outside of the 30‐cm DSV . Distortions on treatment plans and beam arrangements would also introduce dosimetry errors.…”

“…The proposed method reduced the geometric displacements to <2 mm, and studies 42,43 showed that a 2-mm error could cause ≤5% dosimetry uncertainty, which might be considered acceptable for the delivery of the absorbed dose. 22 In this work, pelvis images were acquired from a medium-size male. Although images showed that most of the anatomy of the pelvis was inside of the 30-cm DSV, the accuracy improvement outside of the 30-cm DSV was particularly important for the dosimetry implementation where the body contour had to be geometrically accurate.…”

“…However, the fifth‐order SH may not work optimally for the area at the edges and/or outside of the DSV . In the Australian MRI‐Linac system, as the scanner has a smaller DSV (30‐cm DSV, ±4.05 ppm) than commercial whole‐body MRI scanners (50‐cm DSV, 1 ppm) because of the split structure, the GNL‐induced distortions for the target anatomy at the periphery or outside the DSV need to be resolved …”

“…19,20 In the Australian MRI-Linac system, as the scanner has a smaller DSV (30-cm DSV, AE4.05 ppm) than commercial whole-body MRI scanners 21 (50-cm DSV, 1 ppm) because of the split structure, the GNL-induced distortions for the target anatomy at the periphery or outside the DSV need to be resolved. 22 In this study, we combined the phantom measurement 17 and a designated inverse electromagnetic (EM) method [23][24][25] to characterize the gradient field both inside and outside of the DSV for the Australian MRI-Linac system. A grid phantom was first designed to measure a small number of gradient field samples by using the three-dimensional (3D) Prewitt operator.…”