MRI quality control for low‐field MR‐IGRT systems: Lessons learned

Abstract

Purpose: To present lessons learned from magnetic resonance imaging (MRI) quality control (QC) tests for low-field MRI-guided radiation therapy (MR-IGRT) systems.Methods: MRI QC programs were established for low-field MRI-60 Co and MRI-Linac systems. A retrospective analysis of MRI subsystem performance covered system commissioning, operations, maintenance, and quality control. Performance issues were classified into three groups: (a) Image noise and artifact; (b) Magnetic field homogeneity and linearity; and (c) System reliability and stability.Results: Image noise and artifacts were attributed to room noise sources, unsatisfactory system cabling, and broken RF receiver coils. Gantry angle-dependent magnetic field inhomogeneities were more prominent on the MRI-Linac due to the high volume of steel shielding in the gantry. B 0 inhomogeneities measured in a 24-cm spherical phantom were <5 ppm for both MR-IGRT systems after using MRI gradient offset (MRI-GO) compensation on the MRI-Linac. However, significant signal dephasing occurred on the MRI-Linac while the gantry was rotating. Spatial integrity measurements were sensitive to gradient calibration and vulnerable to shimming.The most common causes of MR-IGRT system interruptions were software disconnects between the MRI and radiation therapy delivery subsystems caused by patient table, gantry, and multi-leaf collimator (MLC) faults. The standard deviation (SD) of the receiver coil signal-to-noise ratio was 1.83 for the MRI-60 Co and 1.53 for the MRI-Linac. The SD of the deviation from the mean for the Larmor frequency was 1.41 ppm for the MRI-60 Co and 1.54 ppm for the MRI-Linac. The SD of the deviation from the mean for the transmitter reference amplitude was 0.90% for the MRI-60 Co and 1.68% for the MRI-Linac. High SDs in image stability data corresponded to reports of spike noise.Conclusions: There are significant technological challenges associated with implementing and maintaining MR-IGRT systems. Most of the performance issues were identified and resolved during commissioning.

Characterization of radiotherapy component impact on MR imaging quality for an MRgRT system

“…The image resolution was altered by the SpatialIntegrityAnalysis2D software, which interpolated the data prior to analysis. Prior studies have utilized MRI image acquisitions with pixel edge lengths of 1.5 mm or greater to report on submillimeter average geometric distortion values for quality assurance and pulse sequence design, including studies using an identical phantom to that used in this study 16,19–22 . In addition, the phantom used in this study cannot replicate the magnetic susceptibility or imaging volume of patient anatomy.…”

“…This sequence is a clinical protocol TrueFISP sequence with TR/TE: 3.4/1.4 ms, flip angle: 60°, rBW: 534 Hz/pixel, FOV: 349 × 349 × 120 mm 3 , imaging matrix 234 × 234 × 80, voxel size: 1.5 × 1.5 × 3 mm 3 . The preupgrade images utilized the MRI gradient offset (MRI‐GO) shim mode, which used predetermined phantom‐based shim adjustments depending on gantry angle 20 . Postupgrade images were acquired using two different shimming modes.…”

Machine QA for the Elekta Unity system: A Report from the Elekta MR‐linac consortium

“…Several publications have considered individual aspects of QA on MR‐linac devices. Measurement equipment performance, 10–18 reference 19 and relative 15,20–23 dosimetry within the presence of a magnetic field, treatment planning system, 24–28 Linac, 22,29–31 MR device commissioning, 32,33 and routine QA 32,34–37 are the broad range of areas addressed. While not considerably different to conventional RT QA, there are several aspects that need considering.…”

Integration of spatial distortion effects in a 4D computational phantom for simulation studies in extra‐cranial MRI‐guided radiation therapy: Initial results

“…It has been shown recently that the behavior of gradient nonlinearities in a low field MR‐linac can change substantially after major repair and re‐shimming, 47 which would require an update of the distortion model before it can be effectively used. Check of spatial distortions in MR‐guided radiotherapy is an integral part for system commissioning and quality assurance 48–50 . Therefore, spatial distortion models could be checked and updated frequently, counting on the availability of repeated measurements during clinical use of MR‐guided systems.…”