Characterizing MR Imaging isocenter variation in MRgRT

Abstract

We characterized MRI isocenter variation at various gantry positions in two 0.35 T MRgRT systems using two independent methods. First, image center-based quantification was employed on 3D volumetric and 2D cine images of a 24 cm diameter spherical phantom at various gantry positions in the MRI QA mode. The center of the phantom images was identified to quantify the variation of the imaging center at each gantry position. Second, image registration-based quantification was used in radiotherapy mode. 3D volumetric MRIs of a cylindrical phantom were acquired and corresponding image registration from MRI to planning CT was performed. The shifts of the couch were identified to quantify the variation of the imaging center. For verification of noticeable MRI isocenter variation, star-shot pattern measurements with five beams were delivered on the radio-chromic film inserted into the phantom after the couch was shifted. The center of the star-shot pattern was identified to quantify the variation of the imaging center. The proposed methods for measuring MRI isocenter variation were demonstrated with MR-LINAC and MR-60 Co systems. Both of the MRgRT systems had field inhomogeneities <5 ppm over a 24 cm diameter spherical volume (DSV) and spatial integrity distortion: <1 mm within 100 mm radius and <2 mm within 175 mm radius. The MRI isocenter of the MR-LINAC system showed noticeable 3D variation (max magnitude: 1.8 mm) compared to that of MR-60 Co system (max magnitude: 0.9 mm) relative to the reference gantry positions. In addition, 2D variations (max magnitude) of the MRI isocenter from sagittal cine images were 0.9 mm for the MR-LINAC system and 0.5 mm for the MR-60 Co system. Two proposed methods quantified the MRI isocenter variation for various gantry positions in two 0.35 T MRgRT systems. The results of significant isocenter variation in the MR-LINAC system requires further investigation to determine the cause.

Evaluation of diffusion‐weighted MRI and geometric distortion on a 0.35T MR‐LINAC at multiple gantry angles

“…The ADC values produced by the 0.35 T system in this study were also significantly lower than those produced on 1.5 T and 3.0 T systems. Additionally, geometric distortion assessment using fiducial markers showed a gantry dependent distortion, similar to those reported by Kim et al 19 . for anatomical imaging sequences used during patient treatment on the MR‐LINAC system.…”

“…An added challenge presented by the MR‐LINAC system is the motion of the LINAC gantry around the MRI. Although there is significant magnetic and RF shielding between the MRI and LINAC subsystems, changes in the gantry angle cause changes in the imaging isocenter and spatial integrity 19 . These changes could adversely impact the accuracy of ADC values and reduce their utility for ART or disease monitoring.…”

Effects of B0 eddy currents on imaging isocenter shifts in 0.35‐T MRI‐guided radiotherapy (MR‐IGRT) system

“…Gantry angle‐dependent shifts in the imaging isocenter were reported on a 0.35‐T MRI‐Linac system. A maximum displacement of 1.8 mm for 3D true fast imaging with (balanced) steady‐state precession (TrueFISP) scans used for treatment planning and patient setup was measured 7 . A maximum displacement of 0.9 mm for 2D cine TrueFISP used in real‐time treatment delivery and beam gating was also observed 7 .…”

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

“…Many components of the LINAC and beam delivery systems have the potential to further reduce the accuracy of the integrated MRI system. The impact of gantry position on radiation‐MR isocenter coincidence has been investigated on MR‐ 60 Co and MR‐LINAC systems, and shown variations in the MR isocenter depending on gantry position 15,16 . Utilizing an MLC for beam shaping introduces mobile metallic material, which is often the RT component closest to the magnet, that may also impact the magnetic field.…”

“…The impact of gantry position on radiation-MR isocenter coincidence has been investigated on MR- 60 Co and MR-LINAC systems, and shown variations in the MR isocenter depending on gantry position. 15,16 Utilizing an MLC for beam shaping introduces mobile metallic material, which is often the RT component closest to the magnet, that may also impact the magnetic field. A computational study by Kolling et al investigated MLC impact on field homogeneity including source-to-isocenter distance, field size, magnetic field strength, and other properties and found that MLC field size caused dynamic changes in field distribution.…”