MRI B ₀ homogeneity and geometric distortion with continuous linac gantry rotation on an Elekta Unity MR-linac

Abstract

This work aimed to quantify any principal magnetic field (B0) inhomogeneity and changes in MR image geometric distortion with continuous linac gantry rotation on an Elekta Unity MR-linac. This situation occurs for around a second between treatment beams during current image guided radiotherapy treatment and would occur frequently in foreseeable real-time adaptive radiotherapy treatment. Pixel by pixel maps of B0 inhomogeneity were obtained via repeated high temporal resolution pulse sequences with the linac gantry static at 36 gantry angles spaced ten degrees apart, and in continuous rotation at both 1 and 2 rpm. Individual B0 maps were subtracted from average maps across all data and the residual peak to peak inhomogeneity was calculated for each. The bulk geometric shift and change in physical extent of a 10 cm diameter spherical flood phantom during continuous linac gantry rotation at 1 and 2 rpm was compared to the static gantry case for two pulse sequences: the real-time clinical monitoring bFFE sequence and a non-clinical EPI sequence, chosen for its susceptibility to geometric distortion. The peak to peak inhomogeneity in the deviation-from-average ppm maps, plotted against gantry angle with the gantry in continuous rotation at 1 and 2 rpm were negligibly different from equivalent data obtained with the gantry static. The real-time clinical monitoring pulse sequence was shown to give negligible geometric distortion during continuous gantry motion, whilst a non-clinical EPI sequence showed bulk shifts of the order of one pixel and gantry angle dependent changes in extent, demonstrating the sensitivity of the chosen method. MR imaging on the Elekta Unity MR-Linac with the gantry in continuous motion is negligibly different from the static gantry case with current clinical pulse sequences. Real-time tracking and treatment plan adaptation using MR images obtained with the linac gantry in motion is possible.

Medical physics challenges in clinical MR-guided radiotherapy

“…Furthermore, most rc-4D-MRI methods cannot accurately assess cycle-to-cycle variations and baseline shifts, as typically only one breathing cycle is reconstructed [25]. During irradiation, the beam-on imaging capabilities of today's clinical MR-linac systems allow real-time intra-fractional monitoring of tumor and OAR motion [12,32,33]. Fast 2D cine MRI sequences like bSSFP [34] or spoiled gradient echo [35] achieve acquisition times down to 150 ms [36].…”

Investigating the impact of patient arm position in an MR‐linac on liver SBRT treatment plans

“…A 15‐beam setup was chosen as a compromise between treatment delivery time and angular coverage. While the MR‐linac gantry supports rotation speeds of up to 6 rpm, minimizing the time penalty of using more beams, we noticed that the segment switching time also puts a practical constraint on the ability to add more beams.…”

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

“…The choice of the axial orientation is justified by the intrinsic features of MR-linac design that determines higher uncertainties in the magnetic field homogeneities. 59 In further studies, the analysis could be repeated by switching the phase and frequency-encoding directions, to achieve the separation of different contributions in the axial plane, and by changing the gradient polarity also along z. As discussed previously, MR image acquisition can be extended to other anatomical orientations, with a further possibility to implement 3D acquisition modalities where two phase-encoding directions are used for k-space sampling.…”