INTRODUCTION

An integrated 1.5 Tesla MRI scanner with a radiotherapy linear accelerator, Unity MR-Linac, has recently been introduced clinically. This MRI guided online adaptive radiotherapy (ART) delivery technology enables the visualization of tumor and organs-at-risk (OAR) throughout all phases of patients’ treatments [1]. Specifically, 1) daily MRI provides highly accurate online ART based on anatomical change or internal motion with sub-millimetric spatial resolution.2) fast MRI with sub-second temporal resolution provides visualization of target and organ movement during RT, allowing real-time radiation beam on/off control; and 3) post-RT functional MRI enables sequential assessment of tumor response, providing guidance to personalized treatment plan adjustment. In our institution, two Unity MRI-Linac systems were simultaneously installed in May and went live in September 2021. In this study, we describe development, implementation, and troubleshooting of acceptance and commissioning testing for the MRI component of both Unity MRI-Linacs.

METHODS

Phantoms: Elekta MR-to-MV QA phantom, Philips 200 mm MR periodic image quality (PIQT) phantom, 7-slab 3D Geometric QA phantom, 400 mm body phantom, ACR MRI QA phantom, and CIRS ZEUS MRgRT motion management QA phantom. Coil: 4-ch anterior and 4-ch posterior surface coils, body coil (QBC).
Phase I: Basic Testing using vendor-provided tools: (I.1) MR-to-MV isocenter check to test the two coordinate systems and transformation; (I.2) PIQT image quality check at different gantry angles to measure spatial resolution, slice profile, field uniformity, SNR, and spatial linearity; (I.3) Coil SNR measurements at different gantry angles with radiation beam on/off; (I.4) Scaling testing to measure transverse and coronal geometric distortion; (I.5) RF spurious test. (I.6) 3D geometric QA to measure distortions for different DSV. (I.7) Center frequency stability.
Phase II: Advanced Testing: (II.1) B0 field inhomogeneity using dual-echo B0 mapping method at different gantry angles and imaging planes; (II.2) Image orientation testing; (II.3) Gradient fidelity by comparing 3D geometric acquisitions with positive and negative readout gradient to decouple gradient- and B0-induced distortion; (II.4) Flip angle accuracy using dual-angle B1 mapping; (II.5) RF interference with conductive line under four Linac status (i.e., power off, magnetron powered but no radiation, MLC moving with no radiation, and radiation on); (II.6) ACR image quality test; and (II.7) EPI testing of ghosting ratio, stability, and distortion.
Phase III: Linac system influence on MRI testing: (III.1) Signal burst-to-noise ratio using dynamic noise scan with RF and gradients off and radiation beam on; (III.2) MRI image isocenter position check at different gantry angles; and (III.3) Cine imaging distortion with radiation beam on at different gantry angles.
Phase IV: End-to-end test using a dynamic motion phantom. The process includes 4D-CT simulation, initial reference plan generation, patient-specific dose verification, daily MR image, adaptive registration, adaptive plan generation, dose delivery, and real-time imaging to detect whether the tumor was out of bounds during radiotherapy.

RESULTS

Both Unity machines passed the MRI acceptance testing. Phase I testing passed the vendor-specific tolerances except on Unity 1, the MR-to-MV isocenter check failed initially due to more than 0.3° rotation angle difference along the B0 direction between two coordinate systems. Image quality measurements were not affected by Linac gantry rotation and radiation on/off. In Phase II, 3D geometric accuracy test, overall total distortion within 200, 300, 400, and 500 mm DSVs was 0.32/0.29, 0.48/0.44, 0.79/0.90, and 1.9/2.1 mm on Unity1/Unity2, respectively. By decoupling the B0-induced distortion, median gradient-induced distortion within 500mm DSV were 0.25/0.23 mm on Unity1/Unity2. B0 homogeneity measured at different gantry angles was about 1ppm on sagittal/coronal planes and 1.5ppm on axial plane over a 400mm body phantom on both systems. Flip angle of 60° was measured accurately. Linac status in RF interference test posed little influence on the SNR measurements. ACR image quality tests were within tolerances [2]. EPI ghost ratio, distortion, and stability were within tolerances required by AAPM [3]. In Phase III, Linac gantry rotation and radiation beam field dependent image noise and distortions on static and cine imaging were found to be negligible. Phase IV end-to-end testing was completed, and real-time motion tracking strategy and procedure were established.

DISCUSSION

MR to MV transform is essential to ensure beams are delivered correctly in relation to patient geometry. One major challenge experienced during acceptance procedure on Unity 1 was the rotation around the head-to-foot axis (Φ) for MR to MV transform was out of tolerance (0.38°), while the vendor specification is less than 0.3°. To fix this issue, the B0 field was ramped down and the Linac gantry and patient table was re-adjusted. To test the outcome after adjustment, the MV phantom was used to test the rotation between the Linac gantry and patient table (-0.02°) to make sure the beam source is aligned with patient table, then 3D geometric phantom was used to test the alignment of the table to the MRI coordinates which implied a deviation of 0.32°, and finally the MR to MV rotation angle was measured as -0.29° which was at the board line.

CONCLUSION

We successfully completed the acceptance procedure for the MRI component on two Unity MR-Linacs. The procedures were adapted from previously publications [4,5], but on top of that, we will focus on providing a step-by-step instruction for site physicists to independently perform the tests.

Acknowledgements

No acknowledgement found.

References

[1] Otazo R, et al. MRI-guided Radiation Therapy: An Emerging Paradigm in Adaptive Radiation Oncology. Radiology. 2021 Feb;298(2):248-260. doi: 10.1148/radiol.2020202747. Epub 2020 Dec 22. PMID: 33350894; PMCID: PMC7924409.[2] https://www.acr.org/-/media/ACR/NOINDEX/QC-Manuals/MR_QCManual.pdf [3] Report No. 100 - Acceptance Testing and Quality Assurance Procedures for Magnetic Resonance Imaging Facilities (2010) https://www.aapm.org/pubs/reports/rpt_100.pdf[4] Roberts, D.A., et al. (2021), Machine QA for the Elekta Unity system: A Report from the Elekta MR-linac consortium. Med. Phys., 48: e67-e85. https://doi.org/10.1002/mp.14764[5] Tijssen RHN, et al. MRI commissioning of 1.5T MR-linac systems - a multi-institutional study. Radiother Oncol. 2019 Mar;132:114-120. doi: 10.1016/j.radonc.2018.12.011. Epub 2018 Dec 31. PMID: 30825959.