Time-course assessment of 3D-image distortion on the 1.5 T Marlin/Elekta Unity MR-LINAC

Andrei Z Damyanovich; Tony Tadic; Warren D Foltz; Salomeh Jelveh; Jean-Pierre Bissonnette

doi:10.1016/j.ejmp.2022.05.009

Time-course assessment of 3D-image distortion on the 1.5 T Marlin/Elekta Unity MR-LINAC

Phys Med. 2022 Aug:100:90-98. doi: 10.1016/j.ejmp.2022.05.009. Epub 2022 Jun 28.

Authors

Andrei Z Damyanovich¹, Tony Tadic², Warren D Foltz², Salomeh Jelveh³, Jean-Pierre Bissonnette⁴

Affiliations

¹ Department of Medical Physics, Princess Margaret Cancer Center, University Health Network, Toronto, Ontario, Canada; Department of Radiation Oncology, University of Toronto, Toronto, Ontario, Canada; Techna Institute, Toronto, Ontario, Canada. Electronic address: Andrei.Damyanovich@rmp.uhn.ca.
² Department of Medical Physics, Princess Margaret Cancer Center, University Health Network, Toronto, Ontario, Canada; Department of Radiation Oncology, University of Toronto, Toronto, Ontario, Canada; Techna Institute, Toronto, Ontario, Canada.
³ Department of Medical Physics, Princess Margaret Cancer Center, University Health Network, Toronto, Ontario, Canada.
⁴ Department of Medical Physics, Princess Margaret Cancer Center, University Health Network, Toronto, Ontario, Canada; Department of Radiation Oncology, University of Toronto, Toronto, Ontario, Canada; Techna Institute, Toronto, Ontario, Canada; Department of Medical Biophysics, University of Toronto, Toronto, Ontario, Canada.

PMID: 35777256
DOI: 10.1016/j.ejmp.2022.05.009

Abstract

Purpose: The efficacy of MR-guided radiotherapy on a MR-LINAC (MR-L) is dependent on the geometric accuracy of its MR images over clinically relevant Fields-of-View (FOVs). Our objectives were to: evaluate gradient non-linearity (GNL) on the Elekta Unity MR-L across time via 76 weekly measurements of 3D-distortion over concentrically larger diameter spherical volumes (DSVs); quantify distortion measurement error; and assess the temporal stability of spatial distortion using statistical process control (SPC).

Methods: MR-image distortion was assessed using a large-FOV 3D-phantom containing 1932 markers embedded in seven parallel plates, spaced 25 mm × 25 mm in- and 55 mm through-plane. Automatically analyzed T1 images yielded distortions in 200, 300, 400 and 500 mm concentric DSVs. Distortion measurement error was evaluated using median absolute difference analysis of imaging repeatability tests.

Results: Over the measurement period absolute time-averaged distortion varied between: dr = 0.30 - 0.49 mm, 0.53 - 0.80 mm, 1.0 - 1.4 mm and 2.28 - 2.37 mm, for DSVs 200, 300, 400 and 500 mm at the 98^th percentile level. Repeatability tests showed that imaging/repositioning introduces negligible error: mean ≤ 0.02 mm (max ≤ 0.3 mm). SPC analysis showed image distortion was stable across all DSVs; however, noticeable changes in GNL were observed following servicing at the one-year mark.

Conclusions: Image distortion on the MR-L is in the sub-millimeter range for DSVs ≤ 300 mm and stable across time, with SPC analysis indicating all measurements remain within control for each DSV.

Keywords: 3D-MR Image Distortion; Elekta Unity; MR-LINAC; Temporal stability of gradient linearity.

MeSH terms

Imaging, Three-Dimensional
Magnetic Resonance Imaging* / methods
Particle Accelerators*
Phantoms, Imaging
Software