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Updated: Sep 6, 2025
Measuring the Complete-arch Distortion of an Optical Dental Impression
Published on: May 30, 2019
Time-course assessment of 3D-image distortion on the 1.5 T Marlin/Elekta Unity MR-LINAC
Andrei Z Damyanovich1, Tony Tadic1, Warren D Foltz1
1Department 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.
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 98th 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.
