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Characterization of Ferromagnetic Influence of a MR-Guided Radiotherapy (MRgRT) System and Vault: Cobalt to Linac

K Mittauer1*, P Yadav1 , S McMain2 , B Paliwal1 , J Bayouth1 , (1) University of Wisconsin-Madison, Madison, WI, (2) ViewRay Inc, Oakwood Village, OH


(Wednesday, 8/1/2018) 10:15 AM - 12:15 PM

Room: Karl Dean Ballroom B1

Purpose: To quantify effects of ferromagnetic material of MR-guided radiotherapy (MRgRT) vault and ferromagnetic gantry components for a MR-guided cobalt versus linac design.

Methods: A clinical MRgRT system was upgraded from tri-cobalt-60 to a 6 MV inline linac within the same vault. Retrofitted MRgRT vault contains ferromagnetic rebar in concrete floor radially-surrounding gantry pit. Six gantry-mounted ferromagnetic steal shields (450 lbs) were installed 87 cm from isocenter on MR-linac to reduce Lorentz force on charge-carrying components. During cobalt-to-linac upgrade, unknown ferromagnetic vault sources (can lights and unistruts) were identified and removed within 5 Gauss, 10’ of isocenter. Metrics for magnetic field homogeneity and geometric distortion were quantified for MR-linac and MR-cobalt of the same magnet and vault acquired.

Results: Inhomogeneity (n=12 gantry angles, 45cm DSV) was reduced for MR-linac (max 16.27 ppm, mean 14.85±0.80 ppm) compared to MR-cobalt (max 19.41 ppm, mean 18.38±0.76 ppm, p<0.0001) for field-camera results. Using the spectral-peak method and post-gradient correction offsets, homogeneity within 24cm DSV was 2.41±0.78ppm(linac) and 1.01±0.09ppm(cobalt). Mean geometric distortion remained similar with 0.81±0.28mm(cobalt) and 0.80± 0.32mm(lianc) for 696 points 35cm DSV, p=0.175. Maximum MR-linac distortion was 1.9 mm in coronal plane, which was statistically different (p<0.0001, mean coronal distortion 1.04±0.25mm linac vs. 0.85±0.33mm cobalt). Y-sine tesseral harmonics of MR-linac were greatest in non-linearity with gantry rotation (range ±2ppm) compared to other directions (range ±0.5ppm). Both MR-linac and MR-cobalt remained within specification for magnetic field homogeneity (<25 ppm, 45cm DSV) and spatial distortion (<2mm, 35cm DSV).

Conclusion: Static ferrous-vault removal within 5 Gauss improved maximum magnetic inhomogeneity by 3 ppm, p<0.0001 for MR-linac compared to MR-cobalt, even with addition of ferromagnetic shields. A marginal increase in coronal spatial distortion was observed (1.04± 0.25mm linac vs. 0.85±0.33mm cobalt), likely due to variability of non-linear Y-harmonics during rotational interplay of MR-linac ferrous-shields and floor-rebar.

Funding Support, Disclosures, and Conflict of Interest: K. Mittauer reports personal fees from ViewRay Inc.; S. McMain is employed by ViewRay Inc.; J. Bayouth reports membership of Advisory Board of ViewRay, Inc.


Image-guided Therapy


IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined Quality Assurance

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