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Impact of Gantry, MLC, and Patient Table Power On An MRgRT System

B Lewis1*, B Gu1, R Klett2, R Lotey3, O Green1, T Kim1, (1) Washington University School of Medicine in St. Louis, Saint Louis, MO, (2) BJC Healthcare, Saint Louis, MO, (3) ViewRay, Oakwood Village, OH


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: High confidence in spatial integrity and imaging isocenter are paramount for patient positioning and adaptive radiotherapy planning. This work investigates the impact of Gantry, MLC, and patient table power of an MR-LINAC system on the MRI image isocenter and image spatial integrity.

Methods: MRI isocenter variation and spatial integrity changes were quantified at multiple gantry angles, and MLC positions, using a square spatial integrity (SI) phantom, and a cylindrical DQA phantom. Volumetric images were acquired with the vendor TrueFISP sequence. The SI phantom was imaged in MR QA mode and analyzed offline, vendor software provided isocenter position and spatial integrity values. The DQA phantom was imaged in clinical mode and registered using the clinical image registration environment to prior acquisitions at gantry home position. MLC position impact was assessed using the SI phantom for multiple square fields, edge length 1.66cm to 27.4cm. The impact of table power was assessed with the SI phantom.

Results: Isocenter shift magnitude measured with the SI phantom was reduced from a maximum of 1.7mm pre-upgrade to 0.9mm post-upgrade. The maximum mean spatial integrity errors were 0.5mm, 0.2mm, and 0.5mm for coronal, axial, and sagittal phantom orientations pre-upgrade, and 0.5mm, 0.3mm, and 0.4mm post-upgrade. Values remained consistent across gantry angles. Isocenter shift magnitude from registration of the DQA phantom found a maximum of 2.5mm at gantry 150° and 1.1mm at gantry 90°, pre- and post-upgrade respectively. All MLC positions resulted in similar mean positional errors, maximum 0.4mm. Couch power caused insignificant changes to mean spatial integrity errors, however SNR was severely degraded with the power on.

Conclusion: Imaging isocenter of the MR-LINAC system can be affected by imaging at gantry angles other than the installation home position, with magnitude varying with gantry position. However, these magnitude changes are less than the acquisition voxel size.

Funding Support, Disclosures, and Conflict of Interest: Dr. Green has received honoraria and travel grants from ViewRay Inc. Washington University in St. Louis has a master research agreement and receives research funding from ViewRay unrelated to this study.


Not Applicable / None Entered.


IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined (general)

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