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Characterization of Diffusion Weighted MRI and Apparent Diffusion Coefficient Calculation On a Low Field MR-LINAC Radiotherapy System at Multiple Gantry Angles

B Lewis1*, A Guta1, S Mackey2, H Gach1, S Mutic1, O Green1, T Kim1, (1) Washington University School of Medicine in St. Louis, Saint Louis, MO, (2) Barnes Jewish Hospital, Saint Louis, MO

Presentations

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

Room: AAPM ePoster Library

Purpose: Diffusion weighted imaging (DWI) provides critical tumor biomarker information for radiotherapy. This work compares apparent diffusion coefficient (ADC) values from DWI echo planar imaging (EPI) acquired at 0.35, 1.5, and 3 T.


Methods: DWI-EPI was acquired using a NIST diffusion phantom with 13 vials of aqueous polyvinylpyrrolidone (PVP), at concentrations ranging from 0% to 50%. Diffusion weighted images were acquired on 3.0T and 1.5T wide bore systems, and two 0.35T MRgRT systems (MR-6°60Co and MR-LINAC b = 0, 500, 900, 2000s/mm²2 for the 1.5T and 3.0T systems, and b = 0, 200, 500, 800s/mm²2 for the MRgRT systems due to gradient limits. Three measurements were performed to assess repeatability. ADC values were calculated using non-linear least squares fitting method of intensity versus b-value. Post processing included denoising utilizing noise floor subtraction and eddy current correction with affine registration.


Results: ADC repeatability was consistent for all field strengths. Low ADC values were measured on both 0.35T systems, e.g., by as much as 14.4% for the lowest PVP concentration vials. Vials with a high concentration of PVP yielded poor ADCs due to complete loss of signal for the b=800s/mm²2 acquisition. Signal loss was due to the high diffusion gradient causing low signal-to-noise ratios. The ADC values and raw signal intensities changed with gantry angle on the MRI-Linac.


Conclusion: ADC values measured at low field are subject to underestimation and gantry angle variations. ADC and signal intensity correction will require future studies.

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

Keywords

Not Applicable / None Entered.

Taxonomy

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

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