H Nusrat^1,2*, L Lawrence^3,4, R Chan³, A Lau^3,4, A Elzibak¹, B Chugh^1,2,5, A Sahgal², B Keller^1,2, (1) Department of Medical Physics, Odette Cancer Centre, Sunnybrook Health Sciences Centre, Toronto, ON, CA, (2) Department of Radiation Oncology, University of Toronto, Toronto, ON, CA, (3) Sunnybrook Research Institute, Toronto, ON, CA, (4) Department of Medical Biophysics, University of Toronto, Toronto, ON, CA, (5) Department of Physics, Ryerson University, Toronto, ON, CA

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  H Nusrat

MO-IePD-TRACK 3-4 (Monday, 7/26/2021) 5:30 PM - 6:00 PM [Eastern Time (GMT-4)]

Purpose: Diffusion weighted imaging (DWI) is sensitive to changes in tumor microstructure and can be used to non-invasively monitor radiotherapy treatment response. Despite being clinically available in MR-scanners, DWI can be further optimized in MR-guided radiotherapy systems. Maximum b-value is an important sequence parameter that controls the amount of diffusion weighting; high diffusion weighting leads to noise and less accurate apparent diffusion coefficient (ADC) estimates. MR-Linac users follow the recommended maximum b-value of 500 s/mm², however, higher maximum b-values are routinely used for DWI in sites such as the brain. The goal of this work was to optimize DWI in an MR-Linac using a standardized phantom.

Methods: DWI sequences for brain, prostate, and abdomen were optimized with respect to maximum b-value and number of signal averages (NEX). DWI scans of the NIST-QIBA phantom (High Precision Devices, Boulder, USA) were conducted in the Elekta Unity MR-Linac’s (Elekta AB, Stockholm, Sweden) Philips Marlin 1.5T MR scanner. ADCs were estimated by fitting with offline reconstruction (MATLAB). Maximum b-value and NEX were varied and differences between measured and manufacturer-specified ADC were reported. The impact of varying NEX on scan duration was also quantified.

Results: For the brain sequence, maximum b-value achieving accurate ADC was 3,000 s/mm² (±4.0% of standard across all diffusion vials). For all three sites, fitting with a subset of b-values below 400 s/mm² reduced ADC accuracy (±17.6%, 22.3%, 24.1% for brain, prostate and abdomen, respectively). Reducing NEX from clinical protocol to the scanner’s minimum resulted in underestimation of ADC by 14.3%(±1.7%), 18.4%(±5.1%), and 17.5%(±4.3%) for the brain, prostate, and abdomen sequences, respectively but reduced average scan duration from 5.8 to 1.1 minutes.

Conclusion: Site-specific DWI sequences were optimized for ADC accuracy with respect to maximum b-value and scan time using a standardized phantom. Future work involves repeating this methodology in vivo.

Funding Support, Disclosures, and Conflict of Interest: FedDev ON, Canada

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Keywords

Diffusion, Image-guided Therapy, MRI

Taxonomy

IM/TH- MRI in Radiation Therapy: MRI protocols for therapy

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