Author: Kaitlyn M Betz, David Dunkerley, Eric Johnson, Kalina V Jordanova, Kathryn E. Keenan, Samuel D Oberdick, Gregory P. Penoncello, Stephen E Russek 👨🔬
Affiliation: Dept of Radiology, Stanford, University of Colorado School of Medicine, NIST, College of Wooster 🌍
Purpose: Examine use of low-field MRI and multiparametric analysis for 3D MRI-readable biomimetic gel dosimetry. Low-field MRIs are compact, inexpensive, and portable. They could be located within radiation oncology units to make MRI-readable dosimetry more accessible. Quantitative MRI allows multiple parameters to be measured such as T1,T2,T1ρ, and water diffusivity. Correlation across several parameters has potential to provide more information and improve dose calibrations.
Methods: 3D-printed head phantoms along with calibration arrays were fabricated with a polymeric radiation sensitive gel Methacrylic and Ascorbic acid in Gelatin Initiated by Copper (MAGIC). The phantoms and calibration arrays were exposed in a Varian LINAC with a photon energy of 6MV. The head phantoms received a 10.4Gy spherical dose while the calibration arrays received uniform doses from 1Gy to 35Gy. The phantoms were imaged in a Siemens 3T Prisma Fit, a preclinical 3T metrology MRI, and a 64mT Hyperfine low-field MRI. T1,T2,T1ρ, and water diffusivity were measured pre and post exposure.
Results: The MAGIC gels have good tissue mimicking properties with T1, T2, T1ρ, water diffusivity, water content, and radiation density all in the range of brain tissue. MAGIC gels showed high radiation sensitivity at 3T with a 5.9%/Gy change in R2=1/T2 after irradiation. While R2 was the most sensitive, R1 and R1ρ showed significant sensitivity, 0.86%/Gy and 5.2%/Gy, respectively, and may be more robust markers. Sensitivities changed at low field: a 34% percent change in T1 in response to 10Gy dose was observed at 64mT, while the same exposed head phantom showed only a 8.2% response at 3T.
Conclusion: Low-field MRI has potential for rapid inexpensive 3D MRI gel dosimetry. A detailed understanding of the accuracy of quantitative MRI at low field is required. Multiple MRI parameters show radiation sensitivity and provide more information on radiation induced changes.