A Quevedo^1,2*, G Luo³ , M Price³ , Z Zu² , P Nicolucci¹ , J Gore² , (1) University of Sao Paulo, Ribeirao Preto, Sao Paulo, (2) Vanderbilt University Institute of Imaging Science, Nashville, Tennessee, (3) Department of Radiation Oncology, Vanderbilt University Medical Center, Nashville, Tennessee

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  A Quevedo

(Wednesday, 8/1/2018) 10:30 AM - 11:00 AM

Room: Exhibit Hall | Forum 2

Purpose: To quantify Nuclear Overhauser Enhancement (NOE) mediated saturation transfer effects in irradiated polymer gels and evaluate their potential for measuring dose distributions.

Methods: MAGIC-f gels were irradiated to doses 0-20 Gy using a 6 MV beam. MRI measurements were performed at 9.4 T. Magnetization transfer (MT) between free water and gel protons was quantified using a continuous wave RF pulse (0.5 μT-1.5 μT, pulse duration 5 s, delay time 2s) with varying frequency offsets (-2000-2000 Hz) to produce Z-spectra. Water longitudinal relaxation rates and semi-solid proton pool sizes were obtained using a quantitative MT approach, a selective inversion recovery sequence with inversion times varying 4 to 6000 ms. The transverse relaxation rate of water was obtained using spin-echoes from 30 to 400 ms and ¹H MR spectra were obtained using a point-resolved spectroscopy sequence.

Results: MR Spectra of the monomers before irradiation revealed narrow resonances at offsets corresponding to the CH₂ and CH₃ protons in Methacrylic Acid which reduced in amplitude with increasing dose. NOE effects at the same offsets were seen as decreases in water Z-spectra. The magnitudes were proportional to dose and demonstrated higher slope-intercept ratio and linearity up to 20 Gy compared to other MRI parameters. MT data showed an increase in the semi-solid immobile proton pool with dose, consistent with mobile monomers forming polymerized particles in which protons are relatively immobile and provide a conduit for through space dipolar coupling with free water. The NOE indicates saturation is transferred from the polymerized protons to the free water.

Conclusion: NOE effects have been used to detect radiation deposited in gels. These results suggest a new MRI method for measuring dose deposition in polymer gels, and provide a newer insights into the dose-response mechanism in irradiated gels.

Funding Support, Disclosures, and Conflict of Interest: Coordination of Superior Level Staff Improvement and Vanderbilt University Medical Center

Keywords

Gel Dosimeter, Radiation Dosimetry, MR

Taxonomy

IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined dose measurement

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