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 Î¼T-1.5 Î¼T, 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 Â¹H 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 CHâ‚‚ and CHâ‚ƒ 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