It is proposed that NMR methods be developed for measuring 3-D dose distributions produced in anthropomorphic phantoms by x-ray and electron beams (single beams or combinations), and by brachytherapy sources. Chemical changes produced in the irradiated medium, which are proportional to the absorbed dose, and which affect proton relaxation times, T, will be mapped in three dimensions by a magnetic resonance imaging (MRI) system. In preliminary trials, gelatin prepared with Fricke dosimeter solution was irradiated by high-energy x rays. An image of the dose distribution results from the reduced T1's of protons in the vicinity of ferric ions which, in turn, were produced by the radiation-induced oxidation of ferrous ions. These experiments demonstrated a linear relationship between the radiation dose and the relaxation rate of protons, and that diffusion of ferric ions in the gel was insignificant in the time between irradiation and imaging. The advantages of this method for measuring dose distribution over those currently employed are: a) a three-dimensional distribution may be recorded in less than one-half hour of accelerator time; b) the recording medium can readily fill anthropomorphic phantoms containing bones and air cavities; c) spatial resolution will be about 1 mm; d) dose distributions produced by combinations of x rays and electrons may be recorded; e) no electronic instrumentation is required beyond an MRI unit.
The specific aims of the project are to: a) investigate the properties of various gels such as polyacrylamide in terms of water or tissue equivalence, and as supporting matrices for the Fricke dosimeter; b) determine the diffusion rate of ferric ions in these gels and, if appropriate, develop corrections schemes for diffusion effects; c) determine the optimum magnetic field strength and pulse sequence for imaging dose distributions; d) develop a low-density, tissue-equivalent gel for the determination of dose distributions in lung; e) measure the dose distributions in anthropomorphic phantoms from x-ray and electron beams and brachytherapy sources; f) explore other radiation-induced chemical reactions, such as the polymerization of acrylamide, that affect T1 but do not require the addition of Fricke dosimeter.
| Maryanski, M J; Gore, J C; Kennan, R P et al. (1993) NMR relaxation enhancement in gels polymerized and cross-linked by ionizing radiation: a new approach to 3D dosimetry by MRI. Magn Reson Imaging 11:253-8 |
| Schulz, R J; deGuzman, A F; Nguyen, D B et al. (1990) Dose-response curves for Fricke-infused agarose gels as obtained by nuclear magnetic resonance. Phys Med Biol 35:1611-22 |
