This project aims to develop a novel, tissue-equivalent gel-dosimeter of ionizing radiation, which is capable of producing highly-resolved and accurate data on three dimensional dose distributions using magnetic resonance imaging (MRI). The dose-response mechanism relies on the enhancement of the proton NMR relaxation of water in the gel at the site of radiation absorption, where acrylic comonomers dispersed in the gel are polymerized. Doses of the order of 10 Gy can be measured with a spatial resolution less than lmm using clinical MR imaging equipment. In addition, the optical turbidity of the transparent gel increases with absorbed dose because the copolymer precipitates from water, and thus the dose distribution can also be optically visualized. The gel will find widespread use in radiation therapy practice, for 3D measurements in homogeneous and anthropomorphic phantoms, confirmation of computerized treatment planning and in quality assurance procedures. The goals of Phase I are to devise and evaluate a protocol for convenient and inexpensive preparation of a prototype polymer-gel dosimeter, and to demonstrate its stability and reproducibility as well as spatial uniformity of the dose- response. These studies will prepare the ground for the development of more efficient polymer-gels in Phase II.