Digital imaging of dose distributions using NMR imager

A method for determining spatial dose distributions in a tissue-equivalent phantom using nuclear magnetic resonance (NMR) is presented. A ferrous sulphate solution (Fricke solution) in a cross-linked dextran gel was used as a medium for detecting ionizing radiation. To measure dose distribution, the gel was made in the phantom which was then irradiated. The phantom was moved into the NMR instrument, and T1 was measured as a function of spatial dose distribution. Several gels with varying compositions were prepared to determine the relationship between absorbed dose and relaxation time. Dose distributions in lung tissues were measured. A lung gel phantom in which small styrofoam cubes were mixed with the gel (ρ=0.3 g/cm3) was made. The T1 signal from the lung phantom was smaller than that from the gel phantom by the ratio of proton densities. It was found that the inversion recovery method of NMR would give a linear signal-to-dose ratio for dose D smaller than 25 Gy. The present method is thus useful for the production of dose distribution images required for treatment planning. (N.K.)