We analyse the diffusion problem in the traditional Fe(II/III) agarose gel system employed in MRI studies of radiation dosimetry. The diffusion coefficient is measured using an inversion recovery null-point imaging method in a model gel/water phantom. The diffusion coefficient of Fe(III) in 1% agarose gel at pH 1.1 is D = 2.7 +/- 0.3 x 10(-6) cm2 s-1. The diffusion coefficient of Fe(II) is D = 3.3 +/- 0.5 x 10(-6) cm2 s-1. Measurement of the diffusion coefficients permits simulation of the MRI signal intensity from phantoms with model radiation dose distributions. We allow for diffusion of both Fe(II) and Fe(III) in our simulations as well as the effect of both relaxation agents on the local spin-lattice relaxation time T1. We also analyse the effects of the physical penumbra on the diffusion problem.

Magnetic resonance imaging was employed to examine spatially and temporally resolved photopolymerization of acrylamide gels. Fast exchange between free and bound water results in single exponential T 2 decay, where 1/T 2 scales linearly with polymer concentration. Measured T 2 s are sensitive to the experimental conditions; however, the 1/T 2 relationship to polymer concentration allows a straightforward interpretation of image contrast changes during photopolymerization. The polymer appears to form at a nearly constant rate until the monomer concentration is significantly depleted. Conventional spin-echo images and quantitative CPMG-weighted spin-echo images were acquired. Photopolymerization of a partially masked sample produced a sharp transition (1 mm width) between polymer and monomer regions of the sample. The image intensity is uniform throughout the illuminated region of the sample, indicating uniform polymer formation. Interrupting the illumination quenches polymer formation.

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