Diffusion in Fe(II/III) radiation dosimetry gels measured by magnetic resonance imaging.

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.