Quantitative model for the interecho time dependence of the CPMG relaxation rate in iron-rich gray matter

A quantitative model is proposed for computing the dependence on the intere cho time of the NMR relaxation rate in iron-rich gray matter obtained with a Carr-Purcell-Meiboom-Gill sequence. The model consists of representing ol igodendrocytes as identical magnetic spheres arranged in a spatially random pattern, and in approximating water diffusion as isotropic and unrestricte d. Predictions of the model are calculated numerically using a Monte Carlo technique and, for the weak field limit, using an analytic formula. The mod el is shown to provide a good fit to experimental measurements of in vitro samples of monkey brain at field levels of 1.0 T and 1.5 T, These field lev els are not sufficient to fully determine the model parameters, but it is a rgued that this may be possible at 3.0 T, The model is potentially of value for multiple-spin-echo MRI studies of iron-related neurodegenerative disor ders, such as Parkinson's disease. In particular, the model can be applied to correlate MRI data with the cellular distribution of iron in gray matter . Magn Reson Med 46:159-165, 2001. (C) 2001 Wiley-Liss, Inc.