Multi-exponential water proton spin-lattice relaxation in biological tissues and its implications for quantitative NMR imaging

This in vitro study was undertaken to examine whether water proton spin-lattice relaxation in biological tissues is adequately described by a single constant T1, to define under what circumstances a multi-exponential approach is indicated, and to study the implications of multi-exponentiality for quantitative NMR imaging. Water proton relaxation curves were measured with the 180-tau-90 method at 60 MHz. Uni- and bi-exponential curves were fitted to the empirical curves using X2 as a criterion for the goodness of fit. An F-test was applied to test the validity of each exponential term as it was added to the fitting function. Taking into account experimental accuracy, the uni-exponential model appeared to be an adequate description of the relaxation data for necrotic tissue. Eyelens and fat showed distinct bi-exponentiality, while liver, spleen, salivary gland, tumour, and muscle presented intermediate cases. The bi-exponential analysis generally yields a minor component with a fast relaxation time, T11 < 20 ms, and a slow relaxation major component with T12 > 300 ms. A simplified bi-exponential model is proposed for implementation in quantitative NMR imaging. (author)