Bi-exponential proton transverse relaxation rate (R2) image analysis using RF field intensity-weighted spin density projection: potential for R2 measurement of iron-loaded liver.

A bi-exponential proton transverse relaxation rate (R(2)) image analysis technique has been developed that enables the discrimination of dual compartment transverse relaxation behavior in systems with rapid transverse relaxation enhancement. The technique is particularly well suited to single spin-echo imaging studies where a limited number of images are available for analysis. The bi-exponential R(2) image analysis is facilitated by estimation of the initial proton spin density signal within the region of interest weighted by the RF field intensities. The RF field intensity-weighted spin density map is computed by solving a boundary value problem presented by a high spin density, long T(2) material encompassing the region for analysis. The accuracy of the bi-exponential R(2) image analysis technique is demonstrated on a simulated dual compartment manganese chloride phantom system with relaxation rates and relative population densities between the two compartments similar to the bi-exponential transverse relaxation behavior expected of iron loaded liver. Results from analysis of the phantoms illustrate the potential of bi-exponential R(2) image analysis with RF field intensity-weighted spin density projection for quantifying transverse relaxation enhancement as it occurs in liver iron overload.