Theoretical prediction and experimental measurement of the field dependence of the apparent transverse relaxation of hyperpolarized noble gases in lungs

In this work, computer modeling based on a finite element method is used to simulate the T2* relaxation of hyperpolarized noble gases (HNG) in the lungs. A physical model of lung airways consisting of a phantom constructed from micro-capillary fibers of diameters similar to the size of lung airways with semi-permeable walls is also presented. The fibers are surrounded by a liquid medium (water) of magnetic susceptibility similar to lung tissue. Theoretical predictions of the field strength dependence of T2* for 129Xe in the phantom and in vivo rat lung are presented. These predictions are in good agreement with experimental T2* values obtained from the phantoms and in vivo rat lungs (160, 19 and 8 ms) at three different field strengths (0.074, 1.89 and 3T, respectively) using hyperpolarized 129Xe. The strong dependence of T2* on field strength is consistent with the theoretical prediction that low fields may be optimal for HNG MR imaging of the lungs as the decreased T2* at high fields necessitates an increase in bandwidth for conventional MR imaging.