Preclinical hyperpolarized Xe MRI: ventilation and T * mapping in mouse lungs at 7 T using multi-echo flyback UTE.

Fast apparent transverse relaxation (short T *) is a common obstacle when attempting to perform quantitative H MRI of the lungs. While T * times are longer for pulmonary hyperpolarized (HP) gas functional imaging (in particular for gaseous Xe), T * can still lead to quantitative inaccuracies for sequences requiring longer echo times (such as diffusion weighted images) or longer readout duration (such as spiral sequences). This is especially true in preclinical studies, where high magnetic fields lead to shorter relaxation times than are typically seen in human studies. However, the T * of HP Xe in the most common animal model of human disease (mice) has not been reported. Herein, we present a multi-echo radial flyback imaging sequence and use it to measure HP Xe T * at 7 T under a variety of respiratory conditions. This sequence mitigates the impact of T relaxation outside the animal by using multiple gradient-refocused echoes to acquire images at a number of effective echo times for each RF excitation. After validating the sequence using a phantom containing water doped with superparamagnetic iron oxide nanoparticles, we measured the Xe T * in vivo for 10 healthy C57Bl/6 J mice and found T * ~ 5 ms in the lung airspaces. Interestingly, T * was relatively constant over all experimental conditions, and varied significantly with sex, but not age, mass, or the O content of the inhaled gas mixture. These results are discussed in the context of T * relaxation within porous media.