On the transverse relaxation enhancement effect in H-MRI of the lung.

Purpose: Presenting a technique to quantify the transverse relaxation time T, which is associated with the diffusion of water molecules through the internal magnetic field gradients of the lung in-vivo.

Methods: A Half-Fourier-Acquired Single-shot Turbo spin-Echo (HASTE) pulse sequence with Hahn-echo preparation was implemented and used for image acquisition. Quantification of T was performed by acquiring multiple images with identical TE, but with a different number of refocusing pulses between excitation and signal acquisition. T was quantified on a voxel-by-voxel basis from the signal attenuation in the different acquisitions. Phantom experiments were performed to evaluate the ability of the proposed technique to discriminate signals with different T. Six samples containing a mixture of water and glass microspheres of different nominal diameters were used. The dependence of T on the sphere diameter was compared with that obtained from the conventional Hahn-echo experiment. In-vivo experiments were performed to investigate the dependence of T on both lung inflation and perfusion. For this, data were acquired in eleven healthy volunteers in different breathing states and different cardiac phases.

Results: Phantom experiments showed a monotonic increase of T with the sphere diameter in agreement with the results of the Hahn-echo experiment, demonstrating an excellent discrimination between signals with different T. In-vivo experiments showed a rather homogeneous distribution of T throughout the lung with a slight dependence on inflation. Mean values obtained in the diastolic cardiac phase resulted in 29 ms at Functional Residual Capacity (FRC) and in 24 ms at Total Lung Capacity (TLC). In the systolic phase the mean value at FRC was 14 ms, indicating a strong dependence of T on perfusion.

Conclusion: The proposed technique allows to quantify T of the lung in a single breath-hold of approximately 10s duration and could help in detecting functional and microstructural injuries of the lung.