On the transverse relaxation enhancement effect in ¹H-MRI of the lung

Presenting a technique to quantify the transverse relaxation time T_2,diff, which is associated with the diffusion of water molecules through the internal magnetic field gradients of the lung in-vivo.

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_2,diff was performed by acquiring multiple images with identical TE, but with a different number of refocusing pulses between excitation and signal acquisition. T_2,diff 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_2,diff. Six samples containing a mixture of water and glass microspheres of different nominal diameters were used. The dependence of T_2,diff 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_2,diff on both lung inflation and perfusion. For this, data were acquired in eleven healthy volunteers in different breathing states and different cardiac phases.

Phantom experiments showed a monotonic increase of T_2,diff with the sphere diameter in agreement with the results of the Hahn-echo experiment, demonstrating an excellent discrimination between signals with different T_2,diff. In-vivo experiments showed a rather homogeneous distribution of T_2,diff 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_2,diff on perfusion.

The proposed technique allows to quantify T_2,diff of the lung in a single breath-hold of approximately 10s duration and could help in detecting functional and microstructural injuries of the lung.