3D Cartesian ultrashort double half-echo imaging of the lung parenchyma for water density imaging.

Purpose: Develop and illustrate a 3D double half-echo Cartesian UTE method for spin-density weighted imaging of the lung parenchyma and calculation of lung water density (LWD).

Methods: A 3D gradient-echo pulse sequence was modified to acquire half-echoes, to enable UTEs (TE/TR = 145 μs/1.2 ms), with an acquired resolution of 3.125 mm by 3.125 mm by 5 mm. Breath-hold (12.9 s) and free-breathing (94 s) acquisitions, using a center of k-space navigator, were compared to a previously validated yarnball UTE sequence (1.5T/2.89T). Apparent SNR in the lung parenchyma was measured for all in-vivo acquisitions. Illustrative clinical cases included heart failure and sarcoidosis with a comparison to CT images.

Results: Lung image quality and calculated LWD was similar for all compared methods at 1.5T and 2.89T for breath-hold and free-breathing acquisitions (N = 10, p > 0.05), with no visible artifacts. The mean lung parenchyma SNR values were 18.4 ± 1.4, 21.8 ± 1.7 and 15.1 ± 1.0 for 1.5T free-breathing, 2.89T free-breathing and 2.89T breath-hold, respectively, and 20.7 ± 1.1 for yarnball acquisitions (2.89T), with corresponding average LWD values of 26.7 ± 2.9%, 27.1 ± 2.5%, 27.1 ± 2.1% and 27.7 ± 2.7%. MRI LWD images and CT scans yielded similar image contrast and normalized signal intensity units. All Cartesian UTE images were reconstructed on the scanner without the requirement for gridding.

Conclusions: A double half-echo Cartesian UTE pulse sequence provides water-density weighted images of the lung parenchyma in a breath-hold or short free-breathing acquisition with sufficient signal to noise for quantification of LWD at 1.5T or 2.89T.