Implementation of the FLORET UTE sequence for lung imaging

Abstract

Purpose: Magnetic resonance imaging of lungs is inherently challenging, but it has become more common with the use of UTE sequences and their relative insensitivity to motion. Spiral UTE sequences have been touted recently as having greater k-space sampling efficiencies than radial UTE, but few are designed for the shorter T₂ * of the lung. In this study, FLORET (Fermat looped, orthogonally encoded trajectories), a recently developed spiral 3D-UTE sequence designed for the short T₂ * species, was implemented in human lungs for the first time and the images were compared with traditional radial UTE images.

Methods: The FLORET sequence was implemented with parameters optimized for lung imaging on healthy and diseased (cystic fibrosis) subjects. On healthy subjects, radial UTE images (3D-radial and 2D-radial with phase encoding) were acquired for comparison to FLORET. Various metrics including SNR, vasculature contrast, diaphragm sharpness, and parenchymal density ratios were acquired and compared among the separate UTE sequences.

Results: The FLORET sequence performed similarly to traditional radial UTE methods with a much shorter total scan time for fully sampled images (FLORET: 1 minute 55 seconds, 3D-radial: 3 minutes 25 seconds, 2D-radial with phase encoding: 7 minutes 22 seconds). Additionally, the FLORET image obtained on the cystic fibrosis subject resulted in the observation of cystic fibrosis lung pathology similar or superior to that of the other UTE-MRI techniques.

Conclusion: The FLORET sequence allows for faster acquisition of high diagnostic-quality lung images and its short T₂ * components without sacrificing SNR, image quality, or tissue/disease quantification.

Keywords: FLORET; UTE; cystic fibrosis; lung; non-Cartesian; spiral.

Figure 1:

Example of the FLORET trajectories implemented in this study. The views on the left are projections along the k_z direction and the views on the right are projections along the k_y direction. Blue trajectories are acquired in the earlier portions of the scan while green trajectories are acquired later. a) and b) show the first, middle and last acquisitions in a set of spirals. c) and d) illustrate all of the acquisitions in a single hub while e) and f) show the full acquisition with 2 hubs.

Figure 2:

Example slice from the sponge phantom images. Top, middle, and bottom rows were acquired by FLORET, 3D-Radial, and 2D-Radial, respectively. The different columns are the different variations of the acquisition parameters used as describe in text (i.e., column 1 refers to variation 1, the readout-time optimized acquisitions).

Figure 3:

Comparisons of fully sampled UTE images. Left: FLORET images (variation 3 in Supporting Information Table S1) acquired using a 1.5 ms read out duration in 2 min 07 sec. Middle: 3D-radial images (variation 1 in Table SI) acquired using a 0.52 ms read out duration in 3 min 49 sec. Right: 2D-radial images acquired (variation 1 in Supporting Information Table S1) using a 0.60 ms read out duration in 4 min 52 sec. Top: Axial slices. Middle: Coronal slices. Bottom: Sagittal slices.

Figure 4:

Example slices using FLORET from the CF subject illustrating the ability to observe typical pathologies including bronchiectasis, bronchial-wall thickening, linear opacities (potential atelectasis) and mucus plugs that are consistent with CF lung pathology.

Figure 5:

Comparisons of UTE images acquired in the same amount of time; 1 min 55 sec (variation 5 in Supporting Information Table S2). Left: Fully sampled FLORET images acquired using a 1.5 ms read out. Middle: 56% sampled 3D-radial images acquired using a 0.52 ms read out duration. Right: 26% sampled 2D-radial images acquired using a 0.60 ms read out duration. Top: Axial slices. Middle: Coronal slices. Bottom: Sagittal slices.

Figure 6:

Cross-sections of vasculature (a) and diaphragm (b) acquired with FLORET, 3D-radial, and 2D-radial. Vasculature cross-sections demonstrate slightly increased vascular contrast in FLORET compared to radial techniques. Diaphragm cross sections indicate similar sharpness of the diaphragm for all sequences with a slight benefit to FLORET and 3D-radial technique.