Improved pulmonary ¹²⁹Xe ventilation imaging via 3D‐spiral UTE MRI

Abstract

Purpose Hyperpolarized 129Xe MRI characterizes regional lung ventilation in a variety of disease populations, with high sensitivity to airway obstruction in early disease. However, ventilation images are usually limited to a single breath‐hold and most‐often acquired using gradient‐recalled echo sequences with thick slices (~10‐15 mm), which increases partial‐volume effects, limits ability to observe small defects, and suffers from imperfect slice selection. We demonstrate higher‐resolution ventilation images, in shorter breath‐holds, using FLORET (Fermat Looped ORthogonally Encoded Trajectories), a center‐out 3D‐spiral UTE sequence. Methods In vivo human adult (N = 4; 2 healthy, 2 with cystic fibrosis) 129Xe images were acquired using 2D gradient‐recalled echo, 3D radial, and FLORET. Each sequence was acquired at its highest possible resolution within a 16‐second breath‐hold with a minimum voxel dimension of 3 mm. Images were compared using 129Xe ventilation defect percentage, SNR, similarity coefficients, and vasculature cross‐sections. Results The FLORET sequence obtained relative normalized SNR, 40% greater than 2D gradient‐recalled echo (P = .012) and 26% greater than 3D radial (P = .067). Moreover, the FLORET images were acquired with 3‐fold‐higher nominal resolution in a 15% shorter breath‐hold. Finally, vasculature was less prominent in FLORET, likely due to diminished susceptibility‐induced dephasing at shorter TEs afforded by UTE sequences. Conclusion The FLORET sequence yields higher SNR for a given resolution with a shorter breath‐hold than traditional ventilation imaging techniques. This sequence more accurately measures ventilation abnormalities and enables reduced scan times in patients with poor compliance and severe lung disease.

In vivo methods and applications of xenon-129 magnetic resonance

“…However, despite the advantages for monitoring of 129 Xe magnetisation dynamics, the SNR of 3D radial 129 Xe MR images has been reported to be lower than a dose equivalent 2D multi-slice SPGR sequence [58] . Novel trajectories for hyperpolarised 129 Xe MRI, including spiral-based techniques for efficient encoding of 3D k -space such as Fermat Looped ORthogonally Encoded Trajectories (FLORET) [84] , are a subject of keen interest. 3D radial UTE sequences show good promise for imaging of pulmonary gas exchange with 129 Xe MRI, where short T 2 *( 129 Xe) is a limiting factor (as discussed in 3.3 , 7 ).…”

Protocols for multi‐site trials using hyperpolarized 129Xe MRI for imaging of ventilation, alveolar‐airspace size, and gas exchange: A position paper from the 129Xe MRI clinical trials consortium

“…3D imaging has several potential advantages, including improved SNR and spatial contiguity of data, 108 but is more prone to motion artifacts from respiratory and cardiac motion. Such artifacts can be mitigated through the use of methods that sample k‐space from the center‐outward, such as radial or spiral imaging 83,109 . Moreover, these techniques are robust to undersampling, which mitigates the concerns of early exhalation.…”

“…Such artifacts can be mitigated through the use of methods that sample k-space from the center-outward, such as radial or spiral imaging. 83,109 Moreover, these techniques are robust to undersampling, which mitigates the concerns of early exhalation. 3D radial and spiral sequences are not typically as fast as slice-selective spirals, but allow improved slice resolution and can provide quantitative corrections for signal depletion.…”

Structural and functional changes in COPD: What we have learned from imaging

“…[7][8][9] Moreover, functional MRI has brought new insights into the disease. [10][11][12] This review will describe advanced imaging modalities available in clinical and research settings that are helping to expand our understanding of COPD pathophysiology.…”

“…Recently, MRI of the lung has benefited from technological developments, especially the ultra‐short echo times (UTE) sequences allowing morphologic millimetric imaging of the lung 7–9 . Moreover, functional MRI has brought new insights into the disease 10–12 …”