Measuring Dynamic Changes in Lung Water Density and Volume Following Supine Body Positioning Using Free-Breathing UTE MRI
Creators: Goodhart, David T
Publisher: name: University of Alberta Library
Year: 2024

<p>An excess of extravascular lung water (EVLW) is known as pulmonary edema, and is associated with dyspnea and poor exercise capacity, heart complications, heart failure, and can be a predictor of poor health outcomes. Recent developments in magnetic resonance imaging (MRI) sequencing and hardware capabilities have enabled free-breathing, 3D isotropic images of the lungs. Previous studies have developed methods to overcome the inherent difficulties in lung MRI, which include low SNR, high signal variations across the image space due to B1 field inhomogeneity, artifacts caused by respiratory and cardiac motion, and a need to normalize signal units within the lung parenchyma to units of lung water content. Through these methods, lung water density (LWD) has been shown to be roughly ~25%-30% in healthy subjects, with a well-documented gradient in LWD in the chest-to-back direction (otherwise known as the “Slinky Effect”). Standard MRI protocol calls for a subject to assume a supine position, which has substantial changes to the thoracic cavity; Changes in global LWD, regional LWD, and global lung volumes over time after assuming supine positioning remain relatively unknown.</p>

<pre><code>This thesis has two goals: 1. Refining and improving upon a previously established Free-Breathing “Yarnball” ultrashort echo time (UTE) non-Cartesian k-space trajectory via maximizing trajectory efficiency (balancing image quality and scan time) and introducing a nnU-Net neural network approach to lung parenchyma and lung vasculature segmentation for increased accuracy in relative LWD (rLWD) calculation and 2. Performing a long-time course series (~25 minutes) of lung MRI scans using the above refined trajectory to establish the time-course changes in global rLWD, regional rLWD, global water content and global lung volumes that occur due to a physiological response triggered by a change in the direction of gravitational forces acting upon the body when supine positioning is assumed. If possible, establishment of the amount of time it takes for the noted “Slinky Effect” to form is a tertiary goal. 

The UTE Yarnball k-space trajectory developed by Meadus et al (2021) was optimized by adjusting the key variables of readout time, TR, voxel size, FOV, and number of averages acquired with image quality, image quantification and total imaging time being taken into consideration. It was determined that a sequence with: FOV = 350 mm, TR = 2.72 ms, TE = 100 μs, 1-degree flip angle, readout time = 1.3 ms, Voxel size = 3.5 mm in all directions, 10 averages and 2738 k-space trajectories was most optimal. 

At the respiratory phase of functional residual capacity (FRC), minimum lung volume with normal tidal breathing, it was found that global rLWD increased significantly (31.8±5.5% to 34.8±6.8%, p=0.001), global lung volumes decreased significantly (2390±620mL to 2130±630mL, p&lt;0.001) and total lung water volume decreased slightly (730±125 mL to 706±126 mL, p=0.028) over a 25-minute period after supine positioning was assumed. The chest-to-back gradient (20.7±4.6% at the chest to 39.9±6.1% at the back) formed prior to first acquisition (~3:54 minutes), and was significant at all time-points. Tidal volume (lung volume change per respiratory cycle) also decreased significantly over time (474±89mL to 382±91 mL, p=0.018) despite respiratory rate remaining constant. 
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<p>UTE Yarnball MRI was refined to “optimally” balance image quality with total image scan time, featuring an increase in number of averages used alongside a reduction in scan time with no significant loss to image quality when compared to the preceding iteration. Global and regional rLWD and lung volumes were shown to change over long periods of time after supine positioning, a phenomenon that should be considered in future lung MRI studies.  In particular, we have shown, for the first time, that the timing of data acquisition following supine positioning will significantly affect the lung water density values, and thus should be taken into consideration in clinical studies.  Furthermore, any free-breathing MRI acquisitions should consider these significant shifts in lung volumes and positions over time following supine positioning (i.e. navigator type sequences would be confounded by the potential drift of lungs, heart and all organs in abdomen over time). Finally, the formation of the chest-to-back gradient was not captured, but can be inferred to occur in &lt; 1 minute, and likely immediately with supine positioning.</p>