Fig 1.
Flow profile measurement and synchronization with PC MRI.
Flow rate of inhaled 129Xe in each of the three subjects beginning at the time the first PC MRI acquisition started. The vertical red lines represent the start of each subsequent dynamic PC MRI image acquisition.
Fig 2.
Example of MRI data acquired from each of the three subjects.
Top row: Sagittal 1H MRI of the head and neck anatomy including the upper airway, captured to create the virtual airway surface for CFD simulations. Middle row: Sagittal magnitude (“Mag”) MR images of inhaled 129Xe in the upper airway at a selected dynamic time (note: slices do not perfectly correspond to 1H MRI slices above). Bottom row: Foot-Head velocity (vFH) maps corresponding to the magnitude images above (one dynamic image), reconstructed from 129Xe gas PC MRI data. Red velocities represent flow in the foot to head direction, blue represents flow in the head to foot direction.
Fig 3.
Example of the processing pipeline to facilitate the comparison of CFD and PC MRI data.
a). The CFD mesh is generated throughout the 3D airway; cells on a sagittal plane are shown here. b). Representative instantaneous CFD 3D velocity field showing high resolution velocity information throughout the airway. c) 3D airway model derived from 1H MRI (red) used to define the 3D virtual surface for CFD, overlaid with cuboids (grey) illustrating the field-of-view and comparatively coarse voxel dimensions of the PC MRI data. These cuboids also illustrate the anisotropic shape of the PC MRI voxels. The mean CFD velocity within each voxel and imaging acquisition period is calculated for comparison to PC MRI.
Fig 4.
Example 129Xe gas PC MRI dataset acquired from subject 1.
The first 5 dynamic images from a 10-image time series are shown left-right, where each dynamic scan corresponds to a 3.1 s interval. Top row: MRI magnitude images showing the 129Xe gas density in the airway. Lower rows: Foot-head (vFH), anterior-posterior (vAP) and right-left (vRL) components of velocity derived from the dynamic PC MRI time series, and the corresponding velocity magnitude (vMAG, vector sum of the three directional components). Colour key: For vFH, red = flow from foot to head, blue = flow from head to foot; for vAP red = flow from anterior to posterior, blue = flow from posterior to anterior; and for vRL red = flow from right to left, blue = flow from left to right. vMAG is non-directional.
Fig 5.
Side-by-side comparison of PC MRI and CFD.
Comparison of the foot-to-head, anterior-to-posterior, and right-to-left components of velocity for, left: A 129Xe gas PC MRI dynamic image (dynamic image #2 in Fig 4); middle: The CFD “slice” resulting from the regional averaging of the 3D CFD volume in to a 2D slice that corresponds to the PC MRI voxel data, as described in Fig 3); right: Voxel-wise difference between PC MRI and CFD data, highlighting regions of good and poor agreement. Labels a-j highlight important flow features highlighted in the main body text.
Fig 6.
Correlation plots and histograms of agreement for each velocity component: PC MRI and CFD.
a) right-left component, b) anterior-posterior component, c) foot-head component. d) data in c) after removal (“erosion”) of the boundary layer of pixels from the 2D maps shown in Fig 5 (Subject 1). Solid blue lines indicate the line of best fit and dashed lines are the identity line. Histograms of velocity values derived from PC MRI and CFD are displayed in blue and red color, respectively.
Fig 7.
Effects of spatial down-sampling on PC MRI and CFD comparison.
a) 2D velocity (vFH) maps for PC MRI and CFD data and the difference between them, and b) correlation plots and histograms presented as in Fig 6, after down-sampling of the PC MRI data in plane by 3x3 voxels and re-sampling the CFD data to the corresponding resolution.
Fig 8.
Bland-Altman plots for vFH between PC MRI and CFD.
a) Bland-Altman plot for the voxel-wise mean and difference in vFH between PC MRI and CFD for subject 1. b) Analogous plot to a) after down-sampling the data in plane by 3x3 voxels.
Fig 9.
Side-by-side comparison of PC MRI and CFD: Additional examples.
Spatial comparison of 3x3 down-sampled foot-head velocity data from PC MRI and CFD, difference maps, and corresponding histograms of velocity values, for a) subject 2 and b) subject 3. Dataset shown is taken from the 3rd temporal dynamic image of the 129Xe PC MRI time series.