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Special Section: Spotlight on the Future–Imaging and Biomechanical Engineering

In Vivo Quantification of Regional Circumferential Green Strain in the Thoracic and Abdominal Aorta by Two-Dimensional Spiral Cine DENSE MRI

[+] Author and Article Information
John S. Wilson

Department of Radiology and Imaging Sciences,
Emory University School of Medicine,
Atlanta, GA 30322
e-mail: john.s.wilson@emory.edu

Xiaodong Zhong

Magnetic Resonance R&D Collaborations,
Siemens Healthcare,
Atlanta, GA 30322;
Department of Radiology and Imaging Sciences,
Emory University School of Medicine,
Atlanta, GA 30322

Jackson Hair

Department of Biomedical Engineering,
Emory University and Georgia
Institute of Technology,
Atlanta, GA 30322

W. Robert Taylor

Department of Biomedical Engineering,
Emory University and Georgia
Institute of Technology,
Atlanta, GA 30322;
Division of Cardiology,
Department of Medicine,
Emory University School of Medicine,
Atlanta, GA 30322;
Division of Cardiology,
Department of Medicine,
Atlanta VA Medical Center,
Decatur, GA 30033

John N. Oshinski

Department of Radiology and Imaging Sciences,
Emory University School of Medicine,
Atlanta, GA 30322;
Department of Biomedical Engineering,
Emory University and Georgia
Institute of Technology,
Atlanta, GA 30322

1Corresponding author.

Manuscript received October 1, 2017; final manuscript received June 25, 2018; published online April 22, 2019. Assoc. Editor: Jonathan Vande Geest.

J Biomech Eng 141(6), 060901 (Apr 22, 2019) (11 pages) Paper No: BIO-17-1442; doi: 10.1115/1.4040910 History: Received October 01, 2017; Revised June 25, 2018

Regional tissue mechanics play a fundamental role in the patient-specific function and remodeling of the cardiovascular system. Nevertheless, regional in vivo assessments of aortic kinematics remain lacking due to the challenge of imaging the thin aortic wall. Herein, we present a novel application of displacement encoding with stimulated echoes (DENSE) magnetic resonance imaging (MRI) to quantify the regional displacement and circumferential Green strain of the thoracic and abdominal aorta. Two-dimensional (2D) spiral cine DENSE and steady-state free procession (SSFP) cine images were acquired at 3T at either the infrarenal abdominal aorta (IAA), descending thoracic aorta (DTA), or distal aortic arch (DAA) in a pilot study of six healthy volunteers (22–59 y.o., 4 females). DENSE data were processed with multiple custom noise reduction techniques including time-smoothing, displacement vector smoothing, sectorized spatial smoothing, and reference point averaging to calculate circumferential Green strain across 16 equispaced sectors around the aorta. Each volunteer was scanned twice to evaluate interstudy repeatability. Circumferential Green strain was heterogeneously distributed in all volunteers and locations. The mean spatial heterogeneity index (standard deviation of all sector values divided by the mean strain) was 0.37 in the IAA, 0.28 in the DTA, and 0.59 in the DAA. Mean (homogenized) peak strain by DENSE for each cross section was consistent with the homogenized linearized strain estimated from SSFP cine. The mean difference in peak strain across all sectors following repeat imaging was −0.1±2.3%, with a mean absolute difference of 1.7%. Aortic cine DENSE MRI is a viable noninvasive technique for quantifying heterogeneous regional aortic wall strain and has significant potential to improve patient-specific clinical assessments of numerous aortopathies, as well as to provide the lacking spatiotemporal data required to refine patient-specific computational models of aortic growth and remodeling.

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Figures

Grahic Jump Location
Fig. 1

(a) Axial locations along the aorta analyzed by DENSE MRI. (IAA, DTA, and DAA) (be) Illustrative example of images acquired at peak systole in the infrarenal abdominal aorta of a healthy 23 year old female: (b) SSFP cine, (c) DENSE magnitude, (d) DENSE x-phase encoding, and (e) DENSE y-phase encoding. (AO—aorta and VT—vertebra)

Grahic Jump Location
Fig. 2

(a) Tracked voxel positions in the y-direction (open circles) and piecewise time-smoothing (lines) for each voxel in the mask over time. (b) Incremental displacement (i.e., displacement since the previous timepoint) in the y-direction for each voxel in the mask over time before time-smoothing. Note the high degree of noise. The thick yellow line represents mean values, and the vertical dashed line identifies tsys.

Grahic Jump Location
Fig. 3

(a) Segmentation of the aortic wall at peak strain from a DENSE magnitude image (blue—luminal border, red—adventitial border). (b) DENSE y-phase image after applying mask and unwrapping the phase data. (c) Diagram of the calculation of Green strain using overlapping sectors from one quadrant of the aortic wall. Dots represent voxels within the mask in the reference configuration at t1. The reported Green strain for the sector in blue (representing a 22.5 deg arc of the aortic wall) is the weighted average (1:2:1) of the Green strains interpolated at the inner midpoint (stars) of the three overlapping quadrilateral elements defined by vertices representing the closest voxels to each radial spoke emanating from the center of the lumen (+). (d) Diagram of the calculation of circumferential linearized strain along the inner (luminal) row of voxels. Linearized strain is calculated as the relative change in length of the line segment between two voxels separated by vlin voxel spaces (here, vlin = 6) in t1. A single value of strain is reported at the midpoint of the wall (blue dot) between the two voxels used in the calculation.

Grahic Jump Location
Fig. 4

Displacement vector map (at peak strain) and plots of circumferential Green strain versus time for all sectors before (ab) and after (cd) spatial smoothing of the displacement vectors. Asterisk* notes focal artifact of the displacement field before smoothing that results in an abnormally high circumferential strain. Red arrow notes artifactual increase in circumferential strain before t12 in (b). Both artifacts improve following smoothing with vsp=2.

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Fig. 5

Circumferential Green strain versus sector position at the time of peak strain following repeat imaging in each volunteer when the postprocessing noise-reduction (N.R.) is either off or on. The sectors are numbered counterclockwise around the aorta with the division between Sectors 1 and 16 representing the aorta–vertebral interface. (IAA, DTA, and DAA)

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Fig. 6

Ribbon heatmaps of circumferential Green strain of the aortic wall at the time of peak strain mapped onto the unadjusted reference configuration at t1 for both scans in each volunteer. Small arrows identify Sector 1 for each scan (near the vertebra). (IAA, DTA, and DAA)

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Fig. 7

Plots of circumferential Green strain (solid lines) from the two-voxel thick segmentation compared to linearized strain (dashed lines) from the inner row of the two-voxel thick segmentation across all sectors at the time of peak strain for each repeated scan

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Fig. 8

Comparison of peak homogenized circumferential strain estimated from SSFP cine images to mean peak circumferential Green strain, mean peak linearized strain from the two-voxel thick segmentation (2-Vox Lin.), and mean peak linearized strain from the single-voxel thick segmentation (1-Vox Lin.) calculated from cine DENSE imaging

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Fig. 9

Interobserver comparisons of peak circumferential Green strain across all sectors

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