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Imaging Ascending Thoracic Aortic Aneurysm Wall Stretch: A Comparison to Biaxial Mechanical Testing
Huiming Dong1, Henrik Haraldsson1, Joseph Leach1, Ang Zhou1, Megan Ballweber1, Chengcheng Zhu2, Yue Xuan3, Zhongjie Wang3, Michael Hope1, Liang Ge3, Frederick H. Epstein4, David Saloner1, Elaine Tseng3, and Dimitrios Mitsouras1
1Department of Radiology and Biomedical Imaging, University of California, San Francisco, San Francisco, CA, United States, 2Department of Radiology, University of Washington, Seattle, WA, United States, 3Department of Surgery, University of California, San Francisco, San Francisco, CA, United States, 4Department of Biomedical Engineering, University of Virginia, Charlottesville, VA, United States

Synopsis

Ascending thoracic aortic aneurysm (aTAA) can result in life-threatening rupture or dissection. Displacement encoding with stimulated echoes (DENSE) is a non-invasive phase-contrast MRI technique that can measure aTAA wall deformation during the cardiac cycle. This study investigated DENSE-derived aTAA wall stretch in patients and found that the ratio between aTAA stretch and descending aorta stretch was different in patients who met surgical repair criteria from those who did not. Moreover, mechanical properties of aTAA specimens from patients who underwent surgery correlated significantly with in vivo DENSE measurements. Our findings suggest DENSE as a potential imaging marker for understanding aTAA progression.

1. Introduction

Rupture and dissection are life-threatening sequelae of ascending thoracic aortic aneurysms (aTAAs). Current management guidelines recommend surgical intervention when aTAA size meets the repair threshold (e.g., 5.5cm for patients without valvular disease) [1]. Aneurysm development is associated with degradative extracellular matrix (ECM) remodeling which reduces the aneurysm wall integrity [2-5]. Aneurysm stiffness is associated with mural ECM composition, suggesting that it could be relevant for understanding the disease progression [6,7].

Displacement encoding with stimulated echoes (DENSE) is a non-invasive phase-contrast MRI technique that measures tissue displacement during its deformation, from which local stretch or strain can be derived to gauge stiffness [8,9]. Although DENSE has been developed for assessing the relatively thick myocardium, feasibility has been recently demonstrated in application to the arterial wall [10-12]. However, no study to date has investigated DENSE-derived aneurysm wall stretch in different aTAA patient groups. Moreover, in vivo aortic DENSE has not been compared with mechanical testing on aTAA specimens.

The aim of this study is to investigate DENSE-measured aTAA wall stretch in bicuspid aortic valve (BAV) and tricuspid aortic valve (TAV) patients who met repair criteria and underwent surgical repair versus those undergoing surveillance. We also sought to compare the in vivo DENSE-derived aneurysm wall stretch to measurements from ex vivo biaxial mechanical testing on aTAA specimens from the same surgical patients.

2. Methods

2.1 Study Population
In this prospective study, in vivo aortic DENSE was performed on 12 male aTAA patients. Three BAV patients and four TAV patients who met repair criteria underwent surgery while five TAV patients who did not the criteria were continued imaging surveillance (Table I). DENSE imaging was performed prior to the surgery. Among all surgical patients, aTAA specimens were collected from four patients for ex vivo mechanical testing.

2.2 Image Acquisition
Data were acquired on 1.5T or 3T scanners (Avanto or Skyra, Siemens Healthcare, Erlangen, Germany). A single imaging plane was prescribed orthogonal to the vessel centerline at the level of maximum aTAA diameter in the tubular portion of the ascending aorta (see Figure 1). CINE steady-state free precession (SSFP) imaging was performed first to determine the time point when the ascending aorta distended maximally. Subsequently, a cardiac and respiratory navigator-gated DENSE acquisition [9] was performed through the same slice. Imaging parameters were TE=1.14 ms, TR=18–27 ms, 8-10 spiral interleaves per image, 1 spiral interleave per heartbeat, FOV=320×320 to 410×410 mm2, slice thickness=8 mm and reconstruction matrix size=148×148 to 180×180. Additional DENSE parameters were: in-plane (xy-plane) and through-plane (z-direction) encoding frequencies of 0.08-0.37 cycles/mm, balanced four-point encoding and three-point phase cycling to suppress spurious free induction decays and artifact-generating echoes [13, 14].

2.3 Image Analysis
DENSE encoding was performed at the time of maximal aortic distension at systole, and the imaging readout was performed 400 ms later in diastole to quantify the wall stretch that occurred between diastole and systole (Figure 2). A purpose-built post-processor [12] was employed to derive the wall stretch for aTAA (λaTAA) and for the remote normal descending aorta (λDA) within the same slice. Additionally, we calculated the ratio of stretch in the aTAA relative to that in the descending aorta as: γ=λaTAA/λDA. The stretch ratio normalizes aTAA with the remote normal aorta for each individual and thus potentially reduces the effect of confounding factors such as age among different patients.

2.4 Biaxial Mechanical Testing
Ex vivo mechanical testing was performed on aTAA specimens harvested from four surgical patients. Specimens were subject to stretch testing using a custom-built planar biaxial stretching system [15, 16]. The specimen deformation at failure with respect to its original length was recorded as failure stretch. For each of the four patients, regional wall stretch was investigated by dividing the aTAA into four quadrants: anterior, posterior, medial and lateral (Figure 1b). DENSE-derived aTAA stretch and the mechanical testing-measured failure stretch were compared for each quadrant.

3. Results

For BAV surgical patients, TAV surgical patients and TAV non-surgical patients, the mean DENSE-derived ATAA wall stretch between diastole and systole was 1.17±0.81%, 1.18±1.58% and 2.23±1.76%, respectively (Figure 3a).

The mean DENSE-derived ratio of aTAA stretch to descending aorta stretch was 0.999±0.004, 0.987±0.013 and 1.017±0.017 for these three groups, respectively (Figure 3b). Moreover, DENSE-derived stretch ratio was significantly lower among TAV surgical patients than the non-surgical groups (p=0.02).

Pooling both BAV and TAV surgical patients, we also observed a significantly lower stretch ratio in the pooled surgical group when compared to the non-surgical patients (p=0.01) as shown in Figure 3c, suggesting less compliant aneurysms in the patients who proceeded to surgical intervention.

The failure stretch of aneurysm wall measured via mechanical testing correlated to both (1) DENSE-derived aTAA stretch (Spearman rho=0.58, p=0.01, Figure 4a) and (2) stretch ratio (Spearman rho=0.64, p=0.003, Figure 4b), demonstrating the potential of DENSE for risk stratification.

4.Conclusion

DENSE-derived aortic stretch ratio was lower for surgical aTAA patients compared to non-surgical patients. In vivo DENSE measurements correlated to mechanical testing-measured the failure property of the aneurysm wall. Future work is warranted to evaluate whether DENSE has utility as a novel imaging marker of aTAA progression.

Acknowledgements

No acknowledgement found.

References

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Figures

Table I. Demographics

Figure 1. Slice Position of DENSE and CINE Imaging. DENSE and CINE data were acquired at the level of the aTAA via a single slice orthogonal to the tubular portion of the ascending aorta (a). The slice captures both the aTAA (red) and the descending portion of the thoracic aorta (blue) (b).

Figure 2. DENSE Encoding and Wall Displacement. DENSE encoding started at a specified trigger delay after detecting the R wave of the ECG signal. This trigger delay was determined from CINE imaging and ensured the aorta was maximally distended. Wall delineation was performed on the DENSE magnitude image. For each point around the aortic circumference, the displacement was derived from DENSE phase data to trace the location of the aTAA wall at diastole, yielding a displacement field for calculating wall stretch.

Figure 3. DENSE Measurements in aTAA Patients. Box and whisker plots displaying DENSE measurements. Whiskers represent the minimum and maximum of the measurements. Significantly different aortic stretch ratios were observed between surgical and non-surgical patients. Surgical ATAAs were overall stiffer than the normal-caliber descending thoracic aorta in surgical patients with a mean stretch ratio<1.

Figure 4. DENSE vs. Mechanical Testing. DENSE-derived aTAA stretch (a) and stretch ratio (b) correlated to the failure stretch.

Proc. Intl. Soc. Mag. Reson. Med. 30 (2022)
4889
DOI: https://doi.org/10.58530/2022/4889