In vivo bistatic dual-aperture ultrasound imaging and elastography of the abdominal aorta

Abstract

Introduction: In this paper we introduce in vivo multi-aperture ultrasound imaging and elastography of the abdominal aorta. Monitoring of the geometry and growth of abdominal aortic aneurysms (AAA) is paramount for risk stratification and intervention planning. However, such an assessment is limited by the lateral lumen-wall contrast and resolution of conventional ultrasound. Here, an in vivo dual-aperture bistatic imaging approach is shown to improve abdominal ultrasound and strain imaging quality significantly. By scanning the aorta from different directions, a larger part of the vessel circumference can be visualized. Methods: In this first-in-man volunteer study, the performance of multi-aperture ultrasound imaging and elastography of the abdominal aortic wall was assessed in 20 healthy volunteers. Dual-probe acquisition was performed in which two curved array transducers were aligned in the same imaging plane. The transducers alternately transmit and both probes receive simultaneously on each transmit event, which allows for the reconstruction of four ultrasound signals. Automatic probe localization was achieved by optimizing the coherence of the trans-probe data, using a gradient descent algorithm. Speckle-tracking was performed on the four individual bistatic signals, after which the respective axial displacements were compounded and strains were calculated. Results: Using bistatic multi-aperture ultrasound imaging, the image quality of the ultrasound images, i.e., the angular coverage of the wall, was improved which enables accurate estimation of local motion dynamics and strain in the abdominal aortic wall. The motion tracking error was reduced from 1.3 mm ± 0.63 mm to 0.16 mm ± 0.076 mm, which increased the circumferential elastographic signal-to-noise ratio (SNRe) by 12.3 dB ± 8.3 dB on average, revealing more accurate and homogeneous strain estimates compared to single-perspective ultrasound. Conclusion: Multi-aperture ultrasound imaging and elastography is feasible in vivo and can provide the clinician with vital information about the anatomical and mechanical state of AAAs in the future.

Keywords: abdominal aorta; coherent compounding; elastography; multi-aperture; ultrasound.

FIGURE 1

Experimental arch set-up used to perform the in vivo measurements of the abdominal aorta.

FIGURE 2

Overview of the probe localization method. A gradient descent algorithm is used to optimize the signal coherence of the T ₁ R ₂ signals from all transmit angles. (A) Coherently compounded T ₁ R ₂ signals from all transmit angles using manual registration of the probe locations (left) and automatic registration of the probe locations based on coherence optimization (right). (B) Optimization function during the gradient descent iteration process.

FIGURE 3

Illustration of the projection angle θ between the axial direction of the receiving transducer, and the radial direction of the aorta, with a color overlay of the mask created by Eq. 4 (Van Hal et al., 2021).

FIGURE 4

Single-perspective (left), and coherent dual-aperture bistatic (right) images of the abdominal aorta in 6 volunteers (V1, V5, V7, V16, V17, and V18). All ultrasound images are displayed using a 50 dB dynamic range. The size of the zoomed windows are 5 cm in the x-direction and 4 cm in the z-direction.

FIGURE 5

Full field-of-view images of the single-perspective (A) and bistatic dual-aperture ultrasound acquisitions (B) of the abdominal aorta in volunteer 1.

FIGURE 6

Regional analysis of the generalized contrast-to-noise ratio (gCNR) between the vessel wall and the lumen of the aorta in all volunteers. (A) gCNR in single-perspective (SP) and multi-aperture bistatic ultrasound imaging in the 8 regions of the vessel wall. (B) Difference in gCNR (ΔgCNR) for bistatic imaging compared to single-perspective ultrasound imaging. Regions in which the differences were statistically significant have been indicated with an asterisk in the label.

FIGURE 7

(A) Coherent dual-aperture bistatic ultrasound images of the abdominal aorta from different volunteers obtained with increasing inter-probe angles. (B) Generalized vessel-lumen contrast-to-noise ratio (gCNR) across the entire circumference of the aorta in each of the 20 volunteers, plotted against the used inter-probe angle during the acquisition.

FIGURE 8

Estimated local circumferential strain in the aortic vessel wall at systole using single-perspective (left), and dual-aperture bistatic (right) ultrasound imaging in 6 volunteers (V1, V5, V7, V16, V17, and V18). The indicated sections at the right and left side of the wall were left out of the analysis. The size of the zoomed windows are 5 cm in the x-direction and 4 cm in the z-direction. A video corresponding to this figure is available as Supplementary Material.

FIGURE 9

Estimated local radial strain in the aortic vessel wall at systole using single-perspective (left), and dual-aperture bistatic (right) ultrasound imaging in 6 volunteers (V1, V5, V7, V16, V17, and V18). The indicated sections at the right and left side of the wall were left out of the analysis. The size of the zoomed windows are 5 cm in the x-direction and 4 cm in the z-direction. A video corresponding to this figure is available as Supplementary Material.

FIGURE 10

Regional analysis of the tracking error (ME) between single-perspective and bistatic imaging configurations in all volunteers. All differences were statistically significant (p $<$ 10^–4), which is indicated with an asterisk in the label.

FIGURE 11

Regional analysis of the elastographic signal-to-noise ratio (SNRe) between single-perspective and bistatic imaging configurations in all volunteers. Regions in which the differences were statistically significant have been indicated with an asterisk in the label. (A) Circumferential SNRe (B) Radial SNRe.