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. 2011 Aug;16(4):302-11.
doi: 10.1177/1358863X11400936. Epub 2011 Mar 29.

The development and potential of acoustic radiation force impulse (ARFI) imaging for carotid artery plaque characterization

Jason D Allen  1 , Katherine L Ham, Douglas M Dumont, Bantayehu Sileshi, Gregg E Trahey, Jeremy J Dahl
Affiliations

The development and potential of acoustic radiation force impulse (ARFI) imaging for carotid artery plaque characterization

Jason D Allen et al. Vasc Med. 2011 Aug.

Abstract

Stroke is the third leading cause of death and long-term disability in the USA. Currently, surgical intervention decisions in asymptomatic patients are based upon the degree of carotid artery stenosis. While there is a clear benefit of endarterectomy for patients with severe (> 70%) stenosis, in those with high/moderate (50-69%) stenosis the evidence is less clear. Evidence suggests ischemic stroke is associated less with calcified and fibrous plaques than with those containing softer tissue, especially when accompanied by a thin fibrous cap. A reliable mechanism for the identification of individuals with atherosclerotic plaques which confer the highest risk for stroke is fundamental to the selection of patients for vascular interventions. Acoustic radiation force impulse (ARFI) imaging is a new ultrasonic-based imaging method that characterizes the mechanical properties of tissue by measuring displacement resulting from the application of acoustic radiation force. These displacements provide information about the local stiffness of tissue and can differentiate between soft and hard areas. Because arterial walls, soft tissue, atheromas, and calcifications have a wide range in their stiffness properties, they represent excellent candidates for ARFI imaging. We present information from early phantom experiments and excised human limb studies to in vivo carotid artery scans and provide evidence for the ability of ARFI to provide high-quality images which highlight mechanical differences in tissue stiffness not readily apparent in matched B-mode images. This allows ARFI to identify soft from hard plaques and differentiate characteristics associated with plaque vulnerability or stability.

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Figures

Figure 1
Figure 1. a–b. The mechanism of ARFI radiation force imaging pulse
(a) Schematic representation of the radiation force field generated by an ARFI imaging pulse. The radiation force is generated within the shape of the transmitted ultrasound beam, approximated by the two curved lines, with the maximum force typically observed at the focal point of the beam. (b) A typical response of the tissue to the radiation force impulse. The tissue at the focal point displaces away from the transducer momentarily and then recovers back to its original position. ARFI images are often created at the point of peak tissue displacement, shown by the dotted line.
Figure 2
Figure 2. a–b. Matched (a) B-mode and (b) ARFI transverse images of a vessel phantom embedded in a gelatin/graphite mix
There is little difference between the vessel phantom and the background in the B-mode image. In the ARFI image, dark blue corresponds to low displacement and red corresponds to high displacement. There is high contrast between the vessel phantom and the background in the ARFI image compared to the B-mode image. Because the vessel phantom displaces significantly less than the background, the vessel phantom is stiffer than the surrounding medium.
Figure 3
Figure 3. a–d. Matched B-Mode (a & c) and ARFI (b & d) images of vessel phantoms with 44% (a & b) and 80% (c & d) soft occlusions
The vessel is indistinguishable from the occlusion in the B-mode images, however the ARFI images display the difference between the stiffer vessel wall and softer occlusion with good contrast
Figure 4
Figure 4. a–c Matched (a) B-Mode and (b) ARFI images from an excised popliteal artery shown in photograph (c)
The photograph of the excised vessel and plaque shows areas of healthy vessel (A & M), calcifications (C) lipid core (L) and a fibrous cap (F) on the distal wall. The ARFI image depicts very low displacement in the location of the calcifications (the dark area beneath the label C) and larger displacement in the region of the lipid core (labeled L). Reprinted with permission from .
Figure 5
Figure 5. a–c An Example of a Healthy Carotid Vessel
(a) Longitudinal sketch of the common carotid artery in the subject. Matched (b) B-Mode and (c) ARFI images from location B the CCA in a healthy 39-year-old male. The ARFI images indicate that healthy vascular tissue shows homogeneous stiffness throughout the vessel.
Figure 6
Figure 6. An Example of a Carotid Artery Containing a Homogeneously Hard Plaque
A (a) Longitudinal sketch of the carotid artery in the subject. Matched (b) B-Mode and (c) ARFI images from the ICA and plaque in the vicinity of location E. The ARFI images here indicate that this plaque (shown in dark blue) is homogenously stiff.
Figure 7
Figure 7. An Example of a Carotid Artery Containing an Intermediate Soft Plaque
(a) Longitudinal sketch of the common carotid artery of the subject. Matched longitudinal (b) B-Mode and (c) ARFI images from the CCA and plaque located in the region of B.
Figure 8
Figure 8. A graph of the Tissue Displacement Responses from different regions of the intermediately soft plaque (from figure 7)
The peak displacement from the soft regions (light green) in both views are 3 times larger than the peak displacement observed in the hard (dark blue) region of the plaque. Both the hard and soft regions exhibit typical displacement responses of tissue. This is contrasted with the partially random displacements observed in the lumen of the vessel, corresponding to the flow of blood.
Figure 9
Figure 9. An Example of a Carotid Artery Containing an Soft Plaque Matched longitudinal
(a) B-Mode and (b) ARFI images at the carotid bifurcation. The plaque is located on the distal wall (DW) of the bifurcation. The plaque appears to wrap around to the proximal wall (PW) in the region of the CCA. The relatively thin cap surrounding the soft region and the size of the soft region place this plaque in the vulnerable category, according to the characteristics described by Ge et al. [14]. Reprinted from permission from .
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