Vascular ultrasound for atherosclerosis imaging

Abstract

Cardiovascular disease is a leading cause of death in the Western world. Therefore, detection and quantification of atherosclerotic disease is of paramount importance to monitor treatment and possible prevention of acute events. Vascular ultrasound is an excellent technique to assess the geometry of vessel walls and plaques. The high temporal as well as spatial resolution allows quantification of luminal area and plaque size and volume. While carotid arteries can be imaged non-invasively, scanning of coronary arteries requires invasive intravascular catheters. Both techniques have already demonstrated their clinical applicability. Using linear array technology, detection of disease as well as monitoring of pharmaceutical treatment in carotid arteries are feasible. Data acquired with intravascular ultrasound catheters have proved to be especially beneficial in understanding the development of atherosclerotic disease in coronary arteries. With the introduction of vascular elastography not only the geometry of plaques but also the risk for rupture of plaques might be identified. These so-called vulnerable plaques are frequently not flow-limiting and rupture of these plaques is responsible for the majority of cerebral and cardiac ischaemic events. Intravascular ultrasound elastography studies have demonstrated a high correlation between high strain and vulnerable plaque features, both ex vivo and in vivo. Additionally, pharmaceutical intervention could be monitored using this technique. Non-invasive vascular elastography has recently been developed for carotid applications by using compound scanning. Validation and initial clinical evaluation is currently being performed. Since abundance of vasa vasorum (VV) is correlated with vulnerable plaque development, quantification of VV might be a unique tool to even prevent this from happening. Using ultrasound contrast agents, it has been demonstrated that VV can be identified and quantified. Although far from routine clinical application, non-invasive and intravascular ultrasound VV imaging might pave the road to prevent atherosclerotic disease in an early phase. This paper reviews the conventional vascular ultrasound techniques as well as vascular ultrasound strain and vascular ultrasound VV imaging.

Keywords: contrast-enhanced ultrasound; elastography; ultrasound imaging; vascular; vulnerable plaque.

Figure 1.

Echogram of the longitudinal view of a common carotid artery using (a) conventional scanning and (b) scanning using spatial compounding. The speckle pattern is diminished while the specular reflections are preserved.

Figure 2.

(a) Angiogram with corresponding IVUS echograms (b–f) of a coronary artery demonstrating that the luminal area as provided by the echogram does not provide information on the presence of plaque (b,c). Furthermore, the presence of (d) a dissection and (e) the adequate positioning and (f) malapposition of a stent can be visualized by IVUS.

Figure 3.

Beam steering methods for vascular strain imaging in transverse planes. (a) Method by Nakagawa et al. and (b) method by Hansen et al.

Figure 4.

Circumferential strain reconstructions for a vulnerable plaque model. (a) The theoretical strain image is shown which was derived by finite element modelling. Ivory, lipids (25 kPa); orange, vessel wall (1000 kPa); yellow, centre of fibrous cap (1250 kPa); red, shoulders of cap (1500 kPa). (b) Strain images derived from simulated ultrasound data are shown. The left image was derived using conventional single angle data. The right image was estimated using beam steering at angles of −30°, 0° and 30°.

Figure 5.

IVUS palpogram of a non-culprit lesion. The patient had a myocardial infarction caused by an occlusion of the left anterior descending coronary artery. Recordings were made at the same location in the right coronary artery, (a) directly after infarct and (b) at three months follow up. The eccentric lesion was not occluding, but characterized by increased strain, which was worse after three months.

Figure 6.

In vivo results in an atherosclerotic rabbit aorta using an ultrasound contrast agent. (a) Fundamental mode at 20 MHz, 10 s after injection where changes in adventitial enhancement are not evident. (b) At 10 s after injection, the harmonic mode (transmit at 20 MHz, receive at 40 MHz) shows significant adventitial enhancement, consistent with the detection of adventitial microvessels. The white dots are contrast agents in the vasa vasorum and the bright ring contrast agents attached to the luminal border. The dynamic range of the fundamental and harmonic images is 40 and 25 dB, respectively.