Vulnerable Plaque Elasticity Imaging

Introduction

Intravascular ultrasound (IVUS) is the only commercially available clinical technique providing real-time cross-sectional images of the coronary artery in patients [30]. IVUS provides information on the severity of the stenosis and the remaining free luminal area. Furthermore, calcified and non-calcified plaque components can be identified. Although many investigators studied the value of IVUS to identify the plaque composition, identification of fibrous and fatty plaque components remains limited [31, 32]. IVUS radiofrequency (RF)-based tissue identification strategies appear to have better performance [32, 33]. However, none of them is yet capable of providing sufficient spatial and parametric resolution to identify a lipid pool covered by a thin fibrous cap.

Identification of different plaque components is of crucial importance to detect the vulnerable plaque since these are characterized by an eccentric plaque with a large lipid pool shielded from the lumen by a thin fibrous cap [13, 34]. Inflammation of the cap by macrophages further increases the vulnerability of these plaques [35]. The mechanical properties of fibrous and fatty plaque components are different [36-38], Furthermore, fibrous caps with inflammation by macrophages are weaker than caps without inflammation [39].

The stress that is applied on an artery by the pulsating blood pressure must balance the circumferentially directed load integrated over the whole arterial wall. To maintain the connection between mechanically different tissue structures (like soft lipid pools and stiff fibrous caps) during arterial deformation, relatively soft regions will therefore carry only a fraction of the total circumferential load and the surrounding stiffer material a greater portion [40, 41], This mechanism causes circumferential stress concentrations in and around the stiff cap, which will rupture if the cap is unable to withstand this stress. This increased circumferential stress will result in an increased radial deformation (strain) of the tissue due to the incompressibility of the material. Therefore, methods that are capable of measuring the radial strain provide information about plaques that may influence clinical decision-making.

In 1991, Ophir et al. [42] proposed a method to measure the elasticity (strain and modulus) of biological tissues using ultrasound. The tissue was deformed by externally applying a stress on it. Different strain values were found in tissues with different material properties. Implementing this method for intravascular purposes has potential to identify the vulnerable plaque by (i) identification of elastically different plaque components and (ii) detection of high radial strain/circumferential stress regions.

This chapter discusses the technique behind the method for and the validation of IVUS strain elastography, which is differentiated into IVUS strain elastography/palpography when the strain is imaged and IVUS modulus elas-tography when the modulus is imaged.

Elasticity Imaging

The Movement Begins

In 1991, Ophir and colleagues [42, 43] developed an elasticity imaging technique called elastography, which is based on (quasi-)static deformation of a linear elastic, isotropic material. The tissue under inspection is deformed by applying stress (i.e., force normalized by area) on a part of its boundary. The resulting distribution of strain (i.e., length of a small block of tissue after deformation, divided by its length before deformation) depends upon (i) the distribution of the tissue's material properties (Young's modulus and Poisson's ratio) and (ii) the displacement or stress conditions on the remaining tissue boundaries. The Young's modulus E [kPa] is a material property, which can be interpreted as the ratio between the normal stress S [kPa] (tensile or compressive) enforced upon a small block of tissue and its resulting strain (elongation or compression). The Poisson's ratio is also a material property and it quantifies a material's local volumetric compressibility. The resulting strain is determined, directly or indirectly using displacement, with ultrasound using two pairs of ultrasound signals, one signal obtained before and the other after deformation [44]. The method was initially developed for detection and characterization of tumors in breast. Nowadays, this principle is also applied to many other biological objects [45]. including prostate, kidney, liver, myocardium, skin, coronary artery and superficial arteries.

Although Ophir et al. [42, 43] never explored the quasi-static approach for intravascular purposes, this approach seems to be the most fruitful concept. In this application, beside knowledge of the material properties of the different plaque components, the strain in itself may be an excellent diagnostic parameter. Furthermore, in intravascular applications, the arterial deformation is naturally present and is caused by the systemic blood pressure. Also user-controlled deformation is possible by inflating an intravascular balloon [46].

IVUS Strain Elastography/Palpography

The principle of IVUS strain elastography is illustrated in figure 3. An echogram of a vessel phantom with a stiff wall and a soft eccentric plaque is acquired at a certain intraluminal pressure using an IVUS catheter. Notice that there is no difference in echogenicity between the wall and the plaque, which

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