Ultrasound elastography in the head and neck. Part I. Basic principles and practical aspects

Abstract

Ultrasound elastography (USE) is a rapidly developing field of imaging that measures and displays tissue elasticity or stiffness properties using ultrasound. In recent years, real-time USE modes have appeared on commercially available clinical ultrasound machines, stimulating an explosion of research into potential oncologic and non-oncologic clinical applications of USE. Preliminary evidence suggests that USE can differentiate benign and malignant conditions accurately in several different tissues. This article presents an overview of the basic principles of different USE technologies that are currently under investigation in the head and neck region. In addition, more practical aspects pertaining to the optimal performance of USE at this site are discussed.

Figures

Figure 1

Strain imaging schematic. A tissue containing a stiff lesion is compressed longitudinally by a transducer while performing US (a). Returning US signals are analysed to measure tissue displacements for different depths (b), which are converted to strain data (displacement/original length) (c). The graphs represent data from a single scan line through the middle of the lesion. The lesion shows minimal displacement and low strain compared with the reference tissue. The resulting strain data for multiple scan lines is displayed graphically as an elastogram corresponding to relative strain (d). This lesion is conspicuous on the elastogram due to its difference in strain compared with the surrounding tissue.

Figure 2

Diagram showing a qualitative scoring system for lesions assessed using strain elastography. The lesion’s margin on the corresponding B mode image is shown by the dotted line. Lesions are visually graded from ES1 to ES5 based on the relative proportion of low strain (red) and high strain (green) areas. Low and high ES scores suggest a soft and stiff lesion compared with the surrounding tissue, respectively.

Figure 3

Strain elastogram with the corresponding gray-scale US image of a benign hyperplastic thyroid nodule (white arrow) showing a colour-coded map and ROIs placed to calculate the strain ratio. An ROI is placed in the nodule and the surrounding parenchyma. The SR was 1.43, which indicates a stiffer lesion than the normal parenchyma.

Figure 4

Schematic illustration of shear wave imaging. (a) An elastography ROI (black box) is placed over a mass on the gray-scale US image. (b) High-intensity acoustic impulses (orange arrows) are transmitted into the tissue, which induce shear waves (blue). (c) Diagnostic impulses (green arrows) track shear wave propagation. In this example, shear waves travel faster in the mass than the background parenchyma. (d) The SWE map displays the mass on the basis of its stiffness contrast compared with the adjacent parenchyma.

Figure 5

Longitudinal US image of a thyroid lobe in a patient with multinodular goitre showing ARFI measurement in a region of normal thyroid parenchyma (white arrow). The ROI for ARFI measurement is a square box of fixed dimensions (∼5 × 5 mm). The mean ARFI velocity for the ROI is 1.79 m/s. This system does not generate elastograms. Image provided courtesy of Dr Chander Lulia, Ria Clinic, Mumbai.

Figure 6

Transverse gray-scale US image with corresponding shear wave elastogram of a hypoechoic thyroid nodule (white arrow), which was confirmed to be a benign hyperplastic degenerative nodule. ROIs are placed in the nodule and adjacent parenchyma (black arrow). The mean SWE stiffness of the nodule is 11.1 kPa and of the parenchyma is 16.4 kPa.

Figure 7

Transverse gray-scale US image with corresponding shear wave elastogram of a hypoechoic parotid nodule (black arrow), which was confirmed to be a pleomorphic adenoma. There is focal stress concentration in the superficial tissues overlying the nodule, seen here as regions of red colour on the elastogram (white arrows). Some of the high stiffness areas appear to extend into the superficial aspect of the nodule. Stress concentration may be problematic for assessing superficial head and neck lesions that produce a marked convex bulge of the skin.