ULTRASOUND of SUBACUTE THYROIDITIS

WHAT MEANS POINT-SHEAR WAVE ELASTOGRAPHY [p-SWE]?

Ultrasound based-elastographic techniques are classified in: strain techniques and shear
wave elastography  techniques. Three  types of elastographic  techniques are  included
in  the  last category: Transient Elastography, point Shear Wave Elastography  (pSWE)
and shear wave elastography (SWE) imaging (including 2D-SWE and 3D-SWE). 

In the pSWE category two techniques are included: Acoustic Radiation Force Impulse (ARFI) elastography and ElastPQ.



Elastographic Techniques Based on Shear Waves Generated by the Acoustic Beam

These techniques have the advantage of being integrated into ultrasound systems; thus, conventional sonography, which is advised every 6 to 12 months in patients with chronic liver disease, could also be performed. As of today, for the assessment of liver stiffness, these techniques are commercially available in high-end ultrasound systems made by Philips Healthcare (Bothell, WA; ElastPQ), Siemens Medical Solutions (Mountain View, CA; Virtual Touch Tissue Quantification [VTTQ]), and SuperSonic Imagine, SA (Aix-en-Provence, France; ShearWave Elastography [SWE]). These techniques generate shear waves inside the liver by using radiation force from a focused ultrasound beam. The shear waves are generated near the region of interest in the liver parenchyma and not on the surface of the body, as happens with external vibration devices. The ultrasound system monitors shear wave propagation using a Doppler-like ultrasound technique and measures its velocity. The shear wave velocity is displayed in meters per second or kilopascals through the Young modulus. Unlike transient elastography, the measurements are not limited by the presence of ascites because the ultrasound beam, which generates the shear waves, propagates through fluids. With the VTTQ and ElastPQ techniques, the readings of the shear wave speed are made by using a small sample box (usually 0.5 × 1 cm); thus, a quantitative estimate of liver stiffness at a single location is obtained (Figures 2 and 3). They have been categorized as point–shear wave elastography.The SWE technique is based on an ultrafast ultrasound imaging approach that allows detailed monitoring of the shear waves in a large area of liver parenchyma with real-time color-coded elasticity imaging inside a sample box, and the measurement is obtained by placing a region of interest inside the sample box (Figure 4). This technique is 2-dimensional elastography.27 In all of the studies that have assessed the accuracy of the different devices in staging liver fibrosis, right intercostal access has been used. The patient is examined in the dorsal decubitus position with the right arm elevated above the head for optimal intercostal access in a resting respiratory position. Measurements are performed at least 1.5 to 2.0 cm beneath the Glisson capsule to avoid reverberation artifacts. In case of physical conditions affecting the signal to-noise ratio, the Philips and Siemens devices do not give any measurement. With the SuperSonic Imagine device, a measurement fails when no/little signals are obtained in the sample box for all of the acquisitions.

Siemens Technique (VTTQ)

The first one available was the Siemens technique, which is commonly referred to as acoustic radiation force impulse in the literature, which is technically the same force that generates shear waves for all 3 available techniques. Moreover, the term acoustic radiation force impulse is rather generic and does not identify shear wave–based methods. In fact, acoustic radiation force impulse push pulses are also used in strain imaging of other organs, such as the breast and thyroid. In recent years, the diagnostic accuracy of the VTTQ technology for quantification of liver stiffness, mainly in patients with chronic hepatitis C, has been investigated in several studies and a meta-analysis. The technology has shown high interobserver agreement, with an intraclass correlation coefficient of 0.86. Operator training does not seem to be required.The cutoff values obtained in a large meta-analysis were 1.34, 1.55, and 1.80 m/s for significant fibrosis (METAVIR fibrosis score of F2 or greater), severe fibrosis (METAVIR fibrosis score of F3 or greater), and cirrhosis (METAVIR fibrosis score of F4), respectively. In this meta-analysis, which included patients with several etiologies of chronic liver disease, the diagnostic accuracy was comparable with that of transient elastography for the assessment of severe fibrosis, whereas higher performance of transient elastography was seen for significant fibrosis and liver cirrhosis. In a study by Rizzo et al, the technique was significantly more accurate than transient elastography for diagnosing significant and severe fibrosis, whereas this difference was only marginal for cirrhosis.

SuperSonic Imagine Technique (SWE)

The reproducibility of the SWE method is very high, with intraobserver intraclass correlation coefficients of 0.95 and 0.93 for an expert and a novice operator, respectively, and interobserver agreement of 0.88. As for conventional sonography, it is user dependent; thus, it is recommended that at least 50 supervised scans and measurements should be performed by a novice operator to obtain consistent measurements. Values obtained in a small series of healthy participants ranged from 4.92 kPa (1.28 m/s) to 5.39 kPa (1.34 m/s). In a pilot study conducted on 121 patients with chronic hepatitis C undergoing liver biopsy, the optimal cutoff values were 7.1 kPa (1.54 m/s) for significant fibrosis (METAVIR fibrosis score of F2 or greater), 8.7 kPa (1.70 m/s) for advanced fibrosis (METAVIR fibrosis score of F3 or greater), and 10.4 kPa (1.86 m/s) for cirrhosis (METAVIR fibrosis score of F4), and the technique was more accurate than transient elastography in assessing significant fibrosis. In another study, with respect to transient elastography, the technique showed higher accuracy in assessing mild and intermediate stages of fibrosis.

Philips Technique (ElastPQ)

The ElastPQ technique was the most recent to enter the market; thus, only a few studies have been published so far. With this technique, liver stiffness values in healthy volunteers have been reported to be less than 4.0 kPa (1.15 m/s). Ling et al found that men had higher values than women (3.8 ± 0.7 versus 3.5 ± 0.4 kPa, or 1.13 ± 0.48 versus 1.08 ± 0.37 m/s) and liver stiffness was comparable with different probe positions, examiners, and age groups. In a series that comprised 88 patients with chronic viral hepatitis and 33 healthy volunteers, the technique compared favorably with transient elastography in staging liver fibrosis, and healthy volunteers showed significantly lower values than patients with nonsignificant fibrosis.

 REFERENCES:

Bamber J, Cosgrove D, Dietrich CF, et al. : EFSUMB guidelines and recommendations on the clinical use of ultrasound elastography, part 1: basic principles and technology. Ultraschall Med 2013; 34:169–184.

Ferraioli et al: Shear Wave Elastography for Evaluation of Liver Fibrosis, J Ultrasound Med 2014; 33:197–203 199