**3.2 Shear-wave elastography**

Shear-wave elastography provides the quantitative measurement that SE does not offer. It is also less dependent on the operator and, consequently, is more reproducible [72]. These dynamic techniques comprise ARFI imaging, point-SWE (pSWE), and 2D-SWE, and rely on acoustic impulses from the US probe that induce tissue movement and generate transverse shear waves. The quantitative elasticity measurement is obtained by assessing the shear wave speed, measured in meters per second or the elasticity index (Young's module) measured in kilopascals [21, 50].

Transient elastography integrates the US transducer and an exterior vibrating "punch" to create shear waves. It is largely employed (FibroScan and Echosens) for evaluating liver fibrosis but is not feasible for thyroid evaluation [73].

#### **Figure 6.**

*2D-SWE elasticity parameters of a thyroid nodule with stiff areas displayed by the Hologic SuperSonic Mach 30 equipment, Aixplorer. Q-box parameters: EI mean = 41.6 kPa; med = 42.3 kPa; min = 23.8 kPa, max = 55.7 kPa; standard deviation (SD) = 6.2 kPa; ROI at 1.4-cm depth; ROI diameter = 10 mm.*

### *Elastography Methods in the Prediction of Malignancy in Thyroid Nodules DOI: http://dx.doi.org/10.5772/intechopen.104261*

In monoplane SWE (point-SWE—pSWE), the ARFI mechanically stimulates the tissue in the ROI applying acoustic push pulses that create local tissue displacement in the axially and shear wave (SW) velocity is estimated (m/s), providing a numerical value (Siemens, VirtualTouch Quantification, VTQ; Phillips ElastPQ ) [73].

Biplane SWE (2D SWE) and 3D SWE provide a real-time imaging of a quantitative color elastogram superimposed over 2B images and an estimation of SW speed. Supersonic shear wave employs focused ultrasonic beams, which spread through the entire imaging region and show on a color map the speed of the SW or plainly the

#### **Figure 7.**

*2D SWE qualitative images: Negative results: A—Entirely soft nodule (homogeneously blue) and B—Mostly soft nodule (heterogeneously blue with green spots), and positive results: C—Nodule with stiff areas (heterogeneous, with patches green, yellow, and red) and D—Completely stiff nodules (heterogeneous multicolored with irregular red, orange, green, and blue areas).*

#### **Figure 8.**

*The nodule-to parenchyma SWE ratio measured on a Hologic Supersonic Mach 30 device—The Q-box ratio. A— Soft nodule, elasticity similar to surrounding healthy thyroid tissue (EI mean = 13.5 kPA and Q-box ratio = 1); B—Nodule with heterogeneous elasticity map, with stiffer areas with EI mean = 50.8 kPa and Q-box ratio = 4.2.*

elasticity index (kPa) for each pixel in the ROI. A set of parameters can be quantified in the ROI: the maximum, minimum, and mean E, and the standard deviation, as displayed in **Figure 6** [72, 73]. Currently accessible available technologies on US equipment include: SuperSonic Imagine—2D-SWE; Siemens—Virtual Touch Imaging Quantification, VTIQ; Toshiba—Acoustic Structure Quantification; Philips—SWE; and GE Healthcare—2D-SWE [73].

#### *Elastography Methods in the Prediction of Malignancy in Thyroid Nodules DOI: http://dx.doi.org/10.5772/intechopen.104261*

The qualitative assessment can be made also for 2D SWE. A group from China proposed a modified four-category scale adapted to the physical characteristics of SWE technique and measurements. Patterns 1 (homogeneous lesion with no meaningful color signal corresponding to high stiffness) and 2 (high stiffness signal limited to the capsule surroundings) are interpreted as low risk. Patterns 3 (marginal stiffness) and 4 (interior stiffness) are viewed as high risk; the authors described a very good diagnostic value with 89.1% sensitivity, 74.6% specificity, and the AUROC of 0.79 [74].

A standardized, systematic assessment of qualitative, color-coded elasticity maps is of great importance for discarding artifacts and ensuring reliable quantitative measurements of elasticity [21, 75]. **Figure 7** illustrates the examples of 2D SWE images for soft (A, B) and hard (C, D) thyroid nodules.

Comparable to SE, an SWE ratio can be generated by comparing the stiffness of the nodule to the bordering normal parenchyma or neighboring muscle [24]. **Figure 8** displays the SWE ratio on a SuperSonic Mach 30 machine (the Q-Box™ ratio).

The cutoff values for the elasticity index reported in SWE studies are also different. For Supersonic 2D SWE, the most accurate parameter, as described in most studies, was the E mean, and a poorer diagnostic value was obtained for the E mean. For the E mean, the following cutoffs were reported: ≥42.1 kPa with 76.9% sensitivity and 71.1% specificity [76]; ≥65 kPa with 71% accuracy [77]; ≥39.2 kPa with 81% accuracy [78]; ≥34.5 kPa with 84% sensitivity, 78% specificity, and 82% accuracy [79]; and ≥ 24.6 kPa with 84% accuracy [80]. The evaluation is not standardized; thus, the number of determinations varied between 3 and 10 per patient; the size of the ROI also varied between 2 mm and 10 mm in the reported studies.

#### **4. Strain versus shear-wave elastography**

Both SE and SWE are efficacious instruments in the stratification of malignancy risk in thyroid nodules, used complementary to grayscale assessment, as specified by the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB) guidelines and proven by diverse studies [24], with a broad range of values for sensitivity and specificity resulting from the comparison of the two elastography methods.

Although the majority of literature data suggest that SE is slightly superior in diagnosing thyroid cancer, there is presently no consensus about which technique is superior, and both SWE and SE demonstrated to present important additional value to the classic US assessment in the preoperative examination strategy for thyroid nodules.

To date, only a few small studies have provided a head-to-head comparison of SE and SWE in the same population. More data are available for comparing the two methods. A large meta-analysis including 71 studies and 16,624 patients revealed that SE is hardly better in discriminating thyroid malignancy. The pooled results included the sensitivity of 82.9% for SE and of 78.4% for SWE and the pooled specificity of 82.8% for SE and of 82.4% for SWE [81]. Another meta-analysis assessing 22 studies revealed a pooled sensitivity of 79% (95% confidence interval (CI): 0.730–0.840) and a specificity of 87% (95% CI: 0.790–0.920) for SWE. On the other hand, a pooled sensitivity of 84% (95% CI: 0.760–0.900) and a specificity of 90% (95% CI: 0.850–0.940) were reported for SE, considerably higher than the values recorded for SWE (p < 0.05) [61].

2D SWE evaluation is superior when it comes to the assessment of nodules that coexist with thyroid autoimmunity, while SE has lower feasibility in this particular setting [81, 82]. The operator's experience in performing each technique is essential, especially for strain elastography evaluation, as SWE proved better reproducibility. Even so, factors such as the manual compression on the US probe may influence the measurements. In SWE, the most common evaluation errors are artifacts generated by the operator. For these reasons, although SWE is easier to learn, it is important that both the elastography techniques are always performed by experienced examiners [83].
