Shear-Wave Elastography in Diffuse Thyroid Diseases

*Cristina Mihaela Cepeha, Andreea Borlea, Corina Paul, Iulian Velea and Dana Stoian*

#### **Abstract**

The diagnosis and evaluation of diffuse thyroid pathologies is often a challenge for clinicians. Ultrasonography has an essential contribution in thyroid imaging, but elastography adds more accuracy. Frequently used in the evaluation of thyroid nodules, elastography has become a necessary tool in assessing the risk of malignancy. Diffuse thyroid pathologies such as Graves' disease, chronic autoimmune thyroiditis, and subacute thyroiditis, are diagnosed based on laboratory tests completed with imaging. Recently it has been shown that elastography is useful in the evaluation and differentiation of these cases due to the differences in elasticity. This chapter describes the general principles of shear-wave elastography, examination technique, features found in diffuse thyroid disease, but also the limitations of this type of investigation for a better understanding of its use in assessing diffuse thyroid pathology.

**Keywords:** thyroid, elastography, shear-wave elastography, ultrasonography, diffuse

#### **1. Introduction**

Optimal thyroid function is necessary for growth and development as well as for reproductive function. Iodine deficiency is the leading cause of thyroid dysfunction. In areas unaffected by iodine deficiency, thyroid dysfunction is due to autoimmunity. Thyroid pathology can be divided into nodular and diffuse pathology. One in 20 Americans will develop a thyroid disorder in their lifetime, with women being seven times more affected than men [1, 2]. In this chapter, we will address diffuse thyroid diseases (DTD) that can be divided into non-autoimmune (subacute thyroiditis, silent thyroiditis) and autoimmune diseases such as Hashimoto thyroiditis (HT) and Graves' disease (GD) [3].

Hashimoto thyroiditis is also known as chronic autoimmune thyroiditis (CAT) is considered the most common endocrine disorder, and autoimmune pathology, as well as it represents the most common cause of hypothyroidism. HT can be divided into primary and secondary based on its etiology. Primary HT includes classic form, juvenile form [4], fibrous form, painless thyroiditis, Hashitoxicosis, and IgG4-related form [5]. Secondary HT is often iatrogenic, for example, caused by interferon [6] or monoclonal antibodies [7]. The diagnosis is established by correlating the clinical manifestations with the presence of antithyroid peroxidase (ATPO) antibodies and

antithyroglobulin (ATG) antibodies correlated with the ultrasound aspect. Symptoms may vary from dysphonia, dysphagia, and dyspnea to systemic symptoms of hypothyroidism or may even be absent. The antibodies listed earlier are found in over 95% of HT cases, thus being an important diagnostic criterion [8]. The ultrasound image usually reveals hypoechogenicity and heterogeneity (**Figure 1**).

The presence of fibrous septae may explain the pseudolobulated appearance of the parenchyma. Micronodules may also be present. Increased vascularity may be observed using color Doppler. The volume of the thyroid gland is often increased, but can be normal or even decreased, atrophic in the final stages of the disease [9, 10].

Graves' disease is characterized by the presence of thyrotoxicosis, ophthalmopathy, and goiter, although not all three characteristics are always present. In iodinesufficient areas, GD accounts for 70–80% of cases of thyrotoxicosis, being more frequent in women over 50 years old. Symptoms of hyperthyroidism include irritability, palpitations, weight loss, shortness of breath, tremor, heat intolerance, sweating, and excessive fatigue. The diagnosis is usually based on anti-TSH-receptor antibodies (TRAB) correlated with the ultrasound appearance [11–13]. The echogenicity is usually decreased, and the appearance is not homogenous. Increased volume and high blood flow may also suggest the diagnosis [10, 12, 14].

**Figure 1.** *US image of a patient with CAT.*

#### *Shear-Wave Elastography in Diffuse Thyroid Diseases DOI: http://dx.doi.org/10.5772/intechopen.102636*

Subacute thyroiditis (SA) is usually caused by a viral infection with symptoms including mild fever, swelling, and pain in the neck area, irradiating to the ear or jaw and fatigability. Thyroid stimulating hormone (TSH) is usually suppressed and inflammatory markers are elevated [15]. The presence of focal or diffuse hypoechoicity together with diminished vascularization may suggest the diagnosis if it is associated with the mentioned symptoms [16].

Post-partum thyroiditis (PPT) occurs in the first year after giving birth and may occur after an induced or spontaneous abortion. PPT prevalence may vary from 1 to 18%, but usually is reported approximatively 5%. In general, PPT has two evolutionary stages, transient thyrotoxicosis due to tissue destruction followed by the phase of hypothyroidism with or without restauration to euthyroidism [17]. Ultrasonography (US) shows the hypoechogenic inhomogeneous texture of the thyroid with decreased vascularity [18].

The ultrasonographic examination is the most sensitive imaging method for evaluating the thyroid. The most important indications of the US are: confirmation of the presence of thyroid nodules, their measurement and characterization, evaluation of diffuse tissue changes, detection of post-operative residual tumors, screening for the patients at high-risk (multiple endocrine neoplasia, history of thyroid cancer, and neck irradiation), and the guidance of the fine needle aspiration (FNA). US is widely available, non-invasive, and reproducible [19–21].

A relatively new imaging technique is elastography, which adds value by assessing tissue elasticity. There are more types of elastography, but the two most used are strain elastography (SE) and shear-wave elastography (SWE). SE evaluates the stiffness by applying external pressure which deforms the tissue. The deformation is named strain. The pressure being exerted by the operator, SE requires longer training to obtain high-quality images. To measure the stiffness, SWE uses shock waves generated by the machine. Both methods have advantages and disadvantages [22, 23]. SWE will be further described in detail, emphasizing the principles, technique, and the value of this method as well as the peculiarities of diffuse thyroid diseases.

### **2. Shear-wave elastography**

#### **2.1 Principles**

SWE technique relies on the production and detection of shear-waves. The wave propagation velocity depends on tissue elasticity. Tissue deformation generated by the production of waves produces changes in the echo pattern. Tissue motion is monitored among the US probe in multiple locations. Shear waves are generated at low frequencies (10–2000 Hz) and their speed (~1–50 m/s) is related to tissue density [22, 24]. SWE is used for evaluation of various tissues: liver [25–27], kidney [28, 29], breast [30, 31], thyroid [32–34], prostate [35], and muscles and tendons [36]. There are three types of dynamic elastography: transient elastography (TE), point shearwave elastography (pSWE), and 2D-SWE [37].

Transient elastography, usually used for measuring liver stiffness, uses a mechanical push. The wave velocity is proportional to tissue fibrosis. The usefulness of TE was demonstrated in many studies [38, 39].

Compared to the previous technique, point shear-wave elastography has the advantage of image guidance so the operator can choose the best acoustic window to perform the measurements. It is an acoustic radiation force impulse (ARFI)-based

method that displays the elasticity as a numerical output (the average speed of shear wave within a region of interest [ROI], expressed in meters per second) [40, 41].

In 2D-SWE, multiple ARFI pulses generate shear waves on a larger area. The machine creates a colored map to display the stiffness. By convention, red is considered stiff and blue is considered soft. Quantitative results are expressed in meters/ second (wave propagation speed) or kilo-Pascals (kPa). 2D-SWE is a reproductible, quantitative, and operator-independent technique (**Figure 2**) [42, 43].
