## 1 Introduction  
Grey scale ultrasound (GSU) is the first-line imaging investigation for the assessment of thyroid nodules [1; 2]. Although certain GSU features have been reported to be associated with thyroid malignancy (microcalcification, hypoechogenicity, irregular shape, tall to width ratio \(>\)1, absent halo sign, and irregular margins) [3; 4; 5], these features may also be found in benign nodules [6; 7]. To date, none of the GSU features are known to exhibit a high sensitivity and specificity in differentiating benign and malignant thyroid nodules [8; 9; 10]. Fine-needle aspiration cytology (FNAC) is recommended by the American Thyroid Association to evaluate thyroid nodules when the GSU findings are inconclusive [11]. However, potential sampling and analytical errors limit its application in establishing correct diagnosis in some cases such as small thyroid nodules [12; 13]. Only about 65-75% of thyroid nodules can be correctly diagnosed by FNAC [14], whilst the remaining 25-35% of nodules yield indeterminate pathology or non-diagnosticresults [15]. Both GSU and FNAC have wide ranges of sensitivity (52-97% and 54-90%, respectively), and specificity (26-83% and 60-98%, respectively) in distinguishing benign and malignant thyroid nodules [10,16]. Power Doppler and color Doppler ultrasound evaluate the vascularity of thyroid nodules. It is generally believed that hypervascularity is a feature of thyroid malignancy [17,18]. Some other studies suggested that vascular pattern of thyroid nodules can be useful to predict the thyroid malignancy e.g., thyroid nodules with dominant central vascularity are frequently associated with malignant thyroid nodules, whereas benign thyroid nodules tend to have peripheral vascularity or appear avascular [19,20]. Ultrasound elastography is a non-invasive imaging technique that measures tissue stiffness and provides color-coded elasticity map. In strain elastography (SE), the elasticity color map and strain ratio (SR) provide qualitative and semi-quantitative estimates of elasticity distribution, respectively. SE has limited clinical value due to the need of subjective interpretation of elastogram and compressive maneuvers. Contrarily, shear wave elastography (SWE) is less operator-dependent and more reproducible than SE, and it provides quantitative assessment in the form of elasticity indices (\(E_{\text{maximum}}\), \(E_{\text{mean}}\), \(E_{\text{minimum}}\)). SWE uses focused ultrasound impulses at varying depths in tissue to induce tissue displacement and results in the generation of shear waves which propagate laterally [21]. Shear waves travel faster in harder medium than in softer counterpart, and their velocity is directly proportional to the square root of Young's modulus. Young's modulus measures the tissue stiffness by demonstrating the relationship between stress (applied force) and strain (resultant deformation) [22,23]. Stiffer tissues exhibit higher Young's modulus as compared to softer tissues. In the current study, a higher Young's modulus for malignant nodules (85.2 \(\pm\) 8.1 kPa) than benign nodule (50.3 \(\pm\) 3.1 kPa) is observed (\(p\) < 0.05). Assuming the tissue density 1 g/cm\({}^{3}\), the Poisson's ratio was 0.5 [24,25]. The propagation speed of shear waves is tracked by the ultrafast sonographic tracking technique [26]. By evaluating the speed of shear waves in the tissue, the tissue stiffness can be determined. In comparison to the normal thyroid parenchyma, most malignant thyroid nodules have firm stroma due to the presence of excessive collagen, excessive myofibblast, and desmoplastic transformation, hence resisting tissue strain upon stress [10,27,28]. Therefore, most of the malignant thyroid nodules tend to be stiffer. As the stiffness of thyroid nodules can be evaluated with shear wave elastography, malignant thyroid nodules tend to have higher shear wave elastography index. In the current study, we hypothesize that malignant thyroid nodules are stiffer than benign nodules on SWE.