Application of Thyroid Ultrasound
Ultrasonography is generally the first choice and the most sensitive imaging modality for diagnosing intrathyroid lesions. Because of its superficial location, the thyroid gland is ideally suited for high-frequency sonography (using 7-13 MHz transducer) which facilitates the detection of clinically non-palpable nodules of 2-3 mm size and allows a more accurate morphological characterization of the lesion. It is also used to determine the size and number of thyroid nodules, to assess the volume of thyroid tissue in cases of thyromegaly, and to differentiate thyroid masses from adjacent non-thyroid masses.
Addition of color and spectral Doppler imaging that determines the vascular pattern of thyroid diseases has been found to be very useful tool in screening the thyroid nodule for malignancy. A nodule with exclusively central vascular pattern is characterized as malignant, while a nodule with a predominantly perinodular pattern is generally benign. The gray scale ultrasonographic pattern associated with thyroid carcinoma includes a solid, hypoechoic mass that is taller than wide, has an irregular margin and microcalcifications but absent halo sign. The improved gray scale and Doppler sonography has increased the accuracy and specificity of ultrasound for thyroid diseases [Figures 1-2]. This combination is also helpful in evaluation of cervical lymphadenopathy. Metastatic lymph nodes appear hypoechoic and show lack of central hilar echogenicity and vascularity.
Figure 1: Hashimoto’s thyroiditis. Transverse gray-scale ultrasound (a) and color Doppler (b) neck, of a 35-year-old female patient, who presented with features of hypothyroidism and had antithyroid antibodies positive for the disease, demonstrates diffuse enlargement of thyroid gland with linear echogenic fibrous bands (arrowheads) but normal vascularity. Note a small hypoechoic lymph node (arrow) in posterior aspect of inferior pole of left lobe of the thyroid gland.
Figure 2: Follicular lesion thyroid. Transverse gray-scale ultrasound (a) and color Doppler (b) neck, of a 40-year-old female patient, shows a large well circumscribed iso-hypoechoic solid thyroid nodule with multiple internal cystic spaces (arrow) and both central and peripheral vascularity.