Imaging of Thyroid Gland Bengi Gürses, M.D. Yeditepe University, Medical Faculty Department of Radiology

Anatomy Located in the anteroinferior part of the neck, outlined by muscle, trachea, esophagus, carotid arteries and jugular veins Two lobes located along either side of the trachea, connected in the midline by the isthmus Size and shape can vary among individuals In adults the mean length is 40-60mm, mean AP diameter is 13-18mm. Mean thickness of the isthmus 4-6 mm

Imaging Modalities Ultrasound Computed Tomography Magnetic Resonance Imaging

Ultrasound Primary radiological imaging modality US is an accurate method to calculate thyroid volume Normal thyroid parenchyma has homogeneous medium-high level echogenicity In color Doppler studies, the gland is seen hypervascular

Ultrasound A signal generator is combined with a transducer. Piezoelectric crystals in the signal generator convert electricity into high-frequency sound waves, which are sent into tissues. The tissues scatter, reflect, and absorb the sound waves to various degrees. The sound waves that are reflected back (echoes) are converted into electric signals. A computer analyzes the signals and displays the information on a screen. Ultrasonography is portable, widely available, and safe. No radiation is used.

Doppler US Doppler US is used to assess blood flow. Doppler ultrasonography uses the Doppler effect (alteration of sound frequency by reflection off a moving object). The moving objects are RBCs in blood. Direction and velocity of blood flow can be determined by analyzing changes in the frequency of sound waves. Changes in frequency of the reflected sound waves are converted into images showing blood flow direction and velocity.

Disadvantages Quality of images depends on the skills of the operator. Obtaining clear images of the target structures can be technically difficult in overweight patients. Ultrasonography cannot be used to image through bone or gas, so certain images may be difficult to obtain.

Thyroid Diseases Congenital Anomalies Nodular Thyroid Diseases Diffuse Thyroid Diseases

Congenital Anomalies Agenesis of one lobe or the whole gland Hypoplasia Ectopia (lingual, suprahyoid,pelvic)

Ectopia

Pyramidal lobe

Thyroid Diseases Congenital Anomalies Nodular Thyroid Diseases Benign nodule(s) Malignant nodule(s) Carcinoma Papillary, Follicular, Medullary, Anaplastic Diffuse Thyroid Diseases

Nodular Thyroid Disease Thyroid nodule  Discrete lesion within the thyroid gland that is sonographically distinguishable from the adjacent parenchyma Thyroid nodules are very common Observed at 50% of the population < 7% of thyroid nodules are malignant The imaging modality of choice for the investigation of thyroid nodules is US.

Thyroid Malignancy

Thyroid Malignancy Papillary Thyroid Carcinoma  Excellent prognosis, 20ys 90-95% Follicular Thyroid Carcinoma  Excellent prognosis, 20ys 70% Medullary Thyroid Carcinoma  More aggressive, 10ys 42-90%. May be familial or as a part of MENII syndrome Anaplastic Thyroid Carcinoma  Poor prognosis, 5ys 5%

US Features Suggestive of Malignancy Suspicious margins, contour and shape Hypoechoic Solid Nodule Calcifications Vascularity Local invasion and lymph node metastases

Margins, Contour and Shape Complete uniform halo around a nodule  Suggestive of benignity (spec 95%) Halo / Hypoechoic rim  Pseudocapsule of fibrous connective tissue, compressed thyroid parenchyma and chronic inflammatory infiltrates HOWEVER!!! A halo is absent in more than half of all benign nodules 10-24 % of papillary carcinomas have complete / incomplete halo

Margins, Contour and Shape Ill-defined nodule  >50% of the border is not clearly demarcated!! Suggests malignant infiltration of the parenchyma No pseudocapsule formation Sens 7-97% Some papillary carcinomas may have well-demarcated margin THEREFORE!! Unless frank invasion is demonstrated US appearance of the nodule margins alone is unreliable for determining malignancy or benignity!!

Margins, Contour and Shape Shape; Not well-described in the literature Oval shape  benign Solid thyroid nodule taller than wide  93% specificity for malignancy Thought to be due to centrifugal tendency in growth

Hypoechoic Solid Nodule Malignant nodules (carcinoma, lymphoma) typically appear solid and hypoechoic When compared with thyroid parenchyma Sens 87%, spec 16-27% Present in also 55% of benign nodules!! When compared with strap muscles (very hypoechoic) Sens 12%, spec 94% Marked hypoechogenicity is very suggestive of malignancy!!

Calcification 2 types Specificity for malignancy 85.8% - 95% Microcalcification Coarse calcification Specificity for malignancy 85.8% - 95% Found in 29%-59% of all thyroid carcinomas, most commonly in papillary type. May be seen in benign conditions such as follicular adenoma, Hashimoto thyroiditis

Calcification Coarse calcification Secondary to tissue necrosis Malignancy rate 75% Cause posterior acoustic shadowing Inspissated colloid calcifications cause reverberation artifact  Benign!! Peripheral calcification in MNG >> malignancy

COMPLETELY AVASCULAR NODULE IS UNLIKELY TO BE MALIGNANT!!! Vascularity Detected with color / power Doppler US Intrinsic hypervascularity  flow in the central part of the tm greater than the surrounding parenchyma 69-72% of all thyroid malignancies However; >50% of hypervascular solid thyroid lesions were benign!! COMPLETELY AVASCULAR NODULE IS UNLIKELY TO BE MALIGNANT!!!

Local invasion and lymph node metastases Highly specific for thyroid malignancy!! Clinically; dyspnea, hoarseness, dysphagia Aggressive local invasion  anaplastic thyroid carcinoma, lymphoma, sarcoma.

Local invasion and lymph node metastases Metastases to regional cervical lymph nodes 19,4% in all thyroid malignancies Most common in papillary type (40-90%) Medullary type (50%) Follicular type – very rare!! Should be a routine part of US evaluation!!

Metastatic lymph nodes Rounded bulging shape Increased size Replaced fatty hilum Irregular margins Heterogeneous echotexture Vascularity throughout the lymph node (instead of normal central hilar vessels)

Nonspecific US features Size of nodule Nodules > 4 cm – more likely to be malignant Benign nodules may be very large in size!! Number of nodules Interval growth of a nodule Poor indicator of malignancy The exception; rapid interval growth!!

US Criteria (Benign vs Malignant) Hyper-isoechoic Halo/Hypoechoic rim Peripheric (eggshell) calcification Peripheral vascularity Oval shape LAP Ø Malignant Marked hypoechoic Irregular border /microlobulation Intranodular microcalcification Internal vascularity Taller than wide LAP 

FNAB of Incidental Thyroid Nodules Controversial!! High prevalence of benign nodules X Low incidence of thyroid carcinoma Low rate of mortality of small thyroid carcinomas Three sets of guidelines for FNAB (fine-needle aspiration biopsy) of thyroid nodules Kim criteria American Association of Clinical Endocrinologists Society of Radiologists in Ultrasound

FNAB of Incidental Thyroid Nodules Kim Criteria A nodule should have at least one of the following findings Marked hypoechogenicity, irregular or microlobulated margins, microcalcifications, length greater than width. American Association of Clinical Endocrinologists A hypoechoic nodule with at least one additional feature should be biopsied Irregular margins, length greater than width, microcalcifications. Society of Radiologists in Ultrasound FNAB should be performed Microcalcifications within a nodule (>1 cm) Coarse calcification within a nodule / Solid nodule (> 1,5 cm) Mixed cystic and solid nodule (> 2 cm) Abnormal lymph node Substantial growth

FNAB of Incidental Thyroid Nodules In a recent study by Ahn et al. (AJR2010;194:31-37) Kim Criteria and American Association of Clinical Endocrinologists are more accurate!! Kim Criteria  higher sensitivity!! AACE  higher specificity!!

Thyroid Diseases Congenital Anomalies Nodular Thyroid Diseases Diffuse Thyroid Diseases Acute suppurative thyroiditis Subacute granulomatous thyroiditis Hashimoto’s (chronic lymphocytic) thyroiditis Painless (silent) thyroiditis Graves’ disease

Thyroiditis Acute suppurative thyroiditis  Rare inflammatory disease, caused by bacterial infection, usually affects children. On US; frank thyroid abscess. Subacute granulomatous (DeQuervain) thyroiditis  Self-limiting viral disease. Fever, enlargement of gland, pain on palpation. Hashimoto’s (chronic lymphocytic) thyroiditis  Most common type. Autoimmune disease. Young or middle-aged woman. Painless, diffuse enlargement of the gland Painless (silent) thyroiditis  Lymphocytic infiltration in thyroid. Variant form of Hashimoto. No tenderness. May be seen in the postpartum period. Graves’ disease  Extensive lymphocytic infiltration of the gland. Usually characterized by thyrotoxicosis.

US Features in Thyroiditis Diffuse glandular enlargement More hypoechoic than normal parenchyma Coarsened parenchymal echotexture Multiple discrete hypoechoic micronodules from 1 to 6 mm in diameter (pseudonodular appearance). Fibrotic septations – pseudolobulated appearance Vascularity   Subacute granulomatous (DeQuervain) thyroiditis Vascularity   Graves’ disease