Thyroid and Neck Ultrasound

Thyroid and neck ultrasound uses a high frequency (10 to 15 MHz, up to 18 MHz) linear probe to image the thyroid gland, parathyroid glands, cervical lymph nodes, salivary glands (parotid and submandibular), neck vessels (carotid and jugular), and soft tissue of the neck. The patient lies supine with a pillow under the shoulders and the neck extended.

Grey scale imaging shows size, echotexture, nodules and margins; colour and power Doppler show vascularity; elastography shows tissue stiffness; and ultrasound control directs fine needle aspiration cytology or core biopsy. The thyroid is measured in three planes (volume by the ellipsoid formula: length x width x depth x 0.52 for each lobe).

The examination takes 10 to 20 minutes, with no radiation and no contrast.

Thyroid nodules are very common: palpable nodules occur in 3 to 7% of adults, and ultrasound shows nodules in 20 to 70% of adults (more in women and the elderly). Only 5 to 10% of nodules are malignant. Thyroid cancer incidence is rising, mostly small papillary cancers.

Ultrasound is the first line test for thyroid disease because it distinguishes cystic, solid and mixed nodules, identifies suspicious features, directs biopsy, and assesses neck nodes; it replaces radionuclide scans for most purposes. Neck ultrasound is also first line for neck lumps, lymphadenopathy, salivary gland disease, and parathyroid localisation.

Indications. Palpable thyroid nodule, goitre or neck swelling. Nodule found incidentally on CT, MR or PET. Abnormal thyroid function (hyperthyroidism or hypothyroidism) with a goitre or nodule, thyroiditis. Staging and follow up of thyroid cancer (neck node mapping, post-operative bed, and recurrence).

Neck lymphadenopathy of unknown cause, lymphoma, metastases and infection (tuberculosis). Primary hyperparathyroidism (localisation of parathyroid adenoma). Salivary gland swelling, stones and masses. Neck masses in children (thyroglossal cyst, branchial cyst, lymphatic malformation, haemangioma). Vocal cord palsy and neck compression symptoms (with CT or MR).

Ultrasound control of fine needle aspiration, core biopsy, ethanol ablation of cysts, and drainage of neck abscess. Surveillance of known benign nodules. Not for routine screening of asymptomatic people.

Contraindications and precautions. None absolute. Patients should remove necklaces and the neck is exposed. Neck extension may be difficult in patients with cervical spine disease, large goitre, kyphosis or respiratory distress, and sitting is used.

Biopsy needs checking of coagulation and anticoagulation (fine needle aspiration is safe on aspirin; anticoagulants are handled as the local rule requires) and consent. The thyroid, carotid and jugular veins are recognised to avoid vascular injury. Doppler pressure should be light to avoid compressing small veins.

Preparation and technique. No fasting is needed. The thyroid is scanned in transverse and longitudinal planes from the isthmus to the upper and lower poles, and nodules are measured in three dimensions and described (composition, echogenicity, margins, shape, calcifications, vascularity, and extrathyroidal extension).

Cervical lymph node levels I to VII are scanned systematically, with the short axis, shape, hilum, cortex, calcifications, cystic change and vascularity noted. Parathyroids are sought posterior to the lower and upper poles of the thyroid. Salivary glands are scanned in two planes with the duct.

Normal appearances. The thyroid has two lobes joined by an isthmus (3 to 6 mm thick). It is homogeneous, with medium-to-high echogenicity (brighter than the adjacent strap muscles), a fine echotexture and a thin capsule. Lobe size is 4 to 6 cm long, 1 to 2 cm wide and 1 to 2 cm deep.

Volume is 6 to 12 mL for each lobe in women and 8 to 14 mL in men (total 10 to 20 mL), larger in iodine deficient regions. Vascularity is mild to moderate on Doppler. Normal parathyroid glands (3 to 5 mm) are rarely seen. Normal lymph nodes are oval, under 10 mm in short axis (up to 15 mm in the submandibular and jugulodigastric regions), with a hypoechoic cortex and echogenic fatty hilum.

The salivary glands are homogeneous, with the parotid brighter than muscle and the submandibular gland similar to the thyroid. The carotid artery is pulsatile, with a thin wall, and the internal jugular vein is compressible.

Interpretation.

• Thyroid nodules are classified by composition (cystic, spongiform, mixed, solid), echogenicity (hyper-, iso-, hypo-, very hypoechoic), shape (taller than wide), margin (smooth, ill defined, lobulated, irregular, extrathyroidal extension) and echogenic foci (macrocalcifications, rim calcifications, punctate echogenic foci). Structured risk stratification systems combine these features into categories that decide on biopsy.
• Benign features: purely cystic nodules, spongiform nodules (over 50% small cystic spaces), isoechoic nodules with a halo and smooth margins, and comet tail artefacts in colloid.
• Suspicious features of malignancy: solid hypoechoic or very hypoechoic nodule, irregular or microlobulated margins, taller than wide shape, microcalcifications (punctate echogenic foci), extrathyroidal extension, and suspicious cervical nodes. Papillary carcinoma is the commonest (about 85%), follicular carcinoma shows a thick halo, medullary carcinoma shows calcifications and associated nodal disease, and anaplastic carcinoma shows a rapidly growing hypoechoic invasive mass.
• Diffuse disease: Graves disease (enlarged, hypoechoic gland with a marked increase in blood flow, "thyroid inferno" on colour Doppler), Hashimoto thyroiditis (heterogeneous hypoechoic gland, pseudonodules, fibrous septa, and micronodules; later small, atrophic gland).
• Subacute thyroiditis (focal hypoechoic painful areas with reduced flow), and multinodular goitre (multiple nodules of varying echogenicity, cysts, calcifications, retrosternal extension).
• Parathyroid adenoma: oval, homogeneous hypoechoic mass posterior to the thyroid with a feeding vessel (polar vessel) on Doppler; hyperplasia involves several glands; the lesion is localised with sestamibi scan or four dimensional CT if ultrasound is negative.
• Cervical lymph nodes: reactive nodes (oval, hilum preserved, mild hypervascularity at the hilum), metastatic nodes (round, short axis over 10 mm, loss of the hilum, cystic change or microcalcifications in papillary thyroid cancer, peripheral vascularity).
• Lymphoma (large, very hypoechoic, "pseudo cystic" with reticular internal echoes and increased vascularity), tuberculous nodes (matted, necrotic centre, calcification, sinus tracts), and nodes with extracapsular spread.
• Neck masses: thyroglossal duct cyst (midline cystic mass near the hyoid), branchial cleft cyst (lateral anechoic or hypoechoic cyst), lymphatic malformation (multiloculated cystic mass), haemangioma (vascular mass), lipoma, carotid body tumour (hypervascular mass at the carotid bifurcation, splaying vessels), and abscess (complex collection).
• Salivary glands: stones (echogenic foci with shadow, dilated duct), sialadenitis (enlarged hypoechoic gland), Sjogren syndrome (multiple small hypoechoic areas), and tumours (pleomorphic adenoma: well defined lobulated hypoechoic mass with posterior enhancement; malignancy: ill defined, heterogeneous mass).
• Vascular: carotid plaque and stenosis, jugular vein thrombosis, and pseudoaneurysm.
• Vocal cord and larynx: limited; cord movement is seen in selected cases.

Critical values and findings needing immediate action.

• Rapidly enlarging neck mass with airway compromise (anaplastic carcinoma, haemorrhage into a nodule, abscess, retrosternal goitre): urgent airway assessment, CT, and surgical or oncological care.
• Deep neck abscess or Ludwig angina: airway protection, intravenous antibiotics and drainage.
• Highly suspicious nodule with suspicious nodes: fine needle aspiration cytology and referral to the thyroid multidisciplinary team.
• Thyroid cancer with extrathyroidal extension or vocal cord palsy: urgent staging and surgery.
• Suspected lymphoma (large, rubbery, hypoechoic nodes with B symptoms): urgent core biopsy, not fine needle aspiration alone.
• Thyrotoxic crisis with a goitre: medical emergency (beta blocker, antithyroid drugs, steroids).
• Haematoma after biopsy or neck trauma with expanding swelling: urgent surgical assessment.
• Jugular vein thrombosis with sepsis (Lemierre syndrome): antibiotics and anticoagulation.
• Carotid dissection or occlusion: stroke team.

Limitations and artefacts.

• Ultrasound cannot reliably separate benign and malignant nodules (follicular adenoma and carcinoma overlap; diagnosis needs histology of capsule and vascular invasion).
• Suspicious features have a sensitivity of 60 to 90% with a false positive rate of 10 to 30%.
• Cytology (fine needle aspiration with reporting categories) is required for nodules over size thresholds (usually 1 to 1.5 cm for suspicious nodules and 2 to 2.5 cm for low risk nodules).
• Size thresholds for biopsy avoid overdiagnosis of small papillary microcarcinomas (under 1 cm) that rarely progress.
• Retrosternal extension, deep structures behind the trachea, clavicle, sternum and air filled spaces are not seen; CT or MR is added.
• Operator dependence is considerable in nodule description, node assessment and measurements.
• Calcification can shadow the posterior part of a nodule; large nodules with partial views may hide solid components.
• Pseudonodules in thyroiditis, parathyroid adenoma, a lymph node or oesophagus (the oesophagus behind the left lobe contains gas) mimic thyroid nodules.
• Reactive lymph nodes are common in the neck (children), and small nodes with preserved hilum seldom need further tests.
• Normal ultrasound does not exclude microscopic nodal metastases.
• Thyroid volume depends on iodine intake, and age.
• Biopsy: non-diagnostic samples (10 to 20% of cases) need repeat, and indeterminate cytology (follicular lesions, 15 to 30%) requires molecular testing or diagnostic surgery.

Special groups.

• Children: thyroid nodules are less common but more likely to be malignant (20 to 25%); neck masses are usually reactive nodes, cysts or malformations; radiation history and family history raise risk.
• Pregnancy: ultrasound is safe and first line; thyroid nodules are managed expectantly and surgery is deferred to the second trimester or after delivery.
• Older adults: multinodular goitre, incidental nodules (follow low risk nodules rather than biopsy) and compressive symptoms.
• Patients with prior head and neck radiation or family history (multiple endocrine neoplasia, familial medullary and papillary cancer): lower thresholds for biopsy and surveillance.
• Patients with hyperparathyroidism: combined thyroid and parathyroid assessment, because thyroid nodules often coexist.
• Regions: iodine deficient areas have endemic goitre and multinodular disease; tuberculous and HIV-related lymphadenopathy, Burkitt lymphoma and nasopharyngeal carcinoma metastases, and late presentation of thyroid cancer are common; ultrasound with fine needle aspiration is often the only available diagnostic route.

Follow up tests.

• Fine needle aspiration cytology or core biopsy of suspicious nodules and nodes
• Thyroid function tests (thyroid stimulating hormone, free thyroxine, free triiodothyronine)
• Thyroid antibodies (anti-thyroid peroxidase, anti-thyroglobulin, thyrotropin receptor antibody)
• Calcitonin (suspected medullary cancer), thyroglobulin (after thyroidectomy)
• Serum calcium, parathyroid hormone and vitamin D (hyperparathyroidism)
• Radionuclide thyroid scan for hyperfunctioning nodules
• CT or MR of the neck and chest for retrosternal extension and staging
• Sestamibi scan or four dimensional CT for parathyroid localisation
• Laryngoscopy for vocal cord movement
• Molecular testing of indeterminate cytology
• Tuberculosis tests and HIV test, full blood count
• Referral to endocrinology, head and neck surgery or endocrine surgery