This capability has expanded the use of ultrasound and resulted in the development of new ultrasound applications for both the diagnosis and therapy of thyroid and parathyroid disorders. Ultrasound guidance for needle biopsy of thyroid nodules has become routine. It is now being used to confirm the ultrasound diagnosis of parathyroid adenoma by measuring parathyroid hormone obtained with the needle placed in the lesion under ultrasound guidance.
This capability has expanded the use of ultrasound and resulted in the development of new ultrasound applications for both the diagnosis and therapy of thyroid and parathyroid disorders. Ultrasound guidance for needle biopsy of thyroid nodules has become routine. It is now being used to confirm the ultrasound diagnosis of parathyroid adenoma by measuring parathyroid hormone obtained with the needle placed in the lesion under ultrasound guidance. Likewise, non-palpable lymph nodes in the neck discovered by ultrasound and suspected of having metastatic carcinoma can be easily biopsied using ultrasound. The material can be submitted for both cytology and thyroglobulin (Tg) to confirm the diagnosis replacing more expensive imaging. Using these same ultrasound guidance techniques, several groups of investigators have developed methods of therapeutic ablation of tissue by chemical or physical means.This has resulted in an alternative to surgery for certain thyroid, parathyroid, and lymph node lesions.
Although high-resolution realtime ultrasound has been available since the early 1980s, it did not make a significant impact in the diagnosis and management of thyroid disease until the mid 1990s. Initially, an ultrasound of the thyroid entailed referring the patient to a radiology department where the ultrasound was performed by a sonographer who took spot films that were interpreted by the radiologist for the clinician.This delay in diagnosis and increased cost of going to another specialist deterred the application of ultrasound to the study of thyroid disorders. Furthermore, the separation of the realtime procedure from the clinician’s physical examination resulted in loss of information and hampered the appreciation of subtle changes that have proven valuable in treating thyroid patients.
Advances in ultrasound engineering and electronic technology in the 1990s make modern ultrasound more user-friendly. These advances coincide with a marked reduction in equipment cost that allows clinicians who treat thyroid patients to have access to a machine dedicated to thyroid ultrasound. The performance of realtime ultrasound by the examining physician who has taken a history, performed a physical examination, and anticipated what may be seen on ultrasound imaging prevents loss of information and allows the ultrasound findings to be integrated with the patients’ clinical palpation findings. The recent introduction of small linear phased-array transducers greatly facilitates ultrasound-guided fine-needle aspiration (UG FNA) biopsy, which decreases the number of inadequate biopsies. The increased convenience and the decreased cost of thyroid ultrasound by not having to refer the patient out to another level of specialty care make thyroid ultrasound an essential part of the examination of the thyroid patient. Thyroid Ultrasound
Realtime ultrasound has proven to be the most sensitive test to detect thyroid nodules. It can recognize nodules that are missed on physical examination, radioiodine scan, computed tomography (CT), or magnetic resonance imaging (MRI). Initially, ultrasound of the thyroid was used primarily to identify and locate nodules within the thyroid. This was practiced following the Chernobyl nuclear accident when ultrasound screening of children detected hundreds of cases of early thyroid cancer and allowed surgical cure. Ultrasound has also been used to identify aberrant anatomy, such as hemiagenesis of the thyroid, and to measure thyroid volume. In children, the thyroid volume correlates with iodine content in the diet and urinary iodine.Thus, it provides a quick and efficient method to identify iodine-deficient areas of the world allowing treatment of endemic goiter.
The primary concern when a patient presents with a thyroid nodule is whether the nodule is benign or malignant.Various ultrasound characteristics of thyroid nodules have proven to have predictive diagnostic value in determining which nodules are malignant and which are not (see Table 1). Among these characteristics of thyroid nodules are echogenecity, regularity of the border or margin around the nodule, presence and type of calcifications, vascular pattern using power Doppler, shape of the nodule, and the presence or absence of enlarged adjacent lymph nodes. The most reliable ultrasound finding that indicates benignity is the presence of a comet tail sign.This artifact caused by the refraction of sound waves by colloid in complex/cystic nodules is pathognomonic of a benign lesion. However, care is needed to differentiate this comet tail artifact from microcalcifications that are highly suspicious of malignancy. Ultrasound also allows observation of changes in thyroid nodules over time that often helps in making the decision regarding surgery. For example, a nodule that is decreasing in size is unlikely to be malignant or require surgical intervention, while a nodule that is increasing in size while on thyroidstimulating hormone (TSH) suppression needs reevaluation. An initial ultrasound examination performed on a patient who presents with a morphologic abnormality of the thyroid often directs what further tests are needed.
UGFNA Biopsy of Thyroid Nodules
Ultrasound alone is not specific enough and cannot be relied upon to diagnose malignancy. FNA has been the standard diagnostic test for evaluating malignancy in a thyroid nodule, but it has limitations. FNA cannot differentiate between follicular adenoma and follicular carcinoma and up to 20% of FNA biopsies yield inadequate material for diagnosis. Ultrasound and FNA compliment each other and, when used together, enhance diagnostic capability.
Many investigators have shown that combiningultrasound and FNA into a single procedure, UG FNA, decreases the number of inadequate specimens to less than 5%. This technique assures precise placement of the needle tip in the target and avoids biopsy of the surrounding normal tissue, which may yield a false negative diagnosis. It prevents puncturing the trachea, common carotid artery, and internal jugular vein, and it often enables avoiding passing the needle through the sternocleiodomastoid muscle, thus markedly reducing the discomfort to the patient. UG FNA is indicated for the biopsy of complex or cystic nodules in order to obtain material from the mural or solid component of the nodule and assure adequate cytology. In solid nodules the best cytology material is usually obtained from the periphery of the nodule rather than the center; this is particularly true if there is any central necrosis. In heterogeneous nodules, the biopsy should be taken from the hypoechoic area of the nodule. When performing FNA of a multinodular goiter, ultrasound is useful in selecting the most suspicious nodule(s) for biopsy by evaluating the nodule’s characteristics. If lymphadenopathy accompanies a thyroid nodule, UG FNA of the lymph node may be more useful than biopsy of the nodule. Sometimes, an irregular thyroid surface is misdiagnosed as a thyroid nodule, and an ultrasound examination can avoid having to perform an FNA. Pseudonodules are often seen in Hashimoto disease, which is easily recognized with ultrasound.
Ultrasound guidance permits biopsy of nodules that were not previously amenable to FNA biopsy. These include many small nodules that are less than 1.5cm and non-palpable nodules such as those located posterior in the thyroid or in the upper mediastinum. Even large nodules may not be palpable in obese or large muscular individuals or in the elderly patient with kyphosis, especially when placed in the supine position.UG FNA permits proper placement of the needle in these patients. Indeed, UG FNA allows tissue sampling of virtually all nodules 0.5cm or greater in size.
Because micronodules (nodules 0.5–1cm) are so common in the population, the question arises when to perform UG FNA.A nodule this size seldom presents a threat to life and they are so numerous that routine biopsy of all such nodules is not practical or costeffective. However, several investigators have shown that the incidence of malignancy in small non-palpable nodules is the same as in palpable nodules. In addition, others have shown that cancers that present less than 1.5cm in size are often as aggressive as larger cancers. Some judgment is required in deciding which micronodules require FNA. Patients who received external radiation during childhood and those with a family history of thyroid cancer (medullary or papillary) should have their micronodules biopsied. The occurrence of a nodule >0.5cm in a patient who had only a hemithyroidectomy for thyroid cancer also requires an UG FNA. Recently, ultrasound criteria have been established to help identify which micronodules are most likely to be malignant and therefore need UG FNA.