Radiology of the Thyroid
The evaluation of thyroid disorders sometimes requires the use of radiology testing. This sections reviews the commonly used imaging tests.
Thyroid and neck ultrasound
An ultrasound is the single best test to evaluate the structure of the thyroid in order to evaluate and measure nodules and goiter. Lymph nodules are small round shaped organs located throughout the body which house infection-fighting cells. The lymph nodules in the neck are generally the first place that a thyroid cancer would spread if it were to metastasize. Ultrasound is one of the best methods to visualize and differentiate normal versus abnormal lymph nodes. Our physicians are trained to perform thyroid and neck ultrasounds within the office. A thyroid ultrasound is used to guide a thyroid fine needle aspiration. There is no ionizing radiation utilized with ultrasound testing. The patient does not need any special preparation for this testing.
Radioactive iodine scanning
Although radioactive iodine (131-I or 123-I) is a method to visualize the function of the thyroid gland and give some structural details, it is not nearly as detailed as thyroid ultrasound. The logic of using radioactive iodine is as follows: 1) iodine is the main building-block of thyroid hormone and not used in significant amounts in the rest of the body 2) the thyroid gland is the predominant gland that uses iodine 3) iodine that is labeled with a small amount of radioactivity can be visualized with radiology methods 4) radioactive iodine will thus be the best test to visualize (normal) thyroid tissue. Radioactive iodine does involve the use of some ionizing radiation. Radioactive iodine is used in three thyroid conditions
Differentiating the causes of hyperthyroidism with a "thyroid nuclear scan and uptake" which only uses a small amount of radioactivity (0.1 -0.2 millicuries) for this diagnostic purpose. Radioactive iodine at larger doses (10-15 millicuries) can be used in the treatment of hyperthyroidism. There are certain risks when using radioactive iodine for hyperthyroidism.
One component of the long term management of thyroid cancer is using a "whole body nuclear thyroid scan." Generally, patients will need to follow a low iodine diet in preparation for this scanning. Radioactive iodine at much larger doses (100 -200 millicuries) are used for the treatment of thyroid cancer. There are certain risks when using radioactive iodine for thyroid cancer.
Prior to the advent of thyroid ultrasound, the thyroid scan was historically used to evaluate thyroid nodules as "cold" or "hot" to help differentiate potentially cancerous versus benign nodules. This is sometimes still utilized in the evaluation of thyroid nodules pathology when read as 'indeterminate' if surgery is being contemplated.
Computer aided tomography (CT) scans are used to evaluate large goiters that cannot be fully visualized by ultrasound and for the management of thyroid cancers in certain patients. It is not helpful with diagnosing smaller thyroid nodules, but it gives good visualization of the other organs in the neck. These scans involve the use of ionizing radiation.
The use of labeled glucose (18 fluorodeoxyglucose -FDG) positron emission tomography (PET) is steadily increasing, but proper use of these tests have not been firmly established. At this time there is NO role for FDG PET/CTin the intial staging for thyroid cancer (preoperative or pre radioactive iodine ablation). FDG is primarily taken up by undifferentiated thyroid cancer cells, which are poorly iodine avid. Thus, it is particularly helpful in the patient with an increasing thyroglobulin but negative radioactive iodine scan. FDG PET is useful in patients with aggressive undifferentiated thyroid cancer, and in patients with differentiated cancer where transformation to undifferented type is suspected. In rarer types of thyroid cancer, such as medullary thyroid cancer, FDG and other tracers such as 99mTc sestamibi, [11C]methionine, [111In]octreotide, and [68Ga]somatostatin receptor binding reagents have been used. PET scans involve use of ionizing radiation.