DOI: 10.1148/rg.244035718
RadioGraphics 2004;24:1101-1115
© RSNA, 2004
Radionuclide Imaging of the Parathyroid Glands: Patterns, Pearls, and Pitfalls
Joel R. Smith, MD and
M. Elizabeth Oates, MD
1 From the Department of Radiology, New England Medical Center and Tufts University School of Medicine, NEMC #299, 750 Washington St, Boston, MA 02111 (J.R.S.); and the Department of Radiology, Boston Medical Center and Boston University School of Medicine (M.E.O.). Received July 16, 2003; revision requested September 12 and received October 31; accepted November 3. Both authors have no financial relationships to disclose. Address correspondence to J.R.S. (e-mail: jsmith9@tufts-nemc.org).
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Abstract
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Optimal parathyroid scintigraphy requires an understanding of (a) the embryologic, anatomic, and physiologic features of the parathyroid glands and (b) the properties of the two common imaging agents, technetium-99m sestamibi and Tc-99m tetrofosmin. Normal parathyroid glands are too small to be visualized, but parathyroid disease often produces visibly enlarged glands. Enlarged parathyroid glands may be found near the thyroid gland or outside their expected locations. Characteristic abnormal scintigraphic patterns may be described as focal or multifocal, usual or ectopic in location, and associated with a normal or abnormal thyroid gland. Patients who are referred for parathyroid imaging should have an abnormal biochemical profile. The first step in evaluating images of a patient suspected to have parathyroid disease is correlating the normal or abnormal scintigraphic patterns with the clinical and surgical history. By integrating the interpretative and technical pearls and pitfalls of parathyroid scintigraphy, the radiologist can be more confident in establishing a correct diagnosis and can precisely guide the surgeon to a single parathyroid adenoma, multiple parathyroid adenomas, or multigland hyperplasia.
© RSNA, 2004
Index Terms: Parathyroid, hyperparathyroidism, 274.531 • Parathyroid, neoplasms, 274.363 • Parathyroid, radionuclide studies, 274.12175 • Parathyroid, SPECT, 274.12162
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LEARNING OBJECTIVES
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After reading this article and taking the test, the reader will be able to:
- Describe the basic embryologic, anatomic, and physiologic features of the parathyroid glands.
- Discuss the principles of dual-phase and dual-isotope parathyroid imaging.
- Identify the common imaging features of pathologic parathyroid glands.
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Introduction
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Parathyroid scintigraphy remains an important tool for guiding clinical and surgical decisions. The primary objective of parathyroid scintigraphy is to identify and localize the culprit parathyroid gland(s) in patients who have biochemical hyperparathyroidism but are often otherwise asymptomatic. Surgeons increasingly expect preoperative identification and precise localization before the first surgery or after a failed exploration. Imaging easily demonstrates culprit glands greater than 500 mg in weight and allows localization of much smaller ones (1). At times, levels of parathyroid hormone (PTH) (also known as "parathormone") are assayed intraoperatively to ensure complete resection, and intraoperative gamma probes can help locate radioactive parathyroid glands, particularly when small or deep (2,3).
This article first reviews the embryologic, anatomic, and physiologic features of the parathyroids. Second, it discusses technetium-99m sestamibi (MIBI) and Tc-99m tetrofosmin (TETR) as the primary imaging agents; at times, adjunctive thyroid imaging with Tc-99m or iodine-123 is helpful or even necessary. Single photon emission computed tomography (SPECT) increases sensitivity and improves localization of enlarged parathyroid glands over planar imaging alone (4).
This article discusses and illustrates characteristic scintigraphic patterns of parathyroid disease, without and with concomitant thyroid disease (Table 1). These case examples provide a framework to share helpful interpretative and technical pearls and pitfalls with the practicing radiologist.
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Principles of Parathyroid Scintigraphy
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Embryologic, Anatomic, and Physiologic Features
The parathyroid glands develop at 6 weeks and migrate caudally at 8 weeks (Fig 1). The paired superior parathyroid glands develop with the thyroid gland from the fourth branchial pouch and are generally consistent in position, residing lateral and posterior to the upper pole of the thyroid at the level of the cricothyroid cartilage. The paired inferior glands descend with the thymus from the third branchial pouch and occasionally migrate to the level of the aortic arch or, rarely, fail to migrate, remaining in the high neck.
The parathyroid glands are usually embedded between the posterior border of the thyroid gland and its fibrous capsule. At times, the parathyroids may be intrathyroidal. They measure 6 x 4 x 2 mm in maximum diameter and weigh 25–40 mg each. Although most patients have four glands, the number can vary from two to six.
The principal function of the parathyroid glands is to regulate calcium ion concentration by release of PTH (Fig 2). At the first author’s institution, the normal range for ionized calcium is 4.2–5.2 mg/dL and the normal range for PTH is 10–65 pg/mL. This PTH range is only a guideline, and individual values should be considered in conjunction with the patient’s history and serum calcium level.
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Figure 2. Regulation of calcium level by the parathyroids. PTH increases the calcium level in the blood by increasing resorption from bone, decreasing excretion by the kidney, and increasing absorption from the gastrointestinal (GI) tract by activating the vitamin D precursor.
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Parathyroid Imaging
Table 2 outlines the protocol for parathyroid scintigraphy. In dual-phase planar imaging, the thyroid and parathyroid glands are imaged at 5 minutes after tracer injection; images are repeated at 2 hours. Tc-99m MIBI and Tc-99m TETR have comparable imaging characteristics (5,6). Often, the choice of imaging agent depends on its availability and the experience of the physician. Tc-99m MIBI clears from the thyroid with a half-life of about 30 minutes but is usually retained by abnormal parathyroid glands. In our experience, Tc-99m TETR may clear more slowly from the thyroid gland. This "differential washout" phenomenon improves target-to-background activity so that abnormal parathyroid tissue should become more visible on the delayed images. This classic finding for parathyroid adenoma is not universal and may occur in only up to 60% of cases (4). SPECT allows identification and localization of a parathyroid adenoma in three dimensions (7). Dual-isotope imaging is sometimes used adjunctively with dual-phase imaging, especially in patients with a history of thyroid disease or thyroid surgery. This approach compares the pattern of Tc-99m MIBI or TETR localization to that of a thyroid-specific radiopharmaceutical to evaluate concordance (related to thyroid tissue) versus discordance (indicative of parathyroid tissue). At times, this dual-isotope approach is critical to proper interpretation.
Parathyroid scintigraphy is approximately 90% sensitive for localizing parathyroid adenomas but only 60% sensitive for hyperplasia (8). The size and cellularity of the glands and the PTH level influence sensitivity and specificity. Small or weakly avid glands may be missed.
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Patterns of Parathyroid Scintigraphy
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Focal Localization and a Normal Thyroid
Except in multiple endocrine neoplasia syndrome, primary hyperparathyroidism is idiopathic, more common in women, and usually detected in asymptomatic individuals at routine blood screening with a prevalence of approximately 1/1,000 (9). Symptoms are related to hypercalcemia and include fatigue, weakness, polyuria, constipation, and depression. Primary hyperparathyroidism is confirmed with an elevated PTH level. More than 80% of the time, there is a single culprit gland (Figs 3–5). Most solitary adenomas occur just posterior to the lower portion of the thyroid gland, and up to 20% are found in ectopic locations (9). Less common causes of primary hyperparathyroidism include parathyroid hyperplasia (10%), multiple adenomas (5%), and carcinoma (1%). Carcinoma is indistinguishable from a functioning adenoma (Fig 6).
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Figure 3a. Focal localization and a normal thyroid. Early (a) and delayed (b) Tc-99m TETR images from a dual-phase study show focal uptake and retention of the tracer in the right inferior aspect of the thyroid (arrowhead). There is complete washout of thyroid activity on the delayed image (b). The diagnosis was a typical solitary adenoma of the right inferior parathyroid. Pearl: The sensitivity of Tc-99m TETR is comparable with that of Tc-99m MIBI for localization of abnormal parathyroid tissue (4,5). Pitfall: Before imaging, the presence of primary hyperparathyroidism should be confirmed by means of elevated calcium and PTH levels to minimize false-positive and false-negative results. Secondary hyperparathyroidism is commonly the result of chronic renal failure but can also be seen with other disorders of vitamin D or phosphate metabolism. Secondary hyperparathyroidism with a normal calcium level should be excluded by means of a normal 25-hydroxyvitamin D level.
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Figure 3b. Focal localization and a normal thyroid. Early (a) and delayed (b) Tc-99m TETR images from a dual-phase study show focal uptake and retention of the tracer in the right inferior aspect of the thyroid (arrowhead). There is complete washout of thyroid activity on the delayed image (b). The diagnosis was a typical solitary adenoma of the right inferior parathyroid. Pearl: The sensitivity of Tc-99m TETR is comparable with that of Tc-99m MIBI for localization of abnormal parathyroid tissue (4,5). Pitfall: Before imaging, the presence of primary hyperparathyroidism should be confirmed by means of elevated calcium and PTH levels to minimize false-positive and false-negative results. Secondary hyperparathyroidism is commonly the result of chronic renal failure but can also be seen with other disorders of vitamin D or phosphate metabolism. Secondary hyperparathyroidism with a normal calcium level should be excluded by means of a normal 25-hydroxyvitamin D level.
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Figure 4a. Focal localization and a normal thyroid. (a, b) Early (a) and delayed (b) Tc-99m TETR images from a dual-phase study show early activity and slight retention of the tracer in the region of the right thyroid lobe (arrow). (c) Tc-99m TETR SPECT images show that the lesion is located just deep to the middle segment of the right thyroid lobe. Ant = anterior, Post = posterior. The diagnosis was a solitary adenoma of the right parathyroid (860 mg, 2 x 1.2 x 0.6 cm). The intraoperative PTH level fell from 82 to 27 pg/mL. Pearl: Eighty-five percent of patients with primary hyperparathyroidism have a solitary parathyroid adenoma. Pearl: Mild residual thyroid activity serves as a landmark for localization. Pitfall: Retention of the tracer does not always occur in parathyroid adenomas and was reported in only 60% in one study in which Tc-99m MIBI was used (4).
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Figure 4b. Focal localization and a normal thyroid. (a, b) Early (a) and delayed (b) Tc-99m TETR images from a dual-phase study show early activity and slight retention of the tracer in the region of the right thyroid lobe (arrow). (c) Tc-99m TETR SPECT images show that the lesion is located just deep to the middle segment of the right thyroid lobe. Ant = anterior, Post = posterior. The diagnosis was a solitary adenoma of the right parathyroid (860 mg, 2 x 1.2 x 0.6 cm). The intraoperative PTH level fell from 82 to 27 pg/mL. Pearl: Eighty-five percent of patients with primary hyperparathyroidism have a solitary parathyroid adenoma. Pearl: Mild residual thyroid activity serves as a landmark for localization. Pitfall: Retention of the tracer does not always occur in parathyroid adenomas and was reported in only 60% in one study in which Tc-99m MIBI was used (4).
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Figure 4c. Focal localization and a normal thyroid. (a, b) Early (a) and delayed (b) Tc-99m TETR images from a dual-phase study show early activity and slight retention of the tracer in the region of the right thyroid lobe (arrow). (c) Tc-99m TETR SPECT images show that the lesion is located just deep to the middle segment of the right thyroid lobe. Ant = anterior, Post = posterior. The diagnosis was a solitary adenoma of the right parathyroid (860 mg, 2 x 1.2 x 0.6 cm). The intraoperative PTH level fell from 82 to 27 pg/mL. Pearl: Eighty-five percent of patients with primary hyperparathyroidism have a solitary parathyroid adenoma. Pearl: Mild residual thyroid activity serves as a landmark for localization. Pitfall: Retention of the tracer does not always occur in parathyroid adenomas and was reported in only 60% in one study in which Tc-99m MIBI was used (4).
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Figure 5a. Focal localization and a normal thyroid. (a, b) Early (a) and delayed (b) Tc-99m TETR images show subtle retention of the tracer at the caudal end of the left thyroid lobe (circle in b). The right submandibular gland (arrow) demonstrates increased activity relative to that in the left submandibular gland. (c) Tc-99m image also shows relatively increased activity in the right submandibular gland (arrow). (d) Coronal Tc-99m TETR SPECT images also show subtle retention of the tracer at the caudal end of the left thyroid lobe (circle) and relatively increased activity in the right submandibular gland (arrow). The diagnosis was a solitary adenoma of the left parathyroid. The intraoperative PTH level fell from 82 to 27 pg/mL. Pitfall: Asymmetric activity in the submandibular glands is a normal variant and may be mistaken for an adenoma of an ectopic undescended parathyroid. The presence of asymmetric activity in the submandibular glands can be confirmed with Tc-99m thyroid imaging.
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Figure 5b. Focal localization and a normal thyroid. (a, b) Early (a) and delayed (b) Tc-99m TETR images show subtle retention of the tracer at the caudal end of the left thyroid lobe (circle in b). The right submandibular gland (arrow) demonstrates increased activity relative to that in the left submandibular gland. (c) Tc-99m image also shows relatively increased activity in the right submandibular gland (arrow). (d) Coronal Tc-99m TETR SPECT images also show subtle retention of the tracer at the caudal end of the left thyroid lobe (circle) and relatively increased activity in the right submandibular gland (arrow). The diagnosis was a solitary adenoma of the left parathyroid. The intraoperative PTH level fell from 82 to 27 pg/mL. Pitfall: Asymmetric activity in the submandibular glands is a normal variant and may be mistaken for an adenoma of an ectopic undescended parathyroid. The presence of asymmetric activity in the submandibular glands can be confirmed with Tc-99m thyroid imaging.
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Figure 5c. Focal localization and a normal thyroid. (a, b) Early (a) and delayed (b) Tc-99m TETR images show subtle retention of the tracer at the caudal end of the left thyroid lobe (circle in b). The right submandibular gland (arrow) demonstrates increased activity relative to that in the left submandibular gland. (c) Tc-99m image also shows relatively increased activity in the right submandibular gland (arrow). (d) Coronal Tc-99m TETR SPECT images also show subtle retention of the tracer at the caudal end of the left thyroid lobe (circle) and relatively increased activity in the right submandibular gland (arrow). The diagnosis was a solitary adenoma of the left parathyroid. The intraoperative PTH level fell from 82 to 27 pg/mL. Pitfall: Asymmetric activity in the submandibular glands is a normal variant and may be mistaken for an adenoma of an ectopic undescended parathyroid. The presence of asymmetric activity in the submandibular glands can be confirmed with Tc-99m thyroid imaging.
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Figure 5d. Focal localization and a normal thyroid. (a, b) Early (a) and delayed (b) Tc-99m TETR images show subtle retention of the tracer at the caudal end of the left thyroid lobe (circle in b). The right submandibular gland (arrow) demonstrates increased activity relative to that in the left submandibular gland. (c) Tc-99m image also shows relatively increased activity in the right submandibular gland (arrow). (d) Coronal Tc-99m TETR SPECT images also show subtle retention of the tracer at the caudal end of the left thyroid lobe (circle) and relatively increased activity in the right submandibular gland (arrow). The diagnosis was a solitary adenoma of the left parathyroid. The intraoperative PTH level fell from 82 to 27 pg/mL. Pitfall: Asymmetric activity in the submandibular glands is a normal variant and may be mistaken for an adenoma of an ectopic undescended parathyroid. The presence of asymmetric activity in the submandibular glands can be confirmed with Tc-99m thyroid imaging.
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Figure 6a. Focal localization and a normal thyroid. SN = sternal notch marker. (a) Early Tc-99m MIBI image shows increased localization of the tracer in the upper pole of the left thyroid lobe. (b) Delayed image shows retention of the tracer in an enlarged parathyroid. The diagnosis was carcinoma of the left superior parathyroid. Pearl: Parathyroid carcinoma, a potential cause of hyperparathyroidism, can demonstrate increased uptake and retention of the tracer, thus mimicking a parathyroid adenoma. The diagnosis is made with pathologic analysis of the surgical specimen.
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Figure 6b. Focal localization and a normal thyroid. SN = sternal notch marker. (a) Early Tc-99m MIBI image shows increased localization of the tracer in the upper pole of the left thyroid lobe. (b) Delayed image shows retention of the tracer in an enlarged parathyroid. The diagnosis was carcinoma of the left superior parathyroid. Pearl: Parathyroid carcinoma, a potential cause of hyperparathyroidism, can demonstrate increased uptake and retention of the tracer, thus mimicking a parathyroid adenoma. The diagnosis is made with pathologic analysis of the surgical specimen.
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With its three-dimensional capability, SPECT is very helpful for directing the surgeon right to the enlarged parathyroid gland and may reveal parathyroid adenomas not seen on planar images alone.
Ectopic Focal Localization and a Normal Thyroid
Parathyroid adenomas are usually located near and posterior to the thyroid gland but may be intrathyroidal or ectopic (Figs 7 –10). The field of view should include the base of the jaw through the heart to search for all potential sites.
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Figure 7. Ectopic focal localization and a normal thyroid. Early Tc-99m MIBI image shows moderately intense focal uptake inferior to the left thyroid lobe (arrow). The diagnosis was a parathyroid adenoma of the left side of the mediastinum. Pearl: Imaging should extend from the base of the jaw to the base of the heart to search for ectopic glands along the embryologic course.
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Figure 8a. Ectopic focal localization and a normal thyroid. (a) Early Tc-99m TETR image shows subtle focal localization of the tracer on the right side (arrow). (b) Delayed Tc-99m TETR image shows no definite retention of the tracer. (c) Coronal Tc-99m TETR SPECT images show subtle focal localization of the tracer inferior to the left thyroid lobe (circle). The diagnosis was a left parathyroid adenoma (0.7 x 0.4 x 0.2 cm) embedded in thymic tissue. The intraoperative PTH level dropped dramatically once the lesion was removed. Pearl: Subtle focal lesions can be identified with Tc-99m TETR or MIBI SPECT, especially if the adenoma is separate from the thyroid, as the background activity is lower and the lesion stands out more clearly.
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Figure 8b. Ectopic focal localization and a normal thyroid. (a) Early Tc-99m TETR image shows subtle focal localization of the tracer on the right side (arrow). (b) Delayed Tc-99m TETR image shows no definite retention of the tracer. (c) Coronal Tc-99m TETR SPECT images show subtle focal localization of the tracer inferior to the left thyroid lobe (circle). The diagnosis was a left parathyroid adenoma (0.7 x 0.4 x 0.2 cm) embedded in thymic tissue. The intraoperative PTH level dropped dramatically once the lesion was removed. Pearl: Subtle focal lesions can be identified with Tc-99m TETR or MIBI SPECT, especially if the adenoma is separate from the thyroid, as the background activity is lower and the lesion stands out more clearly.
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Figure 8c. Ectopic focal localization and a normal thyroid. (a) Early Tc-99m TETR image shows subtle focal localization of the tracer on the right side (arrow). (b) Delayed Tc-99m TETR image shows no definite retention of the tracer. (c) Coronal Tc-99m TETR SPECT images show subtle focal localization of the tracer inferior to the left thyroid lobe (circle). The diagnosis was a left parathyroid adenoma (0.7 x 0.4 x 0.2 cm) embedded in thymic tissue. The intraoperative PTH level dropped dramatically once the lesion was removed. Pearl: Subtle focal lesions can be identified with Tc-99m TETR or MIBI SPECT, especially if the adenoma is separate from the thyroid, as the background activity is lower and the lesion stands out more clearly.
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Figure 9a. Ectopic focal localization and a normal thyroid. (a, b) Early (a) and delayed (b) Tc-99m MIBI images show subtle focal retention of the tracer just inferior to the right thyroid lobe (arrowhead). (c) SPECT images show that the lesion is located far inferior and very deep relative to the right thyroid lobe (circle). The diagnosis was a parathyroid adenoma (1.6 g) deep in the right side of the mediastinum. Pearl: SPECT allows determination of depth, thus guiding the surgeon who said, "I would have missed this one!"
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Figure 9b. Ectopic focal localization and a normal thyroid. (a, b) Early (a) and delayed (b) Tc-99m MIBI images show subtle focal retention of the tracer just inferior to the right thyroid lobe (arrowhead). (c) SPECT images show that the lesion is located far inferior and very deep relative to the right thyroid lobe (circle). The diagnosis was a parathyroid adenoma (1.6 g) deep in the right side of the mediastinum. Pearl: SPECT allows determination of depth, thus guiding the surgeon who said, "I would have missed this one!"
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Figure 9c. Ectopic focal localization and a normal thyroid. (a, b) Early (a) and delayed (b) Tc-99m MIBI images show subtle focal retention of the tracer just inferior to the right thyroid lobe (arrowhead). (c) SPECT images show that the lesion is located far inferior and very deep relative to the right thyroid lobe (circle). The diagnosis was a parathyroid adenoma (1.6 g) deep in the right side of the mediastinum. Pearl: SPECT allows determination of depth, thus guiding the surgeon who said, "I would have missed this one!"
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Figure 10a. Ectopic focal localization and a normal thyroid. (a) Delayed Tc-99m TETR image shows an abnormal focus with central photopenia in the upper right aspect of the mediastinum (circle). (b) Coronal SPECT images show that the lesion is located in the superficial mid- to upper mediastinum. (c) Computed tomographic scan shows a correlative large cystic mass (circle). The diagnosis was a cystic parathyroid adenoma (3 cm in diameter) of the mediastinum. Pearl: A cold center implies cystic degeneration or necrosis.
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Figure 10b. Ectopic focal localization and a normal thyroid. (a) Delayed Tc-99m TETR image shows an abnormal focus with central photopenia in the upper right aspect of the mediastinum (circle). (b) Coronal SPECT images show that the lesion is located in the superficial mid- to upper mediastinum. (c) Computed tomographic scan shows a correlative large cystic mass (circle). The diagnosis was a cystic parathyroid adenoma (3 cm in diameter) of the mediastinum. Pearl: A cold center implies cystic degeneration or necrosis.
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Figure 10c. Ectopic focal localization and a normal thyroid. (a) Delayed Tc-99m TETR image shows an abnormal focus with central photopenia in the upper right aspect of the mediastinum (circle). (b) Coronal SPECT images show that the lesion is located in the superficial mid- to upper mediastinum. (c) Computed tomographic scan shows a correlative large cystic mass (circle). The diagnosis was a cystic parathyroid adenoma (3 cm in diameter) of the mediastinum. Pearl: A cold center implies cystic degeneration or necrosis.
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Multifocal Localization
Primary hyperparathyroidism may be related to multiple adenomas or hyperplasia. Secondary hyperparathyroidism, usually related to renal failure, leads to multigland hyperplasia (Fig 11) and autonomy (tertiary hyperparathyroidism). Renal failure results in hypocalcemia from impaired intestinal absorption of calcium secondary to impaired vitamin D metabolism and phosphate precipitation of calcium, inducing PTH secretion and reactive multigland hyperplasia.
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Figure 11a. Multifocal localization and a normal thyroid. (a) Delayed Tc-99m MIBI image shows at least three and probably four foci of retained tracer (arrows). (b) Coronal SPECT image shows that the foci (arrows) are located just deep to the thyroid. The diagnosis was multigland hyperplasia in renal failure. Pitfall: In general, scintigraphy has a lower sensitivity for hyperplasia than for adenoma.
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Figure 11b. Multifocal localization and a normal thyroid. (a) Delayed Tc-99m MIBI image shows at least three and probably four foci of retained tracer (arrows). (b) Coronal SPECT image shows that the foci (arrows) are located just deep to the thyroid. The diagnosis was multigland hyperplasia in renal failure. Pitfall: In general, scintigraphy has a lower sensitivity for hyperplasia than for adenoma.
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Focal Localization and an Abnormal Thyroid
Visual "subtraction" with adjunctive thyroid-selective scanning (Tc-99m or I-123) may be helpful, or even essential, in patients with goiters or other confounding underlying thyroid disease (Figs 12–15), after thyroid surgery (Fig 16), or in those with a palpable abnormality (10). Thyroid adenomas and carcinomas can coexist and can retain Tc-99m MIBI or TETR, resulting in false-positive study results (Fig 12).
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Figure 12a. Focal localization and an abnormal thyroid. (a, b) Early (a) and delayed (b) Tc-99m TETR images show two abnormal foci: one in the region of the upper right thyroid lobe (circle) and the other in the right side of the neck (arrow). (c) Images from adjunctive Tc-99m pertechnetate imaging performed with two intensities show a hot nodule in the lower right thyroid lobe as well as focal extrathyroidal activity in the right side of the neck (arrow). The upper right lobe lesion is discordant between the two studies, whereas the lower right lobe lesion is concordant and is more intense on the Tc-99m pertechnetate images (c). The lower right lobe lesion has a cold center on the early Tc-99m TETR image (a). There were coexisting diagnoses: (a) an adenoma (1.5 g) of the right superior parathyroid and (b) multinodular goiter with thyroid carcinoma in a large nodule (2.5 cm) of the inferior right pole and multiple metastatic lymph nodes in the right side of the neck. Pearl: Ectopic localization of the tracer raises the suspicion of metastatic thyroid carcinoma. Pitfall: Rarely, thyroid cancer appears hot at thyroid-selective imaging.
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Figure 12b. Focal localization and an abnormal thyroid. (a, b) Early (a) and delayed (b) Tc-99m TETR images show two abnormal foci: one in the region of the upper right thyroid lobe (circle) and the other in the right side of the neck (arrow). (c) Images from adjunctive Tc-99m pertechnetate imaging performed with two intensities show a hot nodule in the lower right thyroid lobe as well as focal extrathyroidal activity in the right side of the neck (arrow). The upper right lobe lesion is discordant between the two studies, whereas the lower right lobe lesion is concordant and is more intense on the Tc-99m pertechnetate images (c). The lower right lobe lesion has a cold center on the early Tc-99m TETR image (a). There were coexisting diagnoses: (a) an adenoma (1.5 g) of the right superior parathyroid and (b) multinodular goiter with thyroid carcinoma in a large nodule (2.5 cm) of the inferior right pole and multiple metastatic lymph nodes in the right side of the neck. Pearl: Ectopic localization of the tracer raises the suspicion of metastatic thyroid carcinoma. Pitfall: Rarely, thyroid cancer appears hot at thyroid-selective imaging.
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Figure 12c. Focal localization and an abnormal thyroid. (a, b) Early (a) and delayed (b) Tc-99m TETR images show two abnormal foci: one in the region of the upper right thyroid lobe (circle) and the other in the right side of the neck (arrow). (c) Images from adjunctive Tc-99m pertechnetate imaging performed with two intensities show a hot nodule in the lower right thyroid lobe as well as focal extrathyroidal activity in the right side of the neck (arrow). The upper right lobe lesion is discordant between the two studies, whereas the lower right lobe lesion is concordant and is more intense on the Tc-99m pertechnetate images (c). The lower right lobe lesion has a cold center on the early Tc-99m TETR image (a). There were coexisting diagnoses: (a) an adenoma (1.5 g) of the right superior parathyroid and (b) multinodular goiter with thyroid carcinoma in a large nodule (2.5 cm) of the inferior right pole and multiple metastatic lymph nodes in the right side of the neck. Pearl: Ectopic localization of the tracer raises the suspicion of metastatic thyroid carcinoma. Pitfall: Rarely, thyroid cancer appears hot at thyroid-selective imaging.
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Figure 13a. Focal localization and an abnormal thyroid. (a) Early Tc-99m MIBI image shows a heterogeneous thyroid. (b) Left: Delayed Tc-99m MIBI image shows subtle retention of the tracer inferior to the right thyroid lobe (arrow). Right: Image from adjunctive Tc-99m thyroid imaging does not show the subtle focus. (c) Top: Axial Tc-99m MIBI SPECT images show the focus (arrow). Bottom: Axial images from adjunctive Tc-99m thyroid SPECT do not show the focus. The diagnosis was a small adenoma (200 mg) of the right inferior parathyroid. Pearl: SPECT is helpful in identification of small or deep lesions and in precisely directing the surgeon. Pearl: In some patients with small perithyroidal parathyroid adenomas, adjunctive thyroid SPECT with careful registration of the image data sets can demonstrate subtle foci. Pitfall: Small parathyroid adenomas may be overlooked with planar imaging alone.
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Figure 13b. Focal localization and an abnormal thyroid. (a) Early Tc-99m MIBI image shows a heterogeneous thyroid. (b) Left: Delayed Tc-99m MIBI image shows subtle retention of the tracer inferior to the right thyroid lobe (arrow). Right: Image from adjunctive Tc-99m thyroid imaging does not show the subtle focus. (c) Top: Axial Tc-99m MIBI SPECT images show the focus (arrow). Bottom: Axial images from adjunctive Tc-99m thyroid SPECT do not show the focus. The diagnosis was a small adenoma (200 mg) of the right inferior parathyroid. Pearl: SPECT is helpful in identification of small or deep lesions and in precisely directing the surgeon. Pearl: In some patients with small perithyroidal parathyroid adenomas, adjunctive thyroid SPECT with careful registration of the image data sets can demonstrate subtle foci. Pitfall: Small parathyroid adenomas may be overlooked with planar imaging alone.
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Figure 13c. Focal localization and an abnormal thyroid. (a) Early Tc-99m MIBI image shows a heterogeneous thyroid. (b) Left: Delayed Tc-99m MIBI image shows subtle retention of the tracer inferior to the right thyroid lobe (arrow). Right: Image from adjunctive Tc-99m thyroid imaging does not show the subtle focus. (c) Top: Axial Tc-99m MIBI SPECT images show the focus (arrow). Bottom: Axial images from adjunctive Tc-99m thyroid SPECT do not show the focus. The diagnosis was a small adenoma (200 mg) of the right inferior parathyroid. Pearl: SPECT is helpful in identification of small or deep lesions and in precisely directing the surgeon. Pearl: In some patients with small perithyroidal parathyroid adenomas, adjunctive thyroid SPECT with careful registration of the image data sets can demonstrate subtle foci. Pitfall: Small parathyroid adenomas may be overlooked with planar imaging alone.
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Figure 14a. Focal localization and an abnormal thyroid. Early (a) and delayed (b) Tc-99m MIBI images show focal localization and retention of the tracer in the expected location of the lower left thyroid lobe (arrow in b). Thyroid activity is suppressed because of thyroid hormone and prolonged lithium use. The diagnosis was a parathyroid adenoma (1.5 cm in diameter) of the left tracheoesophageal groove. Pearl: The sternal notch marker (SN in b) serves as the only landmark. Pearl: If the patient is uncooperative, the early view alone might allow diagnosis. Pitfall: Long-term regimens of some medications may result in nonvisualization of the thyroid at Tc-99m MIBI or TETR imaging.
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Figure 14b. Focal localization and an abnormal thyroid. Early (a) and delayed (b) Tc-99m MIBI images show focal localization and retention of the tracer in the expected location of the lower left thyroid lobe (arrow in b). Thyroid activity is suppressed because of thyroid hormone and prolonged lithium use. The diagnosis was a parathyroid adenoma (1.5 cm in diameter) of the left tracheoesophageal groove. Pearl: The sternal notch marker (SN in b) serves as the only landmark. Pearl: If the patient is uncooperative, the early view alone might allow diagnosis. Pitfall: Long-term regimens of some medications may result in nonvisualization of the thyroid at Tc-99m MIBI or TETR imaging.
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Figure 15a. Focal localization and an abnormal thyroid. (a) Early Tc-99m MIBI image shows an enlarged, heterogeneous thyroid with an asymmetric focus of increased activity superior to the left lobe (arrow). (b) Image from Tc-99m thyroid-selective imaging does not show the focus. (c) Coronal Tc-99m MIBI SPECT images show that the focus is just superior to the left thyroid lobe (arrow). The diagnosis was an adenoma (1.24 g, 2.5 x 1.5 x 0.5 cm) of the left superior parathyroid. Pearl: The planar projection images from the SPECT acquisition, as opposed to the reconstructed tomographic sections, can be useful because they allow separation of the parathyroid focus from the adjacent overlying thyroid tissue. Pearl: Thyroid-selective imaging allows confirmation of the discordant activity in the left upper pole and can be of great help in patients with large goiters.
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Figure 15b. Focal localization and an abnormal thyroid. (a) Early Tc-99m MIBI image shows an enlarged, heterogeneous thyroid with an asymmetric focus of increased activity superior to the left lobe (arrow). (b) Image from Tc-99m thyroid-selective imaging does not show the focus. (c) Coronal Tc-99m MIBI SPECT images show that the focus is just superior to the left thyroid lobe (arrow). The diagnosis was an adenoma (1.24 g, 2.5 x 1.5 x 0.5 cm) of the left superior parathyroid. Pearl: The planar projection images from the SPECT acquisition, as opposed to the reconstructed tomographic sections, can be useful because they allow separation of the parathyroid focus from the adjacent overlying thyroid tissue. Pearl: Thyroid-selective imaging allows confirmation of the discordant activity in the left upper pole and can be of great help in patients with large goiters.
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Figure 15c. Focal localization and an abnormal thyroid. (a) Early Tc-99m MIBI image shows an enlarged, heterogeneous thyroid with an asymmetric focus of increased activity superior to the left lobe (arrow). (b) Image from Tc-99m thyroid-selective imaging does not show the focus. (c) Coronal Tc-99m MIBI SPECT images show that the focus is just superior to the left thyroid lobe (arrow). The diagnosis was an adenoma (1.24 g, 2.5 x 1.5 x 0.5 cm) of the left superior parathyroid. Pearl: The planar projection images from the SPECT acquisition, as opposed to the reconstructed tomographic sections, can be useful because they allow separation of the parathyroid focus from the adjacent overlying thyroid tissue. Pearl: Thyroid-selective imaging allows confirmation of the discordant activity in the left upper pole and can be of great help in patients with large goiters.
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Figure 16a. Focal localization and an abnormal thyroid. (a, b) Delayed Tc-99m MIBI image (a) and image from Tc-99m thyroid-selective imaging (b) obtained after remote thyroid surgery show multiple foci of activity. One discordant focus is just inferior to the remnant of the left thyroid lobe (arrow in a) and represents an enlarged parathyroid. (c) Tc-99m MIBI SPECT images also show that the enlarged parathyroid is near the remnant of the left thyroid lobe (arrow). The diagnosis was a solitary parathyroid adenoma. Pearl: Thyroid-selective imaging is essential in the postoperative patient.
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Figure 16b. Focal localization and an abnormal thyroid. (a, b) Delayed Tc-99m MIBI image (a) and image from Tc-99m thyroid-selective imaging (b) obtained after remote thyroid surgery show multiple foci of activity. One discordant focus is just inferior to the remnant of the left thyroid lobe (arrow in a) and represents an enlarged parathyroid. (c) Tc-99m MIBI SPECT images also show that the enlarged parathyroid is near the remnant of the left thyroid lobe (arrow). The diagnosis was a solitary parathyroid adenoma. Pearl: Thyroid-selective imaging is essential in the postoperative patient.
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Figure 16c. Focal localization and an abnormal thyroid. (a, b) Delayed Tc-99m MIBI image (a) and image from Tc-99m thyroid-selective imaging (b) obtained after remote thyroid surgery show multiple foci of activity. One discordant focus is just inferior to the remnant of the left thyroid lobe (arrow in a) and represents an enlarged parathyroid. (c) Tc-99m MIBI SPECT images also show that the enlarged parathyroid is near the remnant of the left thyroid lobe (arrow). The diagnosis was a solitary parathyroid adenoma. Pearl: Thyroid-selective imaging is essential in the postoperative patient.
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Ectopic Focal Localization and an Abnormal Thyroid
A parathyroid adenoma is usually located posterior to the thyroid lobes but may be intrathyroidal or ectopic along the embryologic course. Surgeons require preoperative imaging to evaluate for unusual locations, particularly in patients with recurrent or residual hyperparathyroidism (Fig 17). SPECT would seem essential in these patients (Fig 18) (11). In patients with ambiguous or negative parathyroid images or with ectopic parathyroid disease, magnetic resonance imaging can aid in surgical planning, particularly when the neck anatomy is distorted (12).
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Figure 17. Ectopic focal localization and an abnormal thyroid. The patient underwent surgical exploration of the neck with right hemithyroidectomy; results of the surgical exploration were negative. Early Tc-99m MIBI image shows an ectopic parathyroid adenoma in the left side of the chest (arrow). The residual left thyroid lobe is seen (arrowheads). The diagnosis was an ectopic parathyroid adenoma of the mediastinum. Pearl: Parathyroid adenomas may be overlooked at initial or repeat surgery because they occur in a distant location. Pitfall: Imaging with a field of view limited to the neck will cause ectopic sites to be overlooked, as in this case. Imaging should extend from the base of the jaw to the base of the heart.
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Figure 18a. Ectopic focal localization and an abnormal thyroid. The patient underwent right hemithyroidectomy during unsuccessful parathyroid surgery. (a) Early Tc-99m MIBI image shows a large left thyroid lobe (arrow) but no abnormal parathyroid focus. (b) Delayed Tc-99m MIBI image shows a subtle focus near the right hemithyroidectomy bed (arrowheads). (c) SPECT images show a parathyroid adenoma located just to the right of midline (arrow). The diagnosis was a parathyroid adenoma (750 mg, 1.5 cm in diameter) in the right tracheoesophageal groove. The intraoperative PTH level fell from 268 to 49 pg/mL. Pearl: SPECT allows confirmation and localization of subtle findings from planar imaging.
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Figure 18b. Ectopic focal localization and an abnormal thyroid. The patient underwent right hemithyroidectomy during unsuccessful parathyroid surgery. (a) Early Tc-99m MIBI image shows a large left thyroid lobe (arrow) but no abnormal parathyroid focus. (b) Delayed Tc-99m MIBI image shows a subtle focus near the right hemithyroidectomy bed (arrowheads). (c) SPECT images show a parathyroid adenoma located just to the right of midline (arrow). The diagnosis was a parathyroid adenoma (750 mg, 1.5 cm in diameter) in the right tracheoesophageal groove. The intraoperative PTH level fell from 268 to 49 pg/mL. Pearl: SPECT allows confirmation and localization of subtle findings from planar imaging.
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Figure 18c. Ectopic focal localization and an abnormal thyroid. The patient underwent right hemithyroidectomy during unsuccessful parathyroid surgery. (a) Early Tc-99m MIBI image shows a large left thyroid lobe (arrow) but no abnormal parathyroid focus. (b) Delayed Tc-99m MIBI image shows a subtle focus near the right hemithyroidectomy bed (arrowheads). (c) SPECT images show a parathyroid adenoma located just to the right of midline (arrow). The diagnosis was a parathyroid adenoma (750 mg, 1.5 cm in diameter) in the right tracheoesophageal groove. The intraoperative PTH level fell from 268 to 49 pg/mL. Pearl: SPECT allows confirmation and localization of subtle findings from planar imaging.
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Conclusions
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Patients referred for evaluation of the parathyroid glands often demonstrate one of the characteristic patterns illustrated in this article. That pattern can be categorized as focal or multifocal localization in or about or distant to the thyroid gland. Concomitant thyroid-selective imaging is essential when there is underlying thyroid disease or a history of neck surgery or occasionally can help uncover a subtle lesion. It behooves radiologists to be familiar not only with the available radiopharmaceuticals and imaging protocols, notably SPECT, but also with the embryologic, anatomic, and physiologic features of the parathyroid gland because they form the foundation for appropriate clinical application and image interpretation. By recognizing the patterns, applying the pearls, and avoiding the pitfalls, radiologists can improve accurate identification and exact localization of abnormal parathyroid glands for successful surgery.
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Acknowledgments
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The authors thank endocrinologist Stephanie L. Lee, MD, PhD, and nuclear radiologist Victor W. Lee, MD, for their helpful suggestions from their respective clinical and radiologic perspectives.
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Footnotes
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Abbreviations: MIBI = sestamibi,
PTH = parathyroid hormone,
TETR = tetrofosmin
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References
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- Wilkinson RH, Jr, Leight GS, Jr, Garner SC, Borges-Neto S. Complementary nature of radiotracer parathyroid imaging and intraoperative parathyroid hormone assays in the surgical management of primary hyperparathyroid disease: case report and review. Clin Nucl Med 2000; 25:173-178.[CrossRef][Medline]
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- Billotey C, Sarfati E, Aurengo A, et al. Advantages of SPECT in technetium-99m-sestamibi parathyroid scintigraphy. J Nucl Med 1996; 37:1773- 1778.[Abstract/Free Full Text]
- Taillefer R, Boucher Y, Potvin C, Lamert R. Detection and localization of parathyroid adenomas in patients with hyperparathyroidism using a single radionuclide imaging procedure with technetium-99m-sestamibi (double-phase study). J Nucl Med 1992; 33:1801-1807.[Abstract/Free Full Text]
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Abstract
LEARNING OBJECTIVES
