Delayed Activity on Dual-Phase Parathyroid Scans

The above images show increased early and delayed uptake inferior to both thyroid glands. SPECT images (not shown) confirmed their position within the parathyroid gland. Ultrasound showed an enlarged parathyroid gland on the left, but nothing on the right.

The principle of dual-phase parathyroid scanning is the clearance of ^99mTc-sestamibi or ^99mTc-tetrofosmin from the thyroid and its delayed washout from abnormal parathyroid glands. This differential washout allows visualization of abnormal parathyroid glands on delayed images. Dual-phase imaging has a sensitivity of 90% for parathyroid adenomas and a sensitivity 60% for parathyroid hyperplasia.

^99mTc-sestamibi and ^99mTc-tetrofosmin can both be used and have similar diagnostic accuracy. The former clears from the thyroid gland with a half life of about 30 minutes, while the latter may clear more slowly.

In dual-phase sestamibi imaging, the thyroid and parathyroid glands are imaged at 5 minutes to 15 minutes and 1 hour - 2 hours after tracer injection. SPECT imaging can localize the abnormality in three dimensions. In cases of equivocal lesions, ^99mTc-pertechnetate can be injected to see if the equivocal area has increased ^99mTc-pertechnetate activity ("concordance" on dual-isotope) and is therefore related to the thyroid. "Discordance" on dual-isotope images indicates parathyroid tissue.

Delayed activity can be due to:

• Parathyroid adenoma: The classic finding is tracer activity on delayed images; however, this may only be seen in up to 60% of cases. In equivocal cases, dual-isotope technique can be used (see above).
• Parathyroid hyperplasia:
• Parathyroid Carcinoma:
• Thyroid adenoma: Follicular adenoma is the most common false positive. SPECT can be used to localize the lesion to the thyroid gland and dual-isotope technique can be used to assess thyroid origin.
• Thyroid carcinoma: SPECT can be used to localize the lesion to the thyroid gland and dual-isotope technique can be used to assess thyroid origin.

References

• Lee VS, Wilkinson RH Jr, Leight GS Jr, Coogan AC, Coleman RE. Hyperparathyroidism in high-risk surgical patients: evaluation with double-phase technetium-99m sestamibi imaging. Radiology. 1995 Dec;197(3):627-33.
• Smith JR, Oates ME. Radionuclide imaging of the parathyroid glands: patterns, pearls, and pitfalls. Radiographics. 2004 Jul-Aug;24(4):1101-15. Review.