Case 6 cont'd
Image findings: -Homogenous distribution of the radiotracer in bilateral thyroid lobe there is a focus of increased uptake inferior to the left thyroid lobe. Retention on delayed phase imaging. 1.4 x 1.1 x 2.3 soft tissue nodule on CT scan with slightly irregular borders inferior to left thyroid lobe.
A. Parathyroid adenomas are most frequently imaged using Tc-99m pertechnetate and thallium-201 (TI-201) subtraction imaging. Tc-99m pertechnetate is only taken up by thyroid tissue, while TI-201 accumulates in both thyroid and parathyroid tissues. The subtraction of the resulting images (TI-201 – Tc-99m) shows the net parathyroid activity. This is especially important as parathyroid adenomas may be present in or behind the thyroid tissue. Subtraction allows to view such adenomas, which would otherwise not be seen by TI-201 imaging alone. Parathyroid adenomas are seen as areas of increased radiotracer uptake.
B. Plummer’s disease is characterized by hot nodule(s) on thyroid scintigraphy (using Tc-99m pertechnetate), therefore greater tracer uptake. Due to these hyperfunctioning nodules, patients present with hyperthyroidism. In Plummer’s disease particularly, these nodules are autonomous, and not under the normal hormonal feedback loop (see Figure 1).
C. Although the majority of thyroid nodules are cold and benign, thyroid malignancies may show up as solitary cold nodules on thyroid scintigraphy. Thyroid carcinomas almost never showing up as hot nodules. However, it is important that the majority of cold nodules are benign and may be cancerous in only 5% of cases. Cold nodules are synonymous with non-functioning and therefore reduced tracer uptake is observed in the nodule. Solitary cold nodules in young people should be thoroughly investigated, as they are more worrisome than in older patients, who have a higher likelihood of developing benign cold nodules due to advanced age. Additionally, incidental nodules are more likely to occur in women than in men, therefore solitary cold nodules in young men are a cause for greater concern.
D. De Quervain’s disease (aka subacute thyroiditis aka granulomatous thyroiditis) is associated with destruction of thyroid gland tissue by inflammatory infiltration. As thyroid cells are destroyed, imaging with Tc-99m pertechnetate shows lack of radiotracer uptake in the affected region/lobe.
Discussion:
In subtraction imaging, TI-201 is usually injected intravenously first. Multiple images are then captured and the view showing the highest likelihood of abnormality is selected. The subject is then repositioned in that view and TI-201 image is obtained (baseline). Tc-99m pertechnetate is then administered to the patient (1-2 mCi), and images are acquired 10 minutes later. A computer is used to subtract both images and generate a view of net parathyroid activity.
Another radiopharmaceutical used for parathyroid imaging is Tc-99m sestamibi (20 mCi, injected intravenously). The scintigraphic technique used in conjunction with this tracer is not subtraction imaging, but rather delayed imaging. Tc-99m sestamibi is taken up by both thyroid and parathyroid tissues, but washout is slow from parathyroid adenomas and hyperplastic parathyroid glands compared to thyroid tissue, which clears the tracer much faster. After IV injection of the tracer, early scans are taken at 15 minutes. Additional images are taken 2 hours later. The initial early scans show uptake in both parathyroid and thyroid tissues. The delayed scans at the 2-hour mark show clearance of thyroid activity. Increased activity demonstrated in delayed images is an accurate indicator of parathyroid adenomas (90% sensitivity).
