Approach to Thyroid and Parathyroid Imaging

Approach to Thyroid and Parathyroid Imaging

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Introduction

Thyroid imaging scans and quantitative thyroid uptake determinations remain a high proportion of the clinical nuclear medicine practice, particularly to characterize nodular thyroid disease and diagnose and treat patients with hyperthyroidism. With the advent of minimally invasive parathyroidectomy techniques in the 1990s, the Tc-99m sestamibi parathyroid scan was developed to localize parathyroid adenomas and direct surgical approach. In recent years, SPECT/CT has been added to parathyroid scan protocols to provide valuable 3D localization.

Thyroid Scan

Thyroid scan with I-123 is indicated for patients with indeterminate thyroid nodules on US &/or biopsy. In the case of indeterminate biopsy results, if the nodule is hot on thyroid scan, a hyperfunctioning nodule is confirmed and malignancy is unlikely. In addition, Tc-99m pertechnetate or I-123 thyroid scans are useful to confirm the etiology of hyperthyroidism and help guide therapy.

Traditionally, planar images with anterior and anterior oblique images over the thyroid were obtained. However, SPECT (± CT) images are now common, as they have better resolution, can be more easily compared to US, and generally take less time than traditional planar imaging.

On thyroid scan, nodular and diffuse thyroid disease can be diagnosed:

• Toxic adenoma appears as solitary hot nodule with relative suppressed uptake of radiotracer in remainder of thyroid gland
• Toxic multinodular goiter presents with multiple hot, warm, and cold nodules in normal-sized or enlarged thyroid gland
• Graves disease shows homogeneous, intense uptake in diffusely enlarged thyroid gland ± pyramidal lobe; physiologic salivary gland uptake is decreased
• Subacute thyroiditis shows little to no uptake in expected location of thyroid

Thyroid Uptake

Thyroid uptake determinations show how much radioactive iodine is absorbed by the thyroid gland. Four or 24 hours after administration of a small amount of I-131 or I-123, a gamma probe over the patient’s neck counts gamma rays emitted for 1 minute. This is compared to counts from a standard dose of radioactive iodine counted in a neck phantom (100% uptake). Physiological background uptake in the patient (gamma probe counts over the thigh) is subtracted. Although normal iodine uptake values can vary depending on regional iodine ingestion, the typical normal value at 24 hours ranges 15-30%.

In patients with hyperthyroidism, uptake percentages help clinicians determine the etiology of hyperthyroidism and the appropriate dose of I-131 for therapy.

• Toxic adenoma: Thyroid uptake determination often low-normal or normal, as most of radioactive iodine uptake occurs in toxic adenoma
• Toxic multinodular goiter: Thyroid uptake determination is often normal or mildly elevated (< 50%)
• Graves disease: Thyroid uptake value usually > 50%, often 80-90%
• Subacute thyroiditis: Thyroid uptake value < 5%, often ~ 1%

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Tags: Diagnostic Imaging Nuclear Medicine

May 7, 2023 | Posted by admin in CARDIOVASCULAR IMAGING | Comments Off

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