See [[Thyroid and Parathyroids]].
Most commonly used to evaluate hyperthyroidism. Solitary nodules best evaluated initially with FNA.
Technitium-99m-pertechnetate (Tc-99m-Osub>4) HL 6h, 140keV, performed at 4-6hrs. Trapped within thyroid follicles, but not organified, hence subsequently washes out. Best for imaging hyperthyroidism, is inexpensive. Lower target-to-background ratio.
Radioiodine is trapped in the thyroid, converted into thyroxine (organified) and stored within colloid cells. Some thyroid tumours (papillary carcinoma; not medullary or anaplastic) also trap iodine, but metabolism is disordered hence is much slower (hence cold cf thyroid gland, but hot cf rest of body). contained in intrathyroidal thyroglobulin within thyroid follicles (physiologic). Best for general thyroid imaging esp nodules, can be used for imaging and RAIU, useful in differentiating substernal thyroid from thymus. Iodine radioactive iodine uptake (RAIU) measurement – percentage of administered dose present in thyroid gland at set time after dose = (neck-thigh cpm)/(standard-background cpm). Performed at 4-6hrs (to check for rapid turnover/organification and excretion in Graves) and 24hr (normal 10-30%). Used for differentiating Graves (high uptake, >35% at 24h) from subacute/factitious hyperthyroidism (<2%); calculations for iodine treatment of Graves; toxic multinodular goiters. Whole-body scans for evaluating residual thyroid and thyroid cancer metastases.Uptake in thyroid bed represents residual thyroid tissue. Iodine secreted by salivary glands, hence uptake in salivary glands, nasopharynx (minor glands), stomach, bowel; also seen in bladder, breast activity physiological. Activity in lungs, skeleton, neck remote from thyroid bed pathologic.
- I-123 HL 13h, 159keV, po 18-24hrs prior to scan, cyclotron produced. Expensive.
- I-131 HL 8d, high energy beta and 364keV, reactor-produced. Inexpensive, high dose per mCi, high energy unsuitable for gamma camera. For thyroid tissue/tumour ablation (beta radiation) and imaging.
Pinhole collimator best, but resolution limited to ~10mm due to inherent gamma camera limitations.
Thallium-201 (Tl-201), Tc-99m-sestamibi/MIBI, and TC-99m-tetrofosmin/Tfos may show non-I-131-avid tumour.
Patients must withdraw from thyroxine and foods with high iodine eg seafood esp seaweed, as these will inhibit Tc or I uptake.
Imaging to localise abnormalities in hyperparathyroidism. No role for imagign in hypoparathyroidism.
Tc-99m-pertechnate/Tl-201 subtraction imaging – thyroid concentrates both, where parathyroid adenomas take up only Tl-201. Tl-201 acquired first, then Tc-99mO4 without moving patient with subsequent subtraction. Residual activity represents presence of parathyroid adenoma (sensitivity 75%, specificity 90%). False positives with Tl-201 uptake in thyroid nodules, sarcoid containing LN, neck metastases. This imaging has been repalced with sestamibi/tetrofosmin.
Sestamibi (Tc-99m-MIBI) and tetrofosmnin (Tc-99m-Tfos) imaging acquires immediate and delayed (1-2hrs) iamges. Parathyroid adenomas may be seen on immediate images, but retain on delayed (mitochondria-rich cells), where normal thyroid washes out. Most are infero-posterior to the thyroid. False-negative in clear cell adenomas (paucity of mitochondria).
I-131-MIBG taken up by adrenal medullary cells eg pheochromocytoma, also tumours of neural crest origin (neuroblastoma, medullary thyroid cancer). Detects neuroblastomas and metastases in >90% of cases.
Indium-111 pentetreotide is a synthetic somatostatin analogue with longer plasma half-life than native somatostatin; useful for imaging neuroendocrine tumours.
I-131-6-iodomethyl-19-norcholesterol (NP59) is a cholesterol analogue (precurser of minerolocorticoids, glucocorticoids, androgens) taken up by adrenal cortex