thyroid scan (I-123)
Thyroid scan (thyroid scintigraphy) is a nuclear medicine examination used to evaluate thyroid tissue.
Clinical indications
- functional status of a thyroid nodule
- thyrotoxicosis: differential diagnosis
- thyroid cancer
- whole body scan for distant metastases
- estimation of local residual thyroid post thyroidectomy
- follow-up for tumor recurrence
Patient preparation
Medications that interfere with thyroid uptake of radioiodine should be discontinued. Review of the history should be carried out to ensure the patient has not received iodine-containing contrast (e.g. for CT or angiography). Patients should be fasted for 4 hours prior to study.
Dose, route of administration and timing
Iodine-123 is the most commonly used radioisotope. It is administered orally in capsule form (3.7-14.8 MBq (100-400 μCi)). Scanning is performed either at 4-6 or 24 hours.
An alternative radioisotope is Tc-99m pertechnetate. Administration is intravenous, and imaging must be done early (maximum uptake at about 20 minutes).The pertechnetate anion, captured by the follicular cells of the thyroid - unlike radio-iodine - is not organificated.
Equipment
- camera: gamma camera
- collimator: 3-6 mm aperture pinhole collimator
- window: 20% energy window centered at 159 keV.
Procedure
The patient is positioned supine with the chin up and the neck extended. The collimator is then positioned so that the thyroid fills about two-thirds of the diameter of the field of view. Mark the chin and suprasternal notch. Note the position and mark palpable nodules and surgical scars. Place marker sources lateral to the thyroid to calibrate size.
Three views are typically obtained: anterior; 45-degree LAO; and 45 degree RAO (move the collimator, if possible, rather than the patient).
Each view should have 100-250,000 counts.
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