Radionuclide imaging of the thyroid is a direct extension of the clinical examination. It is important for the nuclear physician to be familiar with the setting in which the examination is being requested. Patients may be self-medicating with vitamins, kelp or other substances containing iodine and it is recommended to defer the examination for several weeks after discontinuation of these substances (Table 1). Clinical examination of the patient while under the camera allows correlation of palpable features with those of the scan. This ought always to be done by the physician who will report the examination and who should be aware of the presenting complaint, the relevant clinical history and laboratory data. A positive family history will increase the pre-test likelihood for multinodular goitre and Graves's disease. A complaint of pain predisposes toward thyroiditis. A history of radiation exposure with a symptomatic mass increases the probability of a malignancy.
Thyroid imaging is best performed on a camera fitted with a pinhole collimator in order to achieve the highest possible spatial resolution. The disadvantages of pinhole collimation are those of image distortion in depth caused by parallax and of the low sensitivity. Parallax increases as one moves from the center of the field to the periphery and will distort an off-centered image. Externally placed anatomical markers may seem misplaced on account of parallax.
The uptake probe is a nonimaging device that has high sensitivity for counting radiation and is routinely used to measure RAIU by the thyroid. The uptake measurement requires calibration of the probe with the patient dose. When the uptake has stabilized (about 24 hours later) the patient's neck is counted and the percent uptake is calculated with a correction for decay during the interval. In North
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