Which radioisotopes can be used for nuclear medicine imaging of the thyroid gland, and how do they compare physiologically?
The most common radiotracers used in thyroid scintigraphy are the various isotopes of iodine. 123 I is the most commonly used isotope because of its excellent image quality and relatively low radiation exposure. Isotopes 125 I and 127 I are not typically used because of their high radiation exposure and worse image quality. 131 I is still used for thyroid imaging in some patients with thyroid cancer. Its higher photon energy theoretically makes it easier to detect cancer foci in the deep soft tissues in the body. However, 131 I is associated with a higher radiation exposure to patients, and the image quality is not typically as good as with 123 I. 124 I is a positron-producing isotope that can be imaged by positron emission tomography (PET) and has been shown to be valuable in the detection of residual thyroid tissue in patients with thyroid cancer. All of the iodine radiotracers are trapped by the thyroid gland and undergo organification into the thyroid hormones. The other primary radiotracer utilized for thyroid scintigraphy is technetium-99m ( 99m Tc) pertechnetate. This radiotracer is trapped (not organified) by the thyroid gland and can provide high-quality images. The amount of uptake of this radiotracer cannot be used to plan 131 I therapy, in contradistinction to that with the iodine radiotracers.