INTRODUCTION
The most commonly used radiopharmaceutical for in-vivo investigation
of thyroid function and anatomy is sodium iodide, iodine-131.
However,
the radiation dose for usually administered doses is excessively high,
higher than for any other radionuclide examination,
Ready availability at
low cost with good shelf-life accounts for its widespread use.
Thyroid
function tests have been standardized to its use.
In an attempt to improve the quality of imaging procedures and to
reduce radiation dose some have advocated the use of iodine-125 (1-3).
The radiation dose from iodine-125 is still rather high and technical
problems are encountered in its use for quantitative studies.
More recently technetium-99m, as pertechnetate has enjoyed increasing
popularity as an imaging agent (4-7) in addition to its use for functional
evaluation of the thyroid (8-10).
Reasons for its popularity are low cost,
ready availability, high photon yield and low radiation dose.
Some
clinicians remain reluctant to accept it as an agent for assessment of
thyroid function because it is not a natural metabolite of the gland.
number of other drawbacks to its use have been noted.
A
On occasion there
may be a difference between the technetium (trapping) and iodine (binding)
image (11-16).
These differences occur for a variety of reasons and are
@ source of confusion.
When the thyroid gland is large or substernal in |
location, a poor image is obtained,
Often, with low uptakes the image is
not diagnostic due to the low target-nontarget ratio,
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