Nuclear Medicine
CLINICAL DECISION SUPPORT
Na[99mTc]TcO4 ([99mTc]Sodium pertechnetate
Na[99mTc]TcO4 is administrated by i.v. injection and is actively transported in the thyroid gland by the sodium iodide symporter of the thyroid follicular cells. The maximum accumulation is reached about 20 min after injection and then decreases quickly, because Na[99mTc]TcO4 is not organified.
The most common indication in adults and adolescents include
to differentiate congenital hypothyroidism in neonates,
Thyroid scintigraphy has been used for decades to explore thyroid dysfunction. It is the only imaging modality which correlates anatomy and function.
When compared to iodine-123, the advantages of Technetium-99m include wide availability, low cost, and low radiation exposure. Its disadvantages are low thyroid uptake (2-5 %, 20 min after injection of Na[99mTc]TcO4) and higher background due to salivary and vascular activity.
The suggested activity to administer is
In paediatric nuclear medicine, the activities should be modified according to the EANM paediatric dosage card (https://www.eanm.org/publications/dosage-calculator/). The minimum recommended activity to administer is 10 MBq.
The effective dose for Na[99mTc]TcO4 is 13 µSv/MBq [3]. The organ with the highest absorbed dose is the colon wall: 41 µGy/MBq
The effective dose for Na[99mTc]TcO4 is: 1.0 mSv per procedure. In neonates, the ED is about 2.2 mSv when the injected activity is 10 MBq. (EANM paediatric dosage card)
Caveat:
“Effective Dose” is a protection quantity that provides a dose value related to the probability of health detriment to an adult reference person due to stochastic effects from exposure to low doses of ionizing radiation. It should not be used to quantify the radiation risk for a single individual associated with a particular nuclear medicine examination. It is used to characterize a certain examination in comparison to alternatives, but it should be emphasized that if the actual risk to a certain patient population is to be assessed, it is mandatory to apply risk factors (per mSv) that are appropriate for the gender, the age distribution and the disease state of that population."
A normal finding shows bilobed thyroid gland thyroid located at the base of the neck. with homogenous and symmetrical uptake in the lobes, joined inferiorly and medially by the isthmus. Rarely pyramidal lobe is presented.
In a nodular thyroid gland, we evaluate the position of a nodule (upper, middle, lower part of the lobe) and its functionality:
Pitfall
The uptake of Na[99mTc]TcO4 could be decreased by recent administration of iodinated contrast materials, intake of stable iodine in any form, or anti-thyroid agents and other drugs.
There is no specific preparation for the test. Hence, the patient should be questioned carefully regarding diet, History of potentially interfering medications (e.g. thyroid hormones, iodine containing medicines), and radiographic exams (Exposure to iodinated contrast) prior to administering the radiopharmaceutical. If necessary, the examination should be delayed
Additional information can be found in the EANM/SNMMI guideline [147].
Thyroid scintigraphy is usually performed about 20 min post injection. The patient is scanned supine with the neck in hyperextension. or if using a γ camera equipped with a pin-hole collimator it may be performed in sitting position of the patient. Static images in AP position are obtained, in a time range of 5-15 min. The location of palpable nodules is confirmed with a radioactive point source.
In neonates, the anterior images are acquired 15 min post Technetium-99m injection with a LEHS collimator. Given the small size of newborns, the whole body is displayed on a single field. Complementary acquisitions (pinhole or profile) depend on the results of the anterior image.