Thyroid scan

Thyroid Scan
Nuclear Medicine

1. RADIONUCLIDE THYROID IMAGING

2. Indications

3. 1. Assessment of the gland anatomy e.g size and shape. 2. Assessment of the gland function (hyper- or hypo- thyroidism ). 3. Detection of thyroid nodules and differentiate between cold and hot ones.

4. 4. Post-operative (e.g. post- thyroidectomy ) assessment 5. detection of functioning metastatic tissues in known cases of thyroid malignancy  6. Detection of Retrosternal Goiter

5. Contraindications

6. 1 .pregnancy 2 .Patients with Hypersensitivity reaction to iodine 3 . Pertechnetate is excreted in breast milk, so breastfeeding women are advised to express and discard breast milk for 26 hours after injection.

7. Radiopharmaceuticals

8. 99m Tc Pertechnetate : Used mainly in anatomical assessment It carries the properties of 99m Tc Gamma-emitter = 140 Kev Physical half life = 6 hrs. Dose: 3-5 mCi . Route: IV. Advantages: cheap and available

9. 131 I -sodium iodide: Used in full Assessment (Function & Anatomy) Gamma-emitter = 364 Kev Half life: 8 days. Dose: 3 - 5 mCi Route: Orally. Disadvantage: Relatively expensive , Difficult preparation & B particle emitter

10. 123 I -sodium iodide Used in full Assessment (Function & Anatomy) Gamma-emitter = 159 Kev Half life: 13 hours. Dose: 100 mCi Route: orally. Advantage: Pure Gamma emitter (Does not emit B particles) Disadvantage: Very expensive with limited availability

11. Equipment: Gamma camera with pinhole collimator

12. Patient preparation

13. 1. Stoppage of anti-thyroid drugs e.g. Carbimazole for 2 days before scanning. 2. Stoppage of thyroid hormones e.g. thyroxin 4 weeks prior to examination. 3. The patient should avoid iodinated IV contrast media e.g. for IVP for 4 weeks before examination .

14. 4.Stoppage of iodine rich food for one week (fish, cauliflower milk ) 5.patients should be fasted 4 hours prior to examination

15. Patient position: The patient lies supine, with hyper - extended neck and the camera anterior to the neck. Fixed Distance Between Camera and neck (20 cm ) Radio-opaque land mark e.g piece of lead or radioactive tracer is put at chin and suprasternal notch to help detect actual site of the gland and retrosternalgoiter if present

16. In patients who are unable to lie supine, a sitting position may be employed.

17. Views : Anterior Right & Left oblique Whole body scan only in detection of functioning metastatic tissues in known cases of thyroid malignancy ( Anterior- Posterior –oblique)

18. Scanning Time : 1. When Tc-99m is used, imaging should begin 15–30 min after injection. 2. When I-131 is used, the images should be obtained at 2h, 24 hr & 3- 4 days after ingesting the radioiodine. 3.When I-123 is used, images can be obtained as early as after 4-6 H, then at 16 – 24 H

19. Interpretation

20. Normal 99 Tc normal uptake : 1-3 % of total dose Radioactive iodine 131 I normal uptake : Early uptake after 2 h : 10-20 % of total dose Late uptake after 24 hours : 20 – 60% of total dose In case of evaluating functioning cancer thyroid metastasis in whole body scan uptake after 3-4 days will appear as Hot spot any where in the body

21. Gland Should be situated mid-way between symphysis menti & suprasternal notch butterfly shaped Right lobe is somewhat larger than the left In 10% of the patients pyramidal lobe may be present. Homogenous uniform symmetrical uptake all over the gland

22. ANTERIOR R ANT. OBLİQUE L ANT. OBLİQUE

23. Abnormal study : 1. site : ectopic thyroid tissue , retrosternal Goiter 2. size : Enlarged ( Goiter ) 3. Function increased ( hyperthroidism ) , decreased (Hypothyroidism) 4 . Distribution  Diffuse ( Graves Disesae )  Local ( Hot ,cold nodule)

24. Cold Nodule : 80 % (cyst , abcess ,hematoma ) 20% Tumor benign ( Adenoma ) ,Malignant (Non functioning carcinoma) Hot nodule :  Solitary (Autonomous nodule, functioning adenoma or adenocarcinoma )  Multiple ( multiple nodular goiter )

25. Scintigraphy with Tc-99m pertechnetate revealed a diffusely enlarged thyroid gland with homogenous uptake, suggesting diffuse toxic goiter.

26. Retrosternal goiter

27. The activity uptake decreases with variable sizes in hypothyroidism

28. Hot nodule cold nodule

29. Radioactive iodine Uptake may increase in following : H yperthyroidism ,Iodine deficiency ,Pregnancy ,Recovery phase of subacute , silent or postpartum  thyroiditis ,Rebound after withdrawal of antithyroid medication ,Lithium carbonate therapy ,Hashimoto thyroidites May decrease in the following : Hypothrodism , Destructive thyroiditis ( subacute thyroiditis , silent thyroiditis , postpartum thyroiditis )

30. Because of the large radiation dose to the thyroid (approximately one to three rads per mCi administered), the use of I-131 for thyroid scintigraphy should be discouraged (except when a subsequent treatment with I-131 is planned). (Society of nuclear medicine procedure guide lines for thyroid scintigraphy ) I-131 is usually spared for metastatic disease screening for its high radiation dose and inferior image quality. Due to its mode of  beta decay , iodine-131 is notable for causing  mutation  and death in cells that it penetrates, and other cells up to several millimeters away. For this reason, high doses of the isotope are sometimes less dangerous than low doses, since they tend to kill  thyroid tissues that would otherwise become cancerous as a result of the radiation. For example, children treated with moderate dose of I-131 for thyroid adenomas had a detectable increase in thyroid cancer, but children treated with a much higher dose did not. Likewise, most studies of very-high-dose I-131 for treatment of  Graves disease  have failed to find any increase in thyroid cancer, even though there is linear increase in thyroid cancer risk with I-131 absorption at moderate doses. [2]   Thus, iodine-131 is increasingly less employed in small doses in medical use (especially in children), but increasingly is used only in large and maximal treatment doses, as a way of killing targeted tissues. This is known as "therapeutic use."

31. Dr.Mohamed Nader Fouad Dr.Asmaa Youssry Alkasaby