I-123 thyroid imaging
Patient preparation
- Discontinue any medications that interfere with thyroid uptake of radioiodine.
- NPO for 4 hours prior to study.
Radiopharmaceutical
I-123
| Photon energy | 159 keV |
| Adult dose | Routine uptake and scan (100-400 microCi po); thyroid cancer scan (1.5 mCi po) |
| Peds dose | |
| Physical half life | 13.2 hours |
| Biological half life | |
| Normal distribution | Thyroid, nasopharynx, salivary glands, stomach, colon, bladder, lactating breasts |
| Pharmacokinetics | Rapidly absorbed from GI tract; detectable activity within gland in minutes; reaches thyroid follicular lumen in 20-30 min; trapping and organification rapid; delayed imaging for background clearance; image at 6 and 24 hours; 10-30% uptake at 24 hours. |
| Excretion | Renal |
| Target organ | Thyroid, bladder, stomach, SI |
| Other | Secreted in breast milk; crosses placenta; may cause cretinism; discontinue interfering medications |
Dose and route of administration
100-400 microCi orally in capsule form
Time of imaging
At 4-6 or 24 hours
Equipment
- Camera: Gamma camera
- Collimator: 3- to 6-mm aperture pinhole collimator
- Window: 20% energy window centered at 159 keV.
Procedure
- Position the patient supine with the chin up and the neck extended.
- Position the collimator so that the thyroid fills about two-thirds of the diameter of the field of view.
- Obtain anterior, 45-degree LAO and RAO views (move the collimator, if possible, rather than the patient).
- Obtain 100k-250k counts per view.
- Mark the chin and suprasternal notch.
- Note the position and mark palpable nodules and surgical scars.
- Place marker sources lateral to the thyroid to calibrate size.
References
- Ziessman, Harvey A, O'Malley, Janis P, and Thrall, James M. Nuclear Medicine: The Requisites. 3rd ed. Philadelphia, Pennsylvania: Mosby, 2006. ISBN 0323029469