Finding The Ideal Thyroid Imaging Agent

1. Use of ¹³¹I
1. In general, this agent should not be used in a diagnostic setting
1. A long T_1/2 and beta radiation drastically increase the radiation burden to the thyroid and the patient
2. Gamma rays of 364 keV is not suited for a scintillation detector
2. When should ¹³¹I be used for imaging?
1. If a substernal thyroid is suspected the use of higher energy gamma might be suggested since additional attenuation occurs from the sternum. ¹³¹I higher energy gamma will compensate for the increased bone density. However, current literature appears to suggest that ¹²³I should be used for this diagnostic purpose
2. Whole body imaging for the detection of metastatic thyroid cancer
3. Treatment of thyroid cancer

2. Use of ¹²³I
1. In general, this is consider the agent of choice of both uptake and scan
2. Significantly lower radiation burden when compared to ¹³¹I
3. More suitable for gamma camera instrumentation
4. The pure form of ¹²³I is produced via ¹²⁷I(p,5n)¹²³Xe (T_1/2 = 2.1 hrs). Xenon decays to ¹²³I
5. The "dirty" form of ¹²³I can also be produced which yields ¹²⁴I contamination via Te(p,2n)¹²³I
1. ¹²⁴I contamination is considered a contaminant. It is a positron emitter and has T_1/2 of 4.17 days
2. The longer ¹²³I decays, the greater of level ¹²⁴I
3. From a dose of 99 μCi of ¹²³I and 1 μCi of ¹²⁴I, what would the percentage of ¹²⁴I contamination be after 26.4 hours? The T_1/2 for ¹²³I is 13.2 hours
1. At 0 hrs 1/99 * 100 = 1% ¹²⁴I contamination
2. After 26.4 hrs there would be 24.75 μCi of ¹²³I would remain
3. At 26.4 hrs there would be 0.8 μCi of ¹²⁴I
4. At 26.4 hours 0.8/24.75 * 100 = 3% ¹²⁴I contamination
5. The important aspect to appreciate is that as time increases the percent of ¹²⁴I contamination goes up

3. Use of pertechnetate
1. There is an issue regarding a single nodule (suspected thyroid cancer), which maybe hot with pertechnetate, but cold with ¹²³I. Hence ¹²³I should be used with single nodular disease and its possible identification of thyroid cancer
2. In conditions where there is an insult to the thyroid (ex. Thyroiditis) pertechnetate maybe the agent of choice when it comes to imaging the gland. Remember the issue of trapped vs. organification. Therefore the gland may visual with pertechnetate, but not ¹²³I
3. Using pertechnetate will increased count density, therefore improving image resolution when compared to the other thyroid agents
4. Combining pertechnetate with the iodines for uptake and scan maybe suggested
1. If ¹³¹I is used the dose should be around 10 μCi
2. If ¹²³I is used the dose should be around 100 μCi
3. Following the 24 hour uptake inject 5-10 mCi of pertechnetate
4. Image 20-30 post administration
5. Never inject pertechnetate until after the 24 hour uptake is completed. Why?
4. Which iodine should be administered for whole body survey?
   1. It is suggested that ¹³¹I causes stunning of thyroid issue (Ca)
   2. Anderson and colleagues compared possible recurrence thyroid carcinoma with both iodines and concluded that ¹²³I was 90% effective and ¹³¹I was slightly less at 84%
   3. A similar finding was identified by Mandel and colleagues where ¹²³I WB survey found 35 foci and ¹³¹I discovered 32
   4. Bae and colleagues compared the iodines and concluded uptake of post-therapy ¹³¹I was more effective than ¹²³I in diagnosing lung masses
   5. Perhaps the issue might relate to the extended half-life of ¹³¹I
5. Match your serum hormone levels with the percent uptake whenever it is available

1. Hyperthyroidism should general show reduced TSH levels
2. Hypothyroidism should general show increased TSH levels
3. When the percent uptake and the TSH level do not agree further investigation maybe warranted
4. In addition, your free thyroid index should correlate with the thyroid uptake

Here are the normal thyroid hormones levels - Link