**3.3.1 Planar imaging**

6 12 Chapters on Nuclear Medicine

by transforming into the stable Xe-131 in two steps, with gamma decay following rapidly after beta decay. The primary emissions of I-131 decay are beta particles with a maximal energy of 606 keV (89% abundance, others 248–807 keV) and 364 keV gamma rays (81%

I-131 is administered orally with activities of 1–5 mCi or less, with many preferring a range of 1–2 mCi because of the data suggesting that stunning (decreased uptake of the therapy dose of I-131 by the residual functioning thyroid tissue or tumour due to cell death or dysfunction caused by the activity administered for diagnostic imaging) is less likely at the lower activity range. However, detection of more iodine concentrating tissue has been

Type of medication Recommended time interval of withdrawal

Radiographic contrast agents 3 to 6 months, depending on iodide content

Table 2. Recommended time intervals of withdrawal for drugs affecting radioiodine uptake. The time interval can be changed by the administered doses of the medications. The amount of iodine for the drug must also be considered.(Nostrand, Bloom et al. 2004; Silberstein,

I-123 is produced in a cyclotron by proton irradiation of enriched Xe-124 in a capsule and I-123 decays by electron capture with a half-life of 13.22 hours to Te-123 and it emits gamma radiation with predominant energies of 159 keV (the gamma ray primarily used for

I-123 is mainly a gamma emitter with a high counting rate compared with I-131, and I-123 provides a higher lesion-to-background signal, thereby improving the sensitivity and imaging quality. Moreover, with the same administered activity, I-123 delivers an absorbed radiation dose that is approximately one-fifth that of I-131 to the thyroid tissue, thereby lessening the likelihood of stunning from imaging. I-123 is administered orally with activities of 0.4–5.0 mCi,

I-124 is a proton-rich isotope of iodine produced in a cyclotron by numerous nuclear reactions and it decays to Te-124 with a half-life of 4.18 days. Its modes of decay are: 74.4%

which may avoid stunning.(Ma, Kuang et al. 2005; Silberstein, Alavi et al. 2006)

3 to 4 weeks 10 to 14 days

6 weeks

2 to 4 weeks

abundance, others 723 keV).

Natural synthetic thyroid hormone

Lugol's solution, potassium iodide solution

Iodine containing expectorants and anti-

Alavi et al. 2005; Luster, Clarke et al. 2008)

Thyroxine (T4)

(SSKI)

tussives

**3.2.2 I-123** 

**3.2.3 I-124** 

imaging) and 127 keV.

Triiodothyroinine (T3)

reported with higher dosages.(Silberstein, Alavi et al. 2006)

Amiodarone 3 to 6 months Multivitamine 6 weeks

Topical iodine 6 weeks

Iodinated eyedrops and antiseptics 6 weeks

Planar gamma camera imaging can be obtained with gamma emitting I-123 or I-131 for the detection of thyroid cancer tissue expressing the NIS gene which takes up iodine. The main emission energy peak of I-131 is approximately 364 keV, so it requires the use of a high-energy all-purpose collimator for imaging acquisition. The peak of the I-123 is 159 keV, which is close to the 140 keV from Tc-99m for which the gamma camera's design has traditionally been optimized. I-123 can be imaged with a low-energy high-resolution collimator, which is optimized for image acquisition with Tc-99m. (Rault, Vandenberghe et al. 2007)

With radioiodine's avidity for differentiated thyroid cancer tissues, planar radioiodine whole body image has been mainly used for the detection of metastatic thyroid cancer lesions. However, the limited resolution of planar imaging together with the background activity in the radioiodine images can give false-negative results for small lesions. Physiologic uptake of radioiodine is not always easily differentiable from pathologic uptake and it can give false-positive results. (Spanu, Solinas et al. 2009) Therefore, the sensitivity and specificity of planar images for the diagnosis of metastatic thyroid cancer may be limited. (Oh, Byun et al. 2011)
