**3.2.2 I-123**

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 imaging) and 127 keV.

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