Influence of high-energy photons on the spectrum of iodine-123 with low- and medium-energy collimators: consequences for imaging with 123I-labelled compounds in clinical practice.

Using iodine-123 labelled radiotracers, the presence of 2.5% high-energy photons causes image deterioration due to increased scatter. To investigate the influence of these photons on image quality, we measured the spectrum of 123I with a medium-energy (ME), a low-energy all-purpose (LEAP) and a low-energy high-resolution (LEHR) collimator. Even in air, using low-energy collimators a high baseline activity was observed over the total energy detection range of the gamma camera. The 159-keV photopeak to scatter activity ratio fell from 5.9 for ME to 3.6 and 2.9 for LE collimators. Acquisition of images with LEHR collimators with energy windows set at 159 keV and 500 keV demonstrated that the 159-keV LEHR image is a combination of the ME image of the object and of the LEHR 500-keV image. Because of their important septal penetration and greater geometric detection efficiency compared with the 159-keV photons of 123I, the contribution of high-energy photons is dependent on the source-detector distance. For a small source placed in air, the scatter to photopeak activities varied from 17.4% at 80 cm to 37.8% at 5 cm distance from an LEHR collimator. Considering only the scatter problem, ME collimators are the best choice for 123I studies. When using LE collimators for high-resolution tomography with 123I-labelled compounds, scatter contribution from high-energy photons has to be corrected for quantitative analysis or when dual-isotope studies are performed, whether or not these studies are acquired simultaneously.