Our objective was to optimize the quality of 123 I-metaiodobenzylguanidine (MIBG) scans by using a medium-energy collimator to reduce high-energy-photon septal penetration. Methods: In addition to the 159-keV g-ray, 123 I has a small abundance of energies above 400 keV that can compromise the image quality of MIBG studies because of septal penetration. Using a lowenergy ultrahigh-resolution collimator (LEUHR), a low-energy high-resolution collimator (LEHR), and a medium-energy collimator, we obtained and compared SPECT and planar images of a SPECT phantom filled with 123 I. These studies were acquired at a count level comparable to clinical MIBG images, 24,000 counts per view for SPECT and 300,000 counts for planar imaging. Also, we evaluated the sensitivity of the 3 collimators at 0 and 10 cm using the National Electrical Manufacturers Association protocol. Results: The image quality for both SPECT and planar 123 I images using the medium-energy collimator was determined to be substantially better than that using the LEUHR or LEHR collimator. The septa of the medium-energy collimator are thicker than those of the low-energy collimators (1.14 vs. 0.13-0.16 mm), leading to a significant reduction in septal penetration of the high-energy g-rays and a marked improvement in image quality. The sensitivity for the medium-energy collimator did not change with distance (8.00 cpm/kBq), as opposed to the LEUHR collimator (6.59 and 5.51 cpm/kBq for 0 and 10 cm, respectively) and the LEHR collimator (14.32 and 12.30 cpm/kBq for 0 and 10 cm, respectively). This variation in sensitivity for the LEUHR collimator is again due to the presence of high-energy photons. Conclusion: Use of a medium-energy collimator substantially improves the quality of both planar and SPECT 123 I images. We recommend that a medium-energy collimator routinely be used for 123 I-MIBG imaging. In the past several years, many radiopharmaceuticals, including metaiodobenzylguanidine (MIBG), that were formerly labeled with 131 I are now labeled with 123 I because it has better dosimetric and imaging properties (1-3). 123 I decays by electron capture, with a 13.2-h half-life. The primary g-ray of its daughter, 123 Te, has an energy of 159 keV. In addition to the 159-keV g-ray (83.3% abundance), several other g-rays of higher energy and low abundance are also emitted as a result of the decay of 123 I. Several of these emissions are summarized in Table 1 (4). g-rays with energies greater than 400 keV are emitted 2.73% of the time, and g-rays with energies greater than 600 keV are emitted 0.23% of the time. The average energy of the higher-energy g-rays is 507 keV. Although these highenergy g-rays are in low abundance, the low-energy collimators traditionally used for 123 I-MIBG imaging are not effective in stopping them. The result-substantial septal penetration (i.e., the fraction of g-rays that cross the septa separating the collimator holes)-can, in turn, lead to a loss of contrast and image quality. Figure 1 shows a cross section of a parallel-hole collim...