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 γ-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 low-energy 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 γ-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.