MCNP-4c simulations to investigate shielding of scintillation cameras for conjugate imaging of I-123 labeled brain agents

Monte Carlo simulations were performed to characterize various shielding configurations for an iodine-123 (I-123) imaging system. The system comprises two small (10×10 cm) field-of-view (FOV) gamma cameras to be used for conjugate imaging of I-123 labeled brain agents. 83% of I-123 decays result in 159 keV gamma emissions, which can be readily imaged; while 3% result in emissions with energies from 248 to 783 keV. If the cameras have inadequate shielding, a large amount of activity outside the FOV of the cameras can produce significant image noise due to penetration and scatter of the high energy emissions. A 10 mCi injection of an I-123 labeled neuroreceptor ligand generally results in sub-mCi levels of uptake in the brain-μCi levels of uptake in basal ganglia, while most of any injected dose accumulates in the gastrointestinal (GI) tract. Therefore, sufficient shielding is required if accurate quantification is desired. Various prototype camera designs with wall thicknesses ranging from 1/4 to 3/4 inch were tested via Monte Carlo N-Particle Transport code, version 4c (MCNP-4c) simulation of an idealized primate and a small FOV camera. The primate consisted of water-filled shapes for the head, neck, and trunk. High-energy photons were emitted from a uniform distribution in a spherical GI tract region. The camera consisted of tungsten, lead, and aluminum walls; lead distributed uniformly at 22% density representing a typical high-resolution collimator; photomultiplier tube material; a quartz lightguide; and a 0.8×10×10 cm NaI(Tl) scintillation crystal. For each camera configuration, the total energy deposited in the crystal per high-energy source photon was tallied to produce spectra and windowed counts. If a typical high-resolution collimator were to be used in the imaging of a primate head, 10 μCi of focal uptake would result in approximately 12.5 cps in a 159 keV 20% window. For the shielding configurations tested, 10 mCi of GI tract activity would produce, within a 5 cm² region of interest (ROI) encompassing the focal uptake, a 3 to 30% (for 3/4 to 1/4 inch shielding) increase over the base rate of 12.5 cps.