Study of the point spread function (PSF) for 123I SPECT imaging using Monte Carlo simulation.

Area of Science:

• Nuclear Medicine
• Medical Imaging Physics
• Radiochemistry

Background:

• Iterative reconstruction algorithms in brain SPECT require accurate point spread function (PSF) modeling for quantitative imaging.
• Iodine-123 (123I) is commonly used in neurotransmitter SPECT studies, but its decay scheme includes higher energy gammas beyond the primary 159 keV.
• These higher energy photons may contribute to the overall PSF, potentially affecting image reconstruction accuracy.

Purpose of the Study:

• To investigate the contribution of higher energy gamma rays from 123I to the PSF in brain SPECT.
• To evaluate this contribution for parallel and fan beam collimator configurations.
• To assess the impact on image quality metrics such as sensitivity and spatial resolution.

Main Methods:

• Monte Carlo simulations using the PENELOPE code were employed to model radiation transport.
• A custom program tracked photon trajectories through collimator and detector geometries.
• Simulated PSFs were validated using experimental measurements with an 18F source (511 keV photons).

Main Results:

• Higher energy gamma rays constitute a significant portion of detected events (up to ~49% for parallel collimator) within the 159 keV energy window.
• These higher energy photons contribute to the PSF beyond the region dominated by low-energy photons, acting as noise.
• Fan beam collimators demonstrated superior signal-to-noise ratios compared to parallel collimators across analyzed source positions.

Conclusions:

• The contribution of higher energy gamma rays from 123I to the PSF is substantial and must be considered in reconstruction algorithms.
• These high-energy counts should be treated as noise to improve quantitative accuracy in brain SPECT.
• Fan beam collimators offer improved image quality in terms of signal-to-noise ratio for 123I brain SPECT.