Evaluation of penetration and scattering components in conventional pinhole SPECT: Phantom studies using Monte Carlo simulation

In quantitative pinhole SPECT, photon penetration through the collimator edges (penetration), and photon scattering by the object (object scatter) and collimator (collimator scatter) have not been investigated rigorously. Monte Carlo simulation was used to evaluate these three physical processes for different tungsten knife-edge pinhole collimators using uniform, hotspot and donut phantoms filled with ²⁰¹Tl, ^99mTc, ¹²³I and ¹³¹I solutions. For the hotspot phantom, the penetration levels with respect to total counts for a 1 mm pinhole aperture were 78%, 28% and 23% for ¹³¹I, ¹²³I and ^99mTc, respectively. For a 2 mm aperture, these values were 65% for ¹³¹I, 16% for ¹²³I and 12% for ^99mTc. For all pinholes, ²⁰¹Tl penetration was less than 4%. The evaluated scatter (from object and collimator) with a hotspot phantom for the 1 mm pinhole was 24%, 16%, 18% and 13% for ²⁰¹Tl, ^99mTc, ¹²³I and ¹³¹I, respectively. Summation of the object and collimator scatter for the uniform phantom was approximately 20% higher than that for the hotspot phantom. Significant counts due to penetration and object and collimator scatter in the reconstructed image were observed inside the core of the donut phantom. The collimator scatter can be neglected for all isotopes used in this study except for ¹³¹I. Object scatter correction for all radionuclides used in this study is necessary and correction for the penetration contribution is necessary for all radionuclides but ²⁰¹Tl.