title: Photon penetration and scatter in micro-pinhole imaging: a Monte Carlo investigation.
abstract: Pinhole SPECT is rapidly gaining popularity for imaging laboratory animals using gamma-emitting molecules. Penetration and scattering of gamma radiation in the pinhole edge material can account for a significant fraction of the total number of photons detected, particularly if the pinholes have small diameters. This study characterizes the effects of penetration and scatter with micro-pinholes made of lead, tungsten, gold and platinum. Monte Carlo simulations are performed for 1-125 (27-35 keV) and Tc-99m (140 keV) point sources with pinhole diameters ranging from 50 to 500 microm. The simulations account for the effects of photo-electric interaction, Rayleigh scattering, Compton scattering, ionization, bremsstrahlung and electron multiple scattering. As a typical example, in the case of a Tc-99m point source and pinholes with a diameter of 300 microm in gold or platinum, approximately 55% of the photons detected resulted from penetration and approximately 3% from scatter. For pinhole diameters ranging from 100 to 500 microm, the penetration fraction for tungsten and lead was approx a factor of 1.0 to 1.6 higher and the scatter fraction was 1.0 to 1.8 times higher than in case of gold or platinum. Using I-125 instead of Tc-99m decreases the penetration fraction by a factor ranging from 3 to 11 and the scatter fraction by a factor ranging from 12 to 40. For all materials studied, the total amounts of penetrated and scattered photons changed approximately linearly with respect to the pinhole diameter.
authors.affiliation: Image Sciences Institute, UMC Utrecht STR 5.203, Universiteitsweg 100. 3584 CG Utrecht, The Netherlands. f.vanderhave@azu.nl
authors.forename: Frans
authors.initials: F
authors.lastname: van der Have
authors.forename: Freek J
authors.initials: FJ
authors.lastname: Beekman
mesh_terms: D004583:Electrons; D006046:Gold; D007091:Image Processing, Computer-Assisted; D007477:Ions; D007854:Lead; D015233:Models, Statistical; D008962:Models, Theoretical; D009010:Monte Carlo Method; D017785:Photons; D010984:Platinum; D012542:Scattering, Radiation; D013667:Technetium / Q000378:metabolism; D015899:Tomography, Emission-Computed, Single-Photon / Q000379:methods*; D014414:Tungsten
chemical_list: D007477:Ions; D007854:Lead; D010984:Platinum; D013667:Technetium; D006046:Gold; D014414:Tungsten