Primary, scatter, and penetration characterizations of parallel-hole and pinhole collimators for I-123 SPECT

Abstract

Multi-pinhole (MPH) collimators are known to provide better trade-off between sensitivity and resolution for preclinical, as well as for smaller regions in clinical SPECT imaging compared to conventional collimators. In addition to this geometric advantage, MPH plates typically offer better stopping power for penetration than the conventional collimators, which is especially relevant for I-123 imaging. The I-123 emits a series of high-energy (>300 keV, ~2.5% abundance) gamma photons in addition to the primary emission (159 keV, 83% abundance). Despite their low abundance, high-energy photons penetrate through a low-energy parallel-hole (LEHR) collimator much more readily than the 159 keV photons, resulting in large downscatter in the photopeak window. In this work, we investigate the primary, scatter, and penetration characteristics of a single pinhole collimator that is commonly used for I-123 thyroid imaging and our two MPH collimators designed for I-123 DaTscan imaging for Parkinson’s Disease, in comparison to three different parallel-hole collimators through a series of experiments and Monte Carlo simulations. The simulations of a point source and a digital human phantom with DaTscan activity distribution showed that our MPH collimators provide superior count performance in terms of high primary counts, low penetration, and low scatter counts compared to the parallel-hole and single pinhole collimators. For example, total scatter, multiple scatter, and collimator penetration events for the LEHR were 2.5, 7.6 and 14 times more than that of MPH within the 15% photopeak window. The total scatter fraction for LEHR was 56% where the largest contribution came from the high-energy scatter from the back compartments (31%). For the same energy window, the total scatter for MPH was 21% with only 1% scatter from the back compartments. We therefore anticipate that using MPH collimators, higher quality reconstructions can be obtained in a substantially shorter acquisition time for I-123 DaTscan and thyroid imaging.

Recent Insights in Barium-131 as a Diagnostic Match for Radium-223: Cyclotron Production, Separation, Radiolabeling, and Imaging

“…Many of these high-energy photons are assumed to pass through the crystals of the SPECT camera and undergo Compton scattering from the components adjacent to the crystals (e.g., photo-multiplier tubes and associated electronics). These inelastically scattered photons are then detected by re-entering the crystal with a lower energy contaminating the projection data recorded within the photopeak windows [ 29 ].…”

The clinical utilities of multi-pinhole single photon emission computed tomography

“…This is particularly an issue for the case of down-scatter from higher-energy photons, as for those emitted by 123 I with ~2-3% abundance in addition to its primary photon. Könik et al demonstrated that for 123 I DaTscan imaging, total scatter, multiple scatters, and penetration events for the low energy high resolution (LEHR) parallel-hole collimators were 2.5, 7.6 and 14 times more than that of an MPH collimator within the 15% photopeak window and with a 2-cm thick tungsten aperture plate (37).…”

“…Mok et al (37) found that the MPH SPECT for small FOV imaging is able to achieve high detection efficiency without much deterioration in resolution, as MPH provided more angular sampling compared to conventional LEHR and single pinhole collimator. Funk et al (64) proved that MPH collimators with nine pinholes reduced the axial artifacts and distortion compared to single pinhole for the same number of angular views.…”

Inclusion of quasi-vertex views in a brain-dedicated multi-pinhole SPECT system for improved imaging performance

“…In this context, the use of multi-pinhole collimation (MPC) has proven to be successful in significantly enhancing the clinical imaging of the brain, thyroid or heart (Beekman & van der Have 2007, Van Audenhaege et al 2015. MPC offers an excellent sensitivity to spatial resolution trade-off for brain imaging (Chen et al 2019), while reducing the fraction of down-scatter counts (Könik, Auer, De Beenhouwer, Kalluri, Zeraatkar, Furenlid & King 2019). Moreover, MPC provides improved penetration characteristics which makes its use more suited for high-energy photon-emitting isotope imaging (e.g., 111 In, 123 I) (Van Audenhaege et al 2013, Könik, Auer, De Beenhouwer, Kalluri, Zeraatkar, Furenlid & King 2019).…”