High-resolution PET detector design: modelling components of intrinsic spatial resolution

Abstract

The development of dedicated small animal PET (positron emission tomography) scanners has led to significantly higher spatial resolution and comparable sensitivity to clinical scanners. However, it is not clear whether we are approaching the fundamental limit of spatial resolution. This work aims to understand what is currently limiting spatial resolution during data formation and collection and how to apply that knowledge to obtain the best possible resolution for small animal PET without sacrificing sensitivity. Monte Carlo simulations were performed of the interactions of a 511 keV photon in a variety of detector materials to evaluate the modulation transfer function of the materials. Positron range, non-colinearity and pixel size were modelled to determine the contribution of additional components of data formation and collection on the complete modulation transfer function of a PET system. These simulations are shown to predict the intrinsic detector resolution of current high resolution systems very well. They also show that current detectors are not limited by inter-crystal scatter. An intrinsic resolution of 0.5 mm can be achieved, but would require a detector with a pixel size of around 250 µm that can be read out unambiguously. It is shown that a range of different detector materials, both scintillators and semiconductors, can be used in these high-resolution detectors. While this design relies on thin (∼3 mm) pieces of material, stacks of the material are shown to simultaneously provide spatial resolution near 0.5 mm and 60% efficiency. This work has shown that detectors with significantly better resolution and sensitivity can be developed for small animal PET applications.

Latest Advances in Molecular Imaging Instrumentation

“…3C). Feasibility studies focusing on small crystals for PET detectors have shown that, in theory, the smallest achievable resolution is on the order of 0.4 mm in the reconstructed image (33). However, resolutions beyond that are unlikely because the intrinsic physical limitations, such as positron range and noncolinearity of the annihilation g-photons, will dominate.…”

Free Software for PET Imaging

“…Thus in PET, relevant models include -among others-geometric sensitivity, positron range, photon pair non-colinearity, Compton scatter in tissues, detector sensitivity, inter-crystal scatter and penetration and detector dead-times. Additionally, one may have some statistical models Interestingly, system modeling plays a very important role in ET for optimizing detector design, configuration and materials (see Levin & Zaidi (2007) and Stickel & Cherry (2005)) 2 and for assesing acquisition and processing protocols, for example to study differences in image quality when using radionuclides with various positron ranges (Bai et al (2005)) or properties that are not possible to measure directly like the behavior of scattered photons (Dewaraja et al (2000)). Likewise, it is also a valuable tool in the design and assessment of correction and reconstruction methods (Zaidi & Koral (2004), Holdsworth et al (2002)) and in the study of an imaging system response (Alessio et al (2006)).…”

Instrumentation for Molecular Imaging in Preclinical Research: Micro-PET and Micro-SPECT

“…For the most commonly used positron emitters (F, C), this average distance is on the order of 0.5 mm or less. This positron range effect poses a lower limit to the spatial resolution with PET, achievable without the use of sophisticated mathematical models (11)(12)(13)(14). Because every measurement with PET is composed by a large number of annihilations, some of the lost spatial resolution can be recovered with appropriate data treatment that estimates the expected location of origin (10,11).…”