Can positron annihilation lifetime spectroscopy (PALS) be employed to discriminate different types of soft tissues? This work reports our experimental measurements of the three components of positron annihilation lifetime spectra for three types of bovine, non-fixated soft tissues: adipose, hepatic, and muscle. These three components of PALS spectra correspond to contributions from annihilation lifetimes of para-positronium (p-Ps), positron, and ortho-positronium (o-Ps). We also report a benchmark comparison between PALS and X-ray phase-contrast imaging, which is the current state-of-the-art for soft tissue imaging and analysis. Our measurements showed that the joint probability of annihilation from p-Ps and positron in the soft tissues increases with electron density of the tissue and hence correlates well with the mean voxel values measured by X-ray phase-contrast computed tomography. Notably, the o-Ps lifetime in adipose tissue (2.53±0.01 ns) was approximately 25% longer than in hepatic (2.03±0.02 ns) and muscle tissues (2.00±0.01 ns). The significance here is that the o-Ps lifetime is a viable non-invasive probe for analyzing and discriminating the different soft tissues with a strong sensitivity to the lipid content of the tissue.

Abstract:: Monte Carlo algorithms have a growing impact on nuclear medicine reconstruction processes. One of the main limitations of myocardial perfusion imaging (MPI) is the effective mitigation of the scattering component, which is particularly challenging in Single Photon Emission Computed Tomography (SPECT). In SPECT, no timing information can be retrieved to locate the primary source photons. Monte Carlo methods allow an event-by-event simulation of the scattering kinematics, which can be incorporated into a model of the imaging system response. This approach was adopted since the late Nineties by several authors, and recently took advantage of the increased computational power made available by high-performance CPUs and GPUs. These recent developments enable a fast image reconstruction with an improved image quality, compared to deterministic approaches. Deterministic approaches are based on energy-windowing of the detector response, and on the cumulative estimate and subtraction of the scattering component. In this paper, we review the main strategies and algorithms to correct for the scattering effect in SPECT and focus on Monte Carlo developments, which nowadays allow the three-dimensional reconstruction of SPECT cardiac images in a few seconds.

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