Ortho-positronium lifetime for soft-tissue classification

Abstract

The objective of this work is to showcase the ortho-positronium lifetime as a probe for soft-tissue characterization. We employed positron annihilation lifetime spectroscopy to experimentally measure the three components of the positron annihilation lifetime-para-positronium (p-Ps), positron, and ortho-positronium (o-Ps)-for three types of porcine, non-fixated soft tissues ex vivo: adipose, hepatic, and muscle. Then, we benchmarked our measurements with X-ray phase-contrast imaging, which is the current state-of-the-art for soft-tissue analysis. We found that the o-Ps lifetime in adipose tissues (2.54 ± 0.12 ns) was approximately 20% longer than in hepatic (2.04 ± 0.09 ns) and muscle (2.03 ± 0.12 ns) tissues. In addition, the separation between the measurements for adipose tissue and the other tissues was better from o-Ps lifetime measurement than from X-ray phase-contrast imaging. This experimental study proved that the o-Ps lifetime is a viable non-invasive probe for characterizing and classifying the different soft tissues. Specifically, o-Ps lifetime as a soft-tissue characterization probe had a strong sensitivity to the lipid content that can be potentially implemented in commercial positron emission tomography scanners that feature list-mode data acquisition.

Keywords: PALS; Positronium annihilation lifetime spectroscopy; Soft tissue analysis; X-ray phase-contrast imaging.

Figure 1

(a) Illustration of the physics of positronium transport. $_{}^{22}$Na undergoes ${\beta }^{+}$ decay (positron and neutrino emissions) and turns into an excited $_{}^{22}\mathit{Ne}_{}^{\ast }$, which in turn decays by emitting a prompt-$\gamma$ (1274.6 keV gamma emission). Hereafter, the emitted positron is referred to as free positron (FP). FP can either directly annihilate by 2$\gamma$ emission or can form a positronium (Ps); Ps can take a form of a para-positronium (p-Ps) or an ortho-positronium (o-Ps); p-Ps directly annihilates by 2$\gamma$ emission; and o-Ps can either annihilate by 2$\gamma$ emission (pick-off annihilation) or 3$\gamma$ emission. (b) Histological photomicrographs of example soft tissues. Hematoxylin and eosin (H &E) stained histological photomicrographs for adipose, hepatic, and muscle tissues. Scale bars represent 100 $\mathrm{\mu }$m.

Figure 2

Results of PALS analysis and its benchmarking with XPC-CT. (a) The five components Ps-lifetime—p-Ps, fixed component for titanium (Ti) foil encapsulation of the source (248 ps), fixed component for Mylar foil that was used to wrap the tissue samples (382 ps), free-positron (FP), and o-Ps lifetimes—can be decomposed from the measured PALS spectrum (example PALS spectrum and its decomposed components for adipose tissues in a). (b) The comparison of three non-fixed components of the PALS spectrum for the three soft tissue types with 5 repeats for each type (n = 5) showed that o-Ps was the most sensitive out of the three components for discerning the subtle changes between the different types of soft-tissue. Single standard-deviation error bars are added to the plot. (c) Comparison of PALS as a method for soft-tissue analysis with the current state-of-the-art, XPC-CT. PALS probe measurement—o-Ps lifetime—showed less variations across the samples than the mean voxel value measurement through XPC-CT. The adipose tissue was significantly more discernible from the hepatic and the muscle tissues using mean o-Ps lifetime compared to using mean voxel values in XPC-CT. (For the box-and-whiskers plot in (c), the PALS measurements and XPC-CT mean voxel values were normalized to their respective maximums for presentation purposes.)

Figure 3

Methods for PALS analysis and its benchmarking with XPC-CT. (a) The PALS measurement setup: two scintillation detectors with tissue samples in the middle irradiated by the $_{}^{22}$Na source. (b) $_{}^{22}$Na source was placed between two tissue samples that were kept cooled by a Peltier-cell based system. The tissue samples were wrapped with Mylar foils of 2.5 $\mathrm{\mu }$m thickness to avoid these tissue samples from touching the $_{}^{22}$Na source. (c) An example tomographic slice of XPC-CT images. The blue, green, and red boxes represent a 2D slice of the 3D regions-of-interest corresponding to the adipose, hepatic, and muscle tissues, respectively, in the given example XPC-CT slice. (d) Sample fast pulse acquired by the organic scintillation detector, (1) its attenuated version by a factor F, (2) the original pulse delayed by $\mathrm{\Delta }$ ns and inverted, and the (3) bipolar pulse obtained by subtracting (2) from the original pulse. The zero-crossing point corresponds to the pulse time stamp. (e) Energy spectrum and its average energy for 70 kVp X-ray beam from the liquid-metal-jet-based X-ray source used by XPC-CT system.