First Positronium Lifetime Imaging using $^{52}$Mn and $^{55}$Co with a plastic-based PET scanner

TL;DR

It is demonstrated that the applied selection criteria on the data measured with the modular J-PET can be used for PLI studies even with radionuclides with complex decay patterns.

Abstract

Positronium Lifetime Imaging (PLI) extends positron emission tomography by using the lifetime of positronium atoms as a probe of tissue molecular architecture. In this work, we report the first PLI measurements performed with $^{52}$Mn and $^{55}$Co using the modular J-PET. Four samples were studied in each experiment: two Certified Reference Materials (polycarbonate and fused silica) and two human tissues (cardiac myxoma and adipose). The selection of PLI events was based on the registration of two 511~keV annihilation photons and one prompt gamma in triple coincidence. From the resulting lifetime spectra we extracted the mean ortho-positronium lifetime $τ_{\text{oPs}}$ and the mean positron lifetime $ΔT_{\text{mean}}$ for each sample. The measured values of $τ_{\text{oPs}}$ in polycarbonate using both isotopes matches well with the certified reference values. Furthermore, $^{55}$Co reproduced identical results for fused-silica measurements at their respective uncertainty levels. In contrast, measurements with $^{52}$Mn in fused silica show a minor deviation, which could be caused by the Parafilm spacer. In myxoma and adipose tissue, the reduced $τ_{\text{oPs}}$ values are mainly linked to the long storage history of the samples rather than to the choice of isotope. Comparing peak-to-background ratios and spectral purity, $^{55}$Co provides cleaner PLI data under the same experimental conditions. Although $^{52}$Mn offers a longer half-life and a multi gamma cascade enhancing $β^{+}$ + $γ$ coincidences, but at the expense of higher background. In this study, we demonstrate that the applied selection criteria on the data measured with the modular J-PET can be used for PLI studies even with radionuclides with complex decay patterns.

Figures (8)

• Figure 1: (A-C) Decay scheme of $^{44}$Sc (A), $^{52}$Mn (B) and $^{55}$Co (B), where $\beta^{+}$ represents the positron emission yield, EC denotes electron capture contributions, $\gamma$ indicates the prompt gamma ray with its energy shown in parentheses, and the number in pink parentheses shows the fraction of $\beta^{+}+\gamma$ decay relative to all $\beta^{+}$ decays for a given isotope. The delay time, shown in blue text for clarity, represents the average interval between positron emission and prompt gamma emission. (D) Event definition for $^{52}$Mn in the modular J-PET scanner. Two annihilation photons ($t_1$, $\vec{r}_1$) and ($t_2$, $\vec{r}_2$), with a possible cascade of three prompt photons ($t_{i}$, $\vec{r}_{i}$), ($t_{ii}$, $\vec{r}_{ii}$), ($t_{iii}$, $\vec{r}_{iii}$).
• Figure 2: (A) Placement of the 4 samples inside the Modular J-PET scanner (B) Schematic representation of the 4 samples in the modular J-PET detector
• Figure 3: Event Selection for $^{52}$Mn: (A) Distribution of time-over-threshold (TOT$_{Hit}$) for photon identification, with annihilation photons (red) and prompt gammas (blue) marked by distinct ranges. (B) The hit multiplicity ($\mu$) distribution for events is represented by histograms: the red-shaded histogram highlights events with exactly two annihilation photons and one prompt gamma, the blue-shaded histogram indicates events with exactly two annihilation photons and two prompt gammas, and the pink-shaded histogram denotes events with exactly two annihilation photons and three prompt gammas. (C) Distribution of the relative angle ($\theta_{AA}$) between annihilation photon vectors $\vec{r}_1$ and $\vec{r}_2$ (per Fig. \ref{['fig:decay_scheme']}D), with $\theta_{AA} \geq 60^\circ$ (red) as the selection criterion. (D) Distribution of the relative angle ($\theta_{PA}$) between prompt gamma vector $\vec{r}_i$ and annihilation photon vectors $\vec{r}_1$, $\vec{r}_2$ (per Fig. \ref{['fig:decay_scheme']}D), with $\theta_{PA} \geq 30^\circ$ (red) as the restriction.
• Figure 4: Event Selection for $^{55}$Co: (A) Distribution of time-over-threshold (TOT$_{hit}$) for photon identification, with annihilation photons (red) and prompt gammas (blue) marked by distinct ranges. (B) Hit multiplicity ($\mu$) distribution for events, with the red-shaded histogram highlighting selected events containing exactly two annihilation photons and one prompt gamma. (C) Distribution of the relative angle ($\theta_{AA}$) between annihilation photon vectors $\vec{r}_2$ and $\vec{r}_3$ (per Fig. \ref{['fig:decay_scheme']}D), with $\theta_{AA} \geq 60^\circ$ (red) as the selection criterion. (D) Distribution of the relative angle ($\theta_{PA}$) between prompt gamma vector $\vec{r}_i$ and annihilation photon vectors $\vec{r}_2$, $\vec{r}_3$ (per Fig. \ref{['fig:decay_scheme']}D), with $\theta_{PA} \geq 30^\circ$ (red) as the restriction.
• Figure 5: Cross section of the modular J-PET scanner showing background events for $^{52}$Mn. The primary and scattered prompt gamma is depicted as a red solid arrow, with primary and scattered annihilation photons as blue dashed arrows. (A-E) Background events from one decay: (A) One annihilation photon undetected, prompt gamma scatters twice, misidentified as an annihilation photon. (B) One annihilation photon undetected, the other scatters twice, misidentified as an annihilation photon. (C) Annihilation photon undetected, two prompt photons scatter, misidentified as an annihilation photon. (D) One annihilation photon undetected, one unidentified prompt gamma goes through low energy scattering misidentified as annihilation photon. (E) Electron capture without annihilation photons, two prompt photons scatter, misidentified as an annihilation photon. (F-I) Background events from accidentals: (F) Example of the background arising from the accidental coincidence where one annihilation photon from one event and one from another event. (G) Example of the background arising from the accidental coincidence of a prompt gamma from one event and annihilation photon from the other event. (H) Example of the background arising from the accidental coincidence of registering one or two prompt gammas and annihilation photon from one event and one prompt gamma and the prompt scattered registered as annihilation photon from the other event. (I) Example of the background arising from the accidental coincidence of registering two prompt gammas and the scattered prompt registered as annihilation photon and one prompt gamma from the other event.