Performance assessment of the 2 γpositronium imaging with the total-body PET scanners

Abstract

Purpose: In living organisms, the positron-electron annihilation (occurring during the PET imaging) proceeds in about 30% via creation of a metastable ortho-positronium atom. In the tissue, due to the pick-off and conversion processes, over 98% of ortho-positronia annihilate into two 511 keV photons. In this article, we assess the feasibility for reconstruction of the mean ortho-positronium lifetime image based on annihilations into two photons. The main objectives of this work include the (i) estimation of the sensitivity of the total-body PET scanners for the ortho-positronium mean lifetime imaging using 2γ annihilations and (ii) estimation of the spatial and time resolution of the ortho-positronium image as a function of the coincidence resolving time (CRT) of the scanner.

Methods: Simulations are conducted assuming that radiopharmaceutical is labeled with ⁴⁴Sc isotope emitting one positron and one prompt gamma. The image is reconstructed on the basis of triple coincidence events. The ortho-positronium lifetime spectrum is determined for each voxel of the image. Calculations were performed for cases of total-body detectors build of (i) LYSO scintillators as used in the EXPLORER PET and (ii) plastic scintillators as anticipated for the cost-effective total-body J-PET scanner. To assess the spatial and time resolution, the four cases were considered assuming that CRT is equal to 500 ps, 140 ps, 50 ps, and 10 ps.

Results: The estimated total-body PET sensitivity for the registration and selection of image forming triple coincidences (2γ+γ_prompt) is larger by a factor of 13.5 (for LYSO PET) and by factor of 5.2 (for plastic PET) with respect to the sensitivity for the standard 2γ imaging by LYSO PET scanners with AFOV = 20 cm. The spatial resolution of the ortho-positronium image is comparable with the resolution achievable when using TOF-FBP algorithms already for CRT = 50 ps. For the 20-min scan, the resolution better than 20 ps is expected for the mean ortho-positronium lifetime image determination.

Conclusions: Ortho-positronium mean lifetime imaging based on the annihilations into two photons and prompt gamma is shown to be feasible with the advent of the high sensitivity total-body PET systems and time resolution of the order of tens of picoseconds.

Keywords: Medical imaging; PET; Positronium imaging; Total-body PET.

Fig. 1

Pictorial representation of the single detection ring of the positron emission tomography scanner and (not to scale) magnified part of the hemoglobin molecule with pictorial representation of the possible ways of decays of positronium atoms (Ps) trapped in the intramolecular voids. Left-upper (black arrows) and right-upper (red arrows) indicate annihilations in the space free of electrons for para-positronium and ortho-positronium, respectively. Annihilation of positronium through the interaction with the electron from the surrounding molecule is shown in the left-lower corner (violet arrows) while in the right-lower part the conversion of ortho-positronium into para-positronium via interaction with the oxygen molecule and subsequent decay of para-positronium to two photons (magenta arrows) are presented [7]

Fig. 2

Scheme of the time sequence in the processes used for positronium imaging. A ⁴⁴Sc nucleus undergoes β⁺ decay. Next, on the average after about 3 ps, excited ⁴⁴Ca ^∗ emits prompt gamma with energy of 1160 keV (dotted blue arrow). Parallelly, positron travels through matter, thermalizes, and forms an ortho-posironium bound state. Interaction with surrounding molecules or conversion process leads to emission of two photons. The mean ortho-positronium lifetime is in the order of nanoseconds, in contrast to the duration of thermalization [42] and deexcitation [43] processes which are in the order of 10 ps

Fig. 3

Relative gain in sensitivity S as a function of the AFOV of the scanner. The gain is calculated relative to sensitivity for the standard 2γ imaging using LYSO PET with AFOV = 20 cm (blue dot). The gains for 2γ (solid lines) as well as for 2γ+γ_prompt (dashed lines) are shown for LYSO and plastic scintillators as indicated in the legend. The red and black color indicate result for the LYSO and plastic scintillators, respectively

Fig. 4

Registration efficiency (taking into account the geometrical acceptance, probability of gamma quanta registration in the plastic scintillator and J-PET resolution) as a function of applied threshold for the cases of prompt gamma (dashed line), two back-to-back 511 keV photons (dotted line), and two photons simultaneously with the prompt gamma with energy loss higher than 400 keV (solid line)

Fig. 5

Left: Reconstructed distribution of annihilation point spatial coordinates. The voxel size is equal to 5×5×5 mm³. Middle: Reconstructed image of six sources obtained while applying the TOF-FBP algorithm. The voxels: 1.8×1.8×2.9 mm³. Right: Reconstructed source placed at (x,y,z)=(1,0,0) cm. Each row show results with different resolution: CRT =10 ps (top row), CRT =50 ps (second row), CRT =140 ps (third row), and CRT =500 ps (bottom row)

Fig. 6

Distributions of generated positronium lifetimes „ and reconstructed ones, assuming the CRT value of 10 ps (b), 50 ps (c), 140 ps (d) and 500 ps (e). The voxel size is equal to 5×5×5 mm³

Fig. 7

Left: Comparison between generated positronium mean lifetime and reconstructed one assuming different detector CRT resolutions as a function of NEMA position number (see Table 1). Differences between obtained results are in the order of $\mathcal{O}$(10 ps). Right: Resolution of the mean lifetime determination as a function of detected entries in a single voxel