GEANT4-based model and experimental validation of positronium lifetime in organic and inorganic materials

This work presents a feasibility study to demonstrate the use of positronium lifetime in the spectroscopy and imaging of soft tissues. Approximately 40% of positron annihilation results in the production of positronium. Positronium is a metastable subatomic particle encompassing an electron and a positron that can be found in two forms: para-positronium (p-Ps), which decays into two gamma rays with a lifetime of 125 ps in a vacuum, and orthopositronium (o-Ps) that decays into three gamma rays with a lifetime of 142 ns in a vacuum. However, in matter, the positron in o-Ps can annihilate with one of the surrounding lattice electrons emitting two gamma rays, instead of three gamma rays, with a significantly shorter lifetime, through a process known as “pick-off”. The “pick-off” probability depends on the properties of the interacting material.“Pick-off” occurs during the numerous collisions of the o-Ps with the void walls, resulting in a significant reduction of the 142 ns vacuum lifetime to a few nanoseconds in matter. The magnitude of reduction reflects the number of collisions, which in turn is dictated by the dimensions of the void, thus allowing the correlation of the measured o-Ps lifetime to the size of the void. Therefore, positronium lifetime can be used to probe the structure of the interacting material and is often used in material science to detect and characterize the density and size of defects. Recently, several studies have been performed to investigate the application of this principle to the discrimination between different types of biological tissue, with a specific focus on the detection of cancer tissue. Positron emitters are already the radionuclide of choice in Positron Emission Tomography (PET), to produce a functional map of a positron-emitting radiotracer uptake in the patient’s body. To form the image, PET only uses the two annihilation 511-keV gamma rays emitted by positron recombination, whose distribution mainly depends on the metabolic affinity of the tissue for the radionuclide-tagged drug and is independent of the tissue structure. Adding the positronium lifetime measurements in PET can potentially provide additional information about the tissue’s structure. In this thesis, a new physics-based model to simulate positron annihilation and positronii ium decay was investigated. This model was implemented utilizing the GEANT4 simulation toolkit and validated with experimental data. Moreover, we have extended our model to generate a 2D positronium lifetime image of different types of soft tissue by simulating an Inveon small-animal PET scanner. This image is obtained as a function of the three-gamma fraction defined as the ratio between the image reconstructed using the three gamma coincidences and the total number of annihilation events. To validate our model, we performed a positron annihilation lifetime spectroscopy (PALS) experiment to measure the o-Ps lifetime in reference materials: polycarbonate and quartz. The simulated o-Ps lifetime closely matched the measured values with relative errors of <0.00% in polycarbonate and 11.81% in quartz. Furthermore, we measured the o-Ps lifetime in three bovine, non-fixated soft tissue: adipose, hepatic, and muscle to investigate the discrimination capability of different types of tissue utilizing the o-Ps lifetime. The measured o-Ps lifetime in soft tissue yielded distinct results, with lifetimes of 3.39±0.07 ns, 2.72±0.06 ns, and 2.73±0.1 ns in adipose, hepatic, and muscle tissue, respectively. The extended three-dimensional GEANT4 model mimics the Inveon small animal PET scanner to image hepatic and adipose tissue. By reconstructing the spatial distribution of positron emitters using a source with 1 mCi activity and creating o-Ps lifetime images, we demonstrated the effectiveness and feasibility of our modality. This activity is comparable to the one used in the clinical procedures. The resulting image notably revealed a higher o-Ps lifetime in adipose tissue than hepatic, consistent with theoretical expectations and measured results as the electron density of hepatic tissue is higher than adipose tissue. The contrast lifetime image facilitated good discrimination between hepatic and adipose tissue, holding significant promise for advancements in early liver disease diagnosis, like nonalcoholic fatty liver disease.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Nuclear, Plasma, Radiolgc Engr

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Mahmoud, Kholod H.

Contributors dc:contributor

• Fulvio, Angela Di
• Kozlowski, Tomasz
• Avachat, Ashish V

• Geant4
• Positronium
• Pals Positron Annihilation Lifetime Tomography
• Soft Tissue Imaging

Statement dc:rights

• Copyright 2024 Kholod Mahmoud

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/124606

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F124606