Positron emission tomography (PET), an advanced diagnostic imaging technique, exploits the annihilation of positrons (e+) to delineate pathological alterations within diseased tissues. Integral to PET scanners are detector systems that transform gamma photons into fluorescent photons, thereby gleaning insights into the energy, time, and spatial distribution of gamma photons emanating from positron-emitting radiopharmaceuticals. Conventional PET scanners, bear a significant financial burden primarily due to their reliance on LSO (lutetium oxyorthosilicate) or LYSO (lutetium yttrium oxyorthosilicate) scintillation crystals. The exorbitant cost and limited availability of these crystal scintillators impede the widespread adoption of PET scanners. In a departure from conventional PET technology, the prototype J-PET scanner employed in this trial employs plastic scintillators, characterized by unique physical properties. This prototype is further equipped with bespoke software enabling three-photon imaging based on the annihilation of ortho-positronium (o-Ps) generated within diseased tissue. This study delves into the clinical applicability of PET scanners employing plastic scintillators, particularly investigating the feasibility of PET imaging using plastic scintillators where gamma quanta interact by mechanisms other than the photoelectric effect. Furthermore, this study endeavors to contemporaneously acquire and analyze data related to the lifetime of ortho-positronium (o-P) atoms emanating from routine radiopharmaceuticals. Additionally, it seeks to validate the utilization of a novel diagnostic indicator, termed the "positron biomarker," through a prospective study, comparing its efficacy to conventional diagnostic PET scanning methodologies.

Positron emission tomography (PET) is currently one of the basic techniques enabling molecular imaging. This concept means imaging at the level of biochemical processes. The J-PET scanner is the world's first positron tomograph based on plastic strip scintillators to measure the lifetime of the ortho-positronium (o-Ps) atom. This is a modular scanner, designed and installed at the Department of Experimental Particle Physics and Applications of the Jagiellonian University in Krakow. The J-PET scanner is based on technology patented in 2014 and 2016.

Current PET cameras possess remarkable sensitivity, enabling the detection of changes in chemical concentration as subtle as 1E-11 moles. This unprecedented sensitivity allows for the visualization of metabolic alterations, neurotransmitter imbalances, or receptor system dysfunctions at an early stage, often before the onset of clinical symptoms in various diseases. The PET technique relies on radioisotopes that emit positrons, which are the antimatter counterparts of electrons. PET cameras, tasked with monitoring positron distribution, employ detector systems that capture the radiation generated during positron-electron annihilation. This annihilation process occurs in the emission of gamma ray photons, which are detected by the appropriate detector arrays. The computer system particularly records only those events that simultaneously trigger two detectors, ensuring high spatial resolution and precise anatomical localization of the annihilation events. Notably, positron annihilation may be preceded by the formation of positronium, a transient, quasi-stable bound state comprising an electron and its antiparticle, the positron. Due to the mutual arrangement of spins, two states of the positron are distinguished.

• When the electron and positron spins are parallel (triplet state ↑↑); this arrangement is called ortho-positronium (o-Ps). o-Ps decays (annihilation occurs) after an average vacuum lifetime of 142 nanoseconds [ns]. Annihilation produces three gamma ray photons.
• When the spins of the electron and positron are antiparallel (singlet state ↑↓) - the system is called para-positronium (p-Ps). Annihilation produces two gamma-ray photons with an average vacuum lifetime of 125 picoseconds [ps], or 1,136 times shorter.

Distinct from conventional PET scanners employed in diagnostic imaging, the J-PET scanner boasts three remarkable features:

1. Plastic Scintillation: unlike standard PET scanners that use expensive scintillation crystals, the J-PET scanner utilizes plastic scintillators, significantly lowering its cost and making it more affordable.
2. Modular Design: J-PET's modular design allows for easy customization to fit different patient sizes and can be expanded to a whole-body PET scanner. This flexibility caters to a wide range of patient populations and diagnostic needs.
3. Positronium Biomarker: J-PET expands the scope of PET imaging by introducing the detection and analysis of o-Ps.

Ad. 1. Conventional PET scanners use crystal detectors that detect gamma rays using the photoelectric effect. More expensive PET scanners use LSO, LYSO, or BGO crystals. New PET scanners use plastic detectors that detect gamma rays using Compton scattering. This allows for cheaper scanners with the same or better image quality.

Ad. 2. Thanks to the modular design and the use of strip scintillators, the time-of-flight (ToF) parameter is also used to improve image quality or obtain images of the same quality in a shorter examination because it reduces noise.

Ad.3. This capability opens up the possibility of utilizing a novel diagnostic biomarker that holds promising potential but remains underexplored in PET technology. Positronium imaging is applied only in the J-PET scanner. The PET technique uses radioisotopes that emit positron radiation (beta+). Traditional PET scanners image the distribution of gamma ray photons produced by the annihilation of an electron (e-) and a positron (e+). Annihilation may be preceded by the appearance of a positronium atom, which occurs in approximately 30-40% of all annihilations occurring in the patient body.

Working hypothesis:

The J-PET scanner is based on technology using plastic scintillators. If its clinical usefulness is proven, the development of this imaging method may significantly reduce the costs and increase the availability of PET/CT imaging.

Moreover, the J-PET tomograph allows us to determine a new diagnostic indicator, which is the lifetime of positronium atoms.

Aim of the study:

This study aims to demonstrate the clinical feasibility of PET scanners based on plastic scintillators, specifically investigating the performance of three-photon imaging and the use of positronium as a diagnostic biomarker. If the J-PET method allows to record the distribution of a chemical substance acting as a radiopharmaceutical with greater accuracy and - independently, it is possible to record the o-Ps lifetime depending on the biochemical composition of the environment, which is an additional parameter - not yet used in medical imaging.

Inclusion Criteria:

• The patient is referred for a PET/CT scan, in accordance with recognized indications for examining the entire body.
• Age over 18 years
• Informed, voluntary consent to participate in the study

Exclusion Criteria:

• Pregnant women, breastfeeding women
• People with a previously diagnosed allergy to radiopharmaceuticals
• Age under 18 years
• Lack of cooperation with the patient
• Lack of informed consent to participate in the study

The study group will include a maximum of 25 patients aged > 18 years referred for PET/CT examination using [68Ga]Ga-DOTA-TATE or [68Ga]Ga-PSMA or [18F]FDG or [18F ]choline in accordance with recognized oncological clinical indications to the Department of Nuclear Medicine, O/K Department of Endocrinology, Oncological Endocrinology and Nuclear Medicine of the University Hospital in Krakow, who give informed consent to participate in the study.