SPLIT: Statistical Positronium Lifetime Image Reconstruction via Time-Thresholding

Positron emission tomography (PET) is a widely utilized medical imaging modality that uses positron-emitting radiotracers to visualize biochemical processes in a living body. The spatiotemporal distribution of a radiotracer is estimated by detecting the coincidence photon pairs generated through positron annihilations. In human tissue, about 40% of the positrons form positroniums prior to the annihilation. The lifetime of these positroniums is influenced by the microenvironment in the tissue and could provide valuable information for better understanding of disease progression and treatment response. Currently, there are few methods available for reconstructing high-resolution lifetime images in practical applications. This paper presents an efficient statistical image reconstruction method for positronium lifetime imaging (PLI). We also analyze the random triple-coincidence events in PLI and propose a correction method for random events, which is essential for real applications. Both simulation and experimental studies demonstrate that the proposed method can produce lifetime images with high numerical accuracy, low variance, and resolution comparable to that of the activity images generated by a PET scanner with currently available time-of-flight resolution.

Uncovering atherosclerotic cardiovascular disease by PET imaging

Assessing atherosclerosis severity is essential for precise patient stratification. Specifically, there is a need to identify patients with residual inflammation because these patients remain at high risk of cardiovascular events despite optimal management of cardiovascular risk factors. Molecular imaging techniques, such as PET, can have an essential role in this context. PET imaging can indicate tissue-based disease status, detect early molecular changes and provide whole-body information. Advances in molecular biology and bioinformatics continue to help to decipher the complex pathogenesis of atherosclerosis and inform the development of imaging tracers. Concomitant advances in tracer synthesis methods and PET imaging technology provide future possibilities for atherosclerosis imaging. In this Review, we summarize the latest developments in PET imaging techniques and technologies for assessment of atherosclerotic cardiovascular disease and discuss the relationship between imaging readouts and transcriptomics-based plaque phenotyping.

Investigation of the Ability to Detect Electrolyte Disorder Using PET with Positron Annihilation Lifetime Spectroscopy

Various concentrations (8-300 mmol/L) of NaCl, KCl, and NaCl + KCl aqueous solutions were investigated using positron annihilation lifetime spectroscopy (PALS). A strong dependence of the o-Ps intensity as a function of the solution concentration was demonstrated. On this basis, the mean positron lifetime and the sum of counts in a selected time interval were proposed as reliable parameters for detecting disturbances in the ion balance of living organisms. The use of these parameters for differentiating healthy and cancerous tissues allows for the development of auxiliary diagnostic methods in a new generation of PET scanners equipped with a PALS detection module.

Comparative studies of the sensitivities of sparse and full geometries of Total-Body PET scanners built from crystals and plastic scintillators

BACKGROUND

Alongside the benefits of Total-Body imaging modalities, such as higher sensitivity, single-bed position, low dose imaging, etc., their final construction cost prevents worldwide utilization. The main aim of this study is to present a simulation-based comparison of the sensitivities of existing and currently developed tomographs to introduce a cost-efficient solution for constructing a Total-Body PET scanner based on plastic scintillators.

METHODS

For the case of this study, eight tomographs based on the uEXPLORER configuration with different scintillator materials (BGO, LYSO), axial field-of-view (97.4 cm and 194.8 cm), and detector configurations (full and sparse) were simulated. In addition, 8 J-PET scanners with different configurations, such as various axial field-of-view (200 cm and 250 cm), different cross sections of plastic scintillator, and multiple numbers of plastic scintillator layers (2, 3, and 4), based on J-PET technology have been simulated by GATE software. Furthermore, Siemens' Biograph Vision has been simulated to compare the results with standard PET scans. Two types of simulations have been performed. The first one with a centrally located source with a diameter of 1 mm and a length of 250 cm, and the second one with the same source inside a water-filled cylindrical phantom with a diameter of 20 cm and a length of 183 cm.

RESULTS

With regards to sensitivity, among all the proposed scanners, the ones constructed with BGO crystals give the best performance ([Formula: see text] 350 cps/kBq at the center). The utilization of sparse geometry or LYSO crystals significantly lowers the achievable sensitivity of such systems. The J-PET design gives a similar sensitivity to the sparse LYSO crystal-based detectors while having full detector coverage over the body. Moreover, it provides uniform sensitivity over the body with additional gain on its sides and provides the possibility for high-quality brain imaging.

CONCLUSION

Taking into account not only the sensitivity but also the price of Total-Body PET tomographs, which till now was one of the main obstacles in their widespread clinical availability, the J-PET tomography system based on plastic scintillators could be a cost-efficient alternative for Total-Body PET scanners.

Efficiency determination of J-PET: first plastic scintillators-based PET scanner

BACKGROUND

The Jagiellonian Positron Emission Tomograph is the 3-layer prototype of the first scanner based on plastic scintillators, consisting of 192 half-metre-long strips with readouts at both ends. Compared to crystal-based detectors, plastic scintillators are several times cheaper and could be considered as a more economical alternative to crystal scintillators in future PETs. JPET is also a first multi-photon PET prototype. For the development of multi-photon detection, with photon characterized by the continuous energy spectrum, it is important to estimate the efficiency of J-PET as a function of energy deposition. The aim of this work is to determine the registration efficiency of the J-PET tomograph as a function of energy deposition by incident photons and the intrinsic efficiency of the J-PET scanner in detecting photons of different incident energies. In this study, 3-hit events are investigated, where 2-hits are caused by 511 keV photons emitted in [Formula: see text] annihilations, while the third hit is caused by one of the scattered photons. The scattered photon is used to accurately measure the scattering angle and thus the energy deposition. Two hits by a primary and a scattered photon are sufficient to calculate the scattering angle of a photon, while the third hit ensures the precise labeling of the 511 keV photons.

RESULTS

By comparing experimental and simulated energy distribution spectra, the registration efficiency of the J-PET scanner was determined in the energy deposition range of 70-270 keV, where it varies between 20 and 100[Formula: see text]. In addition, the intrinsic efficiency of the J-PET was also determined as a function of the energy of the incident photons.

CONCLUSION

A method for determining registration efficiency as a function of energy deposition and intrinsic efficiency as a function of incident photon energy of the J-PET scanner was demonstrated. This study is crucial for evaluating the performance of the scanner based on plastic scintillators and its applications as a standard and multi-photon PET systems. The method may be also used in the calibration of Compton-cameras developed for the ion-beam therapy monitoring and simultaneous multi-radionuclide imaging in nuclear medicine.

Developing a novel positronium biomarker for cardiac myxoma imaging

PURPOSE

Cardiac myxoma (CM), the most common cardiac tumor in adults, accounts for 50-75% of benign cardiac tumors. The diagnosis of CM is often elusive, especially in young stroke survivors and transthoracic echocardiography (TTE) is the initial technique for the differential diagnostics of CM. Less invasive cardiac computed tomography (CT) and magnetic resonance imaging (MRI) are not available for the majority of cardiac patients. Here, a robust imaging approach, ortho-Positronium (o-Ps) imaging, is presented to determine cardiac myxoma extracted from patients undergoing urgent cardiac surgery due to unexpected atrial masses. We aimed to assess if the o-Ps atom, produced copiously in intramolecular voids during the PET imaging, serves as a biomarker for CM diagnosing.

METHODS

Six perioperative CM and normal (adipose) tissue samples from patients, with primary diagnosis confirmed by the histopathology examination, were examined using positron annihilation lifetime spectroscopy (PALS) and micro-CT. Additionally, cell cultures and confocal microscopy techniques were used to picture cell morphology and origin.

RESULTS

We observed significant shortening in the mean o-Ps lifetime in tumor with compare to normal tissues: an average value of 1.92(02) ns and 2.72(05) ns for CM and the adipose tissue, respectively. Microscopic differences between tumor samples, confirmed in histopathology examination and micro-CT, did not influenced the major positronium imaging results.

CONCLUSIONS

Our findings, combined with o-Ps lifetime analysis, revealed the novel emerging positronium imaging marker (o-PS) for cardiovascular imaging. This method opens the new perspective to facilitate the quantitative in vivo assessment of intracardiac masses on a molecular (nanoscale) level.

Experience and new prospects of PET imaging for ion beam therapy monitoring

Pioneering investigations on the usage of positron-emission-tomography (PET) for the monitoring of ion beam therapy with light (protons, helium) and heavier (stable and radioactive neon, carbon and oxygen) ions started shortly after the first realization of planar and tomographic imaging systems, which were able to visualize the annihilation of positrons resulting from irradiation induced or implanted positron emitting nuclei. And while the first clinical experience was challenged by the utilization of instrumentation directly adapted from nuclear medicine applications, new detectors optimized for this unconventional application of PET imaging are currently entering the phase of (pre)clinical testing for more reliable monitoring of treatment delivery during irradiation. Moreover, recent advances in detector technologies and beam production open several new exciting opportunities which will not only improve the performance of PET imaging under the challenging conditions of in-beam applications in ion beam therapy, but will also likely expand its field of application. In particular, the combination of PET and Compton imaging can enable the most efficient utilization of all possible radiative emissions for both stable and radioactive ion beams, while positronium lifetime imaging may enable probing new features of the underlying tumour and normal tissue environment. Thereby, PET imaging will not only provide means for volumetric reconstruction of the delivered treatment and in-vivo verification of the beam range, but can also shed new insights for biological optimization of the treatment or treatment response assessment.

Multi-photon time-of-flight MLEM application for the positronium imaging in J-PET

We develop a positronium imaging method for the Jagiellonian PET (J-PET) scanners based on the time-of-flight maximum likelihood expectation maximisation (TOF MLEM). The system matrix elements are calculated on-the-fly for the coincidences comprising two annihilation and one de-excitation photons that originate from the ortho-positronium (o-Ps) decay. Using the Geant4 library, a Monte Carlo simulation was conducted for four cylindrical 22Na sources of β+ decay with diverse o-Ps mean lifetimes, placed symmetrically inside the two JPET prototypes. The estimated time differences between the annihilation and the positron emission were aggregated into histograms (one per voxel), updated by the weights of the activities reconstructed by TOF MLEM. The simulations were restricted to include only the o-Ps decays into back-to-back photons, allowing a linear fitting model to be employed for the estimation of the mean lifetime from each histogram built in the log scale. To suppress the noise, the exclusion of voxels with activity below 2% – 10% of the peak was studied. The estimated o-Ps mean lifetimes were consistent with the simulation and distributed quasi -uniformly at high MLEM iterations. The proposed positronium imaging technique can be further upgraded to include various correction factors, as well as be modified according to realistic o-Ps decay models.

Towards high sensitivity and high-resolution PET scanners: imaging-guided proton therapy and total body imaging

Quantitative imaging (i.e., providing not just an image but also the related data) guidance in proton radiation therapy to achieve and monitor the precision of planned radiation energy deposition field in-vivo (a.k.a. proton range verification) is one of the most under-invested aspects of radiation cancer treatment despite that it may dramatically enhance the treatment accuracy and lower the exposure related toxicity improving the entire outcome of cancer therapy. In this article, we briefly describe the effort of the TPPT Consortium (a collaborative effort of groups from the University of Texas and Portugal) on building a time-of-flight positron-emission-tomography (PET) scanner to be used in pre-clinical studies for proton therapy at MD Anderson Proton Center in Houston. We also discuss some related ideas towards improving and expanding the use of PET detectors, including the total body imaging.

Feasibility study of positronium application for blood clots structural characteristics

Positron-electron annihilation in living organisms occurs in about 30% via the formation of a metastable ortho-positronium atom that annihilates into two 511 keV photons in tissues because of the pick-off and conversion processes. Positronium (Ps) annihilation lifetime and intensities can be used to determine the size and quantity of defects in a material’s microstructure, such as voids or pores in the range of nanometers. This is particularly true for blood clots. Here we present pilot investigations of positronium properties in fibrin clots. The studies are complemented by the use of SEM Edax and micro-computed tomography (µCT) to evaluate the extracted thrombotic material’s properties. µCT is a versatile characterization method offering in situ and in operando possibilities and is a qualitative diagnostic tool. With µCT the presence of pores, cracks, and structural errors can be verified, and hence the 3D inner structure of samples can be investigated.