arXiv subjects

Tomislav Bokulić

Publications and source records attributed to Tomislav Bokulić.

5 recordsLinked to original sources

arXiv2026

Imaging performance of a Single-Plane Readout Compton Camera

This work presents the design, development, and characterization of a compact Compton camera employing a novel single-plane readout architecture. The proposed concept simplifies conventional multilayer Compton cameras by optically coupling the scatterer and absorber into a single compact detector element using a light guide, enabling readout exclusively from one side with silicon photomultipliers. The system was validated through Monte Carlo simulations, prototype construction, and experimental characterization using standard gamma-ray sources. Detector performance for 511~keV and 662~keV photons was optimized using simulations based on the Geant4 framework, by guiding event selection strategy. Event reconstruction relied on coincidence detection consistent with Compton scattering kinematics, while image reconstruction was performed using maximum likelihood expectation maximization algorithm. The prototype comprises 64 detector elements arranged in an $8\times8$ matrix, read out by an $8\times8$ silicon photomultiplier array. Each element consists of two GAGG:Ce scintillators coupled by a 20 mm long light guide. Measured energy resolutions were $8.9\%\pm1.9\%$ and $10.8\%\pm1.6\%$ for the front and back layers, respectively. Successful gamma-ray imaging was demonstrated for Na-22 and Cs-137 sources, achieving angular resolutions of $12.4^\circ$--$14.3^\circ$ at 511~keV and $14.3^\circ$--$16.8^\circ$ at 662~keV, confirming the feasibility of the proposed single-plane architecture for compact and cost-effective gamma-ray imaging applications.

physics.ins-det↗

arXiv2026

Positron Emission Tomography with quantum-entangled Compton events: first imaging results at clinically relevant activities

In Positron Emission Tomography, a potential, yet unutilized enhancement, may come from exploiting the quantum entanglement of the annihilation quanta, inscribed in the correlation of their polarizations. To investigate this, we built a PET demonstrator capable of measuring polarization correlations of annihilation quanta by their Compton scattering, based on single-layer scintillator polarimeters. We present a detailed study of the imaging of two $^{68}$Ge line sources, 45 MBq each, to extract the spatial resolution and assess image quality. The results show that a spatial resolution of 2.5$\pm$0.1 mm is obtained using single-pixel events, while resolutions obtained with polarization-correlated Compton events range from 3.6$\pm$0.3 mm to 4.9$\pm$0.3 mm, depending on data selection criteria. We also found that the polarization-correlated Compton events exhibit up to 20% higher average signal to random background ratio compared to the single-pixel events. We also present the first imaging of the NEMA NU-4 phantom filled with a $^{68}$Ga solution of 378 MBq initial activity, successfully combining polarization-correlated events with conventional single-pixel event selection. Based on the extracted spatial resolution, signal-to-background, signal-to-noise, contrast, and contrast-to-noise ratio, we estimate that up to 10% sensitivity increase may be attained by exploiting the polarization-correlated events, while preserving a high image quality.

physics.ins-det↗

arXiv2025

Towards polarization-enhanced PET: Study of random background in polarization-correlated Compton events

Positron Emission Tomography (PET) is a medical imaging modality that utilizes positron-emitting isotopes, such as Ga-68 and F-18, for many diagnostic purposes. The positron annihilates with an electron from the surrounding area, creating two photons of 511 keV energy and opposite momenta, entangled in their orthogonal polarizations. When each photon undergoes a Compton scattering process, the difference of their azimuthal scattering angles reflects the initial orthogonality of their polarizations, peaking at $\pm$90$^{\circ}$. This type of correlation is not yet utilized in conventional PET scanners, but could potentially offer an energy-independent method for background reduction. Measurements of these kinds of correlations can be achieved using Compton polarimeters, built from a single layer of segmented scintillating crystals such as Gadolinium Aluminium Gallium Garnet doped with Cerium (GAGG:Ce), read out by silicon photomultipliers (SiPMs). In this paper, we study the signal-to-random background ratios in measurements of these correlated annihilation photons from coincidence time spectra across clinically relevant source activities, from $\sim$200 MBq to $\sim$378 MBq. These are then compared to the standard single-pixel (photoelectric) measurements. We find that the signal-to-random background ratios (SBRs) obtained from the polarization-correlated events for Compton scattering angles $θ_{1,2}\in[72^{\circ}, 90^{\circ}]$ and azimuthal angle difference $Δϕ=90^{\circ}\pm20^{\circ}$ are consistently higher than those from single-pixel events, with the ratio of their SBR values of 1.23. The SBR of the selected events also increases with the polarimetric modulation factor $μ$, gaining $\sim$50\% in value during the experiment.

physics.ins-det↗

arXiv2024

Investigation of the spatial resolution of PET imaging system measuring polarization-correlated Compton events

Recent studies of positron emission tomography (PET) devices have shown that the detection of polarization-correlated annihilation quanta can potentially reduce the background and creation of false lines of response (LORs) leading to improved image quality. We developed a novel PET demonstrator system, capable of measuring correlated gamma photons with single-layer Compton polarimeters to explore the potential of the method. We tested the system using sources with clinically relevant activities at the University Hospital Centre Zagreb. Here we present, for the first time, the images of two Ge-68 line sources, reconstructed solely from the correlated annihilation events. The spatial resolution at two different diameters is determined and compared to the one obtained from events with photoelectric interaction.

physics.ins-det↗

arXiv2022

Optimization of detector modules for measuring gamma-ray polarization in Positron Emission Tomography

Detection of $γ$-ray polarization in Positron Emission Tomography (PET) is as yet an unexploited feature that could be used as an additional handle to improve signal-to-background ratio in this imaging modality. The $γ$ polarization is related to the azimuthal angle in the Compton scattering process, so the initial correlation of polarizations of the annihilation quanta translates to the correlation of the azimuthal angles in events where both annihilation photons undergo Compton scattering. This results in a modulated distribution of the azimuthal angle difference for true events, while this modulation is lacking for the background events. We present a comprehensive experimental study of five detector configurations based on scintillator matrices and silicon photomultipliers, suitable for measuring the azimuthal modulation. The modules consist of either GaGG:Ce or LYSO:Ce pixels with sizes varying from 1.9x1.9x20 $\mathrm{mm^3}$ to 3x3x20 $\mathrm{mm^3}$. The distinctive feature of the modules is that they can reconstruct the Compton scattering by detecting the recoil electron and the scattered gamma in a single detector layer, which simplifies extension to larger systems. The amplitude modulation of the azimuthal angles' difference is clearly observable in all configurations ranging from $0.26\pm0.01$ to $0.34\pm 0.02$ depending on the event selection criteria. The results suggest that finer detector segmentation plays a leading role in achieving higher modulation factors.

physics.ins-det↗