J-PET (Jagiellonian-PET TOMOGRAPHY)

Jagiellonian PET is the first Positron Emission Tomography scanner build from plastic scintillators.

The plastic scintillators, in contrast to inorganic ones, are relatively cheap and easy to shape. This will allow for preparation of the cost-effective device enabling a symultaneous metabolic imaging of the whole human body.

The premordial aim of the group is to elaborate a technology for:

  • the cost effective whole-body PET,
  • the MR and CT compatible PET insert and
  • a modular and transportable PET with the field of view adjustable to the patient size.
Have a look a the dream solution.

A first full scale J-PET prototype is shown in the photo below. It is used also for studies of the discrete symmetries in the decays of positronium atoms and multi-partite entanglement of photons originating from the decay of positronium.

The Jagiellonian PET collaboration lead by P. Moskal is an interdisciplinary and international group including physicists, chemists, electronic engineers,computer scientists, quantum information physicists as well as bio and medical physicists from the Jagiellonian University, National Centre for Nuclear Research, Maria Curie-Skłodowska University, University of Vienna, National Laboratory in Frascati and from the company Nowoczesna Elektronika.

Plastic scintillators are superior to the crystal ones in terms of their time resolution. The novelty of the J-PET detector lies also in the fact that the information about the place of gamma quanta interaction is extracted solely from the time, rather than the energy deposition measurement.

In each of 196 modules of the J-PET detector there are two photomultipliers mounted at the both ends of a scintillator. Signals are sampled at four voltage levels at the leading and trailing edges with the newly developed, dedicated digital multi-threshold electronics. The place of a gamma quantum interaction in the scintillator (Δl) is determined from the times measured at both ends of the same strip. For the determination of the position of the annihilation point along a line-of-response (Δx) a time-of-flight (TOF) method is used basing on the times registered in a pair of opposite strips. Furthermore, timing of signals is used for suppression of scatterings via the the time-over-threshold (TOT) method.

In order to compensate for the lower gamma quanta registration efficiency in the organic scintillators more detection layers can be used. This together with large light attenuation length in the plastic scintillators allowed for construction of a tomogrph with a large axial field-of-veiw, competitive in terms of spatial resolution and cheaper than commercially available scanners.

Furthermore, the J-PET constitues a high acceptance multi-purpose detecor optimized for the detection of photons from the positron-electron annihilation and can be used in the broad interdisciplinary investigations including, among others:

  • medical imaging,
  • studies of discrete symmetries in the decays of positronium atoms,
  • quantum entanglement of high energy photons originating from the decay of ortho-positronium,
  • research in the field of life- and material-sciences.

A first full scale prototype of the J-PET detector. The J-PET detector is made of three cylindrical layers of EJ-230 plastic scintillator strips (black) with dimension of 7 × 19 × 500 mm3 and Hamamatsu R9800 vacuum tube photomultipliers (grey). The signals from photomultipliers are probed in the voltage domain at four thresholds with the timing accuracy of 30 ps. and the data acquisition is working in the trigger-less mode. The prototype was built with the support from he Polish National Center for Development and Research through grant INNOTECH-K1/IN1/64/159174/NCBR/12.

In the typical detectors with vacuum tube photomultipliers, only few first registered photons contribute to the leading edge of the electrical signal. Therefore, the time resolution may be improved by making a readout allowing to record timestamps from larger number of scintillation photons arriving at the scintillator edge. This can be achieved by preparation of a a read-out in the form of an array with several SiPM photomultipliers. In such a case, a set of all registered scintillation photons is divided into several subgroups and a time of the registration of the first photon in each subgroup is recorded.

Upper scheme indicates a single detection element of the first J-PET detector consisting of the scintillator strip read out on two sides by vacuum tube photomultipliers. Lower part indicates a scheme of an exemplary multi-SiPM readout allowing for determination of timestamps of 20 detected photons (10 on each side).

A single module of the new detection layer of the J-PET tomograph will constitute an independent detection unit which sends information about timing of the signals via optical links to the data acquisition boards. It will consist of 13 plastic scintillator strips each read out at the ends by the matrices of silicone photomultipliers (SiPM). The SiMPs will be connected directly to the newly designed voltage suppliers as well as front-end and digitizing electronics shown symbolically in green. A single modules with the weight of only about 2 kg can be easily handled, and can be connected and disconnected, thus giving a possibility of constructing the tomographic chamber with an adjustable diameter. A new detection layer of the J-PET will be built from 24 modules. It is designed to be inserted into the J-PET prototype.

Due to the application of the matrix of the silicon photomultipliers, the new layer of the J-PET is characterized by even better timing resolution than the first prototype.

Medical diagnostics and PET imaging

PET detector is built from non-magnetic and low-Z material strips. Therefore, it is possible to combine J-PET with CT and J-PET with MR, so that the same part of the body can be scanned simultaneously with both methods. Portable J-PET insert for simultaneous PET-MRI imaging is the subject of work of the LIDER project.

More information can be at the webpage of the LIDER project.

Furthermore, the ability of the J-PET detector to reconstruct the decays of orthopositronium (o-Ps) atoms into three photons was proved. The method is based on trilateration (GPS-like method) and allows for a simultaneous reconstruction of both location and time of the decay. Altough gamma quanta interact in the plastic scintillators predominantly via the Compton effect, making the direct measurement of their energy impossible, it was shown that the J-PET scanner will enable studies of the o-Ps→3γ decays with angular and energy resolution equal to σ(θ) ≈ 0.4o and σ(E) ≈ 4.1 keV, respectively.

Left: A scheme of the J-PET detector with gamma quantum hit positions marked inin black and o-Ps decay plane indicated in gray. Right: Scheme of the decay reconstruction reduced to a two-dimensional problem in the decay plane. Each of hit coordinates constitutes the center of a circle describing possible photon origin point, where radius of the circle depends on the recording time and the unknown o-Ps decay time.

More information can be found in:

Development of J-PET range monitoring for hadron therapy

The main advantage of the proton therapy over the conventional radiotherapy (which uses X-ray or electrones) is the possibility to precisely deposit the radiation dose only in the tumor region. This is the consequence of the characteristic energy loss distribution of ionizing radiation while movement of proton beam through matter. The maximum of energy deposition is localized immediately before the maximum movement range (the Bragg peak).

Altought the range of the proton beam in the homogenous medium of know properties can be precisely calculated, the range reavelled in the patient's body is determined only with limited precision. Therefore, in order to cover the entire range of tumor a marigin of healthy tissues surrounding it is also exposed to radiation.

The availability of the beam range monitoring would allow for more precise therapy planning and full usage of proton therapy benefits.

As a result of proton beam interactions with the atoms in the patient's tissues is the emission of the secondary particles, mainly: neutrons, photons (promt-gamma) and charged particles. Additionally, due to the nuclear interaction isotopes which decay via the β+ decay are created. Emited positrones annihilate with electrones from the patients body and two back to back gamma quanta of minimum energy of 511 keV are emitted. This gamma quanta can escape from the patient's body and be registered. The aim of the project developed in colaboration with Institute of Nuclear Physics of the Polish Academy of Sciences is to test the usefullness of the J-PET technology for the monitoring of the proton beam range.

Bio-medical studies

Positron Annihilation Lifetime Spectroscopy (PALS) is widely used to correlate the mean o-Ps lifetime value with cavity size in which annihilation undergoes. PALS allows to study many properties of materials such as the presence of defects, thermal expansion, temperature of phase transition in polymers, processes of gases, or steams sorption in pores, however it was applied in a very limited number of cases concerning living biological material. PALS method combined with J-PET system will enable the determination of early and advanced stages of carcinogenesis by observing changes in biomechanical parameters between healthy and tumor cells.

The o-Ps lifetime as a function of the water vapor sorption time. Measurement were conducted in four stages: (1) in vacuum, (2) in dried air, (3) with the presence of water vapor, and (4) with drop of water placed in the chamber containing yeast.

Environmental scanning electron microscopy (ESEM) images of lyophilized yeasts (upper) and dried under normal conditions after addition of water (bottom).

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Testing fundamental principles of physics

Positronium is the lightest purely leptonic object decaying into photons. As an atom bound by a central potential, it is a parity eigenstate and as an atom built out of an electron and an anti-electron it is an eigenstate of the charge conjugation operator. Positronium in the ground sub-states with orbital angular momentum L = 0 is formed in a singlet state of the anti-parallel spins orientation (para-positronium, p-Ps), or in a triplet state of parallel spin orientation (ortho-positronium, o-Ps). Due to the symmetry of charge conjugation p-Ps undergoes annihilation with emission of an even number of photons (most often: two), while o-Ps undergoes annihilation with emission of an odd number of photons (most often: three).

The J-PET detector enables to perform tests of discrete symmetries in the leptonic sector via the determination of the expectation values of the discrete-symmetries-odd operators, which may be constructed from the spin of o-Ps and the momenta and polarization vectors of photons originating from its annihilation.

With respect to the previous experiments performed with crystal based detectors, J-PET provides superior time resolution, higher granularity, lower pile-up and opportunity of determining photon’s polarization. These features allow us to expect improvement by more than an order of magnitude in tests of discrete symmetries in decays of positronium atoms.

Scheme of a possible oPs spin direction determination with the J-PET detector. A β+ source is located in the center of a vacuum-filled cylinder covered by aerogel (green band) in which o-Ps formation and decays take place. Red lines denote lines of flight of the three photons used to reconstruct the decay vertex which, in turn, allows us to estimate the positron momentum direction and spin direction of the ortho-positronium. Yellow arrow indicates 1.27 MeV gamma quantum from the Na decay.

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Material science

Together with development of the detector new scintillating materials of properties desired for digital PET are designed, synthesised and investigated with the aim of developing plastic scintillators with better light output and timing properties than comercialy available. The novelty of the concept of the J-PET scintillator lies in application of the 2-(4-styrylphenyl)benzoxazole as a wavelength shifter. The substance has not been used as scintillator dopant before. A dopant shifts the scintillation spectrum towards longer wavelengths making it more suitable for applications in scintillators of long strips geometry and light detection with digital silicon photomultipliers.

Left and middle: The polymerization of scintillating mixture in conducted in the furnace. It leads to obtaining scintillating material in the form of strip. The furnace chamber enables uniform temperature distribution in a whole volume. Precise temperature control (1 °C) and setting whole temperature cycle are possible as well. Right: The photograph of the J-PET scintillator in UV light. The scintillating material is optically homogeneous and does not contain any defects.

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List of articles

Concept First tests of the single and double module prototypes Preliminary estimation of the scatter-fraction and accidental coincidences and discussion on event selection method Estimation of CRT and comparison of FIGURE-OF-MERIT for the whole-body imaging A test of additional WLS layer to improve axial spatial resolution 3-photon imaging with J-PET and concept of the morphometric imaging with positronium atoms Front end electronics is based solely on FPGA More advanced methods for the hit-position reconstruction Proposal for the studies of discrete symmetries in decays of positronium atoms Studies of Quantum entanglement with J-PET