Positron emission tomography (PET) is one of the most important imaging techniques in nuclear medicine, which plays an irreplaceable role in biomedical research and clinical diagnosis. PET technology is a three-dimensional imaging nondestructive testing technique that uses a positron-emitting radioisotope-labeled compound to inject into the organism to measure their spatial distribution and time characteristics in vitro. The detector is the key component of the PET system, which is mainly responsible for detecting annihilated photons and converting the detected cases into electrical signals. The geometrical design of the system alongside the characteristics of the individual PET detector modules contributes to the overall performance of the scanner. The detector performance is mainly influenced by the characteristics of the photo-detector and the scintillation crystal. Scintillation crystals are widely used in PET mainly because of its properties: high density, high light yield, low decay time and absence of hygroscopicity.

The performance of a PET scanner is directly affected by many design factors, including the geometry of the scanner, the detector modules, read-out electronics, data acquisition and processing, and image reconstruction algorithm. Currently, the majority of PET optical converters use inorganic scintillation crystals, which convert gamma photons of 511 keV generated by positron annihilation into low-energy photons suitable for photomultiplier detection and realize photon number amplification. The selection of scintillation crystal by PET mainly depends on the following four points:

  1. In order to minimize the radiation dose received by the subjects and shorten the imaging time, the crystal is required to detect 511 keV gamma photons with high detection efficiency or blocking ability, so the crystal with relatively high effective atomic number and density is preferred.
  2. In order to make the PET scanner work in the state of high count rate, the time count loss is required to be as small as possible, so the crystal should have a relatively short luminescence decay time; Moreover, if the luminescence attenuation time of the crystal is short, the coincidence time window can be reduced and the influence of random coincidence can be reduced.
  3. In order to minimize the statistical errors and obtain good energy resolution and position resolution, the light yield of the crystal should be as high as possible.
  4. In order to cooperate with the photomultiplier tube, the scintillation light wavelength of the crystal should range from 380 to 440 nm.

According to the reports, many crystals have been proven to act as scintillation crystals which can be applied in PET. In the PET system, one scintillation crystal or several different crystals cooperating are used as detector. At the moment, scintillation crystals that can be mass produced are: BGO、LSO、Ce:LYSO、Ce:GaGG、GSO、LuYAP.

The following figure shows the basic structure of the detector for the event circuit. The basic detection unit in the figure is a pair of photon detectors (PMT/APD/MPPC, etc) coupled with scintillation crystals. The electrical signal generated by photons received by the detector is transmitted to the preamplifier for signal amplification.Therefore, the scintillation crystal is required to have a higher light output and a shorter decay time.

Types of scintillator crystal applied in PET

BGO

  • High density
  • High effective atomic number
  • Short radiation length
  • High detection efficiency
  • High stopping-power
  • Excellent energy resolution

scintillator is an attractive material for PET detectors because of its high detection efficiency, large photo fraction, and relatively low cost. In the clinical field, there has been a strong demand for a cost-effective whole-body PET scanner with high sensitivity and this promoted investigations to realize a BGO depth of interaction (DOI) detector. The peak emission wavelength of BGO is – 480 nm which provides a good match to the spectral response of photomultiplier tubes. The effective atomic number of BGO is 75 and density is 7.1g/cm3 which could improve the efficiency of detecting 511 keV gamma photons by minimizing the radiation dose received by the subjects and shortening the imaging time. The radiation length of BGO is 1.12cm,which is beneficial for producing compact detector or probe parts, improving spatial resolution and saving cost.

BGO crystal has a good piezoelectric performance, high photoelectric conversion efficiency and stable thermal performance. The packing fraction of PET detectors based on BGO crystals is usually higher than 90%. The high packing fraction has been one of the design goals for achieving maximum detection sensitivity. Currently, many research institutes cooperate BGO with other scintillation crystals, which can maximize its advantages and overcome its disadvantages of low output light yield and long decay time. In addition, BGO crystal has no afterglow, no dissociation surface, strong anti-irradiation ability and stable chemical properties, which is easy to process and maintain.

References
[1]MPPC Arrays in PET Detectors With LSO and BGO
[2] A DOI Detector With Crystal Scatter Identification Capability for High Sensitivity and High Spatial Resolution PET Imaging
[3] Performance Characteristics of BGO Detectors for a Low Cost Preclinical PET Scanner

Ce:LYSO

Wavelength(Max. emission) (nm)410
Decay time (ns)40
Light yield (photons/keV)25
Light output relative to Nal(Tl)  (%)75
Refractive index1.82@410nm

The design of a positron emission tomography (PET) scanner is specially challenging since it should not compromise high spatial resolution, high sensitivity, high count-rate capability, and good energy and time resolution. At the detector level, the scintillation crystal and the photo-detector are the two main components playing key roles, which are configured either by use of small pixelated scintillator elements or large monolithic crystals. In both approaches, the main goals are high spatial resolution over the entire field of view (FOV), high sensitivity, high count-rate performance, and good energy resolution and time resolution. Ce :LYSO is the most widely used scintillator in PET application because of its properties: high density, high light yield, low decay time and absence of hygroscopicity.

Ce :LYSO is a mixed LSO/YSO non-hygroscopic crystal that offers high density (7.1 , 10%Y), high light output (26000–32000 ph/MeV), good energy resolution ( ~10%) and short decay time (40 ns). Its maximum emission wavelength is 420nm, which can match well with the photocathode of photomultiplier tube. Ce :LYSO crystals coupled to SiPMs have been widely incorporated into PET systems recently. However, it is important to note that the intrinsic radiation from 176Lu results in gamma ray emissions at 88, 202, and 307 keV, which contribute to a significant radiation background as the number of crystals increase in the PET scanner.

References
[1] Performance of FBK high-density SiPM technology coupled to Ce:LYSO and CeGAGG for TOF-PET [2] Characterization of 1.2×1.2 mm2 silicon photomultipliers with Ce:LYSO, Ce:GAGG, and Pr:LuAG scintillation crystals as detector modules for positron emission tomography [3] Luminescence Emission Properties of (Lu; Y)2SiO5Ce (LYSO:Ce) and (Lu; Y)AlO3Ce (LuYAPCe) Single Crystal Scintillators Under Medical Imaging Conditions

Ce:GAGG

Wavelength(Max. emission) (nm)520
Wavelength range (nm)475-800
Decay time (ns)90
Light yield (photons/keV)50
Refractive index1.9@540nm

Positron Emission Tomography (PET) is a well-established method of obtaining information on the biochemical function of organs, and useful for the early stage detection of cancers and diagnosis of neurological disease. Typical PET scanners have detectors composed of thick scintillation crystals and photodetectors, such as photomultiplier tubes (PMT). These applications demand scintillators with high light yield, good energy resolution, high effective atomic number, fast scintillation response and chemical stability.

The Ce:GAGG scintillator is a novel inorganic scintillator having a high light yield of 46 000 photons=MeV. Its high light yield is expected to improve the energy resolution and accuracy of crystal identification by the DOI detector modules. s. GAGG:Ce scintillators emit a light-yellowish color light and its effective atomic number is equal to 54.4. Moreover, GAGG:Ce does not contain natural radioactivity which may improve the minimum detectable activity limits of a system. In addition, cooperated with SiPMs, GAGG:Ce crystal shows excellent timing resolution (below 200 ps) which makes them suitable for Time-of-Flight (ToF) PET applications. The mean energy resolution for the GAGG:Ce array was calculated equal to 10.5% under 511 keV irradiation.

References

[1]Performance of FBK high-density SiPM technology coupled to CeLYSO and CeGAGG for TOF-PET
[2] Characterization of 1.2×1.2 mm2 silicon photomultipliers with Ce LYSO, Ce GAGG, and Pr LuAG scintillation crystals as detector modules for positron emission tomography
[3] Development of prototype PET scanner using dual-sided readout DOI-PET modules

Ce:YAP

Wavelength(Max. emission) (nm)370
Decay time (ns)28
Light yield (photons/keV)25
Light output relative to Nal(Tl)  (%)60-70
Refractive index1.95@370nm

YAP : Ce (Yttrium alumium perosvkite doped with Cerium) has proven to be a good candidate for PET (Positron Emission Tomography) small animals. Its fast scintillation, good light output, reasonably high Z%&&, high density, and excellent physical properties have allowed the growth of long thin crystals. Put together in bundles and optically coupled to a position sensitive readout these detectors have given promising results. The main disadvantage for human PET applications of human PET applications of YAP:Ce is the low photoelectric fraction (~4%) at 511 keV.