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Impact on timing and light extraction of a photonic crystal as measured on a half patterned LYSO crystal: implications for time of flight PET imaging
Abstract
The improvement of the energy and of the timing resolution is always a challenge in scintillation-based detectors. A large fraction of the photons produced by scintillation remains trapped inside the crystal. Photonic crystals have been suggested as a solution to improve light extraction. Here we will present results obtained with a nanostructured TiO2 coating on a 50×50 mm2 LYSO crystal. The objective of the present paper is to characterize the performance of this coating in both light extraction and timing performance as both parameters are critical to spatial resolution of PET systems. To avoid tricky calibration problems, especially for timing, we have manufactured a monolithic crystal with one half patterned with a photonic crystal and with one half bare. We are rotating the crystal π/2 relative to the photo-detector between each measure. We have chosen a digital Si-PM Philips DPC 3200 as photo-detector due to its excellent timing precision and stability. The impact on light extraction of the photonic crystal is very strong as 30% of the light only escaped through the naked face vs 70% through the textured face for each position. The timing effect is much more subtle and quite at odd with previous results. By averaging the measurements on four positions, we are detecting a time lag effect of photon extraction with a probability of 98%. The average lag is only of 17 ps on the detection in the textured part of the crystal. This effect, although without practical consequences for PET imaging, is nevertheless perplexing as we were foreseeing a faster exit of photons on the textured face. We propose an explanation for the effect observed.
... [29][30][31] Introducing photon crystal microstructures holds promise for significantly improving the light output of PET detectors. [32][33][34][35] Mainstream methods include the indirect fabrication of photonic crystal structures on crystal surfaces using dielectric materials. For example, Lecoq et al. fabricated Si 3 N 4 dielectric photonic crystals on LSO crystal surfaces, resulting in a 56% enhancement in light output. ...
Background
Gamma‐ray detection plays a crucial role in the fields of biomedicine, space exploration, national defense, and security. High‐precision gamma photon detection relies on scintillation crystals, which attenuate gamma rays through mechanisms such as photoelectric effect and Compton scattering. These interactions generate light signals within the scintillation crystal, which are subsequently converted into electronic signals using photodetectors, enabling accurate readout, and analysis.
Purpose
Improving the readout efficiency of visible photons in crystal detectors can significantly improve the efficiency of gamma‐ray detection. Scintillator crystals are usually hard and brittle materials, which are difficult to process. In this paper, we innovatively propose the method of using a femtosecond laser to process grooved microstructures on the light output surface of scintillator crystals to improve the detection efficiency, and thus enhance the comprehensive performance of gamma‐ray detection.
Methods
Optical simulation software is first used to explore the enhancement of the light output performance by the grooved microstructures. Subsequently, a 5‐dimension system for femtosecond laser processing of scintillator crystals was constructed, which can achieve accurate processing of grooved structures. Finally, the feasibility of the study was verified by applying grooved microstructure on crystal bars and crystal arrays.
Results
TracePro simulation results showed an average efficiency improvement in light output of 33.56% within the groove parameters: spacing from 20 to 140 µm, depth from 8 to 28 µm, and width from 10 to 30 µm. A custom‐designed readout electronic system for gamma detection and a laser processing platform was then constructed to evaluate the feasibility of applying grooved structures to the lutetium‐yttrium oxyorthosilicate (LYSO) crystal surface. According to the simulation results, 12 groups of crystal bars were fabricated with spacings from 60 to 140 µm, depths from 7 to 16 µm, and widths from 11 to 14 µm. Experimental results showed an average improvement of 20.4% in light output for the crystal bars, and that of the crystal arrays can be improved by 6.85% on average.
Conclusions
This study introduces a method of using femtosecond lasers to fabricate grooved microstructures on LYSO crystal surfaces, which has demonstrated a significant improvement in light output in both simulation and experimentation. This method can be applied to the production of crystal arrays at a low cost and on a large scale, showing promising potential for common gamma detection applications, such as medical imaging, industry, and astrophysics.
... The next steps concerning the PhC will consist in extending the experimental characterization to crystal needles as well as measuring the possible impact of the PhC on the spatial reconstruction on one side, and the temporal on the other in terms of timing performance. We expect an improvement of CRT as also suggested in [18,21,43,44]. Another interesting, though challenging perspective is represented by the simulation of the light interaction with the entire scintillator-photodetector interface and the scintillating crystal itself, to optimize the output gain. ...
Scintillators play a key role in the detection chain of several applications which rely on the use of ionizing radiation, and it is often mandatory to extract and detect the generated scintillation light as efficiently as possible. Typical inorganic scintillators do however feature a high index of refraction, which impacts light extraction efficiency in a negative way. Furthermore, several applications such as preclinical Positron Emission Tomography (PET) rely on pixelated scintillators with small pitch. In this case, applying reflectors on the crystal pixel surface, as done conventionally, can have a dramatic impact of the packing fraction and thus the overall system sensitivity. This paper presents a study on light extraction techniques, as well as combinations thereof, for two of the most used inorganic scintillators (LYSO and BGO). Novel approaches, employing Distributed Bragg Reflectors (DBRs), metal coatings, and a modified Photonic Crystal (PhC) structure, are described in detail and compared with commonly used techniques. The nanostructure of the PhC is surrounded by a hybrid organic/inorganic silica sol-gel buffer layer which ensures robustness while maintaining its performance unchanged. We observed in particular a maximum light gain of about 41% on light extraction and 21% on energy resolution for BGO, a scintillator which has gained interest in the recent past due to its prompt Cherenkov component and lower cost.
The use of monolithic scintillator-based photon detectors in positron emission tomography (PET) has emerged as an attractive alternative to traditional pixelated array designs. Monolithic-based detector designs employ the scintillation light distribution (LD) shape to provide a single 3D photon interaction position per event, enabling high spatial resolution throughout the crystal volume. Since there are no intercrystal gaps, monolithic designs provide higher intrinsic detection efficiency compared to pixelated designs. However, in order to make the monolithic detector design practical for clinical PET systems, some major drawbacks need to be addressed such as the timeconsuming and complex calibration procedures to obtain precise spatial and timing information. This paper gives a historical review of monolithic-based PET detectors, a description of their main advantages and challenges, describes the state-of-the-art, including their use in current commercial system, and ends with a future prospective.
Scintillators are central for detection of γ-ray, x-ray, and high energy particles in various applications, all seeking higher scintillation yield and rate. However, these are limited by the intrinsic isotropy of spontaneous emission of the scintillation light and its inefficient outcoupling. We propose a new design methodology for scintillators that exploits the Purcell effect to enhance their light emission. As examples, we show 1D photonic crystals from scintillator materials that achieve directional emission and fivefold enhancement in the number of detectable photons per excitation.
Gamma-ray detectors based on thick monolithic scintillator crystals can achieve spatial resolutions <2 mm full-width-at-half-maximum (FWHM) and coincidence resolving times (CRTs) better than 200 ps FWHM. Moreover, they provide high sensitivity and depth-of-interaction (DOI) information. While these are excellent characteristics for clinical time-of-flight (TOF) positron emission tomography (PET), the application of monolithic scintillators has so far been hampered by the lengthy and complex procedures needed for position- and time-of-interaction estimation. Here, the algorithms previously developed in our group are revised to make the calibration and operation of a large number of monolithic scintillator detectors in a TOF-PET system practical. In particular, the k-nearest neighbor (k-NN) classification method for x,y-position estimation is accelerated with an algorithm that quickly preselects only the most useful reference events, reducing the computation time for position estimation by a factor of ~200 compared to the previously published k-NN 1D method. Also, the procedures for estimating the DOI and time of interaction are revised to enable full detector calibration by means of fan-beam or flood irradiations only. Moreover, a new technique is presented to allow the use of events in which some of the photosensor pixel values and/or timestamps are missing (e.g. due to dead time), so as to further increase system sensitivity. The accelerated methods were tested on a monolithic scintillator detector specifically developed for clinical PET applications, consisting of a 32 mm × 32 mm × 22 mm LYSO : Ce crystal coupled to a digital photon counter (DPC) array. This resulted in a spatial resolution of 1.7 mm FWHM, an average DOI resolution of 3.7 mm FWHM, and a CRT of 214 ps. Moreover, the possibility of using events missing the information of up to 16 out of 64 photosensor pixels is shown. This results in only a small deterioration of the detector performance.
In this paper we present the first fully digital implementation of the Silicon Photomultiplier. The chip design is based on the technology demonstrator chip presented in [3]. The new sensor represents a self-contained detector including a JTAG controller for configuration and test, single-ended and differential clock and test input signals, an integrated acquisition controller and two serial data outputs. The sensor is based on a single photon avalanche photodiode (SPAD) technology integrated in a standard CMOS process flow. Photons are detected directly by sensing the voltage at the SPAD terminal using a dedicated cell electronics block next to each diode. This block also contains active quenching and recharge circuits as well as a one bit memory for the selective activation of individual detector cells. A balanced trigger network is used to propagate the trigger signal from all cells to the two integrated time-to-digital converters. Photons are detected and counted as digital signals, thus making the sensor less susceptible to temperature variations and electronic noise. The resulting data packets are transferred to the readout system through a serial data interface. In this paper, we discuss the new sensor architecture and evaluate its performance.
This paper presents a characterization method to experimentally determine the angular distribution of scintillation light. By exciting LYSO crystals with a radioactive source, we measured the light angular profiles obtained with samples of different geometries in different conditions of wrapping. We also measured the angular distribution of light emitting in glue and compared it with the one emitting in air. Angular distribution of light output of photonic crystals is also provided. Consistency of the measurements is verified with conventional light output measurements.
Photonic crystals (PhCs) and quantum optics phenomena open interesting perspectives to enhance the light extraction from scintillating media with high refractive indices as demonstrated by our previous work. By doing so, they also influence the timing resolution of scintillators by improving the photostatistics. The present contribution will demonstrate that they are actually doing much more. Indeed, photonic crystals, if properly designed, allow the extraction of fast light propagation modes in the crystal with higher efficiency, therefore contributing to increasing the density of photons in the early phase of the light pulse. This is of particular interest to tag events at future high-energy physics colliders, such as CLIC, with a bunch-crossing rate of 2 GHz, as well as for a new generation of time-of-flight positron emission tomographs (TOFPET) aiming at a coincidence timing resolution of 100 ps FWHM. At this level of precision, good control of the light propagation modes is crucial if we consider that in a 2 × 2 × 20-mm3 LSO crystal, the time spread (peak to peak) of extracted photons can be as large as 400 ps considering simple light bouncing only. This paper presents a detailed analysis of the light propagation and extraction modes in a LSO crystal combining the LITRANI light ray tracing and the CAMFR PhC simulation codes. Ongoing measurement results are shown with an attempt to unfold the contribution from the improved photostatistics that result from the total enhanced light output on one side and from the improved contribution of fast propagation mode extraction on the other side. Some results are also shown on a new and more industrial process to produce PhCs by the use of nano-imprint technology.
Photonic crystals (PhCs) are optical materials which can affect the propagation of light in multiple ways. In recent years PhCs contributed to major technological developments in the field of semiconductor lasers, light emitting diodes or photovoltaic applications. In our case we are investigating the capabilities of photonic crystal slabs with the aim to improve the performance of heavy inorganic scintillators. To study the combination of scintillators and PhCs we use a Monte-Carlo program to simulate the light propagation inside a scintillator and a rigorous coupled wave analysis (RCWA) framework to analyze the optical PhC properties. The simulations show light output improvements of a wide range of scintillating materials due to light scattering effects of the PhC slabs. First samples have been produced on top of 1:2mm×2:6mm×5mm LSO (cerium-doped Lutetium Oxyorthosilicate, Lu2SiO5:Ce3+) scintillators using electron beam lithography and reactive ion etching. Our samples are showing a 30–60% light output improvement when compared to unstructured reference crystals which is in close accordance with our simulation results.
The impact of photonic crystal (PhC) slabs on the extraction of light from the heavy inorganic scintillators LuYAP and LYSO is evaluated by combining numerical transmission calculations of a scintillator with PhC coupling face with simulations of the light propagation inside the scintillator. The transmission of the scintillator-PhC coupling face is determined by means of a scattering-matrix algorithm. The PhC slab is assumed to consist of a bulk material with a triangular pattern of air holes that is sandwiched between the scintillator substrate and a layer of optical grease. By folding these data with the angular distribution of the scintillation photons arriving at the coupling face, the light collection efficiency η L of the system is estimated. The results indicate that a scintillator coupling face equipped with a PhC slab can exhibit a significant gain in η L. This gain is due to the extraction of photons that are lost in a scintillator with plain exit surface due to total internal reflection. The largest simulated gains of up to a factor of two are observed for small scintillators and for PhC coupling faces with n bulk ≪ n sub, n avg ≈ √n sub · n grease, and d ≤ a, where n bulk, n sub and n grease are the respective refractive indexes of the PhC bulk, the scintillator substrate, and the optical grease, a the lattice constant of the PhC pattern, d the thickness of the PhC slab, and n avg the average refractive index of the PhC slab determined by n bulk and the filling factor f. Due to the approximations and idealizations of the model, these gains in light collection efficiency may be lower in practical applications and are expected to be achieved only with specular reflectors with reflectivities above 90%, and PhC bulk materials with absorption coefficients α abs ≤ 10 3-10 4cm -1 over the whole wavelength range of emission of the scintillator.
The renewal of interest in Time of Flight Positron Emission Tomography (TOF-PET), as well as the necessity to precisely tag events in high energy physics (HEP) experiments at future colliders are pushing for an optimization of all factors affecting the time resolution of the whole acquisition chain comprising the crystal, the photo detector, and the electronics. The time resolution of a scintillator-based detection system is determined by the rate of photo electrons at the detection threshold, which depends on the time distribution of photons being converted in the photo detector. The possibility to achieve time resolution of about 100 ps Full Width at Half Maximum (FWHM) requires an optimization of the light production in the scintillator, the light transport and its transfer from the scintillator to the photo detector. In order to maximize the light yield, and in particular the density of photons in the first nanosecond, while minimizing the rise time and decay time, particular attention must be paid to the energy transfer mechanisms to the activator as well as to the energy transition type at the activator ion. Alternatively other light emission mechanisms can be considered. We show that particularly Cerenkov emission can be used for this purpose. Special emphasis was put on the light transport within the crystal and at its interface with the photo detector. Since light is produced isotropically in the scintillator the detector geometry must be optimized to decrease the optical path-length to the photo detector. Moreover light bouncing within the scintillator, affecting about 70% of the photons generated in currently used crystals, must be reduced as much as possible. We also investigate photonics crystals that are specifically designed to favor specific light propagation modes at the limit of total reflection inside and outside of the crystal and how they might increase the light transfer efficiency to the photo detector and hence improve time resolution. Exampl-
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es for the production and deposition of photonics crystals as layers on Lutetium Yttrium Ortho-Silicate (LYSO) and Lutetium Yttrium Aluminum Perovskite (LuYAP) crystals are shown here, as well as first results on an improved light extraction resulting from this method.
The high refractive index of current scintillating materials puts severe restrictions on their effective light yield. In this paper, we describe an approach that uses a photonic crystal pattern machined into the coupling face of the scintillator to partly overcome the problem of total internal reflection. Simulations are performed for 2 mm times 2 mm times 8 mm LuAP and LSO pixels with and without photonic crystal and different types of wrapping. It is shown that by tuning the structure of the photonic crystal and the size of its elements, the extraction efficiency of the surface can be significantly improved compared to a plain exit surface.
Improved light extraction efficiency on 2 inches LYSO with nanopatterned TiO 2 photonic crystals, posters at the IEEE Nucl
- S Zanettini
S. Zanettini et al., Improved light extraction efficiency on 2 inches LYSO with nanopatterned TiO 2
photonic crystals, posters at the IEEE Nucl. Sci. Symp. Med. Imag. Conf., October 29-November 5,
Strasbourg, France (2016).
Temporal imaging for PET: observation and precise localisation of photo-electric events inside a monolithic 20 mm LYSO plate with a Phillips digital Si-PM
- A Iltis
A. Iltis et al., Temporal imaging for PET: observation and precise localisation of photo-electric events
inside a monolithic 20 mm LYSO plate with a Phillips digital Si-PM, (2016).