Continuous Scintillation Crystals Research Articles

Evaluation of LFS continuous scintillation crystals for PET

Abstract The suitability of monolithic Lutetium Fine Silicate (LFS) continuous scintillation crystals coupled to 64-channel SiPM-TSV matrices for small animal PET has been evaluated. Three crystals of size 25.8 × 25 . 8 × 15 mm 3 were tested: LYSO white coating, LFS white coating (LFS-W) and LFS specular coating (LFS-S). Electronically collimated measurements were carried out to compare the performance of LYSO and LFS crystals. Mean energy resolutions of 14%, 17% and 19% were measured for LYSO, LFS-S and LFS-W respectively. Similar intrinsic spatial resolutions were found for all three crystals in the central region. Different versions of the light model used to estimate the interaction position of the photons in the detector have been studied. Further measurements with two detector heads of each crystal type in coincidence have also been carried out with a moveable 22Na source. A ML-MLEM image reconstruction code is used to reconstruct the data. Final position resolutions for all three configurations show similar values. Taking into account that LYSO and LFS present similar performance levels (and considering that LFS has better timing resolution than LYSO) we conclude that LFS is a valid substitute for LYSO.

Retroreflector arrays for better light collection efficiency ofγ-ray imaging detectors with continuous scintillation crystals without DOI misestimation

A method to improve light collection efficiency of γ-ray imaging detectors by using retroreflector arrays has been tested, simulations of the behaviour of the scintillation light illuminating the retroreflector surface have been made. Measurements including retroreflector arrays in the setup have also been taken. For the measurements, positron emission tomography (PET) detectors with continuous scintillation crystals have been used. Each detector module consists of a continuous LSO-scintillator of dimensions 49x49x10 mm3 and a H8500 position-sensitive photo-multiplier (PSPMT) from Hamamatsu. By using a continuous scintillation crystal, the scintillation light distribution has not been destroyed and the energy, the centroids along the x- and y-direction and the depth of interaction (DOI) can be estimated. Simulations have also been run taking into account the use of continuous scintillation crystals. Due to the geometry of the continuous scintillation crystals in comparison with pixelated crystals, a good light collection efficiency is necessary to correctly reconstruct the impact point of the γ-ray. The aim of this study is to investigate whether micro-machine retro-reflectors improve light yield without misestimation of the impact point. The results shows an improvement on the energy and centroid resolutions without worsening the depth of interaction resolution. Therefore it can be concluded that using retroreflector arrays at the entrance side of the scintillation crystal improves light collection efficiency without worsening the impact point estimation.

Study of position reconstruction of a LaBr3:Ce continuous scintillation crystal for medical applications

Many modern molecular imaging techniques, based on radiopharmaceuticals, can take advantage of sophisticated devices but are still based on the scintillation detector mechanism proposed by Anger. These devices can perform with millimeter spatial resolution and high detection efficiency, but the final performance is strongly affected by the algorithm used for the scintillation position detection.In this work, a detailed comparison of the effect on the imaging performances of three new position detection algorithms, in terms of spatial resolution, detection linearity and useful Field of View is performed on a prototype gamma detector. The detector, built by the authors, is based on a continuous LaBr3:Ce scintillation crystal coupled to an Hamamatsu MA-PMT H8500 and a single anode readout electronics. The experimental data are obtained scanning the detector surface with a Tc99m collimated source (0.4 mm ϕ) at 1.5 mm step.The overall imaging performances of the device are also tested by mean of a bar phantom. We conclude that the Anger-like algorithms give a 50% uFoV with a 1.30 mm ±0.05 mm spatial resolution while the proposed algorithms give a 80% uFoV and 1.10 mm ±0.06 mm spatial resolution.

Spectrometric performances of high quantum efficiency multi and single anode PMTs coupled to LaBr3(Ce) crystal

High quantum efficiency semiconductor photodetectors have recently drawn the attention of the scientific community for their potential in the realization of a new class of scintillation imagers with very high energy and spatial resolution performance. However, this goal does not seem within easy reach, due to various technological issues such as, for example, the difficulty to scale the characteristics of a single detector to an imager with suitable dimensions. Lately a definite technical improvement in increasing quantum efficiency up to 42% for position sensitive photomultipliers was achieved. The aim of this work is thus to test this new technological progress and to study the possible implications in imaging applications. Four Hamamatsu PMTs were tested: two multi anode photomultipliers, one with a bialkali (27% quantum efficiency) and the other one with a super-bialkali photocathode (38% quantum efficiency), and two 1×1 in. PMTs, both equipped with an ultra bialkali photocathode (42% quantum efficiency). In particular one of the ultra bialkali PMT has also an increased efficiency of first dynode charge collection. The results were compared with the ones obtained with a reference PMT (Hamamatsu R6231), mainly used in spectroscopy. The PMTs were coupled to LaBr3(Ce), NaI(Tl) and LSO(Ce) continuous scintillation crystals. The tests were done using two independent electronic chains: one dedicated for spectroscopic application and a second one, using a multi wire 64 channel readout, for imaging applications. The super-bialkali MA-PMTs have shown high energy resolution, both with spectroscopic and imaging setup, highlighting the appropriateness of these devices for the development of imaging devices with high spectroscopic performance.

Combining Surface Treatments With Shallow Slots to Improve the Spatial Resolution Performance of Continuous, Thick LYSO Detectors for PET

Positron emission tomography (PET) detectors based on continuous scintillation crystals can achieve very good performance and have a number of practical advantages compared to detectors based on a pixelated array of crystals. Our goal is to develop a thick continuous detector with high energy and spatial resolution, along with high γ-photon capture efficiency. We examine the performance of two crystal blocks: a 46 × 46 × 14 mm3 and a 48 × 48 × 25 mm3 block of LYSO (Lutetium Yttrium Orthosilicate). Using Maximum Likelihood (ML) positioning based upon the light response function (LRF) in the 14 mm thick crystal, we measure a spatial resolution of 3 mm in the central region of the crystal with degradation near the edges due to reflections off the crystal sides. We also show that we can match the spatial resolution achieved using a 14 mm thick crystal by using a 25 mm thick crystal with slots cut into the gamma entrance surface to narrow the LRF. We also find that we can improve the spatial resolution performance near the detector edges by reducing the reflectivity of the crystal sides, albeit with some loss in energy resolution.

First Compton telescope prototype based on continuous LaBr3-SiPM detectors

A first prototype of a Compton camera based on continuous scintillator crystals coupled to silicon photomultiplier (SiPM) arrays has been successfully developed and operated. The prototype is made of two detector planes. The first detector is made of a continuous 16×18×5mm3 LaBr3 crystal coupled to a 16-elements SiPM array. The elements have a size of 3×3mm 3 in a 4.5×4.05mm2 pitch. The second detector, selected by availability, consists of a continuous 16×18×5mm3 LYSO crystal coupled to a similar SiPM array. The SPIROC1 ASIC is employed in the readout electronics. Data have been taken with a 22Na source placed at different positions and images have been reconstructed with the simulated one-pass list-mode (SOPL) algorithm.Detector development for the construction of a second prototype with three detector planes is underway. LaBr3 crystals of 32×36mm2 size and 5/10mm thickness have been acquired and tested with a PMT. The resolution obtained is 3.5% FWHM at 511keV. Each crystal will be coupled to four MPPC arrays. Different options are being tested for the prototype readout.

Design and Simulation of a Gamma-Ray Camera for PET with a Continuous Scintillation Crystal

One of the main advantages of continuous scintillators is the excellent depth of interaction identification capability. We present the results of the Monte Carlo program OPTICS simulating the light response of a gamma-ray detector module comprising a continuous scintillator and an avalanche photodiode array to determine the effect of scintillator surface treatment on the ability to determine the three-dimensional position of interaction. We investigated the effect of optical coupling compounds, surface treatment, and depth of interaction on the distribution of scintillation photons reaching the photodetector pixels. We show that a black coating material without an air gap leads to improvement.

AMIC: An Expandable Front-End for Gamma-Ray Detectors With Light Distribution Analysis Capabilities

A novel CMOS integrated front-end architecture is presented throughout this paper. It is designed to be used with detectors based on continuous scintillation crystals plus position sensitive photomultiplier. Its structure aims at carrying out an analog computation which extracts fundamental information of the detected event. This fact allows us to avoid an individual acquisition of every input channel so that a large increase of inputs is feasible. In order to accomplish the processing task, a current buffer delivers a copy of each input signal to several computation blocks. Those processing units implement current mode analog filtering operations with a digitally programmable 8-bit precision coefficient for each front-end input. Output currents are summed and sent to the output stage where a buffered current output is provided. A voltage signal is also available by means of a rail-to-rail transresistance amplifier. The final goal is to obtain several moments of the light distribution on the detector surface. Each one provides useful information, such as energy, position, depth of interaction (from the light distribution width), skewness (deformation due to border effect), etc. Since the computation is purely additive, the current outputs can be used as inputs to other equal devices thus creating a fully expandable architecture.

Maximum-likelihood scintillation detection for EM-CCD based gamma cameras

Gamma cameras based on charge-coupled devices (CCDs) coupled to continuous scintillation crystals can combine a good detection efficiency with high spatial resolutions with the aid of advanced scintillation detection algorithms. A previously developed analytical multi-scale algorithm (MSA) models the depth-dependent light distribution but does not take statistics into account. Here we present and validate a novel statistical maximum-likelihood algorithm (MLA) that combines a realistic light distribution model with an experimentally validated statistical model. The MLA was tested for an electron multiplying CCD optically coupled to CsI(Tl) scintillators of different thicknesses. For 99mTc imaging, the spatial resolution (for perpendicular and oblique incidence), energy resolution and signal-to-background counts ratio (SBR) obtained with the MLA were compared with those of the MSA. Compared to the MSA, the MLA improves the energy resolution by more than a factor of 1.6 and the SBR is enhanced by more than a factor of 1.3. For oblique incidence (approximately 45°), the depth-of-interaction corrected spatial resolution is improved by a factor of at least 1.1, while for perpendicular incidence the MLA resolution does not consistently differ significantly from the MSA result for all tested scintillator thicknesses. For the thickest scintillator (3 mm, interaction probability 66% at 141 keV) a spatial resolution (perpendicular incidence) of 147 µm full width at half maximum (FWHM) was obtained with an energy resolution of 35.2% FWHM. These results of the MLA were achieved without prior calibration of scintillations as is needed for many statistical scintillation detection algorithms. We conclude that the MLA significantly improves the gamma camera performance compared to the MSA.

Development of a variable-radius pinhole SPECT system with a portable gamma camera

Objective To develop a small-animal SPECT system using a low cost commercial portable gamma camera equipped with a pinhole collimator, a continuous scintillation crystal and a position-sensitive photomultiplier tube. Material and methods The gamma camera was attached to a variable radius system, which enabled us to optimize sensitivity and resolution by adjusting the radius of rotation to the size of the object. To investigate the capability of the SPECT system for small animal imaging, the dependence of resolution and calibration parameters on radius was assessed and acquisitions of small phantoms and mice were carried out. Results Resolution values, ranging from 1.0 mm for a radius of 21.4 mm and 1.4 mm for a radius of 37.2 mm were obtained, thereby justifying the interest of a variable radius SPECT system. Conclusions The image quality of phantoms and animals were satisfactory, thus confirming the usefulness of the system for small animal SPECT imaging.

Position correction with depth of interaction information for a small animal PET system

In this work we study the effects on the spatial resolution when depth of interaction (DOI) information is included in the parameterization of the line of response (LOR) for a small animal positron emission tomography (PET) system. One of the most important degrading factors for PET is the parallax error introduced in systems that do not provide DOI information of the recorded γ -rays . Our group has designed a simple and inexpensive method for DOI determination in continuous scintillation crystals. This method is based, on one hand, in the correlation between the scintillation light distribution width in monolithic crystals and the DOI, and, on the other hand, on a small modification of the widely applied charge dividing circuits (CDR). In this work we present a new system calibration that includes the DOI information, and also the development of the correction equations that relates the LOR without and with DOI information. We report the results obtained for different measurements along the transaxial field of view (FOV) and the image quality enhancement achieved specially at the edge of the FOV.

Investigation of high resolution compact gamma camera module based on a continuous scintillation crystal using a novel charge division readout method

The objective of this study is to investigate a high performance and lower cost compact gamma camera module for a multi-head small animal SPECT system. A compact camera module was developed using a thin Lutetium Oxyorthosilicate (LSO) scintillation crystal slice coupled to a Hamamatsu H8500 position sensitive photomultiplier tube (PSPMT). A two-stage charge division readout board based on a novel sub-tractive resistive readout with a truncated center-of-gravity (TCOG) positioning method was developed for the camera. The performance of the camera was evaluated using a flood 99mTc source with a four-quadrant bar-mask phantom. The preliminary experimental results show that the image shrinkage problem associated with the conventional resistive readout can be effectively overcome by the novel subtractive resistive readout with an appropriate fraction subtraction factor. The response output area (ROA) of the camera shown in the flood image was improved up to 34%, and an intrinsic spatial resolution better than 2 mm of detector was achieved. In conclusion, the utilization of a continuous scintillation crystal and a flat-panel PSPMT equipped with a novel subtractive resistive readout is a feasible approach for developing a high performance and lower cost compact gamma camera.

Revisited position arithmetics for LaBr 3:Ce continuous crystals

The development of the molecular imaging technique based on radiopharmaceuticals is requiring sophisticated devices with sub-millimeter spatial resolution and high detection efficiency. Recently, in the field of scintillation gamma camera, it has been demonstrated that continuous crystals may improve spatial resolution with respect to the use of scintillation arrays. In this work we propose a new algorithm that improves spatial resolution. It calculates the position from a single scintillation event by squaring the charge collected on a multi-anodes photomultiplier tube (MA-PMT). It is able to remove the position linearity distortion due to the light reflections and the truncations at the crystal edge. We present measurements from a compact gamma camera based on a 51 mm × 51 mm × 4.0 mm LaBr 3 :Ce continuous scintillation crystal, coupled to a MA-PMT Hamamatsu H8500. The results show a strong improvement in spatial resolution and confirm the high potential in using of LaBr continuous crystal for molecular imaging applications.

Theoretical study of three-dimensionally position-sensitive scintillation detector based on continuous crystal

In the field of nuclear imaging (PET, Compton camera, etc.) the application of detectors that are position sensitive in all the three dimensions could increase spatial resolution of images by avoiding the parallax error. In the present paper the viability of a three-dimensionally position-sensitive scintillation detector using continuous scintillator crystal is theoretically studied. The feasibility of building such a detector is based on the recognition that both the spatial position and the radiant flux of a point-like isotropic radiation source can be determined by performing flux density measurements at four appropriately selected points of the space. On the basis of this one can construct a position-sensitive scintillation detector where the scintillation crystal is surrounded with four or more photodetectors, supposing that the point of interaction (POI) of the gamma ray within the scintillator could be considered as a point-like isotropic light source. The POI coordinates can then be determined relying on the finding that the ratio of two arbitrary linear combinations of the photodetector responses is equal to the same ratio constructed from the solid angles subtended by the photodetectors as seen from the POI. The results of theoretical calculations describing the relations between the photodetector responses and the spatial coordinates of the POI are presented for three different photodetector–scintillator combinations.

Readout of a LaBr3:Ce continuous crystal by a MPPC array: First results

In this work preliminary results of single-photon measurements made with a LaBr3:Ce continuous scintillation crystal (BrilLanCe 380™, by Saint-Gobain Crystals, FR) coupled to an array of Multi-Pixel Photon Counters (MPPC™, by Hamamatsu Photonics, JP) are presented. The array of 7 MPPCs was realized as the innermost element of the hexagonal Anger-like structure suitable for growing to larger areas by adding peripheral elements. The design of MPPC electronic readout for position determination was based on Anger logic. Wide-band amplifiers (by MITEQ®, US) were used for signals conditioning. Digitized traces of background, 57Co and 133Ba collimated irradiation were captured, characterized and stored by using the data-logging capabilities of a WaveMaster Oscilloscope (by LeCroy, US). Results are presented in terms of energy response, position sensing and signals fall and rise times.

A micro-machined retro-reflector for improving light yield in ultra-high-resolution gamma cameras

High-resolution imaging of x-ray and gamma-ray distributions can be achieved with cameras that use charge coupled devices (CCDs) for detecting scintillation light flashes. The energy and interaction position of individual gamma photons can be determined by rapid processing of CCD images of individual flashes. Here we investigate the improvement of such a gamma camera when a micro-machined retro-reflector is used to increase the light output of a continuous scintillation crystal. At 122 keV we found that retro-reflectors improve the intrinsic energy resolution (full width at half maximum (FWHM)) by 32% (from 50% to 34%) and the signal-to-noise (SNR) ratio by 18%. The spatial resolution (FWHM) was improved by about 4%, allowing us to obtain a resolution of 159 µm. The full width at tenth maximum (FWTM) improvement was 13%. Therefore, this enhancement is a next step towards realizing compact high-resolution devices for imaging gamma emitters.

PESIC: An Integrated Front-End for PET Applications

An ASIC front-end has been developed for multianode photomultiplier based nuclear imaging devices. Its architecture has been designed to improve resolution and decrease pile-up probability in positron emission tomography systems which employ continuous scintillator crystals. Analog computation elements are isolated from the photomultiplier by means of a current sensitive preamplifier stage. This allows digitally programmable adjustment of every anode gain, also providing better resolution in gamma event position calculation and a shorter front-end deadtime. The preamplifier stage also offers the possibility of using other types of photomultiplier devices such as SiPM. The ASIC architecture includes measurement of the depth of interaction of the gamma event based on the width of the light distribution in order to reduce parallax error and increase spatial resolution during image reconstruction stage. An output stage of transresistance amplifiers offer voltage output signals which may be introduced in the A/D conversion stage with no further processing.

Front-end circuit for position sensitive silicon and vacuum tube photomultipliers with gain control and depth of interaction measurement

Silicon Photomultipliers, though still under development for mass production, may be an alternative to traditional Vacuum Photomultipliers Tubes (VPMT). As a consequence, electronic front-ends initially designed for VPMT will need to be modified. In this simulation, an improved architecture is presented which is able to obtain impact position and depth of interaction of a gamma ray within a continuous scintillation crystal, using either kind of PM. A current sensitive preamplifier stage with individual gain adjustment interfaces the multi-anode PM outputs with a current division resistor network. The preamplifier stage allows to improve front-end processing delay and temporal resolution behavior as well as to increase impact position calculation resolution. Depth of interaction (DOI) is calculated from the width of the scintillation light distribution, which is related to the sum of voltages in resistor network input nodes. This operation is done by means of a high-speed current mode scheme.

Performance tests of two portable mini gamma cameras for medical applications

We have developed two prototypes of portable gamma cameras for medical applications based on a previous prototype designed and tested by our group. These cameras use a CsI(Na) continuous scintillation crystal coupled to the new flat-panel-type multianode position-sensitive photomultiplier tube, H8500 from Hamamatsu Photonics. One of the prototypes, mainly intended for intrasurgical use, has a field of view of 44 x 44 mm2, and weighs 1.2 kg. Its intrinsic resolution is better than 1.5 mm and its energy resolution is about 13% at 140 keV. The second prototype, mainly intended for osteological, renal, mammary, and endocrine (thyroid, parathyroid, and suprarenal) scintigraphies, weighs a total of 2 kg. Its average spatial resolution is 2 mm; it has a field of view of 95 x 95 mm2, with an energy resolution of about 15% at 140 keV. The main advantages of these gamma camera prototypes with respect to those previously reported in the literature are high portability and low weight, with no significant loss of sensitivity and spatial resolution. All the electronic components are packed inside the mini gamma cameras, and no external electronic devices are required. The cameras are only connected through the universal serial bus port to a portable PC. In this paper, we present the design of the cameras and describe the procedures that have led us to choose their configuration together with the most important performance features of the cameras. For one of the prototypes, clinical tests on melanoma patients are presented and images are compared with those obtained with a conventional camera.

Medium field of view multiflat panel-based portable gamma camera

A portable gamma camera based on the multianode technology has been built and tested. The camera consists in optically coupling four “Flat Panel” H8500 PSPMTs to a 100×100×4 mm 3 CsI(Na) continuous scintillation crystal. The dimensions of the camera are 17×12×12 cm 3 including the pinhole collimator and it weighs a total of 2 kg. Its average spatial resolution is 2 mm, its energy resolution is about 15%, and it shows a field of view of 95 mm. Because of its portability, its FOV and its cost, it is a convenient choice for osteological, renal, mammary, and endocrine (thyroid, parathyroid and suprarenal) scintigraphies, as well as other important applications such as intraoperatory detection of lymph nodes and surgical oncology. We describe the simulations performed which explain the crystal choice, the mechanical design of the camera and the method of calibration and algorithms used for position, energy and uniformity correction. We present images taken from phantoms. We plan to increase the camera sensitivity by using a four-holes collimator in combination with the MLEM algorithm, in order to decrease the exploration time and to reduce the dose given to the patient.