Hamamatsu R2486 Research Articles

Position and Energy Resolution of a γ-Camera based on a Position Sensitive Photomultiplier Tube

Studies of the spatial- and energy-resolution of a small field, high resolution γ-Camera system currently being developed in our laboratory are presented here. The system is based on a cylindrical Position Sensitive Photo-Multiplier Tube (HAMAMATSU R2486) with 32 crossed-wired anodes, arranged in two orthogonal groups. The anode outputs are connected to a resistive current divider network and after pre-amplification are guided to a local digitizing system. A PCI-Analog to Digital Converter card with a maximum sampling rate of 20 MHz and the rest of the Data Acquisition System is controlled by software running on the LabVIEW environment. Position and induced charge of the incident light pulses can be easily reconstructed through a variety of offline algorithms. In the first part of this study the intrinsic response of the PMT and especially its energy and position resolution is presented. The experimental procedure utilizes the controlled pulse light output of a LED guided through fiber glass directly to the PMT’s entrance. The accumulated charge distribution and charge spread as a function of the incident number of photons is studied. In the second part, the response of the integrated γ-Camera system after the application of typical scintillation crystals for 99Tc radioactive phantoms is measured and analyzed.

A Small Field, High Resolution γ-Camera System dedicated to SPECT Imaging

A small field, high resolution γ-Camera system dedicated to radiopharmaceutical research and other clinical SPECT (Single Photon Emission Computed Tomography) applications is currently being developed in our group. The system is equipped with the 3” HAMAMATSU R2486 Position Sensitive PhotoMultiplier Tube (PSPMT) with a 16X+16Y-crossed wire anode and various pixelated and ho- mogeneous scintillation crystals. Planar images are created from the recorded charge signals by applying the resistive chain technique. The main part of this work focuses on the development of new correction methods for the improvement of the spatial resolution and the uniformity of the γ-Camera. The spatial distortion correction technique is based on lookup tables with the coordinates of reference points which are selected during the calibration phase of the system for a given set of collimator and scintillation crystal. The applied algorithm incorporates 2D-interpolation tech- niques and has been developed on a full automated graphics environment making use of the HIGZ (High Level Interface to Graphics and ZEBRA) program libraries from CERN. Both correction methods for the spatial distortion and non-uniformity have been applied to phantom images using several combinations of small capillaries filled with water solution of 99mTc. Comparative studies are shown on planar im- ages for different phantom geometries. The method is also extended to tomographic images and the obtained SPECT improvement in resolution is discussed.

Achieving reliable coincidence resolving time measurement of PET detectors using multichannel waveform digitizer based on DRS4 chip

Coincidence resolving time (CRT) is one of the most important physical-performance measures for positron emission tomography (PET), as reconstruction with accurate time-of-flight information enhances the lesion detectability in patient studies. Accordingly, various PET detector designs and high-performance front-end readout circuits have been actively investigated to improve timing performance. The resulting PET detectors are often evaluated using multichannel waveform digitizers for versatile data analysis of the output signals. However, we have found that inappropriate data acquisition (DAQ) using a multichannel waveform digitizer based on the domino-ring-sampler 4 (DRS4) chip can lead to a considerable error when determining CRT. To address this issue, we performed CRT measurements using a pair of Hamamatsu R9800 photomultiplier tube based PET detectors. Then, considering intra- and inter-chip sampling, we employed four different combinations of input channels into the CAEN DT5742B waveform digitizer and obtained 2D CRT maps according to the leading-edge discriminator threshold for assessing each DAQ scheme. The intra-chip CRT measurement exhibited unusual streak patterns in the 2D CRT map and yielded the artificially-low CRT information in PET detector pairs, whereas the inter-chip CRT measurement provided the reliable estimation of timing resolution. Further, we could prevent the high-frequency signal crosstalk among input channels within the DRS4 chip using the inter-chip CRT measurement. We expect that our findings will also be useful for achieving the reliable CRT measurements when using other single-chip-based multichannel waveform digitizers.

VECC array for Nuclear fast Timing and angUlar corRElation studies (VENTURE)

The VECC array for Nuclear fast Timing and angUlar corRElation studies (VENTURE) has been developed using several fast Cerium-Bromide (CeBr3) scintillators coupled to Hamamatsu R9779 Photomultiplier tubes. The CeBr3 detector has been characterised for the spectroscopic properties like energy response, energy resolution, timing resolution and detection efficiency. The response and efficiency of the detector have been compared with the results obtained from a Monte Carlo simulation with GEANT3 package. A time resolution of 144(1) ps and 109(1) ps was obtained for a single detector using 622–512 keV and 1173–1332 keV cascades respectively. The Generalised Centroid Difference (GCD) method has been employed with CeBr3 detectors by measuring the level lifetimes for the 511.9 keV level of 106Pd and the 160.6 and 383.8 keV levels of 133Cs. The angular correlation measurement was performed for the 1173–1332 keV cascade in 60Ni and the 228–49 keV cascade of 132I nucleus, populated from the decay of 132Te produced via 238U(α, f) reaction.

Reduced γ–γ time walk to below 50 ps using the multiplexed-start and multiplexed-stop fast-timing technique with LaBr3(Ce) detectors

The electronic γ–γ fast-timing technique using arrays consisting of many LaBr3(Ce) detectors is a powerful method to determine lifetimes of nuclear excited states with a lower limit of about 5ps. This method requires the determination of the energy-dependent time walk of the zero time which is represented by the centroid of a prompt γ–γ time distribution. The full-energy peak versus full-energy peak prompt response difference which represents the linearly combined mean γ–γ time walk of a fast-timing array consisting of 8 LaBr3(Ce) detectors was measured using a standard 152Eu γ-ray source for the energy region of 40–1408keV. The data were acquired using a “multiplexed-start and multiplexed-stop” analogue electronics circuitry and analysed by employing the generalized centroid difference method. Concerning the cylindrical 1.5in.×1.5in. LaBr3(Ce) crystals which are coupled to the Hamamatsu R9779 photomultiplier tubes, the best fast-timing array time resolution of 202(3)ps is obtained for the two prompt γ lines of 60Co by using the leading-edge timing principle. When using the zero-crossover timing principle the time resolution is degraded by up to 30%, dependent on the energy and the shaping delay time of the constant fraction discriminator model Ortec 935. The smallest γ–γ time walk to below 50ps is obtained by using a shaping delay time of about 17ns and an optimum “time-walk adjustment” needed for detector output pulses with amplitudes smaller than 400mV.

Enhanced time response of 1-in. LaBr3(Ce) crystals by leading edge and constant fraction techniques

We have characterized in depth the time response of three detectors equipped with cylindrical LaBr3(Ce) crystals with dimensions of 1-in. in height and 1-in. in diameter, and having nominal Ce doping concentration of 5%, 8% and 10%. Measurements were performed at 60Co and 22Na γ-ray energies against a fast BaF2 reference detector. The time resolution was optimized by the choice of the photomultiplier bias voltage and the fine tuning of the parameters of the constant fraction discriminator, namely the zero-crossing and the external delay. We report here on the optimal time resolution of the three crystals. It is observed that timing properties are influenced by the amount of Ce doping and the crystal homogeneity. For the crystal with 8% of Ce doping the use of the ORTEC 935 CFD at very shorts delays in addition to the Hamamatsu R9779 PMT has made it possible to improve the LaBr3(Ce) time resolution from the best literature value at 60Co photon energies to below 100ps.

Quadratic Programming Time Pickoff Method for Multivoltage Threshold Digitizer in PET

Multi-voltage threshold (MVT) digitization, which has been implemented in our preclinical scanner–Trans-PET BioCaliBurn, is a low-power sampling solution with a reasonable cost for fast scintillation pulses. This MVT digitizing scheme employs a few comparators with programmable reference voltages for determining the time points when the scintillation pulse crosses the user-defined voltage thresholds. After obtaining the digital time samples, use of various sophisticated linear or nonlinear algorithms to improve the accuracy of timing information becomes possible. In the previous implementation of MVT digitizers, the arrival time of a scintillation pulse was determined by the linear fitting (LF) algorithm, which assumes leading edge as a straight line, and not involves the transition time on the falling edge. It is not unreasonable to expect achieving a better timing resolution in MVT-based PET detectors by the combinatorial optimization of MVT samples both on leading and falling edges. In this paper, a novel method, referred to as quadratic programming (QP) method is therefore proposed to mark the time stamps of sampling event pulses. In this method, the arrival time is directly expressed as a parametric combination of the MVT samples. The parameters in the combination are obtained by the quadratic programming which minimizes variation of timing error. To evaluate the performance of QP/MVT method and other time pickoff methods, we setup two gamma ray detectors using $LaBr_{3}:Ce$ crystal and Hamamatsu R9800 PMT. The scintillation pulses are directly read out by Tektronics DPO 71604 digital storage oscilloscope. CTR of 175 ps was obtained by QP/MVT, and 191 ps by LF/MVT, when four thresholds were employed in each of the two channels. The experimental results indicate the potential advantage of QP/MVT in timing determination. Meanwhile, the assumption of linear leading edge, which is the basis of LF method, was demonstrated to be improper in the data analysis. For QP method, probably of even greater significance is the manner to define the parameters rather than the resulting detector-specified parameters.

Performance of the Tachyon Time-of-Flight PET Camera.

We have constructed and characterized a time-of-flight Positron Emission Tomography (TOF PET) camera called the Tachyon. The Tachyon is a single-ring Lutetium Oxyorthosilicate (LSO) based camera designed to obtain significantly better timing resolution than the ~ 550 ps found in present commercial TOF cameras, in order to quantify the benefit of improved TOF resolution for clinically relevant tasks. The Tachyon's detector module is optimized for timing by coupling the 6.15 × 25 mm2 side of 6.15 × 6.15 × 25 mm3 LSO scintillator crystals onto a 1-inch diameter Hamamatsu R-9800 PMT with a super-bialkali photocathode. We characterized the camera according to the NEMA NU 2-2012 standard, measuring the energy resolution, timing resolution, spatial resolution, noise equivalent count rates and sensitivity. The Tachyon achieved a coincidence timing resolution of 314 ps +/- ps FWHM over all crystal-crystal combinations. Experiments were performed with the NEMA body phantom to assess the imaging performance improvement over non-TOF PET. The results show that at a matched contrast, incorporating 314 ps TOF reduces the standard deviation of the contrast by a factor of about 2.3.

Fundamental study of inorganic–organic hybrid scintillator using Pr:Lu3Al5O12 and plastic scintillator

Read out test of inorganic–organic hybrid scintillator; Pr:LuAG single crystal covered with plastic scintillator BC-499 is demonstrated. Emission peaks of the hybrid scintillator was observed around 430 nm, which suit to the sensitive wavelength of photomultiplier tube (PMT). The Pr:LuAG sample coated with BC-499 showed the better light output than the that of Pr:LuAG itself. Light output was increased up to 30% sing PMT (Hamamatsu R9800). Decay curve of the hybrid scintillator was also measured and successfully modeled. Corporation.

A TOF-PET Detector based on Quadrant-Sharing PMTs and Optimized Leading-edge Timing Method

A time-of-flight positron emission tomography (TOF-PET) detector was developed based on a 6×9 LYSO array and four single channel photomultipliers (Hamamatsu R9800). Leading-edge timing circuit with optimized parameter was used instead of the constant fraction discriminator. The results showed that all 54 elements in the flood histogram could be identified clearly. The average coincidence resolving time was 402 ps FWHM.

Timing Performance Study of New Fast PMTs With LYSO for Time-of-Flight PET

Most efforts at developing time-of-flight (TOF) positron emission tomography (PET) instruments have focused on improving their time resolution, which is affected by many factors. The aim of this study was to evaluate the performance of two new fast photomultiplier tubes (PMT) (Hamamatsu R9800 and R11194) for use as a TOF-PET detector, and to investigate the correlation of time resolution with time discrimination methods, the properties of the PMTs, the locations of the scintillation crystal on the PMT, and the size of the scintillation crystal. The PMTs evaluated in this study have fast rise time and short time jitter. The performance evaluation of the PMTs was conducted using a LYSO scintillation crystal. A <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sup> Na source was placed between the testing detector and a reference detector (with a time resolution of 200 ps), and data were acquired by using the coincidence trigger of the two detectors. To determine the optimal time discrimination method, time resolutions were obtained using a leading-edge discriminator (LED) with various thresholds and a constant-fraction discriminator (CFD) with various CFD delays. The effects on time resolution of PMT properties, the crystal position on the PMT, and the size of the crystal were observed. Based on the comparison of time resolutions obtained by various time discrimination setups, the optimal setup was determined to be CFD with 1.0 ns CFD delay. When these PMTs were coupled with 4 mm × 4 mm ×10 mm LYSO crystal and CFD with 1.0 ns delay was used for time discrimination, the average time resolution (FWHM) for 7 R9800 was 200.5 ps and for 7 R11194 was 227.8 ps. The average energy resolutions were 11.1% and 11.8%, respectively. Time resolutions at the edge of the PMT were degraded due to light collection loss and worse transit time jitter. Time resolutions were inversely proportional to the square root of the numbers of photoelectrons per pulse. The numbers of photoelectrons increased proportionately with the blue sensitivity of individual PMTs. Time resolutions have a distinct dependence as a function of the length and cross-section of the crystal. The new fast PMT coupled with LYSO crystals allow for the improvement of timing performance in TOF-PET detectors. The results of this study will be of value as a guideline for optimizing the TOF-PET detector design using the fast PMTs.

Fast timing study of a CeBr3 crystal: Time resolution below 120 ps at 60Co energies

We report on the time response of a novel inorganic scintillator, CeBr3. The measurements were performed using a cylindrical crystal of 1-in. in height and 1-in. in diameter at 22Na and 60Co photon energies. The time response was measured against a fast reference BaF2 detector. Hamamatsu R9779 and Photonis XP20D0 fast photomultipliers (PMTs) were used. The PMT bias voltages and Constant Fraction Discriminator settings were optimized with respect to the timing resolution. The Full Width at Half Maximum (FWHM) time resolution for an individual CeBr3 crystal coupled to Hamamatsu PMT is found here to be as low as 119ps at 60Co energies, which is comparable to the resolution of 107ps reported for LaBr3(Ce). For 511keV photons the measured FWHM time resolution for CeBr3 coupled to the Hamamatsu PMT is 164ps.

A Dual-Layer TOF-DOI Detector &lt;newline/&gt;Block for Whole-Body PET

We developed a dual-layer high-resolution detector block for whole-body positron-emission tomography systems. The top layer is a 13 × 13 array of lutetium yttrium orthosilicate (LYSO) crystals (2.85 × 2.85 × 10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) and the bottom layer a 13 × 13 array of lutetium gadolinium oxyorthosilicate (LGSO) crystals (2.85 2.85 10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ). The LGSO layer was coupled to four Hamamatsu R11194HA photomultiplier tubes (PMTs) by a photomultiplier quadrant-sharing configuration. This block has both time-of-flight and depth-of-interaction functions. Using a Na-22 point source, the performance of this block was evaluated by coinciding with a reference detector which consisted of a 4 × 4 × 10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> LYSO crystal coupled to a Hamamatsu R9779 PMT. A total of 4.7% of the counts were mixed among the two layers. The LYSO layer had 11.4% energy resolution and 329 ps coincidence time resolution against the reference detector. The LGSO layer had 14.8% energy resolution and 356 ps coincidence time resolution against the reference detector.

Evaluation of a fast photomultiplier tube for time-of-flight PET

Purpose Adding time-of-flight (TOF) information to positron emission tomography (PET) can lead to an improvement in image quality because TOF information allows the estimation of gamma ray annihilation positions in three dimensions. For accurate position estimation, precise time resolution is the most important characteristic required of TOF-PET detectors. The aims of this study were to establish the optimal setup for timing measurements and to evaluate the timing performance of a fast photomultiplier tube (PMT) (Hamamatsu R9800) coupled with a lutetium yttrium orthosilicate (LYSO) scintillation crystal for TOF-PET detector development.

A compact positron annihilation lifetime spectrometer

Using LYSO scintillator coupled on HAMAMATSU R9800 (a fast photomultiplier) to form the small size γ-ray detectors, a compact lifetime spectrometer has been built for the positron annihilation experiments. The system time resolution FWHM=193 ps and the coincidence counting rate ∼8 cps/μCi were achieved. A lifetime value of 219±1 ps of positron annihilation in well annealed Si was tested, which is in agreement with the typical values published in the previous lectures.

Investigation of a Multi-Anode Microchannel Plate PMT for Time-of-Flight PET

We report on an investigation of a mulit-anode microchannel plate PMT for time-of-flight PET detector modules. The primary advantages of an MCP lie in its excellent timing properties (fast rise time and low transit time spread), compact size, and reasonably large active area, thus making it a good candidate for TOF applications. In addition, the anode can be segmented into an array of collection electrodes with fine pitch to attain good position sensitivity. In this paper, we investigate using the Photonis Planacon MCP-PMT with a pore size of 10 µm to construct a PET detector module, specifically for time-of-flight applications. We measure the single electron response by exciting the Planacon with pulsed laser diode. We also measure the performance of the Planacon as a PET detector by coupling a 4 mm × 4 mm × 10 mm LSO crystal to individual pixel to study its gain uniformity, energy resolution, and timing resolution. The rise time of the Planacon is 440 ps with pulse duration of about 1 ns. A transit time spread of 120 ps FWHM is achieved. The gain is fairly uniform across the central region of the Planacon, but drops off by as much as a factor of 2.5 around the edges. The energy resolution is fairly uniform across the Planacon with an average value of 18.6±0.7% FWHM. While the average timing resolution of 252±7 ps FWHM is achieved in the central region of the Planacon, it degrades to 280±9 ps FWHM for edge pixels and 316±15 ps FWHM for corner pixels. We compare the results with measurements performed with a fast timing conventional PMT (Hamamatsu R-9800). We find that the R9800, which has significantly higher PDE, has a better timing resolution than the Planacon. Furthermore, we perform detector simulations to calculate the improvement that can be achieved with a higher PDE Planacon. The calculation shows that the Planacon can achieve significantly better timing resolution if it can attain the same PDE as the R-9800, while only a 30% improvement is needed to yield a similar timing resolution as the R-9800.

A Time Resolution Study of a Continuous Crystal Detector for TOF PET

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The majority of current Positron Emission Tomography (PET) systems are based on block-detectors consisting of many scintillating pixels read by a smaller number of photomultipliers (PMTs). An improvement of the time resolution, using a common light readout from a cluster of PMTs, was proposed by Kuhn <etal/>, and tested by us earlier with LSO crystals. This, triggered an idea to design a new PET detector optimized for Time of Flight (TOF) systems, based on continuous crystals. In the present work, we report on optimization of timing with a 20<formula formulatype="inline"><tex Notation="TeX">$\,\times\,$</tex></formula>20<formula formulatype="inline"><tex Notation="TeX">$\,\times\,$</tex></formula>20 mm<formula formulatype="inline"><tex Notation="TeX">$^{3}$</tex></formula> LYSO crystal coupled to a 16-channel photomultiplier H8711-200MOD from Hamamatsu. First, measurements were performed of the transit time jitter, the number of photoelectrons and the time resolution using a small 10<formula formulatype="inline"><tex Notation="TeX">$\,\times\,$</tex></formula>10<formula formulatype="inline"> <tex Notation="TeX">$\,\times\,$</tex></formula>5 mm<formula formulatype="inline"> <tex Notation="TeX">$^{3}$</tex></formula> LSO crystal coupled to a H8711-200MOD PMT. Results were compared with data collected from fast timing photomultipliers like Photonis XP1020, XP3060, XP20D0 or Hamamatsu R9800. In the second part of the study, the time resolution measurements and the optimization of the system were made with a continuous LYSO crystal. The final results are discussed in terms of the measured photoelectron number and the requirements for the TOF-PET scanners. </para>

A compact gamma camera with scintillation array and parallel-hole collimator

A new compact gamma camera for small object imaging has been developed. It consists of a pixelized Nal(T1) scintillator array coupled to a position sensitive photomultiplier tube (Hamamatsu R2486) with a parallel-hole lead collimator. The compact camera has better spatial resolution than Anger camera. The average value of intrinsic spatial resolutions is 2.3 mm (FWHM). The overall spatial resolution (FWHM) is 3, 5 and 6 mm at 0, 2.5 and 3 mm SCD (source-to-collimator distance), respectively. The phantom studies with the compact camera have demonstrated that parallel-hole collimator gamma camera is a practical technique for nuclear medicine application.

A multi-threshold sampling method for TOF-PET signal processing

As an approach to realizing all-digital data acquisition for positron emission tomography (PET), we have previously proposed and studied a multi-threshold sampling method to generate samples of a PET event waveform with respect to a few user-defined amplitudes. In this sampling scheme, one can extract both the energy and timing information for an event. In this paper, we report our prototype implementation of this sampling method and the performance results obtained with this prototype. The prototype consists of two multi-threshold discriminator boards and a time-to-digital converter (TDC) board. Each of the multi-threshold discriminator boards takes one input and provides up to eight threshold levels, which can be defined by users, for sampling the input signal. The TDC board employs the CERN HPTDC chip that determines the digitized times of the leading and falling edges of the discriminator output pulses. We connect our prototype electronics to the outputs of two Hamamatsu R9800 photomultiplier tubes (PMTs) that are individually coupled to a 6.25 × 6.25 × 25 mm 3 LSO crystal. By analyzing waveform samples generated by using four thresholds, we obtain a coincidence timing resolution of about 340 ps and an ∼ 18 % energy resolution at 511 keV. We are also able to estimate the decay-time constant from the resulting samples and obtain a mean value of 44 ns with an ∼ 9 ns FWHM. In comparison, using digitized waveforms obtained at a 20 GSps sampling rate for the same LSO/PMT modules we obtain ∼ 300 ps coincidence timing resolution, ∼ 14 % energy resolution at 511 keV, and ∼ 5 ns FWHM for the estimated decay-time constant. Details of the results on the timing and energy resolutions by using the multi-threshold method indicate that it is a promising approach for implementing digital PET data acquisition.

Dynode-Timing Method for PET Block Detectors

The focus of this paper is the investigation of a new dynode-timing technique optimized for PET block detectors. This method allows utilization of dynode signals from single but especially multiple photo-multiplier tubes (PMTs), operated with negative high-voltage. The technique will provide an event-timing trigger without deteriorating the anode signal. A printed circuit board has been developed and built for this investigation. Benchmark measurements have been performed, comparing timing of the anode signal with timing of the inverted last-dynode signal and timing of the dynode signal extracted via a newly developed LVPECL-logic based board. Timing measurements were performed with plastic as well as LSO scintillators. From single PMT measurements we find a 30 ps improvement with the dynode-timing method compared to the standard anode timing with two Photonis XP2020Q PMTs with LSO (10 mmtimes10 mmtimes10 mm). For a quad-PMT block detector, assembled of four Hamamatsu R9800 with a Hi-Rez block, the timing-resolution improves ~10%, by 43 ps compared to the standard anode timing.