Multichannel Photomultiplier Tube Research Articles

Remote Sensing of Seawater Temperature and Salinity Profiles by the Brillouin Lidar Based on a Fizeau Interferometer and Multichannel Photomultiplier Tube

Brillouin spectroscopy is a powerful tool to measure the water temperature and salinity profiles of seawater. Considering the insufficiency of the current spectral measurement methods in real-time, spectral integrity, continuity, and stability, we developed a new lidar system for spectrum measurement on an airborne platform that is based on a Fizeau interferometer and multichannel photomultiplier tube. In this approach, the lidar system uses time-of-flight information to measure the depth and relies on Brillouin spectroscopy as the temperature and salinity indicator. In this study, the system parameters were first optimized and analyzed. Based on the analysis results, the performance of the system in terms of detection depth and accuracy was evaluated. The results showed that this method has strong anti-interference ability, and under a temperature measurement accuracy of 0.5 °C and a salinity measurement accuracy of 1‱, the effective detection depth exceeds 40.51 m. Therefore, the proposed method performs well and will be a good choice for achieving Brillouin lidar application in seawater remote sensing.

Simulation analysis of a lidar performance for atmospheric temperature profile measurements based on the Fizeau interferometer and multichannel photomultiplier tube

AbstractIn this study, a lidar system based on the Fizeau interferometer and photomultiplier tube array is designed to analyze the entire Rayleigh‐Brillouin spectrum aiming at obtaining the atmospheric temperature profiles. The laser wavelength emitted by the lidar is 355 nm with the pulse energy of 20 mJ, and the diameter of the receiving telescope is 0.6 m. We analyze the influence of the cumulative average of different pulse numbers on the accuracy of temperature inversion. The temperature inversion results in different height ranges are calculated and the results show that if the integration time is 100 s, the temperature deviation is less than 3 K in the altitude range below 24 km.

Detector-specific issues in Satellite Laser Ranging to Swarm-A/B/C satellites

Satellite Laser Ranging (SLR) to Low Earth Orbiters (LEOs) is traditionally used to validate orbits derived by microwave techniques, e.g., Global Positioning System (GPS). For most LEOs, SLR is the only possibility to independently assess the GPS orbit quality, to calibrate onboard instruments, and to reveal otherwise unnoticed systematic errors. However, the detector-specific errors in SLR measurements may corrupt the SLR observations and derived parameters. We found that the stations employing Compensated Single-Photon Avalanche Diode (CSPAD) detectors are characterized by the lowest, 15 and 21 mm root-mean-square residuals for reduced-dynamic and kinematic orbits, respectively, and show almost no dependency on the satellite nadir angle nor the observation acquisition time, as opposed to stations with Multi-Channel Plates (MCP) and Photomultiplier Tube (PMT) detectors. SLR observations to Swarm-A/B/C can also be used for the determination of SLR station coordinates characterized by 27, 15, 18 mm interquartile range repeatability for the Up, North, and East components, respectively.

Measurement of the Parametrized Single-Photon Response Function of a Large Area Picosecond Photodetector for Time-of-Flight PET Applications

We have used a subpicosecond laser with an output trigger synched to a 4-GHz oscilloscope sampling at 25 GSa/s to characterize the temporally resolved response of a large area picosecond photodetector (LAPPD) to single photons/photoelectrons. The LAPPD is a large area (20 cm ×20 cm) photosensor with a nanosecond-width time response arising from its underlying multichannel plate photomultiplier tube (MCP-PMT) structure. We used optical filters to attenuate the laser pulse intensity to a single-photon level. LAPPD pulses are nearly identical in shape, particularly for pulses whose peak amplitude is a large multiple of our system's 0.6-mV rms baseline electronic noise. Three waveform timing algorithms were compared in their measurements of the transit time spread (TTS) Gaussian fitted to the distribution of measured single-photoelectron (SPE) pulse times under zero-width laser excitation: 1) a constant fraction algorithm; 2) a pulse centroid timing algorithm; and 3) a pulse model-fitting algorithm. The measured SPE values with the three algorithms are: [79±17, 67±15, and 82±20] ps Gaussian sigma, respectively. The LAPPD provides independent voltage control for its photocathode (PC), and at the entry and entry faces of its chevron pair of MCPs. We found no or minimal change in TTS with PC voltage for pulses >15 mV.

The Coherent Particle-Oscillation Excitation System at the VEPP-4M Collider

A system for excitation of coherent oscillations of a bunch of particles was created to study the beam dynamics at the VEPP-4M electron-positron collider; it consisted of a pulse generator, a beam deflection device (kicker), and a control and synchronization system. The system has two independent channels for the excitation of horizontal and vertical oscillations and makes it possible to excite oscillations of a given bunch of particles. For observations of beam oscillations, a system for measuring the position of the beam in the reverse mode is used, as well as a fast multichannel photomultiplier tube (profilometer). The design and results of operation of high-voltage nanosecond shock generators with a current pulse duration of 200–250 ns at the base, a voltage up to 25 kV and a current up to 2 kA are described. Using constructive solutions, the durations of the fronts of the output pulses were reduced, which made it possible to avoid the excitation of neighboring bunches. Testing the generators for their working loads gave the following results: a horizontal impact pulse of 30 ns and a vertical impact of 50 ns, a horizontal impact pulse tip duration of ~20 ns, a maximum pulse amplitude of 25 kV, and a time instability (jitter) of ±1 ns. The results of measurements of the beam parameters obtained using this system are presented.

Experimental setup for the laboratory investigation of micrometeoroid ablation using a dust accelerator.

A facility has been developed to simulate the ablation of micrometeoroids in laboratory conditions. An electrostatic dust accelerator is used to generate iron particles with velocities of 10-70 km/s. The particles are then introduced into a chamber pressurized with a target gas, where the pressure is adjustable between 0.01 and 0.5 Torr, and the particle partially or completely ablates over a short distance. An array of biased electrodes above and below the ablation path is used to collect the generated ions/electrons with a spatial resolution of 2.6 cm along the ablating particles' path, thus allowing the study of the spatiotemporal evolution of the process. For completely ablated particles, the total collected charge directly yields the ionization coefficient of a given dust material-target gas combination. The first results of this facility measured the ionization coefficient of iron atoms with N2, air, CO2, and He target gases for impact velocities >20 km/s, and are reported by Thomas et al. [Geophys. Res. Lett. 43, 3645 (2016)]. The ablation chamber is also equipped with four optical ports that allow for the detection of the light emitted by the ablating particle. A multichannel photomultiplier tube system is used to observe the ablation process with a spatial and temporal resolution of 0.64 cm and 90 ns. The preliminary results indicate that it is possible to calculate the velocity of the ablating particle from the optical observations, and in conjunction with the spatially resolved charge measurements allow for experimental validation of ablation models in future studies.

Research and analysis on lidar performance with intrinsic fluorescence biological aerosol measurements

Biological aerosols which could cause diseases of human beings, animals and plants, are living particles suspended in the atmosphere. Ultraviolet laser induced fluorescence has been developed as a standard technique used to discriminate between biological and non-biological particles. As an effective tool of remote sensing, fluorescence lidar is capable of detecting concentration of biological aerosols with high spatial and temporal resolutions. Intrinsic fluorescence, one of the most important characteristics of biological aerosols, has quite a large effect on the performances of fluorescence lidar. To investigate the effects of intrinsic fluorescence on biological aerosols, we design an ultraviolet laser induced fluorescence lidar at an excited wavelength of 266 nm, with a repetition rate of 10 Hz. Fluorescence signals are collected by a Cassegrain telescope with a diameter of 254 mm, in which fluorescence spectra of 300-800 nm are mainly considered. A spectrograph and a multichannel photomultiplier tube (PMT) array detector are employed to achieve the fine separation and highefficiency detection of fluorescence signals. According to the present configuration, we perform a series of simulations to estimate the measurement range and the concentration resolution of biological aerosols, with a certain pulse energy. With a relative error less than 10%, theoretical analysis shows that designed fluorescence lidar is able to detect the biological aerosols within a range of 1.5 km at a concentration of 1000 particles·L-1. When the detection distance enlarges to 2.1 km, detectable wavelength range is limited to 300-310 nm. In addition, the lidar is capable of identifying minimum concentrations of biological aerosols with 2 particles·L-1 and 4 particles·L-1 at fluorescence wavelengths of 350 nm and 600 nm, respectively, where the induced pulse energy is set to be 60 mJ and detected range 0.1 km. With setting energies of 40 mJ and 20 mJ, minimum concentrations of biological aerosols decrease to 3 particles·L-1 and 6 particles·L-1, respectively, at a fluorescence wavelength of 350 nm. The relative error of minimum concentration resolution is about 2 particles·L-1, increasing rapidly with range. For a fluorescence wavelength of 600 nm, both the minimum concentration and the relative error show relatively high values, 5 particles·L-1 at 40 mJ and 10 particles·L-1 at 20 mJ, where the relative errors are found to be 2 particles·L-1 and 4 particles·L-1, respectively. The results prove that a shorter intrinsic fluorescence wavelength has a better effect on biological aerosol measurement. We believe that a proper intrinsic fluorescence wavelength will further improve the detection accuracy of biological aerosols.

Field experiments of multi-channel oceanographic fluorescence lidar for oil spill and chlorophyll-a detection

A Multi-channel Oceanographic Fluorescence Lidar (MOFL), with a UV excitation at 355 nm and multiple receiving channels at typical wavelengths of fluorescence from oil spills and chlorophyll-a (Chl-a), has been developed using the Laser-induced Fluorescence (LIF) technique. The sketch of the MOFL system equipped with a compact multi-channel photomultiplier tube (MPMT) is introduced in the paper. The methods of differentiating the oil fluorescence from the background water fluorescence and evaluating the Chl-a concentration are described. Two field experiments were carried out to investigate the field performance of the system, i.e., an experiment in coastal areas for oil pollution detection and an experiment over the Yellow Sea for Chl-a monitoring. In the coastal experiment, several oil samples and other fluorescence substances were used to analyze the fluorescence spectral characteristics for oil identification, and to estimate the thickness of oil films at the water surface. The experiment shows that both the spectral shape of fluorescence induced from surface water and the intensity ratio of two channels (I-495/I-405) are essential to determine oil-spill occurrence. In the airborne experiment, MOFL was applied to measure relative Chl-a concentrations in the upper layer of the ocean. A comparison of relative Chl-a concentration measurements by MOFL and the Moderate Resolution Imaging Spectroradiometer (MODIS) indicates that the two datasets are in good agreement. The results show that the MOFL system is capable of monitoring oil spills and Chl-a in the upper layer of ocean water.

Detector Performance Characterization for High Sensitivity Single-Photon Imaging

We are developing detectors for an un-collimated scanner for very high sensitivity single-photon imaging. The scanner consists of two large, thin, closely spaced, pixelated scintillation detectors of either thallium-doped sodium iodide (NaI(Tl)), sodium-doped cesium iodide (CsI(Na)), or bismuth germanate (BGO). The scintillator arrays are interchangeable to optimize performance for a wide range of isotope energies and imaging subjects. For each scintillator, flood histograms were obtained utilizing test gamma sources: <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">109</sup> Cd, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">241</sup> Am, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">57</sup> Co, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">133</sup> Ba, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sup> Na, and beta source <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">32</sup> Si. Using a 3 mm thick 21 ×41 NaI(Tl) pixelated array ( 2 mm ×2 mm pixels on a 2.2 mm pitch), read out by two adjacent Hamamatasu H8500 multi-channel photomultiplier tubes (MCPMTs), the energy resolution for <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">57</sup> Co was 12.2% over the center of the MCPMTs and 45.3% for pixels over the gap, with flood histogram peak-to-valley ratios > 6.1 for the center of the MCPMTs and > 4.2 for the gap between the two MCPMTs. The NaI(Tl) detector consistently produced peak-to-valley ratios of > 1.6 in the center of the MCPMTs for all radioisotopes studied. CsI(Na) and BGO detectors had flood histograms of similar quality to NaI(Tl) at intermediate to high-energies, allowing for optimal imaging across a wide energy range. The measured sensitivity for point sources placed 10 mm from the detector approached 35% for low-energy isotopes and all scintillator materials, which results in an expected system sensitivity (two opposing detectors) of ~ 70% for these isotopes. The 5 mm thick BGO arrays had the greatest efficiency for high-energy isotopes due to the increased stopping power of this material. The measured spatial resolution for <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">99m</sup> Tc for the CsI(Na) detector (1.6 mm pitch) was 7.5 mm FWHM at the aluminum entrance window (1.5 mm from scintillator face), decreasing to 15 mm at a distance of 5 mm from the scintillator face. The NaI(Tl) detector showed similar spatial resolution. This spatial resolution is sufficient for imaging well-localized radiotracer distributions in thin objects, and the very high sensitivity can be used for fast dynamic imaging, or imaging using very low injected doses.

MEMS scanner based handheld fluorescence hyperspectral imaging system

We demonstrate a micromirror based hand-held hyperspectral fluorescence imaging microsystem for skin cancer detection, where multiple narrow-band spectral images can be captured simultaneously across the area under examination. In addition to the advantages of combined functional imaging and spectroscopy, the system demonstrates the fast scanning over large field-of-view (FOV) provided by a CMOS compatible 2-axis microelectromechanical system (MEMS) scanning mirror in the probe. Spectral information from 15 wavelengths was acquired simultaneously using a multichannel photomultiplier tube (PMT), rendering a spectral resolution of 20nm. FOV up to 0.8cm×0.5cm with 50μm lateral resolution was acquired, with a hyperspectral imaging rendering rate of 2s per frame.

Novel positioning method using Gaussian mixture model for a monolithic scintillator-based detector in positron emission tomography

We developed a high precision position decoding method for a positron emission tomography (PET) detector that consists of a thick slab scintillator coupled with a multichannel photomultiplier tube (PMT). The DETECT2000 simulation package was used to validate light response characteristics for a 48.8 mm×48.8 mm×10 mm slab of lutetium oxy- orthosilicate coupled to a 64 channel PMT. The data are then combined to produce light collection histograms. We employed a Gaussian mixture model (GMM) to parameterize the composite light response with multiple Gaussian mixtures. In the training step, light photons acquired by N PMT channels was used as an N-dimensional feature vector and were fed into a GMM training model to generate optimal parameters for M mixtures. In the positioning step, we decoded the spatial locations of incident photons by evaluating a sample feature vector with respect to the trained mixture parameters. The average spatial resolutions after positioning with four mixtures were 1.1 mm full width at half maximum (FWHM) at the corner and 1.0 mm FWHM at the center section. This indicates that the proposed algorithm achieved high performance in both spatial resolution and posi- tioning bias, especially at the corner section of the detector. C � 2011 Society

A Multi-Slice Dual Layer MR-Compatible Animal PET System

A multi-slice depth-of-interaction (DOI) MR-compatible PET system for imaging small animals such as mice or rats was developed and tested It employs DOI block detectors made of mixed lutetium silicates (MLSs), optical fibers, and multi-channel position sensitive photo-multiplier tubes (PSPMTs). The size of a single MLS was 2.5 mm times 3.5 mm times 3.5 mm and eight MLSs were combined to form a DOI block detector using the Anger principle and connected to four optical fibers with light guides. Thirty-two sets of block detectors were arranged in a 51 mm diameter circular ring. The scintillation light signals from the DOI detectors are read from the side of the detector ring. The configuration makes the diameter of the detector ring small and easy to use in the small imaging area of the MRI. Position histograms showed good separation for all block detectors. The timing resolution was 6 ns FWHM. The measured transaxial spatial resolution was 2.8 mm FWHM at the center of the image and the axial resolution was 3 mm FWHM. Sensitivity was around 0.23% at the center of the axial field of view (FOV). Simultaneous imaging of PET and MRI was also tested using a 0.15 T permanent magnet system with a 30-mm FOV. The detector part of the PET system was set inside the gradient coil of the MRI system and the RF coil was positioned inside the FOV of the PET system. No difference was found in the PET images among outside MRI, inside MRI without gradient and RF, and inside MRI with the gradient and the RF sequences. Also there is no noticeable change of the MRI images with and without the detector ring of the PET system. We also successfully obtained simultaneously measured rat brain with the PET and MRI. These results indicate that the developed MR-compatible PET system can be used for simultaneous measurements with an MRI system.

Investigation of Depth of Interaction Encoding for a Pixelated LSO Array With a Single Multi-Channel PMT

A new approach to depth of interaction (DOI) encoding for a pixelated LSO array using a single multi-channel PMT was investigated. In this method the DOI information was estimated by taking advantage of optical crosstalk between LSO elements and examining the standard deviation (spread) of signals on all channels of the PMT. Unpolished and polished 6×6 LSO arrays with a crystal size of 1.3×1.3×20 mm(3) were evaluated on a Hamamatsu H7546 64-channel PMT. The arrays were placed on the center of the PMT and the central 16 channels of the PMT were individually read out and digitized. For the unpolished array, all crystals were resolved in the flood histogram. An average DOI resolution of 8 mm was obtained. The energy resolution was ∼25% after the signal amplitude was corrected using the measured DOI information. For the polished array, the flood histogram was superior to the unpolished array, however no DOI information could be measured. Using unpolished crystals, this method could be a practical way to achieve limited DOI information in PET detectors. The standard deviation of all PMT channels can be readily obtained using a resistor network. Only five signals (four signals to determine the x-y position and one signal measuring the standard deviation) need to be digitized, and this method only requires a single photon detector to read out the array. Unlike phoswich detectors, the method does not require segmenting the scintillator array into layers. The measured DOI resolution was much worse than that obtained with the dual-ended readout method, however, it was similar to that obtained with a two-layer phoswich detector.

Physical Performance of an Intraoperative Beta Probe Dedicated to Glioma Radioguided Surgery

The precise delineation and excision of brain tumor extent allows to improve survival outcome and quality of life of surgically treated patients. In order to refine the resection of gliomas, we are developing a novel intraoperative probe specifically dedicated to the localization of residual tumor after the bulk has been excised. The probe, built around clear and plastic scintillating fibers, was designed to detect positrons emitted from radiolabeled brain tissue in order to discriminate more specifically neoplastic from normal tissues. The probe was also built to be directly coupled to the excision tool leading to simultaneous detection and removal of tumor. We report here performances of the first radio-isotopic configuration of the intraoperative probe which consists of a detection head composed of eight detection elements held around the excision tool in a closed packed annular arrangement. This head is coupled to an optic fiber bundle that exports the scintillating light to a multi-channel photomultiplier tube. The gamma ray background generated by the annihilation of beta+ in tissues is eliminated by a real-time subtraction method. The detector exhibits a beta sensitivity of 139 cps/kBq and a gamma ray rejection efficiency of 99.5%. The ability of the probe to detect residual lesions was evaluated with a realistic brain phantom representing the surgical cavity and the boundaries of the tumor. We showed that lesions as small as 5 mm in diameter can be detected for tumor to normal tissue uptake ratios of fluorinated tracers greater than 3.5. This ratio is achieved with radiopharmaceuticals like 18F-FET or 18F-choline. These promising results suggest that the features of our system are compatible with in situ localization of residual radiolabeled tumors.

Development of a positron-imaging detector with background rejection capability

Intra-operative probes have recently become important instruments in nuclear medicine. In such an application, the radiopharmaceutical F-18-fluorodeoxyglucose (FDG) is promising. For the FDG-guided surgery, we developed and tested a positron-imaging detector with background rejection capability. The detector consists of an array of phoswich scintillators, a multi-channel position-sensitive photo-multiplier tube (PSPMT) and an electronic circuit. The scintillators and the PSPMT are encased in a tungsten shield and replaceable collimators are mounted on the top of the detector. Positrons are detected by the plastic scintillators while annihilation photons are detected by the BGOs. By employing a pulse-shape analysis, we can distinguish the true events (positrons) from background gamma events. The dimensions of each plastic scintillator are 2 mm x 2 mm x 3 mm and those of the BGO are 2 mm x 2 mm x 15 mm. These scintillators are optically coupled to each other and combined in an 8 x 8 array, which is optically coupled to a 1-inch square 8 x 8 multi-channel PSPMT via optical fibers. Position determination of the positrons is performed by 64-channel threshold circuits while the pulse shape analysis is applied for the summing signal. The spatial resolution was measured by positioning an F-18 point source onto one pixel of the detector and found than the spillover to the neighbor pixel was less than 20%. The background count rate was less than 2 cps for a 20-cm diameter, 20-cm long cylinder phantom containing 3.7 MBq of F-18. These results indicated that the developed positron-imaging detector will be useful for FDG-guided surgery.

Development of a GSO positron/single-photon imaging detector

We have developed and tested a GSO (gadolinium oxyorthosilicate) position-sensitive gamma detector which can be used with positron and single-photon radionuclides for imaging breast cancer or sentinel lymph node detection. Because GSO has a relatively good energy resolution for annihilation gammas as well as low energy gamma photons, and does not contain any natural radioisotopes, it can be used for positron imaging and lower energy single-photon imaging. The imaging detector consists of a GSO block, 2 inch square multi-channel position-sensitive photo-multiplier tube (PSPMT), and associated electronics. The size of a single GSO element was 2.9 mm × 2.9 mm × 20 mm and these elements were arranged into 15 × 15 matrixes to form a block that was optically coupled to the PSPMT. It was possible to separate all GSO crystals into a two-dimensional position histogram for annihilation gammas (511 keV) and low energy gamma photons (122 keV). The typical energy resolution was 24% FWHM and 37% FWHM for 511 keV and 122 keV gamma photons, respectively. For the positron imaging, coincidence between the imaging detector and a single gamma probe is measured. For the single-photon imaging, a tungsten collimator is mounted in front of the imaging detector. With this configuration, it was possible to image both positron radionuclides and low energy single-photon radionuclides. We measured spatial resolution and sensitivity as well as image quality of the developed imaging detector. Results indicated that the developed imaging detector has the potential to be a new and useful instrument for nuclear medicine.

A GSO positron/single-photon imaging detector

We have developed and tested a GSO position sensitive gamma detector which can be used with positron and single-photon radionuclides for imaging breast cancer or sentinel lymph node detection. Because GSO has a relatively good energy resolution for annihilation gammas as well as low energy gamma photons, and does not contain any natural radioisotopes it can be used for positron imaging and lower energy single-photon imaging. The imaging detector consists of a GSO block, 2-inch square multichannel position sensitive photo-multiplier tube (PSPMT), and associated electronics. For the positron imaging, coincidence between the imaging detector and a single gamma probe is measured. For the single-photon imaging, a tungsten collimator is mounted in front of the imaging detector. With this configuration, it was possible to image both positron radionuclides and low energyu single-photon radionuclides.

Design and development of an MR-compatible PET scanner for imaging small animals

A single-slice magnetic resonance (MR) compatible positron emission tomography (PET) system with a transaxial field of view /spl sim/6 cm has been designed and is currently under construction. It will consist of four concentric rings of lutetium oxyorthosilicate (LSO) scintillation crystals, each coupled to one of eight multi-channel photomultiplier tubes (PMTs) via 3.5 m optical fibers. The annulus containing the crystals has an outside diameter of 12 cm and is placed within the MR field of view whilst the PMTs are located outside the main magnetic field. A highly reproducible method that optimizes the amount of scintillation light that reaches the PMTs has been established. Two small sections of the scanner, each containing 4 by 4 crystal arrays, demonstrated good flood position histograms with all sixteen channels clearly identifiable. The light loss through a fiber of length of 3.25 m was approximately 70%. The spatial resolution of the two arrays in coincidence was measured at 1.6 mm full-width half-maximum (FWHM), and the temporal resolution of one array in coincidence with a single LSO crystal was measured to be 10.9 ns. The design incorporates a novel technique for improving sampling at the center of the field of view within the scanner. The concentric rings are offset with respect to one another by one quarter of the crystal width between layers resulting in significantly improved sampling. The results indicate that the PET scanner will have a performance comparable with that of current small animal systems and it will be used to investigate the utility and potential applications of combining PET and MR in small animals.

Development of a Small-Animal PET System

Developments in small-animal positron emission tomography (PET) technology aim at reaching spatial resolution in the submillimeter range and increasing the system sensitivity. Various detector schemes have been implemented in complete tomographic systems for in vivo animal imaging. These include small scintillation crystals read out by multichannel-photomultiplier tubes with optical fibers to guide the light, phoswich detectors, and direct coupling of semiconductor detectors to small scintillation crystals. One design uses avalanche photodiodes (APDs) for the detection of scintillation light. A prototype sector scanner was the first to combine fast lutetium oxyorthosilicate (LSO) crystals with APDs. Based on the results from this tomograph, a new system is being built with smaller detector elements for improved spatial resolution and sensitivity. In this overview paper, the concepts, technical realization, and expected performance of this system are described.

Readout of the optical PET (OPET) detector

The design of an imaging system capable of detecting both high-energy γ-rays and optical wavelength photons is underway at the Crump Institute for Molecular Imaging. This system will noninvasively image small animal models in vivo for the presence of positron emission tomographic (PET) and optical signals. The detector will consist of modules of multichannel photomultiplier tubes (MC-PMT) coupled to arrays of scintillator crystals. The MC-PMT will detect both the photons produced due to bioluminescence and the photons generated by the interaction of γ-rays within the crystals. The long wavelength photons produced through bioluminescence are only slightly attenuated by these crystals and are detected directly at the photocathode of the MC-PMT, resulting in signals of small (5-10 mV) short (~15 ns) pulses. In contrast, annihilation (511 keV) γ-rays interacting in the scintillator crystal send large bursts of photons to the PMT, and result in pulses that can be as large as 500 mV and > 200 ns duration. The processing of pulses with such different characteristics in a single circuit requires significant alteration of the standard pulse processing circuitry used in PET scanners. In this paper, we discuss the requirements of such a circuit and show the results of implementation of one design using single and multiple channel PMTs.