PET Detector Module Research Articles

A sub-millimeter resolution PET detector module using a multi-pixel photon counter array

A PET block detector module using an array of sub-millimeter lutetium oxyorthosilicate (LSO) crystals read out by an array of surface-mount, semiconductor photosensors has been developed. The detector consists of a LSO array, a custom acrylic light guide, a 3 × 3 multi-pixel photon counter (MPPC) array (S10362-11-050P, Hamamatsu Photonics, Japan) and a readout board with a charge division resistor network. The LSO array consists of 100 crystals, each measuring 0.8 × 0.8 × 3 mm3 and arranged in 0.86 mm pitches. A Monte Carlo simulation was used to aid the design and fabrication of a custom light guide to control distribution of scintillation light over the surface of the MPPC array. The output signals of the nine MPPC are multiplexed by a charge division resistor network to generate four position-encoded analog outputs. Flood image, energy resolution and timing resolution measurements were performed using standard NIM electronics. The linearity of the detector response was investigated using gamma-ray sources of different energies. The 10 × 10 array of 0.8 mm LSO crystals was clearly resolved in the flood image. The average energy resolution and standard deviation were 20.0% full-width at half-maximum (FWHM) and ±5.0%, respectively, at 511 keV. The timing resolution of a single MPPC coupled to a LSO crystal was found to be 857 ps FWHM, and the value for the central region of detector module was 1182 ps FWHM when ±10% energy window was applied. The nonlinear response of a single MPPC when used to read out a single LSO was observed among the corner crystals of the proposed detector module. However, the central region of the detector module exhibits significantly less nonlinearity (6.5% for 511 keV). These results demonstrate that (1) a charge-sharing resistor network can effectively multiplex MPPC signals and reduce the number of output signals without significantly degrading the performance of a PET detector and (2) a custom light guide to permit light sharing among multiple MPPC and to diffuse and direct scintillation light can reduce the nonlinearity of the detector response within the limited dynamic range of a typical MPPC. As a result, the proposed PET detector module has the potential to be refined for use in high-resolution PET insert applications.

Development of a position decoder circuit for PET consisting of GAPD arrays

Use of channel reduction techniques that reduce the number of signals transmitted to a data acquisition (DAQ) module can lead to more efficient use of the DAQ module for PET with numerous readout channels. The purpose of this study was to develop a position decoder circuit (PDC) with a capacity to output the digital address and analog pulse of one interacted channel from numerous PET outputs. A PDC capable of reducing the number of readout channels by a factor of 32 was designed and fabricated. PET detector modules consisting of an LYSO scintillator, a 4×4 array Geiger-mode avalanche photodiode (GAPD), and a 16-channel preamplifier were also constructed to evaluate the performance of the PDC developed for this study. The output signal from the PET detector module was transmitted to the DAQ module after a 45ns delay by the PDC. Using the gain correction circuit implemented in the PDC, gain uniformity for all channels of the PET detector module was improved by 80%. Energy resolution in the PET detector was 20.4% with the PDC and 20.8% without the PDC. Timing resolution was 2.2ns with the PDC and 1.5ns without the PDC. A hot-rod phantom image was successfully acquired using proof-of-principle PET with the PDC developed for this study. Experimental results indicate that the PDC developed for this study is not only useful for the reduction of the readout channel number from the PET detector module consisting of GAPD arrays, but also for PET signal processing and PET imaging.

Statistical LOR estimation for a high-resolution dMiCE PET detector

We develop a statistical line of response (LOR) estimator of the three-dimensional interaction positions of a pair of annihilation photons in a PET detector module with depth of interaction capability. The three-dimensional points of interaction of a coincidence pair of photons within the detector module are estimated by calculation of an expectation of the points of interaction conditioned on the signals measured by the photosensors. This conditional expectation is computed from estimates of the probability density function of the light collection process and a model of the kinetics of photon interactions in the detector module. Our algorithm is capable of handling coincidences where each annihilation photon interacts any number of times within the detector module before being completely absorbed or escaping. In the case of multiple interactions, our algorithm estimates the position of the first interaction for each of the coincidence photons. This LOR estimation algorithm is developed for a high-resolution PET detector capable of providing depth-of-interaction information. Depth of interaction is measured by tailoring the light shared between two adjacent detector elements. These light-sharing crystal pairs are referred to as dMiCE and are being developed in our lab. Each detector element in the prototype system has a 2 × 2 mm2 cross section and is directly coupled to a micro-pixel avalanche photodiode (MAPD). In this set-up, the distribution of the ratio of light shared between two adjacent detector elements can be expressed as a function of the depth of interaction. Monte Carlo experiments are performed using our LOR estimation algorithm and compared with Anger logic. We show that our LOR estimation algorithm provides a significant improvement over Anger logic under a variety of parameters.

DOI resolution measurement and error analysis with LYSO and APDs

Spatial resolution degradation in PET occurs at the edge of Field Of View (FOV) due to parallax error. To improve spatial resolution at the edge of FOV, Depth-Of-Interaction (DOI) PET has been investigated and several methods for DOI positioning were proposed. In this paper, a DOI–PET detector module using two 8×4 array avalanche photodiodes (APDs) (Hamamatsu, S8550) and a 2 cm long LYSO scintillation crystal was proposed and its DOI characteristics were investigated experimentally. In order to measure DOI positions, signals from two APDs were compared. Energy resolution was obtained from the sum of two APDs’ signals and DOI positioning error was calculated. Finally, an optimum DOI step size in a 2 cm long LYSO were suggested to help to design a DOI–PET.

Comparison of four depth-encoding PET detector modules with wavelength shifting (WLS) and optical fiber read-out

We propose detectors for a laboratory positron emission tomography scanner specific for mouse imaging that utilizes fewer detectors and channels of electronics compared with existing designs. The detectors are based on lutetium oxyorthosilicate arrays, read out by orthogonal optical fibers placed on the top and bottom of the arrays. Depth of interaction (DOI) information is obtained from the ratio of the signals at either end of the array. Four different detector modules were evaluated, using different reflector materials and two types of optical fibers (wavelength shifting (WLS) fibers and clear optical fibers). The modules were compared in terms of flood histograms, energy resolution, DOI resolution and timing resolution. Energy resolution for single crystals at one irradiation depth was around 65% full-width half-maximum (FWHM). A DOI resolution of ∼6 mm was obtained for the modules. Timing resolution was in the range of 5.1–7.8 ns. An array assembled in the laboratory and coupled with WLS fibers had the best DOI resolution; the same array with clear fibers had the best timing resolution and a commercially manufactured array and coupled with WLS fibers had the best energy resolution.

Simultaneous MRI and PET imaging of a rat brain

Multi-modality imaging is rapidly becoming a valuable tool in the diagnosis of disease and in the development of new drugs. Functional images produced with PET fused with anatomical structure images created by MRI will allow the correlation of form with function. Our group is developing a system to acquire MRI and PET images contemporaneously. The prototype device consists of two opposed detector heads, operating in coincidence mode. Each MRI–PET detector module consists of an array of LSO detector elements coupled through a long fibre optic light guide to a single Hamamatsu flat panel position-sensitive photomultiplier tube (PSPMT). The use of light guides allows the PSPMTs to be positioned outside the bore of a 3T MRI scanner where the magnetic field is relatively small. To test the device, simultaneous MRI and PET images of the brain of a male Sprague Dawley rat injected with FDG were successfully obtained. The images revealed no noticeable artefacts in either image set. Future work includes the construction of a full ring PET scanner, improved light guides and construction of a specialized MRI coil to permit higher quality MRI imaging.

Initial Characterization of a Nonpixelated Scintillator Detector in a PET Prototype Demonstrator

Previous experimental results have demonstrated that a PET detector module based on a 20times10times10 mm LSO block read out by a Hamamatsu S8550 APD array is able to determine the impinging position of perpendicularly incident 511-keV photons with a resolution better than 2-mm FWHM. A prototype PET demonstrator using two of these detector heads was build and evaluated. The axial and transaxial resolution measured in two-dimensional (2-D) OSEM and 2-D FBP reconstructed images is better than 1.5- and 2-mm FWHM, respectively, for radial distances less than 15 mm. A Monte Carlo simulation showed that a four-ring PET scanner based on these monolithic scintillator blocks has a sensitivity which is about 2.5 times higher than the sensitivity of a similar system equipped with detector heads using a 4times8 matrix of 2times2times10 mm LSO pixels

Initial tests of a prototype MRI-compatible PET imager

Multi-modality imaging is rapidly becoming a valuable tool in the diagnosis of disease and in the development of new drugs. Functional images produced with PET fused with anatomical structure images created by MRI, will allow the correlation of form with function. Our group (a collaboration of West Virginia University and Jefferson Lab) is developing a system to acquire MRI and PET images contemporaneously. The prototype device consists of two opposed detector heads, operating in coincidence mode with an active FOV of 5×5×4 cm 3. Each MRI–PET detector module consists of an array of LSO detector elements (2.5×2.5×15 mm 3) coupled through a long fiber optic light guide to a single Hamamatsu flat panel PSPMT. The fiber optic light guide is made of a glued assembly of 2 mm diameter acrylic fibers with a total length of 2.5 m. The use of a light guides allows the PSPMTs to be positioned outside the bore of the 3 T General Electric MRI scanner used in the tests. Photon attenuation in the light guides resulted in an energy resolution of ∼60% FWHM, interaction of the magnetic field with PSPMT further reduced energy resolution to ∼85% FWHM. Despite this effect, excellent multi-plane PET and MRI images of a simple disk phantom were acquired simultaneously. Future work includes improved light guides, optimized magnetic shielding for the PSPMTs, construction of specialized coils to permit high-resolution MRI imaging, and use of the system to perform simultaneous PET and MRI or MR-spectroscopy .

Corrected position estimation in PET detector modules with multi-anode PMTs using neural networks

This paper studies the use of Neural Networks (NNs) for estimating the position of impinging photons in gamma ray detector modules for PET cameras based on continuous scintillators and Multi-Anode Photomultiplier Tubes (MA-PMTs). The detector under study is composed of a 49/spl times/49/spl times/10 mm/sup 3/ continuous slab of LSO coupled to a flat panel H8500 MA-PMT. Four digitized signals from a charge division circuit, which collects currents from the 8/spl times/8 anode matrix of the photomultiplier, are used as inputs to the NN, thus reducing drastically the number of electronic channels required. We have simulated the computation of the position for 511 keV gamma photons impacting perpendicularly to the detector surface. Thus, we have performed a thorough analysis of the NN architecture and training procedures in order to achieve the best results in terms of spatial resolution and bias correction. Results obtained using GEANT4 simulation toolkit show a resolution of 1.3 mm/1.9 mm FWHM at the center/edge of the detector and less than 1 mm of systematic error in the position near the edges of the scintillator. The results confirm that NNs can partially model and correct the non-uniform detector response using only the position-weighted signals from a simple 2D DPC circuit. Linearity degradation for oblique incidence is also investigated. Finally, the NN can be implemented in hardware for parallel real time corrected Line-of-Response (LOR) estimation. Results on resources occupancy and throughput in FPGA are presented.

A high-resolution PET detector based on continuous scintillators

We have compared the performance of PET detector modules using continuous scintillator blocks with modules based on a large number of small crystal pixels. We show that a good spatial resolution can be obtained in undivided scintillators. One of the main advantages of the undivided scintillators is the excellent depth of interaction identification capability. There is almost no degradation of the spatial resolution upto incidence angles of 40° away from normal incidence.

Study of spatial resolution and depth of interaction of APD-based PET detector modules using light sharing schemes

Using 4 /spl times/ 8 Hamamatsu S8550 APD matrices with 1.6 mm /spl times/ 1.6 mm pixels and a 2.3 mm pitch, we compared the spatial resolution of light sharing schemes using block detectors to detector modules using 1-to-1 coupling between individual 2 mm /spl times/ 2 mm /spl times/ 10 mm LSO crystals and APD pixels. In addition, depth of interaction (DOI) measurement capabilities of the block detector were studied. Using neural network methods to determine the interaction position in a 10 mm thick LSO block, we achieved an intrinsic spatial resolution of 1.5 mm FWHM compared to 2.06 mm for individual readout. The DOI resolution varied between 1.8 and 3.0 mm FWHM.

Spatial resolution and depth-of-interaction studies with a PET detector module composed of LSO and an APD array

Detailed results are given for the study of the spatial resolution obtained with different configurations of lutetium oxyorthosilicate (LSO) scintillation crystals read out by a Hamamatsu 2 /spl times/ 8 avalanche photodiode (APD) array. Block detectors as well as small-sized individual LSO crystals have been coupled to the APD matrix. Both one-to-one coupling and light-sharing schemes were investigated. The spatial resolution is derived for each configuration. Two different configurations with two layers of LSO crystals shifted by half a crystal size with respect to each other are used for investigations on the depth-of-interaction information. Prior to the spatial resolution measurements, a comparison of the light output for various crystal surface treatments and different optical coupling materials has been made.

An LSO scintillator array for a PET detector module with depth of interaction measurement

Presents construction methods and performance results for a production scintillator array of 64 optically isolated, 3 mm/spl times/3 mm/spl times/30 mm sized LSO crystals. This scintillator array has been developed for a PET detector module consisting of the 8/spl times/8 LSO array coupled on one end to a single photomultiplier tube (PMT) and on the opposite end to a 64 pixel array of silicon photodiodes (PD). The PMT provides an accurate timing pulse and initial energy discrimination, the PD identifies the crystal of interaction, the sum provides a total energy signal, and the PD/(PD+PMT) ratio determines the depth of interaction (DOI). Unlike the previous LSO array prototypes, the authors now glue Lumirror reflector material directly onto 4 sides of each crystal to obtain an easily manufactured, mechanically rugged array with the authors' desired depth dependence. With 511 keV excitation, the authors' obtain a total energy signal of 3600 electrons, pulse-height resolution of 25% FWHM, and 6-15 mm FWHM DOI resolution.

Design studies of a high resolution PET detector using APD arrays

The authors evaluated a compact, high resolution PET detector module using avalanche photodiode (APD) arrays to replace bulky position sensitive PMTs. The newly developed APD array is a planar processed 4/spl times/4 array which has a 2/spl times/2 mm/sup 2/ pixel size with 0.4 mm gaps between pixels, about 60% quantum efficiency at 420 nm wavelength, and uniform high gain (>1000) across all channels. A 4/spl times/4 array of 2/spl times/2/spl times/10 mm/sup 3/ LSO crystals was coupled to an APD array. Different readout electronics and signal multiplexing schemes were explored. All crystals in the detector array were clearly identified in the flood source histogram, with average peak-to-valley ratios of about 12:1 using a charge sharing resistor network. The energy resolution was measured to be /spl sim/14% at 511 keV in the detector array. The measured timing resolution was 2.6 ns in coincidence with a LSO/PMT detector. By optimizing the readout electronics currently being used, it is likely that detector performance can be further improved. The authors have also determined depth-of-interaction (DOI) by reading out two APD arrays connected to the ends of a 2/spl times/2/spl times/22 mm/sup 3/ LSO crystal. Preliminary measurements show good DOI measurement capability with DOI positioning uncertainty between 4 and 6.5 mm.

Depth identification accuracy of a three layer phoswich PET detector module

We describe a PET detector module that provides three levels of depth-of-interaction (DOI) information. The detector is a 9/spl times/9 array of 2 mm/spl times/2 mm/spl times/12 mm deep phoswich crystal elements, each consisting of 4 mm long LSO (entrance layer), GSO (middle layer) and BGO (exit layer) crystals joined optically together end-to-end. The BGO exit layer is directly coupled to a miniature position-sensitive photomultiplier tube (PSPMT). Delayed charge integration, a method that exploits differences in the light decay times of these scintillators, is used to determine the layer-of-interaction. DOI accuracy, measured by scanning a slit source of 511 keV radiation along the length of the module was 86% for the LSO layer, 80% for the GSO layer and 84% for the BGO layer. Energy resolution at 511 keV was 19% for LSO, 21% for GSO and 40% for BGO. Apparent gain differed between layers in the ratios 2.7:1.9:1.0 (LSO:GSO:BGO). Crystal separation was good between crystals in the LSO layer, acceptable between crystals in the GSO layer and poor between crystals in the BGO layer due, primarily, to the pronounced spatial nonlinearity of the PSPMT. The delayed charge integration method, however, does appear suitable for obtaining multi-level depth information when DOI effects are particularly significant, e.g. in very small ring diameter PET scanners for small animal imaging.

First results with high-resolution PET detector modules using wavelength-shifting fibers

The authors have constructed and tested prototype PET detector modules incorporating wavelength-shifting fiber ribbons alternating with thin CsI(Na) scintillator plates in multilayer stacks. Modules with cross-sectional area of 11 cm/spl times/11 cm and with 9 mm of total crystal thickness have been tested. A reconstructed spatial resolution of 3 mm FWHM has been obtained for a line source, with an energy resolution of <20% and a time resolution of 25 ns FWHM. Energy measurements and coincidence timing are provided by 4 photomultipliers in an Auger array on one 11 cm/spl times/11 cm module face. Multiplexed fiber readout has been implemented with multianode photomultipliers coupled to several fiber ribbons. The methods and resolution achieved are readily extended to larger systems in a straightforward and cost-effective manner. Results of tests with several phantoms are presented.

Calibration of a PET detector module that measures depth of interaction

We investigate two in situ calibration techniques (using gamma rays incident from the patient side) for PET detector modules that measure interaction depth via an analog ratio. For each crystal element in the camera, we need to determine two unknowns: the gain ratio K of the two detector (PMT and PD) signals and the depth dependence /spl alpha/ of the signals (defined as the ratio of the signal observed when the crystal is excited at the end closest to and furthest from the detector). The light collection is assumed to be linearly dependent on distance from the detector end, in agreement with experimental results. The first method extracts K and /spl alpha/ using (a) the ratio of the PMT and PD signals for interactions that occur in the detector end closest to the patient (the most probable depth) and (b) the shape of the PMT pulse height distribution which reflects the exponential attenuation length in the detector. The second method utilizes the fact that E=PD+PMT (the total energy estimator) is independent of /spl Gamma/=PD/(PD+PMT) (the depth estimator) when K is correct, with a distribution position that is /spl alpha/ dependent. On simulated data, both the gain ratio and depth dependence are determined with errors of 3% rms and 2% rms respectively, resulting in minimal degradation of energy and depth resolution.

Design of a depth of interaction (DOI) PET detector module

A method to determine depth of interaction (DOI) from a PET detector module is described and evaluated. The basic element of the DOI detector module is a two crystal detector unit. The hypothesis is that by controlling how light is shared between two crystals (A and B) DOI information can be extracted from the ratio of light collected using simple Anger type logic [A/(A+B)]. The interface between crystals is designed so that a significant amount of light is shared when a photon interacts near the front face of a crystal and very little light is shared when an interaction occurs near the back of a crystal. The effects of surface treatment (e.g., polished, roughened) and optical coupling compounds are investigated. BGO, GSO and LSO detector units have been evaluated. A DOI uncertainty of /spl sim/6 mm was attained for the front section (/spl sim/4 mm) of a 2/spl times/2/spl times/20 mm LSO detector unit. A method to decode a 64 crystal detector module (32 detector units) using a 16 channel multi-anode photomultiplier tube is described.

A high-speed and high-precision Winner-Select-Output (WSO) ASIC

The design and performance characteristics of a 16-channel Winner-Select-Output (WSO) ASIC are presented. The WSO ASIC does a fast comparison of 16 analog input voltages, outputs the maximum signal, the second maximum (partner) signal, and their addresses. The WSO ASIC chip is a key component of an analog electronics system being developed for a depth of interaction (DOI) PET detector module. The basic cell of the WSO ASIC is a winner take all (WTA) circuit. The precision of the WSO ASIC is enhanced by using a cascade structure WTA cell. The chip requires a single +5 V supply and consumes 35 mW of power. The WSO ASIC is sensitive to voltage differences as small as 10 mV. The propagation delay of the chip is less than 30 nsec for voltage differences of >50 mW (typical for proposed application).

Characterization of a 64 channel PET detector using photodiodes for crystal identification

The authors present performance results for a prototype PET detector module consisting of 64 LSO scintillator crystals (3/spl times/3/spl times/20 mm) coupled on one end to a single photomultiplier tube and on the opposite end to a 64 pixel array of 3 mm square silicon photodiodes (typical pixel parameters are 5 pF capacitance, 300 pA dark current, and 73% quantum efficiency at 415 nm). The photomultiplier tube (PMT) provides an accurate timing pulse and energy threshold for all crystals in the module, the silicon photodiodes (PD) identify the crystal of interaction, the sum (PD+PMT) provides a total energy signal, and the PD/(PD+PMT) ratio determines the depth of interaction. With 32 of the channels instrumented, the detector module correctly identifies the crystal of interaction (where correct includes the adjacent 4 crystals) 79/spl plusmn/4% of the time with high detection efficiency. The timing resolution for a single LSO detector module is 750 ps fwhm, while its pulse height resolution at 511 keV is 24/spl plusmn/3% fwhm. The depth of interaction measurement resolution is 8/spl plusmn/1 mm fwhm.