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The main aim of this work is to provide a method to retrieve the intrinsic resolution of detector blocks based on monolithic crystals in a fully assembled scanner. This method suggests a software collimation to the original data. The results are compared with the traditional approach of separating two detector blocks far enough, resulting in geometrical collimation.An empirical equation has been deduced to fit the experimental data in which the detector intrinsic resolution follows a Gaussian distribution and the contribution of the source, given the small size of 0.25 mm in diameter, follows a Lorentzian profile. The experiments resulted in an average detector intrinsic spatial resolution of 0.6 mm FWHM, with a standard deviation error of 0.1 mm. These tests show a method to determine the intrinsic resolution of monolithic-based detector blocks, once assembled in the PET system, with high accuracy.

The monolithic scintillator block approach for gamma detection in the Positron Emission Tomography (PET) avoids estimating Depth of Interaction (DOI), reduces dead zones in detector and diminishes costs of detector production. Neural Networks (NN) are very efficient to determine the entrance point of a gamma incident on a scintillator block. This paper presents results on the robustness of the spatial resolution as a function of the random fraction in the data, temperature and HV fluctuations. This is important when implementing the method in a real scanner. Measurements were done with two Hamamatsu S8550 APD arrays, glued on a 20×20×10 mm3 monolithic LSO crystal block.

Vsing a nano-focus x-ray source we have been able to visualize structures down to 150 nm in x-ray projection images. Because of their unlimited depth of focus, these sources don't restrict the specimen size in case of 3D tomographic imaging. Simulation studies have shown that optimization of the detector response curve and switching from a reflective x-ray target to a transmission target might allow us to reach sub-100 nm resolutions.

We are currently developing a prototype of monolithic scintillator PET detector module based on neural network position estimators. The detector module comprises of a 25.5mm×25.5mm×10mm LYSO crystal coupled to Hamamatsu 64 channels multi-anode PMT H7546B. Comparing with classical pixelated detectors, the prominent drawbacks of neural network based detector module are lower signal to noise ratio and more complicated signal readout scheme and data processing. The former needs low noise multi-channel readout front-end electronics with high dynamic range and high resolution signal digitization; meanwhile the latter can resort to modern FPGA technology. This paper describes our design and implementation of electronics for single block crystal PET detector modules which we are building. We choose to digitize 16 readout signals compressed from the 64 PMT channels after the simulating optimization of several possible signal readout geometries. Benefiting from the capacity and the flexibility of resource allocation of modern general class of FPGA, we implement all necessary digital nuclear signal processing techniques into a FPGA. Specially, the neural network position calculation is realized also in the FPGA. The validation and performance tests show that the system functions well with 15.3M events on-line processing throughput. The modular detector prototypes based on this electronics system can be a good platform for future physical experiments.

The optimal pixel size of SiPMT arrays to read out monolithic 20x20x10 mm LSO scintillator block detectors has been studied. The influence of different SiPMT parameters on the achievable spatial resolution was compared. It was concluded that a SiPMT array with 3*3 mm pixel size, pitch 3.4 mm pixel, 30% detection efficiency, 100 ìm *100 ìm micro-cell size and crosstalk smaller than 20% is a good and realistic design that achieves better spatial and energy resolution than our current monolithic scintillator block readout based on Hamamatsu S8550 APD arrays.

We are presently developing a novel PET scanner for human brain functional imaging based on monolithic scintillator crystals read by APD matrices, capable of being inserted into an MRI system. In this work we report on the detailed study that has been made of the design of the detector blocks, aiming at defining the most suitable geometrical and readout configuration for optimizing the overall performance of the entire scanner. Both parallel and trapezoidal geometries have been simulated, featuring two layers of active scintillator material with different or similar thickness and APD readout on the front or back side. Results of this study indicate that a trapezoidal geometry with equal thickness of both layers is the best solution for improving the expected scanner performance.

In this work we present experimental results obtained with a PET prototype module based on monolithic block detectors of LYSO:Ce coupled to 2 Hamamatsu S8550-02 APD arrays and a dedicated ASIC front end readout. This module has been designed to be compatible with strong magnnetic fields and will be used on a research prototype for PET/MRI human brain imaging. The front-end electronics presented is based on a new ASIC: the VATA240. This ASIC implements the sum of the signals provided by the 8 pixels of each column and row of the detector array, composed of two APD modules. Using these 16 summed channels instead of the output of the 64 single pixels. the signal to noise ratio of the overall detector response is enhanced. To estimate the impinging photon direction we use Neural Networks (NN). The use of the summed channels also reduces the complexity of the NN algorithms implemented. The characterization of the VATA240 ASIC and the adjustment of key parameters have been carried out, and both the spatial and energy resolution with an electronically-collimated 22Na radioactive source have been experimentally evaluated.

The CRYSTAL CLEAR collaboration, in short CCC, is a consortium of 12 academic institutions, mainly from Europe, joining efforts in the area of developing instrumentation for nuclear medicine and medical imaging. In the framework of the CCC a high performance small animal PET system, called ClearPET, was developed by using new technologies in electronics and crystals in a phoswich arrangement combining two types of lutetium- based scintillator materials: LSO:Ce and LuYAP:Ce. Our next aim will be the development of hybrid image systems. Hybrid MR-PET imaging has many unique advantages for brain research. This has sparked a new research line within CCC for the development of novel MR-PET compatible technologies. MRI is not as sensitive as PET but PET has poorer spatial resolution than MRI. Two major advantages of PET are sensitivity and its ability to acquire metabolic information. To assess these innovations, the development of a 9.4T hybrid animal MR-PET scanner is proposed based on an existing 9.4T MR scanner that will be adapted to enable simultaneous acquisition of MR and PET data using cutting- edge technology for both MR and PET.

We propose and evaluate an improved design at the level of PET detector blocks based on monolithic crystals that will eventually be used on a research prototype for human brain PET/MRI imaging - the BrainPET scanner. These new detector blocks, when compared with pixilated designs, feature simpler mechanics, lower cost, larger sensitive volume, better energy and spatial resolutions, all of which contribute to improvements in PET detector technology. Moreover, the magnetic compatibility of all the materials composing the block makes it suitable for operation inside an MRI scanner. Results from both experimental data and Monte Carlo simulations allow an evaluation of the performance of the detector blocks, illustrating their potential for high-sensitivity PET imaging of the human brain.

Recent years have seen an increased interest in high resolution, high sensitivity detectors for small animal positron emission tomography (PET). Detectors based on large (e.g. several cm/sup 3/ of LSO) monolithic scintillators optically coupled to one or more avalanche photodiode (APD) array(s) minimize loss of sensitivity due to dead space. In such detectors, the primary event position and depth of interaction (DOI) correction are derived from the measured distribution of scintillation light over the APD array(s). The performance of detectors consisting of a monolithic LYSO scintillator read out by two Hamamatsu S8550 APD arrays is investigated. Monte Carlo simulations indicate that the scanner sensitivity and uniformity are improved when trapezoidal crystals are used instead of rectangular ones. The performance of both types of detectors was studied. Experiments show that the influence of the angle of incidence on the spatial resolution of both detector types is small, ranging from /spl sim/2.0 mm FWHM at 0/spl deg/ to /spl sim/2.4 mm FWHM at 30/spl deg/. An energy resolution of 11.5% at 511 keV and a time resolution of 2.4 ns was measured. Monte Carlo simulations predict a sensitivity of /spl sim/21% for a point source at the center of the field of view for a four-ring scanner based on trapezoidal monolithic detectors.

In this work results of ongoing research are presented for the LYSO/LuYAP phoswich solution adapted in the ClearPETtrade scanner with depth-of-interaction capability. The use of artificial neural network has been found as a very efficient phoswich crystal layer identification method that works for scintillators used at present, as well as on prospective materials. The LYSO/LuAP and LYSO/GSO phoswiches have been identified as a promising candidates for future PET scanners

The ClearPET/spl trade/ Neuro is the first full ring scanner within the Crystal Clear Collaboration (CCC). It consists of 80 detector modules allocated to 20 cassettes. LSO and LuYAP:Ce crystals in phoswich configuration in combination with position sensitive photomultiplier tubes are used to achieve high sensitivity and realize the acquisition of the depth of interaction (DOI) information. The complete system has been tested concerning the mechanical and electronical stability and interplay. Moreover, suitable corrections have been implemented into the reconstruction procedure to ensure high image quality. We present first results which show the successful operation of the ClearPET/spl trade/ Neuro for artefact free and high resolution small animal imaging. Based on these results during the past few months the ClearPET/spl trade/ Neuro System has been modified in order to optimize the performance.

Minimizing dead space is one way to increase the detection efficiency of small-animal PET scanners. By using monolithic scintillator crystals (e.g., 20 mm/spl times/10 mm/spl times/10 mm LSO), loss of efficiency due to inter-crystal reflective material is minimized. Readout of such crystals can be performed by means of one or more avalanche photo-diode (APD) arrays optically coupled to the crystal. The entry point of a gamma photon on the crystal surface can be estimated from the measured distribution of the scintillation light over the APD array(s). By estimating the entry point, correction for the depth-of-interaction (DOI) is automatically provided. We are studying the feasibility of such detector modules. To this end, a 64-channel test setup has been developed. Experiments to determine the effect on the spatial resolution of crystal surface finish and detector geometry have been carried out. The first results of these experiments are presented and compared to simulation results. The crystal surface finish has only a small influence on the spatial resolution. The spatial resolution of 20 mm/spl times/10 mm/spl times/10 mm detectors is significantly better when read out on the front side than when read out on the back side. With a 20 mm/spl times/10 mm/spl times/20 mm crystal coupled to two APD arrays, a very small resolution degradation of only /spl sim/0.2 mm is observed for an incidence angle of 30/spl deg/ compared to normal incidence.

Monolithic scintillator detectors, consisting of several cm3 of scintillating material coupled to one or more Hamamatsu S8550 avalanche photodiode (APD) arrays, have been proposed as detectors for high resolution positron emission tomography (PET). These detectors eliminate the dead space caused by reflective material between crystal pixels, and provide good depth of interaction (DOI) information. The entry point of impinging annihilation photons on the front surface of the detector is derived from the scintillation light distribution on the APD array(s). In this work, the various contributions to the energy and timing resolutions were analysed, as well as the influence of several contributions to the spatial resolution. Energy resolutions of around 11.5% FWHM at 511 keV were measured, the major part of which was found to be due to the intrinsic photon variance of the scintillator. A timing resolution of 1.6 ns FWHM was measured against a BaF2-PMT detector. The timing resolution was found to be strongly gain dependent. Spatial resolutions of around 1.75 mm FWHM were measured, fairly independent of the APD gain. This figure still needs to be corrected for the ~0.9 mm FWHM measurement beam.

We are investigating the possibility of using monolithic scintillator blocks as detectors for small animal positron emission tomography (PET). These detectors consist of several cm/sup 3/ of scintillating material, read out by one or more avalanche photo-diode (APD) arrays. The entry point of an incoming gamma photon is estimated from the distribution of the scintillation light over the APD pixels. To optimize the detector design, the influence of different design parameters is investigated using Geant4 simulations. To make it possible to study the influence of individual design parameters on the intrinsic spatial resolution of the detector, the use of a performance measure is proposed that is independent of the algorithm used to estimate the entry point, namely the Cramer-Rao lower bound on the estimation of the coordinates of a point source of light inside the crystal. To illustrate the use of this method, the influence of optical transport inside the detector is investigated for different detector designs, surface finishes and APD pixel sizes. A comparison with resolutions obtained from simulations involving beams of 511 keV annihilation photons indicates that this approach gives valid results.

The paper describes the LSO/LuYAP phoswich detector head developed for the ClearPET small animal PET scanner demonstrator that is under construction within the Crystal Clear Collaboration. The detector head consists of a dual layer of 8/spl times/8 LSO and LuYAP crystal arrays coupled to a multi-channel photomultiplier tube (Hamamatsu R7600-M64). Equalisation of the LSO/LuYAP light collection is obtained through partial attenuation of the LSO scintillation light using a thin aluminium deposit of 20-35 nm on LSO and appropriate temperature regulation of the phoswich head between 30 to 60 /spl deg/C. At 511 keV, typical FWHM energy resolutions of the pixels of a phoswich head amounts to (28/spl plusmn/2)% for LSO and (25/spl plusmn/2)% for LuYAP. The LSO versus LuYAP crystal identification efficiency is better than 98%. Six detector modules have been mounted on a rotating gantry. Axial and tangential spatial resolutions were measured up to 4 cm from the scanner axis and compared to Monte Carlo simulations using GATE. FWHM spatial resolution ranges from 1.3 mm on axis to 2.6 mm at 4 cm from the axis.

A 2nd generation high performance small animal PET scanner, called ClearPET/spl trade/, has been designed and a first prototype is built by working groups of the Crystal Clear Collaboration (CCC). In order to achieve high sensitivity and maintain good uniform spatial resolution over the field of view in high resolution PET systems, it is necessary to extract the depth of interaction (DOI) information and correct for spatial degradation. The design of the first ClearPET/spl trade/ Demonstrator based on the use of the multi-anode photomultiplier tube (Hamamatsu R7600-M64) and a LSO/LuYAP phoswich matrix. The two crystal layers of 8*8 crystals (2*2*10 mm/sup 3/) are stacked on each other and mounted without light guide as one to one on the PMT. A unit of four PMTs arranged in-line represents one of 20 sectors of the ring design. The opening diameter of the crystal ring is 137 mm, the axial detector length is 110 mm. The PMT pulses are digitized by free-running ADCs and digital data processing determines the gamma energy, the phoswich layer and even the pulse arrival time. Single gamma interactions are recorded and coincidences are found by software. The gantry allows rotation of the detector modules around the field of view. The measurements have been done using the first LSO/LuYAP detector cassettes.

A 3D small animal PET scanner, designed and built at the University of Brussels, has been operational since the beginning of 1996. The scanner has a transaxial field of view (FOV) of 110 mm and an axial FOV of 51 mm. The absolute sensitivity is around 35.000 coincidences/s/MBq for a point source at the center of the scanner and the time resolution is 29 ns FWHM. To measure the achievable spatial resolution, a thin /sup 22/Na source was scanned at various distances from the scanner axis. The resolution in a reconstructed image for a source close to the center is 3.0 mm FWHM. Finally, to assess the overall image quality, scans were made of a cylindrical phantom with holes of varying diameter.

We have obtained optically clear samples of LuAP:Ce from the Institute of Physical Research, Ashtarak, Armenia, measuring up to 5/spl times/5/spl times/40 mm/sup 3/. The photo electron yield for a number of these samples was measured with a Philips XP2020Q photomultiplier tube, and 556 photo electrons per MeV was obtained for the best samples. This is comparable to what is obtained from BGO crystals under the same condition, but a factor of 5 less than what is obtained with an LSO sample of similar dimensions. The light in LuAP:Ce increases with cerium concentration between 0.13 and 0.47%.

The authors have designed and built a small high resolution PET scanner using BaF/sub 2/ scintillation crystals and photosensitive wire chambers using TMAE as photosensitive agent. The very first measurements performed on the detector are presented. >

The LuAlO/sub 3/:Ce/sup 3+/ system was shown to be a promising scintillator for medical imaging devices. Recently, efforts were focused on the improvement of its scintillating properties. Several large size crystals with various cerium concentrations were grown. Emission spectra, fluorescence decay times and light yields, both under /spl gamma/-ray and X-ray excitation, were measured under various experimental conditions. A reabsorption process is shown to take place in this material. This process is responsible for the observed decrease of the light yield when increasing the size of the sample.

The possibility of extracting depth-of-interaction (DOI) has been studied for block detector configurations, consisting of either a block of LSO or a saw-shaped crystal coupled to a Hamamatsu Si-avalanche photodiode (APD) array 1370. The results of this study, as well as previous results on DOI, obtained with individual crystal configurations consisting of two layers of lutetium oxyorthosilicate (LSO) crystals shifted over half a crystal width with respect to each other, are applied in corrections on the spatial resolution obtained with these Positron Emission Tomography (PET) detector modules. Further improvement in the composition of crystal matrices has been realized. The energy resolution and the light yield for crystals in such a crystal matrix have been evaluated.

Detailed results are given for the study of the spatial resolution obtained with different configurations of LSO scintillation crystals read out by a Hamamatsu 2/spl times/8 APD array. Block detectors as well as small sited 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 (DOI) 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.

A program of work has been undertaken using Monte Carlo simulations and experimental measurements with the RPET scanner to evaluate the role of a dedicated high resolution positron camera for breast imaging. The simulated detector consists of 20 mm long crystals arranged in a ring of diameter 220 mm by 52 mm long. The sides of the crystal faces were varied from 1 mm to 4.5 mm. The breast was simulated by a cylinder 69 mm in diameter by 65 mm long containing a lesion varying from 2 mm to 10 mm in diameter with an SUV in the range 2-8. The number of simulated counts varied from 1-10 million. Preliminary measurements with the RPET camera have also been made using the same size phantom/lesion combination. After these measurements are completed it is hoped to evaluate a hybrid photo diode detector for its usefulness as a dedicated PET breast scanner. The simulations show that a 4 mm diameter lesion with an SUV of 6 (typical of measured values) can be detected with 1 million acquired counts using 2.5 mm crystals. The detection of smaller lesions is severely limited by sensitivity. Using smaller crystals does not help detect smaller lesions but increasing acquired counts does. Experimental measurements show the importance of minimising the background from the heart.