Showing papers in "IEEE Transactions on Nuclear Science in 1990"

Measuring PET scanner sensitivity: relating countrates to image signal-to-noise ratios using noise equivalents counts

Abstract: True coincidence count (TCC) and noise equivalent count (NEC) curves were measured with a standardized 20-cm-diameter nylon cylinder for five different CTI/Siemens PET (positron emission tomography) scanners with several scanner-collimator combinations: (1) 831/08-12 with 1-mm collimator septa; (2) 933/08-12 and 933/08-16 with 3 to 1-mm tapered collimator septa; and (3) 931/08-12 with 3 to 1-mm tapered and a 1-mm collimator septa and the 931/08-16 with 3 to 1-mm tapered collimator septa In addition, TCC and NEC curves on the 933/08-12 were compared with those from an Alderson brain phantom In general, it is found that the TCC curves indicated peak count rates and activity levels that were as much as 50% higher than the corresponding values from NEC curves The primary factor causing this difference is the noise effect of the randoms component It is demonstrated that, compared to the Alderson brain phantom, the standard 20-cm cylinder is a poor predictor of count rate performance for PET brain imaging >

3-D phantom to simulate cerebral blood flow and metabolic images for PET

Abstract: A three-dimensional brain phantom has been developed to simulate the activity distributions found in the human brain in the cerebral blood flow and metabolism studies employed in PET (positron emission tomography). The phantom has a single contiguous chamber and utilizes thin layers of lucite to provide apparent relative concentrations of 5, 1, and 0 for grey matter, white matter, and ventricles, respectively, in the brain. The phantom and an ideal image set were created from the same set of data. Thus, the user has a basis for comparing measured images with an ideal image set, which enables the user to make quantitative evaluation of the errors in PET studies with a data set similar to that obtained in patient studies. >

Modeling the anneal of radiation-induced trapped holes in a varying thermal environment

Abstract: The anneal of radiation-induced trapped holes in MOS transistors is found to be thermally activated. A quantitative, physical model based on thermal emission and tunneling is developed. It accurately predicts the anneal of radiation-induced trapped holes in constant or time-varying thermal environments. Data are presented which quantitatively verify the accuracy of the model for temperatures between 25 and 160 degrees C. This model provides the basis for developing accurate quantitative screens for the rebound failure mechanism. >

Scintillation characteristics of pure and Tl-doped CsI crystals

Abstract: The scintillation characteristics of pure and Tl-doped CsI were investigated. The scintillation light was detected with photomultipliers and silicon photodiodes. The influence of the Tl concentration on the light yield and on the decay times was studied upon excitation with gamma -rays and alpha -particles. The origin of the different emission components is discussed. >

Field dependence of interface-trap buildup in polysilicon and metal gate MOS devices

Abstract: The electric field dependence of radiation-induced oxide- and interface-trap charge ( Delta V/sub ot/ and Delta V/sub it/) generation for polysilicon- and metal-gate MOS transistors is investigated at electric fields (E/sub ox/) from -42 MV/cm to +47 MV/cm If electron-hole recombination effects are taken into account, the absolute value of Delta V/sub ot/ and the saturated value of Delta V/sub it/ for both polysilicon- and metal-gate transistors are shown to follow an approximate E/sup -1/2/ field dependence for E/sub ox/>or=04 MV/cm An E/sup -1/2/ dependence for the saturated value of Delta V/sub it/ was also observed for negative-bias irradiation followed by a constant positive-bias anneal The E/sup -1/2/ field dependence observed suggests that the total number of interface traps created in these devices may be determined by hole trapping near the Si/SiO/sub 2/ interface for positive-bias irradiation or near the gate/SiO/sub 2/ interface for negative bias irradiation, though H/sup +/ drift remains the likely rate-limiting step in the process Based on these results, a hole-trapping/hydrogen transport model-involving hole trapping and subsequent near-interfacial H/sup +/ release, transport, and reaction at the interface-is proposed as a possible explanation of Delta V/sub it/ buildup in these polysilicon- and metal-gate transistors >

Calculation of the rotational centers in computed tomography sinograms

Abstract: An efficient method for accurately calculating the center-of-rotation, or projection center, for parallel computed tomography projection data, or sinograms, is described. This method uses all the data in the sinogram to estimate the center by a least-squares technique and requires no previous calibration scan. The method also finds the object's center-of-mass without reconstructing its image. Since the method uses the measured data, it is sensitive to noise in the measurements, but that sensitivity is relatively small compared to other techniques. Examples of its use on simulated and actual data are included. For fan-beam data over 360 degrees , two related methods are described to find the center in the presence or absence of a midline offset. >

Physical characteristics of TTV03, a new high spatial resolution time-of-flight positron tomograph

Abstract: TTV03, a novel high-spatial-resolution TOF-PET (time-of-flight positron emission tomography) system, is presented. The machine consists of four 89-cm-diameter rings made of 7-mm-wide, 18-mm-high, 45-mm-deep BaF/sub 2/ crystals (324 per ring) individually coupled to 11-mm-diameter phototubes. The transaxial resolution (full width at half maximum or FWHM) in the reconstructed image is 5.7 mm at center, 5.7 mm (tangential), and 6.5 mm (radial) at 10 cm. The axial resolution is 8.8 mm (direct plane) and 6.7 mm (crossplane) at center, and 10.3 mm (direct) and 11.2 mm (cross) at 10 cm from center. With a 150-keV threshold, the TOF resolution is 750 ps (FWHM), and, with a 4.18-ns coincidence window, the sensitivity (true evts/s per mu Ci/cm/sup 3/) is 9800 (direct plane) and 14300 (crossplane). At 1 mu Ci/cm/sup 3/, the ratio of random events to true events is .20 for the total and .03 per image element, and the scatter fraction is 22%. Dead time losses are 8.3% at 2.5 mu Ci/cm/sup 3/ (four rings), but the tomograph can handle activities over 50 mu Ci/cm/sup 3/ in small regions with negligible losses. These characteristics make TTV03 particularly suitable for dynamic brain and heart H/sub 2//sup 15/ bolus studies. >

Evaluation of the Karolinska new positron camera system; the Scanditronix PC2048-15B

Abstract: The PC2048-15B is the brain version of the new generation of Scanditronix positron camera systems, now being installed at the Karolinska Hospital in Stockholm, Sweden. The system has eight rings with 256 BGO crystals per ring. The dimensions of the crystals are 6*12*30 mm. Data corresponding to 15 image slices are simultaneously recorded. The detector rings are based on block-detector units, each with 16 scintillation crystals mounted on two dual photomultiplier tubes. The system is described, and test results including spatial resolution, sensitivity to true and random coincidences, scatter correction, count rate linearity, spatial independence, and reproducibility are presented. Clinical results are discussed, including rCBF studies and receptor studies, to cover the two most extreme situations met in brain PET (positron emission tomography) studies, i.e. high count rate and high resolution requirements. >

Sensor validation for power plants using adaptive backpropagation neural network

Abstract: Signal validation and process monitoring problems in many cases require the prediction of one or more process variables in a system. The feasibility of using neural networks to characterize one variable as a function of other related variables is studied. The backpropagation network (BPN) is used to develop models of signals from both a commercial power plant and the Experimental Breeder Reactor-II (EBR-II). Several innovations are made in the algorithm, the most significant of which is the progressive adjustment of the sigmoidal threshold function and weight updating terms. >

Prospects for new inorganic scintillators

Abstract: It is shown that it is possible to use synchrotron X-radiation to measure fast fluorescent emission in compounds not readily available in single-crystal form. Of 85 compounds tested, about half were found to have fluorescence below the limit of the technique used, which was about 0.05% of Bi/sub 4/Ge/sub 3/O/sub 12/ (BGO). Others had emissions in the 0.1 to 1% range, and a few had emissions that were both fast and above 1% of BGO. Scintillation from CeF/sub 3/ and PbCO/sub 3/ was discovered. Since only a small fraction of known compounds have ever been tested for scintillation properties, it is concluded that many of these may be useful scintillators. >

Post-irradiation behavior of the interface state density and the trapped positive charge

Abstract: The postirradiation behavior of the energy distribution of interface states, D/sub it/, and of the trapped positive charge, N/sub ot/, of MOS devices is studied. The shift of interface state density from the broad peak at 0.7 eV to below midgap is found to be reversible. The direction of the shift is determined by the surface potential. The postirradiation introduction of hydrogen into the gate oxide increases the interface state density across the bandgap and decreases N/sub ot/ by a similar amount. Following the introduction of hydrogen, the reversibility of N/sub ot/ is increased by an order of magnitude. These effects show the connection between the experimental conditions and the type of postirradiation behavior that is observed. Models for the hydrogen-induced D/sub it/ buildup and for the reversibility of D/sub it/ and N/sub ot/ are discussed. >

Effect of radiation-induced charge on 1/f noise in MOS devices

Abstract: The 1/f noise in MOS transistors is measured as a function of gate and drain bias, total ionizing dose, and postirradiation biased annealing time. The transistors tested varied in size, radiation hardness, and process technology. The radiation-induced 1/f noise correlates strikingly with the oxide trap charge through irradiation and anneal, but not with interface-trap charge, for frequencies up to 10 kHz. This implies that oxide trapped charge is the predominant factor which leads to the increased 1/f noise in irradiated MOS devices. >

Total-dose radiation-induced degradation of thin film ferroelectric capacitors

Abstract: The radiation hardness of thin-film PbZr/sub y/Ti/sub 1-y/O/sub 3/ (PZT) ferroelectric capacitors is explored. Ferroelectric capacitors were irradiated using X-ray and Co-60 sources to dose levels up to 16 Mrad(Si). The capacitors were characterized for their memory properties both before and after irradiation. The radiation hardness was process dependent. Three out of four processes resulted in capacitors that showed less than 30% radiation-induced degradation in retained polarization charge and remnant polarization after irradiating to 16 Mrad(Si). On the other hand, one of the processes showed significant radiation-induced degradation in retained polarization charge and remanent polarization at dose levels above 1 Mrad(Si). A model for simulating the observed degradation is developed. The model indicates that the data are consistent with trapping of radiation-induced charge in the ferroelectric material. The radiation hardness levels indicate that ferroelectric devices can be fabricated that can survive radiation exposures well in excess of 10 Mrad(Si). >

Observation of H/sup +/ motion during interface trap formation

Abstract: The time dependence of changes in the oxide trapped charge during interface trap formation is investigated. Changes in MOSFET threshold voltage V/sub th/ and number of interface traps N/sub it/ are measured in the same sample as a function of time following pulsed irradiation. When the gate bias during irradiation V/sub gl/ is positive, the initial mod Delta V/sub th/ mod is large due to trapping of radiation-induced holes at the Si-SiO/sub 2/ interface and the postirradiation time dependence of Delta V/sub th/ is dominated by hole detrapping, as expected. When V/sub gl/ is negative, interfacial hole trapping is minimized. In this case, an unusual peak in the Delta V/sub th/ vs. time curve provides evidence of the involvement of H/sup +/ ions in the N/sub it/ formation process. >

A brief survey of radiation effects on polymer dielectrics

Abstract: Future space power needs are extrapolated to be at least three to four orders of magnitude more than is currently available. This long-term reliable power will be required on missions such as the Space Station, Pathfinder, Space Plane, and high-powered satellites, and for national defense. Electrical insulation and dielectrics are the key electrical materials needed to support these power systems, where a single-point system failure could prove catastrophic or even fatal for the whole mission. Therefore, the impact of radiation, an environmental stress, on the properties and performance of insulation and dielectrics must be understood. The influence of radiation on polymer dielectrics, the insulating materials most commonly used for power transmission and storage, is reviewed. The effects of the type of radiation, dose, rate, and total exposure on the key electrical, mechanical, and physical properties of polymer dielectrics are described and explained. >

Time-dependent hole and electron trapping effects in SIMOX buried oxides

Abstract: The back-channel threshold shift associated with the buried oxide layers of separation by implanted oxygen (SIMOX) and zone-melted recrystallization (ZMR) field-effect transistors (FETs) was measured following pulsed irradiation as a function of temperature and back-gate bias using a fast time-resolved I-V measurement technique. The SIMOX FETs showed large initial negative voltage shifts at 0.2 ms after irradiation followed by temperature- and bias-dependent additional negative shifts to 800 s. Analysis and modeling of the results indicated: (1) efficient deep trapping of radiation-generated holes in the bulk of the oxide, (2) substantial initial trapping of radiation-generated electrons in the oxide, and (3) rapid removal of the trapped electrons by a thermal detrapping process. The ZMR FETs showed evidence of substantial trapping of holes alone in the oxide bulk. >

Two parameter Bendel model calculations for predicting proton induced upset (ICs)

Abstract: The two-parameter model of W.L. Bendel and E.L. Petersen (1984) represents an improvement over the existing single-parameter model in terms of the goodness of fit to actual proton upset data. It especially gives a better fit to the data from devices with small feature dimensions, which ultimately leads to a more accurate proton error rate prediction. Small feature sized devices have a proton upset energy dependence that cannot be accurately described with the one-parameter model and only one data point. There are no substantial differences in proton error rate predictions from one- and two-parameter approaches for older devices with larger feature sizes. >

The effect of ionizing radiation on sol-gel ferroelectric PZT capacitors

Abstract: Ferroelectric (FE) thin-film capacitors are irradiated to 100 Mrad(Si) with 10-keV X-rays. Some of the FE hysteresis loops show distortion at 5 Mrad(Si). The type and degree of distortion are dependent on the polarization state and/or the applied field during irradiation. Preliminary results indicate that a fraction of the radiation-induced damage can be removed simply by cycling the FE capacitor with a 20-kHz square wave. The amount of damage removed is dependent upon the radiation conditions. >

Scintillation properties of GSO

Abstract: The timing properties of Gd/sub 2/SiO/sub 5/:Ce (GSO) single-crystal scintillators have previously been evaluated for positron emission tomography applications. The measured time resolution, however, was worse than expected from calculations based on photoelectron yield and a 60-ns exponential decay constant, leading the authors to further investigate GSO's basic properties. With a time-correlated-single-photon technique, they found two decay components, one of 56 ns and one of 600 ns, the latter containing 10-15% of the total scintillation output. This may explain the difference between the experimental and theoretical time resolutions and confirms a previous hypothesis of a long decay component. In addition, it was found that the decay constant of each component strongly depends on the cerium concentration. The primary component varies from approximately 20 ns to approximately 190 ns and the secondary component varies from approximately 70 ns to approximately 1200 ns as the cerium concentration is varied from 5.0 mol.% to 0.1 mol.%. >

High resolution block detectors for PET

Abstract: New high-resolution blocks detectors in which BGO scintillators are coupled to a position-sensitive photomultiplier tube (PMT), were developed for positron emission tomography (PET) applications. The block detectors, which use two different BGO scintillator structures, were constructed and evaluated: one is a 1.7-mm-wide BGO array which provides a coincidence detector resolution of less than 2 mm FWHM (full width at half maximum) and the other is a BGO block having comb-shaped slits in it, which has a potential capability of improving the resolution uniformity of a PET system because of its ability to detect the depth-of-interaction of gamma -rays in the scintillator. The application of these detectors to a PET system for animal studies is discussed. >

A study of reconstruction artifacts in cone beam tomography using filtered backprojection and iterative EM algorithms

Abstract: Reconstruction artifacts in cone beam tomography are studied for filtered backprojection (Feldkamp) and iterative EM algorithms. The filtered backprojection algorithm uses a voxel-driven, interpolated backprojection to reconstruct the cone beam data, whereas the iterative EM algorithm performs ray-driven projection and backprojection operations for each iteration. Two weighting schemes for the projection and backprojection operations in the EM algorithm are studied. One weights each voxel by the length of the ray through the voxel and the other equates the value of a voxel to the functional value of the midpoint of the line intersecting the voxel, which is obtained by interpolating between eight neighboring voxels. Cone beam reconstruction artifacts such as rings, bright vertical extremities, and slice-to-slice cross-talk are not found with parallel beam and fan beam geometries. When using filtered backprojection and iterative EM algorithms, the line-length weighting is susceptible to ring artifacts which are improved by using interpolated projector-backprojectors. >

Fast methods for including attenuation in the EM algorithm

Abstract: A fast approach to including attenuation in iterative maximum-likelihood and least-squares algorithms for single-photon-emission computed tomography (SPECT) is presented. Ray-tracing and summing of attenuation coefficients are replaced by the use of two lookup tables, one to compute attenuated ray path integrals based on a set of polar grid points and one to perform polar-to-rectangular transformations. The resulting algorithm implements a spatial average which is comparable in accuracy to ray-tracing with rectangular pixels, yet requires less than one sixteenth the CPU time. >

A 4096 cell switched capacitor analog waveform storage integrated circuit

Abstract: A full-custom switched-capacitor transient analog waveform storage and reconstruction integrated circuit containing 4096 sample and hold cells has been designed, fabricated, and tested. The switched capacitor array (SCA) is organized as 16 parallel channels of 256 serially addressed samples per channel with multiplexed and buffered analog output. Signal sampling frequencies of up to 100 MHz have been accomplished, with a dynamic range, measured at 10 MHz, of at least 8000 to 1. The circuit operation and performance are described, limits on performance are examined, and future implementations are discussed. >

Proton-induced displacement damage distributions and extremes in silicon microvolumes charge injection device

Abstract: An analytic approach for determining the pixel-to-pixel distribution of particle-induced damage and damage extremes in microvolumes representative of focal plane array pixel geometries is presented. Comparisons between predicted and measured dark current distributions in a silicon charge injection device (CID) show excellent agreement for 12- and 63-MeV proton-induced damage. The calculated and measured damage extremes are compared using extreme value statistical analysis. The calculations reveal how high-energy recoils from proton-induced nuclear reactions strongly influence the pixel-to-pixel variation in damage as well as the damage extremes. A comparison between Si and GaAs pixels with equal volumes and equal 12-MeV proton fluences indicates that both the average damage and its variance are significantly greater in GaAs. >

A TPC detector for the study of high multiplicity heavy ion collisions

Abstract: The design of a time projection chamber (TPC) detector with complete pad coverage is presented. The TPC allows the measurements of high multiplicity ( approximately 200 tracks) relativistic nucleus-nucleus collisions initiated with the heaviest, most energetic projectiles available at the LBL (Lawrence Berkeley Laboratory) BEVALAC accelerator facility. The front-end electronics, composed of over 15000 time sampling channels, is located on the chamber. The highly integrated, custom-designed electronics and the VME-based data acquisition system are described. >

Radiation induced defects in CVD-grown 3C-SiC

Abstract: Radiation-induced defects in 3C-SiC epitaxially grown by a chemical vapor deposition method were studied with the electron spin resonance (ESR) technique. A 15-line ESR spectrum was observed in 2-MeV proton and 1-MeV electron irradiated 3C-SiC when the magnetic field was applied parallel to the axis. This spectrum, T1, which has an isotropic g-value of 2.0029+or-0.0001, was interpreted by simultaneous hyperfine interactions of a paramagnetic electron with the surrounding /sup 13/C at four carbon sites and /sup 29/Si at 12 silicon sites. The T1 spectrum appeared to arise from a point defect at a silicon site. The observed hyperfine interactions with neighboring /sup 13/C and /sup 29/Si nuclei are discussed in terms of a simple molecular-orbital treatment of the defect. >

Predicting switched-bias response from steady-state irradiations MOS transistors

Abstract: A novel semiempirical model of radiation-induced charge neutralization is presented. This model is combined with 12 heuristic guidelines derived from studies of oxide- and interface-trap charge ( Delta V/sub ot/ and Delta V/sub it/) buildup and annealing to develop a method to predict MOS switched-bias response from steady-state irradiations, with no free parameters. For n-channel MOS devices, predictions of Delta V/sub ot/, Delta V/sub it/, and mobility degradation differ from experimental values through irradiation by less than 30% in all cases considered. This is demonstrated for gate oxides with widely varying Delta V/sub ot/ and Delta V/sub it/ and for parasitic field oxides. Preliminary results suggest that n-channel MOS Delta V/sub ot/ annealing and Delta V/sub it/ buildup following switched-bias irradiation and through switched-bias annealing also may be predicted with less than 30% error. The p-channel MOS response at high frequencies (>1 kHz) is more difficult to predict. >

A study of data loss and mispositioning due to pileup in 2-D detectors in PET

Abstract: The effect of event pileup was investigated with a PET (positron emission tomography) system which employs a 2-D detector-array system. Event pileup caused losses of resolution of 9%, 19.8%, and 32% of the FWHM (full width at half maximum) for 10, 20, and 30 mCi of positron emitter in the field of view (FOV), respectively. 9.4%, 18.7%, and 28.1% of the data from the outer planes of the module were incorrectly placed in the inner planes for 5, 10, and 15 mCi of the positron emitter in the FOV, respectively. A simple model of the process was formulated and found to be adequate to describe the major features of event pileup for a number of source configurations and a wide range of activities. >

Restoration of combined conjugate images in SPECT: comparison of a new Wiener filter and the image-dependent Metz filter

Abstract: Two image-dependent restoration filters were applied to projection image sets obtained with single-photon-emission computed tomography (SPECT). Wiener and Metz restorations of combined conjugate views are compared to each other, Wiener restoration of individual projection images, and one-dimensional Butterworth smoothing. The combined view restoration filters adapt to the average thickness of the object by estimating a modulation transfer function (MTF) for that thickness. Simulated Tc-99m liver-spleen studies with randomly placed cold spot tumors, a projector which accounts for the spatially variant blurring in SPECT, and a Poisson noise generator are used to compute simulated projection image sets. These sets are filtered and reconstructed using the method of intrinsic attenuation correction of Tanaka et al. (1984). Cold spot contrast, automated receiver operator characteristic (ROC) analysis of cold spot detectability, and the normalized mean squared error (NMSE) are used to compare the four processing methods. Little difference is noted between the three restoration methods. However, the restoration filters all yield noticeably better ROC and NMSE results than the one-dimensional smoothing. Inspection of the three-dimensional MTF derived from reconstructions of point sources indicates that the restoration filters substantially reduce the low-frequency degradation which is primarily due to scatter. >

A projector, backprojector pair which accounts for the two-dimensional depth and distance dependent blurring in SPECT

Abstract: The speed and accuracy of iterative reconstruction routines for single-photon-emission computed tomography (SPECT) are highly dependent on the projection and backprojection algorithms used. A projector-backprojector pair which accounts for the two-dimensional, spatially dependent blurring encountered in SPECT and which reduces the computational load has been developed. These algorithms assume that the attenuator is a uniform elliptical cylinder that is invariant in the axial direction. They further assume that the blur can be modeled using the sum of several separable functions, e.g. separate Gaussian functions for the geometric and scatter components. When implemented on an array processor of moderate computational power, a projection or backprojection of a SPECT study, with 64 64-pixel*64-pixel projection images and a 64*64*64 reconstruction space, can be completed in less than 2.4 h, or 2.3 min per transverse slice. >