Time-of-flight Resolution Research Articles

An Evaluation of Image Noise Variance for Time-of-Flight PET

We describe a simple theoretical framework for linear time-of-flight (TOF) PET reconstruction and evaluate several alternative TOF filters. We implement a capability for direct computation of TOF noise variance images and evaluate the gain in image variance for TOF vs. non-TOF for 1.26 nsec TOF resolution on the Siemens biograph HiRez PET/CT. The variance calculation is validated by comparing to pixel variances computed from replicated synthetic data. TOF reconstruction does not give a uniform improvement in image noise variance, but reduces variance more toward the periphery of an object than at its center. The TOF performance gain depends in part on how random coincidences are processed, and is greater when the randoms fraction is higher, or when randoms smoothing is not employed. We confirm that the TOF gain is greater for larger objects than for smaller ones, but find discrepancies with the D/Deltax rule of thumb. With 1.26 nsec resolution on the HiRez, TOF image noise gain factors (non-TOF/TOF variance) range from 1.5 to 2.2 on the measured data at hand. We predict that an improvement in TOF resolution by a factor of 2 (to 600 psec) could improve image variance on this machine by an additional factor of 1.8. We find that confidence weighted TOF reconstruction under-performs non-TOF reconstruction for objects whose diameters are small compared to the TOF resolution.

A Front-End Electronics Card Comprising a High Gain/High Bandwidth Amplifier and a Fast Discriminator for Time-of-Flight Measurements

During the development of the Multi-Strip Multi-Gap Resistive Plate Chambers (MMRPCs) for the time-of-flight (ToF) upgrade of the FOPI detector system, we have designed different versions of the front-end electronics (FEE). The signals from a MMRPC are read out on each side of the anode strips by an amplifier followed by a leading-edge discriminator. They are digitized in a time-to-amplitude converter (TAC) followed by a charge-to-digital converter (QDC). These counters are designed to provide a ToF resolution below sigmat les 60 ps requiring an amplifier-diecriminator stage with an intrinsic electronic resolution below sigmat(FEE) les 20 ps. We describe the design steps of the FEE-card from a 4-channel version (FEE1) for the R&D phase to the final 16-channel card (FEE5) for the readout of 4800 channels of the total detector array. Through these design steps we have kept the following key characteristics: low noise sigman les 25 muV RMS (to the amplifier input), high gain (200) and a high bandwidth (1 GHz).

Fast reconstruction of 3D time-of-flight PET data by axial rebinning and transverse mashing

Faster scintillators like LaBr3 and LSO have sparked renewed interest in PET scanners with time-of-flight (TOF) information. The TOF information adds another dimension to the data set compared to conventional three-dimensional (3D) PET with the size of the projection data being multiplied by the number of TOF bins. Here we show by simulations and analytical reconstruction that angular sampling for two-dimensional (2D) TOF PET can be reduced significantly compared to what is required for conventional 2D PET. Fully 3D TOF PET data, however, have a wide range of oblique and transverse angles. To make use of the smaller necessary angular sampling we reduce the 3D data to a set of 2D histoprojections. This is done by rebinning the 3D data to 2D data and by mashing these 2D data into a limited number of angles. Both methods are based on the most likely point given by the TOF measurement. It is shown that the axial resolution loss associated with rebinning reduces with improved timing resolution and becomes less than 1 mm for a TOF resolution below 300 ps. The amount of angular mashing that can be applied without tangential resolution loss increases with improved TOF resolution. Even quite coarse angular mashing (18 angles out of 324 measured angles for 424 ps) does not significantly reduce image quality in terms of the contrast or noise. The advantages of the proposed methods are threefold. Data storage is reduced to a limited number of 2D histoprojections with TOF information. Compared to listmode format we have the advantage of a predetermined storage space and faster reconstruction. The method does not require the normalization of projections prior to rebinning and can be applied directly to measured listmode data.

129I in precipitation using a new TOF system for AMS measurements

A new time-of-flight set-up for accelerator mass spectrometry with 129I was installed at the Maier-Leibnitz laboratory in Garching (Germany). The detector system consists of a TOF-system and an ionization chamber. A TOF resolution of 440 ps is obtained, the energy resolution is about 2%. The sensitivity of the complete system is 10 −14 for the ratio of 129I/ 127I. The measurements of rain water samples from the time period between July 2003 and October 2004 showed a decrease of 129I concentration, of 129I deposition flux and of 129I/ 127I isotopic ratio compared to previous measurements of rain water in Southern Germany from 1988, 1989 and 1995. This decrease can be explained by a substantial drop in gaseous 129I emissions from the French reprocessing facilities.

A large acceptance lead-scintillator time-of-flight wall for neutral and charged particles

A large acceptance lead-scintillator time-of-flight (TOF) wall has been developed to detect photons, neutrons, and charged particles produced in meson-photoproduction experiments on the proton and on the neutron. A TOF resolution of 600 ps and a position resolution of 11×18 cm 2 (full-width at half-maximum) have been achieved. The wall has been successfully used in η and π + photoproduction experiments as a photon and a neutron detector. This paper reports its design, calibration and performance.

Surface physics with low-energy ion scattering

Four versions of low-energy ion scattering (LEIS) measurements are briefly reviewed. In the first version discussed here the charged fraction of scattered noble-gas ions is detected with an electrostatic analyser. The composition of exclusively the outermost layer is probed in this way, even when the surface of the sample is not smooth. The second method probes the surface step density, and thereby growth oscillations, with grazing-angle specular scattering, whereby the scattered particles are measured without energy resolution. An example is given of the observation of oscillations during the growth of Ag on Ag (0 0 1) at several temperatures. The oscillations are compared with oscillations obtained from simulations of the growth process, combined with LEIS simulations. This second method is also sensitive to the structure of the outermost layer. There is no sensitivity to the composition of the surface. In the third method either the beam direction, or the direction of the detector, makes a grazing angle with the surface, and atoms scattered over a large (non-specular) angle are detected with time-of-flight (TOF) resolution. This provides sensitivity only to atoms protruding from the surface, like adatoms, or atoms at step edges. An example is presented whereby this method is used to measure adatom mobilities. Finally the conventional method of measuring with TOF resolution azimuth scans of scattered or recoiled particles in a shadowing or blocking geometry is described. The scans are compared with the results of computer simulations carried out for trial structures. The properties of the program MATCH that can be used efficiently for these simulations are summarised. As an example of the application of this method the determination of the surface structure of Fe 3O 4 (0 0 1) is presented. The features of the LEIS versions presented are compared with MEIS and HEIS features, with emphasis on the possibility to obtain information on the composition and structure, separately for the first few layers of a sample.

The BELLE TOF system

The BELLE TOF system provides π/K separation for particle momentum below 1.2 GeV/c . A novel Time Stretcher TDC scheme is used to measure flight times with a 25 ps LSB in the high rate environment of KEKB. The TOF resolution is found to be 100 ps. The TOF system also provides precise event timing to the BELLE trigger system.

Cosmic ray antiproton spectrum measured by BESS in 1993–1995

Balloon-borne experiments using a superconducting spectrometer were carried out successfully in northern Canada, in summer 1993 to 1995. In '93 flight, seven antiprotons were detected in the kinetic range between 200 and 600 MeV. The corresponding cosmic ray antiproton flux at the top of the atmosphere is found to be 6.4 −3.5 +5.5×10 −3(m 2 sr s GeV) −1. In '95 flight, improvement on the TOF resolution resulted to the observation of the forty antiprotons in the kinetic energy region 175 MeV to 1.3 GeV. The analysis on '95 flight is under way.

High resolution TOF detector for hypernuclei lifetime measurement

Abstract We have tested several types of TOF counters based on scintillators and photomultiplier tubes in order to measure the lifetime of hypernuclei. With the advantage of small counter size, we obtained less than 30 ps timing resolution, and about 50 ps overall TOF resolution. The various factors in this resolution have been studied. The timing resolution dependence on the beam intensity is also studied.

Neutron interferometry at a pulsed source

A compact setup providing vibration damping support as well as thermal shielding for a classical LLL interferometer was developed and installed at the IBR-2 pulsed reactor at JINR Dubna. Despite the low incident neutron flux successful experiments were performed with two different LLL-type interferometer crystals and simultaneous observation of the interference patterns on two reflection orders was demonstrated. This first experience leads to the conclusion that routine experiments in the multi-wavelength regime are feasible, but a careful optimization of the setup regarding the opposing demands on TOF resolution, incident flux and background would be desirable to compensate at least partially for the rapid decrease of diffracted intensity at higher reflection orders.

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

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. >

Time-of-flight positron imaging and the resolution improvement by an iterative method

A simple time-of-flight (TOF) positron imaging method is proposed in which the spatial resolution obtained with TOF information is improved by a novel iterative algorithm. The prototype has a pair of opposed detectors using BaF/sub 2/ (2*2*2 cm/sup 3/) and photomultiplier tubes (18.5-mm diameter). The measured TOF resolution full width at half maximum was 330 ps, which corresponds to 5.0 cm of spatial resolution for source localization between the detectors. Eight iterations with the algorithm improve the spatial resolution by a factor of about two for a point source. The algorithm is based on modifications of a Bayesian deconvolution algorithm. For more complex sources, it takes more iterations (e.g. 80); however the algorithm is about five times faster than the original Bayesian algorithm. >

Characteristics of Small Barium Fluoride (BaF2) Scintillator for High Intrinsic Resolution Time-of-Flight Positron Emission Tomography

The implementation of BaF2 for high spatial and TOF resolution PET cameras has been investigated. Intrinsic characteristics relating to the exclusive use of its fast 220 nm component, such as self-absorption, reflection material, optical coupling, energy resolution, timing resolution, number of fast photons, photofractrion, coincidence efficiency, and photomultiplier are presented. Extrinsic characteristics such as intrinsic spatial resolution variation with radius, the need for tungsten partition and the optimization of detector ring radius and tungsten partition thickness were investigated. Schemes of using multiple scintillators on a single PMT due to the absence of small PMT with small transit time spread and economic reasons are also presented.

Measurement of acoustoelastic coefficients of Rayleigh waves in steel alloys

An ultrasonic technique to determine the acoustoelastic coefficients of Rayleigh waves in steel alloys is described. The technique is based on the measurement of the time of flight of Rayleigh waves over a fixed surface distance as a function of applied stress. Measurements on AISI 1080 carbon steel, AISI 4130 alloy steel, and 316L stainless steel specimens are reported. Time of flight resolution and repeatability as well as temperature effects are discussed insofar as they influence the applicability of ultrasonic methods to the measurement of applied and residual biaxial surface stresses in steel.

Crossed molecular beams kinetics: SnO recoil velocity spectra from Sn+O2

Laboratory (LAB) angular and time-of-flight (TOF) recoil velocity spectra of SnO product produced by crossing a thermal Sn beam with an O2 nozzle beam have been measured in a molecular beam apparatus equipped with an electron bombardment ionizer–quadrupole mass filter ’’universal’’ detector. Product center-of-mass (c.m.) recoil angle and energy distributions have been fit to the data by convoluting the c.m.→LAB transformation over the measured speed distributions in both beams and the instrumental TOF resolution. The results establish that the Sn+O2 bimolecular reaction proceeds via formation of an osculating or long-lived SnO2 collision intermediate. In addition, comparison of observed LAB product signals have established approximate relative total cross sections for reaction of Sn with O2, CO2, SO2, N2O, CH3OH, and CH3NO2.

Reactive scattering of Sn from Cl 2 and Br 2 and a lower limit on D00 (SnBr)

Laboratory (lab) angular and time-of-flight (TOF) recoil velocity spectra of SnCl product formed by crossing a thermal Sn beam with a Cl 2 nozzle beam have been measured in a molecular beam apparatus equipped with an electron bombardment ionizer—quadrupole mass filter detector unit. Product center of mass (c.m.) recoil angle and energy distributions have been fit to the data by convoluting the c.m. → lab transformation over measured beam speed distributions and the instrumental TOF resolution. The inferred distributions for the Sn + Cl 2 reaction are strikingly similar to those reported for the Li + Cl 2 reaction. In addition, SnBr product was observed from crossed beams of Sn and Br2 which establishes a lower limit for D 0 0 (SnBr) of 131 kJ/mole.