William W. Moses

Time of flight in PET revisited

PET scanners based on LSO have the potential for significantly better coincidence timing resolution than the 6 ns FWHM typically achieved with BGO. This study analyzes the performance enhancements made possible by improved timing as a function of the coincidence time resolution. If 500 ps FWHM coincidence timing resolution can be achieved in a complete PET camera, the following four benefits can be realized for whole-body FDG imaging: 1) the random event rate can be reduced by using a narrower coincidence timing window, increasing the peak NECR by /spl sim/50%; 2) using time-of-flight (TOF) in the reconstruction algorithm will reduce the noise variance by a factor of 5; 3) emission and transmission data can be acquired simultaneously, reducing the total scan time; and 4) axial blurring can be reduced by using TOF to determine the correct axial plane of origin for each event. While TOF was extensively studied in the 1980s, practical factors limited its effectiveness at that time and little attention has been paid to timing in PET since then. As these potential improvements are substantial and the advent of LSO PET cameras gives us the means to obtain them without other sacrifices, efforts to improve PET timing should resume after their long dormancy.

Strontium and barium iodide high light yield scintillators

Europium-doped strontium and barium iodide are found to be readily growable by the Bridgman method and to produce high scintillation light yields.

Prospects for time-of-flight PET using LSO scintillator

We present measurements of the timing properties of lutetium orthosilicate (LSO) scintillator crystals coupled to a photomultiplier tube (PMT) and excited by 511 keV photons. These crystals have dimensions suitable for use in PET cameras (3/spl times/3/spl times/30 mm/sup 3/). Coincidence timing resolution of 475 ps fwhm is measured between detectors utilizing two such crystals, significantly worse than the 300 ps fwhm predicted based on first principles for small crystals and measured in 3 mm cubes. This degradation is found to be caused by the scintillation light undergoing multiple reflections at quasi-random angles within the scintillator crystal, which has two effects. First, it slows down the effective information propagation speed within the crystal (to an effective n/spl circ/=3.9-5.3). Since the incident annihilation photon travels with n=1, information from interactions at different depths arrives at the PMT with different time delays. Second, the random nature of the reflection angles (and path lengths) introduce dispersion and so a 10%-90% rise time of 1 ns to the optical signal.

Current trends in scintillator detectors and materials

The last decade has seen a renaissance in inorganic scintillator development for gamma ray detection. Lead tungstate (PbWO4) has been developed for high energy physics experiments, and possesses exceptionally high density and radiation hardness, albeit with low luminous efficiency. Lutetium orthosilicate or LSO (Lu2SiO5:Ce) possesses a unique combination of high luminous efficiency, high density, and reasonably short decay time, and is now incorporated in commercial positron emission tomography (PET) cameras. There have been advances in understanding the fundamental mechanisms that limit energy resolution, and several recently discovered materials (such as LaBr3:Ce) possess energy resolution that approaches that of direct solid state detectors. Finally, there are indications that a neglected class of scintillator materials that exhibit near band-edge fluorescence could provide scintillators with sub-nanosecond decay times and high luminescent efficiency.

List-mode maximum-likelihood reconstruction applied to positron emission mammography (PEM) with irregular sampling

Presents a preliminary study of list-mode likelihood reconstruction of images for a rectangular positron emission tomograph (PET) specifically designed to image the human breast. The prospective device consists of small arrays of scintillation crystals for which depth of interaction is estimated. Except in very rare instances, the number of annihilation events detected is expected to be far less than the number of distinguishable events. If one were to histogram the acquired data, most histogram bins would remain vacant. Therefore, it seems natural to investigate the efficacy of processing events one at a time rather than processing the data in histogram format. From a reconstruction perspective, the new tomograph presents a challenge in that the rectangular geometry leads to irregular radial and angular sampling, and the field of view extends completely to the detector faces. Simulations are presented that indicate that the proposed tomograph can detect 8-mm-diameter spherical tumors with a tumor-to-background tracer density ratio of 3:1 using realistic image acquisition parameters. Spherical tumors of 4-mm diameter are near the limit of detectability with the image acquisition parameters used. Expressions are presented to estimate the loss of image contrast due to Compton scattering.

Nonproportionality of Scintillator Detectors: Theory and Experiment. II

We report measurements of electron response of scintillators, including data on 29 halides, oxides, organics, and fluorides. We model the data based on combining the theories of: Onsager, to account for formation of excitons and excited activators; Birks, to allow for exciton-exciton annihilation; Bethe-Bloch, to relate electron stopping to its energy; and Landau, to describe how fluctuations in the linear energy deposited (dE/dx) lead to nonproportionalitys contribution to resolution. In general there is satisfactory agreement with experiment, in terms of fitting the electron response data and reproducing the literature values of resolution. We find that the electron response curve shapes are more affected by the host lattice than by the activator or its concentration.

Measurements of the intrinsic rise times of common inorganic scintillators

The intrinsic rise times of a number of common inorganic scintillators are determined using ultrafast measurements of luminescence following pulsed X-ray excitation. A Ti-sapphire mode-locked laser and a light-excited X-ray tube are used to produce X-ray pulses with 60 ps fwhm. Fluorescence photons are detected with a microchannel phototube and the response of the phototube and electronics is 45 ps fwhm. Samples are either powders or thin crystals painted black on five sides to reduce delayed scattered photons. The intrinsic scintillators CeF/sub 3/, CdWO/sub 4/, Bi/sub 4/Ge/sub 3/O/sub 12/, and CsI have rise times /spl les/30 ps, indicating that electrons are promptly captured to form the excited states. The activated scintillators CaF/sub 2/:Eu, ZnO:Ga, and Lu/sub 2/SiO/sub 5/:Ce have rise times /spl les/40 ps, indicating that the luminescent centers are excited by rapid sequential hole capture- electron capture. The activated scintillators CsI:Tl and YAlO/sub 3/:Ce have slower rise times due to processes that delay the formation of excited states. It is shown that for practical scintillation detectors, internal reflections in the crystal can degrade observed rise times by hundreds of ps depending on size, reflector, and index of refraction.

A positron tomograph with 600 BGO crystals and 2.6 mm resolution

A description is given of the imaging performance of the Donner 600-Crystal Positron Tomograph, a single 60-cm-diameter ring of 3-mm-wide bismuth germanate (BGO) crystals coupled individually to 14-mm phototubes. With a pulse height threshold of 200-keV and a slice thickness of 5 mm, the sensitivity is 7024 events/s per mu Ci/Ml in a 20-cm cylinder of water. The measured rates for 18 mu Ci/ml are 95000 trues/s plus 20000 randoms/s. A 0.3-mm-diameter /sup 22/Na line source near the center of the tomograph has a circular point-spread function (PSF) with a full-width at half-maximum (FWHM) of 2.6 mm. At 5 cm from the center the PSF is elliptical with a FWHM of 2.7 mm tangential*3.2 mm radial. At 10 cm the PSF has a FWHM of 2.8 mm tangential*4.8 mm radial. Attenuation data have been accumulated with a 20 mCi /sup 68/Ge orbiting transmission source, and 100 million coincident events have been collected in 200 s. >

Crystal Growth and Scintillation Properties of Strontium Iodide Scintillators

Single crystals of SrI₂:Eu and SrI₂:Ce/Na were grown from anhydrous iodides by the vertical Bridgman technique in evacuated silica ampoules. Growth rates were of the order of 5-30 mm/day. Radioluminescence spectra of SrI₂:Eu and SrI₂:Ce/Na exhibit a broad band due to Eu²⁺ and Ce³⁺ emission, respectively. The maximum in the luminescence spectrum of SrI₂:Eu is found at 435 nm. The spectrum of SrI₂:Ce/Na exhibits a doublet peaking at 404 and 435 nm attributed to Ce³⁺ emission, while additional impurity-or defect-related emission is present at approximately 525 nm. The strontium iodide scintillators show very high light yields of up to 120 000 photons/MeV, have energy resolutions down to 3% at 662 keV (Full Width Half Maximum) and exhibit excellent light yield proportionality with a standard deviation of less than 5% between 6 and 460 keV.

Temperature dependence of CsI(Tl) gamma-ray excited scintillation characteristics

The gamma-ray excited, temperature dependent scintillation characteristics of CsI(Tl) are reported over the temperature range of −100 to + 50°C. The modified Bollinger-Thomas and shaped square wave methods were used to measure the rise and decay times. Emission spectra were measured using a monochromator and corrected for monochromator and photocathode spectral efficiencies. The shaped square wave method was also used to determine the scintillation yield as was a current mode method. The thermoluminescence emissions of CsI(Tl) were measured using the same current mode method. At room temperature, CsI(Tl) was found to have two primary decay components with decay time constants of τ1 = 679±10 ns (63.7%) and τ2 = 3.34±0.14 μs (36.1%), and to have emission bands at about 400 and 560 nm. The τ1 luminescent state was observed to be populated by an exponential process with a resulting rise time constant of 19.6±1.9 ns at room temperature. An ultra-fast decay component with a < 0.5 ns decay time was found to emit about 0.2% (about 100 photons/MeV) of the total scintillation light. Except for the ultra-fast decay time, the rise and decay time constants were observed to increase exponentially with inverse temperature. At −80°C τ1 and τ2 were determined to be 2.22±0.33 μs and 18.0±2.59 μs, respectively, while the 400 nm emission band was not observed below −50°C. At +50°C the decay constants were found to be 628 ns (70.5%) and 2.63 μs (29.3%) and both emission bands were present. The scintillation yield of CsI(Tl) was observed to be only slightly temperature dependent between −30 and +50°C, peaking at about −30°C (about 6% above the room temperature yield) and monotonically decreasing above and below this temperature. Four different commercially available CsI(Tl) crystals were used. Minimal variations in the measured scintillation characteristics were observed among these four crystals. Thermoluminescence emissions were observed to have peak yields at −90, −65, −40, +20, and possibly −55°C. The relative magnitudes and number of thermoluminescence peaks were found to vary from crystal to crystal.