Gamma-ray Imaging Detector Research Articles

SPECT-CT system for small animal imaging

The Detector Group at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) and the Biology, Physics, and Applied Sciences Departments at the College of William and Mary are collaborating on the development of a miniature dual modality SPECT-CT system for mouse imaging. The detector heads of the SPECT sub-system are designed to be capable of imaging the gamma- and X-ray emissions (28-35 keV) of the radioactive isotope iodine-125 (I-125). Two different sets of I-125 imaging detectors are configured on a gantry that has an open-barrel type design. One set of detector heads is based on the 1-in square Hamamatsu R5900-M64 position sensitive photomultiplier tube coupled to crystal scintillator arrays. The other detector heads configured on the gantry are two 5-in diameter Hamamatsu R3292-based compact gamma cameras. The X-ray radiographic projections are obtained using a LIXI Inc. model LF-85-503-OS X-ray imaging system that has an active area of 5.5 cm in diameter. The open-barrel shaped gantry facilitates the positioning of various mini gamma-ray imaging detectors and the X-ray system. The data acquisition and gantry control is interfaced through a Macintosh G3 workstation. Preliminary SPECT reconstruction results using the R5900 based detector are presented.

Detector design issues for compact nuclear emission cameras dedicated to breast imaging

Abstract Certain gamma ray and positron emitting radiotracers have shown great promise for use in the detection, diagnosis and staging of breast cancer. Unfortunately, standard nuclear emission cameras (SPECT, PET) found in the clinic are not practical for breast imaging of these emissions due to inadequate spatial and energy resolutions and sensitivity, large and awkward size, and relatively high cost per study. High spatial and energy resolutions and sensitivity are needed for good lesion detectability. Due to these limitations of standard cameras, there has been recent research into the development of small, compact nuclear emission imagers dedicated for close-proximity breast imaging. The small detector head size means a variety of exotic detectors or collimators may be implemented to improve spatial and energy resolution and sensitivity performances at a reasonable cost. In this paper, we will present some of the compact gamma ray and annihilation photon imaging detector designs that have been proposed and/or developed for dedicated breast imaging. We will review the physics and discuss the advantages and disadvantages of various detector configurations. Finally we will estimate the fundamental spatial resolution potential available with close-proximity nuclear emission imaging and discuss how one may approach those limits through proper detector design.

Gamma-ray imaging detectors based on silicon drift detectors arrays coupled to a single scintillator

Abstract Arrays of Silicon Drift Detectors (SDDs) coupled to a single scintillator, according to the Anger Camera scheme, can be successfully employed in gamma-ray imaging. The low value of output capacitance of a SDD allows to reach a lower electronics noise with respect to conventional silicon photodiodes used in scintillation detection. A small prototype of gamma camera with sub-millimeter resolution has been realized by using a monolithic array of small SDDs (5 mm2 each unit) with on-chip JFET. For the realization of gamma cameras of larger areas based on single units assembled in array, SDDs of 30 mm2 of area with external JFET have been also experimented.

A monolithic array of silicon, drift detectors for high-resolution gamma-ray imaging

In this work, the preliminary results obtained with a high position resolution gamma-ray imaging detector are presented. The detector consists of a single CsI(Tl) scintillator crystal optically coupled to a monolithic array of 19 silicon drift detectors (SDDs) used as photodetectors. In this array, having a total sensitive area of about 1 cm/sup 2/, each SDD has a front-end JFET directly integrated on the detector chip, close to the collecting anode. This architecture reduces the electronic noise of each single photodetector to a value small enough to obtain a submillimeter position resolution at 122 keV gamma-ray energy. The present detector module represents the development of a previous prototype based on a very limited numbers of SDDs.

Simulation of the AGILE gamma-ray imaging detector performance: part I

We present in this paper the results of a comprehensive GEANT simulation of the Gamma-Ray Imaging Detector (GRID) being developed for the AGILE space astrophysics mission. The GRID is designed to be sensitive in the ∼30 MeV – 50 GeV energy range, with excellent imaging and timing capabilities, and with a very large field-of-view ( ∼3 sr ). In this paper (Paper I) we present the GRID baseline geometry, a model for the charged particle and albedo-photon backgrounds for an equatorial orbit of 550– 600 km altitude, and the main results of the first level (Level-1 Trigger) on-board data processing. Our simulations show that the GRID Level-1 data processing is expected to be capable of decreasing by a factor of ∼20 the charged particle background (from ∼2 kHz to below 100 Hz ), and by a factor of ∼30 the albedo-photon background. The gamma-ray photon detection efficiency by the imaging GRID is simulated to be particularly efficient, varying between 39% and 17% depending on photon energies and incident directions.

Simulation of the AGILE gamma-ray imaging detector performance: Part II

In this paper (Paper II) we complete our discussion on the results of a comprehensive GEANT simulation of the scientific performance of the AGILE Gamma-Ray Imaging Detector (GRID), operating in the ∼30 MeV – 50 GeV energy range in an equatorial orbit of height near 550 km . Here we focus on the on-board Level-2 data processing and discuss possible alternative strategies for event selection and their optimization. We find that the dominant particle background components after our Level-2 processing are electrons and positrons of kinetic energies between 10 and 100 MeV penetrating the GRID instrument from directions almost parallel to the Tracker planes (incidence angles θ≳90°) or from below. The analog (charge) information available on-board in the GRID Tracker is crucial for a reduction by almost three orders of magnitude of protons (and heavier ions) with kinetic energies near 100 MeV . We also present in this paper the telemetry structure of the GRID photon and particle events, and obtain the on-board effective area for photon detection in the energy range ∼30 MeV – 50 GeV .

Gamma-ray imaging detectors based on SDDs coupled to scintillators

Silicon Drift Detectors (SDDs) have been recently employed succesfully in scintillation detection. Owing to the low value of output capacitance, a SDD is in fact characterized by a lower electronics noise with respect to a conventional silicon photodiode. This feature could allow a detector based on a CsI(Tl)-SDD assembly to reach high energy and position resolution in gamma detection. In this work, we present a small prototype of gamma detector for 2-D position measurements, based on a single scintillator coupled to an array of SDDs. The first experimental results are reported.

Scintillation light read-out by thin photodiodes in silicon wells

Several applications of X-ray and gamma ray imaging detectors, e.g. in medical diagnostics, require millimeter or sub-millimeter spatial resolution and good energy resolution. In order to achieve such features we have proposed a new type of camera, which takes advantage of micromachining technology. It consists of an array of scintillator crystals encapsulated in silicon wells with photodiodes at the bottom. Several parameters of the photodiode need to be optimised: uniformity and efficiency of the light detection, gain, electronic noise and breakdown voltage. In order to evaluate these parameters we have processed 3×3 arrays of 1.8mm2, ∼10μm thick photodiodes using (100) wafers etched in a KOH solution. Their optical response at 675nm wavelength is comparable to that of a 500μm thick silicon PIN diode. Their low light detection efficiency is compensated by internal amplification. Several scintillator materials have been positioned in the wells on top of the thin photodiodes, i.e. a 200μm thick film of structured CsI(Tl), single crystals of CsI(Tl) and Lu2S3(Ce3+). First experiments of γ-ray detection have been performed.

A new compact position-sensitive PMT for scintillation detectors

A new compact position-sensitive photomultiplier tube (PS-PMT), Hamamatsu R7600-C12, has been developed for scintillation detectors. The PS-PMT has 11 stages of metal channel dynodes [2] and 6(X)+6(Y) crossed plate anodes in a 25.7 mm square/spl times/20 mm high metal can package, where the photo-sensitive area is 22 mm square. The performance of the PS-PMT was evaluated in terms of applicability to radiation imaging systems. In comparison with the former type of PS-PMT (Hamamatsu R5900-C8), the new PS-PMT provides smaller light spread and better position response. Also, by removing the flange at the bottom of the PS-PMT, the ratio of the effective area to the outward area is increased. The spatial resolution capability was demonstrated by imaging a stratified LSO array having an element of 1.8 mm/spl times/1.8 mm/spl times/10 mm. Each crystal element is clearly identified on the image map with 662 keV uniform gamma-ray irradiation. This paper describes the characteristics of the new PS-PMT and the experimental results for a gamma-ray imaging detector.

Signals induced in semiconductor gamma-ray imaging detectors

The signal induced in a readout circuit connected to a pixel electrode in a semiconductor gamma-ray imaging array is calculated by solving the Laplace equation. Two approaches are presented that use Green functions in solving the boundary value problem: decomposition into basis functions, and construction of an infinite series of image charges. Another approach is developed based on the Ramo–Shockley theorem, which makes use of weighting potentials. These potentials may be readily calculated in three dimensions using a Fourier-transform propagation technique. An analytic solution is found for the special two-dimensional case of a strip detector. Experiments on CdZnTe square-pixel test structures using alpha radiation confirm the expected trends in pulse shape as a function of pixel size. Signals observed simultaneously on adjacent pixels also follow the predicted division of currents. Trends with pixel size are also confirmed in the shape of pulse-height spectra taken using a Tc99m source.

Optical fiber readout of scintillator arrays using a multi-channel PMT: a high resolution PET detector for animal imaging

The authors report the results from a new high resolution gamma ray imaging detector designed for use in a positron emission tomography (PET) system dedicated to small animal imaging. The detectors consist of an 8/spl times/8 array of 2/spl times/2/spl times/10 mm bismuth germanate (BGO) crystals coupled by 2 mm diameter double clad optical fibers to a 64 pixel multi-channel photomultiplier tube (MC-PMT). A charge division readout board is used to convert the 64 output channels into four position sensitive signals which determine the crystal of interaction. Measurements with a pair of these detectors demonstrate an intrinsic spatial resolution of 1.4 mm, a coincidence timing resolution of 15 ns and an energy resolution ranging between 35 and 60%. Based on these encouraging results, the design for a dedicated animal PET tomograph is proposed and simulations of this system project a reconstructed resolution of less than 2 mm within a 5 cm diameter transaxial field of view.

Simple charge division readouts for imaging scintillator arrays using a multi-channel PMT

Three simple charge division circuits were assembled and tested as 2-D position encoding readouts for multi-channel photomultiplier tubes (MC-PMT). They were evaluated with an 8/spl times/8 array of individual scintillators (2/spl times/2/spl times/10 mm BGO) coupled to a 64 channel MC-PMT (Philips XP1722) via 25 cm long, 2 mm diameter, double clad, optical fibers (Kuraray). This type of gamma-ray imaging detector has many potential applications in medical and industrial imaging. Though independent channel readout would allow for the discrimination of scatter within the array, and higher count rates, it would also require an excessive amount of supporting electronics. This is especially true for systems comprised of many MC-PMTs. In this study, the number of channels being read out was reduced from 64 to 4 using three different simple resistor networks. These circuits take advantage of the discretized nature of the scintillator array, the low interchannel crosstalk of the MC-PMT and low input impedance current-sensitive preamplifiers. For each circuit, the scintillator identification accuracy was compared. The identification accuracy as a function of deposited energy was also determined by exposure to various gamma-ray emitters. It was found that the preamplifier circuit noise contributed the most to the degradation of the detector's spatial response so several low noise op amps were evaluated. It was also determined that keeping the preamplifier input impedance small was necessary for accurate positioning. The coincidence timing resolution of the detector (15 ns) is only slightly degraded by the readout circuit.

Recursive calculation of the two-dimensional maximum likelihood position estimate for a scintillation camera

Maximum likelihood position estimation of a scintillation event in a gamma-ray imaging detector has been described using two independent one-dimensional estimates in x and y. This approximation is only valid when the sampled light-spread function is separable, and spatial distortion is observed when this constraint is violated. Recursive calculation of the 2-D position estimate using a sequence of one-dimensional estimates was investigated. Mean phototube response functions were computed from Monte Carlo simulations of the light-spread function, and position was estimated for scintillation events using recursive estimation. After three steps the maximum errors were mod delta x mod =0.04 mm and mod delta y mod =0.06 mm for the case of noise-free data and when mean response functions were known at each estimated location. Using response functions interpolated over 6-mm intervals gave increased distortion with average x and y errors of 0.097 and 0.082 mm, respectively.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Multiple electron scattering in gamma ray imaging detectors

The effect of converter thickness on spatial resolution and detection efficiency of gamma ray imaging detectors was investigated by Monte Carlo simulation of photon and electron transport in thin slabs. The simulations were carried out by employing a multiple electron scattering theory to simulate successive changes in electron trajectory during propagation through the converter and detector materials. For comparison, results were also obtained using the approximate method of assuming straight-line paths for the electrons. The straight-line path simulations indicate that both spatial resolution and detection efficiency can be improved by the use of thicker converters, while the multiple electron scattering simulations indicate that, without the use of pulse height selection, detection efficiency can be improved only at the expense of spatial resolution. The calculated spatial resolution and detection efficiency for 0.662 MeV photons incident on a 0.18 cm thick mylar converter in juxtaposition with a 1.27 cm thick argon slab detector (NTP) are 2.1 cm and 0.39% and 1.3 cm and 0.64% for the multiple electron scattering and straight-line path analyses, respectively.