Monte Carlo simulation of breast tumor imaging properties withcompact, discrete gamma cameras

Describes Monte Carlo simulation results for breast tumor imaging using a compact, discrete gamma camera. The simulations were designed to analyze and optimize camera design, particularly collimator configuration and detector pixel size. Simulated planar images of 5-15 mm diameter tumors in a phantom patient (including a breast, torso, and heart) were generated for imaging distances of 5-55 mm, pixel sizes of 2×2-4×4 mm², and hexagonal and square hole collimators with sensitivities from 4000 to 16,000 counts/mCi/sec. Other factors considered included T/B (tumor-to-background tissue uptake ratio) and detector energy resolution. Image properties were quantified by computing the observed tumor FWHM (full-width at half-maximum) and S/N (sum of detected tumor events divided by the statistical noise). Results suggest that hexagonal and square hole collimators perform comparably, that higher sensitivity collimators provide higher tumor S/N with little increase in the observed tumor FWHM, that smaller pixels only slightly improve tumor FWHM and S/N, and that improved detector energy resolution has little impact on either the observed tumor FWHM or the observed tumor S/N     

Imaging Properties of a Positron Tomograph with 280 Bgo Crystals

The basic imaging properties of the Donner 280-BGO-Crystal positron tomograph were measured and compared with the same system when it was equipped with 280 NaI(T1) crystals. The NaI(T1) crystals were 8 mm × 30 mm × 50 mm deep, sealed in 10 mm wide stainless steel cans. The BGO crystals are 9.5 mm wide × 32 mm × 32 mm deep and as they are not hygroscopic do not require sealed cans. With a shielding gap of 3 cm (section thickness 1.7 cm FWHM) the sensitivity of the BGO system is 55,000 events per sec for 1 ?Ci per cm3 in a 20 cm cylinder of water, which is 2.3 times higher than the NaI(T1) system. For a 200 ?Ci/cm line source on the ring axis in a 20 cm diameter water cylinder, the BGO system records 86% of the scatter fraction and 66% of the accidental fraction of the NaI(T1) system. The lower light yield and poorer time resolution of BGO requires a wider coincidence timing window than NaI(T1); however, the ability to use full-energy pulse height selection with a 2.3-fold improvement in sensitivity results in an overall reduction in the fraction of accidental events recorded. The in-plane resolution of the BGO system is 9-10 mm FWHM within the central 30 cm diameter field, and the radial elongation at the edge of the field in the NaI(T1) system has been nearly eliminated.     

Resolution and sensitivity as a function of energy and incident geometry for germanium detectors

The use of modeling programs such as MCNP to predict the response of HPGe detectors is increasing in importance. Accurate simulation of germanium detectors to incident gamma rays relies on knowledge of the performance of the detector in different detector–source geometries. Two important performance parameters are the resolution and sensitivity. The resolution is the FWHM and FW.1M/FWHM ratio. The IEEE 325-1996 standard only specifies the FWHM measurement at one geometry and two energies. Nearly all measurements are made in a different geometry and at other energies. Other investigators [Specifications for Today’s Coaxial HPGe Detectors, 2001 ANS Annual Meeting, Milwaukee, WI; Metzger, private communication, see also: Radionuclide Depth Distribution by Collimated Spectroscopy, 2002 ANS Topical Meeting, Santa Fe, NM], have shown that the sensitivity and resolution change with position of the incident gamma ray on the front of the detector. Such variability has possible implications for the accuracy of peak shape and area determination, since the calibration is potentially a function of angle of incidence. To quantify the sensitivity and resolution variation as a function of energy and point of incidence, measurements have been made on several coaxial detectors of various crystal types and sizes in different source–detector geometries. The full-energy peaks from 59 keV to 2.6 MeV were used. The detectors were placed in a low-background shield to reduce any contribution from external sources. None of the detectors tested was a low-background type. The sources used were an ²⁴¹Am source, ⁶⁰Co source and a natural thorium oxide sample. The ²⁴¹Am 59 keV gamma rays were collimated by a 2 cm thick, 1 mm diameter lead collimator. Several gamma rays from the thorium source were used and collimated by a 10 cm thick and 2 mm diameter tungsten collimator. These collimated sources were used to collect spectra for the incident beam on the front and sides of the detectors. The peak widths were calculated using the methods outlined in IEEE 325-1996. Data are presented to show that the peak shape and sensitivity change with incident beam position and full peak energy.     

Performance Characteristics of BGO Detectors for a Low Cost Preclinical PET Scanner

PETbox is a low-cost benchtop PET scanner dedicated to high throughput preclinical imaging that is currently under development at our institute. This paper presents the design and characterization of the detectors that are used in the PETbox system. In this work, bismuth germanate scintillator was used for the detector, taking advantage of its high stopping power, high photoelectric event fraction, lack of intrinsic background radiation and low cost. The detector block was segmented into a pixelated array consisting of 20 × 44 elements, with a crystal pitch of 2.2 mm and a crystal cross section of 2 mm × 2 mm. The effective area of the array was 44 mm × 96.8 mm. The array was coupled to two Hamamatsu H8500 position sensitive photomultiplier tubes, forming a flat-panel type detector head with a sensitive area large enough to cover the whole body of a typical laboratory mouse. Two such detector heads were constructed and their performance was characterized. For one detector head, the energy resolution ranged from 16.1% to 38.5% full width at half maximum (FWHM), with a mean of 20.1%; for the other detector head, the energy resolution ranged from 15.5% to 42.7% FWHM, with a mean of 19.6%. The intrinsic spatial resolution was measured to range from 1.55 mm to 2.39 mm FWHM along the detector short axis and from 1.48 mm to 2.33 mm FWHM along the detector long axis, with an average of 1.78 mm. Coincidence timing resolution for the detector pair was measured to be 4.1 ns FWHM. These measurement results show that the detectors are suitable for our specific application.     

Experimental Results of the Dichotomic Sampling in Circular Ring Positron Emission Tomograph

Experimental verification of the new dichotomic sampling scheme was attempted and applied to a circular ring PET system to improve the sampling thereby the overall system resolution and its performances. In the experiment, two different types of aperture collimators were adopted for high resolution (HR) and very high resolution (VHR) imaging. In HR mode, a resolution of 6.5 mm FWHM was obtained without appreciable degradation in overall sensitivity which represents a threefold resolution improvement over the original system. In phantom studies with HR mode a sensitivity of 4500 counts/sec/?Ci/cc was obtained for a 20 cm diameter uniform phantom filled with water. VHR mode experiment was also conducted to observe the ultimate resolution capability of DICHOTOM-I system and resolution of 4.2 mm FWHM was obtained at the expense of sensitivity reduction by a factor of four from HR mode experiment.     

Miniature Photomultiplier for Scintillation and Space Use

A rugged miniature photomultiplier has been developed which is only 8 mm in diameter. The electron multiplying element is a single channel multiplier. Photoelectrons from the 5 mm diameter cathode are electrostatically focused into the channel multiplier without need for a funnel or cathode-to-channel bias. This photomultiplier is a two-terminal device and uses no external resistors. The high gain characteristics (~108 at 3,000V), and the single-electron pulse height distribution (FWHM ?0.6), make the tube ideal for pulse-counting applications. The small diameter of this tube suggests applications in close-packed arrays and mosaics for scintillation imaging. This tube can be used as a scintillation spectum analyzer with good resolution even at high gains where there are greater than 109 anode electrons per scintillation event. The structure can also be employed in a windowless configuration as a particle or x-ray detector. In this modification apertures as large as 5 × 7 mm2 are possible. One use is as the detector in an Auger apparatus, where the high-gain output pulses allow precision digital data handling techniques and long drift-free integration time.     

A New Tomograph for Quantitative Positron Emission Computed Tomography of the Brain

A new system (NEUROECAT) has been designed and built for quantitative positron emission computed tomography (PCT) of the brain. It consists of three octagonal arrays of bismuth germanate detectors (BGO) providing 5 simultaneous image planes. The design utilizes unique shielding and detector geometries which optimize uniformity in image and axial resolution and also minimize scatter and accidental coincidence rates. The basic system image resolution is 11.0±.4 mm FWHM with a total system sensitivity of 270,000 counts/sec/microcurie/cc uniformly dispersed in a 20 cm diameter cylinder.     

Wafer-Bonded Silicon Gamma-Ray Detectors

A wafer-bonded silicon power transistor has been shown to function as an x-ray detector. The device consists of two thin device wafers bonded onto either side of a 2 mm-thick high-resistivity silicon wafer. The hydrophobic bonding process was performed at 400deg C. This low temperature wafer bonding technique should enable the development of large-area, position-sensitive detectors, using thick, high-resistivity intrinsic silicon bonded to thin readout wafers fabricated using conventional CMOS technology. These devices should enable fabrication of thicker intrinsic silicon detectors than currently available. Thick, position-sensitive detectors based on double-sided strip detectors and pixellated detectors are possible. To demonstrate this, a 1 mm thick gamma-ray detector was created from two 0.5 mm thick wafers that were patterned with gamma-ray strip detectors. The energy resolution of the detector is 8.9 keV FWHM for 60 keV gamma rays at room temperature with a leakage of 0.9 nA while operating at 700 V and fully depleted. Improvements in the technique should allow for thicker detectors with better energy resolution.     

A new thermal neutron detector for protein crystallography

A new position-sensitive detector is being developed for protein crystallography studies at a spallation source. Based on eight, independent, wire proportional chamber segments housed in a curved pressure vessel, the device covers a scattering angle of 120 degrees, and has a collecting area of 1.5 m by 20 cm. The position resolution will be about 1.3 mm FWHM, with a total counting rate in excess of one million per second. Timing resolution, essential for a spallation source application, is of order 1 μs and provides neutron energy determination that is well suited for crystallography. Advanced features of this device include a digital centroid finding scheme, a seamless readout between segments, and a wire array design that minimizes anode modulation. Details of the mechanical design are given, together with digital centroid measurements that illustrate accurate, uniform response     

Development of a compact high resolution gamma-ray detector for E-CT applications

A compact gamma-ray detector with good spatial resolution for emission computed tomography (E-CT) applications has been developed. The detector is composed of NaI(Tl) scintillation pixels array and Hamamastu R2486-05 PSPMT. Having a pixel size of 2 mm × 2 mm and an overall dimension of 48.2 mm × 48.2mm × 5 mm, it has 484 pixels in a 22×22 matrix. An average spatial resolution of 2.5mm (FWHM) was achieved. The slope of position linearity is constant within 10% in a range of 40mm. After corrections, the average value of differential non-linearity and absolute non-linearity were 0.16mm and 0.535mm respectively, and a 17% at FWHM of total energy resolution for 241Am was obtained.     

Study of PET Detector Performance with Varying SiPM Parameters and Readout Schemes

The spatial resolution performance characteristics of a monolithic crystal PET detector utilizing a sensor on the entrance surface (SES) design is reported. To facilitate this design, we propose to utilize a 2D silicon photomultiplier (SiPM) array device. Using a multi-step simulation process, we investigated the performance of a monolithic crystal PET detector with different data readout schemes and different SiPM parameters. The detector simulated was a 49.2mm by 49.2mm by 15mm LYSO crystal readout by a 12 by 12 array of 3.8mm by 3.8mm SiPM elements. A statistics based positioning (SBP) method was used for event positioning and depth of interaction (DOI) decoding. Although individual channel readout provided better spatial resolution, row-column summing is proposed to reduce the number of readout channels. The SiPM parameters investigated include photon detection efficiency (PDE) and gain variability between different channels; PDE and gain instability; and dark count noise. Of the variables investigated, the PDE shift of -3.2±0.7% and gain shift of -4±0.9% between detector testing and detector calibration had the most obvious impact on the detector performance, since it not only degraded the spatial resolution but also led to bias in positioning, especially at the edges of the crystal. The dark count noise also had an impact on the intrinsic spatial resolution. No data normalization is required for PDE variability of up to 12% FWHM and gain variability of up to 15% FWHM between SiPM channels. Based upon these results, a row-column summing readout scheme without data normalization will be used. Further, we plan to cool our detectors below room temperature to reduce dark count noise and to actively control the temperature of the SiPMs to reduce drifts in PDE and gain.     

Caliste 64, an Innovative CdTe Hard X-Ray Micro-Camera

A prototype 64 pixel miniature camera has been designed and tested for the Simbol-X hard X-ray observatory to be flown on the joint CNES-ASI space mission in 2014. This device is called Caliste 64. It is a high performance spectro-imager with event time-tagging capability, able to detect photons between 2 keV and 250 keV. Caliste 64 is the assembly of a 1 or 2 mm thick CdTe detector mounted on top of a readout module. CdTe detectors equipped with aluminum Schottky barrier contacts are used because of their very low dark current and excellent spectroscopic performance. Front-end electronics is a stack of four IDeF-X Vl.l ASICs, arranged perpendicular to the detection plane, to read out each pixel independently. The whole camera fits in a 10 times 10 times 20 mm³ volume and is juxtaposable on its four sides. This allows the device to be used as an elementary unit in a larger array of Caliste 64 cameras. Noise performance resulted in an ENC better than 60 electrons rms in average. The first prototype camera is tested at -10degC with a bias of -400 V. The spectrum summed across the 64 pixels results in a resolution of 697 eV FWHM at 13.9 keV and 808 eV FWHM at 59.54 keV.     

Measurement of HETP of SUS Dixon Ring and Porcelain Packing in Small-Scale Water Distillation Column for H2O-HTO Isotope Separation

Series of water distillations in a total reflux mode have been performed in a 100cm height column of 1.6cm I. D. in order to measure values of HETP for various packings, that is, one brass or three SUS Dixon rings and three porcelain packings. The HETP were measured by changing the vapor flow rate within the column. The SUS Dixon ring of 1.5 mm diameter and the porcelain packing of 1.2 mm O. D. had a small HETP (～5cm), but could not meet a large vapor flow rate because of a large pressure drop. The SUS Dixon ring of 6.0 mm diameter had a small HETP (～6cm) in the vapor flow rate under 2g/min, but the HETP value increased with increasing the vapor flow rate. The pressure drop for the ring, however, was almost constant in the range of these measurements.     

Schottky junctions on semi-insulating LEC gallium arsenide for X and γ-ray spectrometers operated at and below roomtemperature

This work deals with the study of a Schottky junction used as an X- and γ-ray detector in a spectrometer operated in the temperature range from -30°C to +22°C. The device (7 mm² active area and 100 μm thickness), fabricated on liquid encapsulated Czochralski (LEG) semi-insulating Gallium Arsenide, is designed with a noninjecting ohmic contact which allows biasing voltages up to 550 V. At room temperature (22°C) the energy resolution is found to be relatively poor (15.5-keV full-width at half-maximum (FWHM) at 59.5 keV) due to the large junction reverse current, whose density (7-37 nA/mm² at V_bias=100-500 V) is within the typical values for Schottky junctions on SI LEC GaAs. By cooling of the detector to -30°C, the noise of the reverse current is drastically lowered, thus achieving electronic noise levels around 160-180 rms electrons (1.6-1.8 keV FWHM), At 500-V bias, the ²⁴¹Am spectrum has been resolved down to an energy of 4 keV with charge collection efficiency of cce=97% and a resolution of about 2-keV FWHM for the Np L lines and 2.4-keV FWHM for the 59.5-keV γ photons. The linearity of the detector has been measured to be better than ±0.6% within the explored energy range (14-59 keV). From the experimental spectra, it has been analyzed how either the electronic noise or the trapping of the signal charge contribute to the energy resolution of the spectrometer. The result is that despite the high measured cce. The trapping gives a contribution higher than 1.5 keV FWHM for the 59.5-keV spectral line. A comparison between the experimental results and Monte Carlo simulations, based on the Hecht model of charge trapping in detectors, is shown to give a satisfactory justification of the observed phenomena. A total mean drift length of carriers has been experimentally derived, finding an exponential dependence upon the bias voltage applied to the detector     

一种高性能双维位置灵敏平行板雪崩探测器

本文描述了为兰州放射性离子束流线（RIBLL）研制的一种高性能平行板雪崩计数器（PPAC）。它由中心阳极和X、Y位置阴极构成，阳极间距3mm。阳极为1.5mm厚Mylar膜，双面镀金。阴极为φ25μm镀金钨丝，丝距1mm，位置读出采用电荷分除法。工作气体异丁烷。当工作气压为700Pa，阳极电压为500V时，对3组分α粒子位置分辨0.76mm(FWHM)，探测效率约99.1％。     

Monte Carlo optimization of depth-of-interaction resolution in PETcrystals

The light distribution along one edge of a PET (positron emission tomography) scintillation crystal was investigated by Monte Carlo simulation. This position-dependent light distribution can be used to measure the position of the 500-keV photon interaction, in the crystal on an event-by-event basis, thus reducing radial elongation. The predicted full width at half maximum (FWHM) of the light distribution on the 3×30 mm surface of a 3×10×30 mm bismuth germanate (BGO) crystal surrounded by diffuse reflector is 3.0 mm. This light distribution is constant for widths (originally 3 mm) between 1 and 6 mm, but decreases from 3.0 to 1.8 mm FWHM as the height is reduced from 10 to 3 mm. Other geometrical modifications were simulated, including corner reflectors on the opposing 3×30 mm surface. A promising geometry is a 2.2×5×30 mm BGO crystal, for which a 2.2 mm FWHM light distribution is predicted, which should yield a PET detector module with a depth of interaction measurement resolution of 3.6 mm FWHM     

A New Instrument Concept for Gamma Ray Astronomy

A new instrument concept that utilizes the principle of the Total Absorption Shower Cascade (TASC) detector is proposed for gamma ray astronomy in space. Using the TASC detector to measure the gamma ray energy and wire chamber arrays to measure the incident angle, an instrument package designed to measure gamma rays having energies greater than 100 MeV is being proposed for a large satellite such as NASA's proposed High Energy Astronomy Observatory. The proposed instrument has a minimal flux sensitivity of 2 × 10-10 photons/cm2 sr sec which will allow gamma rays with energies up to 104 GeV to be measured. An energy resolution (FWHM) for the TASC detector of ~l percent at 10 GeV has been measured and an angular resolution of ~1 degree appears to be obtainable for the wire chamber arrays.     

Large-area APDs and monolithic APD arrays

In this paper, development of large-area planar avalanche photodiodes (APDs) and monolithic APD arrays for X-ray and scintillation detection is discussed. Single APDs with areas as large as 10 cm² have been fabricated and tested with a CsI(Tl) scintillator (3.8 cm diameter, 2.5 cm height). The resolution of the 662 keV photopeak has been measured to be 9% (FWHM). The X-ray detection performance, gain, and noise of these large APDs have been characterized. Multielement APD arrays have also been fabricated in various formats, such as 4×4 to 14×14 elements (2 mm pixels), and the uniformity of gain, noise, and sensitivity has been evaluated for 4×4 arrays using an ⁵⁵Fe source. Timing properties have been measured. Packaging issues related to the APD arrays are discussed     

Pressure Dependence of the Relativistic Rise in Neon and Highest Attainable Ionization Sampling Resolution in Neon, Argon, Ethylene and Propane

A full-scale detector segment with 128 samples of 2 cm and a 50 cm drift space was used to measure precisely the relativistic rise of ionization from 5 to 70 GeV/c in Ne at 1, 2 and 4 atm. The pressure dependence of dE/dx resolution and of particle separation efficiency was determined at 15 GeV/c in several Ne/CnHm mixtures, in Ar/CH4, pure C2H4 and pure C3H8. The results are used for optimization of parameters of a compact colliding beam detector suitable for particle identification up to the 30-50 GeV/c momentum range. The ?/K/p separation is found to be better in Ne, but extends to higher momenta in Ar. Ar is more efficient for e/? separation. The gain in resolution at higher pressures is to certain extent compensated by reduction of the relativistic rise slope, so that the separation efficiency is reduced when the detector dimensions are scaled down with pressure. At 4-5 atm. we obtained slightly better resolution than the 6.0% FWHM (? = 2.5%) record performance of the EPI. In pure C3H8 a resolution below ? = 2.0% was reached. Measurements of drift velocities in various Ne mixtures are presented, showing saturated behaviour already at low E/p values.     

Clustering Method to Process Signals from a CdZnTe Detector

The poor mobility of holes in a compound semiconductor detector results in the imperfect collection of the primary charge deposited in the detector. Furthermore the fluctuation of the charge loss efficiency due to the change in the hole collection path length seriously degrades the energy resolution of the detector. Since the charge collection efficiency varies with the signal waveform, we can expect the improvement of the energy resolution through a proper waveform signal processing method. We developed a new digital signal processing technique, a clustering method which derives typical patterns containing the information on the real situation inside a detector from measured signals. The obtained typical patterns for the detector are then used for the pattern matching method. Measured signals are classified through analyzing the practical waveform variation due to the charge trapping, the electric field and the crystal defect etc. Signals with similar shape are placed into the same cluster. For each cluster we calculate an average waveform as a reference pattern. Using these reference patterns obtained from all the clusters, we can classify other measured signal waveforms from the same detector. Then signals are independently processed according to the classified category and form corresponding spectra. Finally these spectra are merged into one spectrum by multiplying normalization coefficients. The effectiveness of this method was verified with a CdZnTe detector of 2 mm thick and a ¹³⁷Cs gamma-ray source. The obtained energy resolution was improved to about 8 keV (FWHM). Because the clustering method is only related to the measured waveforms, it can be applied to any type and size of detectors and compatible with any type of filtering methods.