Position-sensitive Gamma-ray Detector Research Articles

Extended sources reconstructions by means of coded mask aperture systems and deep learning algorithm

Diagnostics and monitoring of radiological scenes are critical to the field of nuclear safety and here, the localization of radioactive hotspots is mandatory and remains a critical challenge. In order to perform gamma-ray imaging, one main method relies on indirect imaging by means of coded mask aperture associated with a position sensitive gamma-ray detector and a dedicated deconvolution algorithm. However, the deconvolution problem is non-injective, which implies limitations of the reconstruction performance, especially for spatially extended radioactive sources with respect to the angular resolution. In this paper, we present and evaluate a new method based on a deep learning algorithm with a convolutional neural network to overcome this limitation, in comparison with a classical iterative algorithm. Our deep learning algorithm is trained on simulated data of extended sources that may imply an intrinsic regularization of the neural network. We test it on real data acquired with a gamma camera system based on Caliste, a CdTe detector for high-energy photons.

Study of a small scale position-sensitive scintillator detector for γ-ray spectroscopy

Precision measurement of the energy and direction of gamma-rays plays a key role in many fields such as medical imaging, nuclear spectroscopy, and astrophysics. In astrophysics position sensitive gamma-ray detectors are used to study diffuse and point-like sources. These space-born gamma-ray telescopes, however, are mainly based on large arrays of detectors which are expensive, complex, and take long time to build. A different approach is the use of detector with similar energy and angular resolution, but small enough to be deployed on nanosatellites. We research a design based on monolithic scintillator and multi-pixel photo-sensor. At first we are focused on characterization of CeBr3 scintillator detectors. Bench test experiments were performed with CeBr3 crystal with square surfaces and thicknesses of 10 mm and 25 mm. The crystals were coupled to a single-anode PMT and the outcome compared to Geant4 simulation.

A novel and fast technique for evaluation of plastic rod scintillators as position sensitive gamma-ray detectors using artificial neural networks

The problem of how to precisely estimate the radiation interaction position is an important parameter in medical and industrial imaging systems. This paper presents a new methodology for prediction of the incident position of the gamma rays based attenuation technique and multilayer perceptron (MLP) neural network system. The detection system is comprised of a gamma-ray source (60Co or 137Cs) and a plastic rod scintillator (BC400) coupled with just one PMT at one side. The experimental setup provides the required data for training and testing the network. Using this proposed method, the radiation interaction position was predicted in plastic rod scintillator with a mean absolute error less than 0.8 and 0.5 for 137Cs and 60Co sources, respectively. The mean relative error percentage was calculated less than 3.9% and 3.5%. Also, the correlation coefficient was measured 0.999 and 0.998, respectively. The results showed that the predicted interaction position using the ANN method is in good agreement with real data. The techniques and set up used in the previous position sensitive detectors were fairly complicated whereas the new set-up and proposed method are really very simple. Also, the radiation safety, cost and shielding, and electronics requirements are minimized and optimized.

Technical aspects of real time positron emission tracking for gated radiotherapy.

Respiratory motion can lead to treatment errors in the delivery of radiotherapy treatments. Respiratory gating can assist in better conforming the beam delivery to the target volume. We present a study of the technical aspects of a real time positron emission tracking system for potential use in gated radiotherapy. The tracking system, called PeTrack, uses implanted positron emission markers and position sensitive gamma ray detectors to track breathing motion in real time. PeTrack uses an expectation-maximization algorithm to track the motion of fiducial markers. A normalized least mean squares adaptive filter predicts the location of the markers a short time ahead to account for system response latency. The precision and data collection efficiency of a prototype PeTrack system were measured under conditions simulating gated radiotherapy. The lung insert of a thorax phantom was translated in the inferior-superior direction with regular sinusoidal motion and simulated patient breathing motion (maximum amplitude of motion ±10 mm, period 4 s). The system tracked the motion of a (22)Na fiducial marker (0.34 MBq) embedded in the lung insert every 0.2 s. The position of the was marker was predicted 0.2 s ahead. For sinusoidal motion, the equation used to model the motion was fitted to the data. The precision of the tracking was estimated as the standard deviation of the residuals. Software was also developed to communicate with a Linac and toggle beam delivery. In a separate experiment involving a Linac, 500 monitor units of radiation were delivered to the phantom with a 3 × 3 cm photon beam and with 6 and 10 MV accelerating potential. Radiochromic films were inserted in the phantom to measure spatial dose distribution. In this experiment, the period of motion was set to 60 s to account for beam turn-on latency. The beam was turned off when the marker moved outside of a 5-mm gating window. The precision of the tracking in the IS direction was 0.53 mm for a sinusoidally moving target, with an average count rate ∼250 cps. The average prediction error was 1.1 ± 0.6 mm when the marker moved according to irregular patient breathing motion. Across all beam deliveries during the radiochromic film measurements, the average prediction error was 0.8 ± 0.5 mm. The maximum error was 2.5 mm and the 95th percentile error was 1.5 mm. Clear improvement of the dose distribution was observed between gated and nongated deliveries. The full-width at halfmaximum of the dose profiles of gated deliveries differed by 3 mm or less than the static reference dose distribution. Monitoring of the beam on/off times showed synchronization with the location of the marker within the latency of the system. PeTrack can track the motion of internal fiducial positron emission markers with submillimeter precision. The system can be used to gate the delivery of a Linac beam based on the position of a moving fiducial marker. This highlights the potential of the system for use in respiratory-gated radiotherapy.

Comparison of LSO and BGO block detectors for prompt gamma imaging in ion beam therapy

A major weakness of ion beam therapy is the lack of tools for verifying the particle range in clinical routine. The application of the Compton camera concept for the imaging of prompt gamma rays, a by-product of the irradiation correlated to the dose distribution, is a promising approach for range assessment and even three-dimensional in vivo dosimetry. Multiple position sensitive gamma ray detectors arranged in scatter and absorber planes, together with an imaging algorithm, are required to reconstruct the prompt gamma emission density map. Conventional block detectors deployed in Positron Emission Tomography (PET), which are based on Lu2SiO5:Ce (LSO) and Bi4Ge3O12 (BGO) scintillators, are suitable candidates for the absorber of a Compton camera due to their high density and absorption efficiency with respect to the prompt gamma energy range (several MeV). We compare experimentally LSO and BGO block detectors in clinical-like radiation fields in terms of energy, spatial and time resolution. The high energy range compensates for the low light yield of the BGO material and boosts significantly its performance compared to the PET scenario. Notwithstanding the overall superiority of LSO, BGO catches up in the field of prompt gamma imaging and can be considered as a competitive alternative to LSO for the absorber plane due to its lower price and the lack of intrinsic radioactivity.

Test of Compton camera components for prompt gamma imaging at the ELBE bremsstrahlung beam

In the context of ion beam therapy, particle range verification is a major challenge for the quality assurance of the treatment. One approach is the measurement of the prompt gamma rays resulting from the tissue irradiation. A Compton camera based on several position sensitive gamma ray detectors, together with an imaging algorithm, is expected to reconstruct the prompt gamma ray emission density map, which is correlated with the dose distribution. At OncoRay and Helmholtz-Zentrum Dresden-Rossendorf (HZDR), a Compton camera setup is being developed consisting of two scatter planes: two CdZnTe (CZT) cross strip detectors, and an absorber consisting of one Lu2SiO5 (LSO) block detector. The data acquisition is based on VME electronics and handled by software developed on the ROOT framework.The setup has been tested at the linear electron accelerator ELBE at HZDR, which is used in this experiment to produce bunched bremsstrahlung photons with up to 12.5 MeV energy and a repetition rate of 13 MHz. Their spectrum has similarities with the shape expected from prompt gamma rays in the clinical environment, and the flux is also bunched with the accelerator frequency.The charge sharing effect of the CZT detector is studied qualitatively for different energy ranges. The LSO detector pixel discrimination resolution is analyzed and it shows a trend to improve for high energy depositions.The time correlation between the pulsed prompt photons and the measured detector signals, to be used for background suppression, exhibits a time resolution of 3 ns FWHM for the CZT detector and of 2 ns for the LSO detector. A time walk correction and pixel-wise calibration is applied for the LSO detector, whose resolution improves up to 630 ps. In conclusion, the detector setup is suitable for time-resolved background suppression in pulsed clinical particle accelerators. Ongoing tasks are the quantitative comparison with simulations and the test of imaging algorithms. Experiments at proton accelerators have also been performed and are currently under analysis.

Proximity Electrode Signal Readout of High-Purity Ge Detectors

Proximity electrode signal readout of semiconductor- based radiation detectors is accomplished by measuring the charge induced on electrodes positioned close, but not electrically connected, to the detector material. This technology can be applied to high-purity Ge (HPGe) to create a position sensitive gamma-ray detector. The proximity readout technique offers the advantages of simplified detector fabrication, expanded electrode geometry options, and greatly improved position resolution through simple signal interpolation. We have produced small, HPGe prototype detectors that utilize strip proximity electrodes for gamma-ray interaction energy and position measurement. With these detectors we have collected event data sets under a variety of detector and source conditions. With this data we have explored energy and position reconstruction methods and have demonstrated sub-strip-pitch position determination. In this paper, we summarize the results of our investigation.

Source Detection Performance Prediction for a CdZnTe Array

The complex system response of 3D position-sensitive gamma-ray detectors complicates modeling the recorded measurements and makes exact expressions for detection performance intractable. This makes source detection performance difficult and expensive to compute. Asymptotic analysis has the potential to simplify detection performance prediction with complex systems and has previously been applied to detection performance prediction with simulated gamma-ray detectors. In this work, we use asymptotic performance prediction methods to predict points on the receiver operating characteristic (ROC) curve for the illustrative task of detecting a 137Cs source in background with an 18-detector CdZnTe array. We assume that the source position, source energy, background spectrum, and background spatial distribution are known. Although these assumptions are not always valid in practice, the efficiency of the prediction method makes it attractive for detection system design problems. Our results show that the asymptotic performance prediction method accurately predicts the empirically observed performance with real data recorded with a real system. Our results also characterize the performance of the detector array for the task of source detection. The accuracy and computational efficiency of the asymptotic detection performance prediction method make it a viable alternative to empirical performance evaluation.

Comparison of two position sensitive gamma-ray detectors based on continuous YAP and pixellated NaI(TI) for nuclear medical imaging applications**Supported by National Nature Science Foundation of China (10275063)

Dedicated position sensitive gamma-ray detectors based on position sensitive photomultiplier tubes (PSPMTs) coupled to scintillation crystals, have been used for the construction of compact gamma-ray imaging systems, suitable for nuclear medical imaging applications such as small animal imaging and single organ imaging and scintimammography. In this work, the performance of two gamma-ray detectors: a continuous YAP scintillation crystal coupled to a Hamamastu R2486 PSPMT and a pixellated NaI(TI) scintillation array crystal coupled to the same PSPMT, is compared. The results show that the gamma-ray detector based on a pixellated NaI(TI) scintillation array crystal is a promising candidate for nuclear medical imaging applications, since their performance in terms of position linearity, spatial resolution and effective field of view (FOV) is superior than that of the gamma-ray detector based on a continuous YAP scintillation crystal. However, a better photodetector (Hamamatau H8500 Flat Panel PMT, for example) coupled to the continuous crystal is also likely a good selection for nuclear medicine imaging applications.

A compact gamma-ray detector using wavelength-shifting fibers coupled to YAP scintillation crystal

The performance of a compact position sensitive gamma-ray detector based on wavelength-shifting fibers coupled to YAIO 3:Ce scintillation crystal was evaluated using a Monte-Carlo simulation method. The simulation model has been setup using the GEANT4 codes. Compared with the gamma-ray detector based on the YAIO 3:Ce scintillation crystal coupled to Hamamastu R2486 position sensitive photomultiplier tube, the results indicate that the gamma-ray detector based on wavelength-shifting fibers readout has good position linearity, good spatial resolution and larger effective field of view. The image and point spread function of measured point were presented. The spatial resolution response as a function of position was obtained. The factors influencing spatial resolution and position linearity were discussed.

Determination of spatial resolution of plastic scintillation fiber array with a simple method

The spatial resolution of a position sensitive gamma-ray detector configuration based on plastic scintillation fiber array was measured using a Monte Carlo simulation method. Both point spread function and modulation transfer function (MTF) were presented. The factors that influence the spatial resolution were also discussed. The results of the simulation showed that the intrinsic spatial resolution was consistent with the size of the physical pixels and a few centimeters spatial resolution could be obtained under certain circumstances.

An Advanced Preamplifier for Highly Segmented Germanium Detectors

We present a fast low-noise hybrid charge-sensitive preamplifier for germanium position-sensitive gamma-ray detectors. In conjunction with a bulky 36-fold segmented detector it provided an excellent resolution of 1.71/1.77 keV fwhm on the 1.17/1.33MeV60Co lines. The preamplifier rise time, as measured at the test bench, is as fast as 7.5 ns, with a detector capacitance of 21 pF and with a 5 m 50 Omega twisted-pair cable connected at its output. The dynamic range of the preamplifier input stage is as large as 92 dB, ranging from 0.275 fC to 9.9 pC, i.e., from 5 keV to 180 MeV in terms of photon energy. On signals larger than 2 MeV a fast reset (~ 10 MeV/mus) is enforced to reduce the system dead time. An estimate of the amplitude of such large signals is derived from the reset time, still obtaining a high resolution. Using this technique we achieved an energy resolution of 0.3% at 16.7 MeV

Monte Carlo simulation of a position sensitive gamma ray detector

The Monte Carlo method was employed to obtain the response of a position sensitive gamma ray detector. The basic unit consists of a scintillator bar coupled to two photon detectors. Determination of the position of interaction is obtained from the ratio of the light intensity measured at each extremity of the bar. Association of two or more detectors of this type can be used to build a system with potential use in positron emission tomography. For practical purposes, a CsI(Tl) bar with dimensions of 15×1×1 \thinspacecm³ was considered. Simulations were performed with the GEANT4 toolkit, using the physical processes of low-energy electromagnetic extension. Light attenuation coefficient and energy resolution of CsI(Tl) bars were included according to data found in the literature. Results obtained from detector irradiation with 511 keV annihilation photons indicate that the position of interaction can be determined with uncertainty of 3 mm. Simulations of positron sources placed between two detectors were also performed and two dimensional images were obtained.

Feasibility study for a low-cost 3D gamma-ray camera

A novel gamma-ray camera is proposed based on Positron Annihilation Compton Scattering Imaging (PACSI). The camera comprises a positron source and a fast, position-sensitive gamma-ray detector. Monte Carlo simulations of the camera have demonstrated that it should be capable of forming 3-D images of objects that can only be viewed from one side. A simple 1-D PACSI camera has been constructed and tested in the laboratory. Prospects for the construction of a full 3-D prototype camera are discussed.

Pulse processing for the PET liquid xenon multiwire ionisation chamber

The readout and data acquisition systems designed for a first prototype of liquid xenon position sensitive gamma ray detector for Positron Emission Tomography is described, The problem of measurement of the gamma ray energy in the gridless multiwire ionisation chamber is considered. A previously suggested drift time based correction algorithm, allowing correction of the amplitude spectra for the contribution of positive ions, is experimentally tested. First ionisation spectra, with an energy resolution of 15-17%, are presented. A pulse shape discrimination method for rejecting the induction cross-talk between the wires, improving energy resolution and allowing the unambiguous identification of Compton scattered events in the detector, is described.

Effect of refraction index and thickness of the light guide in the position-sensitive gamma-ray detector using compact PS-PMTs

We constructed a position-sensitive gamma-ray detector consisting of an array of BGO scintillators, a light guide and compact PS-PMTs. The effects of refractive index and thickness of the light guide of a glass plate on the detector performance were investigated. A light guide with higher refractive index and smaller thickness is found better for a good spatial resolution.

A method for correcting the depth-of-interaction blurring in PET cameras

A method is presented for correcting PET images for the blurring caused by variations in the depth-of-interaction in position-sensitive gamma ray detectors. The method uses an empirically determined, tabulated relationship between depth-of-interaction and most probable pulse-height to estimate the unknown depth from the measured pulse-height for each detected gamma ray. In the case of one fine-cut 50x50x30 mm BGO block detector, the method is shown to improve the detector resolution by 25%, averaged over the 50x50 mm face, measured in the geometry corresponding to detection at the edge of the field-of-view. Strengths and weaknesses of the method are discussed and its potential usefulness for improving the images of future PET cameras is assessed.

Data acquisition for a medical imaging MWPC detector

Multiwire proportional chambers, combined with drilled Pb converter stacks, are used as position sensitive gamma-ray detectors for medical imaging at Queen's University. This paper describes novel features of the address readout and data acquisition system. To obtain the interaction position, induced charges from wires in each cathode plane are combined using a three-level encoding scheme into 16 channels for amplification and discrimination, and then decoded within 150 ns using a lookup table in a 64 Kbyte EPROM. A custom interface card in an AT-class personal computer provides handshaking, rate buffering, and diagnostic capabilities for the detector data. Real-time software controls the data transfer and provides extensive monitor and control functions. The data are then transferred through an Ethernet link to a workstation for subsequent image analysis.

Design of an efficient position sensitive gamma ray detector for nuclear medicine

The shortcomings of conventional scintillation cameras are analysed theoretically with a view towards improving performance at gamma ray energies above 140 keV. A camera design is proposed which incorporates several new features to obtain good spatial resolution from thicker crystals of sodium iodide. Computer simulations show that in addition to having good efficiency and spatial resolution, the new design allows parallax error correction and (possibly) Compton scattering correction at gamma energies up to 511 keV.

Position-Sensitive Detectors for Shielding Tests of Spent Fuel Shipping Casks

A system for measuring spatial distributions of dose rates around the surface of shipping casks is described. A He-3 counter surrounded by a 30 mm thick moderator is used as a position-sensitive neutron detector based on the charge division method. A one-dimensional array of ten NaI(Tl) scintillation counters is used as a position-sensitive gamma-ray detector.