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Nuclear Science, IEEE Transactions on

Issue 1  Part 1 • Date Feb. 2012

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  • [Front cover]

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  • IEEE Transactions on Nuclear Science publication information

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  • Table of contents

    Page(s): 1 - 2
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  • Use of Cramer–Rao Lower Bound for Performance Evaluation of Different Monolithic Crystal PET Detector Designs

    Page(s): 3 - 12
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1519 KB) |  | HTML iconHTML  

    We have previously reported on continuous miniature crystal element (cMiCE) PET detectors that provide depth of interaction (DOI) positioning capability. A key component of the design is the use of a statistics-based positioning (SBP) method for 3D event positioning. The Cramer-Rao lower bound (CRLB) expresses limits on the estimate variances for a set of deterministic parameters. We examine the CRLB as a useful metric to evaluate the performance of our SBP algorithm and to quickly compare the best possible resolution when investigating new detector designs. In this work, the CRLB is first reported based upon experimental results from a cMiCE detector using a 50X50X15-mm3 LYSO crystal readout by a 64-channel PMT (Hamamatsu H8500) on the exit surface of the crystal. The X/Y resolution is relatively close to the CRLB, while the DOI resolution is more than double the CRLB even after correcting for beam diameter and finite X (i.e., reference DOI position) resolution of the detector. The positioning performance of the cMiCE detector with the same design was also evaluated through simulation. Similar with the experimental results, the difference between the CRLB and measured spatial resolution is bigger in DOI direction than in X/Y direction. Another simulation study was conducted to investigate what causes the difference between the measured spatial resolution and the CRLB. The cMiCE detector with novel sensor-on-entrance-surface (SES) design was modeled as a 49.2X49.2X15 mm3 LYSO crystal readout by a 12X12 array of 3.8X3.8-mm2 silicon photomultiplier (SiPM) elements with 4.1-mm center-to-center spacing on the entrance surface of the crystal. The results show that there are two main causes to account for the differences between the spatial resolution and the CRLB. First, Compton scatter in the crystal degrades the spatial resolution. The DOI resolution is degraded more than the X/Y resolution since small angle scatter is preferred. Secon- , our maximum likelihood (ML) clustering algorithm also has limitations when developing 3D look up tables during detector calibration. View full abstract»

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  • A High-Speed Multi-Channel Readout for SSPM Arrays

    Page(s): 13 - 18
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2049 KB) |  | HTML iconHTML  

    Solid-state photomultiplier (SSPM) arrays are a new technology that shows great promise to be used in PET detector modules. To reduce the number of channels in a PET scanner, it is attractive to use resistor dividers, which multiplex the number of channels in each module down to four analog output channels. It is also attractive to have SSPMs with large pixels (3×3 or 4×4 mm2). However, large area SSPMs have correspondingly large capacitances (up to 1 nF) and directly coupling them to a resistive network will create a low-pass filter with a high RC time constant. In order to overcome this, we have developed an application specific integrated circuit (ASIC) that “hides” the intrinsic capacitance of the SSPM array from a resistive network with current buffers, significantly improving the rise time of the SSPM signals when connected to the resistive network. The ASIC is designed for a wide range of SSPM sizes, up to 1 nF (equivalent to 4×4 mm2), and for input currents of 1 to 20 mA per channel. To accommodate various sizes of SSPM pixels, the ASIC uses adjustable current sources (to keep the feedback loop stable). A test ASIC has been fabricated that has 16 input channels, an internal resistor divider array that produces four analog outputs, 16 buffers that isolate the SSPM capacitance from the resistor array, and four output buffers that can drive 100 ohm loads. Thus, detector modules based on SSPMs and this ASIC should be compatible with the block detector readout electronics found in many PET cameras. Tests of this ASIC show that its rise time is <; 2 ns (and it will thus not significantly degrade the ~7 ns rise time of the SSPM pixels) and that the analog decoding circuitry functions properly. View full abstract»

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  • Towards High-Resolution X-Ray Imaging Using a Structured Scintillator

    Page(s): 19 - 23
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1732 KB) |  | HTML iconHTML  

    Structured scintillators, by light-guiding secondary emitted visible photons to a pixel in a CCD or CMOS image sensor, improve the lateral resolution of X-ray imaging detectors. In this work we have fabricated pore arrays in a silicon wafer and subsequently filled them with CsI(Tl) by a melting process. The goal was to down-scale the pore geometry for increased resolution. The results show that although pore depth must be reduced to comply with achievable aspect ratio of the Inductively Coupled Plasma (ICP) etching, melting into the pores is possible. The time and temperature has, however, to be optimized to prevent thallium loss during the melting. By correlating light yield measurements with the X-ray absorption in samples of various geometries, we find that the efficiency is slightly reduced for pore diameters approaching one micron. Nevertheless, the increased absorption in deep pores will lead to a significantly improved quantum efficiency compared to thin films currently used to achieve the same lateral resolution. View full abstract»

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  • Novel Point-Like ^{68}{\rm Ge}/^{68}{\rm Ga} Radioactive Source With Spherical Positron Absorber

    Page(s): 24 - 29
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (747 KB) |  | HTML iconHTML  

    Purpose: In the conventional techniques for determining the calibration factors of positron emission tomography (PET) scanners, cylindrical water and resin phantoms with radioisotope 18F or 68Ge/68Ga are used. In these methods, however, the results depend on attenuation and scatter correction. The purpose of this study is to develop a point-like 68Ge/68Ga radioactive source that can be used to determine the calibration factors of PET scanners without the uncertainty of attenuation and scatter correction. Methods: A spherical absorber design was employed to realize a symmetric angular distribution of emitted annihilation photons. A Geant4-based Monte Carlo simulation code was used to compare physics characteristics of point-like sources with various absorber materials. On the basis of this simulation, a point-like 68Ge/68Ga source with a spherical aluminum absorber was manufactured. Its radioactivity was calibrated at an accredited national calibration facility. A calibration factor of a clinical PET scanner was then obtained with a point-like source and compared with that obtained by a standard cross-calibration method. Results: The emission probability of 0.511 MeV annihilation photon pairs per positron decay was typically 0.6-0.8. The fraction of background photon pairs was 6-8% in the energy region of 0.4-0.6 MeV. Considering these two figures, lower density materials such as aluminum and pol(ymethyl methacrylate) (PMMA) were preferable. For the aluminum absorber, a diameter of 8 mm was suitable to prevent positrons from escaping. The calibration factor obtained with the point-like source agreed with that obtained by the standard method within 2-3%. Conclusion: A point-like 68Ge/68Ga radioactive source was successfully designed, manufactured, and used for determining a calibration factor of a PET scanner. It can be considered a practical tool for calibrating and ev- luating the quantitative aspects of PET scanners. View full abstract»

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  • A High-Rate Energy-Resolving Photon-Counting ASIC for Spectral Computed Tomography

    Page(s): 30 - 39
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    We describe a high-rate energy-resolving photon-counting ASIC aimed for spectral computed tomography. The chip has 160 channels and 8 energy bins per channel. It demonstrates a noise level of ENC =214 electrons at 5 pF input load at a power consumption of <; 5 mW/channel. Maximum count rate is 17 Mcps at a peak time of 40 ns, made possible through a new filter reset scheme, and maximum read-out frame rate is 37 kframe/s. View full abstract»

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  • Real-Time Imaging System for the OpenPET

    Page(s): 40 - 46
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1020 KB) |  | HTML iconHTML  

    The OpenPET and its real-time imaging capability have great potential for real-time tumor tracking in medical procedures such as biopsy and radiation therapy. For the real-time imaging system, we intend to use the one-pass list-mode dynamic row-action maximum likelihood algorithm (DRAMA) and implement it using general-purpose computing on graphics processing units (GPGPU) techniques. However, it is difficult to make consistent reconstructions in real-time because the amount of list-mode data acquired in PET scans may be large depending on the level of radioactivity, and the reconstruction speed depends on the amount of the list-mode data. In this study, we developed a system to control the data used in the reconstruction step while retaining quantitative performance. In the proposed system, the data transfer control system limits the event counts to be used in the reconstruction step according to the reconstruction speed, and the reconstructed images are properly intensified by using the ratio of the used counts to the total counts. We implemented the system on a small OpenPET prototype system and evaluated the performance in terms of the real-time tracking ability by displaying reconstructed images in which the intensity was compensated. The intensity of the displayed images correlated properly with the original count rate and a frame rate of 2 frames per second was achieved with average delay time of 2.1 s. View full abstract»

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  • Detecting Small Liver Tumors With ^{111} In-Pentetreotide SPECT—A Collimator Study Based on Monte Carlo Simulations

    Page(s): 47 - 53
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1003 KB) |  | HTML iconHTML  

    In 111In-pentetreotide single-photon emission computed tomography (SPECT), the tumor-to-background-uptake ratio is generally high. The noise is, however, also usually on a high level, and in combination with the low spatial resolution of SPECT, this may lead to difficulties in the detection of small tumors. This is especially the case in regions with a relatively high background activity, such as in the liver, which is a common region for somatostatin-positive metastases. Visually detecting the small tumors is important for a successful treatment of the cancer disease. In this paper, we compare three different parallel-hole collimators for 111In-pentetreotide SPECT regarding contrast as a function of image noise for a phantom simulating small tumors in liver background. The corresponding contrast-to-noise ratios are also presented. All raw-data projections are produced using Monte Carlo simulations. The collimators are of type low-energy general-purpose (LEGP), extended LEGP (ELEGP), and medium-energy general-purpose (MEGP). Reconstructions were performed with OSEM both with and without model-based compensation. Of the investigated collimators, the ELEGP collimator proved to be the most optimal for the smallest tumors, both with and without model-based compensation included in the reconstruction. It is also shown that model-based compensation outperforms the conventional reconstruction technique. View full abstract»

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  • X-Ray Fluorescence Imaging With the Medipix2 Single-Photon Counting Detector

    Page(s): 54 - 61
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    Material-resolved X-ray imaging or colour X-ray imaging is of a great interest for many applications ranging from physics, industry to medicine and biology. X-ray fluorescence offers a method for producing such images if the energies and positions of origin of the fluorescent photons can be adequately resolved. View full abstract»

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  • Finding Optimized Conditions of Slit-Slat and Multislit-Slat Collimation for Breast Imaging

    Page(s): 62 - 69
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1221 KB) |  | HTML iconHTML  

    In order to develop a breast-imaging system for Single Photon Emission Computed Tomography (SPECT) using slit-slat and multislit-slat collimators, we searched for optimized geometric parameters of the collimators. For this study, we employed two independent metrics to validate each result: 1)Signal to Noise Ratio (SNR) based on the Cramer-Rao lower Bound (CRB) and 2) contrast at the same noise level from an ensemble. We calculated SNR values using forward-projection data of an anthropomorphic digital phantom containing two lesions in the breast (one at the chest wall and the other at the center) with a simulated slit-slat collimator as a function of the collimator's geometric parameters. We also calculated contrast values from reconstructed images with noise. Based on the results from the slit-slat case, we investigated angular range, SNR, and contrast for the multislit-slat. We saw similar trends of the two metrics. One interesting property of the multislit-slat is that the imaging performance depends on the orientation of the field of view (FOV) of the side slits. When we compared the metric values for the slit-slat and multislit-slat, improvement was seen only when the lesion was in the FOV of the side slits. Therefore, tuning the parameters of the multislit-slat to optimally detect lesions at the chest wall might be a sensible option since the slit-slat already provides good image quality for center and superficial lesions. View full abstract»

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  • Demonstration of in-vivo Multi-Probe Tracker Based on a Si/CdTe Semiconductor Compton Camera

    Page(s): 70 - 76
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    By using a prototype Compton camera consisting of silicon (Si) and cadmium telluride (CdTe) semiconductor detectors, originally developed for the ASTRO-H satellite mission, an experiment involving imaging multiple radiopharmaceuticals injected into a living mouse was conducted to study its feasibility for medical imaging. The accumulation of both iodinated (131I) methylnorcholestenol and 85Sr into the mouse's organs was simultaneously imaged by the prototype. This result implies that the Compton camera is expected to become a multi-probe tracker available in nuclear medicine and small animal imaging. View full abstract»

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  • An Experiment-Based Approach for Predicting Positron Emitter Distributions Produced During Therapeutic Ion Irradiation

    Page(s): 77 - 87
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1307 KB) |  | HTML iconHTML  

    In-beam positron emission tomography (PET) is a valuable method for a beam-delivery independent dose monitoring in radiation therapy with ion beams. Up to now, its clinical feasibility has been demonstrated for patient irradiation with carbon ions. From radiobiological point of view it is highly desirable to perform tumor irradiation also with other light ions. To extend the application of in-beam PET also to these ions, extensive knowledge about positron emitter production via nuclear fragmentation reactions during ion irradiation is necessary. To model the positron emitter production correctly, cross sections for all possible nuclear reactions occurring in the tissue during irradiation and leading to positron emitters are required. Since these cross sections are available only for a few reaction channels in the required energy range, a novel approach for estimating the positron emitter production from experimental data is introduced. The prediction of positron emitter distributions is based on depth dependent thick target yields, which are derived by linear superposition of measured yields in water, graphite and polyethylene as reference materials. Results on the prediction of positron emitter distributions in polymethyl methacrylate (PMMA) as well as inhomogeneous targets induced by lithium and carbon irradiation are presented. By comparison with data deduced from experiments, it is shown that a rather accurate prediction of positron emitter distribution is feasible using this method. View full abstract»

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  • Evaluation of a New Regularization Prior for 3-D PET Reconstruction Including PSF Modeling

    Page(s): 88 - 101
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (4248 KB) |  | HTML iconHTML  

    The response of a PET system can be described by its characteristic Point Spread Function (PSF) representing the spatial degradation of a point source due to physical effects and system design. If the PSF is accounted for in the reconstruction algorithm, better image quality and spatial resolution may be achieved. Un- fortunately, a common behaviour of unregularized iterative reconstruction techniques is represented by the increase of noise as the iterations proceed, while-on the other hand-a high number of iterations is usually needed to recover a significant percentage of the signal and to reach convergence, especially when resolution modelling is used in the reconstruction to recover the degraded signal. Moreover, a recognized effect of PSF-based reconstructions is the overenhancement of sharp transitions (edges) in the reconstructed images. In an attempt to solve both these problems, regularization strategies can be employed: a) to control the noise amplification as the iterations proceed and b) to reduce the edge overenhancement effect. In this work, a new prior for variational Maximum a posteriori regularization is proposed to be used in a 3-D One-Step-Late (OSL) reconstruction algorithm which also accounts for the PSF of the PET system. The new regularization prior is characterized by a strong smoothing component for regions in the image with a magnitude of the gradient below a given threshold (set to discriminate between background and signal), while preserving transitions above this threshold. The new algorithm has been validated on phantom and clinical data. The results showed that the use of the proposed regularization prior allows: a) a better control of the noise compared to un- regularized reconstructions, while maintaining high enough signal recovery thanks to the PSF action, and b) the control and reduction of the edge overenhancement, with a contemporary good preservation of spatial resolution. View full abstract»

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  • Automatic Detection of Lung and Liver Lesions in 3-D Positron Emission Tomography Images: A Pilot Study

    Page(s): 102 - 112
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (758 KB) |  | HTML iconHTML  

    Positron emission tomography (PET) using fluorine-18 deoxyglucose (18F-FDG) has become an increasingly recommended tool in clinical whole-body oncology imaging for the detection, diagnosis, and follow-up of many cancers. One way to improve the diagnostic utility of PET oncology imaging is to assist physicians facing difficult cases of residual or low-contrast lesions. This study aimed at evaluating different schemes of computer-aided detection (CADe) systems for the guided detection and localization of small and low-contrast lesions in PET. These systems are based on two supervised classifiers, linear discriminant analysis (LDA) and the nonlinear support vector machine (SVM). The image feature sets that serve as input data consisted of the coefficients of an undecimated wavelet transform. An optimization study was conducted to select the best combination of parameters for both the SVM and the LDA. Different false-positive reduction (FPR) methods were evaluated to reduce the number of false-positive detections per image (FPI). This includes the removal of small detected clusters and the combination of the LDA and SVM detection maps. The different CAD schemes were trained and evaluated based on a simulated whole-body PET image database containing 250 abnormal cases with 1230 lesions and 250 normal cases with no lesion. The detection performance was measured on a separate series of 25 testing images with 131 lesions. The combination of the LDA and SVM score maps was shown to produce very encouraging detection performance for both the lung lesions, with 91% sensitivity and 18 FPIs, and the liver lesions, with 94% sensitivity and 10 FPIs. Comparison with human performance indicated that the different CAD schemes significantly outperformed human detection sensitivities, especially regarding the low-contrast lesions. View full abstract»

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  • Realistic and Efficient Modeling of Radiotracer Heterogeneity in Monte Carlo Simulations of PET Images With Tumors

    Page(s): 113 - 122
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2046 KB) |  | HTML iconHTML  

    Monte Carlo simulations are extensively used in PET to evaluate the accuracy with which PET images can yield reliable estimates of parameters of interest. For such applications, the simulated images should be as realistic as possible so that conclusions can be extrapolated to clinical PET images. In this work, we describe a method for introducing realistic modeling of radiotracer heterogeneity into Monte Carlo simulations of patient PET scans. The modeling of the complex physiological activity distribution in healthy regions is directly based on real patient PET/CT images, and realistic tumor shapes can be included into these regions. This method represents a competitive alternative to the use of complex anthropomorphic phantoms such as the XCAT, that require a fixed activity per structure. The method is extended to the simulation of serial PET scans with tumor changes, as acquired in the context of therapy monitoring, and this extension is validated using a patient study. Using the proposed method, very realistic patient PET images can be produced for evaluation purposes.In addition, a strategy to efficiently simulate many sets of pathological cases, based on a unique background physiological activity distribution, is described and carefully assessed using a numerical phantom. The background activity is simulated only once, while tumors are simulated separately. The data are then recombined in a specific way so that the final image has the same properties as images produced by simulating pathological and tumor activities at the same time. View full abstract»

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  • Evaluation of the Accuracy and Robustness of a Motion Correction Algorithm for PET Using a Novel Phantom Approach

    Page(s): 123 - 130
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    We introduce the use of a novel physical phantom to quantify the performance of a motion-correction algorithm. The goal of the study was to assess a PET-PET image registration, the final output of which is a motion-corrected high-count PET image volume, a procedure called Reconstruct, Register and Average (RRA). Methods: A phantom was constructed using 5 ~ 2 mL Ge-68 filled spheres suspended in a water-filled tank via lightweight fishing line and driven by a periodic motion. Comparison of maximum and mean activity concentration and sphere volume was performed. Ground truth data were measured using “no motion.” With motion, five replicate datasets of 3-minute phase-gated data for each of three different periods of motion were acquired. Gated PET images were registered using a multi-resolution level-sets-based non-rigid registration (NRR). The NRR images were then averaged to form a motion-corrected, high-count image volume. Spheres from all images were segmented and compared across the imaging conditions. Results: The average center-of-mass range of motion was 7.35, 5.83, and 2.66 mm for the spheres over the three periods of 8, 6 and 4 seconds. The center-of-mass for all spheres in all conditions was corrected to within 1 mm on average using NRR as compared to the gated data. For the RRA data, the sphere maximum activity concentration (MAC) was on average 40.2% higher (-4.0% to 116.7%) and sphere volume was on average 12.0% smaller (-8.2% to 28.1%) as compared to the ungated data with motion. The RRA results for MAC were on average 70% more accurate and for sphere volume 80% more accurate as compared to the ungated data. Conclusions: The results show that the novel phantom setup and analysis methods are a promising evaluation technique for the assessment of motion correction algorithms. Benefits include the ability to compare against ground truth data without motion but with control of the statistical data quality and background variability. Use of a- non-moving object adjacent to spheres in motion, the spatial extent of the motion correction algorithm was confirmed to be local to the induced motion and to not affect the stationary object. View full abstract»

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  • 2012 IEEE membership form

    Page(s): 131 - 132
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  • IEEE Transactions on Nuclear Science information for authors

    Page(s): C3
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  • Affiliate Plan of the IEEE Nuclear and Plasma Sciences Society

    Page(s): C4
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IEEE Transactions on Nuclear Science focuses on all aspects of the theory and applications of nuclear science and engineering, including instrumentation for the detection and measurement of ionizing radiation; particle accelerators and their controls; nuclear medicine and its application; effects of radiation on materials, components, and systems; reactor instrumentation and controls; and measurement of radiation in space.

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