基于广义Gibbs先验的优质PET成像

 最大后验方法(Maximum a posteriori, MAP)已经广泛应用于解决图像重建中的病态问题,正电子发射成像(Positron emission tomography, PET)便是其中之一。本文基于MAP方法,针对PET成像提出一新的基于图像相似结构信息的广义Gibbs先验形式,新先验能在有效地抑制噪声的同时,鲁棒地保持锐利的边缘信息。但由于新先验的引入,使得重建模型的求解趋于复杂。为解决模型解的收敛性问题,我们提出两步式的局部线化优化迭代重建策略,并结合抛物线替代坐标上升(Paraboloidal surrogate coordinate ascent,PSCA)算法进行求解。新算法分别对PET模拟数据和真实数据进行重建实验,结果表明本文提出的基于广义Gibbs先验的PET成像可以获得优质的重建图像。     

PET成像前端读出芯片研究进展

在过去的半个多世纪里,虽然正电子发射断层(PET)成像设备在外型上没有多大变化,但在技术和方法上发生了多次革命性的飞跃。微电子技术在PET成像领域的应用将进一步推动PET向更小体积、更高性能、更低成本等方向发展。从PET系统成像原理出发,详细综述了PET前端读出芯片技术的研究进展。将PET探测器信号的前端读出和信号处理分为光电转换、信号采集、脉冲高度分析、峰值探测和保持、信号数字化和数字信号处理等环节,给出了各个环节的微电子电路实现。然后,描述了PET前端读出大规模专用集成电路的研发进展,指出采用数字电路的方法来处理探测器前端读出和模拟信号处理已经成为PET前端电子的发展趋势,而且集成PET专用DSP的多通道智能前端读出电路已经成为一个重要的方向。     

The Integration of Positron Emission Tomography With Magnetic Resonance Imaging

A number of laboratories and companies are currently exploring the development of integrated imaging systems for magnetic resonance imaging (MRI) and positron emission tomography (PET). Scanners for both preclinical and human research applications are being pursued. In contrast to the widely distributed and now quite mature PET/computed tomography technology, most PET/MRI designs allow for simultaneous rather than sequential acquisition of PET and MRI data. While this offers the possibility of novel imaging strategies, it also creates considerable challenges for acquiring artifact-free images from both modalities. This paper discusses the motivation for developing combined PET/MRI technology, outlines the obstacles in realizing such an integrated instrument, and presents recent progress in the development of both the instrumentation and of novel imaging agents for combined PET/MRI studies. The performance of the first-generation PET/MRI systems is described. Finally, a range of possible biomedical applications for PET/MRI are outlined.     

The Effect of Accidental Coincidences in Time-of-Flight Positron Emission Tomography

An accidental coincidence is defined as the erroneous registration of two photons, originating from separate positron annihilations, as having originated from the same positron annihilation. Previous analyses which did not consider accidental coincidences indicated that for a certain radioactivity distribution a gain in image signal-to-noise ratio of about 5 dB is achieved by the time-of-flight method over the conventional method. Subsequent experiments have validated this prediction in low counting rate situations. For higher, typical counting rates these experiments showed a significantly larger gain of about 9 dB, which was attributed to the way in which the time-of-flight method suppresses the degrading effects of accidental coincidences. We present an analytical model, extended from a previous model, which considers accidental coincidences. Calculations of signal-to-noise ratio, using this model, compare well with the experiments and show that the additional gain is indeed due to the treatment of accidental coincidences. An understanding of the model leads to an intuitive explanation of the gain mechanism and a determination of an effective coincidence-timing window that is achieved by the time-of-flight method.     

New Imaging Technologies to Enhance the Molecular Sensitivity of Positron Emission Tomography

Positron emission tomography (PET) is used in the clinic and in vivo small animal research to study certain molecular processes associated with diseases such as cancer, heart disease, and neurological disorders and guide the discovery and development of new treatments. New PET molecular probes and associated small animal imaging assays are under development to target, visualize, and quantify subtle molecular and cellular processes such as protein-protein interactions in signal transduction pathways, cancer cell trafficking, therapeutic stem cells and their progeny, interaction of the immune system and tumor cells, and gene delivery and expression in living animals. These next-generation PET molecular imaging assays require an order of magnitude increase in PET's ability to detect, visualize, and quantify low concentrations of probe interacting with its target, which we will refer to as molecular sensitivity , in order to study the subtle signatures associated with these molecular processes. The molecular sensitivity is determined by a combination of the probe and biological/physiological properties of the subject that determine its specificity for the target, and the performance capabilities of the imaging system that determine how well the resulting signal can be measured. This paper focuses on the second aspect: the challenges of advancing PET technology and some of the new imaging system technologies under investigation to substantially enhance PET's molecular sensitivity. If successful, these novel imaging system technology advances, together with new probe molecules that target specific molecular processes associated with disease, will substantially enhance the molecular sensitivity of PET and thus increase its role in preclinical and clinical research as well as evaluating and managing disease in the clinic.     

一种放射CT重建的鲁棒估计算法

放射CT的重建问题可以归结为一种基于泊松分布的最大似然估计（MuxnimumLikelihood,ML），其中最著名的如EM算法[1]，这种算法存在的问题是鲁棒性不强，当观测数据中存在一些出格点（outlier）（如2％）时，EM算法的性能就变得很差。本文分析了算法性能变差的原因，将泊松分布近似为高斯分布后，仿照鲁律估计中的M-估计，构造了一种鲁棒性很强的EM改进算法。实验结果表明，即使在投影数据中混有10％的出格点，仍能重建出满意的结果来。     

Generalized EM-Type Reconstruction Algorithms for Emission Tomography

We provide a general form for many reconstruction estimators of emission tomography. These estimators include Shepp and Vardi's maximum likelihood (ML) estimator, the quadratic weighted least squares (WLS) estimator, Anderson's WLS estimator, and Liu and Wang's multi-objective estimator, and others. We derive a generic update rule by constructing a surrogate function. This work is inspired by the ML-EM (EM, expectation maximization), where the latter naturally arises as a special case. A regularization with a specific form can also be incorporated by De Pierro's trick. We provide a general and quite different convergence proof compared with the proofs of the ML-EM and De Pierro. Theoretical analysis shows that the proposed algorithm monotonically decreases the cost function and automatically meets nonnegativity constraints. We have introduced a mechanism to provide monotonic, self-constraining, and convergent algorithms, from which some interesting existing and new algorithms can be derived. Simulation results illustrate the behavior of these algorithms in term of image quality and resolution-noise tradeoff.     

Considerations for Optical Receiver Design

The low dispersion and low attenuation properties of fibers between 1.3 μm and 1.6 μm make them more attractive for optical fiber transmission applications. In the absence of efficient avalanche photodiodes in this wavelength range, considerable research and development efforts on detectors as well as on the lownoise amplifier devices have been carried out. This paper reviews the present status of optical detectors and low-noise amplifier devices and discusses the low-noise amplifier design process by using a couple of examples at DS-1 and DS-4 bit rates.     

Quad-Detector Arrangement and Sampling Characteristics in Rotary Positron Tomography: POSITOLOGICA II

This paper clarifies that a method of rotary positron-emission computed tomography (PCT) with quad (tri or bi)-detector units can provide the satisfactory sampling characteristics with negligible sacrifice (less than 1 percent) of the detector packing ratio. This paper also presents the array design of quad-BGO (bismuth germanate) units and the sampling characteristics of a rotary PCT system named POSITOLOGICA II. In the rotary PCT, detector units are mounted unequally spaced on a ring so as to provide fine and uniform linear sampling during a 360 degrees rotation of the ring around its center. This principle was applied to a system named POSITOLOGICA I, but it has unidetector units [one detector-one photomultiplier tube (PMT)]. The quad-detector unit consists of four BGO crystals, two PMT's, and a circuit positioning the address of the crystal by which a gamma ray is detected. Merits of the quad units are an effective coupling between rectangular crystals and a photocathode of PMT and reduction of the number of PMT's.     

Design Considerations for Monolithic Microwave Circuits

Monolithic microwave integrated circuits based on silicon-on-sapphire (SOS) and gallium arsenide technologies are being considered seriously as viable candidates for satellite communication systems, airborne radar, and other applications. The low-loss properties of sapphire and semi-insulating GaAs substrates, combined with the excellent microwave performance of metal-semiconductor FET's (MESFET's), allows, for the first time, a truly monolithic approach to microwave integrated circuits. By monolithic we mean an approach wherein all passive and active circuit elements and interconnections are formed into the bulk, or onto the surface of the substrate by some deposition scheme, such as epitaxy, ion implantation, sputtering, evaporation, and other methods. The importance of this development is that microwave applications such as airborne phased-array systems based on a large number of identical circuits and requiring small physical volume and/or light weight, may, finally, become cost effective. The paper covers in some detail the design considerations that must be applied to monolithic microwave circuits in general, and to gallium arsenide circuits in particular. The important role being played by computer-aided design techniques is stressed. Numerous examples of monolithic circuits and components which illustrate the design principles are described. These provide a cross section of the world-wide effort in this field. A glimpse into the future prospects of monolithic microwave circuits is made.     

数字音频放大器设计需要考虑的因素

在设计新的数字音频放大器电路时,音响设备设计师需要考虑的因素远远不止是音频放大控制器IC所提供的输出功率和音调、音色以及过载功能。设计成功的关键是要仔细地考察三个关键方面:控制器的DSP功能、整个解决方案可达到的模拟性能以及充分发挥放大器性能所需要的系统设计支持。当然还包括所选择的放大器类型。D类放大器消费者总是需要更多的音箱、更大的功率,以及能够提供更高音质而体积更小的音响设备,因此有理由认为不久以后AB类音频放大器技术将无法提供可行的解决方案。看一下一个典型应用就可以找出问题所在。家庭影响需要六声道…     

Design Considerations for Paralleling Bipolar Transistors

A simple design procedure for direct paralleling of bipolar junction transistors (BJT's) is proposed. It is based on the matching of transfer characteristics (Ic versus VCE) at a low collector voltage. The design procedure further addresses the base and collector circuit's layout requirements, optimal base-driven conditions, and thermal design requirements for reliable and efficient operation of BJT's in parallel. The influence of a snubber circuit is also discussed. The procedure is verified experimentally by performing dynamic and reversebias safe operating area (RBSOA) testings.     

USB电路保护的设计考虑

在USB的功耗管理方面,USB规范要求产品具备电流限流和/或电源切换功能.规范指出,可复位的聚合物正温度系数(PPTC)器件和固态开关可以用作过流保护措施.在出现故障的情况下,可复位限流措施不仅有助于防止电路受损和相关系统的失效,而且有助于系统的安全性达到UL标准.……     

高安全等级嵌入式应用的设计考虑

介绍了安全嵌入式系统设计考虑,提出了几种防入侵措施.     

Design Considerations for Long-Haul Lightwave Systems

The following design considerations for long-haul lightwave systems are reviewed: outage and error rate requirements, terminal architecture, regenerator span design and performance specification, capacity upgrade alternatives, and broad-gauge economic factors.     

层析SAR系统基线优化设计

自然场景3维影像重构是层析合成孔径雷达(TomoSAR)的重要应用。传统方法在沿高程维进行层析处理时,均通过对等距线阵模型进行加权的方式,以主瓣展宽为代价来抑制方向图旁瓣水平。针对该问题,该文建立一种基于非等距线阵的峰值旁瓣比极小极大优化模型,即在约束主瓣宽度一定的情况下,通过阵元位置的优化配置来获取观测场景视角范围内任意指向的最优旁瓣水平;提出一种目标函数离散栅格化方法,进而采用序列二次规划(SQP)方法并结合差分进化算法特有的全局记忆能力,以获取最优阵元配置方案。对PolSARpro全极化层析SAR仿真数据的处理结果表明该方法能够有效地应用于自然场景的3维影像重构。     

Emission Spectral Tomography Reconstruction Based on Maximum Entropy Interpolation

To improve the performance of optical computed tomography (OpCT) reconstruction in the case of limited projection views, maximum entropy (ME) algorithms were proposed and can achieve better results than traditional ones. However, in the discrete iterative process of ME, the variables of the iterative function are continuous. Hence, interpolation methods ought to be used to improve the precision of the iterative function values. Here, a sinc function interpolation approach was adopted in ME algorithm (SINCME) and its reconstruction results for OpCT with limited views were studied using four typical phantoms. Compared results with ME without interpolation, traditional medical CT back-projection algorithm (BP), and iterative algorithm algebraic reconstruction technique (ART) showed that the SINCME algorithm achieved the best reconstruction results. In an experiment of emission spectral tomography reconstruction, SINCME was also adopted to calculate the three-dimensional distribution of physical parameters of a candle flame. The studies of both algorithm and experiment demonstrated that SINCME met the demand of limited-view OpCT reconstruction.     

Imaging Performance of a Dynamic Positron Emission Tomograph: Positome IIIp

This paper documents modifications to an older PET system to improve its dead time, scatter fraction, and spatial resolution in high count rate, short duration studies. A new dual-tapered collimator reduces scatter by 33 percent while providing excellent resolution uniformity in all slices. A data encoding scheme produces uniformly sampled parallel projections from the coincidence data in real time while the detector array executes an orbital motion. The image uniformity, scatter compensation and high count-rate performance have been validated up to 40 kBq/cc in a 20 cm flood source. The errors in image quantification due to counting statistics, live time, and random counts are estimated from repeated measurements on a contrast phantom at high count-rates. The effects of two methods of scatter compensation on image contrast are shown in contrast phantoms and a typical glucose utilization study. Blood flow measurements using 0-15 labeled water bolus method, made under different physiological conditions, reflected the changes expected. The true count efficiency of 75 kcps/(uCi/cc) permits these studies to be done with only 500-900 MBq (13-25 mCi) injected activity.     

电磁兼容性(EMC)设计需要考虑的问题

阐述了电磁兼容（EMC）设计的原则，指出了产品EMC现象的若干主要原因，提出了相应的方法和措施。