Sharif Digital Repository

Purpose: Presence of photon attenuation severely challenges quantitative accuracy in single-photon emission computed tomography (SPECT) imaging. Subsequently, various attenuation correction methods have been developed to compensate for this degradation. The present study aims to implement an attenuation correction method and then to evaluate quantification accuracy of attenuation correction in small-animal SPECT imaging. Methods: Images were reconstructed using an iterative reconstruction method based on the maximum-likelihood expectation maximization (MLEM) algorithm including resolution recovery. This was implemented in our designed dedicated small-animal SPECT (HiReSPECT) system. For... 

In this paper the design of a digital step attenuator with simultaneous low phase and gain error characteristics is investigated. First, the loading effect of the consecutive blocks of an N-bit attenuator on the precision of the attenuation levels is analyzed. Then a modified structure to decrease the loading effect as well as the phase error of the attenuator blocks is presented. A comprehensive analysis of the circuit is performed and some design guidelines have described. Finally, a 6-bit attenuator with attenuation range of 0.5–31.5 dB and resolution of 0.5 dB is implemented in 0.18 µm complementary metal–oxide-semiconductor (CMOS) technology. The root mean square (RMS) gain error and... 

We investigate hot spot effects on increasing THz emission of the frequency modes in a square mesa structure with constant hot spot size in different positions, without applying magnetic field. We show that in the presence of hot spot at any position, the average of emitted power intensity level is increased. The frequency modes that the position of their field concentration is more matched with hot spot position are more excited. Accordingly, in the case with hot spot located at center of a-axis or b-axis, frequency modes TM(m,n) with even indices m or n is more excited and in the case in which hot spot is located at the corner of a-axis or b-axis, frequency modes TM(m,n) with odd indices m... 

Online monitoring of HV XLPE cables is a major interest for utilities. Due to severe and rapid risk of life of this type of cable with occurrence of partial discharges, studies are carried out for detection and location of PD. However there is a lack of models that can help to accurately detect the partial discharge and its location in installed cables, due to the relatively long length and the load condition of cable. Detection and location of any partial discharge signal requires: A verified knowledge of phenomena which contributes to P.D. propagation and its deformation, measured data of P.D. type signal propagation on different cable test setups, accurate frequency dependent cable model... 

A 1-V CMOS current steering digital to analog converter with enhanced static and dynamic linearity is presented. The 14-bit static linearity is achieved by a novel background analog self calibration technique which is suitable for low voltage applications and does not require error measurement and correction circuits. To improve dynamic linearity at high frequencies, a new low power track/attenuate output stage is used at the DAC output. Integral and differential nonlinearities of the proposed DAC corresponding, to 14-bit specification are less than 0.35LSB and 0.25LSB, respectively. The DAC is functional up to 400MS/S with SFDR better than 71dB in the Nyquist band. The circuit has been... 

The locking of two mutually coupled microwave oscillators is considered. The common frequency of oscillation is obtained both without and in the presence of a non-linear reactance. The effect of the non-linear reactance on the locking bandwidth is studied. It is shown analytically that, unlike the common injection locked oscillator case, the locking bandwidth can be either increased or decreased by the non-linearity. The variations in amplitude of oscillations is investigated. It is demonstrated that the total power delivered by the active devices decreases in interinjection locked oscillators  

Composites of unsaturated polyester containing 5 wt% nanoclay and different amounts of lead monoxide particles (0, 10, 20, and 30 wt%) were prepared. XRD patterns showed the exfoliation of nanoclay layers in the polymer. Morphological properties of the composites were studied using SEM micrographs. The prepared composites were investigated for their thermal resistance and mechanical properties using thermogravimetric analysis and tensile testing method, respectively. Addition of lead monoxide to the polymer worsened its thermal resistance and tensile properties, whereas the observed negative effects could be moderated by the clay nanoparticle. Gamma attenuation performance of the composites... 

In this paper the fault-Tolerant control problem for uncertain linear systems in the presence of L, disturbances and actuator saturation is studied. The conflict between enlarging the domain of attraction and attenuating the effect of L, disturbances is tackled by proposing a non-constant state feedback controller. The feedback gains are calculated off-line by using linear matrix inequalities. In addition, the proposed method is capable of tolerating time-varying faults. The suggested approach is implemented on a sample model and the result is compared with other works  

The purpose of this article is to investigate the shielding performance of the B2O3-Bi2O3-ZnO-Li2O glasses against the gamma-ray. To this end, the attenuation parameters of the gamma-ray inside the B2O3-Bi2O3-ZnO-Li2O glasses were evaluated in the energy range of 200–1500 keV. It was found that the mass attenuation coefficient of the composites increases with increasing the Li2O content. The relative percent deviation between the results of simulation using the MCNPX and XCOM database in most cases was less than 2%. Additionally, the Mean Free Path (MFP) and Half-Value Layer (HVL) parameters were evaluated, and the shielding efficiency of samples was compared with some conventional shielding... 

One of the miscellaneous units that are frequently used in water and wastewater systems is flush tank. Flush tank increases the self cleaning velocity in the conduit, and helps to clear the conduits from deposits and contaminations. While these units have been used for many years, there is not enough information about the hydraulic performance of these units in respect to the discharge, distance from the tank and the pipe hydraulics. In this paper the experimental results from observed hydrographs at different distances from the flush tank in a pipe has been presented. The variables include; volume of releasing water (or inlet hydrograph), upstream head, pipe diameter (3 cm), pipe length and... 

In this paper, we consider a low-complexity iterative receiver for decoding multi-user information data for the bandwidth efficient convolutionally coded UWB-TH-CDMA system, in a multipath channel. The proposed receiver structure consists of a bank of soft interference cancellers (SIC), followed by a block of MMSE filters. The outputs of these filters, are feeded to a bank of likelihood calculators (LC), each followed by a soft-input soft-output (SISO) convolutional decoder. Information is fed back from SISO decoders to SICs for interference cancellation. This receiver requires the knowledge of the attenuation and delay of the strongest path of each active user. We evaluate the performance... 

The use of adaptive feedforward controllers has proven to be a very successful strategy for controlling noise and vibration in a variety of applications. One reason is that the feedforward controller is an open loop controller, which can be designed to cancel the undesired noise in one position with any accuracy. However, the feedforward controller requires an input signal, called a reference signal, correlated to the noise source. As a consequence, a single reference controller can only reduce noise radiated from a single noise source. In many applications, there is a need to attenuate noise produced by several noise sources. In this paper, three different structures, single, modulating and... 

In this paper we present a method of localizing multiple wideband acoustic sources based on both signal attenuation pattern and delays in the reception of the signal. Moreover, an additional order of flexibility is added to the problem by considering the attenuation model as an unknown function with reasonable prior assumptions about its functional space. Such approach is capable of enhancing the localization performance compared to when only the signal attenuation data or the time delays are used. A cost function is then formed in terms of the sources locations and the attenuation model parameters. Having different types of unknowns in the cost function, convergence concerns, and the... 

Repetitive decomposing of the objective concept hierarchically, developed for design involves a human intentional activity, ranging from management to physical works. This model proved the empirical rules in Industrial Engineering. In addition to these "design" process characteristics, the "test" process is found to be a defect attenuating process with the attenuation rate of its error rate of the second kind. As these are applicable to any software process, they are very useful  

A 1-V CMOS current steering digital to analog converter with enhanced static and dynamic linearity is presented. The 14-bit static linearity is achieved by a background analog self calibration technique which is suitable for low voltage applications and does not require error measurement and correction circuits. To improve dynamic linearity at high frequencies a track/attenuate output stage is used at the DAC output. Integral and differential nonlinearities of the proposed DAC corresponding to 14-bit specification are less than 0.35 and 0.25 LSB respectively. The DAC is functional up to 400MS/s with SFDR better than 71 dB in the Nyquist band. The circuit has been designed and simulated in a... 

This Regular evaluation and analysis of river quality helps accurate management to reach acceptable quality. In this study, water quality indices (WQI) method applied to evaluate quality of Jajrood River which is one of the main sources of Tehran drinking water. It helps to subsidize management actions in Jajrood watershed. Sampling carried out for 9 years during 2001-2010 in 10 stations along the river in wet and dry stations to identify water quality and the main source of pollution. Results show water quality in all stations classifies as medium according to National Sanitation Foundation (NSF) and water quality index in wet season is better than the corresponding value in dry seasons.... 

This paper is concerned with the disturbance attenuation problem of fuzzy large-scale systems which consist of N interconnected subsystems which are represented by Takagi-Sugeno fuzzy models. Using Lyapunov function and linear matrix inequalities (LMIs), a criterion is proposed to have a prescribed level of disturbance attenuation. A numerical example is given to illustrate the control design procedure and its effectiveness  

Optical coherence tomography (OCT) is a noninvasive imaging modality that provides cross-sectional images from microstructures of tissues. This optical imaging system works based on Michelson interferometry principle and has intermediate resolution and penetration depth which makes it appropriate for imaging thin tissues such as skin and eye. OCT images suffer from three main artifacts that make images difficult to be analyzed. The first one is small grainy structures called speckle which degrade image quality and decreases axial and lateral resolution. The second one is light intensity attenuation which is a function of depth. It happens because of absorbing and scattering nature of tissue... 

Lead silicate glasses (LSGs) have high gamma-ray attenuation but low chemical durability properties. In this work, LSGs with (55.5-68.5 wt%) PbO content containing ZrO2 and TiO2 additions were produced. The chemical corrosion of various produced LSGs in 0.5 N HNO3 aqueous solution and determination of their gamma-ray attenuation coefficients for 60Co and 137Cs sources were investigated. The weight loss measurements, the SEM micrographs, the EDS analysis of the sample surfaces and the ICP analysis of solution were used to characterize the dissolution process. The effects of PbO content, ZrO2 and TiO2 additives on chemical corrosion, and also the effect of PbO on gamma-ray attenuation... 

This paper presents a highly-linear transceiver core chip for X-band phased-array systems with two RX and one TX channels. Implemented in a standard 0.18-μm CMOS technology, the core chip provides 6-bit phase control (with rms error <2°) and 6-bit gain control (with rms error <0.6 dB) both within the 8.5–11.5 GHz frequency band. Improved accuracy is also available by digital calibration in narrowband applications. The receivers achieve a gain of 13.5 dB, an IIP3 of +10.3 dBm, and a noise figure of 8.2 dB, while drawing 170 mA per channel from the 3.3 V supply. The chip also provides an additional low-gain mode which further enhances IIP3 to +19.1 dBm and the input-referred P1dB to +11.4 dBm....