Optical architectures for compressive imaging

We compare three optical architectures for compressive imaging: sequential, parallel, and photon sharing. Each of these architectures is analyzed using two different types of projection: (a) principal component projections and (b) pseudo-random projections. Both linear and nonlinear reconstruction methods are studied. The performance of each architecture-projection combination is quantified in terms of reconstructed image quality as a function of measurement noise strength. Using a linear reconstruction operator we find that in all cases of (a) there is a measurement noise level above which compressive imaging is superior to conventional imaging. Normalized by the average object pixel brightness, these threshold noise standard deviations are 6.4, 4.9, and 2.1 for the sequential, parallel, and photon sharing architectures, respectively. We also find that conventional imaging outperforms compressive imaging using pseudo-random projections when linear reconstruction is employed. In all cases the photon sharing architecture is found to be more photon-efficient than the other two optical implementations and thus offers the highest performance among all compressive methods studied here. For example, with principal component projections and a linear reconstruction operator, the photon sharing architecture provides at least 17.6% less reconstruction error than either of the other two architectures for a noise strength of 1.6 times the average object pixel brightness. We also demonstrate that nonlinear reconstruction methods can offer additional performance improvements to all architectures for small values of noise.     

Reversible back-propagation imaging algorithm for postprocessing of ultrasonic array data

The paper describes a method for processing data from an ultrasonic transducer array. The proposed algorithm is formulated in such a way that it is reversible, i.e., the raw data set can be recovered from the image. This is of practical significance because it allows the raw-data to be spatially filtered using the image to extract, for example, only the raw data associated with a particular scatterer. The method is tested on experimental data obtained from a commercial 64-element, 5.5-MHz array on an aluminum specimen that contains several machined slots and side-drilled holes. The raw transmitter-receiver data corresponding to each scatterer is extracted, and the scattering matrices of different scatterers are reconstructed. This allows the signals from 1-mm-long slot and a 1-mm-diameter hole to be clearly distinguished and the orientation and the size of the slots to be determined.     

High-accuracy standard specimens for the line-focus-beam ultrasonic material characterization system

We prepared standard specimens for the line-focus-beam ultrasonic material characterization system to obtain absolute values of the propagation characteristics (phase velocity and attenuation) of leaky surface acoustic waves (LSAWs). The characterization system is very useful for evaluating and analyzing specimen surfaces. The calibration accuracy of these acoustic parameters depends on the accuracy of acoustical physical constants (elastic constants, piezoelectric constants, dielectric constants, and density) determined for standard specimens. In this paper, we developed substrates of non piezoelectric single crystals (viz., gadolinium gallium garnet [GGG], Si, and Ge) and an isotropic solid (synthetic silica [SiO₂] glass) as standard specimens. These specimens can cover the phase velocity range of 2600 to 5100 m/s for Rayleigh-type LSAWs. To determine the elastic constants with high accuracy, we measured velocities by the complex-mode measurement method and corrected diffraction effects. Measurements of bulk acoustic properties (bulk wave velocity and density) were conducted around 23°C, and bulk wave velocities were obtained with an accuracy of within ±0.004%. We clearly detected differences in acoustic properties by comparing the obtained results with the previously published values; the differences were considered to be due to differences of the specimens used. We also detected differences in acoustic properties among four SiO₂ substrates produced by different manufacturers     

Fluorescence lifetime imaging by using time-gated data acquisition

The use of the time gating technique for lifetime reconstruction in the Fourier domain is a novel technique. Time gating provides sufficient data points in the time domain for reliable application of the Fourier transform, which is essential for the time deconvolution of the system of the integral equations employed in the reconstruction. The Fourier domain telegraph equation is employed to model the light transport, which allows a sufficiently broad interval of frequencies to be covered. Reconstructed images contain enough information needed for recovering the lifetime distribution in a sample for any given frequency within the megahertz-gigahertz band. The use of this technique is essential for recovering time-dependent information in fluorescence imaging. This technique was applied in reconstruction of the lifetime distribution of four tubes filled with Rhodamine 6G embedded inside a highly scattering slab. Relatively accurate fluorescence lifetime reconstruction demonstrates the effectiveness and the potential of the proposed technique.     

A digital hardware correlation system for fast ultrasonic data acquisition in peak power limited applications

The theoretical and experimental evaluation of a digital hardware correlation system for low-power ultrasonic applications is presented. The system, which incorporates dual Golay code matched filtering, is capable of 20-MHz processing rates with a signal-to-noise-ratio enhancement (SNRE) of 23 dB over a conventional pulse-echo system operating at the same peak power levels. The effects of digitization have been investigated, and a TTL (transistor transistor logic)-based hardware correlator has been developed. For many applications, low-voltage driving followed by differential detection is sufficient, permitting the system to be used in a number of power-limited environments. Sample tests conducted on three different transducers have demonstrated that the system is operational over a wide variety of probe configurations.     

Optimizing the speckle noise for maximum efficacy of data acquisition in coherent imaging

The impact of multiplicative speckle noise on data acquisition in coherent imaging is studied. This demonstrates the possibility to optimally adjust the level of the speckle noise in order to deliberately exploit, with maximum efficacy, the saturation naturally limiting linear image sensors such as CCD cameras, for instance. This constructive action of speckle noise cooperating with saturation can be interpreted as a novel instance of stochastic resonance or a useful-noise effect.     

Subaperture processing for ultrasonic imaging

Ultrasonic subaperture processing using aperture synthesis and beam space interpolation is presented. The number of beam lines scanning the image plane for a given transmit-receive subaperture combination is chosen according to the spatial sampling criteria for that combination. On each beam line, echo signals over the entire array are collected through electronic multiplexing of array channels, where the transmit subaperture at the transducer center is fired K successive times, with K equal to the number of nonoverlapping receive subapertures. For every receive subaperture, the number of beam lines is increased through digital interpolation using a linear filter with spatial frequency band associated with the subaperture. Interpolated beam lines from all receive subapertures are then added to obtain a high resolution sector image. The efficiency of subaperture processing for different system configurations is tested on experimental rf data acquired from two different phantoms using a 3.5 MHz, 128-element transducer array. The proposed subaperture processing reduces the number of firings for data acquisition, and thus allows real-time imaging with very low susceptibility to motion artifacts.     

Efficient data acquisition for engineers

A method of simple modular data acquisition is described. The approach is currently used by engineers for a wide variety of applications. It is particularly useful for engineers who have a limited knowledge of computer programming.     

Fastbus data acquisition for CDF

All CDF event data are collected in a multilevel FASTBUS network. At the lowest level of this network, MEP/MX and SSP scanners read and buffer data from RABBIT and FASTBUS front end systems. Operation of these front end scanners is coordinated by the Trigger Supervisor module which initiates parallel readout after receiving Level 1 and Level 2 triggers. Dataflow from scanners to consumer processes on host VAX computers is supervised by the Buffer Manager which directs an Event Builder to collect and format data from a set of scanner modules. This system is designed to allow partitioning into semi-independent sections for parallel development and calibration studies.     

Dry-contact technique for high-resolution ultrasonic imaging

To accomplish a high-resolution ultrasonic imaging without wetting a sample, the efficiency of the dry-contact ultrasonic transmission is discussed. In this study, a dry-contact interface is formed on a sample by inserting a thin film between water and a sample, and the pressure is working on the interface by evacuating the air between the film and the sample. A model of dry-contact ultrasonic transmission is presented to assess the signal loss accompanied with the transmission. From the determination of the signal loss caused by the transmission using various films, it was found that the higher frequency ultrasound is transmitted effectively into the sample by selecting an optimum film, which can keep the displacement continuity between the film and the sample during ultrasonic transmission. Finally, ultrasonic imaging with the sufficient signal-to-noise ratio (SNR) and high lateral resolution was performed on the delamination in a package and the jointing interface of the ball-grid-array package without wetting the packages.     

Personal computer-basedcamac system for data acquisition

A single board auxiliarycamac crate controller for a Z80-A based personal computer ZX-spectrum to drive thecamac crate housing multipleadc, tdc, dac and 16 I/O ports has been developed. Thecamac crate controller used in this system is of A2 type which supports multiple auxiliary crate controllers. Acamac exerciser/training software for the use ofcamac commands in process control as well as data acquisition has been developed.     

Rotary encoding for intravascular ultrasonic imaging systems

Images produced with an intravascular ultrasound system (IVUS) can be distorted because of uncertainty in the instantaneous angular position of a rotating ultrasonic transducer. A rotary encoder placed in proximity to the transducer is required to detect the problem; however, size constraints make a conventional electromechanical or optomechanical encoder difficult to implement. Measurements that test the feasibility of a software-derived encoder, based of the rate of decorrelation of ultrasonic RF lines with angle, are reported. Provided that the instantaneous angular velocity of the transducer can be measured, adjustments can be made to the pulse rate of the transducer, which would eliminate the image distortion.     

Beam transformation techinques for ultrasonic medical imaging

Data acquisition rates in pulsed ultrasound scanners are limited by the speed of sound in the human body. This poses severe limitations to the design of future ultrasound equipment, such as 3-D imaging scanners. The authors describe a technique for higher data acquisition rates based on the simultaneous transmission of multiple beams. By using a linear combination of the received beams, interbeam interference due to the sidelobe energy of the transmitting beams can be significantly reduced. The transformation coefficients are found by using a least squares minimization criterion. A simulation environment used for the evaluation of the authors' methodologies and various simulation results are presented.     

Coded pulse excitation for ultrasonic strain imaging

Decorrelation strain noise can be significantly reduced in low echo-signal-to-noise (eSNR) conditions using coded excitation. Large time-bandwidth-product (>30) pulses are transmitted into tissue mimicking phantoms with 2.5-mm diameter inclusions that mimic the elastic properties of breast lesions. We observed a 5-10 dB improvement in eSNR that led to a doubling of the depth of focus for strain images with no reduction of spatial resolution. In high eSNR conditions, coded excitation permits the use of higher carrier frequencies and shorter correlation windows to improve the attainable spatial resolution for strain relative to that obtained with conventional short pulses. This paper summarizes comparative studies of strain imaging in noise-limited conditions obtained by short pulses and four common aperiodic codes (chirp, Barker, suboptimal, and Golay) as a function of attenuation, eSNR and applied strain. Imaging performance is quantified using SNR for displacement (SNRd), local modulation transfer function (LMTF), and contrast-to-noise ratio for strain (CNRepsilon). We found that chirp and Golay codes are the most robust for imaging soft tissue deformation using matched filter decoding. Their superior performance is obtained by balancing the need for low-range lobes, large eSNR improvement, and short-code duration.     

Airborne ultrasonic imaging system for parallelepipedic objectlocalization

An ultrasonic image device was designed for use in robotic applications where parallelepipedic objects need to be manipulated. It is based on ranging measurement by an array of ten identical airborne ultrasonic transducers with an operating frequency of 200 kHz and a detection cone angle of approximately 7 degrees. An electronic scanning associated with a mechanical displacement of the array covers an area of 0.4×0.4 m in less than 0.5 seconds (100 measurement points). The system was tested in an automatic packaging line. It allowed a real-time localization of cubic objects as small as 2 cm and the determination of an emplacement in which a new object can be put by a robot arm     

Piezo-polymer transducers for ultrasonic imaging in air

Airborne polyvinylidenefluoride transducers have been designed for robotic applications in air. Characteristics of transducer prototypes are: working frequencies from 61 kHz to 86 kHz, quality factor Q from 4 to 6, and two-way insertion loss of about 90 dB. The small dimension, the lightness, and the low-cost fabrication technology allow the development of arrays or matrices for ultrasonic imaging systems in air. In this work two different image reconstruction algorithms are proposed: the first carries out a combined spectral and aperture synthesis for detecting isolated scatterers with a spatial resolution of about 2 mm; the second is based on an accurate ranging algorithm with sub-millimeter resolution at distances up to 50 cm. Finally, this work's application to the reconstruction of three-dimensional object profiles is discussed.     

High-accuracy three-dimensional shape acquisition of a large-scale object from multiple uncalibrated camera views

Acquiring a high-accuracy three-dimensional (3D) shape of a large-scale object from multiple uncalibrated camera views remains a big challenge, since a considerable number of images is required to cover the entire surface; the use of multiple images could, however, result in accumulative errors from each processed image. Here error propagation rules in the 3D reconstruction process have been deduced on the basis of the traditional dual-view reconstruction method. We propose a method that can control the accumulative errors by reducing the times of coordinate transformation with common-view-based dual-view reconstruction. This method involves constructing an image network composed of many image groups, each of which contains a common view. A baseline threshold method is introduced to construct a high-quality image network, and the sums or reprojection residual of all the common points is proposed to assess the validity of the solutions of the orientation. Experiments carried out with both synthetic and real images demonstrate that the proposed method can handle the accumulative error problem with robust and highly accurate results.     

A modular acquisition system for coincidence data

A modular extendable acquisition system for coincidence data is developed, which uses commercially available high-stability high-precision ADCs. The basic functions in the system are incorporated each in a specific type of module, which offers maximum flexibility. The user can define the relevant types of coincidences by making the connections between the different modules. Many tools are provided that make it easy to set up and to monitor the system. Data latching, a fast clear, and the possibility of pileup rejection are implemented. Extension possibilities, included in the concept, are discussed. The maximum throughput of the system is such that the conversion-time of the ADCs and the maximum data-rate of a magnetic-tape unit are the limiting factors.