Synthetic Focusing Research Articles

A Scalable Architecture for Real-Time Synthetic-Focus Imaging

A scalable architecture for forming real-time synthetic focus images is described, and the design of a 256-channel system using currently available technology is presented as an example implementation of the architecture. The parallelism of the system scales directly with the number of array elements, and the image computation rate for a given image size (in pixels) stays constant as the number of array elements is increased. The system leverages earlier work in the real-time generation of the required time-of-flight surfaces and allows either real-time image generation or iterative adaptive image generation from a single complete dataset.

Bistatic Radar Imaging of the Marine Environment—Part I: Theoretical Background

We describe in detail the theoretical and practical implementation aspects of a simulation for marine radars which can, in particular, be used in multistatic configurations. Since the simulator is intended to deliver pseudoraw signals, it can be used later as a tool to benchmark and improve postprocessing algorithms such as bistatic synthetic aperture radar focusing algorithms and ship wake detection algorithms. The work is divided into two parts. This paper reviews and recalls theoretical prerequisites necessary in implementing such a simulator. Included are the full derivation of the bistatic radar equation from the transmitter to the receiver, accounting also for the transmit-receive time, a description of the sea state phenomenology, a review of the theory of electromagnetic scattering from the sea surface, and the presentation and validation of the method used in the simulation. A companion paper discusses the practical implementation aspects of the simulator as well as an analysis of our results.

A Novel Clutter Suppression Algorithm for Landmine Detection With GPR

In this paper, we propose a new algorithm for the enhancement of plastic-cased antipersonnel mine detection using a video-impulse ground-penetrating radar (GPR). The algorithm is implemented as a nonlinear signal processor, which searches for the presence of a reference waveform in a 1D GPR echo return. The reference waveform represents a class of targets within a certain environment. The processor marks the presence of all responses similar to the reference waveform with a sharp mono-cycle. Simultaneously, responses with different waveforms, which presumably correspond to clutter, are suppressed. The reference waveform and other algorithm parameters are determined from training data sets acquired in a controlled environment. After training, the algorithm can be successfully applied at sites where soil, targets, and measurement scenarios are similar but not identical to those of the training site. The processor is integrated into an automated data processing and mine detection scheme as an additional clutter suppression step. The scheme consists of clutter suppression, synthetic aperture radar focusing, construction of a confidence map, and automated detection in it. The suggested algorithm is tested on experimental data, and its performance is compared against schemes where clutter suppression is organized by means of background removal and the cross correlation with a reference wavelet. The performance comparison is done in terms of receiver operating characteristic curves. It has been found that the suggested algorithm reduces the false alarm rate in about two and a half times in comparison to the cross-correlation-based clutter suppression.

Model-Based Correction of Diffraction Effects of the Virtual Source Element

A method for ultrasonic synthetic aperture imaging using finite-sized transducers is introduced that is based on a virtual source (VS) concept. In this setup, a focused transducer creates a VS element at its focal point that facilitates the use of synthetic aperture focusing technique (SAFT). It is shown that the performance of the VS method may be unsatisfactory due to the distortion introduced by the diffraction effects of the aperture used for creating the VS element. A solution to this problem is proposed that consists of replacing the classical SAFT by the extended synthetic aperature focusing technique (ESAFT) algorithm presented in our earlier works. In ESAFT, the full geometry of the VS is modeled, instead of applying the simplified point source approximation used when VS is combined with classical SAFT. The proposed method yields a substantial improvement in spatial resolution compared to that obtained using SAFT. Performance of the proposed algorithm is first demonstrated on simulated data, then verified on real data acquired with an array system.

Pulse-compression ultrasound imaging using a 40-MHz annular array

High-frequency ultrasound allows fine-resolution imaging at the expense of limited depth-of-field and acoustic penetration depth. Coded-excitation imaging allows a significant increase in the signal-to-noise ratio (SNR) and acoustic penetration depth. A 40-MHz, five-element annular array with a focal length of 12 mm and a total aperture of 6 mm was made using a 9-μm thick PVDF-TrFe membrane. The transducer was excited with an optimized, custom design, 4-μs, linear chirp spanning 15 to 65 MHz that was deduced from ex vivo experiments and computer simulations. Images of a 12-μm wire were generated to quantify lateral and axial resolutions. All 25 transmit/receive signal combinations were digitized and post-processed for compression and synthetic focusing. Compression consisted of linearly filtering the signals with the time-reversed excitation chirp modulated by a Dolph-Chebyshev window. Results showed that resolutions were not significantly degraded when compared to a conventional monocycle excitation and that SNR was improved by more than 14 dB. Images of mouse embryos showed marked improvements in image-quality. [Work supported in part by NIH Grant R21 EB006509.]

High-Frequency Ultrasonic Imaging of the Anterior Segment Using an Annular Array Transducer

Very high-frequency ultrasound (VHFU; >35 megahertz [MHz]) allows imaging of anterior segment structures of the eye with a resolution of less than 40 microm. The low focal ratio of VHFU transducers, however, results in a depth of field (DOF) of less than 1 mm. The aim was to develop a high-frequency annular array transducer for ocular imaging with improved DOF, sensitivity, and resolution compared with conventional transducers. Experimental study. Cadaver eyes, ex vivo cow eyes, in vivo rabbit eyes. A spherically curved annular array ultrasound transducer was fabricated. The array consisted of 5 concentric rings of equal area, had an overall aperture of 6 mm, and a geometric focus of 12 mm. The nominal center frequency of all array elements was 40 MHz. An experimental system was designed in which a single array element was pulsed and echo data were recorded from all elements. By sequentially pulsing each element, echo data were acquired for all 25 transmit-and-receive annuli combinations. The echo data then were focused synthetically and composite images were produced. Transducer operation was tested by scanning a test object consisting of a series of 25-microm diameter wires spaced at increasing range from the transducer. Imaging capabilities of the annular array were demonstrated in ex vivo bovine, in vivo rabbit, and human cadaver eyes. Depth of field, resolution, and sensitivity. The wire scans verified the operation of the array and demonstrated a 6.0-mm DOF, compared with the 1.0-mm DOF of a conventional single-element transducer of comparable frequency, aperture, and focal length. B-mode images of ex vivo bovine, in vivo rabbit, and cadaver eyes showed that although the single-element transducer had high sensitivity and resolution within 1 to 2 mm of its focus, the array with synthetic focusing maintained this quality over a 6-mm DOF. An annular array for high-resolution ocular imaging has been demonstrated. This technology offers improved DOF, sensitivity, and lateral resolution compared with single-element fixed focus transducers currently used for VHFU imaging of the eye.

High-frequency pulse-compression ultrasound imaging using an 18-MHz annular array

High-frequency ultrasound (HFU) allows fine-resolution imaging at the expense of limited depth-of-field (DOF) and shallow acoustic penetration. Coded-excitation imaging allows a significant increase in the signal-to-noise ratio (SNR) and acoustic penetration depth. A 18-MHz, five-element annular array with a focal length of 31 mm and a total aperture of 10 mm was made using a 25-μm thick membrane. To excite the PVDF-TrFe transducer, an optimized 8-μs linear chirp spanning 6 to 30 MHz was uploaded into a waveform generator. The optimal chirp design was deduced from ex vivo experiments and computer simulations. Images of a seven-wire phantom were generated. The 25-μm wires were spaced in 1-mm axial and lateral increments. All 25 transmit/receive signal combinations were digitized and postprocessed for compression and synthetic focusing. Compression consisted of linearly filtering the signals with a mismatched filter. The mismatched filter was the time-reversed excitation chirp modulated by a Dolph-Chebyshev window. Marked image-quality improvements were demonstrated in images of ex vivo cow eyes. Results of this study demonstrate that the DOF can be improved by a factor of 3 and SNRs, after compression, can be improved by 24 dB.

Synthetic focusing and simulation of dual laterolog tool in axisymmetric subsurface models

The laterolog tools for electrical resistivity measurement are largely employed by the petroleum industry especially in boreholes filled with salt water-base mud. This work develops a synthetic focusing for many electrical tools, such as LLS, LLD and LL7. This is done by applying the principle of superposition to each normal arrangement. A finite element method (FEM) is used to simulate the tool response in axisymmetric subsurface models crossed vertically by the well with constant radius. Also, the logs are corrected for the borehole effects that are, in some cases, important. Despite the application of the synthetic focusing has been accomplished in two-dimensional models, it is possible to apply it in more complex simulations: three-dimensional models with deviated boreholes (or dipping beds) as well as anisotropic systems.

Light field microscopy

By inserting a microlens array into the optical train of a conventional microscope, one can capture light fields of biological specimens in a single photograph. Although diffraction places a limit on the product of spatial and angular resolution in these light fields, we can nevertheless produce useful perspective views and focal stacks from them. Since microscopes are inherently orthographic devices, perspective views represent a new way to look at microscopic specimens. The ability to create focal stacks from a single photograph allows moving or light-sensitive specimens to be recorded. Applying 3D deconvolution to these focal stacks, we can produce a set of cross sections, which can be visualized using volume rendering. In this paper, we demonstrate a prototype light field microscope (LFM), analyze its optical performance, and show perspective views, focal stacks, and reconstructed volumes for a variety of biological specimens. We also show that synthetic focusing followed by 3D deconvolution is equivalent to applying limited-angle tomography directly to the 4D light field.

Operational verification of a 40-MHz annular array transducer

An experimental system to take advantage of the imaging capabilities of a 5-ring polyvinylidene fluoride (PVDF)-based annular array is presented. The array has a 6-mm total aperture and a 12-mm geometric focus. The experimental system is designed to pulse a single element of the array and then digitize the received data of all array channels simultaneously. All transmit/receive pairs are digitized and then the data are post-processed with a synthetic-focusing technique to achieve an enhanced depth of field (DOF). The performance of the array is experimentally tested with a wire phantom consisting of 25-microm diameter wires diagonally spaced at 1-mm by 1-mm intervals. The phantom permitted the efficacy of the synthetic-focusing algorithm to be tested and was also used for two-way beam characterization. Experimental results are compared to a spatial impulse response method beam simulation. After synthetic focusing, the two-way echo amplitude was enhanced over the range of 8 to 19 mm and the 6-dB DOF spanned from 9 to 15 mm. For a wire at a fixed axial depth, the relative time delays between transmit/receive ring pairs agreed with theoretical predictions to within +/- 2 ns. To further test the system, B-mode images of an excised bovine eye were rendered.

Extended wavenumber-domain synthetic aperture radar focusing with integrated motion compensation

Modern synthetic aperture radar (SAR) systems are continually developing in the direction of higher spatial resolution. This requires the usage of high range bandwidths combined with long azimuth integration intervals. High-quality SAR processing methods, which are able to deal with such sensor parameters, are necessary for focusing the raw data of such sensors. Wavenumber-domain (ω–k) processing is commonly accepted as the ideal solution to the SAR focusing problem. However, it is only applicable to spaceborne SAR data where a straight sensor trajectory is given. In the case of airborne data, wavenumber-domain processing is limited because of its inability to perform high-precision motion compensation. Here, the extended chirp scaling (ECS) algorithm has proven to be very powerful, although it has certain limitations concerning long aperture syntheses and highly squinted geometries. In the paper, a new stripmap SAR data-processing algorithm, called extended ω–k (EOK), is analytically derived. The EOK algorithm aims to combine the high focusing accuracy of the wavenumber-domain algorithm with the high-precision motion compensation of the ECS algorithm. The new EOK algorithm integrates a three-step motion compensation correction in the general formulation of the wavenumber-domain algorithm, leading to a new airborne SAR processing scheme, which is also very robust in the cases of long synthetic apertures and high squint angles. As demonstrated, it offers the possibility of processing wide-band, low-frequency airborne SAR data up to near-wavelength resolution. The performance and accuracy of the new EOK SAR data-processing algorithm are demonstrated using simulated data in different data collection scenarios and geometries as well as using interferometric data acquired by the airborne experimental SAR system of DLR at L-band (Horn, 1996; Scheiber, 1999).

Ophthalmic imaging with a 40-MHz annular array

High-frequency annular arrays provide a means for improving the depth of field versus single-element transducers. An annular array with five equal-area rings has been designed and an experimental system has been assembled to test it. The transducer is fabricated from a 9-μm polyvinylidene fluoride (PVDF) membrane bonded to a copper-clad polyimide film. The transducer has a total aperture of 6 mm, a geometric focus of 12 mm, and a nominal center frequency of 40 MHz with a 6-dB fractional bandwidth of 33%. The device is pressed into a spherically curved geometry and then backfilled with epoxy. Radio-frequency data were acquired for all 25 transmit/receive ring pairs, and then postprocessed with a synthetic focusing algorithm to improve the image depth of field (DOF). The resulting processed data improved DOF by a factor of 6 compared to a single-element transducer of the same total aperture and geometric focus. To test the system, rf data were acquired from excised bovine eyes, human cadaver eyes, and anesthetized rabbits. The tests revealed enhanced resolution away from the geometric focus and a more uniform image brightness than can be achieved with high-frequency, single-element devices. [This work was supported by NIH Grant EY014371.]

Synthetically focused ultrasonic diagnostic imaging system for tissue and flow imaging

A synthetic focus ultrasound system is described in which individual transducer elements are activated to insonify the image field and the resultant echoes are received by a plurality of elements of the array. The echo signals are stored and the complete echo set is used to produce motion maps for different velocity vectors over the image field. Various ones of the motion maps are used to produce a wide variety of ultrasound image modes, including grey-scale images, colorflow images, and spectral flow displays.

Synthetic aperture focusing techniques in time and frequency domains for photoacoustic imaging

Synthetic aperture focusing techniques (SAFT) have been applied to B-scan images of artificial defects produced by a scanning photoacoustic probe operating under water. The probe consisted of a fibre-optic laser beam delivery system combined with a polymer (PVDF) transducer mounted at the tip of the probe for ultrasonic reception. This system had a broadband frequency response in the 1-10 MHz range. Such an integral probe was designed to optically transmit and receive near on-axis ultrasonic transients. A frequency domain synthetic aperture method was successfully applied using phantom samples to produce refined 2D images from initial B-scans. In this paper, the phantom consisted of a black nylon rectangular sample with a 1.0 mm circular defect at a depth of 5.9 mm from the surface. A comparison was made of conventional B-scan images with SAFT images. Results showed that synthetic focusing apertures, in time or frequency domains, offer better signal-to-noise ratios with improved capabilities in lateral resolution.

Extending nature's leads: the anticancer agent ellipticine.

The natural plant product ellipticine was isolated in 1959 from the Australian evergreen tree of the Apocynaceae family. This compound was found to be an extremely promising anticancer drug. The planar polycyclic structure was found to interact with DNA through intercalation, exhibiting a high DNA binding affinity (10(6) M(-1)). The presence of protonatable ring nitrogens distinguished ellipticine from other simple intercalators. Both monocationic and uncharged species were found to be present under physiological conditions. The positive charge stabilized the binding of ellipticine to nucleic acids, while the more lipophilic uncharged compound was shown to readily penetrate membrane barriers. The structural nature of these compounds offers a plausible basis for the implication of multiple modes of action, including DNA binding, interactions with membrane barriers, oxidative bioactivation and modification of enzyme function; most notably that of topoisomerase II and telomerase. Pharmacologically, a number of toxic side effects have been shown to be problematic, but the amenability of ellipticine towards systematic structural modification has permitted the extensive application of rational drug design. A number of successful ellipticine analogs have been designed and synthesized with improved toxicities and anticancer activities. More recently the synthetic focus has broadened to include the design of hybrid compounds, as well as drug delivery conjugates. Considerable research efforts have been directed towards gaining a greater understanding of the mechanism of action of these drugs that will aid further in the optimization of drug design.

Apparatus and system for real-time synthetic focus ultrasonic imaging

An apparatus for use in an ultrasonic imaging system adapted for generating synthetic focus images in real-time. The apparatus employs an integrated circuit architecture which is scalable and can be employed to construct real-time synthetic focus ultrasonic imaging systems having a large number of transducer array elements. The apparatus utilizes one field programmable gate array per image channel for data storage and sub-image generation. Each field programmable gate array includes on-chip block memories to store digitized ultrasonic signal return echo data and at least one block memory used to store data associated with each ultrasonic transmitter-receiver array pair. Logic inside the field programmable gate array calculates memory addresses during image generation to generate a time-of-flight surface and to form sub-images.

A scalable architecture for real-time synthetic-focus imaging.

A scalable architecture for forming real-time synthetic focus images is described and the design of a 256-channel system using currently-available technology is presented as an example implementation of the architecture. The parallelism of the system scales directly with the number of array elements and the image computation rate for a given image size (in pixels) stays constant as the number of array elements is increased. The system leverages earlier work in the real-time generation of the required time-of-flight surfaces and allows either real-time image generation or iterative adaptive image generation from a single complete data set.

Condensation of Glycidoxypropyltrimethoxysilane to Afford Siloxane Pre-polymers useful in the Preparation of Hardcoating Resins

This paper reports on the formation of polysiloxanes (3) from glycidoxypropyltrimethoxysilane using alkali catalysis that exclude the formation of tri-condensed siloxane units during synthesis. The synthetic focus was on the elucidation of the species formed, and maximizing pre-polymerisation of the siloxane without introducing tri-condensed species into the polymer chains. The siloxanes synthesised here were used to prepare hardcoating resins. The physical properties of the hardcoatings obtained from the cure of these resins were characterized by the Bayer abrasion test. It was found that siloxanes of the formula (3), when combined with polydimethoxysiloxane formula (4), afforded hardcoatings with a Bayer abrasion ratio of 6 to 8—an improvement of nearly two fold over most commercially available ophthalmic lens hardcoatings. Crazing of the hardcoating during cure was found to be significantly reduced, and resin potlives were found to be longer.

Diagnostic ultrasound imaging method and system with improved frame rate

A medical diagnostic ultrasonic imaging system acquires receive beams from spatially distinct transmit beams. The receive beams alternate in type between at least first and second types across the region being imaged. The first and second types of receive beams differ in at least one scan parameter other than transmit and receive line geometry, and can for example differ in transmit phase, transmit or receive aperture, system frequency, transmit focus, complex phase angle, transmit code or transmit gain. Receive beams associated with spatially distinct ones of the transmit beams (including at least one beam of the first type and at least one beam of the second type) are then combined. In this way, many two-pulse techniques, including, for example, phase inversion techniques, synthetic aperture techniques, synthetic frequency techniques, and synthetic focus techniques, can be used while substantially reducing the frame rate penalty normally associated with such techniques.

Two-dimensional synthetic focusing using large apertures—Deconvolution of the spatial impulse response

The synthetic aperture focusing technique (SAFT) has been used successfully in both medical and nondestructive evaluation (NDE) applications of ultrasound. Transducer size is an important issue when implementing SAFT algorithms. The classical SAFT method is based on the assumption that a point source is used for emitting ultrasonic waves, which means in practice that the transducer aperture is so small compared to the wavelength that it emits spherical waves. If the transducer aperture is large its spatial impulse response (SIR) significantly differs from a Dirac pulse and nonspherical waves will be generated. Therefore, to successfully perform synthetic focusing using large apertures, the transducer’s SIR must be taken into account. A new method for compensating transducer SIR in synthetic focusing is presented in the paper. The method employs a stochastic, time-domain Wiener filtering technique for two-dimensional synthetic focusing. Results of the experiments performed in water using a linear phased array are presented to demonstrate the performance of the proposed method. The use of an array enabled altering the aperture without changing the electrical characteristics of the whole system. The results obtained using the proposed technique for finite apertures are compared to those obtained with a classical SAFT algorithm.