共查询到20条相似文献,搜索用时 15 毫秒
1.
Busse LJ 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1992,39(2):174-179
A frequency-domain method for implementing the synthetic aperture focusing technique is developed and demonstrated using computer simulation. As presented, the method is well suited to reconstructing ultrasonic reflectivity over a volumetric region of space using measurements made over an adjacent two-dimensional aperture. Extensive use is made of both one- and two-dimensional Fourier transformations to perform the temporal and spatial correlation required by the technique, making the method well suited to general-purpose computing hardware. Results are presented demonstrating both the lateral and axial resolution achieved by the method. The effect of limiting the reconstruction bandwidth is also demonstrated. 相似文献
2.
Yamani A. 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1997,44(4):943-947
The drawbacks of the multifrequency automatic focusing technique (MF-AFT) have been eased considerably when the scanned-frequency signals are transmitted simultaneously in a pulse. While the focusing capabilities of the MF-AFT are maintained, substantial reduction in data processing is obtained when the new procedure is used. In addition, the derived pulse can be tailored to further enhance the quality of the image produced by this new procedure. Simulation results are included to demonstrate the focusing capabilities of the procedure for both two- and three-dimensional objects 相似文献
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Li ML Guan WJ Li PC 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》2004,51(1):63-70
Synthetic aperture focusing using a virtual source was used previously to increase the penetration and to extend the depth of focus in high-frequency ultrasonic imaging. However, the performance of synthetic aperture focusing is limited by its high sidelobes. In this paper, an adaptive weighting technique based on a focusing-quality index is introduced to suppress the sidelobes. The focusing-quality index is derived from the spatial spectrum of the scan-line data along the mechanical scan direction (i.e., the synthetic aperture direction) after focusing delays relative to the virtual source have been applied. The proposed technique is of particular value in high-frequency ultrasound in which dynamic focusing using array transducers is not yet possible. Experimental ultrasound data from a 50-MHz imaging system with a single-crystal transducer (f-number=2) are used to demonstrate the efficacy of the proposed technique on both wire targets and speckle-generating objects. An in vivo experiment also is performed on a mouse to further demonstrate the effectiveness. Both 50-MHz fundamental imaging and 50-MHz tissue harmonic imaging are tested. The results clearly demonstrate the effectiveness in sidelobe reduction and background-noise suppression for both imaging modes. The principles, experimental results, and implementation issues of the new technique are described in this paper. 相似文献
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We have improved the resolution of our laser optical feedback imaging (LOFI) setup by using a galvanometric scanning and two-dimensional angular synthetic aperture (SA) process. The experimental resolution of the classical LOFI images and of the SA LOFI images are compared for different working distances, and we show that the SA LOFI method is able to balance the degradation of the images resolution with increasing distance. We also show that the resolution of the SA LOFI images can be controlled by choosing the position of the galvanometric scanner between the laser and the target under investigation. Theoretical and experimental SA resolutions are compared. 相似文献
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Directional synthetic aperture flow imaging 总被引:1,自引:0,他引:1
Jensen JA Nikolov SI 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》2004,51(9):1107-1118
A method for flow estimation using synthetic aperture imaging and focusing along the flow direction is presented. The method can find the correct velocity magnitude for any flow angle, and full color flow images can be measured using only 32 to 128 pulse emissions. The approach uses spherical wave emissions with a number of defocused elements and a linear frequency-modulated pulse (chirp) to improve the signal-to-noise ratio. The received signals are dynamically focused along the flow direction and these signals are used in a cross-correlation estimator for finding the velocity magnitude. The flow angle is manually determined from the B-mode image. The approach can be used for both tissue and blood velocity determination. The approach was investigated using both simulations and a flow system with a laminar flow. The flow profile was measured with a commercial 7.5 MHz linear array transducer. A plastic tube with an internal diameter of 17 mm was used with an EcoWatt 1 pump generating a laminar, stationary flow. The velocity profile was measured for flow angles of 90 and 60 degrees. The RASMUS research scanner was used for acquiring radio frequency (RF) data from 128 elements of the array, using 8 emissions with 11 elements in each emission. A 20-micros chirp was used during emission. The RF data were subsequently beamformed off-line and stationary echo canceling was performed. The 60-degree flow with a peak velocity of 0.15 m/s was determined using 16 groups of 8 emissions, and the relative standard deviation was 0.36% (0.65 mm/s). Using the same setup for purely transverse flow gave a standard deviation of 1.2% (2.1 mm/s). Variation of the different parameters revealed the sensitivity to number of lines, angle deviations, length of correlation interval, and sampling interval. An in vivo image of the carotid artery and jugular vein of a healthy 29-year-old volunteer was acquired. A full color flow image using only 128 emissions could be made with a high-velocity precision. 相似文献
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Efficient synthetic aperture imaging from a circular aperture with possible application to catheter-based imaging 总被引:1,自引:0,他引:1
O'Donnell M Thomas LJ 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1992,39(3):366-380
Phased-array imaging, including complete dynamic focus, is explored for imaging using a circular aperture. Based on the constraints of catheter-based systems, an efficient synthetic aperture method has been developed for imaging using a single wire connection between the imaging array and external electronics. The method employs a highly sampled array with an element pitch small compared to the acoustic wavelength. On any given firing of the array, however, a large number of channels are electrically connected on both transmission and reception. From firing to firing, one element is dropped and one new element is included, in analogy to a classic linear array system. Using an optimal filtering approach for synthetic aperture reconstruction, a dynamically focused image exhibiting diffraction limited resolution is produced. The results of detailed simulations are presented demonstrating the capabilities of the method. In addition, the prospects for real-time implementation of the reconstruction are discussed. 相似文献
10.
Lockwood GR Talman JR Brunke SS 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1998,45(4):980-988
A method for real-time three-dimensional (3-D) ultrasound imaging using a mechanically scanned linear phased array is proposed. The high frame rate necessary for real-time volumetric imaging is achieved using a sparse synthetic aperture beamforming technique utilizing only a few transmit pulses for each image. Grating lobes in the two-way radiation pattern are avoided by adjusting the transmit element spacing and the receive aperture functions to account for the missing transmit elements. The signal loss associated with fewer transmit pulses is minimized by increasing the power delivered to each transmit element and by using multiple transmit elements for each transmit pulse. By mechanically rocking the array, in a way similar to what is done with an annular array, a 3-D set of images can be collected in the time normally required for a single image. 相似文献
11.
We have improved the resolution of our laser optical feedback imaging (LOFI) setup by using a synthetic aperture (SA) process. We report a two-dimensional (2D) SA LOFI experiment where the unprocessed image (i.e., the classical LOFI image) is obtained point by point, line after line using full 2D galvanometric scanning. The 2D superresolved image is then obtained by successively computing two angular SA operations while a one-dimensional angular synthesis is preceded by a frequency synthesis to obtain a 2D superresolved image conventionally in the synthetic aperture radar (SAR) method and their corresponding laser method called synthetic aperture ladar. The numerical and experimental results are compared. 相似文献
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Bandyopadhyay A Stepanov A Schulkin B Federici MD Sengupta A Gary D Federici JF Barat R Michalopoulou ZH Zimdars D 《Journal of the Optical Society of America. A, Optics, image science, and vision》2006,23(5):1168-1178
The stand-off imaging properties of a terahertz (THz) interferometric array are examined. For this application, the imaged object is in the near-field region limit of the imaging array. In this region, spherical and circular array architectures can compensate for near-field distortions and increase the field of view and depth of focus. Imaging of THz point sources is emphasized to demonstrate the imaging method and to compare theoretical predictions to experimental performance. 相似文献
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Tong JH Chiu CL Wang CY 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》2010,57(11):2512-2521
A useful nondestructive testing tool for civil engineering should immediately reveal defects inside concrete structures at the construction sites. To date, there are few effective methods to image defects inside concrete structures. In this paper, a new nondestructive testing method using elastic waves for imaging possible defects inside concrete is developed. This method integrates the point-source/point receiver scheme with the synthetic aperture focusing technique (SAFT) to increase functioning depth and enhance received signals. To improve image quality, received signals are processed by Hilbert-Huang transform (HHT) to get time-frequency curves for the SAFT process. Compared with conventional SAFT method processing with time-amplitude signals, this new method is capable of providing a better image of defects not only in the numerical simulation but also in the experimental result. The image can reveal the number of defects and their locations and front-end profiles. The results shown in this paper indicate that this new elastic-wave-based method exhibits high capability in imaging the defects of in situ concrete structures. 相似文献
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Nikolov SI Jensen JA Tomov BG 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》2008,55(8):1755-1767
This paper describes the design and implementation of a real-time delay-and-sum synthetic aperture beamformer. The beamforming delays and apodization coefficients are described parametrically. The image is viewed as a set of independent lines that are defined in 3D by their origin, direction, and inter-sample distance. The delay calculation is recursive and inspired by the coordinate rotation digital computer (CORDIC) algorithm. Only 3 parameters per channel and line are needed for their generation. The calculation of apodization coefficients is based on a piece- wise linear approximation. The implementation of the beamformer is optimized with respect to the architecture of a novel synthetic aperture real-time ultrasound scanner (SARUS), in which 4 channels are processed by the same set of field-programmable gate arrays (FPGA). In synthetic transmit aperture imaging, low-resolution images are formed after every emission. Summing all low-resolution images produces a perfectly focused high-resolution image. The design of the beamformer is modular, and a single beamformation unit can produce 4600 low-resolution images per second, each consisting of 32 lines and 1024 complex samples per line. In its present incarnation, 3 such modules fit in a single device. The summation of low-resolution images is performed internally in the FPGA to reduce the required bandwidth. The delays are calculated with a precision of 1/16th of a sample, and the apodization coefficients with 7-bit precision. The accumulation of low-resolution images is performed with 24-bit precision. The level of the side- and grating lobes, introduced by the use of integer numbers in the calculations and truncation of intermediate results, is below -86 dB from the peak. 相似文献
15.
O'Donnell M Wang Y 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》2005,52(2):171-176
Peak acoustic power limits the signal-to-noise ratio (SNR) of real-time ultrasound images. For most conventional scan formats, however, the average power is well below heating limits. This means the SNR can be significantly increased using coded excitation. A coded system transmits a broadband, temporally elongated excitation pulse with a finite time-bandwidth product. The received signal must be decoded to produce an imaging pulse with improved SNR resulting from the higher average power in the elongated excitation. Decoding can produce significant range side lobes, however, greatly reducing image quality. All practical coding designs, therefore, represent a trade-off between SNR gain and range side lobes. A specific coding scheme appropriate for synthetic aperture imaging is presented. A 14.5 dB SNR improvement with acceptable range side lobes is demonstrated on a forward-looking imaging system appropriate for intravascular applications. 相似文献
16.
Ylitalo J.T. Ermert H. 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1994,41(3):333-339
An ultrasound synthetic aperture imaging method based on a monostatic approach was studied experimentally. The proposed synthetic aperture method offers good dynamical resolution along with fast numerical reconstruction. In this study complex object data were recorded coherently in a two-dimensional hologram using a 3.5 MHz single transducer with a fairly wide-angle beam. Image reconstruction which applies the wavefront backward propagation method and the near-field curvature compensation was performed numerically in a microcomputer using the spatial frequency domain. This approach allows an efficient use of the FFT-algorithms. Because of the simple and fast scanning scheme and the efficient reconstruction algorithms the method can be made real-time. The image quality of the proposed method was studied by evaluating the spatial and dynamical resolution in a waterbath and in a typical tissue-mimicking phantom. The lateral as well as the range resolution (-6 dB) were approximately 1 mm in the depth range of 30-100 mm. The dynamical resolution could be improved considerably when the beam width was made narrower. Although it resulted in a slightly reduced spatial resolution this compromise has to be done for better resolution of low-contrast targets such as cysts. The study showed that cysts as small as 2 mm by diameter could be resolved 相似文献
17.
Daher NM Yen JT 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》2006,53(5):912-924
A two-dimensional (2-D) array of 256 X 256 = 65,536 elements, with total area 4 X 4 = 16 cm2, serves as a flexible platform for developing acquisition schemes for 3-D rectilinear ultrasound imaging at 10 MHz using synthetic aperture techniques. This innovative system combines a simplified interconnect scheme and synthetic aperture techniques with a 2-D array for 3-D imaging. A row-column addressing scheme is used to access different elements for different transmit events. This addressing scheme is achieved through a simple interconnect, consisting of one top, one bottom single-layer, flex circuits that, compared to multilayer flex circuits, are simpler to design, cheaper to manufacture, and thinner so their effect on the acoustic response is minimized. We present three designs that prioritize different design objectives: volume acquisiton time, resolution, and sensitivity, while maintaining acceptable figures for the other design objectives. For example, one design overlooks time-acquisition requirements, assumes good noise conditions, and optimizes for resolution, achieving -6 dB and -20 dB beamwidths of less than 0.2 and 0.5 mm, respectively, for an F/2 aperture. Another design can acquire an entire volume in 256 transmit events, with -6 dB and -20 dB beamwidths in the order of 0.4 and 0.8 mm, respectively. 相似文献
18.
We present a scheme for radar imaging of satellites by the inverse synthetic aperture radar technique. We include some general principles in image formation when dealing with microwave-scattering experiments, such as radar observation of a satellite, so that the article is not confined to the radar community. We propose an original motion compensation algorithm for image autofocusing: This one-stage algorithm is faster than usual two-stage algorithms and turns to be quite accurate, which can be measured by quality criteria and visual inspection of reconstructed images. We show its application to imaging of the Russian space station Mir with two different radar systems. © 1998 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 9, 24–28, 1998 相似文献
19.
Glastre W Lacot E Jacquin O Hugon O de Chatellus HG 《Journal of the Optical Society of America. A, Optics, image science, and vision》2012,29(4):476-485
In this paper, we compare the sensitivity of two imaging configurations, both based on laser optical feedback imaging (LOFI). The first one is direct imaging, which uses conventional optical focalization on target, and the second one is made by a synthetic aperture (SA) laser, which uses numerical focalization. We show that SA configuration allows us to obtain good resolutions with high working distance and that the drawback of SA imagery is that it has a worse photometric balance in comparison to a conventional microscope. This drawback is partially compensated by the important sensitivity of LOFI. Another interest of SA relies on the capacity of getting three-dimensional information in a single x-y scan. 相似文献
20.
Because the existing range-Doppler algorithm in inverse synthetic aperture sonar (ISAS) is based on target model of uniform motion, it may be invalidated for maneuvering targets due to the time-varying changes of both individual scatter′s Doppler and imaging projection plane. To resolve the problem, a new range-instantaneous Doppler imaging method is proposed for imaging maneuvering targets based on time-frequency analysis. The proposed approach is verified using real underwater acoustic data. 相似文献