The emergence of ultrafast frame rates in ultrasonic imaging has been recently made possible by the development of new imaging modalities such as transient elastography. Data acquisition rates reaching more than thousands of images per second enable the real-time visualization of shear mechanical waves propagating in biological tissues, which convey information about local viscoelastic properties of tissues. The first proposed approach for reaching such ultrafast frame rates consists of transmitting plane waves into the medium. However, because the beamforming process is then restricted to the receive mode, the echographic images obtained in the ultrafast mode suffer from a low quality in terms of resolution and contrast and affect the robustness of the transient elastography mode. It is here proposed to improve the beamforming process by using a coherent recombination of compounded plane-wave transmissions to recover high-quality echographic images without degrading the high frame rate capabilities. A theoretical model is derived for the comparison between the proposed method and the conventional B-mode imaging in terms of contrast, signal-to-noise ratio, and resolution. Our model predicts that a significantly smaller number of insonifications, 10 times lower, is sufficient to reach an image quality comparable to conventional B-mode. Theoretical predictions are confirmed by in vitro experiments performed in tissue-mimicking phantoms. Such results raise the appeal of coherent compounds for use with standard imaging modes such as B-mode or color flow. Moreover, in the context of transient elastography, ultrafast frame rates can be preserved while increasing the image quality compared with flat insonifications. Improvements on the transient elastography mode are presented and discussed.

Coherent plane-wave compounding for very high frame rate ultrasonography and transient elastography

Although the use of ultrasonic plane-wave transmissions rather than line-per-line focused beam transmissions has been long studied in research, clinical application of this technology was only recently made possible through developments in graphical processing unit (GPU)-based platforms Far beyond a technological breakthrough, the use of plane or diverging wave transmissions enables attainment of ultrafast frame rates (typically faster than 1000 frames per second) over a large field of view This concept has also inspired the emergence of completely novel imaging modes which are valuable for ultrasound-based screening, diagnosis, and therapeutic monitoring In this review article, we present the basic principles and implementation of ultrafast imaging In particular, present and future applications of ultrafast imaging in biomedical ultrasound are illustrated and discussed

Ultrafast imaging in biomedical ultrasound

For pt.I see ibid., vol.38, no.2, p.100-8 (1991). The authors describe the design, application, and evaluation of parallel processing to the high-speed volumetric ultrasound imaging system. The scanner produces images analogous to an optical camera or the human eye and supplies more information than conventional sonograms. Potential medical applications include improved anatomic visualization, tumor localization, and better assessment of cardiac function. The system uses pulse-echo phased array principles to steer a 2-D array transducer of 289 elements in a pyramidal scan format. Parallel processing in the receive mode produces 4992 scan lines at a rate of approximately 8 frames/s. Echo data for the scanned volume is presented online as projection images with depth perspective, stereoscopic pairs, or multiple tomographic images. The authors also describe the techniques developed for the online display of volumetric images on a conventional CRT oscilloscope and show preliminary volumetric images for each display mode. >

High-speed ultrasound volumetric imaging system. II. Parallel processing and image display

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.

Real-time 3-D ultrasound imaging using sparse synthetic aperture beamforming

The major problem facing the development of 2-D arrays for imaging is the complexity arising from the large number of elements anticipated in such transducers. The authors have undertaken a theoretical investigation of the focusing and steering properties of pulsed 2-D arrays to characterize the parameters required for medical imaging, such as element size, spacing, and number of elements. Details of the computational methods employed are presented, as well as a discussion of the steered beam properties of wideband 2-D arrays. The effects of apodization and element cross-coupling on the beam properties of a 2-D transducer array are examined. The beam properties of various sparse arrays with elements randomly distributed over the aperture of the transducer are discussed. >

Beam steering with pulsed two-dimensional transducer arrays

Ultrafast cardiac‐valve ultrasonic tomography requires parallel multichannel processing of received echoes. In parallel processing the level of secondary ’’ghost’’ images due to spatial undersampling is much higher than in slower series processors which use a selective field insonification. The paper describes a 20‐channel moving‐focus parallel‐processing analog electronic system, which is realized in our laboratory. It is shown analytically that the secondary‐image level is reduced and remains unchanged when the receiver angular aperture (aperture relative to distance) is limited and kept constant during the whole observation time.

Acoustical image reconstruction in parallel‐processing analog electronic systems

In acoustics, the analysis of pressure distribution in a field of surface sources is most frequently performed using the assumption that the source constitutes a part of an infinite planar rigid baffle (Rayleigh’s approximation). However, in many practical cases of ultrasonic echography, assumption of a soft pressure‐release baffle (the Rayleigh‐Sommerfeld approximation) or of free‐field conditions (the Kirchhoff approximation) seemed to be better matched to real conditions. A theoretical survey of the planar baffles problem is given in this paper, and its practical aspects in acoustic source radiation are discussed. Some experiments, showing the influence of different boundary conditions onto radiation patterns, verify the theoretical predictions.

The infinite planar baffles problem in acoustic radiation and its experimental verification

In order to improve ultrasonic image quality, transducers, similar to those currently used in nondestructive testing and medical imaging applications, have been studied extensively. Attention has been given to single isolated transducers, for which anomalous radiation patterns have been obtained, and to transducers integrated in an array structure, where additional problems occur due to coupling effects between nearest neighbors. An experimental study of these phenomena has made it possible in both cases to find the origin of the perturbations with respect to the theoretical situations which occur in the radiation pattern. Solutions for increasing these perturbations are also suggested.

Anomalous behavior in the radiation pattern of piezoelectric transducers induced by parasitic Lamb wave generation

Acoustic diffraction of plane impulsive waves is considered for free‐field, baffled, and pressure release boundary conditions, in the case of an arbitrary spatial distribution of combined, amplitude‐time delay modulation in an aperture. The method is based on the impulse response analysis of parallel aperture lines, the line impulse responses being then integrated to give an aperture impulse response. A closed‐form, analytical expression is derived for lines having an arbitrary amplitude modulation. In the case of nonmodulated apertures of an arbitrary shape, the aperture impulse response is also of an analytical form, directly involving the aperture contour line. The analysis is performed with emphasis on maintaining clear physical sense of successive evaluation steps.

B. Delannoy

Papers

Acoustical image reconstruction in parallel‐processing analog electronic systems

The infinite planar baffles problem in acoustic radiation and its experimental verification

Anomalous behavior in the radiation pattern of piezoelectric transducers induced by parasitic Lamb wave generation

Acoustic diffraction analysis by the impulse response method: A line impulse response approach

Electrical coupling effects in an ultrasonic transducer array