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Crossed electrode arrays address some of the challenges associated with 3-D ultrasound imaging because of the significant reduction in the number of elements (2N versus N2). However, creating a two-way focused 3-D image in real time is difficult with these arrays because azimuth and elevation dimensions cannot be beamformed at the same time. This work describes a new 3-D imaging technique that uses the flexibility of bias-sensitive substrates to create a high-quality elevation focus on a crossed electrode array. The principle behind this technique is to perform conventional compound imaging with an azimuth set of electrodes while implementing a bias controllable elevation lens with an elevation set of electrodes. On transmit, the biases are chosen to mimic a Fresnel lens. Then, on receive, the Hadamard coding is implemented along the elevation dimension. After decoding, we gain the RF data for each element across the elevation aperture even though there is effectively only one channel in that dimension. A 30-MHz, 128-element crossed electrode relaxor array was fabricated on a 1–3 electrostrictive composite substrate and was used to demonstrate the performance of the imaging technique. The on-axis −6-dB beamwidths were simulated to be 175 and 150 μm in the azimuth and elevation directions, respectively, and the focus remained isotropic in the furthest elevation slice. Images were generated of a wire phantom to confirm the performance of the azimuth and elevational radiation patterns with good agreement between simulation and experiment. High-resolution 3-D volumetric images were generated of an ex vivo rat brain. Images of the cerebellum showed that the white and gray matter tracts could clearly be visualized with isometric resolution in both the azimuth and elevation dimensions. [ABSTRACT FROM AUTHOR]

We present a new transmit pulse encoding scheme for ultrafast phased-array imaging called sparse orthogonal diverging wave imaging (SODWI). In SODWI, Hadamard encoding is used to selectively invert transmit pulse phases beamformed with a diverging wave delay profile. This approach has the advantage of delivering energy to a much wider field of view than conventional Hadamard-encoded multielement synthetic transmit aperture (HMSTA), making it more suitable for phased-array applications. With SODWI, we use a synthetic transmit element delay insertion (STEDI) approach which produces significant improvements in resolution, grating lobe level, and signal-to-noise ratio (SNR) over HMSTA. We also show how in SODWI a subset of the Hadamard codes can be sparsely selected to increase the imaging frame rate at the expense of image quality. SODWI is then compared with a variety of beamforming schemes for phased-array applications, including HMSTA, STEDI-HMSTA, diverging wave imaging (DWI), synthetic aperture (SA), and focused imaging. We present the results by implementing this technique on a 64-channel custom beamforming platform with a 40-MHz phased array. When a full set of codes is used, SODWI outperforms focused imaging contrast and SNR by 2.7 and 1.8 dB in addition to an $8\times $ increase in frame rate, respectively. [ABSTRACT FROM AUTHOR]

In the medical fields, ultrasound detection is often performed with piezoelectric arrays that enable one to simultaneously map the acoustic fields at several positions. In this work, we develop a novel method for transforming a single-element ultrasound detector into an effective detection array by spatially filtering the incoming acoustic fields using a binary acoustic mask coded with cyclic Hadamard patterns. By scanning the mask in front of the detector, we obtain a multiplexed measurement data set from which a map of the acoustic field is analytically constructed. We experimentally demonstrate our method by transforming a single-element ultrasound detector into 1-D arrays with up to 59 elements. [ABSTRACT FROM AUTHOR]

Hadamard-encoded multipulses (HEM) transmit has recently been utilized for tissue background suppression in ultrasound contrast imaging to enhance the contrast-to-tissue ratio (CTR). Nonetheless, the second-harmonic component in HEM transmit results in residual tissue background after decoding and, thus, compromises the detection of contrast microbubbles. Theoretically, high-order HEM transmit can produce harmonic-free background but the memory effect, which considers the nonlinear contribution of previous bit waveform into the next one in the progress of harmonic generation, may limit the achievable tissue suppression. In this paper, three possible harmonic-free pairs using time-shifted subtraction (SH1, SH2, and SH3) in the fourth-order Hadamard decoding are analyzed and experimentally compared using hydrophone measurement and B-mode imaging. Moreover, the orthogonal decoding (OD) of HEM transmit is also proposed with pulse-inversion harmonic suppression (PIHS) to remedy memory effect on the tissue background. Results show that SH3, which utilizes the third and fourth rows for decoding, provides the lowest magnitude of tissue background among all possible decoding pairs and performs comparably to the reference PI and amplitude-modulation sequence in terms of CTR. For PIHS-OD, the pulse subtraction effectively removes the harmonic interferences from memory effect and, thus, further improves the CTR by 5.4 dB compared to SH3. For high-order HEM transmit, PIHS-OD can help to eliminate the residual tissue background due to memory effect and is comparable to Hadamard decoding in temporal resolution and possible motion artifacts. [ABSTRACT FROM AUTHOR]

Contrast-enhanced ultrasound (CEUS) imaging has great potential for use in many new ultrasound clinical applications. We recently proposed a novel CEUS imaging sequence, Hadamard-encoded multipulses (HEM), to improve the signal-to-noise ratio (SNR) and the contrast-to-tissue ratio (CTR) as compared to other classic CEUS methods. HEM increases microbubble responses by using longer coded transmit pulses and the fast polarity change between coded pulses. In this study, we propose to combine the HEM pulse with multiplane wave (MW) imaging technique to further improve CEUS imaging SNR and contrast-to-noise ratio. During MW-HEM transmissions, the microbubbles undergo multiple fast pulse polarity changes, leading to significantly improved microbubble nonlinear responses, and thus further enhanced SNR and CTR as compared to HEM alone or other CEUS sequences. This improvement may facilitate more robust CEUS imaging for deep abdominal organs and the heart. [ABSTRACT FROM AUTHOR]

The development of contrast-enhanced ultrasound (CEUS) imaging offers great opportunities for new ultrasound clinical applications such as myocardial perfusion imaging and abdominal lesion characterization. In CEUS imaging, the contrast agents (i.e., microbubbles) are utilized to improve the contrast between blood and tissue based on their high nonlinearity under low ultrasound pressure. In this paper, we propose a new CEUS pulse sequence by combining Hadamard-encoded multipulses (HEM) with fundamental frequency bandpass filter (i.e., filter centered on transmit frequency). HEM consecutively emits multipulses encoded by a second-order Hadamard matrix in each of the two transmission events (i.e., pulse-echo events), as opposed to conventional CEUS methods which emit individual pulses in two separate transmission events (i.e., pulse inversion (PI), amplitude modulation (AM), and PIAM). In HEM imaging, the microbubble responses can be improved by the longer transmit pulse, and the tissue harmonics can be suppressed by the fundamental frequency filter, leading to significantly improved contrast-to-tissue ratio (CTR) and signal-to-noise ratio (SNR). In addition, the fast polarity change between consecutive coded pulse emissions excites strong nonlinear microbubble echoes, further enhancing the CEUS image quality. The spatial resolution of HEM image is compromised as compared to other microbubble imaging methods due to the longer transmit pulses and the lower imaging frequency (i.e., fundamental frequency). However, the resolution loss was shown to be negligible and could be offset by the significantly enhanced CTR, SNR, and penetration depth. These properties of HEM can potentially facilitate robust CEUS imaging for many clinical applications, especially for deep abdominal organs and heart. [ABSTRACT FROM AUTHOR]