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Publications

"If I have seen a little further it is by standing on the shoulders of giants." - Isaac Newton

Under Review

Y. Zhang, et al., L.V. Wang*. “Rotational ultrasound and photoacoustic tomography of the human body.” under review.

Y. Zhang, et al., L.V. Wang*. “Functional photoacoustic noninvasive Doppler angiography in humans.” under review.

Y. Zhang, et al., W.-N. Lee*. “Ultrasound angiography with tissue echoes filtering and adaptive image formation,” under review.

Transcranial photoacoustic computed tomography of human brain function

arXiv preprint, Jun. 2022. In preparation for peer reviewed article.

Herein we report the first known in-human transcranial imaging of brain function using photoacoustic computed tomography. Functional responses to benchmark motor tasks were imaged on both the skull-less and the skull-intact hemispheres of a hemicraniectomy patient. The observed brain responses in these preliminary results demonstrate the potential of photoacoustic computed tomography for achieving transcranial functional imaging. (Read full article)

[Y. Zhang, S. Na, K. Sastry, JJ. Russin], P. Hu, L. Lin, X. Tong, KB. Jann, DJ. Wang, C. Y. Liu*, L.V. Wang*. “Transcranial photoacoustic computed tomography of human brain function.” arXiv preprint, 2022. 

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Photoacoustic vector tomography for deep hemodynamic imaging

Nature Biomedical Engineering, 2023.

Non-invasive imaging of deep blood vessels for mapping hemodynamics remains an open quest in biomedical optical imaging. Although pure optical imaging techniques offer rich optical contrast of blood and have been reported to measure blood flow, they are generally limited to surface imaging within the optical diffusion limit of about one millimeter. Herein, we present photoacoustic vector tomography (PAVT), breaking through the optical diffusion limit to image deep blood flow with speed and direction quantification. PAVT synergizes the spatial heterogeneity of blood and the photoacoustic contrast; it compiles successive single-shot, wide-field photoacoustic images to directly visualize the frame-to-frame propagation of the blood with pixel-wise flow velocity estimation. We demonstrated in vivo that PAVT allows hemodynamic quantification of deep veins at five times the optical diffusion limit (more than five millimeters), leading to vector mapping of blood flow in humans. By offering the capability for deep hemodynamic imaging with optical contrast, PAVT may become a powerful tool for monitoring and diagnosing vascular diseases and mapping circulatory system function. (Read full article)

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[Y. Zhang, J. O. Gibson], A. Khadria, L.V. Wang*. “Photoacoustic vector tomography for deep hemodynamic imaging.” Nature Biomedical Engineering, 2023.

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This work is highlighted by Nature Reviews Cardiology: Fernández-Ruiz, I. Photoacoustic method enables deep imaging of blood flow. Nat Rev Cardiol 21, 72 (2024).

Cross-ray ultrasound tomography and photoacoustic tomography of cerebral hemodynamics in rodents

Advanced Science, 2022.

Recent advances in functional ultrasound imaging (fUS) and photoacoustic tomography (PAT) offer powerful tools for studying brain function. Complementing each other, fUS and PAT, respectively, measure the cerebral blood flow (CBF) and hemoglobin concentrations, allowing synergistic characterization of cerebral hemodynamics. Here, cross-ray ultrasound tomography (CRUST) and its combination with PAT are presented. CRUST employs a virtual point source from a spherically focused ultrasonic transducer (SFUST) to provide widefield excitation at a 4-kHz pulse repetition frequency. A full-ring-shaped ultrasonic transducer array whose imaging plane is orthogonal to the SFUST's acoustic axis receives scattered ultrasonic waves. Superior to conventional fUS, whose sensitivity to blood flow is angle-dependent and low for perpendicular flow, the crossed transmission and panoramic detection fields of CRUST provide omnidirectional sensitivity to CBF. Using CRUST-PAT, the CBF, oxygen saturation, and hemoglobin concentration changes of the mouse brain during sensory stimulation are measured, with a field of view of ≈7 mm in diameter, spatial resolution of ≈170 µm, and temporal resolution of 200 Hz. The results demonstrate CRUST-PAT as a unique tool for studying cerebral hemodynamics. (Read full article)

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Y. Zhang, et, al. “ xx of the human body,” Nature Biomedical Engineering, under review.

[S. Na, Y. Zhang], L.V. Wang*. “Cross-Ray ultrasound tomography and photoacoustic tomography of cerebral hemodynamics in rodents,” Advanced Science, 2022.

Imaging heart dynamics with ultrafast cascaded-wave ultrasound

IEEE Transactions on Ultrason. Frerroelectr. Freq. Control, 2019.

The heart is an organ with highly dynamic complexity, including cyclic fast electrical activation, muscle kinematics, and blood dynamics. Although ultrafast cardiac imaging techniques based on pulsed-wave ultrasound (PUS) have rapidly emerged to permit mapping of heart dynamics, they suffer from limited sonographic signal-to-noise ratio (SNR) and penetration due to insufficient energy delivery and inevitable attenuation through the chest wall. We hereby propose ultrafast cascaded-wave ultrasound (uCUS) imaging to depict heart dynamics in higher SNR and larger penetration than conventional ultrafast PUS. To solve the known tradeoff between the length of transmitted ultrasound signals and spatial resolution while achieving ultrafast frame rates (>1000 Hz), we develop a cascaded synthetic aperture (CaSA) imaging method. In CaSA, an array probe is divided into subapertures; each subaperture transmits a train of diverging waves. These diverging waves are weighted in both the aperture (i.e., spatial) and range (i.e., temporal) directions with a coding matrix containing only +1 and -1 polarity coefficients. A corresponding spatiotemporal decoding matrix is designed to recover backscattered signals. The decoded signals are thereafter beamformed and coherently compounded to obtain one high-SNR beamformed image frame. For CaSA with M subapertures and N cascaded diverging waves, sonographic SNR is increased by 10× log10(N×M) (dB) compared with conventional synthetic aperture (SA) imaging. The proposed uCUS with CaSA was evaluated with conventional SA and Hadamard-encoded SA (H-SA) methods in a calibration phantom for B-mode image quality and an invivo human heart in a transthoracic setting for the quality assessment of anatomical, myocardial motion, and chamber blood power Doppler images. Our results demonstrated that the proposed uCUS with CaSA (4 subapertures, 32 cascaded waves) improved SNR (+20.46 dB versus SA, +14.83 dB versus H-SA) and contrast ratio (+8.44 dB versus SA, +7.81 dB versus H-SA) with comparable spatial resolutions to and at the same frame rates as benchmarks. (Read full article)

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Y. Zhang, H. Li, W.-N. Lee*. “Imaging heart dynamics with ultrafast cascaded-wave ultrasound,” IEEE Transactions on Ultrason. Ferroelectr. Freq. Control, vol. 66, No. 9, pp. 1465-1479, 2019.

Ultrafast ultrasound imaging with cascaded dual-polarity waves

IEEE Transactions on Medical Imaging, 2018.

Ultrafast ultrasound imaging using plane or diverging waves, instead of focused beams, has advanced greatly the development of novel ultrasound imaging methods for evaluating tissue functions beyond anatomical information. However, the sonographic signal-to-noise ratio (SNR) of ultrafast imaging remains limited due to the lack of transmission focusing, and thus insufficient acoustic energy delivery. We hereby propose a new ultrafast ultrasound imaging methodology with cascaded dual-polarity waves (CDWs), which consists of a pulse train with positive and negative polarities. A new coding scheme and a corresponding linear decoding process were thereby designed to obtain the recovered signals with increased amplitude, thus increasing the SNR without sacrificing the frame rate. The newly designed CDW ultrafast ultrasound imaging technique achieved higher quality B-mode images than coherent plane-wave compounding (CPWC) and multiplane wave (MW) imaging in a calibration phantom, ex vivo pork belly, and in vivo human back muscle. CDW imaging shows a significant improvement in the SNR (10.71 dB versus CPWC and 7.62 dB versus MW), penetration depth (36.94% versus CPWC and 35.14% versus MW), and contrast ratio in deep regions (5.97 dB versus CPWC and 5.05 dB versus MW) without compromising other image quality metrics, such as spatial resolution and frame rate. The enhanced image qualities and ultrafast frame rates offered by CDW imaging beget great potential for various novel imaging applications. (Read full article)

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Y. Zhang, Y. Guo, W.-N. Lee*. “Ultrafast ultrasound imaging with cascaded dual-polarity waves,” IEEE Transactions on Medical Imaging, vol. 37, No. 4, pp. 906-917, 2018.

Ultrafast ultrasound imaging using combined transmissions with cross-coherence based reconstruction

IEEE Transactions on Medical Imaging, 2018.

Plane-wave-based ultrafast imaging has become the prevalent technique for non-conventional ultrasound imaging. The image quality, especially in terms of the suppression of artifacts, is generally compromised by reducing the number of transmissions for a higher frame rate. We hereby propose a new ultrafast imaging framework that reduces not only the side lobe artifacts but also the axial lobe artifacts using combined transmissions with a new coherence-based factor. The results from simulations, in vitro wire phantoms, the ex vivo porcine artery, and the in vivo porcine heart show that our proposed methodology greatly reduced the axial lobe artifact by 25±5 dB compared with coherent plane-wave compounding (CPWC), which was considered as the ultrafast imaging standard, and suppressed side lobe artifacts by 15 ± 5 dB compared with CPWC and coherent spherical-wave compounding. The reduction of artifacts in our proposed ultrafast imaging framework led to a better boundary delineation of soft tissues than CPWC. (Read full article)

Y. Zhang, Y. Guo, W.-N. Lee*. “Ultrafast ultrasound imaging using combined transmissions with cross-coherence based reconstruction” IEEE Transactions on Medical Imaging, vol. 37, No. 2, pp. 337-348, 2018.

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Improving the accuracy of time difference measurement by reducing the impact of baseline shift

IEEE Transactions on Instrum. Meas., 2015.

Many time-difference measurement devices such as time-to-digital converters (TDCs) are designed on the basis of threshold or zero cross detection technology. However, baseline shift, which means that the detected signals may have a shift of amplitude of the whole signal, is common in electronic circuits and has a great influence on the accuracy and stability of the time-difference measured with conventional posedge or negedge detection methods. In general, the baseline shifts can be classified into two cases which are parallel and oblique baseline shifts according to their different origins. Parallel baseline shift means that the baselines of two detected signals are parallel but in different levels, while oblique baseline shift is that the baselines of two detected signals are not parallel and the baselines have a nearly linear relationship with the time in the duration of signals. In this paper, detection methods using multiple edges, namely, double-edge and triple-edge methods, are proposed to reduce the impact of relative baseline shift between two signals for high-precision time-difference measurement because they require more information of the signal waveforms than conventional methods. The simulation results show that the proposed methods have a better performance than conventional posedge or negedge method. The experiment results based on a TDC chip, i.e., TDC-GP21, also demonstrate the validity of proposed methods when baseline shifts exist. (Read full article)

Y. Zhang, Z. Li*, “Improving the accuracy of time difference measurement by reducing the impact of baseline shift,” IEEE Transactions on Instrum. Meas., vol. 64, No. 11, pp. 3013-3020, 2015.

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