Coherent Flow Power Doppler Research Articles

Real-Time In Vivo Imaging of Human Liver Vasculature Using Coherent Flow Power Doppler: A Pilot Clinical Study.

Power Doppler (PD) is a commonly used technique for flow detection and vessel visualization in radiology clinics. Despite its broad set of applications, PD suffers from multiple noise sources and artifacts, such as thermal noise, clutter, and flash artifacts. In addition, a tradeoff exists between acquisition time and Doppler image quality. These limit the ability of clinical PD imaging in deep-lying and small-vessel detection and visualization, particularly among patients with high body mass indices (BMIs). To improve the Doppler vessel detection, we have previously proposed coherent flow PD (CFPD) imaging and demonstrated its performance on porcine vasculature. In this article, we report on a pilot clinical study of CFPD imaging on healthy human volunteers and patients with high BMI to assess the clinical feasibility of the technique in liver imaging. In this study, we built a real-time CFPD imaging system using a graphical processing unit (GPU)-based software beamformer and a CFPD processing module. Using the real-time CFPD imaging system, the liver vasculature of 15 healthy volunteers with normal BMI below 25 and 15 patients with BMI greater than 25 was imaged. Both PD and CFPD image streams were produced simultaneously. The generalized contrast-to-noise ratio (gCNR) of the PD and CFPD images was measured to provide the quantitative evaluation of image quality and vessel detectability. Comparison of PD and CFPD image shows that gCNR is improved by 35% in healthy volunteers and 28% in high BMI patients with CFPD compared to PD. Example images are provided to show that the improvement in the Doppler image gCNR leads to greater detection of small vessels in the liver. In addition, we show that CFPD can suppress in vivo reverberation clutter in clinical imaging.

A Spatial Coherence Beamformer Design for Power Doppler Imaging.

Acoustic clutter is a primary source of image degradation in ultrasound imaging. In the context of flow imaging, tissue and acoustic clutter signals are often much larger in magnitude than the blood signal, which limits the sensitivity of conventional power Doppler in SNR-limited environments. This has motivated the development of coherence-based beamformers, including Coherent Flow Power Doppler (CFPD), which have demonstrated efficacy in mitigating sources of diffuse clutter. However, CFPD uses a measure of normalized coherence, which incurs a non-linear relationship between image intensity and the magnitude of the blood echo. As a result, CFPD is not a robust approach to study gradation of blood signal energy, which depicts the fractional moving blood volume. We propose the application of mutual intensity, rather than normalized coherence, to retain the clutter suppression capability inherent in coherence beamforming, while preserving the underlying signal energy. Feasibility of this approach was shown via Field II simulations, phantoms, and in vivo human liver data. In addition, we derive an adaptive statistical threshold for the suppression of residual noise signals. Overall, this beamformer design shows promise as an alternative technique to depict flow volume gradation in cluttered imaging environments.

Suppression of in vivo flow artifacts in power Doppler through spatial and angular coherence beamforming

Power Doppler imaging is traditionally the flow imaging technique utilized clinically in the task of flow detection. Due to the relatively low pulse repetition frequencies used, power Doppler ensemble sizes are small to allow real-time imaging. The small ensemble size, however, makes power Doppler imaging subject to the so-called flash artifact due to limited stationary clutter filtering capabilities. In addition, with the increasing population of overweight and obese individuals, power Doppler imaging is subject to higher amounts of thermal noise and reverberation artifact that pass through the stationary clutter filters. We present coherence beamforming techniques, applied to in vivo human imaging, to demonstrate their ability to reduce flow artifacts and noise in power Doppler imaging. Using coherence beamforming techniques in Coherent Flow Power Doppler (CFPD), we show that artifacts due to reverberation and thermal noise are reduced, corresponding to an increase of 7.5 dB in signal-to-noise ratio, in liver imaging of overweight and obese individuals. Similarly, we demonstrate coherence beamforming techniques in CFPD to reduce flash artifacts in videos of the neonatal brain.

Combining Slow Flow Techniques With Adaptive Demodulation for Improved Perfusion Ultrasound Imaging Without Contrast.

Noncontrast perfusion ultrasound imaging remains challenging due to spectral broadening of the tissue clutter signal caused by patient and sonographer hand motion. To address this problem, we previously introduced an adaptive demodulation scheme to suppress the bandwidth of tissue prior to high-pass filtering. Our initial implementation used single plane wave power Doppler imaging and a conventional tissue filter. Recent advancements in beamforming and tissue filtering have been proposed for improved slow flow imaging, including coherent flow power Doppler (CFPD) imaging and singular value decomposition (SVD) filtering. Here, we aim to evaluate adaptive demodulation in conjunction with improvements in beamforming and filtering using simulations, single-vessel phantoms, and an in vivo liver tumor embolization study. We show that simulated blood-to-background contrast-to-noise ratios are highest when using adaptive demodulation with CFPD and a 100-ms ensemble, which resulted in a 13.6-dB average increase in contrast-to-noise ratio compared to basic IIR filtering alone. We also show that combining adaptive demodulation with SVD and with CFPD + SVD results in 9.3- and 19-dB increases in contrast-to-noise ratios compared to IIR filtering alone at 700- and 500-ms ensembles for phantom data with 1- and 5-mm/s average flows, respectively. In general, combining techniques resulted in higher signal-to-noise, contrast-to-noise, and generalized contrast-to-noise ratios in both simulations and phantoms. Finally, adaptive demodulation with SVD resulted in the largest qualitative and quantitative changes in tumor-to-background contrast postembolization.

Ultrasonic non-contrast perfusion imaging with adaptive demodulation

Ultrasonic non-contrast perfusion imaging remains challenging due to spectral broadening of the tissue clutter signal caused by patient and sonographer hand motion. Simply, the velocity of the slowest moving blood has similar spectral support to the moving tissue. To address this problem, we developed an adaptive demodulation (AD) scheme to suppress the bandwidth of tissue prior to high-pass filtering. The method works by directly estimating the modulation imposed by tissue motion, and then removing that motion from the signal. Our initial implementation used single plane wave power Doppler imaging sequence combined with a conventional high-pass IIR tissue filter. However, other recent advancements in beamforming and tissue filtering have been proposed for improved slow-flow imaging, including coherent flow power Doppler (CFPD) and singular value decomposition (SVD). Here, we aim to evaluate AD separately as well as in comparison and in conjunction with improvements in beamforming and filtering using simulations and an in vivo muscle contraction experiment. We show that simulated blood-to-background SNR is highest when using AD + CFPD and a 100 ms ensemble, which resulted in a 9.88 dB increase in SNR compared to CFPD by itself. Additionally, AD + SVD resulted in a 54.6% increase in mean power within the in vivo muscle after contraction compared to a 6.84% increase with AD and a conventional IIR filter.

Enhanced sensitivity of flow detection in small vessels with slow flow by coherent-flow power Doppler

Power Doppler imaging is a commonly used method for the detection of flow. However, the sensitivity of power Doppler in small vessel detection is limited in clinical ultraound scanners by a combination of factors, including the small-diameter of vessels, slow flow through the vessels, the size of the Doppler ensemble, the presence of motion, the presence of noise, and the type of filter used. We have developed an alternative power Doppler imaging technique that enhances flow detection of small vessels and slow flow. This technique utilizes the same principles as power Doppler, but uses the coherence of backscattered blood signal rather than the backscattered signal power. We show that this technique, called coherent flow power Doppler (CFPD), is less sensitive to clutter-based signals and thermal noise and improves signal-to-noise ratio of the flow signal. We show through simulations, phantoms, and in vivo experimentation in human liver that this additional signal-to-noise ratio (7.5-12.5 dB) can be utilized to detect slower flow, improved frame rate (smaller Doppler ensemble), or improve the Doppler image profile. In addition, the technique demonstrates better signal-to-noise ratio for small-sized vessels (less than 2 mm).

Visualization of Small-Diameter Vessels by Reduction of Incoherent Reverberation With Coherent Flow Power Doppler.

Power Doppler (PD) imaging is a widely used technique for flow detection. Despite the wide use of Doppler ultrasound, limitations exist in the ability of Doppler ultrasound to assess slow flow in the small-diameter vasculature, such as the maternal spiral arteries and fetal villous arteries of the placenta and focal liver lesions. The sensitivity of PD in small vessel detection is limited by the low signal produced by slow flow and the noise associated with small vessels. The noise sources include electronic noise, stationary or slowly moving tissue clutter, reverberation clutter, and off-axis scattering from tissue, among others. In order to provide more sensitive detection of slow flow in small diameter vessels, a coherent flow imaging technique, termed coherent flow PD (CFPD), is characterized and evaluated with simulation, flow phantom experiment studies, and an in vivo animal small vessel detection study. CFPD imaging was introduced as a technique to detect slow blood flow. It has been demonstrated to detect slow flow below the detection threshold of conventional PD imaging using identical pulse sequences and filter parameters. In this paper, we compare CFPD with PD in the detection of blood flow in small-diameter vessels. The results from the study suggest that CFPD is able to provide a 7.5-12.5-dB increase in the signal-to-noise ratio (SNR) over PD images for the same physiological conditions and is less susceptible to reverberation clutter and thermal noise. Due to the increase in SNR, CFPD is able to detect small vessels in high channel noise cases, for which PD was unable to generate enough contrast to observe the vessel.