Ultrafast Vector Doppler Using RF Sub-Nyquist Sampling

International audienceBackground, Motivation and ObjectiveUltrafast ultrasound has contributed to a renewed interested in vector Doppler. Sampling RF signals at a rate 4 times the carrier frequency is the standard procedure since this rule complies with the Nyquist sampling theorem, regardless of the transducer bandwidth. RF acquisition with a high-performance multi-channel system generates massive datasets,especially in 3-D ultrafast ultrasound. The objective of this in vitro and in vivo study was to demonstrate that sub-Nyquist sampling can lead to substantial lossless data reduction in vector Doppler.Statement of Contribution/MethodsVector Doppler was generated from unsteered plane waves (5-MHz linear array). Two receive sub-apertures were used, with receive angles of ±15°. A staggered dual-PRF sequence (PRF2 = ⅔ PRF1) doubled the Nyquist velocity. The effect of RF data undersampling on vector Doppler was investigated in a rotating disc and in a carotid bifurcation. The RF signalswere sampled at 20 MHz (center frequency × 4), then downsampled to simulate sub-Nyquist sampling. We used downsampling ratios up to 13; a ratio of 13 means that the RF signals were sampled at 20/13 = 1.54 MHz. The ratios were chosen so that the positive and negative frequency components did not overlap within the -10 dB bandwidth. After I/Qdemodulation and beamforming, the Doppler velocities were estimated using an auto-correlator. The Doppler-derived velocity vectors were compared with the actual vector fields. The in vitro phantom rotated at angular velocities up to 15 RPS (maximum outer speed = 1.5 m/s). In vivo vector flow images of the carotid bifurcation were produced in onevolunteer. We used a high-frame-rate duplex sequence (B-mode + color Doppler) based on plane wave imaging. A spatially-adaptive polynomial regression filter was used to remove the clutter components. Since no in vivo reference was available, the velocity vectors produced from the undersampled RF signals were compared with those obtained from thestandard Nyquist sampling.Results/DiscussionThe velocity vector errors due to sub-Nyquist sampling were marginal, which illustrates that vector Doppler can be correctly computed with a drastically reduced amount of RF samples. In our study, a 11-fold data reduction was obtained. Sub-Nyquist sampling can be a method of choice in vector Doppler to avoid information overload and reduce data transfer and storage

Color and Vector Flow Imaging in Parallel Ultrasound With Sub-Nyquist Sampling

International audienceRF acquisition with a high-performance multichannel ultrasound system generates massive data sets in short periods of time, especially in "ultrafast" ultrasound when digital receive beamforming is required. Sampling at a rate four times the carrier frequency is the standard procedure since this rule complies with the Nyquist-Shannon sampling theorem and simplifies quadrature sampling. Bandpass sampling (or undersampling) outputs a bandpass signal at a rate lower than the maximal frequency without harmful aliasing. Advantages over Nyquist sampling are reduced storage volumes and data workflow, and simplified digital signal processing tasks. We used RF undersampling in color flow imaging (CFI) and vector flow imaging (VFI) to decrease data volume significantly (factor of 3 to 13 in our configurations). CFI and VFI with Nyquist and sub-Nyquist samplings were compared in vitro and in vivo. The estimate errors due to undersampling were small or marginal, which illustrates that Doppler and vector Doppler images can be correctly computed with a drastically reduced amount of RF samples. Undersampling can be a method of choice in CFI and VFI to avoid information overload and reduce data transfer and storage. Index Terms— Color Doppler, parallel ultrasound imaging, sub-Nyquist sampling, undersampling, vector Doppler, vector flow imaging (VFI)

Atherosclerotic carotid bifurcation phantoms with a stenotic soft inclusion for flow-structure ultrasound imaging analysis

As the complexity of ultrasound signal processing algorithms increases, it becomes more difficult to demonstrate their added value. In the context of quantitative measurements such as vascular elastography, vector flow mapping, and ultrafast tissue Doppler, innovative validation strategies are required. Phantoms have been widely used but they do not correspond to the geometry of vulnerable carotid atherosclerotic plaques containing a large lipid pool embedded within the vessel wall of an anthropomorphic lumen geometry.We propose a method of manufacturing such phantoms for applications in flow imaging and elastography. The internal carotid geometry was based on a computed tomography scan of a healthy individual. During the fabrication process, a soft inclusion mimicking a stenotic lipid pool was embedded within the vascular wall. The phantom wall and soft inclusion were made of polyvinyl alcohol (PVA) that undergone different numbers of freezing-thawing cycles to produce different mechanical property.Mechanical testing measured Young's moduli of the vascular wall and soft inclusion at 342 ± 25 kPa and 17 ± 3 kPa, respectively. Strain elastography results on the lipid pool mimicking inclusion, fibrous cap and remaining phantom wall showed greater strain in the lipid pool, which is consistent with expected results. Because of their realistic geometries and mechanical properties, those phantoms may become advantageous for fluid-structure experimental studies and validation of new ultrasound-based imaging technologies