ASAP: Super-Contrast Vasculature Imaging Using Coherence Analysis and High Frame-Rate Contrast Enhanced Ultrasound

Abstract

The very high frame rate afforded by ultrafast ultrasound, combined with microbubble contrast agents, opens new opportunities for imaging tissue microvasculature. However, new imaging paradigms are required to obtain superior image quality from the large amount of acquired data while allowing real-time implementation. In this paper, we report a technique-acoustic sub-aperture processing (ASAP)-capable of generating very high contrast/signal-to-noise ratio (SNR) images of macro-and microvessels, with similar computational complexity to classical power Doppler (PD) imaging. In ASAP, the received data are split into subgroups. The reconstructed data from each subgroup are temporally correlated over frames to generate the final image. As signals in subgroups are correlated but the noise is not, this substantially reduces the noise floor compared to PD. Using a clinical imaging probe, the method is shown to visualize vessels down to $200~\mu \text{m}$ with a SNR of 10 dB higher than PD and to resolve microvascular flow/perfusion information in rabbit kidneys noninvasively in vivo at multiple centimeter depths. With careful filter design, the technique also allows the estimation of flow direction and the separation of fast flow from tissue perfusion. ASAP can readily be implemented into hardware/firmware for real-time imaging and can be applied to contrast enhanced and potentially noncontrast imaging and 3-D imaging.

3-D Microvascular Imaging Using High Frame Rate Ultrasound and ASAP Without Contrast Agents: Development and Initial In Vivo Evaluation on Nontumor and Tumor Models

“…The * denotes the complex conjugate and |−| denotes the modulus operator. ASAP was developed to reduce the noise in PD images by assuming that the noise between channels is uncorrelated [26]. With the same receive channel data, two set of images were reconstructed by splitting the channels into two nonoverlapping sub-apertures with non-common elements.…”

“…This however may cause grating lobes to appear in the images as the pitch of the sampling of the new sub-aperture increases by twofold. To reduce this problem, the sub-apertures can be formed by alternating the channel sampler after every 4 elements, as previously discussed in [26].…”

“…To evaluate the image quality we compared the ASAP images with the PD images using two metrics, similarly to our previous study [26]. The signal to noise ratio (SNR) and the contrast to noise ratio (CNR) were evaluated as follow:…”

“…We recently developed acoustic sub-aperture processing (ASAP) for super-contrast vasculature imaging, and demonstrated its feasibility for improving 2D HFR-CEUS imaging [26], [27]. In this study, we aim to demonstrate the applicability of ASAP for improving the quality of volumetric microvascular images acquired without the use of contrast agents.…”

High-Contrast 3D in Vivo Microvascular Imaging Using Scanning 2D Ultrasound and Acoutic Sub-Aperture Processing (ASAP)

“…In order to reduce the noise background of ultrafast Doppler, we recently developed acoustic sub-aperture processing (ASAP) for super-contrast vasculature imaging, and demonstrated its feasibility for improving 2D HFR-CEUS imaging [9], [10]. However, 2D imaging is insufficient to represent the volumetric structure of the vascular environment.…”