Resolution limits of ultrafast ultrasound localization microscopy

Abstract

As in other imaging methods based on waves, the resolution of ultrasound imaging is limited by the wavelength. However, the diffraction-limit can be overcome by super-localizing single events from isolated sources. In recent years, we developed plane-wave ultrasound allowing frame rates up to 20,000 fps. Ultrafast processes such as rapid movement or disruption of ultrasound contrast agents (UCA) can thus be monitored, providing us with distinct punctual sources that could be localized beyond the diffraction limit. We previously showed experimentally that resolutions beyond λ/10 can be reached in ultrafast ultrasound localization microscopy (uULM) using a 128 transducer matrix in reception. Higher resolutions are theoretically achievable and the aim of this study is to predict the maximum resolution in uULM with respect to acquisition parameters (frequency, transducer geometry, sampling electronics). The accuracy of uULM is the error on the localization of a bubble, considered a point-source in a homogeneous medium. The proposed model consists in two steps: determining the timing accuracy of the microbubble echo in radiofrequency data, then transferring this time accuracy into spatial accuracy. The simplified model predicts a maximum resolution of 40 μm for a 1.75 MHz transducer matrix composed of two rows of 64 elements. Experimental confirmation of the model was performed by flowing microbubbles within a 60 μm microfluidic channel and localizing their blinking under ultrafast imaging (500 Hz frame rate). The experimental resolution, determined as the standard deviation in the positioning of the microbubbles, was predicted within 6 μm (13%) of the theoretical values and followed the analytical relationship with respect to the number of elements and depth. Understanding the underlying physical principles determining the resolution of superlocalization will allow the optimization of the imaging setup for each organ. Ultimately, accuracies better than the size of capillaries are achievable at several centimeter depths.

Microbubble Axial Localization Errors in Ultrasound Super-Resolution Imaging

“…It is important to note that the work presented in this paper does not aim to investigate the fundamental limit to which a single, linear, point scatterer can be localised with a given localisation method, as has been investigated in other work [28]. The work presented here differs for a number of reasons.…”

Improved Super-Resolution Ultrasound Microvessel Imaging With Spatiotemporal Nonlocal Means Filtering and Bipartite Graph-Based Microbubble Tracking

“…This is in good agreement with the observations reported in Errico et al . [1] and Desailly et al [9]. …”

“…[1] proposed a seminal method, ultrasound localization microscopy (ULM), that uses ultrafast ultrasound imaging to record spatially isolated microbubble blinking events from decorrelated microbubble signals that can be generated by microbubble disruption, dissolution and motion. ULM achieved ~λ/10 subwavelength spatial resolution with a wide dynamic range of blood flow speed measurement (1 mm/s to several cm/s) that is Doppler angle-independent [1, 9]. In an in vivo murine brain study, ULM demonstrated visualization of ~10 μm microvessels over the entire depth of the mouse brain (~10 mm) [1].…”

3-D Super-Resolution Ultrasound Imaging Using a 2-D Sparse Array with High Volumetric Imaging Rate

“…By localizing spatially isolated microbubbles through multiple frames, super-resolution ultrasound (SR-US) images can be generated. Even at ultrasonic frequencies in the low MHz range, a localization precision of a few micrometers can be achieved [1], [2]. In the absence of sample motion, it is this localization precision that determines the maximum achievable resolution in super-resolution images.…”