Three-dimensional super-resolution ultrasound imaging of chicken embryos - A validation framework for analysis of microvascular morphology

Advancements in Three-Dimensional Super-Resolution Ultrasound Imaging: A Narrative Review

The lack of sensibility of traditional ultrasound (US) imaging to the slow blood flow in small vessels resulted in the development of microbubble (MB) contrast agents. These MBs are given intravenously, and US imaging can detect them quite effectively. This noninvasive imaging method, known as contrast-enhanced US (CEUS), now makes it possible to accurately assess tissue perfusion and blood flow. Though CEUS offers several benefits, diffraction restricts the spatial resolution of all US imaging systems to length scales equal to roughly half the wavelength of the transmitted US beam. Based on individual MB detection and localization, the recently developed super-resolution US (SRUS) imaging method has shown unprecedentedly high spatial resolution exceeding the physical diffraction limit. It is now possible to visualize the microvasculature beyond the diffraction-limited resolution by localizing spatially isolated MBs across several frames. The highest resolution possible at clinical US frequencies can be on the order of several micrometers when tissue and probe motion are not present. Enhancing the functional study of tissue microvascular networks with structural data could lead to improved disease management. Through the localization and tracking of MBs, SRUS may reconstruct images of the microvasculature with resolution exceeding the diffraction limit in both 2-dimensional (2D) and 3-dimensional (3D) space. In contrast to the 2D approach, 3D SRUS imaging does not suffer from out-of-plane motion and can offer volumetric coverage with super-resolution in all three dimensions. Research has used two primary methods for 3D SRUS imaging including arrays that can electronically gather volumetric information or mechanically scanning the volume with a linear probe to produce a stack of 2D SRUS images. This manuscript aims to offer a comprehensive review of 3D SRUS imaging, clarifying methodologies, clinical applications, and notable challenges that could motivate future research and help facilitate clinical translation.

Evaluation of Nonlinear Contrast Pulse Sequencing for Use in Super-Resolution Ultrasound Imaging

The use of super-resolution ultrasound (SR-US) imaging greatly improves visualization of microvascular structures, but clinical adoption is limited by long imaging times. This method depends on detecting and localizing isolated microbubbles (MBs), forcing the use of a dilute contrast agent concentration. Contrast-enhanced ultrasound (CEUS) image acquisition times as long as minutes arise as the localization of thousands of MBs are acquired to form a complete SR-US image. In this article, we explore the use of nonlinear CEUS strategies using nonlinear fundamental contrast pulse sequencing (CPS) to increase the contrast-to-tissue ratio (CTR) and compare MB detection effectiveness to linear B-mode CEUS imaging. The CPS compositions of amplitude modulation (AM), pulse inversion (PI), and a combination of the two (AMPI) were studied. A simulation study combined the Rayleigh-Plesset-Marmottant (RPM) model of MB characteristics and a nonlinear tissue model using the k-Wave toolbox for MATLAB (MathWorks Inc., Natick, MA, USA). Validation was conducted using an in vitro flow phantom and in vivo in the rat hind-limb. Imaging was performed with a programmable US scanner (Vantage 256, Verasonics Inc., Kirkland, WA, USA) and customized to transmit a set of basis US pulses from which both B-mode US (frame rate (FR) of 800 Hz) and multiple nonlinear CPS compositions (FR of 200 Hz) could be assessed from identical in vitro and in vivo datasets using a near simultaneous method. The simulations suggest that MB characteristics, such as diameter and motion, help to predict which US imaging strategy will enhance MB detection. The in vitro and in vivo US imaging studies revealed that different subpopulations of polydisperse MB contrast agents were detected by linear imaging and by each different nonlinear CPS composition. The most effective single imaging strategy at a 200-Hz FR was found to be B-mode US imaging. However, a combination of B-mode US imaging with a nonlinear CPS imaging strategy was more effective in detecting MBs in vivo at all depths and was shown to shorten image acquisition time by an average of 33.3%-76.7% when combining one or more CPS sequences.

Three-dimensional visualization and improved quantification with super-resolution ultrasound imaging - validation framework for analysis of microvascular morphology using a chicken embryo model

The purpose of this study was to improve the morphological analysis of microvascular networks depicted in three-dimensional (3D) super-resolution ultrasound (SR-US) images. This was supported by qualitative and quantitative validation by comparison to matched brightfield microscopy and traditional B-mode ultrasound (US) images. Contrast-enhanced US (CEUS) images were collected using a preclinical US scanner (Vevo 3100, FUJIFILM VisualSonics Inc.) equipped with an MX250 linear array transducer. CEUS imaging was performed after administration of a microbubble (MB) contrast agent into the vitelline network of a developing chicken embryo. Volume data was collected by mechanically scanning the US transducer throughout a tissue volume-of-interest in 90μm step increments. CEUS images were collected at each increment and stored as in-phase/quadrature data (2000 frames at 152 frames per sec). SR-US images were created for each cross-sectional plane using established data processing methods. All SR-US images were then used to reconstruct a final 3D volume for vessel diameter (VD) quantification and for surface rendering. VD quantification from the 3D SR-US data exhibited an average error of 6.1% ± 6.0% when compared with matched brightfield microscopy images, whereas measurements from B-mode US images had an average error of 77.1% ± 68.9%. Volume and surface renderings in 3D space enabled qualitative validation and improved visualization of small vessels below the axial resolution of the US system. Overall, 3D SR-US image reconstructions depicted the microvascular network of the developing chicken embryos. Improved visualization of isolated vessels and quantification of microvascular morphology from SR-US images achieved a considerably greater accuracy compared to B-mode US measurements.