Ultrasound localization microscopy

Commercially Available Ultrasound Contrast Agents: Factors Contributing to Favorable Outcomes With Ultrasound-Mediated Drug Delivery and Ultrasound Localization Microscopy Imaging

ABSTRACT

Ultrasound contrast agents (UCAs) are microbubbles comprising an inert gas core stabilized by an encapsulating shell, which serves to increase the signal-to-noise ratio of blood-to-tissue in diagnostic ultrasound imaging. More recently, research has investigated the use of UCAs to combine both diagnostics and therapeutic outcomes in an amalgamated approach, designated 'theranostics.' Two examples of theranostic based approaches include the use of super-resolution imaging with ultrasound localized microscopy (ULM) and ultrasound-mediated drug delivery (UMDD). Both ULM and UMDD have been shown to have the potential to improve both patient care and clinical outcomes. Currently, there are 4 commercially available global UCAs licensed for clinical use. The physico-chemical properties of each of these UCAs influence its potential theranostic efficacy. Because of differences in their composition and/or manufacturing processes, each UCA has different characteristics that contribute to different in vivo resonance behavior, which in turn influences their effective clinical applications. This review highlights the key physico-chemical characteristic differences of the 4 commercially available contrast agents, with specific emphasis on their gaseous core, shell composition, and microbubble volume distribution, while providing novel insights into their benefits for supporting emerging clinical technologies, specifically ULM and UMDD.

Contrast-Enhanced Ultrasound to Assess Kidney Quality During Ex Situ Normothermic Machine Perfusion

Normothermic machine perfusion (NMP) provides opportunity for viability assessment of donated kidneys. Diminished microvascular perfusion, despite adequate total blood flow, is a key pathophysiology in ischaemia-mediated acute kidney injury. Contrast-enhanced ultrasound (CEUS) could allow objective assessment of microvascular perfusion during renal NMP. Blood-based NMP was performed on porcine kidneys (circulatory death model) and human kidneys declined for transplant (preclinical). CEUS was performed with a contrast bolus into the NMP circuit arterial limb. Microvascular perfusion quality was quantified and z-score normalisation allowed combination of metrics and regions into an overall "CEUS-score." In porcine kidneys, inferior microvascular perfusion of cortex and medulla correlated with increased urinary NGAL (Neutrophil gelatinase-associated lipocalin) and histological DNA-fragmentation (a hallmark of apoptosis). In human kidneys, CEUS-score at 2 h was correlated with histological DNA-fragmentation (r = -0.937; P = 0.019) and predicted urinary NGAL at 24 h of NMP (r = -0.925; P = 0.024). Total renal flow was not correlated with these outcomes. An open-source web application (stingle.shinyapps.io/Time_intensity_analysis) and R package ("tican") were developed for quantitative time-intensity curve analysis. CEUS allows objective point-of-care microvascular perfusion assessment during NMP. As 2-hour CEUS-score predicts NGAL at 24 h, CEUS warrants future clinical investigation as a potential tool to assess kidney quality in assessment and reconditioning centres.

Optimizingin vivodata acquisition for robust clinical microvascular imaging using ultrasound localization microscopy

Objective. Ultrasound localization microscopy (ULM) enables microvascular imaging at spatial resolutions beyond the acoustic diffraction limit, offering significant clinical potentials. However, ULM performance relies heavily on microbubble (MB) signal sparsity, the number of detected MBs, and signal-to-noise ratio (SNR), all of which vary in clinical scenarios involving bolus MB injections. These sources of variations underscore the need to optimize MB dosage, data acquisition timing, and imaging settings in order to standardize and optimize ULM of microvasculature. This pilot study aims to investigate the temporal changes in MB signals during bolus injections in both pig and human models to optimize data acquisition for clinical ULM.Approach.Quantitative indices, mainly including individual MB SNR, normalized cross-correlation (NCC) of the MB signal with the point-spread function, and the number of localizable MBs, were developed to evaluate MB signal quality and guide the selection of acquisition timing. The effects of transmitted voltage and dosage on signal quality for MB localization were also explored.Main results. In both pig and human studies, MB localization quality (primarily indicated by NCC) reached a minimum at peak MB concentration, then improved as MB counts decreased during the wash-out phase. An optimal acquisition window was identified by balancing localization quality (empirically, NCC > 0.57) and MB concentration. In the pig model, a relatively short time window (approximately 10 s) for optimal acquisition was identified during the rapid wash-out phase, highlighting the need for real-time MB signal monitoring during data acquisition. The slower wash-out phase in humans allowed for a more flexible imaging window of 1-2 min, while trade-offs were observed between localization quality and MB density (or acquisition length) at different wash-out phase timings. Guided by these findings, robust ULM imaging was achieved in both pig and human kidneys using a short period of data acquisition (3.6 s and 9.6 s of data), demonstrating its feasibility in clinical practice.Significance.This study provides insights into optimizing data acquisition for consistent and reproducible ULM, paving the way for its standardization and broader clinical applications.

A Tracking Prior to Localization Workflow for Ultrasound Localization Microscopy

Ultrasound Localization Microscopy (ULM) has proven effective in resolving microvascular structures and local mean velocities at sub-diffraction-limited scales, offering high-resolution imaging capabilities. Dynamic ULM (DULM) enables the creation of angiography or velocity movies throughout cardiac cycles. Currently, these techniques rely on a Localization-and-Tracking (LAT) workflow consisting in detecting microbubbles (MB) in the frames before pairing them to generate tracks. While conventional LAT methods perform well at low concentrations, they suffer from longer acquisition times and degraded localization and tracking accuracy at higher concentrations, leading to biased angiogram reconstruction and velocity estimation. In this study, we propose a novel approach to address these challenges by reversing the current workflow. The proposed method, Tracking-and-Localization (TAL), relies on first tracking the MB and then performing localization. Through comprehensive benchmarking using both in silico and in vivo experiments and employing various metrics to quantify ULM angiography and velocity maps, we demonstrate that the TAL method consistently outperforms the reference LAT workflow. Moreover, when applied to DULM, TAL successfully extracts velocity variations along the cardiac cycle with improved repeatability. The findings of this work highlight the effectiveness of the TAL approach in overcoming the limitations of conventional LAT methods, providing enhanced ULM angiography and velocity imaging.

Quantitative Analysis on Vessel Stiffness and Vector Flow Imaging in the Assessment of Carotid Artery Structural and Functional Changes in Patients With Type 2 Diabetes

OBJECTIVE

To explore the value of RF-data-based quantitative analysis on vessel stiffness (R-QVS) combined with dynamic vector flow imaging (VFI) in evaluating structural and functional changes in the carotid arteries of patients with type 2 diabetes mellitus (T2DM).

METHODS

A prospective study was conducted between October 2022 and April 2024, including 275 consecutive subjects (50 volunteers as controls, 108 patients with T2DM and normal carotid intima-media thickness (CIMT), and 117 patients with T2DM and thickened CIMT). Carotid intima-media thickness (IMT) was measured using real-time intima-media thickness (RIMT) technology, while R-QVS was employed to measure the systolic diameter (Diam), displacement (Dist), hardness coefficient (HC), and pulse wave velocity (PWV) of the distal segment of the carotid artery. VFI was used to measure the maximum wall shear stress (WSSmax), mean wall shear stress (WSSmean), and maximum instantaneous velocity (Vmax) of the vessel wall. Differences in ultrasound parameters among the three groups were compared, and receiver operating characteristic (ROC) curves were plotted to calculate the area under the curve (AUC), evaluating the efficacy of these parameters in assessing structural and functional changes in the carotid arteries of patients with T2DM.

RESULTS

There were statistically significant differences in carotid IMT, Diam, Dist, HC, PWV, WSSmax, and Vmax among the three groups (all p < 0.01). The AUCs for evaluating structural and functional changes in the carotid arteries of patients with T2DM using carotid ultrasound parameters Diam, Dist, HC, PWV, WSSmax, and Vmax were 0.64, 0.68, 0.83, 0.88, 0.86, and 0.82, respectively. Multiple linear regression analysis identified Dist., HC, PWV, WSSmax, and WSSmean as influencing factors for CIMT in T2DM patients (with β values of -0.406, 0.515, 0.564, -0.472, and -0.438, respectively; all p < 0.05).

CONCLUSION

R-QVS and VFI techniques contribute to the early assessment of structural and functional changes in the carotid arteries of patients with type 2 diabetes mellitus. Compared with controls, T2DM patients exhibit more advanced functional changes than morphological changes despite normal CIMT. The enhanced sensitivity, reproducibility, and detailed assessment capabilities of these methods make them valuable tools in the early detection and intervention of cardiovascular risk in T2DM.

Optimizing In Vivo Data Acquisition for Robust Clinical Microvascular Imaging Using Ultrasound Localization Microscopy

Ultrasound localization microscopy (ULM) enables microvascular imaging at spatial resolutions beyond the acoustic diffraction limit, offering significant clinical potentials. However, ULM performance relies heavily on microbubble (MB) signal sparsity, the number of detected MBs, and signal-to-noise ratio (SNR), all of which vary in clinical scenarios involving bolus MB injections. These sources of variations underscore the need to optimize MB dosage, data acquisition timing, and imaging settings in order to standardize and optimize ULM of microvasculature. This pilot study investigated temporal changes in MB signals during bolus injections in both pig and human models to optimize data acquisition for clinical ULM. Quantitative indices were developed to evaluate MB signal quality, guiding selection of acquisition timing that balances the MB localization quality and adequate MB counts. The effects of transmitted voltage and dosage were also explored. In the pig model, a relatively short window (approximately 10 seconds) for optimal acquisition was identified during the rapid wash-out phase, highlighting the need for real-time MB signal monitoring during data acquisition. The slower wash-out phase in humans allowed for a more flexible imaging window of 1-2 minutes, while trade-offs were observed between localization quality and MB density (or acquisition length) at different wash-out phase timings. Guided by these findings, robust ULM imaging was achieved in both pig and human kidneys using a short period of data acquisition, demonstrating its feasibility in clinical practice. This study provides insights into optimizing data acquisition for consistent and reproducible ULM, paving the way for its standardization and broader clinical applications.

Influence of Image Discretization and Patch Size on Microbubble Localization Precision

For ultrasound localization microscopy, the localization of microbubbles (MBs) is an essential part to obtain super-resolved maps of the vasculature. This article analyzes the impact of image discretization and patch size on the precision of different MB localization methods to reconcile different observations from previous studies, provide an estimate of feasible localization precision, and derive guidelines for an optimal parameter selection. For this purpose, the images of MBs were simulated with Gaussian point-spread functions (PSFs) of varying width parameter at randomly generated subpixel positions, and Rician distributed noise was added. Four localization methods were tested on the patches of different sizes (number of pixels ): Gaussian fit (GF), radial symmetry (RS) method, calculation of center of mass (CoM), and peak detection (PD). Additionally, the Cramér-Rao lower bound (CRLB) for the given estimation problem was calculated. Our results show that the localization precision is strongly influenced by the ratio of the PSF width parameter to the pixel size , as well as the patch size N. The best parameter combination depends on the localization method. Generally, very small ratios as well as large ratios in combination with small N lead to performance degradation. The GF as a representative of a model-based fit comes close to the CRLB and always performs best for the ratios given by image discretization if N is adapted to the PSF. To achieve good results with the GF and the RS method, a good rule of thumb is to set the pixel sizes and the patch sizes .

Volumetric Ultrasound Localization Microscopy

Super-resolution ultrasound (SRUS) has evolved significantly with the advent of ultrasound localization microscopy (ULM). This technique enables subwavelength resolution imaging using microbubble contrast agents. Initially confined to 2-D imaging, ULM has progressed toward volumetric approaches, allowing for comprehensive 3-D visualization of microvascular networks. This review explores the technological advancements and challenges associated with volumetric ULM, focusing on key aspects such as transducer design, acquisition speed, data processing algorithms, or integration into clinical practice. We discuss the limitations of traditional 2-D ULM, including dependence on precise imaging plane selection and compromised resolution in microvasculature quantification. In contrast, volumetric ULM offers enhanced spatial resolution and allows motion correction in all directions, promising transformative insights into microvascular pathophysiology. By examining current research and future directions, this review highlights the potential of volumetric ULM to contribute significantly to diagnostic across various medical conditions, including cancers, arteriosclerosis, strokes, diabetes, and neurodegenerative diseases.

Ultrasound Localization Microscopy Precision of Clinical 3-D Ultrasound Systems

Ultrasound localization microscopy (ULM) is becoming well established in preclinical applications. For its translation into clinical practice, the localization precision achievable with commercial ultrasound (US) scanners is crucial-especially with volume imaging, which is essential for dealing with out-of-plane motion. Here, we propose an easy-to-perform method to estimate the localization precision of 3-D US scanners. With this method, we evaluated imaging sequences of the Philips Epiq 7 US device using the X5-1 and the XL14-3 matrix transducers and also tested different localization methods. For the X5-1 transducer, the best lateral, elevational, and axial precision was 109, 95, and m for one contrast mode, and 29, 22, and m for the other. The higher frequency XL14-3 transducer yielded precisions of 17, 38, and m using the harmonic imaging mode. Although the center of mass was the most robust localization method also often providing the best precision, the localization method has only a minor influence on the localization precision compared to the impact by the imaging sequence and transducer. The results show that with one of the imaging modes of the X5-1 transducer, precisions comparable to the XL14-3 transducer can be achieved. However, due to localization precisions worse than m, reconstruction of the microvasculature at the capillary level will not be possible. These results show the importance of evaluating the localization precision of imaging sequences from different US transducers or scanners in all directions before using them for in vivo measurements.

Ultrasound technology assisted colloidal nanocrystal synthesis and biomedical applications

Non-invasive and high spatiotemporal resolution mythologies for the diagnosis and treatment of disease in clinical medicine promote the development of modern medicine. Ultrasound (US) technology provides a non-invasive, real-time, and cost-effective clinical imaging modality, which plays a significant role in chemical synthesis and clinical translation, especially in in vivo imaging and cancer therapy. On the one hand, the US treatment is usually accompanied by cavitation, leading to high temperature and pressure, so-called "hot spot", playing a significant role in sonochemical-based colloidal synthesis. Compared with the classical nucleation synthetic method, the sonochemical synthesis strategy presents high efficiency for the fabrication of colloidal nanocrystals due to its fast nucleation and growth procedure. On the other hand, the US is attractive for in vivo and medical treatment, with applications increasing with the development of novel contrast agents, such as the micro and nano bubbles, which are widely used in neuromodulation, with which the US can breach the blood-brain barrier temporarily and safely, opening a new door to neuromodulation and therapy. In terms of cancer treatment, sonodynamic therapy and US-assisted synergetic therapy show great effects against cancer and sonodynamic immunotherapy present unparalleled potentiality compared with other synergetic therapies. Further development of ultrasound technology can revolutionize both chemical synthesis and clinical translation by improving efficiency, precision, and accessibility while reducing environmental impact and enhancing patient care. In this paper, we review the US-assisted sonochemical synthesis and biological applications, to promote the next generation US technology-assisted applications.