Kirsten Christensen-Jeffries

3-D In Vitro Acoustic Super-Resolution and Super-Resolved Velocity Mapping Using Microbubbles

The structure of microvasculature cannot be resolved using standard clinical ultrasound (US) imaging frequencies due to the fundamental diffraction limit of US waves. In this work, we use a standard clinical US system to perform in vivo sub-diffraction imaging on a CD1, female mouse aged eight weeks by localizing isolated US signals from microbubbles flowing within the ear microvasculature, and compare our results to optical microscopy. Furthermore, we develop a new technique to map blood velocity at super-resolution by tracking individual bubbles through the vasculature. Resolution is improved from a measured lateral and axial resolution of 112 μm and 94 μm respectively in original US data, to super-resolved images of microvasculature where vessel features as fine as 19 μm are clearly visualized. Velocity maps clearly distinguish opposing flow direction and separated speed distributions in adjacent vessels, thereby enabling further differentiation between vessels otherwise not spatially separated in the image. This technique overcomes the diffraction limit to provide a noninvasive means of imaging the microvasculature at super-resolution, to depths of many centimeters. In the future, this method could noninvasively image pathological or therapeutic changes in the microvasculature at centimeter depths in vivo.

Microbubble Axial Localization Errors in Ultrasound Super-Resolution Imaging

Acoustic super-resolution imaging has allowed the visualization of microvascular structure and flow beyond the diffraction limit using standard clinical ultrasound systems through the localization of many spatially isolated microbubble signals. The determination of each microbubble position is typically performed by calculating the centroid, finding a local maximum, or finding the peak of a 2-D Gaussian function fit to the signal. However, the backscattered signal from a microbubble depends not only on diffraction characteristics of the waveform, but also on the microbubble behavior in the acoustic field. Here, we propose a new axial localization method by identifying the onset of the backscattered signal. We compare the accuracy of localization methods using in vitro experiments performed at 7-cm depth and 2.3-MHz center frequency. We corroborate these findings with simulation results based on the Marmottant model. We show experimentally and in simulations that detecting the onset of the returning signal provides considerably increased accuracy for super-resolution. Resulting experimental cross-sectional profiles in super-resolution images demonstrate at least 5.8 times improvement in contrast ratio and more than 1.8 times reduction in spatial spread (provided by 90% of the localizations) for the onset method over centroiding, peak detection, and 2-D Gaussian fitting methods. Simulations estimate that these latter methods could create errors in relative bubble positions as high as

Two Stage Sub-Wavelength Motion Correction in Human Microvasculature for CEUS Imaging

900~\mu \text{m}

Acoustic wave sparsely-activated localization microscopy (AWSALM): super-resolution ultrasound imaging using acoustic activation and deactivation of nanodroplets

at these experimental settings, while the onset method reduced the interquartile range of these errors by a factor of over 2.2. Detecting the signal onset is, therefore, expected to considerably improve the accuracy of super-resolution.

Localisation of multiple non-isolated microbubbles with frequency decomposition in super-resolution imaging

The structure of microvasculature cannot be resolved using clinical B-mode or contrast-enhanced ultrasound (CEUS) imaging due to the fundamental diffraction limit at clinical ultrasound frequencies. It is possible to overcome this resolution limitation by localizing individual microbubbles through multiple frames and forming a super-resolved image. However, ultrasound super-resolution creates its unique problems since the structures to be imaged are on the order of 10s of μm. Tissue movement much larger than 10 μm is common in clinical imaging, which can significantly reduce the accuracy of super-resolution images created from microbubble locations gathered through hundreds of frames. This study investigated an existing motion estimation algorithm from magnetic resonance imaging for ultrasound super-resolution imaging. Its correction accuracy is evaluated using simulations with increasing complexity of motion. Feasibility of the method for ultrasound super-resolution in vivo is demonstrated on clinical ultrasound images. For a chosen microvessel, the super-resolution image without motion correction achieved a sub-wavelength resolution; however after the application of proposed two-stage motion correction method the size of the vessel was reduced to half.

The subharmonic amplitude of SonoVue increases with hydrostatic pressure at low incident acoustic pressures

Photo-activated localization microscopy (PALM) has revolutionized the field of fluorescence microscopy by breaking the diffraction limit in spatial resolution. In this study, “acoustic wave sparsely activated localization microscopy (AWSALM),” an acoustic counterpart of PALM, is developed to super-resolve structures which cannot be resolved by conventional B-mode imaging. AWSALM utilizes acoustic waves to sparsely and stochastically activate decafluorobutane nanodroplets by acoustic vaporization and to simultaneously deactivate the existing vaporized nanodroplets via acoustic destruction. In this method, activation, imaging, and deactivation are all performed using acoustic waves. Experimental results show that sub-wavelength micro-structures not resolvable by standard B-mode ultrasound images can be separated by AWSALM. This technique is flow independent and does not require a low concentration of contrast agents, as is required by current ultrasound super resolution techniques. Acoustic activation and deactivation can be controlled by adjusting the acoustic pressure, which remains well within the FDA approved safety range. In conclusion, this study shows the promise of a flow and contrast agent concentration independent super-resolution ultrasound technique which has potential to be faster and go beyond vascular imaging.

Ultrasound super-resolution with microbubble contrast agents

Sub-diffraction imaging, also known as ultrasound localization microscopy, is a novel method that can overcome the fundamental diffraction limit by localizing spatially isolated microbubbles. This method requires the use of a low concentration of microbubbles to ensure that they are spatially isolated. For in vivo microvascular imaging, especially for cancer tissue with high microvascular density, spatial isolation cannot be always achieved, since vessels are close to each other and the speed of flow is slow. This study proposes a frequency decomposition method that uses the polydisperse nature of commercial contrast agents to separate spatially non-isolated microbubbles with different acoustic signatures. Zero-phase filters were applied to ensure that there is no relative phase delay between decomposed signals. Results showed that a super-resolution image after frequency decomposition can be generated with three times lower number of acquisitions without sacrificing image quality.

Super-resolution imaging of microbubble contrast agents

Physiologically important pressures in the heart and aorta are currently assessed with invasive pressure catheters. The subharmonic signal from ultrasound contrast agents, however, may be exploited to estimate pressures non-invasively. The objective of this work was (i) to develop a static phantom from commercially-available components for easy replication across different laboratories, and (ii) to investigate the subharmonic response of the ultrasound contrast agent SonoVue (Bracco, Milan, Italy) at physiological pressures within the phantom.

Acoustic super-resolution with ultrasound and microbubbles

Ultrasound super-resolution imaging can be achieved by localizing spatially isolated microbubble contrast agents over multiple imaging frames. In vivo images with resolutions of ∼10–20 microns in deep tissue have been demonstrated. The technique has the potential to revolutionize the way micro-circulation can be visualized and quantified, and has implications in a wide range of clinical applications including cancer, diabetes and beyond. In this paper we describe the principle of the technique with in vivo results demonstrating the superior resolution achieved compared with existing ultrasound imaging. We also discuss the challenges and opportunities in the area of 3D imaging including, imaging speed, tissue motion and microbubble localization errors.

Investigation of microbubble detection methods for super-resolution imaging of microvasculature

Many diseases, such as those associated with diabetes, coronary heart disease, and the growth and metastasis of tumors exhibit architectural changes in microvascular structure, as well as variations in vascular flow. Current clinical imaging modalities such as MRI, PET, CT and Ultrasound cannot easily resolve the microvasculature. Although optical light microscopy is able to resolve structures on the scale of microvasculature, the restricted imaging depth of this approach is a major limitation. Thus, there is a clinical need for the development of new imaging techniques that can fill this resolution gap.