Micro-Ultrasound Molecular Imaging

__Abstract__ Perhaps using sound to identify objects, those which remained mysterious to sight and touch senses, is one of the very early diagnostic tools mankind has ever used. We all can distinguish the sound of a plastic cup from one made of glass when tapped by a finger nail no matter how similar they look. The sounds that we hear and those that we don’t, are mechanical vibrations (acoustic waves) traveling through a compressible and expandable (elastic) medium such as air or water. We can recognize the sound of a glass cup from a plastic one because of the fact that such mechanical vibrations of the particles of materials with dissimilar properties are unlike. Perhaps the most characteristic properties of an acoustic wave are its amplitude and its pitch (frequency). The amplitude of a sound determines how loud the sound is. The frequency of a sound is an indicator of how fast are the mechanical vibrations of the medium substances

Quantification of bound microbubbles in ultrasound molecular imaging

Molecular markers associated with diseases can be visualized and quantified noninvasively with targeted ultrasound contrast agent (t-UCA) consisting of microbubbles (MBs) that can bind to specific molecular targets. Techniques used for quantifying t-UCA assume that all unbound MBs are taken out of the blood pool few minutes after injection and only MBs bound to the molecular markers remain. However, differences in physiology, diseases, and experimental conditions can increase the longevity of unbound MBs. In such conditions, unbound MBs will falsely be quantified as bound MBs. We have developed a novel technique to distinguish and classify bound from unbound MBs. In the post-processing steps, first, tissue motion was compensated using block-matching (BM) techniques. To preserve only stationary contrast signals, a minimum intensity projection (MinIP) or 20th-percentile intensity projection (PerIP) was applied. The after-flash MinIP or PerIP was subtracted from the before-flash MinIP or PerIP. In this way, tissue artifacts in contrast images were suppressed. In the next step, bound MB candidates were detected. Finally, detected objects were tracked to classify the candidates as unbound or bound MBs based on their displacement. This technique was validated in vitro, followed by two in vivo experiments in mice. Tumors (n = 2) and salivary glands of hypercholesterolemic mice (n = 8) were imaged using a commercially available scanner. Boluses of 100 μL of a commercially available t-UCA targeted to angiogenesis markers and untargeted control UCA were injected separately. Our results show considerable reduction in misclassification of unbound MBs as bound ones. Using our method, the ratio of bound MBs in salivary gland for images with targeted UCA versus control UCA was improved by up to two times compared with unprocessed images

Microbubble Composition and Preparation for High-Frequency Contrast-Enhanced Ultrasound Imaging: In Vitro and in Vivo Evaluation

Although high-frequency ultrasound imaging is gaining attention in various applications, hardly any ultrasound contrast agents (UCAs) dedicated to such frequencies (>15 MHz) are available for contrast-enhanced ultrasound (CEUS) imaging. Moreover, the composition of the limited commercially available UCAs for high-frequency CEUS (hfCEUS) is largely unknown, while shell properties have been shown to be an important factor for their performance. The aim of our study was to produce UCAs in-house for hfCEUS. Twelve different UCA formulations A-L were made by either sonication or mechanical agitation. The gas core consisted of C4F10 and the main coating lipid was either 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC; A-F formulation) or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC; G-L formulation). Mechanical agitation r

Targeted ultrasound contrast agents for ultrasound molecular imaging and therapy

Ultrasound contrast agents (UCAs) are used routinely in the clinic to enhance contrast in ultrasonography. More recently, UCAs have been functionalised by conjugating ligands to their surface to target specific biomarkers of a disease or a disease process. These targeted UCAs (tUCAs) are used for a wide range of pre-clinical applications including diagnosis, monitoring of drug treatment, and therapy. In this review, recent achievements with tUCAs in the field of molecular imaging, evaluation of therapy, drug delivery, and therapeutic applications are discussed. We present the different coating materials and aspects that have to be considered when manufacturing tUCAs. Next to tUCA design and the choice of ligands for specific biomarkers, additional techniques are discussed that are applied to improve binding of the tUCAs to their target and to quantify the strength of this bond. As imaging techniques rely on the specific behaviour of tUCAs in an ultrasound field, it is crucial to understand the characteristics of both free and adhered tUCAs. To image and quantify the adhered tUCAs, the state-of-the-art techniques used for ultrasound molecular imaging and quantification are presented. This review concludes with the potential of tUCAs for drug delivery and therapeutic applications

Lamb Waves and Adaptive Beamforming for Aberration Correction in Medical Ultrasound Imaging

Phase aberration in transcranial ultrasound imaging (TUI) caused by the human skull leads to an inaccurate image reconstruction. In this article, we present a novel method for estimating the speed of sound and an adaptive beamforming technique for phase aberration correction in a flat polyvinylchloride (PVC) slab as a model for the human skull. First, the speed of sound of the PVC slab is found by extracting the overlapping quasi-longitudinal wave velocities of symmetrical Lamb waves in the frequency-wavenumber domain. Then, the thickness of the plate is determined by the echoes from its front and back side. Next, an adaptive beamforming method is developed, utilizing the measured sound speed map of the imaging medium. Finally, to minimize reverberation artifacts caused by strong scatterers (i.e., needles), a dual probe setup is proposed. In this setup, we image the medium from two opposite directions, and the final image can be the minimum intensity projection of the inherently co-registered images of the opposed probes. Our results confirm that the Lamb wave method estimates the longitudinal speed of the slab with an error of 3.5% and is independent of its shear wave speed. Benefiting from the acquired sound speed map, our adaptive beamformer reduces (in real time) a mislocation error of 3.1, caused by an 8 mm slab, to 0.1 mm. Finally, the dual probe configuration shows 7 dB improvement in removing reverberation artifacts of the needle, at the cost of only 2.4-dB contrast loss. The proposed image formation method can be used, e.g., to monitor deep brain stimulation procedures and localization of the electrode(s) deep inside the brain from two temporal bones on the sides of the human skull

Frequency Analysis of the Photoacoustic Signal Generated by Coronary Atherosclerotic Plaque

The identification of unstable atherosclerotic plaques in the coronary arteries is emerging as an important tool for guiding percutaneous coronary interventions and may enable preventive treatment of such plaques in the future. Assessment of plaque stability requires imaging of both structure and composition. Spectroscopic photoacoustic (sPA) imaging can visualize atherosclerotic plaque composition on the basis of the optical absorption contrast. It is an established fact that the frequency content of the photoacoustic (PA) signal is correlated with structural tissue properties. As PA signals can be weak, it is important to match the transducer bandwidth to the signal frequency content for in vivo imaging. In this ex vivo study on human coronary arteries, we combined sPA imaging and analysis of frequency content of the PA signals. Using a broadband transducer (-3-dB one-way bandwidth of 10-35 MHz) and a 1-mm needle hydrophone (calibrated for 1-20 MHz), we covered a large frequency range of 1-35 MHz for receiving the PA signals. Spectroscopic PA imaging was performed at wavelengths ranging from 1125 to 1275 nm with a step of 2 nm, allowing discrimination between plaque lipids and adventitial tissue. Under sPA imaging guidance, the frequency content of the PA signals from the plaque lipids was quantified. Our data indicate that more than 80% of the PA energy of the coronary plaque lipids lies in the frequency band below 8 MHz. This frequency information can guide the choice of the transducer element used for PA catheter fabrication

Erratum: Lamb Waves and Adaptive Beamforming for Aberration Correction in Medical Ultrasound Imaging: Lamb Waves and Adaptive Beamforming for Aberration Correction in Medical Ultrasound Imaging (IEEE Trans.Ultrason., Ferroelectr., Freq. Control, early access (2020) DOI: 10.1109/TUFFC.2020.3007345)

In the above article [1], we mentioned that the superposition of the different symmetric (S) modes in the frequencywavenumber (f-k) domain results in a high-intensity region where its slope corresponds to the longitudinal wave speed in the slab. However, we have recently understood that this highintensity region belongs to the propagation of a wave called lateral wave or head wave [2]-[5]. It is generated if the longitudinal sound speed of the aberrator (i.e., the PVC slab) is larger than that of water and if the incident wavefront is curved. When the incidence angle at the interface between water and PVC is near the critical angle, the refracted wave in PVC reradiates a small part of its energy into the fluid (i.e., the head wave). As discussed in [4], if the thickness of the waveguide is larger than the wavelength, the first arriving signal is the head wave. This is also the case in our study [1] where the ultrasound wavelength of a compressional wave in PVC was close to 1 mm, and a PVC slab with a thickness of 8 mm was used. In this Erratum, numerical simulations (with SimSonic solver [5]) and experimental measurements (with the same PVC slab used in [1]) are conducted to investigate the propagation of the Lamb waves and head wave in detail, for the specific configuration studied in [1]. The pitch and element width of the P4-1 probe were used to assemble the numerical signals [see Fig. 1(a)]. If all the data simulated for the P4-1 probe is used, there indeed is a region with a slope [see Fig. 1(b)], but this has a low intensity, meaning that the head wave has a relatively low amplitude compared to the specular reflections. Once the head wave is isolated, the sound speed can be estimated with a 0.3% error from the f-k domain plot [see Fig. 1(c)]. No significant difference is observed between Fig. 1(b) and (d), in which the head wave is muted. Our experimental results show that if only the head wave (the first arriving signal) is used [see Fig. 2(b)], the slope of the linear fitting in the f-k domain also yields the longitudinal sound speed of the PVC with a 0.3% error. Of note, the signal processing (i.e., linear fitting in the f-k domain) used in our study [1] still works for the head wave and is correct provided that the aberrator is parallel to the probe [6]. Also, in [1, p. 6], it is mentioned that “the curved structure of the skull might lead to other types of modes, such as the torsional modes.” Here, we acknowledge that this sentence is not correct, as torsional modes only exist in cylindrical waveguides or rectangular bars. We would like to mention that Guillaume Renaud is added as a coauthor to acknowledge his contribution to the findings reported in this Erratum.</p

Subharmonic, non-linear fundamental and ultraharmonic imaging of microbubble contrast at high frequencies

There is increasing use of ultrasound contrast agent in high-frequency ultrasound imaging. However, conventional contrast detection methods perform poorly at high frequencies. We performed systematic in vitro comparisons of subharmonic, non-linear fundamental and ultraharmonic imaging for different depths and ultrasound contrast agent concentrations (Vevo 2100 system with MS250 probe and MicroMarker ultrasound contrast agent, VisualSonics, Toronto, ON, Canada). We investigated 4-, 6- and 10-cycle bursts at three power levels with the following pulse sequences: B-mode, amplitude modulation, pulse inversion and combined pulse inversion/amplitude modulation. The contrast-to-tissue (CTR) and contrast-to-artifact (CAR) ratios were calculated. At a depth of 8 mm, subharmonic pulse-inversion imaging performed the best (CTR = 26 dB, CAR = 18 dB) and at 16 mm, non-linear amplitude modulation imaging was the best contrast imaging method (CTR = 10 dB). Ultraharmonic imaging did not result in acceptable CTRs and CARs. The best candidates from the in vitro study were tested in vivo in chicken embryo and mouse models, and the results were in a good agreement with the in vitro findings. (C) 2015 World Federation for Ultrasound in Medicine & Biology

Phase aberration correction for focused ultrasound transmission by refraction compensation

Phase aberration of focused ultrasound by tissue structure causes focus degradation and reduces the quality of B-mode images. Refraction at the boundary between subcutaneous fat and muscle is one of the dominant factors behind such degradation. To correct this, we propose a refraction compensation method in which ultrasound is transmitted and received twice. The boundary shape between different tissues is detected by the first ultrasound transmission. Next, ultrasound rays from probe elements to the target are calculated taking refraction into account. Corrected delay times are calculated from the length of the rays and the sound velocity of the medium. Finally, ultrasound is transmitted a second time using the corrected delay time and a B-mode image is created. We evaluate the correction effect of the proposed method by numerical simulation and experiments with non-compensated and refraction-compensated cases of intensity distribution of the focused ultrasound. Results show that focus degradation is effectively corrected by the proposed method.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.ImPhys/Acoustical Wavefield Imagin