Nonlinear intravascular ultrasound contrast imaging

Nonlinear contrast agent imaging with intravascular ultrasound (IVUS) is investigated using a prototype IVUS system and an experimental small bubble contrast agent. The IVUS system employed a mechanically scanned single element transducer and was operated at a 20 MHz transmit frequency (F20) for second harmonic imaging (H40), and at a 40 MHz transmit frequency (F40) for subharmonic imaging (SH20). Characterization experiments were performed with agent and tissue phantom signals acquired during transducer rotation. The suppression of transmit frequency tissue signals was achieved using a combination of pulse-inversion and bandpass filtering. H40 was found to improve the contrast-to-tissue signal ratio (CTR) by up to 22 dB relative to F20, but suffered from tissue propagation harmonics at higher pressures (>0.3 MPa). SH20 was also shown to be possible at a range of pressures (approximately 0.25 to 1.8 MPa), with tissue signals suppressed to near the noise floor. Coronary phantom experiments demonstrated the detection of agent in 1 mm diameter vessels outside a larger 4 mm diameter vessel in which the IVUS catheter was situated. These results suggest the feasibility of harmonic IVUS contrast imaging, which may have applications in coronary lumen boundary detection and vasa vasorum imaging.     

Subharmonic contrast intravascular ultrasound for vasa vasorum imaging

The feasibility of subharmonic contrast intravascular ultrasound (IVUS) imaging was investigated using a prototype nonlinear IVUS system and the commercial contrast agent Definity™. The system employed a mechanically scanned commercial catheter with a custom transducer element fabricated to have sensitivity at both 15 and 30 MHz. Experiments were conducted at a fundamental frequency of 30 MHz (F30; 25% bandwidth), with on-axis pressures ranging from 0.12 to 0.79 MPa, as measured with a needle hydrophone. In vitro characterization experiments demonstrated the detection of 15 MHz subharmonic signals (SH15) when pressure levels reached 360 kPa. The formation of SH15 images was shown, with tissue signals suppressed to near the noise floor and contrast to tissue ratios were improved by up to 30 dB relative to F30. In vivo experiments were performed using the atherosclerotic rabbit aorta model. Following the bolus injection of contrast agent upstream of the imaging catheter, agent was detected within the aorta, vena cava and within the perivascular space. These results provide a first in vivo demonstration of subharmonic contrast IVUS and suggest its potential as a new technique for imaging vasa vasorum. (E-mail: goertz@sri.utoronto.ca)     

Contrast-enhanced intravascular ultrasound: combining morphology with activity-based assessment of plaque vulnerability

Acute coronary syndromes are the result of coronary plaque rupture in the majority of cases. Available diagnostic techniques that focus on the early detection of plaques that are prone to rupture are still limited. Increased neovascularization in the vasa vasorum of the atherosclerotic plaque has been identified recently as a common feature of inflammation and plaque vulnerability. Microbubbles, which have been used for ultrasound imaging, can be used to trace neovascularization. We present recent advances in contrast agents and contrast-enhanced intravascular ultrasound that may be used for the detection of vasa vasorum, including fundamental and harmonic contrast imaging. Identification of vasa vasorum proliferation in atherosclerotic plaques presents important clinical implications; in particular it could provide a means to detect vulnerability in vivo, thereby guiding targeted treatments.     

Contrast-enhanced intravascular ultrasound: combining morphology with activity-based assessment of plaque vulnerability

Acute coronary syndromes are the result of coronary plaque rupture in the majority of cases. Available diagnostic techniques that focus on the early detection of plaques that are prone to rupture are still limited. Increased neovascularization in the vasa vasorum of the atherosclerotic plaque has been identified recently as a common feature of inflammation and plaque vulnerability. Microbubbles, which have been used for ultrasound imaging, can be used to trace neovascularization. We present recent advances in contrast agents and contrast-enhanced intravascular ultrasound that may be used for the detection of vasa vasorum, including fundamental and harmonic contrast imaging. Identification of vasa vasorum proliferation in atherosclerotic plaques presents important clinical implications; in particular it could provide a means to detect vulnerability in vivo, thereby guiding targeted treatments.     

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.     

Imaging Microvasculature with Contrast-Enhanced Ultraharmonic Ultrasound

Atherosclerotic plaque neovascularization was shown to be one of the strongest predictors of future cardiovascular events. Yet, the clinical tools for coronary wall microvasculature detection in vivo are lacking. Here we report an ultrasound pulse sequence capable of detecting microvasculature invisible in conventional intracoronary imaging. The method combines intravascular ultrasound with an ultrasound contrast agent, i.e., a suspension of microscopic vascular acoustic resonators that are small enough to penetrate the capillary bed after intravenous administration. The pulse sequence relies on brief chirp excitations to extract ultraharmonic echoes specific to the ultrasound contrast agent. We implemented the pulse sequence on an intravascular ultrasound probe and successfully imaged the microvasculature of a 6 days old chicken embryo respiratory organ. The feasibility of microvasculature imaging with intravascular ultrasound sets the stage for a translation of the method to studies of intra-plaque neovascularization detection in humans.     

Investigation of vaporized submicron perfluorocarbon droplets as an ultrasound contrast agent

Acoustically activated submicron droplets of liquid perfluorocarbon are investigated as a new class of ultrasound contrast agent. In the liquid state, intravascular droplets can extravasate within tumours. Activation is then accomplished by using bursts of ultrasound to vaporize the droplets. We use acoustical and optical techniques to assess the characteristics of vaporized droplets and the resulting microbubbles in vitro, including size, conversion threshold, echogenicity and nonlinearity. Under exposure to single 5-50 cycle bursts of ultrasound at 7.5 MHz and mechanical index <1.0, droplets with mean diameter of 400 nm convert into microbubbles with mean diameter of 1.4 μm at 1 ms after vaporization, expanding to 5.6 μm by 1 s. The growth of microbubbles produced by vaporization causes a characteristic time-dependent increase in linear and nonlinear echogenicity, enabling selective detection with conventional bubble-specific imaging. These results suggest that submicron perfluorocarbon droplets, activated in situ, may be a candidate for an extravascular ultrasound contrast agent.     

Broadband Reduction of the Second Harmonic Distortion During Nonlinear Ultrasound Wave Propagation

Ultrasound contrast harmonic imaging and detection techniques are hampered by the harmonic distortion of the ultrasound wave caused by the nonlinearities of the medium. To increase the discrimination between the tissue and ultrasound contrast agents at higher harmonics, we investigate a tissue harmonic suppression technique. The main attention of the research is the signal that is introduced at the source and is constructed out of several discrete frequency components from the second harmonic band. Therefore, this method was coined as the multiple component second harmonic reduction signal or multiple component SHRS. By adjusting the amplitude and phase of discrete components and simultaneously propagating multiple component SHRS with the imaging signal, the nonlinear distortion of the ultrasound waveform is considerably reduced. Using the numerical simulation, the optimal parameters for multiple component SRHS were deduced. The simulations results were corroborated in the water tank experiments and showed 40 dB reduction with respect to the fundamental, covering up to 75% of the entire second harmonic band. In the other series of experiments with the clinically used contrast agent, the uniform increase in agent-to-tissue ratio of 7.4 dB over a relatively large region of imaging was observed. The use of the proposed method in the everyday clinical practice can improve discrimination between the tissue and the contrast agent in harmonic imaging. (E-mail: mirza.pasovic@creatis.insa-lyon.fr)     

Ultrasound contrast imaging based on a novel algorithm combined pulse inversion with wavelet transform

In this article, an ultrasound contrast imaging method that combines the pulse-inversion technique with wavelet transform has been proposed to enhance the contrast between bubbles and surrounding tissues. In this technique, wavelet transform is utilized to analyze the correlation between mother wavelet and the received echoes from a pair of inverted transmit pulses, respectively. To obtain a better correlation, a new mother wavelet named “bubble wavelet” is constructed according to the modified Herring equation. Radio-frequency (RF) data were acquired from a modified digital diagnostic ultrasound system that transmits two identical pulses with opposite polarity. The proposed method was validated by simulations. Experiments were performed on an ultrasound flow phantom and results showed that the contrast-to-tissue ratio (CTR) was improved by up to 28 dB depending on types of mother wavelet, scales and depths, compared with that obtained using pulse-inversion-based second-harmonic imaging. Experiments in vivo were also conducted out using kidneys of rabbits and results showed that the signals of surrounding tissues can be well suppressed compared with that of bubbles. The proposed method was compared with the quadratic pulse inversion (QPI) imaging on the same set of experimental data. Further improvements might be achieved with optimized bubble wavelet and imaging algorithm.     

Collagen and Chondrocyte Concentrations Control Ultrasound Scattering in Agarose Scaffolds

Ultrasound imaging has been proposed for diagnostics of osteoarthritis and cartilage injuries in vivo. However, the specific contribution of chondrocytes and collagen to ultrasound scattering in articular cartilage has not been systematically studied. We investigated the role of these tissue structures by measuring ultrasound scattering in agarose scaffolds with varying collagen and chondrocyte concentrations. Ultrasound catheters with center frequencies of 9 MHz (7.1–11.0 MHz, −6 dB) and 40 MHz (30.1–45.3 MHz, −6 dB) were applied using an intravascular ultrasound device. Ultrasound backscattering quantified in a region of interest starting right below sample surface differed significantly (p < 0.05) with the concentrations of collagen and chondrocytes. An ultrasound frequency of 40 MHz, as compared with 9 MHz, was more sensitive to variations in collagen and chondrocyte concentrations. The present findings may improve diagnostic interpretation of arthroscopic ultrasound imaging and provide information necessary for development of models describing ultrasound propagation within cartilage.     

Acoustic Characterization and Pharmacokinetic Analyses of New Nanobubble Ultrasound Contrast Agents

In contrast to the clinically used microbubble ultrasound contrast agents, nanoscale bubbles (or nanobubbles) may potentially extravasate into tumors that exhibit more permeable vasculature, facilitating targeted molecular imaging and drug delivery. Our group recently presented a simple strategy using the non-ionic surfactant Pluronic as a size control excipient to produce nanobubbles with a mean diameter of 200 nm that exhibited stability and echogenicity on par with microbubbles. The objective of this study was to carry out an in-depth characterization of nanobubble properties as compared with Definity microbubbles, both in vitro and in vivo. Through use of a tissue-mimicking phantom, in vitro experiments measured the echogenicity of the contrast agent solutions and the contrast agent dissolution rate over time. Nanobubbles were found to be more echogenic than Definity microbubbles at three different harmonic frequencies (8, 6.2 and 3.5 MHz). Definity microbubbles also dissolved 1.67 times faster than nanobubbles. Pharmacokinetic studies were then performed in vivo in a subcutaneous human colorectal adenocarcinoma (LS174T) in mice. The peak enhancement and decay rates of contrast agents after bolus injection in the liver, kidney and tumor were analyzed. No significant differences were observed in peak enhancement between the nanobubble and Definity groups in the three tested regions (tumor, liver and kidney). However, the decay rates of nanobubbles in tumor and kidney were significantly slower than those of Definity in the first 200-s fast initial phase. There were no significant differences in the decay rates in the liver in the initial phase or in three regions of interest in the terminal phase. Our results suggest that the stability and acoustic properties of the new nanobubble contrast agents are superior to those of the clinically used Definity microbubbles. The slower washout of nanobubbles in tumors suggests potential entrapment of the bubbles within the tumor parenchyma.     

Acoustic characterization of tissue-mimicking materials for ultrasound perfusion imaging research

Materials with well-characterized acoustic properties are of great interest for the development of tissue-mimicking phantoms with designed (micro)vasculature networks. These represent a useful means for controlled in-vitro experiments to validate perfusion imaging methods such as Doppler and contrast-enhanced ultrasound (CEUS) imaging. In this work, acoustic properties of seven tissue-mimicking phantom materials at different concentrations of their compounds and five phantom case materials are characterized and compared at room temperature. The goal of this research is to determine the most suitable phantom and case material for ultrasound perfusion imaging experiments. The measurements show a wide range in speed of sound varying from 1057 to 1616 m/s, acoustic impedance varying from 1.09 to 1.71 × 10⁶ kg/m²s, and attenuation coefficients varying from 0.1 to 22.18 dB/cm at frequencies varying from 1 MHz to 6 MHz for different phantom materials. The nonlinearity parameter B/A varies from 6.1 to 12.3 for most phantom materials. This work also reports the speed of sound, acoustic impedance and attenuation coefficient for case materials. According to our results, polyacrylamide (PAA) and polymethylpentene (TPX) are the optimal materials for phantoms and their cases, respectively. To demonstrate the performance of the optimal materials, we performed power Doppler ultrasound imaging of a perfusable phantom, and CEUS imaging of that phantom and a perfusion system. The obtained results can assist researchers in the selection of the most suited materials for in-vitro studies with ultrasound imaging.     

Investigation of the Relationship of Nonlinear Backscattered Ultrasound Intensity with Microbubble Concentration at Low MI

The aim of this study was to measure the relationship of image intensity with contrast agent concentration. In vitro experiments were performed with a flow phantom and a sulphur hexafluoride filled microbubble contrast agent (SonoVue) at different concentrations (0.004‰ to 4‰) covering the range commonly encountered in clinical practice. The concentration of microbubbles in the contrast agent solutions was confirmed optically. Images were collected with a diagnostic ultrasound system (iU22, Phillips Medical Systems, Bothell, WA, USA) and with a nonlinear imaging technique (power modulation) at low mechanical index (MI = 0.05) to avoid bubble destruction. The mean intensity within a region of interest was measured to produce time-intensity curves from linearized (absolute scale) data. The relationship of linearized image intensity to contrast agent concentration was found to be linear up to 1‰ and reached a plateau at approximately 2‰. To operate in the linear range of the intensity-concentration relationship the contrast agent dose should be adjusted to avoid those high values in vivo and the highest dynamic range of the ultrasound system should be used to avoid unnecessary signal saturation. (E-mail: maverk@ucy.ac.cy).     

Ultrasound Time-Reversal MUSIC Imaging of Extended Targets

Ultrasound time-reversal imaging with multiple signal classification (TR-MUSIC) can produce images with subwavelength spatial resolution when the targets are point scatterers. In this experimental study, we evaluate the performance of the TR-MUSIC algorithm when the interrogated medium contains extended targets that cannot be considered as point scatterers, i.e., the size of the targets is on the order of the ultrasound wavelength or larger. We construct four tissue-mimicking phantoms, each of which contains glass spheres of a given size. We show that the quality of the phantom images obtained using the TR-MUSIC algorithm decreases with increasing sphere size. However, significant improvement is achieved when the image plane is divided into subregions, where each subregion is imaged separately. In this method, the TR-MUSIC calculations are performed on the windowed backscattered signals originating from each subregion. Our study demonstrates that the TR-MUSIC algorithm with time windowing can accurately locate extended targets but cannot provide the shape and reflectivity of the targets. We scan an inhomogeneous commercial tissue-mimicking phantom using an investigational synthetic-aperture ultrasound system, and show that the TR-MUSIC algorithm is capable of detecting small targets with high spatial resolution in inhomogeneous media.     

Ultrasound imaging of atherosclerotic plaques

The development of techniques for imaging the molecular mediators of atherosclerosis is an area of great interest. The ability to image vascular phenotype will create new opportunities for assessing patient risk for aggressive disease at a very early stage and for choosing appropriate treatment strategies in late stages of disease. Ultrasound will undoubtedly play an important role in molecular imaging because of its practicality as a screening test and because intravascular imaging approaches are already widely used as an adjunct to angiographic procedures. This review focuses on the biophysical principles for the diverse set of tools used to evaluate atherosclerosis. General strategies for imaging vascular phenotype include: 1) assessment of histomorphometry by radiofrequency analysis of high-frequency ultrasound; 2) assessment of plaque content by vascular elastic properties; 3) detection of remodeling of the vasa vasorum by contrast ultrasound; and 4) imaging endothelial molecular phenotype with targeted ultrasound contrast agents.     

Very Low Frequency Radial Modulation for Deep Penetration Contrast-Enhanced Ultrasound Imaging

Contrast-enhanced ultrasound imaging allows vascular imaging in a variety of diseases. Radial modulation imaging is a contrast agent-specific imaging approach for improving microbubble detection at high imaging frequencies (≥7.5 MHz), with imaging depth limited to a few centimeters. To provide high-sensitivity contrast-enhanced ultrasound imaging at high penetration depths, a new radial modulation imaging strategy using a very low frequency (100 kHz) ultrasound modulation wave in combination with imaging pulses ≤5 MHz is proposed. Microbubbles driven at 100 kHz were imaged in 10 successive oscillation states by manipulating the pulse repetition frequency to unlock the frame rate from the number of oscillation states. Tissue background was suppressed using frequency domain radial modulation imaging (F-RMI) and singular value decomposition-based radial modulation imaging (S-RMI). One hundred-kilohertz modulation resulted in significantly higher microbubble signal magnitude (63–88 dB) at the modulation frequency relative to that without 100-kHz modulation (51–59 dB). F-RMI produced images with high contrast-to-tissue ratios (CTRs) of 15 to 22 dB in a stationary tissue phantom, while S-RMI further improved the CTR (19–26 dB). These CTR values were significantly higher than that of amplitude modulation pulse inversion images (11.9 dB). In the presence of tissue motion (1 and 10 mm/s), S-RMI produced high-contrast images with CTR up to 18 dB; however, F-RMI resulted in minimal contrast enhancement in the presence of tissue motion. Finally, in transcranial ultrasound imaging studies through a highly attenuating ex vivo cranial bone, CTR values with S-RMI were as high as 23 dB. The proposed technique demonstrates successful modulation of microbubble response at 100 kHz for the first time. The presented S-RMI low-frequency radial modulation imaging strategy represents the first demonstration of real-time (20 frames/s), high-penetration-depth radial modulation imaging for contrast-enhanced ultrasound imaging.     

The Delayed Onset of Subharmonic and Ultraharmonic Emissions from a Phospholipid-Shelled Microbubble Contrast Agent

Characterizing the non-linear response of microbubble contrast agents is important for their efficacious use in imaging and therapy. In this article, we report that the subharmonic and ultraharmonic response of lipid-shelled microbubble contrast agents exhibits a strong temporal dependence. We characterized non-linear emissions from Targestar-p microbubbles (Targeson Inc., San Diego, CA, USA) periodically for 60 min, at 10 MHz excitation frequency. The results revealed a considerable increase in the subharmonic and ultraharmonic response (nearly 12–15 and 5–8 dB) after 5–10 min of agent preparation. However, the fundamental and the harmonic response remained almost unchanged in this period. During the next 50 min, the subharmonic, fundamental, ultraharmonic, and harmonic responses decreased steadily by 2–5 dB. The temporal changes in the non-linear behavior of the agent appeared to be primarily mediated by gas-exchange through the microbubble shell; temperature and prior acoustic excitation based mechanisms were ruled out. Further, there was no measurable change in the agent size distribution by static diffusion. We envisage that these findings will help obtain reproducible measurements from agent characterization, non-linear imaging, and fluid-pressure sensing. These findings also suggest the possibility for improving non-linear imaging by careful design of ultrasound contrast agents.     

Acoustic and kinetic behaviour of definity in mice exposed to high frequency ultrasound

Microbubble contrast agents have shown clinical potential for characterising blood flow using 1 to 10 MHz ultrasound; however, scaling their use for similar applications in the mouse with high frequency ultrasound (20 to 60 MHz) has not been addressed. The goal was to determine the utility of microbubbles for mouse imaging with 30 MHz ultrasound by investigating their attenuation and backscatter characteristics as a function of concentration in vitro and dose response in vivo. The agent was exposed to a 30 MHz, 20% bandwidth pulse with a peak negative pressure of 244 kPa. In vitro results showed that the attenuation and backscatter increased linearly for concentrations between 2.8 x 10(6) and 28 x 10(6) bubbles per mL of deionized water. In vivo experiments where performed in the jugular vein of CD-1 mice and time intensity curves were acquired for doses between 10 and 100 microL kg(-1). These doses corresponded to the range of concentrations used in vitro. In vivo results showed that the peak enhancement of the agent increased linearly for doses between 10 and 60 microL kg(-1), the duration of enhancement varied between 200 to 300 s and the integrated enhancement (area under the curve) increased linearly up to 100 microL kg(-1). A maximum enhancement of 13 dB over the blood pool was observed for a dose of 100 microL kg(-1). The intra- and inter-mouse variabilities were 10% to 40% and indicate that further optimisations are required. These results suggest that quantitative contrast flow studies in the mouse using high frequency ultrasound are possible for doses between 10 and 60 microL kg(-1).     

The effect of preactivation vial temperature on the acoustic properties of Definity™

Definity^TM is a widely available clinically approved ultrasound contrast agent. The manufacturer’s instructions indicate that the refrigerated vial should be allowed to reach room temperature prior to its 45 s mechanical agitation activation process. Activation results in vial heating and it has been previously observed that “smaller” bubbles are formed later in this process (>10 s) when the vial temperature is elevated. The objective of this work was to examine the effects of preactivation vial temperature on the size distribution, frequency dependent attenuation (1.5–27 MHz) and nonlinear imaging performance of Definity^TM. Experiments were conducted with vials at refrigerator temperature (2°C), room temperature (22°C) or 37°C at the outset of the activation procedure. The size distributions were found to be strongly dependent on preactivation vial temperature and the attenuation results indicated considerable differences in the frequency response of the agent, most notably the appearance of a peak at 4 MHz for the 2°C case. Nonlinear imaging results performed using a 1–5 MHz transducer probe with a wall-less vessel phantom indicated that 2°C vials produced a signal enhancement 5.1 dB greater than for 22°C Definity^TM (p < 0.05). These results, therefore, indicate that not permitting the vial to reach room temperature has a considerable impact on the imaging performance of Definity^TM. Conversely, activating a cooled vial can provide a means by which to improve contrast enhancement when using low frequency clinical transducers.     

A Review of Tissue Substitutes for Ultrasound Imaging

The characterization and calibration of ultrasound imaging systems requires tissue-mimicking phantoms with known acoustic properties, dimensions and internal features. Tissue phantoms are available commercially for a range of medical applications. However, commercial phantoms may not be suitable in ultrasound system design or for evaluation of novel imaging techniques. It is often desirable to have the ability to tailor acoustic properties and phantom configurations for specific applications. A multitude of tissue-mimicking materials and phantoms are described in the literature that have been created using a variety of materials and preparation techniques and that have modeled a range of biological systems. This paper reviews ultrasound tissue-mimicking materials and phantom fabrication techniques that have been developed over the past four decades, and describes the benefits and disadvantages of the processes. Both soft tissue and hard tissue substitutes are explored. (E-mail: mculjat@mednet.ucla.edu)