Subharmonic signal generation from contrast agents in simulated neovessels

Characterization of Adnexal Masses Using Contrast-Enhanced Subharmonic Imaging: A Pilot Study

OBJECTIVES

This pilot study evaluated whether contrast-enhanced subharmonic imaging (SHI) could be used to characterize adnexal masses before surgical intervention.

METHODS

Ten women (with 12 lesions) scheduled for surgery of an ovarian mass underwent an SHI examination of their adnexal region using a modified LOGIQ E9 scanner (GE Healthcare, Waukesha, WI) with an endocavitary transducer, in which digital clips were acquired by pulse destruction-replenishment SHI across the lesions. Time-intensity curves were created offline to quantitatively evaluate SHI parameters (fractional tumor perfusion, peak contrast intensity, time to peak contrast enhancement, and area under the time-intensity curve), which were compared to pathologic characterizations of the lesions.

RESULTS

Of the 12 masses, 8 were benign, and 4 were malignant. A qualitative analysis of the SHI images by an experienced radiologist resulted in diagnostic accuracy of 70%, compared to 56% without contrast, whereas an inexperienced radiologist improved from 50% to 58% accuracy, demonstrating the benefit of SHI. A quantitative analysis of SHI parameters produced diagnostic accuracy as high as 81%. Peak contrast intensity was significantly greater in malignant than benign masses (mean ± SD, 0.109 ± 0.088 versus 0.046 ± 0.030 arbitrary units; P = .046). Malignant masses also showed significantly greater perfusion than benign masses (24.79% ± 25.34% versus 7.62% ± 6.50%; P = .045). When the radiologist reads were combined with the most predictive quantitative SHI parameter (percent perfusion), diagnostic accuracy improved to 84% for the experienced radiologist and 96% for the novice radiologist.

CONCLUSIONS

Results indicate that SHI for presurgical characterization of adnexal masses may improve the determination of malignancy and diagnostic accuracy, albeit based on a small sample size.

Quantitative Nonlinear Contrast-Enhanced Ultrasound of the Breast

OBJECTIVE

Breast cancer is the most frequent type of cancer among women (25% of all cancers). The angiogenic process that fuels the growth of tumors is a potential early indicator for differentiating between malignant and benign tumors. Recently, the use of microbubble-based contrast agents combined with ultrasound has allowed the development of contrast agent-specific imaging modes that provide visualization of tumor neovascularity.

CONCLUSION

Contrast-enhanced Doppler, harmonic, and subharmonic imaging are some of the imaging modes that have been investigated for visualizing and quantifying the vascularity in breast tumors.

Interpreting attenuation at different excitation amplitudes to estimate strain-dependent interfacial rheological properties of lipid-coated monodisperse microbubbles

Broadband attenuation of ultrasound measured at different excitation pressures being different raises a serious theoretical concern, because the underlying assumption of linear and independent propagation of different frequency components nominally requires attenuation to be independent of excitation. Here, this issue is investigated by examining ultrasound attenuation through a monodisperse lipid-coated microbubble suspension measured at four different acoustic excitation amplitudes. The attenuation data are used to determine interfacial rheological properties (surface tension, surface dilatational elasticity, and surface dilatational viscosity) of the encapsulation according to three different models. Although different models result in similar rheological properties, attenuation measured at different excitation levels (4-110 kPa) leads to different values for them; the dilatation elasticity (0.56 to 0.18 N/m) and viscosity (2.4 × 10(-8) to 1.52 × 10(-8) Ns/m) both decrease with increasing pressure. Numerically simulating the scattered response, nonlinear energy transfer between frequencies are shown to be negligible, thereby demonstrating the linearity in propagation and validating the attenuation analysis. There is a second concern to the characterization arising from shell properties being dependent on excitation amplitude, which is not a proper constitutive variable. It is resolved by arriving at a strain-dependent rheology for the encapsulation. The limitations of the underlying analysis are discussed.

Quantitative analysis of vascular heterogeneity in breast lesions using contrast-enhanced 3-D harmonic and subharmonic ultrasound imaging

Ability to visualize breast lesion vascularity and quantify the vascular heterogeneity using contrast-enhanced 3-D harmonic (HI) and subharmonic (SHI) ultrasound imaging was investigated in a clinical population. Patients (n = 134) identified with breast lesions on mammography were scanned using power Doppler imaging, contrast-enhanced 3-D HI, and 3-D SHI on a modified Logiq 9 scanner (GE Healthcare). A region of interest corresponding to ultrasound contrast agent flow was identified in 4D View (GE Medical Systems) and mapped to raw slice data to generate a map of time-intensity curves for the lesion volume. Time points corresponding to baseline, peak intensity, and washout of ultrasound contrast agent were identified and used to generate and compare vascular heterogeneity plots for malignant and benign lesions. Vascularity was observed with power Doppler imaging in 84 lesions (63 benign and 21 malignant). The 3-D HI showed flow in 8 lesions (5 benign and 3 malignant), whereas 3-D SHI visualized flow in 68 lesions (49 benign and 19 malignant). Analysis of vascular heterogeneity in the 3-D SHI volumes found benign lesions having a significant difference in vascularity between central and peripheral sections (1.71 ± 0.96 vs. 1.13 ± 0.79 dB, p < 0.001, respectively), whereas malignant lesions showed no difference (1.66 ± 1.39 vs. 1.24 ± 1.14 dB, p = 0.24), indicative of more vascular coverage. These preliminary results suggest quantitative evaluation of vascular heterogeneity in breast lesions using contrast-enhanced 3-D SHI is feasible and able to detect variations in vascularity between central and peripheral sections for benign and malignant lesions.

Encapsulated microbubbles and echogenic liposomes for contrast ultrasound imaging and targeted drug delivery

Micron- to nanometer-sized ultrasound agents, like encapsulated microbubbles and echogenic liposomes, are being developed for diagnostic imaging and ultrasound mediated drug/gene delivery. This review provides an overview of the current state of the art of the mathematical models of the acoustic behavior of ultrasound contrast microbubbles. We also present a review of the in vitro experimental characterization of the acoustic properties of microbubble based contrast agents undertaken in our laboratory. The hierarchical two-pronged approach of modeling contrast agents we developed is demonstrated for a lipid coated (Sonazoid™) and a polymer shelled (poly D-L-lactic acid) contrast microbubbles. The acoustic and drug release properties of the newly developed echogenic liposomes are discussed for their use as simultaneous imaging and drug/gene delivery agents. Although echogenicity is conclusively demonstrated in experiments, its physical mechanisms remain uncertain. Addressing questions raised here will accelerate further development and eventual clinical approval of these novel technologies.

Delineation of atherosclerotic plaque using subharmonic imaging filtering techniques and a commercial intravascular ultrasound system

The ability to delineate atherosclerotic plaque from the surrounding tissue using custom-developed subharmonic imaging (SHI) digital filtering techniques was investigated in vivo using a commercially available system. Atherosclerosis was induced in the aorta of two Watanabe Heritable Hyperlipidemic rabbits following which injections of an ultrasound contrast agent (UCA) Definity (Lantheus Medical Imaging, N Billerica, Massachusetts) were administered. Imaging was performed using a Galaxy intravascular ultrasound (IVUS) scanner (Boston Scientific, Natick, Massachusetts) equipped with an Atlantis® SR Pro Imaging Catheter (Boston Scientific). Four preliminary band-pass filters were designed to isolate the subharmonic signal (from surrounding tissue) and applied to the radio-frequency (RF) data. Preliminary filter performances were compared in terms of vessel-tissue contrast-to-tissue ratio (CTR) and visual examination. Based on preliminary results, a subharmonic adaptive filter and a stopband (SB) filter were designed and applied to the RF data. Images were classified as fundamental, SHI, and SB. Four readers performed qualitative analysis of 168 randomly selected images (across all three imaging modes). The images were scored for overall image quality, image noise, plaque visualization, and vessel lumen visualization. A Wilcoxon signed-rank test was used to compare the scores followed by intraclass correlation (ICC) evaluation. Quantitative analysis was performed by calculating the CTRs for the vessel-to-plaque and vessel-to-tissue (compared using a paired student's t test). Qualitative analysis showed SHI and SB to have significantly less image noise relative to the fundamental mode (p < 0.001). Fundamental mode scored significantly higher than SHI and SB for the remaining three categories. ICC showed mixed results among reader evaluation for delineation of plaque. However, quantitatively, SHI produced the best vessel-plaque CTR.

In vitro measurement of attenuation and nonlinear scattering from echogenic liposomes

Echogenic liposomes (ELIP) are an excellent candidate for concurrent imaging and drug delivery applications. They combine the advantages of liposomes-biocompatibility and ability to encapsulate both hydrophobic and hydrophilic drugs-with strong reflections of ultrasound. The objective of this study is to perform a detailed in vitro acoustic characterization - including nonlinear scattering that has not been studied before - along with an investigation of the primary mechanism of echogenicity. Both components are critical for developing viable clinical applications of ELIP. Mannitol, a cryoprotectant, added during the preparation of ELIP is commonly believed to be critical in making them echogenic. Accordingly, here ELIP prepared with varying amount of mannitol concentration are investigated for their pressure dependent linear and non-linear scattered responses. The average diameter of these liposomes is measured to be 125-185nm. But they have a broad size distribution including liposomes with diameters over a micro-meter as observed by TEM and AFM. These larger liposomes are critical for the overall echogenicity. Attenuation through liposomal solution is measured with four different transducers (central frequencies 2.25, 3.5, 5, 10MHz). Measured attenuation increases linearly with liposome concentration indicating absence of acoustic interactions between liposomes. Due to the broad size distribution, the attenuation shows a flat response without a distinct peak in the range of frequencies (1-12MHz) investigated. A 15-20dB enhancement with 1.67 μg/ml of lipids is observed both for the scattered fundamental and the second harmonic responses at 3.5MHz excitation frequency and 50-800kPa amplitude. It demonstrates the efficacy of ELIP for fundamental as well as harmonic ultrasound imaging. The scattered response however does not show any distinct subharmonic peak for the acoustic excitation parameters studied. Small amount of mannitol proves critical for echogenicity. However, mannitol concentration above 100mM shows no effect.

In vivo characterization of ultrasound contrast agents: microbubble spectroscopy in a chicken embryo

The dynamics of coated microbubbles was studied in an in vivo model. Biotinylated lipid-coated microbubbles were prepared in-house and were injected into a chick embryo chorioallantoic membrane (CAM) model on the fifth day of incubation. The microbubbles, ranging between 1.0 and 3.5 μm in diameter, were insonified in the frequency range of 4-7 MHz. Two amplitudes of acoustic pressure were applied: 300 kPa and 400 kPa. The fundamental and subharmonic responses were recorded optically with an ultra-fast camera (Brandaris 128) at 20 million frames per second. A subharmonic response was observed for 44% of the studied bubbles. From the data the frequency of the maximum fundamental and subharmonic response was derived for each individual bubble and resulted in the resonance curves of the microbubbles. All the bubbles showed shell (strain) hardening behavior for a higher acoustic pressure. We conclude that the subharmonic oscillations observed in this study belonged to the transmit at resonance (TR) regime.

Parametric subharmonic imaging using a commercial intravascular ultrasound scanner: an in vivo feasibility study

OBJECTIVES

The feasibility of visualizing atherosclerotic plaque using parametric subharmonic intravascular ultrasound (IVUS) was investigated in vivo.

METHODS

Atherosclerosis was induced in the aorta of 2 rabbits. Following injection of Definity (Lantheus Medical Imaging, North Billerica, MA), radiofrequency IVUS signals were acquired at 40 MHz with a Galaxy IVUS scanner (Boston Scientific/Scimed, Natick, MA). Subharmonic imaging (SHI; receiving at 20 MHz) was performed offline by applying an 8-order equalization filter. Contrast-to-tissue ratios (CTRs) were computed for the vessel relative to the plaque area over 4 time points. Contrast-to-tissue ratios were also calculated for the plaque-tissue and vessel-tissue from 4 tissue regions of interest at 4 time points. Finally, parametric images showing the cumulative maximum intensity (CMI), time to peak, perfusion (PER), and time-integrated intensity (TII) were generated for the fundamental and subharmonic data sets, and CTR measurements were repeated.

RESULTS

Injection of the contrast agent resulted in improved delineation between plaque and the vessel lumen. Subharmonic imaging resulted in noticeable tissue suppression, although the intensity from the contrast agent was reduced. No significant improvement in the plaque to vessel lumen CTR was observed between the subharmonic and fundamental IVUS (2.1 ± 3.64 versus 2.2 ± 4.20; P = .5). However, the CTR for plaque-tissue was improved (11.8 ± 7.32 versus 9.9 ± 7.06; P < .0001) for SHI relative to fundamental imaging. Cumulative-maximum-intensity and TII maps of both fundamental and subharmonic data provided increased CTRs relative to nonparametric data sets (P < .002). Additionally, the CMI, PER, and TII of SHI IVUS showed significantly improved vessel-plaque CTRs for SHI relative to the fundamental (P < .04).

CONCLUSIONS

Parametric SHI IVUS of atherosclerotic plaque is feasible and improves the visualization of the plaque.

Simultaneous grayscale and subharmonic ultrasound imaging on a modified commercial scanner

OBJECTIVE

To demonstrate the feasibility of simultaneous dual fundamental grayscale and subharmonic imaging on a modified commercial scanner.

MOTIVATION

The ability to generate signals at half the insonation frequency is exclusive to ultrasound contrast agents (UCA). Thus, subharmonic imaging (SHI; transmitting at f(0) and receiving at f(0)/2) provides improved visualization of UCA within the vasculature via suppression of the surrounding tissue echoes. While this capability has proven useful in a variety of clinical applications, the SHI suppression of surrounding tissue landmarks (which are needed for sonographic navigation) also limits it use as a primary imaging modality. In this paper we present results using a commercial ultrasound scanner modified to allow imaging in both grayscale (f(0)=4.0 MHz) and SHI (f(0)=2.5 MHz, f(0)/2=1.25 MHz) modes in real time.

METHODS

A Logiq 9 ultrasound scanner (GE Healthcare, Milwaukee, WI) with a 4C curvilinear probe was modified to provide this capability. Four commercially available UCA (Definity, Lantheus Medical Imaging, North Billerica, MA; Optison, GE Healthcare, Princeton, NJ; SonoVue, Bracco Imaging, Milan, Italy; and Sonazoid, GE Healthcare, Oslo, Norway) were all investigated in vitro over an acoustic output range of 3.34 MPa. In vivo the subharmonic response of Sonazoid was investigated in the portal veins of four canines (open abdominal cavity) and four patients with suspected portal hypertension.

RESULTS

In vitro, the four UCA showed an average maximum subharmonic amplitude of 44.1±5.4 dB above the noise floor with a maximum subharmonic amplitude of 48.6±1.6 dB provided by Sonazoid. The average in vivo maximum signal above the noise floor from Sonazoid was 20.8±2.3 dB in canines and 33.9±5.2 dB in humans. Subharmonic amplitude as a function of acoustic output in both groups matched the S-curve behavior of the agent observed in vitro. The dual grayscale imaging provided easier sonographic navigation, while the degree of tissue suppression in SHI mode varied greatly on a case by case basis.

CONCLUSIONS

These results demonstrate the feasibility of dual grayscale and SHI on a modified commercial scanner. The ability to simultaneously visualize both imaging modes in real time should improve the applicability of SHI as a future primary clinical imaging modality.

Combined optical and acoustical detection of single microbubble dynamics

A detailed understanding of the response of single microbubbles subjected to ultrasound is fundamental to a full understanding of the contrast-enhancing abilities of microbubbles in medical ultrasound imaging, in targeted molecular imaging with ultrasound, and in ultrasound-mediated drug delivery with microbubbles. Here, single microbubbles are isolated and their ultrasound-induced radial dynamics recorded with an ultra-high-speed camera at up to 25 million frames per second. The sound emission is recorded simultaneously with a calibrated single element transducer. It is shown that the sound emission can be predicted directly from the optically recorded radial dynamics, and vice versa, that the nanometer-scale radial dynamics can be predicted from the acoustic response recorded in the far field.

Modeling subharmonic response from contrast microbubbles as a function of ambient static pressure

Variation of subharmonic response from contrast microbubbles with ambient pressure is numerically investigated for non-invasive monitoring of organ-level blood pressure. Previously, several contrast microbubbles both in vitro and in vivo registered approximately linear (5-15 dB) subharmonic response reduction with 188 mm Hg change in ambient pressure. In contrast, simulated subharmonic response from a single microbubble is seen here to either increase or decrease with ambient pressure. This is shown using the code BUBBLESIM for encapsulated microbubbles, and then the underlying dynamics is investigated using a free bubble model. The ratio of the excitation frequency to the natural frequency of the bubble is the determining parameter--increasing ambient pressure increases natural frequency thereby changing this ratio. For frequency ratio below a lower critical value, increasing ambient pressure monotonically decreases subharmonic response. Above an upper critical value of the same ratio, increasing ambient pressure increases subharmonic response; in between, the subharmonic variation is non-monotonic. The precise values of frequency ratio for these three different trends depend on bubble radius and excitation amplitude. The modeled increase or decrease of subharmonic with ambient pressure, when one happens, is approximately linear only for certain range of excitation levels. Possible reasons for discrepancies between model and previous experiments are discussed.

Subharmonic behavior of phospholipid-coated ultrasound contrast agent microbubbles

Coated microbubbles, unlike tissue are able to scatter sound subharmonically. Therefore, the subharmonic behavior of coated microbubbles can be used to enhance the contrast in ultrasound contrast imaging. Theoretically, a threshold amplitude of the driving pressure can be calculated above which subharmonic oscillations of microbubbles are initiated. Interestingly, earlier experimental studies on coated microbubbles demonstrated that the threshold for these bubbles is much lower than predicted by the traditional linear viscoelastic shell models. This paper presents an experimental study on the subharmonic behavior of differently sized individual phospholipid coated microbubbles. The radial subharmonic response of the microbubbles was recorded with the Brandaris ultra high-speed camera as a function of both the amplitude and the frequency of the driving pulse. Threshold pressures for subharmonic generation as low as 5 kPa were found near a driving frequency equal to twice the resonance frequency of the bubble. An explanation for this low threshold pressure is provided by the shell buckling model proposed by Marmottant et al. [J. Acoust. Soc. Am. 118, 3499-3505 (2005)]. It is shown that the change in the elasticity of the bubble shell as a function of bubble radius as proposed in this model, enhances the subharmonic behavior of the microbubbles.

The influence of fragmentation on the acoustic response from shrinking bubbles

Bubble disruption is associated with the response of ultrasound contrast agents (UCAs) exposed to high acoustic pressures. This behavior is important for bubble detection techniques as well as flow quantitation and some proposed therapeutic applications. Previous work has measured acoustically the disruption threshold and postdisruption echo from populations of microbubbles. This suggests a model for UCA disruption whereby ultrasound breaks their shell, leaving free gas bubbles. Diffusion of gas causes the bubbles to shrink and, consequently, reduces the measured backscatter echo over time. In this work, similar bubbles containing three different gases were measured and their echo behavior with time compared with a simple simulation based on diffusion of gas out of the bubble. It was found that, in general, the simulations and experiments compared well at low disruption pressures. Incorporating bubble fragmentation in the simulation model brought its results closer to experiment.

Breast lesions: imaging with contrast-enhanced subharmonic US--initial experience

PURPOSE

To prospectively compare accuracy of gray-scale subharmonic imaging (SHI) with that of standard gray-scale ultrasonography (US), power Doppler US (with and without contrast material), and mammography for the diagnosis of breast cancer, with histopathologic or clinical follow-up results as the reference standard.

MATERIALS AND METHODS

This HIPAA-compliant pilot study had institutional review board approval; all subjects gave written informed consent. Fourteen women (age range, 37-66 years) had 16 biopsy-proved breast lesions. In SHI, pulses are transmitted at one frequency, but only echoes at half that frequency (the subharmonic) are received. A US scanner was modified to perform gray-scale SHI (transmitting at 4.4 and receiving at 2.2 MHz). Precontrast imaging (gray-scale US and power Doppler) was followed by contrast material-enhanced power Doppler and gray-scale SHI. A reader blinded to mammographic and pathologic findings assessed diagnosis on a six-point scale. Sensitivity, specificity, accuracy, and receiver operating characteristic (ROC) curves were computed for mammography, gray-scale and power Doppler imaging (pre- and postcontrast), and SHI.

RESULTS

Of the 16 lesions, four (25%) were malignant. Mammography had 100% sensitivity and 20% specificity. Sensitivity and specificity, respectively, were 50% and 92% for precontrast imaging and 75% and 75% for contrast-enhanced power Doppler. SHI had 75% sensitivity and 83% specificity. Specificity was higher for all US modes than for mammography (P<.04). There were no significant differences in specificity among US modes or in sensitivity (P>or=.50). Area under the ROC curve for the diagnosis of breast cancer was 0.64 for standard gray-scale US and power Doppler US, 0.67 for contrast-enhanced power Doppler US, 0.76 for mammography, and 0.78 for SHI (P>.20). Contrast enhancement was better with SHI than with power Doppler (100% vs 44% of lesions with good or excellent enhancement; P=.004).

CONCLUSION

SHI appears to improve the diagnosis of breast cancer relative to conventional US and mammography, albeit on the basis of results in a very limited number of subjects.

Fundamental study on subharmonic imaging by irradiation of amplitude-modulated ultrasound waves

The second harmonic and subharmonic components, the frequencies of which are twice and one half the fundamental frequency, are included in echoes from contrast agents. An imaging method, which employs a second harmonic (second harmonic imaging), is widely used in medical diagnoses. On the other hand, subharmonic is expected to provide a higher contrast between biological tissues and blood flow because echo signals are generated only from blood containing the contrast agents. However, the subharmonic component echo signal power from contrast agents is relatively low. This has resulted in little progress in the field of subharmonic imaging. In this study, a new imaging method is proposed using amplitude-modulated waves as transmitted waves combined with the pulse inversion method to enhance subharmonic echo signals. Two optimal frequencies are set, including the modulated waves, F(1) and F(2), so that the subharmonic frequency of F(1) and the second harmonic frequency of F(2) may result in the same value. This allows a more powerful signal at the frequency band because the second harmonic and subharmonic components are integrated. Furthermore, a B-mode ultrasound image of an agar phantom that imitated biological tissue and showed the effectiveness of our method was reconstructed. As a result, the echo power of the subharmonic component was enhanced by approximately 11.8 dB more than the conventional method and the signal to noise ratio showed an improvement of 7.6 dB.

The natural frequency of nonlinear oscillation of ultrasound contrast agents in microvessels

Ultrasound contrast agents (UCAs) are under intensive investigation for their applications in physiological and molecular imaging and drug delivery. Prediction of the natural frequency of the oscillation of UCAs in microvessels has drawn increasing attention. To our knowledge, the existing models to predict the natural frequency of oscillation of UCAs in microvessels all apply the linear approximation and treat the blood vessel wall as a rigid boundary. In the potential applications of ultrasound imaging drug and gene delivery, the compliance of small vessels may play an important role in the bubble's oscillation. The goal of this work is to provide a lumped-parameter model to study the natural frequency of nonlinear oscillation of UCAs in microvessels. Three types of the blood vessel conditions have been considered: i.e., rigid vessels, normal compliable vessels and vessels with increasing stiffness that could correspond to tumor vasculature. The corresponding bubble oscillation frequencies in vessels with a radius less than 100 microm are examined in detail. When a bubble with a radius of 4 microm is confined in a compliable vessel (inner radius 5 microm and length 100 microm), the natural frequency of bubble oscillation increases by a factor of 1.7 compared with a bubble in an unbounded field. The natural frequency of oscillation of a bubble in a compliable vessel increases with decreasing vessel size while decreasing with increasing values of vessel rigidity. This model suggests that contrast agent size, blood vessel size distribution and the type of vasculature should comprehensively be considered for choosing the transmitted frequency in ultrasound contrast imaging and drug delivery.

Methodology for quantifying interactions between perfusion evaluated by DCE-US and hypoxia throughout tumor growth

The objective was to validate a combination of two new technologies to depict tumor physiology both temporally and spatially with dynamic contrast-enhanced sonography and an oximeter. Human cancer prostate tumors xenografted onto mice were followed for three weeks using dynamic contrast-enhanced ultrasonography (DCE-US) to detect tumor perfusion. Time intensity curves in linear data were quantified on four regions-of-interest (ROI, main tumor section and its anterior, central and posterior intra-tumoral areas) to extract three indices of perfusion. An oxygen sensor was guided by sonography to obtain accurate pO(2) measurements in the three predefined areas of tumors during their development. No impact on tumor growth of subsequent pO(2) probe insertion was detected. Among the four ROIs studied, the local central tumor showed significant perfusion and oxygenation variations throughout the experiment. A correlation was observed between local central tumor perfusion and pO(2), both of them decreasing through time (p = 0.0068; r = 0.66). The methodology which we developed demonstrated the potential of combining DCE-US with direct tissue pO(2) measurements, improving the description of complex intratumoral dynamic behavior.

Contrast imaging with chirped excitation

Coded excitation has been successfully used in imaging to increase the signal-to-noise ratio (SNR) and penetration depth. With a contrast agent, wideband signals have been hypothesized to increase the contrast-to-tissue ratio (CTR). However, nonlinear properties of contrast agents make decoding difficult when applying coded excitation to contrast imaging. We propose two chirped excitation methods to image contrast agents, with a mechanical index (MI) ranging from 0.05 to 0.34. In the single chirp method, one chirp is transmitted, followed by a clutter filter to reject tissue echoes, then a matched filter is used to recover range resolution. In the chirp sequence method, an increasing and decreasing chirp sequence is transmitted followed by subtraction of the compressed echoes to reject tissue echoes (assuming tissue is a linear scatterer at low MI). Ten independent acoustic experiments were performed to evaluate the CTR for chirp and tone burst insonation, with the same spatial peak temporal averaged intensity (I(SPTA)). A significant increase in CTR, ranging from 4 dB to 8 dB, is observed for chirped excitation as compared with tone burst insonation, at an I(SPTA) of 0.1 and 0.3 mW/cm2 (P < or = 5e-3). To achieve the same CTR of 15 dB, the spatial peak pulse averaged intensity (I(SPPA)) can be decreased by 6 dB for chirp insonation as compared with tone burst insonation (P < 1e-5). Additionally, an increase of more than 10 dB in tissue rejection ratio (TRR) is observed for a chirp sequence insonation compared to tone burst phase inversion for this set of parameters (P < or = 1e-9). Deconvolution of the linear microbubble response from the received echoes is proposed as a method to recover spatial resolution. The difference in the axial resolution resulting from chirp and three-cycle tone burst insonation is approximately 220 microm. The difference in the mainlobe width between experimental and predicted compressed echoes is less than 20%. The side-lobe amplitude is 9 dB to 16 dB below the mainlobe with a transmitted I(SPTA) from 0.1 to 6.6 mW/cm2.

Theoretical predictions of harmonic generation from submicron ultrasound contrast agents for nonlinear biomedical ultrasound imaging

Submicron ultrasound contrast agents have aroused attention for their significant promise in ultrasonic contrast/molecular imaging, targeted therapy and echo particle imaging velocimetry. However, nonlinear acoustic properties of submicron encapsulated gas bubbles for ultrasonic applications are still not clearly understood. In this paper, nonlinear acoustic emission characteristics from submicron bubbles were examined using a numerical study. The modified RP equation incorporating viscosity, acoustic radiation, thermal effects and encapsulated shell was used to study single bubble dynamics. Further, a size integration method, shown previously to be useful in prediction of backscatter spectra from groups of bubbles, was applied to analyse response from a bubble population. We show that bubbles with radii (200-500 nm) produce significant subharmonic and ultraharmonic components of the backscatter spectrum, while smaller bubbles (<200 nm) provide substantial second harmonic components. Additionally, nanoscale bubbles (<100 nm) produce very low backscatter amplitudes and thus may not be useful with the use of current ultrasound technology. Analysing optimal ultrasound driving pressures and bubbles size ranges for maximal subharmonic and ultraharmonic signals showed that sub and ultraharmonic mode nonlinear imaging methods may be potentially competitive for larger size bubbles (>200 nm) in providing proper contrast-to-tissue signal ratios.

In vivo perfusion estimation using subharmonic contrast microbubble signals

OBJECTIVE

The purpose of this study was to quantify perfusion in vivo using contrast-enhanced subharmonic imaging (SHI).

METHODS

A modified LOGIQ 9 scanner (GE Healthcare, Milwaukee, WI) operating in gray scale SHI mode was used to measure SHI time-intensity curves in vivo. Four dogs received intravenous contrast bolus injections (dose, 0.1 mL/kg), and renal SHI was performed. After 3 contrast agent injections, a microvascular staining technique based on stable (nonradioactive) isotope-labeled microspheres (BioPhysics Assay Laboratory Inc, Worcester, MA) was used to quantify the degree of perfusion in 8 sections of each kidney. Low perfusion states were induced by ligating surgically exposed segmental renal arteries followed by contrast agent injections and microvascular staining. Digital clips were transferred to a personal computer, and SHI time-intensity curves were acquired in each section using Image-Pro Plus software (Media Cybernetics, Silver Spring, MD). Subharmonic fractional blood volumes were calculated, and the perfusion was estimated from the initial slope of the fractional blood volume uptake averaged over 3 injections. Subharmonic perfusion data were compared with the gold standard (ie, the microspheres) using linear regression analysis.

RESULTS

In vivo gray scale SHI clearly showed flow and, thus, perfusion in the kidneys with almost complete suppression of tissue signals. In total, 270 SHI time-intensity curves were acquired, which reduced to 94 perfusion estimates after averaging. Subharmonic perfusion estimates correlated significantly with microsphere results (r = 0.57; P < .0001). The best SHI perfusion estimates occurred for high perfusion states in the anterior of the kidneys (r = 0.73; P = .0001). The corresponding root mean square error was 2.4%.

CONCLUSIONS

Subharmonic perfusion estimates have been obtained in vivo. The perfusion estimates were in reasonable to good agreement with a microvascular staining technique.

High-frequency dynamics of ultrasound contrast agents

Ultrasound contrast agents enhance echoes from the microvasculature and enable the visualization of flow in smaller vessels. Here, we optically and acoustically investigate microbubble oscillation and echoes following insonation with a 10 MHz center frequency pulse. A high-speed camera system with a temporal resolution of 10 ns, which provides two-dimensional (2-D) frame images and streak images, is used in optical experiments. Two confocally aligned transducers, transmitting at 10 MHz and receiving at 5 MHz, are used in acoustical experiments in order to detect subharmonic components. Results of a numerical evaluation of the modified Rayleigh-Plesset equation are used to predict the dynamics of a microbubble and are compared to results of in vitro experiments. From the optical observations of a single microbubble, nonlinear oscillation, destruction, and radiation force are observed. The maximum bubble expansion, resulting from insonation with a 20-cycle, 10-MHz linear chirp with a peak negative pressure of 3.5 MPa, has been evaluated. For an initial diameter ranging from 1.5 to 5 microm, a maximum diameter less than 8 microm is produced during insonation. Optical and acoustical experiments provide insight into the mechanisms of destruction, including fragmentation and active diffusion. High-frequency pulse transmission may provide the opportunity to detect contrast echoes resulting from a single pulse, may be robust in the presence of tissue motion, and may provide the opportunity to incorporate high-frequency ultrasound into destruction-replenishment techniques.

Time and pressure dependence of acoustic signals radiated from microbubbles

Encapsulated microbubbles are considered to be microsensors for in vivo blood pressure measurements in the cardiovascular system. To study the potential of this method, we developed a simulation and an experimental set-up that relate various characteristics of radiated acoustic signals from the microbubbles to the varying ambient pressure. Both the simulation and the experiment show that the radiated pressure from microbubbles generates a significant subharmonic component, which is modulated by changes in the ambient pressure. A time-dependent decrease of the steady-state radii within a population of microbubbles causes a phase reversal phenomenon, which explains the observed time delay in the build-up of the subharmonic modulation response. Additionally, we identify a frequency-capturing effect that indicates the termination of the nonlinear behavior of the microbubbles. Our research suggests that these subharmonic signals can be used for in vivo blood pressure measurements and highlights some of the considerations that need to be addressed in developing such techniques.

In vivo pressure estimation using subharmonic contrast microbubble signals: proof of concept

Changes in ambient pressure affects the reflectivity of ultrasound contrast microbubbles leading to an excellent correlation between subharmonic signals and hydrostatic pressure. The aortas of two dogs were scanned with an experimental pulse-echo system to validate in vivo pressure estimation based on subharmonic microbubble signals. Results matched well with instantaneous pressure measurements (from 20-60 mmHg) obtained simultaneously with a pressure catheter (root mean square errors <27%).

Multi-frequency harmonic arrays: initial experience with a novel transducer concept for nonlinear contrast imaging

Nonlinear contrast imaging modes such as second harmonic imaging (HI) and subharmonic imaging (SHI) are increasingly important for clinical applications. However, the performance of currently available transducers for HI and SHI is significantly constrained by their limited bandwidth. To bypass this constraint, a novel transducer concept termed multi-frequency harmonic transducer arrays (MFHA's) has been designed and a preliminary evaluation has been conducted. The MFHA may ultimately be used for broadband contrast enhanced HI and SHI with high dynamic range and consists of three multi-element piezo-composite sub-arrays (A-C) constructed so the center frequencies are 4f(A) = 2f(B) = f(C) (specifically 2.5/5.0/10.0 MHz and 1.75/3.5/7.0 MHz). In principle this enables SHI by transmitting on sub-array C receiving on B and, similarly, from B to A as well as HI by transmitting on A receiving on B and, likewise, from B to C. Initially transmit and receive pressure levels of the arrays were measured with the elements of each sub-array wired in parallel. Following contrast administration, preliminary in vitro HI and SHI signal-to-noise ratios of up to 40 dB were obtained. In conclusion, initial design and in vitro characterization of two MFHA's have been performed. They have an overall broad frequency bandwidth of at least two octaves. Due to the special design of the array assembly, the SNR for HI and SHI was comparable to that of regular B-mode and better than commercially available HI systems. However, further research on multi-element MFHA's is required before their potential for in vivo nonlinear contrast imaging can be assessed.