Improved flow measurement using microbubble contrast agents and disruption-replenishment: clinical application to tumour monitoring

Imaging Methods for Ultrasound Contrast Agents

Microbubble contrast agents were introduced more than 25 years ago with the objective of enhancing blood echoes and enabling diagnostic ultrasound to image the microcirculation. Cardiology and oncology waited anxiously for the fulfillment of that objective with one clinical application each: myocardial perfusion, tumor perfusion and angiogenesis imaging. What was necessary though at first was the scientific understanding of microbubble behavior in vivo and the development of imaging technology to deliver the original objective. And indeed, for more than 25 years bubble science and imaging technology have evolved methodically to deliver contrast-enhanced ultrasound. Realization of the basic bubbles properties, non-linear response and ultrasound-induced destruction, has led to a plethora of methods; algorithms and techniques for contrast-enhanced ultrasound (CEUS) and imaging modes such as harmonic imaging, harmonic power Doppler, pulse inversion, amplitude modulation, maximum intensity projection and many others were invented, developed and validated. Today, CEUS is used everywhere in the world with clinical indications both in cardiology and in radiology, and it continues to mature and evolve and has become a basic clinical tool that transforms diagnostic ultrasound into a functional imaging modality. In this review article, we present and explain in detail bubble imaging methods and associated artifacts, perfusion quantification approaches, and implementation considerations and regulatory aspects.

Effects of ultrasound pulse parameters on cavitation properties of flowing microbubbles under physiologically relevant conditions

Acoustic cavitation from ultrasound-driven microbubbles can induce diverse bioeffects that are useful in clinical therapy. However, lack of control over the cavitation activity of flowing microbubbles results in unwanted treatment regions in the targeted tissue, which influences the therapeutic efficacy and bio-safety. The aim of this study is to understand the relationship between the ultrasound pulse parameters and cavitation properties of flowing microbubbles, including the type (and transition between types), threshold, intensity and temporal distribution of cavitation. An in vitro physiological-flow phantom was fabricated, in which the microbubbles had a constant velocity, and were sonicated to a 1-MHz focused transducer at a wide range of peak negative pressures (PNPs) (0.10-1.28 MPa), pulse repetition frequencies (PRFs) (1-200 Hz) and pulse lengths (PLs) (10-400 μs). The signals from the flowing bubbles were passively detected by another 7.5-MHz plane transducer. From detailed time- and frequency-domain analysis, we found 1). The occurrence of stable cavitation (SC) and inertial cavitation (IC) depended on PNP and PL when the PRF was below a critical value (PRF threshold) that related to the fluid velocity and PNP full width at half maximum diameter of the transducer. 2) Below the PRF threshold, the PL had no influence on the temporal distribution of SC intensity; however, above the PRF threshold, the SC properties depended on the PL because of acoustically-driven diffusion. Specifically, at shorter PLs, the SC intensity had a uniform temporal distribution and was independent of the PRF; at longer PLs, the SC intensity correlated negatively with the PRF. 3) Below the PRF threshold, the IC properties were independent of the PRF. Increasing the PRF above the PRF threshold caused the IC intensity to decrease with a non-uniform temporal distribution. These results indicate that the fluid velocity and a pulsed acoustic field influence the number and properties of the replenished bubbles into the targeted region, resulting in the change of cavitation properties. In future therapeutic applications, the physiological fluid conditions must be taken into consideration to design reasonable pulse parameters and achieve desirable cavitation properties.

Compound speckle model detects anti-angiogenic tumor response in preclinical nonlinear contrast-enhanced ultrasonography

PURPOSE

This paper proposes a method for analyzing the first-order speckle statistics of nonlinear contrast-enhanced ultrasound images from tumors.

METHODS

Contrast signal intensity is modeled as a compound distribution of exponential probability density functions with a gamma weighting function. The gamma probability weighting function serves as an approximation for log-normally distributed flow velocities in a vascular network. The model was applied to sub-harmonic bolus-injection images acquired from a mouse breast cancer xenograft model treated with murine version bevacizumab.

RESULTS

The area under curve produced using the compound statistical model could more accurately discriminate anti-VEGF-treated tumors from untreated tumors than conventional contrast-enhanced ultrasound image processing. This result was validated with gold standard histological measures of microvascular density. Fractal vessel geometry was estimated using the gamma weighting function and tested against micro-CT perfusion casting. Treated tumors had a significantly lower vascular fractal dimension than control tumors. Vascular complexity estimated using the ultrasound compound statistical model performed similarly to micro-CT fractal dimension for discriminating treated from control tumors.

CONCLUSION

The proposed technique can quantify tumor perfusion and provide an index of vascular complexity, making it a potentially useful addition for clinical detection of vascular normalization in anti-angiogenic trials.

Ultrasound-based imaging methods of the kidney-recent developments

In recent years, several novel ultrasound (US)-based techniques have emerged for kidney diagnostic imaging, including tissue stiffness assessment with elastography, Ultrasensitive Doppler techniques, and contrast-enhanced ultrasonography to assess renal microvascularization. Renal elastography has become available with the development of noninvasive quantitative techniques, following the rapidly growing field of liver fibrosis diagnosis. With the increased incidence of chronic kidney disease, noninvasive diagnosis of renal fibrosis can be of critical value. However, it is difficult to simply extend the application of US elastography from one organ to the other due to anatomic and technical issues. Today, renal elastography appears to be a promising application that, however, still requires optimization and validation. New ultrasensitive Doppler techniques improve the detection of slow blood flow and can be used alone or after administration of US contrast agents. These microbubble-based agents are extremely well tolerated and can be administered even in cases of impaired renal function. Despite the lack of approval, they improve the characterization of atypical renal masses, complex cystic renal masses, and peripheral vascular disorders. Dynamic contrast-enhanced US is based on quantification of the signal intensity from region of interest and mathematical fits of the time-intensity curves. Perfusion-related parameters can be extracted for the monitoring of vascular changes in the renal parenchyma and in tumors in order to evaluate drug response. This estimation of renal perfusion depends on many parameters that should be kept constant for follow-up studies, and, when possible, an internal reference should be used to normalize the measurements.

Mathematical Models of Contrast Transport Kinetics for Cancer Diagnostic Imaging: A Review

Angiogenesis plays a fundamental role in cancer growth and the formation of metastasis. Novel cancer therapies aimed at inhibiting angiogenic processes and/or disrupting angiogenic tumor vasculature are currently being developed and clinically tested. The need for earlier and improved cancer diagnosis, and for early evaluation and monitoring of therapeutic response to angiogenic treatment, have led to the development of several imaging methods for in vivo noninvasive assessment of angiogenesis. The combination of dynamic contrast-enhanced imaging with mathematical modeling of the contrast agent kinetics enables quantitative assessment of the structural and functional changes in the microvasculature that are associated with tumor angiogenesis. In this paper, we review quantitative imaging of angiogenesis with dynamic contrast-enhanced magnetic resonance imaging, computed tomography, and ultrasound.

Detection of Early Tumor Response to Axitinib in Advanced Hepatocellular Carcinoma by Dynamic Contrast Enhanced Ultrasound

This study aimed to evaluate the utility of dynamic contrast-enhanced ultrasound (DCE-US) in measuring early tumor response of advanced hepatocellular carcinoma to axitinib. Twenty patients were enrolled (aged 18-78 y; median 65). DCE-US was performed with bolus injection and infusion/disruption replenishment. Median overall survival was 7.1 mo (1.8-27.3) and progression free survival was 3.6 mo (1.8-17.4). Fifteen patients completed infusion scans and 12 completed bolus scans at 2 wk. Among the perfusion parameters, fractional blood volume at infusion (INFBV) decreased at 2 wk in 10/15 (16%-81% of baseline, mean 47%) and increased in 5/15 (116%-535%, mean 220%). This was not significantly associated with progression free survival (p = 0.310) or progression at 16 wk (p = 0.849), but was borderline statistically significant (p = 0.050) with overall survival, limited by a small sample size. DCE-US is potentially useful in measuring early tumor response of advanced hepatocellular carcinoma to axitinib, but a larger trial is needed.

Impact of Acquisition Method and Region of Interest Placement on Inter-observer Agreement and Measurement of Tumor Response to Targeted Therapy Using Dynamic Contrast-Enhanced Ultrasound

This study evaluated the impact of different acquisition methods, user-directed region of interest placement and post-processing steps on the quantification of dynamic contrast-enhanced ultrasound measurements of blood volume in 29 patients with renal cancer, pre- and post-treatment. Specifically, we compared tumor quantification using multiple planes versus a single plane, breathhold versus free breathing and large region of interest versus a region targeting the area of highest vascularity. Performance was evaluated using area under the receiver operating characteristic curves to identify the method that best predicts progression-free survival. The intra-class correlation coefficient was also used to investigate how the same parameters affect inter-observer agreement. Of the different methods used to quantify blood volume in this study, the combination that had the highest level of inter-observer agreement (intra-class correlation coefficient = 0.8-0.97) and was the best predictor of progression-free survival was the change in blood volume measured (area under receiver operating characteristic curve = 0.77, p = 0.04) by a multiplane average, acquired during quiet breathing, quantified using a region of interest that encompassed the entire tumor.

Assessment of Hepatic Vascular Network Connectivity with Automated Graph Analysis of Dynamic Contrast-enhanced US to Evaluate Portal Hypertension in Patients with Cirrhosis: A Pilot Study

PURPOSE

To test whether graph analysis of vascular images obtained with hepatic dynamic contrast material-enhanced (DCE) ultrasonography (US) allows calculation of the degree of organization of the liver circulation and whether graph properties are correlated to the severity of portal hypertension.

MATERIALS AND METHODS

Institutional review board approval and written informed consent were obtained. Fifteen patients with liver cirrhosis (nine men; mean age ± standard deviation, 55 years ± 8) who underwent DCE US and hepatic venous pressure gradient (HVPG) measurement and four healthy subjects (two men and two women; mean age, 34 years ± 4) were included between January 2007 and December 2008. Individual graph models ("vascular connectomes") were computed on the basis of time series analysis of video sequences of DCE US examinations (conducted with the disruption-reperfusion technique). Graph analysis was performed, and the clustering coefficient C was calculated. Correlations between clustering coefficient and HVPG were assessed.

RESULTS

Healthy subjects had a high clustering coefficient of vascular connectome (C = 0.4447; interquartile range [IQR], 0.3864-0.4679), suggesting a highly organized hepatic vascular network. Conversely, patients with cirrhosis showed a low clustering coefficient, indicating disruption of normal anatomy (C = 0.0288; IQR, 0.0157-0.0861; P = .001 vs healthy subjects). The clustering coefficient decreased as HVPG increased, with a clustering coefficient of 0.0237 (IQR, 0.0066-0.0378) in patients with HVPG of at least 10 mm Hg versus 0.1180 (IQR, 0.0987-0.1414) in those with HVPG of less than 10 mm Hg (P = .006). The correlation between the best model derived from the distribution of the clustering coefficient (10 bins) of vascular connectome and HVPG had a Pearson correlation of 0.977 (root mean squared error, 1.57 mm Hg; P < .0001).

CONCLUSION

This pilot study demonstrates that graph modeling of vascular connectivity based on video processing of liver DCE US examinations and subsequent graph analysis enable calculation of personalized parameters that reflect the degree of organization of the hepatic microvascular network and are correlated to the severity of portal hypertension in cirrhosis.

Reducing tumor growth and angiogenesis using a triple therapy measured with Contrast-enhanced ultrasound (CEUS)

Background

To evaluate the in vivo response by detecting the anti-angiogenic and invasion-inhibiting effects of a triple-combination-therapy in an experimental-small-animal-squamous-cell-carcinoma-model using the “flash-replenishment” (FR) method to assess tissue hemodynamics via contrast-enhanced-ultrasound (CEUS).

Methods

Human hypopharynx-carcinoma-cells were subcutaneously injected into the left flank of 22-female-athymic-nude-rats. After seven days of subcutaneous tumor growth, FR-measurements were performed on each rat. Treatment-group and control-group were treated every day for a period of one week, with the treatment-group receiving solvents containing a triple therapy of Upamostat®, Celecoxib® and Ilomastat® and the control-group solvents only. On day seven, follow-up measurements were performed using the same measurement protocol to assess the effects of the triple therapy. VueBox® was used to quantify the kinetic parameters and additional immunohistochemistry analyses were performed for comparison with and validation of the CEUS results against established methods (Proliferation/Ki-67, vascularization/CD31, apoptosis/caspase3).

Results

Compared to the control-group, the treatment-group that received the triple-therapy resulted in a reduction of tumor growth by 48.6% in size. Likewise, the immunohistochemistry results showed significant decreases in tumor proliferation and vascularization in the treatment-group in comparison to the control-group of 26%(p≤0.05) and 32.2%(p≤0.05) respectively. Correspondingly, between the baseline and follow-up measurements, the therapy-group was associated with a significant(p ≤ 0.01) decrease in the relative-Blood-Volume(rBV) in both the whole tumor(wt) and hypervascular tumor(ht) areas (p≤0.01), while the control-group was associated with a significant (p≤0.01) increase of the rBV in the wt area and a non-significant increase (p≤0.16) in the ht area. The mean-transit-time (mTT) of the wt and the ht areas showed a significant increase (p≤0.01) in the follow-up measurements in the therapy group.

Conclusion

The triple-therapy is feasible and effective in reducing both tumor growth and vascularization. In particular, compared with the placebo-group, the triple-therapy-group resulted in a reduction in tumor growth of 48.6% in size when assessed by CEUS and a significant reduction in the number of vessels in the tumor of 32% as assessed by immunohistochemistry. As the immunohistochemistry supports the CEUS findings, CEUS using the “flash replenishment”(FR) method appears to provide a useful assessment of the anti-angiogenic and invasion-inhibiting effects of a triple combination therapy.

Dynamic contrast-enhanced ultrasound for quantification of tissue perfusion

Dynamic contrast-enhanced ultrasound (US) imaging, a technique that uses microbubble contrast agents with diagnostic US, has recently been technically summarized and reviewed by a European Federation of Societies for Ultrasound in Medicine and Biology position paper. However, the practical applications of this imaging technique were not included. This article reviews and discusses the published literature on the clinical use of dynamic contrast-enhanced US. This review finds that dynamic contrast-enhanced US imaging is the most sensitive cross-sectional real-time method for measuring the perfusion of parenchymatous organs noninvasively. It can measure parenchymal perfusion and therefore can differentiate between benign and malignant tumors. The most important routine clinical role of dynamic contrast-enhanced US is the prediction of tumor responses to chemotherapy within a very short time, shorter than using Response Evaluation Criteria in Solid Tumors criteria. Other applications found include quantifying the hepatic transit time, diabetic kidneys, transplant grafts, and Crohn disease. In addition, the problems involved in using dynamic contrast-enhanced US are discussed.

Quantifying vascular heterogeneity using microbubble disruption-replenishment kinetics in patients with renal cell cancer

PURPOSE

The purposes of this study were to establish the physiological interpretation of the shape parameter of the dynamic contrast-enhanced ultrasound (DCE-US) lognormal perfusion model and to evaluate the clinical significance of the parameter in a sample of patients undergoing antiangiogenic therapy for metastatic renal cell carcinoma (mRCC).

MATERIALS AND METHODS

The physiological interpretation of the lognormal shape parameter was explored using computer simulations of disruption-replenishment in fractal models of the microcirculation generated by a piecewise iterative algorithm in MATLAB. Architectural variety was accomplished by introducing random perturbations to the diameter, length, and branching angles to the growing vascular tree. The shape parameter was extracted from the time-intensity curves and compared with the transit time distributions calculated directly from the simulations. Dynamic contrast-enhanced ultrasound data were obtained from 31 consenting patients with mRCC being treated with antiangiogenic therapy. Lognormal parameters related to the blood volume, mean flow speed, and vascular morphology/heterogeneity extracted before, during, and after therapy were correlated with progression-free survival (PFS). Cox proportional hazard ratios were calculated alongside receiver operator characteristics for different combinations of the vascular parameters to determine their ability to distinguish patients who would progress early (less than the median PFS) versus late (greater than the median PFS).

RESULTS

The lognormal shape parameter correlated strongly to the width of the transit time distribution calculated directly from the simulations, and by extension, to the morphology/heterogeneity of the microvascular network (Spearman r = 0.80, P < 0.001, n = 28). Shorter time to progression was predicted by higher baseline heterogeneity (P = 0.003) and a reduction in tumor blood volume less than 43% (P = 0.002) after 2 weeks of treatment. Combining baseline parameters with changes that occur shortly after starting treatment increased the sensitivity and specificity of DCE-US to identify which patients would progress/resist therapy early versus late compared with when the vascular parameters were considered in isolation.

CONCLUSIONS

The DCE-US shape parameter from the lognormal perfusion model is representative of microvascular morphology/heterogeneity and may be used to noninvasively characterize the vascular architecture of cancer lesions. A more abnormal flow distribution at baseline predicts for poorer outcome for patients treated with antiangiogenic therapy for metastatic renal cell cancer. Combining pretreatment and on-treatment measurements of vascularity can improve the performance of DCE-US to predict which patients will progress earlier versus later when on antiangiogenic therapy for mRCC.

On the acoustic properties of vaporized submicron perfluorocarbon droplets

The acoustic characteristics of microbubbles created from vaporized submicron perfluorocarbon droplets with fluorosurfactant coating are examined. Utilizing ultra-high-speed optical imaging, the acoustic response of individual microbubbles to low-intensity diagnostic ultrasound was observed on clinically relevant time scales of hundreds of milliseconds after vaporization. It was found that the vaporized droplets oscillate non-linearly and exhibit a resonant bubble size shift and increased damping relative to uncoated gas bubbles due to the presence of coating material. Unlike the commercially available lipid-coated ultrasound contrast agents, which may exhibit compression-only behavior, vaporized droplets may exhibit expansion-dominated oscillations. It was further observed that the non-linearity of the acoustic response of the bubbles was comparable to that of SonoVue microbubbles. These results suggest that vaporized submicron perfluorocarbon droplets possess the acoustic characteristics necessary for their potential use as ultrasound contrast agents in clinical practice.

Echo-power estimation from log-compressed video data in dynamic contrast-enhanced ultrasound imaging

Ultrasound (US) scanners typically apply lossy, non-linear modifications to the US data for visualization purposes. The resulting images are then stored as compressed video data. Some system manufacturers provide dedicated software for quantification purposes to eliminate such processing distortions, at least partially. This is currently the recommended approach for quantitatively assessing changes in contrast-agent concentration from clinical data. However, the machine-specific access to US data and the limited set of analysis functionalities offered by each dedicated-software package make it difficult to perform comparable analyses with different US systems. The objective of this work was to establish if linearization of compressed video images obtained with an arbitrary US system can provide an alternative to dedicated-software analysis of machine-specific files for the estimation of echo-power. For this purpose, an Aplio 50 system (Toshiba Medical Systems, Tochigi, Japan), coupled with dedicated CHI-Q (Contrast Harmonic Imaging Quantification) software by Toshiba Medical Systems, was used. Results were compared with two approaches that apply algorithms to estimate relative echo-power from compressed video images: commercially available VueBox software by Bracco Suisse SA (Geneva, Switzerland) and in-laboratory software called PixPower. The echo-power estimated by CHI-Q analysis indicated a strong linear relationship versus agent concentration in vitro (R(2) ≥ 0.9996) for dynamic range (DR) settings of DR60 and DR80, with slopes between 9.22 and 9.57 dB/decade (p = 0.05). These values approach the theoretically predicted dependence of 10.0 dB/decade (equivalent to 3 dB for each concentration doubling). Echo-power estimations obtained from compressed video images with VueBox and PixPower also exhibited strong linear proportionality with concentration (R(2) ≥ 0.9996), with slopes between 9.30 and 9.68 dB/decade (p = 0.05). On an independent in vivo data set (N = 24), the difference in echo-power estimation between CHI-Q and each of the other two approaches was calculated after excluding regions that contain pixels affected by saturated or thresholded pixel values. The mean difference in estimates (expressed in decibels) was -0.25 dB between VueBox and CHI-Q (95% confidence interval: -0.75 to 0.26 dB) and -0.17 dB between PixPower and CHI-Q (95% confidence interval: -0.67 to 0.13 dB). To achieve linearization of data, one of the approaches (VueBox) requires calibration files provided by the software manufacturer for each machine type and setting. The other (PixPower) requires empirical correction of the imaging dynamic range based on ground truth data. These requirements could potentially be removed if US system manufacturers were willing to make relevant information on the applied processing publically available. Reliable echo-power estimation from linearized data would facilitate inclusion of different US systems in multicentric studies and more widespread implementation of emerging techniques for quantitative analysis of contrast ultrasound.

The efficiency and stability of bubble formation by acoustic vaporization of submicron perfluorocarbon droplets

Submicron droplets of liquid perfluorocarbon converted into microbubbles with applied ultrasound have been studied, for a number of years, as potential next generation extravascular ultrasound contrast agents. In this work, we conduct an initial ultra-high-speed optical imaging study to examine the vaporization of submicron droplets and observe the newly created microbubbles in the first microseconds after vaporization. It was estimated that single pulses of ultrasound at 10 MHz with pressures within the diagnostic range are able to vaporize on the order of at least 10% of the exposed droplets. However, only part of the newly created microbubbles survives immediately following vaporization - the bubbles may recondense back into the liquid droplet state within microseconds of nucleation. The probability of bubble survival within the first microseconds of vaporization was shown to depend on ultrasound excitation pressure as well as on bubble coalescence during vaporization, a behavior influenced by the presence of coating material on the newly created bubbles. The results of this study show for the first time that although initial vaporization of droplets is necessary to create echogenic bubbles, additional factors, such as coalescence and bubble shell properties, are important and should be carefully considered for the production of microbubbles for use in medical imaging.

Ultrasound contrast plane wave imaging

BACKGROUND

Monitoring the accumulation of microbubbles within tissue vasculature with ultrasound allows both molecular and perfusion imaging. Unfortunately, conventional imaging with focused pulses can destroy a large fraction of the microbubbles it is trying to follow. Using coherent synthetic summation, ultrafast plane wave imaging could attain similar image quality, while reducing the peak acoustic pressure and bubble disruption.

METHOD

In these experiments, microbubbles were flowed in a wall-less vessel phantom. Images were obtained on a programmable clinical scanner with a set of line-per-line focused pulses for conventional contrast imaging and with compounded plane wave transmission adapted for nonlinear imaging. Imaging was performed between 14 and 650 kPa peak negative pressure at 7.5 MHz. The disruption of the microbubbles was evaluated by comparing the microbubble intensity before and after acquisition of a set of 100 images at various pressures.

RESULTS

The acoustic intensity required to disrupt 50% of the microbubbles was 24 times higher with plane-wave imaging compared with conventional focused pulses. Although both imaging approaches yield similar resolution, at the same disruption level, plane-wave imaging showed better contrast. In particular, at similar disruption ratio (50% after 100 images), contrast-pulse sequencing (CPS) performed with plane waves displayed an improvement of 11 dB compared with conventional nonlinear imaging.

CONCLUSION

In each resolution cell of the image, plane-wave imaging spread the spatial peak acoustic intensity over more pulses, reducing the peak pressure and, hence, preserving the microbubbles. This method could contribute to molecular imaging by allowing the continuous monitoring of the accumulation of microbubbles with improved contrast.

Spatiotemporal evolution of cavitation dynamics exhibited by flowing microbubbles during ultrasound exposure

Ultrasound and microbubble-based therapies utilize cavitation to generate bioeffects, yet cavitation dynamics during individual pulses and across consecutive pulses remain poorly understood under physiologically relevant flow conditions. SonoVue(®) microbubbles were made to flow (fluid velocity: 10-40 mm/s) through a vessel in a tissue-mimicking material and were exposed to ultrasound [frequency: 0.5 MHz, peak-rarefactional pressure (PRP): 150-1200 kPa, pulse length: 1-100,000 cycles, pulse repetition frequency (PRF): 1-50 Hz, number of pulses: 10-250]. Radiated emissions were captured on a linear array, and passive acoustic mapping was used to spatiotemporally resolve cavitation events. At low PRPs, stable cavitation was maintained throughout several pulses, thus generating a steady rise in energy with low upstream spatial bias within the focal volume. At high PRPs, inertial cavitation was concentrated in the first 6.3 ± 1.3 ms of a pulse, followed by an energy reduction and high upstream bias. Multiple pulses at PRFs below a flow-dependent critical rate (PRF(crit)) produced predictable and consistent cavitation dynamics. Above the PRF(crit), energy generated was unpredictable and spatially biased. In conclusion, key parameters in microbubble-seeded flow conditions were matched with specific types, magnitudes, distributions, and durations of cavitation; this may help in understanding empirically observed in vivo phenomena and guide future pulse sequence designs.

Optical studies of vaporization and stability of fluorescently labelled perfluorocarbon droplets

Droplets of liquid perfluorocarbon (PFC) are under study as the next generation of contrast agents for ultrasound (US). These droplets can be selectively vaporized into echogenic gas bubbles in situ by externally applied US, with numerous applications to diagnosis and therapy. However, little is known about the mechanisms of droplet vaporization and the stability of the bubbles so produced. Here we observe optically the vaporization of fluorescent PFC droplets and the stability of the newly created bubbles. Fluorescent markers were used to label selectively either the liquid PFC core or the shell of the droplets. It was found that, following vaporization, the fluorescent marker is quickly expelled from the core of the newly created bubble and is retained on the gas-liquid interface. At the same time, it was shown that bubbles retain the original shells encapsulating their droplet precursors. The efficiency of encapsulation was found to depend strongly on the nature of the stabilizing material itself. These results provide direct evidence of droplet encapsulation post-vaporization, and suggest that the behaviour of the vaporized droplets is strongly dependent on the choice of the stabilizing material for the emulsion.

Heterogeneous staining in the liver parenchyma after the injection of perflubutane microbubble contrast agent

This study aimed to characterize the features of heterogeneous staining in the liver after injection of perflubutane microbubble agent (Sonazoid(TM), 0.0075 mL/kg). Digitized hepatic contrast sonograms from 906 subjects were reviewed to assess time-related changes in heterogeneous staining and the possible association between this effect and the clinical backgrounds was analyzed. Heterogeneous staining was found in seven subjects (0.77%) on 15-min phase sonograms. The staining initially appeared as hyper-enhanced circular spots in the liver 10 min or later after the agent injection. The number of spots increased gradually with unequally-spaced distribution. Although the staining pattern did not improve during the examination, there were no abnormal findings in vital signs or symptoms on the day and blood test results or sonograms on the following day. Heterogeneous staining is a side effect that impedes ultrasound examination. However, at present, the precise causes and underlying mechanisms of this event are unknown.

Contrast ultrasound imaging for identification of early responder tumor models to anti-angiogenic therapy

Agents targeting vascular endothelial growth factor (VEGF) have been validated as cancer therapeutics, yet efficacy can differ widely between tumor types and individual patients. In addition, such agents are costly and can have significant toxicities. Rapid noninvasive determination of response could provide significant benefits. We tested if response to the anti-VEGF antibody bevacizumab (BV) could be detected using contrast-enhanced ultrasound imaging (CEUS). We used two xenograft model systems with previously well-characterized responses to VEGF inhibition, a responder (SK-NEP-1) and a non-responder (NGP), and examined perfusion-related parameters. CEUS demonstrated that BV treatment arrested the increase in blood volume in the SK-NEP-1 tumor group only. Molecular imaging of α(V)β(3) with targeted microbubbles was a more sensitive prognostic indicator of BV efficacy. CEUS using RGD-labeled microbubbles showed a robust decrease in α(V)β(3) vasculature following BV treatment in SK-NEP-1 tumors. Paralleling these findings, lectin perfusion assays detected a disproportionate pruning of smaller, branch vessels. Therefore, we conclude that the response to BV can be identified soon after initiation of treatment, often within 3 days, by use of CEUS molecular imaging techniques. The use of a noninvasive ultrasound approach may allow for earlier and more effective determination of efficacy of antiangiogenic therapy.

Correlation and agreement between contrast-enhanced ultrasonography and perfusion computed tomography for assessment of liver metastases from endocrine tumors: normalization enhances correlation

We studied correlation and agreement between perfusion parameters derived from contrast-enhanced ultrasonography (CEUS) and computed tomography (CT). Both techniques were performed in 16 patients with proven liver metastases from endocrine tumor. Replenishment study after ultrasound-induced destruction of microbubbles was used for CEUS quantification. CEUS-derived relative values of blood flow, blood volume and mean transit time were compared with perfusion CT-derived parameters measured in the same tumors. Significant correlation was observed between CEUS normalized values and CT absolute tumor values for blood flow (r = 0.58; p = 0.018), blood volume (r = 0.61; p = 0.012) and mean transit time (r = 0.52; p = 0.037). Correlation was not significant for non-normalized values. Agreement between CEUS normalized values and perfusion CT relative values was significant (p < 0.04). Estimated bias between CEUS and CT for relative perfusion values was -1.38 (-5.02; 2.27) for blood flow, +0.26 (-0.79; 1.31) for blood volume and +0.21 (-0.46; 0.87) for mean transit time. We conclude that normalization markedly increased correlation between CEUS- and CT-derived perfusion values and allowed agreement assessment.

The lognormal perfusion model for disruption replenishment measurements of blood flow: in vivo validation

Dynamic contrast enhanced ultrasound (DCE-US) is evolving as a promising tool to noninvasively quantify relative tissue perfusion in organs and solid tumours. Quantification using the method of disruption replenishment is best performed using a model that accurately describes the replenishment of microbubble contrast agents through the ultrasound imaging plane. In this study, the lognormal perfusion model was validated using an exposed in vivo rabbit kidney model. Compared against an implanted transit time flow meter, longitudinal relative flow measurement was (×3) less variable and correlated better when quantification was performed with the lognormal perfusion model (Spearman r = 0.90, 95% confidence interval [CI] = 0.05) vs. the prevailing mono-exponential model (Spearman r = 0.54, 95% CI = 0.18). Disruption-replenishment measurements using the lognormal perfusion model were reproducible in vivo to within 12%.