Microbubble contrast agents: a new era in ultrasound

Quantum-dot-modified microbubbles with bi-mode imaging capabilities

The aim of this paper was to develop a novel bi-mode ultrasound/fluorescent imaging agent through stepwise layer-by-layer deposition of poly(allylamine hydrochloride) (PAH) and CdTe quantum dots (QDs) onto ST68 microbubbles (MBs) produced by sonication of a mixture of surfactants (Span 60 and Tween 80). The experiments using photoluminescence spectroscopy and confocal laser scanning microscopy confirmed that CdTe nanoparticles were successfully adsorbed on the outer surface of the MBs. The static light scattering measurements showed that size distributions of MBs before and after QD deposition met the size requirements for clinical application. The in vitro and in vivo ultrasonography indicated that the QD-modified MBs maintained good contrast enhancement properties as the original MBs. Furthermore, the in vitro ultrasound-targeted microbubble destruction (UTMD) experiment of the QD-MB composites was carried out to validate the ability of MBs to deliver QDs for fluorescent imaging. The results showed that the QD-modified MBs not only maintained the capability of ultrasound imaging, but also could be used as a targeted-drug controlled-release system to deliver the QDs for cell and tissue fluorescent imaging by UTMD. The novel dual-functional imaging agent has potential for a variety of biological and medical applications.

Ultrasonography in kidney transplantation: values and new developments

Renal transplant is performed on patients with end-stage renal disease. Gray-scale renal sonography combined with color Doppler has become the main noninvasive imaging method for evaluating a kidney transplant, as it provides information about the kidney anatomy and its vascular flow. In this article, we discuss the utility of sonography in renal transplants and describe the ultrasound findings in early and chronic graft pathology. Also, we explain new developments in ultrasound imaging with contrast media and its utility in renal transplantation, proposing that contrast-enhanced sonography be incorporated as a method to evaluate graft status because of its capability to evaluate cortical capillary blood flow.

Contrast-enhanced Coded Phase-inversion Harmonic Sonography of Knee Synovitis Correlates with Histological Vessel Density: 2 Automated Digital Quantifications

Objective.

To use contrast-enhanced coded phase-inversion harmonic B-mode sonography to assess the acoustic enhancement of the synovial area of the knee; and to compare the data with the histological vessel density.

Methods.

Eleven patients eligible for a knee arthroscopy were studied. Acoustic quantification was carried out by a digital image analysis program that detects the time-dependent increase [intensity (time) = k × time + C] of gray-level intensity in all the pixels of a specific region of interest (ROI) following intravenous injection of the microbubble contrast agent sulfur hexafluoride. Echo-guided synovial biopsies were carried out in the same ROI. Synovial vessel areas were quantified after Factor VIII immunostaining of synovial biopsies using an automated digital image analysis.

Results.

Significant (p < 0.05) correlations were observed between histological vessel density and percentage of the synovial area with a k value > 0.01 (r = 0.93) and kmaxvalues (r = 0.79), as well as between the 2 latter parameters (r = 0.72). The histological vessel density and the 2 acoustic parameters were also significantly correlated with the logarithm of erythrocyte sedimentation rate (r = 0.77, r = 0.87, r = 0.67, respectively) and with log C-reactive protein serum concentration (r = 0.69, r = 0.83, r = 0.62, respectively).

Conclusion.

Contrast-enhanced coded phase-inversion harmonic B-mode sonography coupled with an appropriate data analysis method is a new tool to identify and quantify vessel density in knee synovitis.

Dual-mode intracranial catheter integrating 3D ultrasound imaging and hyperthermia for neuro-oncology: feasibility study

In this study, we investigated the feasibility of an intracranial catheter transducer with dual-mode capability of real-time 3D (RT3D) imaging and ultrasound hyperthermia, for application in the visualization and treatment of tumors in the brain. Feasibility is demonstrated in two ways: first by using a 50-element linear array transducer (17 mm x 3.1 mm aperture) operating at 4.4 MHz with our Volumetrics diagnostic scanner and custom, electrical impedance-matching circuits to achieve a temperature rise over 4 degrees C in excised pork muscle, and second, by designing and constructing a 12 Fr, integrated matrix and linear-array catheter transducer prototype for combined RT3D imaging and heating capability. This dual-mode catheter incorporated 153 matrix array elements and 11 linear array elements diced on a 0.2 mm pitch, with a total aperture size of 8.4 mm x 2.3 mm. This 3.64 MHz array achieved a 3.5 degrees C in vitro temperature rise at a 2 cm focal distance in tissue-mimicking material. The dual-mode catheter prototype was compared with a Siemens 10 Fr AcuNav catheter as a gold standard in experiments assessing image quality and therapeutic potential and both probes were used in an in vivo canine brain model to image anatomical structures and color Doppler blood flow and to attempt in vivo heating.

Comparison of one-dimensional and two-dimensional least-squares strain estimators for phased array displacement data

In this study, the performances of one-dimensional and two-dimensional least-squares strain estimators (LSQSE) are compared. Furthermore, the effects of kernel size are examined using simulated raw frequency data of a widely-adapted hard lesion/soft tissue model. The performances of both methods are assessed in terms of root-mean-squared errors (RMSE), elastographic signal-to-noise ratio (SNRe) and contrast-to-noise ratio (CNRe). RMSE analysis revealed that the 2D LSQSE yields better results for phased array data, especially for larger insonification angles. Using a 2D LSQSE enabled the processing of unfiltered displacement data, in particular for the lateral/horizontal strain components. The SNRe and CNRe analysis showed an improvement in precision and almost no loss in contrast using 2D LSQSE. However, the RMSE images for different kernel sizes revealed that the optimal 2D kernel size depends on the region-of-interest and showed that the LSQ kernel size should be limited to avoid loss in resolution.

Therapeutic potential of low-intensity ultrasound (part 2): biomolecular effects, sonotransfection, and sonopermeabilization

Part one of this review focused on the thermal and mechanical effects of low-intensity ultrasound (US). In this second and final part of the review, we will focus on and discuss various aspects of low-intensity US, with emphasis on the biomolecular effects, US-mediated gene transfection (sonotransfection), and US-mediated permeabilization (sonopermeabilization). Sonotransfection of different cell lines in vitro and target tissues in vivo have been reported. Optimization experiments have been done and different mechanisms investigated. It has also been found that several genes can be up-regulated or down-regulated by sonication. As to the potential therapeutic applications, systemic or local sonotransfection might also be a safe and effective gene therapy method in effecting the cure of local and systemic disorders. Gene regulation of target cells may be utilized in modifying cellular response to a treatment, such as increasing the sensitivity of diseased cells while making normal cells resistant to the side effects of a treatment. Advances in sonodynamic therapy and drug sonopermeabilization also offer an ever-increasing array of therapeutic options for low-intensity US.

Therapeutic potential of low-intensity ultrasound (part 1): thermal and sonomechanical effects

In this first part of the review, we will focus on and discuss various aspects of low-intensity ultrasound (US), with emphasis on mild thermal effects, apoptosis induction, and sonomechanical effects. Mild thermal effects of US have been commonly applied to physical therapy. Though US has clear beneficial effects, the advantage of using US over other heating modalities remains unclear. US has also been used in vivo and clinically in the treatment of wounds and fractures, with promising results. On the biomolecular level, studies have shown that US can induce apoptosis and that certain conditions can provide optimal apoptosis induction. As to potential therapeutic applications, in addition to the thermal and other physical effects, apoptosis induction by US may offer direct and rapid treatment of tumors or cancer tissues. Technological advances and rapidly accelerating research in this field are providing an ever-increasing array of therapeutic options for lowintensity US.

Imaging hemodynamics

Microvascular permeability is a pharmacologic indicator of tumor response to therapy, and it is expected that this biomarker will evolve into a clinical surrogate endpoint and be integrated into protocols for determining patient response to antiangiogenic or antivascular therapies. This review discusses the physiological context of vessel permeability in an imaging setting, how it is affected by active and passive transport mechanisms, and how it is described mathematically for both theoretical and complex dynamic microvessel membranes. Many research groups have established dynamic-enhanced imaging protocols for estimating this important parameter. This review discusses those imaging modalities, the advantages and disadvantages of each, and how they compare in terms of their ability to deliver information about therapy-associated changes in microvessel permeability in humans. Finally, this review discusses future directions and improvements needed in these areas.

Sonodynamic therapy

Recently, there have been numerous reports on the application of non-thermal ultrasound energy for treating various diseases in combination with drugs. Furthermore, the introduction of microbubbles and nanobubbles as carriers/enhancers of drugs has added a whole new dimension to therapeutic ultrasound. Non-thermal mechanisms for effects seen include various forms of energy due to cavitation, acoustic streaming, micro jets and radiation force which increases possibilities for targeting tissue with drugs, enhancing drug effectiveness or even chemically activating certain materials. Examples such as enhancement of thrombolytic agents by ultrasound have proven to be beneficial for acute stroke patients and peripheral arterial occlusions. Non-invasive low intensity focused ultrasound in conjunction with anti-cancer drugs may help to reduce tumor size and lessen recurrence while reducing severe drug side effects. Chemical activation of drugs by ultrasound energy for treatment of atherosclerosis and tumors is another new field recently termed as "Sonodynamic therapy". Lastly, advances in molecular imaging have aroused great expectations in applying ultrasound for both diagnosis and therapy simultaneously. Microbubbles or nanobubbles targeted at the molecular level will allow medical doctors to make a final diagnosis of a disease using ultrasound imaging and then immediately proceed to a therapeutic ultrasound treatment.

Analysis of muscle microcirculation in advanced diabetes mellitus by contrast enhanced ultrasound

AIMS

Contrast enhanced ultrasound (CEUS) was recently established to quantify perfusion deficits in peripheral arterial disease (PAD). However, this approach was not suitable to assess microangiopathy of skeletal muscle, a major contributor to PAD in diabetic patients. We hypothesized that an optimized methodology would detect impaired microcirculation.

METHODS

Ten patients with advanced diabetes mellitus (mean diabetes duration 21 years), 10 PAD patients, and 10 control subjects were enrolled consecutively. The arrival times of the contrast agent Sonovue after intravenous injection were assessed selectively in a small artery, muscle tissue and a muscle vein of the calf muscle. Contrast transit times (CTTs) were calculated as the differences between arrival times.

RESULTS

The median CTT for artery-vein was significantly higher in the diabetes group (43 s) than in the PAD (22 s, p=0.007) and control groups (11 s, p<0.001, no value overlap). CTTs for artery-muscle and muscle-vein were shorter with highest median values in the diabetes group.

CONCLUSIONS

We validated improved CEUS as consistent method to detect changes in the microvascular bed. This method may become a valuable tool to quantify impaired microcirculation in diabetes and help to improve patient care.

Diagnosis of liver cirrhosis by transit-time analysis at contrast-enhanced ultrasonography

PURPOSE

The aims of this prospective study were to evaluate analysis of sulfur-hexafluoride-filled microbubble contrast agent (Sonovue) transit times as a tool for differentiating liver cirrhosis from the noncirrhotic stage of liver disease and to compare its performance with that of conventional B-mode and Doppler ultrasonography (US).

MATERIALS AND METHODS

Contrast-enhanced hepatic ultrasonography with the US contrast agent Sonovue was performed on 38 patients with diagnoses of hepatic cirrhosis based on unequivocal clinical signs or liver biopsy findings (Child-Pugh classes A in 19, B in 16 and C in three), 31 patients with noncirrhotic diffuse liver disease (biopsy confirmed) and 14 controls without diffuse liver disease. Time curves of hepatic-vein signal intensity were analysed using objective criteria to determine the time of enhancement onset (hepatic-vein arrival time) and peak enhancement (hepatic-vein peak enhancement). Accuracy in diagnosing cirrhosis was compared with that based on B-mode and Doppler data.

RESULTS

Hepatic-vein arrival time in cirrhotic patients was significantly shorter (p < 0.01) than in noncirrhotic (chronic liver disease and controls) patients. Peak enhancement times in these three groups were not significantly different. An arrival-time cutoff of 17 s distinguished cirrhotic from noncirrhotic patients with high accuracy (100% sensitivity, 93.3% specificity, positive and negative predictive values 92.6% and 100%, respectively) and excellent reproducibility (kappa coefficients of 1.0 and 0.93 for intraand interobserver agreement). Contrast-enhanced US showed better sensitivity than the B-mode and Doppler data.

CONCLUSIONS

Analysis of the time of onset of US contrast enhancement of the hepatic vein appears to be a potentially useful noninvasive supplement to conventional sonography and Doppler in the follow-up of patients with chronic diffuse liver disease.

Bioeffects considerations for diagnostic ultrasound contrast agents

Diagnostic ultrasound contrast agents have been developed for enhancing the echogenicity of blood and for delineating other structures of the body. Approved agents are suspensions of gas bodies (stabilized microbubbles), which have been designed for persistence in the circulation and strong echo return for imaging. The interaction of ultrasound pulses with these gas bodies is a form of acoustic cavitation, and they also may act as inertial cavitation nuclei. This interaction produces mechanical perturbation and a potential for bioeffects on nearby cells or tissues. In vitro, sonoporation and cell death occur at mechanical index (MI) values less than the inertial cavitation threshold. In vivo, bioeffects reported for MI values greater than 0.4 include microvascular leakage, petechiae, cardiomyocyte death, inflammatory cell infiltration, and premature ventricular contractions and are accompanied by gas body destruction within the capillary bed. Bioeffects for MIs of 1.9 or less have been reported in skeletal muscle, fat, myocardium, kidney, liver, and intestine. Therapeutic applications that rely on these bioeffects include targeted drug delivery to the interstitium and DNA transfer into cells for gene therapy. Bioeffects of contrast-aided diagnostic ultrasound happen on a microscopic scale, and their importance in the clinical setting remains uncertain.

High-speed optical observations and simulation results of SonoVue microbubbles at low-pressure insonation

Abstract-Modified Rayleigh-Plesset models are commonly used to characterize the acoustic response of microbubbles under ultrasound exposure. In most instances these models have been parameterized through acoustic measurements taken from bulk suspensions of microbubbles. The aim of this study was to parameterize the Hoff model for the commercial contrast agent SonoVue using optically observed oscillations from individual microbubbles recorded with a high-speed camera. The shell elasticity model term was tuned to fit simulation data to the measured oscillations while the shell viscosity parameter was held constant at 1 Pa??s. The results demonstrate a limited ability of the model to predict the microbubble behavior. The shell elasticity parameter was found to vary proportionally between 10 and 80 MPa with the initial microbubble diameter, implying the viscoelastic shell terms are not a constant property of the shell material. Further analysis using a moving window optimization to probe the microbubble responses suggests that the elasticity of the shell can increase by up to 50% over the course of insonation, particularly for microbubbles oscillating nearer to their resonant frequency. Microbubble oscillations were modeled more successfully by incorporating a varying elasticity term into the model.

Clinical applications of perfluorocarbon nanoparticles for molecular imaging and targeted therapeutics

Molecular imaging is a novel tool that has allowed non-invasive diagnostic imaging to transition from gross anatomical description to identification of specific tissue epitopes and observation of biological processes at the cellular level. This technique has been confined to the field of nuclear imaging; however, recent advances in nanotechnology have extended this research to include ultrasound (US) and magnetic resonance (MR) imaging. The exploitation of nanotechnology for MR and US molecular imaging has generated several candidate contrast agents. One multimodality platform, targeted perfluorocarbon (PFC) nanoparticles, is useful for noninvasive detection with US and MR, targeted drug delivery, and quantification.

Contrast-enhanced ultrasound measurement of microvascular perfusion relevant to nutrient and hormone delivery in skeletal muscle: a model study in vitro

Contrast-enhanced ultrasound (CEU) has been used to measure muscle microvascular perfusion in vivo in response to exercise and insulin. In the present study we address whether CEU measurement of capillary volume is influenced by bulk flow and if measured capillary filling rate allows discrimination of different flow pattern changes within muscle. Three in vitro models were used: (i) bulk flow rate was varied within a single length of capillary tubing; (ii) at constant bulk flow, capillary volume was increased 3-fold by joining lengths of capillary in series, and compared to a single length; and (iii) at constant bulk flow, capillary volume was increased by sharing flow between a number of lengths of identical capillaries in parallel. The contrast medium for CEU was gas-filled albumin microbubbles. Pulsing interval (time) versus acoustic-intensity curves were constructed and from these, capillary volume and capillary filling rate were calculated. CEU estimates of capillary volume were not affected by changes in bulk flow. Furthermore, as CEU estimates of capillary volume increased, measures of capillary filling rate decreased, regardless of whether capillaries were connected in series or parallel. Therefore, CEU can detect a change in filling rate of the microvascular volume under measurement, but it can not be used to discriminate between different flow patterns within muscle that might account for capillary recruitment in vivo.

Arteriogenesis: basic mechanisms and therapeutic stimulation

Pharmacological attempts to stimulate the growth of collateral arteries (arteriogenesis) are evolving towards a new treatment option for patients with vascular occlusive diseases. This enlargement of small pre-existing anastomoses towards large conductance arteries takes place independent of local oxygen tension and is driven by changes in luminal shear stress and infiltration of circulating cells. With the increasing knowledge regarding the distinct differences between capillary sprouting (angiogenesis) and arteriogenesis, several cytokines and growth factors have been demonstrated to stimulate the growth of arterial blood vessels in preclinical models of vascular disease. However, the translation towards clinical practice remains difficult and first in-man trials show limited success. Intensive research especially regarding new drug delivery platforms and the potentially serious side effects of pro-arteriogenic therapeutics is warranted before stimulation of arteriogenesis could become a significant treatment option for vascular occlusive diseases. This review focuses on the recent advances in the field of collateral artery growth. In addition, possible means to overcome the hurdles that have hampered the clinical implementation of pro-arteriogenic therapies will be discussed.

Contrast-enhanced ultrasound to evaluate the severity of chronic hepatitis C

BACKGROUND

Non-invasive techniques are being developed to assess the severity of liver disease. Haemodynamic changes in the hepatic circulation during the development of liver disease can be evaluated with contrast-enhanced ultrasound.

AIM

To evaluate the possible correlation between ultrasound contrast-agent transit times and different stages of chronic hepatitis C.

PATIENTS

Sixteen patients with clinically evident hepatitis C virus-related cirrhosis, 22 non-cirrhotic patients with chronic hepatitis C and 14 controls with no clinical evidence of liver disease were studied.

METHODS

Contrast-enhanced hepatic ultrasonography was performed with a sulphur hexafluoride-filled microbubble contrast agent, and time curves of hepatic vein signal intensity were analysed to determine the time of enhancement onset (hepatic vein arrival time) and peak enhancement (hepatic vein peak enhancement).

RESULTS

Hepatic vein arrival time in cirrhotic patients was significantly shorter (p<0.001) than in non-cirrhotic patients and controls. Within the group with chronic hepatitis C, METAVIR scores of fibrosis and necro-inflammatory changes had no significant effect on hepatic vein arrival times.

CONCLUSION

Analysis of the time of onset of ultrasound contrast enhancement of the hepatic vein appears to be a simple, non-invasive method for reliably excluding cirrhosis with signs of portal hypertension, but not for assessing the severity of either chronic hepatitis C or cirrhosis.

Cavitation threshold of microbubbles in gel tunnels by focused ultrasound

The investigation of inertial cavitation in micro-tunnels has significant implications for the development of therapeutic applications of ultrasound such as ultrasound-mediated drug and gene delivery. The threshold for inertial cavitation was investigated using a passive cavitation detector with a center frequency of 1 MHz. Micro-tunnels of various diameters (90 to 800 microm) embedded in gel were fabricated and injected with a solution of Optison(trade mark) contrast agent of concentrations 1.2% and 0.2% diluted in water. An ultrasound pulse of duration 500 ms and center frequency 1.736 MHz was used to insonate the microbubbles. The acoustic pressure was increased at 1-s intervals until broadband noise emission was detected. The pressure threshold at which broadband noise emission was observed was found to be dependent on the diameter of the micro-tunnels, with an average increase of 1.2 to 1.5 between the smallest and the largest tunnels, depending on the microbubble concentration. The evaluation of inertial cavitation in gel tunnels rather than tubes provides a novel opportunity to investigate microbubble collapse in a situation that simulates in vivo blood vessels better than tubes with solid walls do.

Value of contrast-enhanced ultrasound in rheumatoid arthritis

The purpose of this review is to describe the spectrum of sonographic findings in rheumatic diseases with respect to the diagnostic potential using US contrast media which prove activity or inactivity in synovial tissue where new treatment regimes target. Synovial activity can be found in non-erosive and erosive forms of primary and secondary osteoarthritis, and in inflammatory forms of joint diseases like rheumatoid arthritis and peripheral manifestations of spondyloarthritis including, ankylosing spondylitis, Reiter's syndrome, psoriatic arthritis and enteropathic arthritis. It can also be present in metabolic and endocrine forms of arthritis, in connective tissue arthropathies like systemic lupus erythematosus or scleroderma and in infectious arthritis. Ultrasound should be used as first-line imaging modality in suspected early cases of RA and other forms of arthritis, whereas contrast-enhanced ultrasound (CEUS) can further enable for sensitive assessment of vascularity which correlates with disease activity.

Second-generation sonographic contrast agents in the evaluation of renal trauma

PURPOSE

The purpose of this study was to define the indications, diagnostic accuracy and limitations of second-generation sonographic contrast agents in the evaluation of patients with renal trauma.

MATERIALS AND METHODS

Between March 2004 and April 2005, 277 patients with blunt abdominal trauma were evaluated. Twenty-eight out of 277 patients had renal lesions, the severity of which was graded according to the organ injury severity scale of the American Association for the Surgery of Trauma (AAST). All the patients enrolled in the study had minor trauma and were evaluated with baseline ultrasound (US), contrast-enhanced US after injection of a second-generation contrast agent (SonoVue) and, if positive, with multiphasic multidetector computed tomography (MDCT).

RESULTS

Five out of 28 traumatic parenchymal lesions with perirenal fluid collection were identified at baseline US. All 28 renal parenchymal lesions, with or without perirenal or retroperitoneal haematoma, were identified at contrast-enhanced US. Multiphase MDCT confirmed all the cases that were positive at contrast-enhanced US and demonstrated the integrity of the urinary tract in the delayed phase.

CONCLUSIONS

Our experience confirmed the diagnostic accuracy of second-generation sonographic contrast material both for diagnosis and for appropriate patient management. In particular, contrast-enhanced sonography proved to be a reliable technique for the evaluation and follow-up of low-grade renal injuries. Its main advantage is reduced radiation exposure, as fewer MDCT examinations are needed, whereas its limitation is the high cost of the technique if used in unselected patients.

Nano- and microparticle-based imaging of cardiovascular interventions: overview

The rapid progress of nanoscience and the application of nanotechnology are changing the foundations of diagnosis, treatment, and prevention of cardiovascular diseases. As the core of nanotechnology, nano- and microparticles offer "three-in-one" functions as imaging agents, target probes, and therapeutic carriers. While nano- and microparticle-based imaging of cardiovascular interventions is still in its developing phase, it has already presented the exciting potential to monitor primary interventional procedures for precise therapeutic delivery, enhance the effectiveness of delivered therapeutics, and monitor therapeutic efficiency after interventions performed to treat cardiovascular diseases. This article provides an overview of the current status of the application of nano- and microparticles in the imaging of cardiovascular interventions.

Impact of microbubbles on shock wave-mediated DNA uptake in cells in vitro

Gas-filled microbubbles have been successfully used as gene delivery reagents in combination with diagnostic ultrasound. Although shock wave exposure has been shown to transfect cells with naked DNA in vitro, it has not been tested whether the addition of microbubbles would augment DNA uptake under those conditions. Therefore, the aim of this study was to test the impact of microbubbles on transgene expression in vitro under shock wave exposure conditions. HEK 293 cells were treated with 60 or 120 pulses of shock waves at varying energy levels. Cells were mixed with either 100 microg/mL luciferase expressing plasmid DNA or with microbubbles that were produced with the same amount of this DNA. Cell death was evaluated after 1 h and transgene expression, after 24 h. In the presence of microbubbles, transgene expression was significantly higher (as much as 29-fold) relative to that obtained without microbubbles. Cells exposed to 120 pulses demonstrated higher transgene expression (as high as 2.7-fold) compared with cells exposed to 60 pulses. The use of microbubbles resulted in greater cell death, varying from 26% (low energy) to 78% (high energy). In conclusion, DNA-loaded microbubbles can significantly increase shock wave mediated gene transfer. However, this effect is associated with increased levels of cell destruction.

Contrast-Enhanced Ultrasound for Blunt Abdominal Trauma

Sonography is widely used in the initial diagnostic assessment of blunt abdominal trauma in adults and children. It has been formally incorporated worldwide into the routine armamentarium available for emergency diagnosis and treatment as a means of rapid detection of free abdominal fluid, normally referred to as FAST (Focused Assessment with Sonography in Trauma). However, there is some controversy regarding its value because free abdominal fluid may be lacking in patients with abdominal organ injuries from blunt trauma. More recently, a new ultrasound technique has been developed using contrast agents. Contrast-enhanced ultrasound performs better than the non-contrast-enhanced technique for the detection of abdominal solid organ injuries and can play an important role in the prompt evaluation of patients with blunt trauma. Furthermore, contrast-enhanced ultrasound can be used in the follow-up of patients who have solid organ lesions and are managed with nonoperative treatment, avoiding radiation and iodinated contrast medium exposure.

New perspectives for the use of contrast-enhanced liver ultrasound in clinical practice

The introduction of second-generation microbubble ultrasound contrast agents and the development of contrast specific ultrasound techniques have improved the ability of contrast enhanced ultrasound in detecting and characterising liver lesions, offering new perspectives for its exploitation in clinical hepatology. Indeed, several studies have demonstrated a high diagnostic accuracy in focal lesion characterisation (85-96%) in patients either with or without underlying chronic liver disease. This review article describes the basic principles of contrast enhanced ultrasound, defines the different vascular features of benign and malignant liver lesions, and assesses its clinical impact in different clinical scenarios, according to the guidelines of the European Federation of Societies for Ultrasound in Medicine and Biology, contrast enhanced ultrasound enables the characterisation of focal liver lesions, regardless of the presence or absence of underlying chronic liver disease. Contrast enhanced ultrasound is also useful in staging and follow-up of cancer patients and in monitoring local ablative treatment. Contrast enhanced ultrasound is expected to be considerably increased and replace many computed tomography and magnetic resonance imaging examinations in near future, according to the European Federation of Societies for Ultrasound in Medicine and Biology guidelines. Therefore, it is necessary to take measures in order to meet the demand for an increasing number of these procedures.

Cardiac imaging: The biological effects of diagnostic cardiac ultrasound

Diagnostic cardiac ultrasounds are an environment-friendly and non-ionising imaging technology. However, ultrasounds are not biologically inert, and their use might have profound clinical impact. This paper summarizes the known effects of cardiac ultrasound--compared to other major imaging techniques--to exposed patients and to clinically exposed physicians practising ultrasound imaging. Furthermore, this review also provides an overview of the evidences on the biological effects of diagnostic ultrasound--which suggest that ultrasound with frequency, intensity and duration fully in the diagnostic range have significant molecular, cellular and organ effects. A better understanding of these effects may improve our understanding of the complex interactions between ultrasound and biological tissues and may open new avenues to therapeutic applications based on the ultrasound-modulated cell functions, such as membrane transduction, apoptosis, cell permeability and thrombolysis.

Neue Entwicklungen in der Gelenksonographie

Musculoskeletal ultrasonography has become an established imaging technique in rheumatology. The ability of ultrasonography to visualize soft tissue changes provides a possibility for differentiating between exudative and proliferative synovial tissue changes. Superficial cartilage and bone lesions can be detected earlier by ultrasonography than by conventional radiography. The application of Doppler and power Doppler ultrasonography is helpful for the detection of early inflammation. Current studies with ultrasound contrast media demonstrate its benefit in the differentiation of inflammatory processes. Musculoskeletal ultrasonography is helpful in the diagnosis of early arthritis, especially in patients with inconspicuous conventional radiography or suspicious clinical findings. It is a convenient method for follow-up analysis, and therefore has an impact on the monitoring of therapy. It is patient-friendly and is an important tool for the diagnostic work-up of arthritis.

Acoustic response of compliable microvessels containing ultrasound contrast agents

The existing models of the dynamics of ultrasound contrast agents (UCAs) have largely been focused on an UCA surrounded by an infinite liquid. Preliminary investigations of a microbubble's oscillation in a rigid tube have been performed using linear perturbation, under the assumption that the tube diameter is significantly larger than the UCA diameter. In the potential application of drug and gene delivery, it may be desirable to fragment the agent shell within small blood vessels and in some cases to rupture the vessel wall, releasing drugs and genes at the site. The effect of a compliant small blood vessel on the UCA's oscillation and the microvessel's acoustic response are unknown. The aim of this work is to propose a lumped-parameter model to study the interaction of a microbubble oscillation and compliable microvessels. Numerical results demonstrate that in the presence of UCAs, the transmural pressure through the blood vessel substantially increases and thus the vascular permeability is predicted to be enhanced. For a microbubble within an 8 to 40 microm vessel with a peak negative pressure of 0.1 MPa and a centre frequency of 1 MHz, small changes in the microbubble oscillation frequency and maximum diameter are observed. When the ultrasound pressure increases, strong nonlinear oscillation occurs, with an increased circumferential stress on the vessel. For a compliable vessel with a diameter equal to or greater than 8 microm, 0.2 MPa PNP at 1 MHz is predicted to be sufficient for microbubble fragmentation regardless of the vessel diameter; however, for a rigid vessel 0.5 MPa PNP at 1 MHz may not be sufficient to fragment the bubbles. For a centre frequency of 1 MHz, a peak negative pressure of 0.5 MPa is predicted to be sufficient to exceed the stress threshold for vascular rupture in a small (diameter less than 15 microm) compliant vessel. As the vessel or surrounding tissue becomes more rigid, the UCA oscillation and vessel dilation decrease; however the circumferential stress is predicted to increase. Decreasing the vessel size or the centre frequency increases the circumferential stress. For the two frequencies considered in this work, the circumferential stress does not scale as the inverse of the square root of the acoustic frequency va as in the mechanical index, but rather has a stronger frequency dependence, 1/va.

Ultrasound contrast agents: an overview

With the introduction of microbubble contrast agents, diagnostic ultrasound has entered a new era that allows the dynamic detection of tissue flow of both the macro and microvasculature. Underpinning this development is the fact that gases are compressible, and thus the microbubbles expand and contract in the alternating pressure waves of the ultrasound beam, while tissue is almost incompressible. Special software using multiple pulse sequences separates these signals from those of tissue and displays them as an overlay or on a split screen. This can be done at low acoustic pressures (MI<0.3) so that the microbubbles are not destroyed and scanning can continue in real time. The clinical roles of contrast enhanced ultrasound scanning are expanding rapidly. They are established in echocardiography to improve endocardial border detection and are being developed for myocardial perfusion. In radiology, the most important application is the liver, especially for focal disease. The approach parallels that of dynamic CT or MRI but ultrasound has the advantages of high spatial and temporal resolution. Thus, small lesions that can be indeterminate on CT can often be studied with ultrasound, and situations where the flow is very rapid (e.g., focal nodular hyperplasia where the first few seconds of arterial perfusion may be critical to making the diagnosis) are readily studied. Microbubbles linger in the extensive sinusoidal space of normal liver for several minutes whereas they wash out rapidly from metastases, which have a low vascular volume and thus appear as filling defects. The method has been shown to be as sensitive as three-phase CT. Microbubbles have clinical uses in many other applications where knowledge of the microcirculation is important (the macrocirculation can usually be assessed adequately using conventional Doppler though there are a few important situations where the signal boost given by microbubbles is useful, e.g., transcranial Doppler for evaluating vasospasm after subarachnoid haemorrhage). An important situation where demonstrating tissue devitalisation is important is in interstitial ablation of focal liver lesions: using microbubble contrast agents at the end of a procedure allows immediate evaluation of the adequacy of the ablation which can be extended if needed; this is much more convenient and cost-saving than moving the patient to CT and perhaps needing an additional ablation session at a later date. Similar considerations suggest that contrast-enhanced ultrasound might have a role in abdominal trauma: injury to the liver, spleen and kidneys can be assessed rapidly and repeatedly if necessary. Its role here alongside dynamic CT remains to be evaluated. Infarcts or ischaemia and regions of abnormal vascularity, especially in malignancies, in the kidneys and spleen seem to be useful and improved detection of the neovascularisation of ovarian carcinomas is promising. Similar benefits in the head-and-neck and in the skin while the demonstration of the neovascularisation of atheromatous plaques and of aggressive joint inflammation offer interesting potentials.

Encapsulated ultrasound microbubbles: Therapeutic application in drug/gene delivery

Encapsulated gas microbubbles are well known as ultrasound contrast agents for medical ultrasound imaging. Nonetheless, not only do these microbubbles help to image, but they can also be used as drug/gene carriers. The microbubbles as drug/gene carriers have an average size less than that of red blood cells, i.e. they are capable of penetrating even into the small blood capillaries and releasing drug and genes under the action of ultrasound field. The application of ultrasound and microbubbles to targeted drug and gene delivery has been the subject of intense experimental research. Under exposure of sufficiently high-amplitude ultrasound, these targeted microbubbles would rupture, spewing drugs or genes, which are contained in its encapsulating layer, to targeted cells or tissues. Recently, targeting ligands are attached to the surface of the microbubbles (i.e. targeted-microbubbles), which have been widely used in cardiovascular system and tumor diagnosis and therapy. In this paper, the characterization of novel targeted ultrasonic contrast agents or microbubbles and their potential applications in drug delivery or gene therapy are reviewed.

Physical parameters affecting ultrasound/microbubble-mediated gene delivery efficiency in vitro

Ultrasound (US)/microbubble-mediated gene delivery is a technology with many potential advantages suited to clinical application. Previous studies have demonstrated transfection but many are unsatisfactory in respect to the exposure apparatus, lack of definition of the US field or the limitations on parameters that can be explored using clinical diagnostic US machines. We investigated individual exposure parameters using a system minimising experimental artefacts and allowing control of many parameters of the US field. Using a 1-MHz transducer we systematically varied US parameters, the duration of exposure and the microbubble and DNA concentrations to optimise gene delivery. Delivery was achieved, using lipid microbubbles (SonoVue) and clinically acceptable US exposures, to adherent cells at efficiencies of approximately 4%. The acoustic pressure amplitude (0.25 MPa peak-negative), pulse repetition frequency (1-kHz) and duration of exposure (10 s) were important in optimising gene delivery with minimal impact on cell viability. These findings support the hypothesis that varying the physical parameters of US-mediated gene delivery has an affect on both efficiency and cell viability. These data are the first in terms of their thorough exploration of the US parameter space and will be the basis for more-informed approaches to developing clinical applications of this technology.

Blunt abdominal trauma: emergency contrast-enhanced sonography for detection of solid organ injuries

OBJECTIVE

The objective of our study was to prospectively compare the diagnostic value of sonography and contrast-enhanced sonography with CT for the detection of solid organ injuries in blunt abdominal trauma patients.

SUBJECTS AND METHODS

Sonography, contrast-enhanced sonography, and CT were performed to assess possible abdominal organ injuries in 69 nonconsecutive hemodynamically stable patients with blunt abdominal trauma and a strong clinical suspicion of abdominal lesions. Sonography and contrast-enhanced sonography findings were compared with CT findings, the reference standard technique.

RESULTS

Thirty-two patients had 35 abdominal injuries on CT (10 kidney or adrenal lesions, seven liver lesions, 17 spleen lesions, and one retroperitoneal hematoma). Sixteen lesions were detected on sonography, and 32 were seen on contrast-enhanced sonography. The sensitivity and specificity of sonography were 45.7% and 91.8%, respectively, and the positive and negative predictive values were 84.2% and 64.1%, respectively. Contrast-enhanced sonography had a sensitivity of 91.4%, a specificity of 100%, and positive and negative predictive values of 100% and 92.5%, respectively.

CONCLUSION

Contrast-enhanced sonography was found to be more sensitive than sonography and almost as sensitive as CT in the detection of traumatic abdominal solid organ injuries. It can therefore be proposed as a useful tool in the assessment of blunt abdominal trauma.

Role of microbubble ultrasound contrast agents in the non-invasive assessment of chronic hepatitis C-related liver disease

Patients who are chronically infected with the hepatitis C virus often develop chronic liver disease and assessment of the severity of liver injury is required prior to considering viral eradication therapy. This article examines the various assessment methods currently available from gold standard liver biopsy to serological markers and imaging. Ultrasound is one of the most widely used imaging modalities in clinical practice and is already a first-line diagnostic tool for liver disease. Microbubble ultrasound contrast agents allow higher resolution images to be obtained and functional assessments of microvascular change to be carried out. The role of these agents in quantifying the state of hepatic injury is discussed as a viable method of determining the stage and grade of liver disease in patients with hepatitis C. Although currently confined to specialist centres, the availability of microbubble contrast-enhanced ultrasound will inevitably increase in the clinical setting.

Sonographie synovialer und erosiver entzündlicher Veränderungen

High-frequency sonography enables excellent detection of early erosions and synovial proliferations. Power Doppler sonography (PDUS) allows for an improved characterization of articular and peritendinous augmented volume, because detection of hypervascularity correlates with inflammatory activity and further is helpful in differentiation from effusion and inactive pannus. The use of contrast media improves the sensitivity of vascularity detection, because they allow for a delineation of vessels at the microvascular level. This is of increased interest, as the development of new therapeutic options targeting the microvascular level calls for earlier diagnosis and optimal assessment of disease activity. Because of good availability, cost effectiveness, and patient acceptance, sonography facilitates early diagnosis of synovial proliferations and erosions as well as therapy follow-up.

Microbubbles assist goat liver ablation by high intensity focused ultrasound

High intensity focused ultrasound (HIFU) has been introduced to treat cancers. However, this therapy is a time-consuming procedure; destructing a deeper volume is also difficult as ultrasonic energy attenuates exponentially with increasing depth in tissues. The aim of the present study was to investigate the effects of introducing microbubbles on liver HIFU ablation. Seventeen goats were divided into groups A (n=8) and B (n=9). The livers in both groups were ablated using HIFU (1.0 MHz, 22,593 W/cm2) performed in the manner of a clinical regime using a clinical device. A microbubble agent was bolus-injected intravenously before HIFU exposure in group B. All animals in group A and seven goats in group B were euthanased to evaluate the ablation efficiency 24 h after HIFU. The necrosis rate (mm3/s), which was the volume of necrosis tissue per second of HIFU exposure, was used to judge the ablation efficiency. Pathological examinations were performed to determine whether there were residual intact tissues within the exposed volume. The other two goats in group B were used to determine the delayed pathological changes 7 days after ultrasonic ablation. The necrosis rate (mm3/s) was increased in group B (14.4647+/-4.1960 versus 33.5302+/-12.4484, P=0.0059). Pathological examinations confirmed that there were no residual unaffected tissue focuses within the exposed volume. Two remarkable changes occurred in the other two goats in group B 7 days after HIFU: there were ghost-cell islands at the periphery of the ablated tissues, and surrounding adjacent tissues outside the reactive zone necrotized. These findings showed that microbubbles could be used to assist liver HIFU ablation.

The use of a micro-bubble agent to enhance rabbit liver destruction using high intensity focused ultrasound

Liver tissues in New Zealand rabbits were ablated using high intensity focused ultrasound (HIFU, 14300 W/cm(2), 1.0 MHz). The animals were intravenously administered 0.2 ml of micro-bubble agent in the experimental (n=20) group and an isovolumetric normal saline solution in the control (n=27) group before HIFU treatment which was performed as a linear scan. In both groups, the preselected tissue volumes were destroyed without harming the overlying tissues. Necrosis rate (NR, cm(3)/s) was used to reflect the ablation efficiency, which was the tissue volume of occurring coagulative necrosis per 1s HIFU exposure. NR was improved in the experimental group (0.0570+/-0.0433 vs 0.0120+/-0.0122, P=0.0002). Pathological studies confirmed that there were no residual intact targets within the exposed volume. These findings suggested that the introduction of the micro-bubble agent enhanced HIFU liver destruction.

Targeted Therapeutic Applications of Acoustically Active Microspheres in the Microcirculation

The targeted delivery of intravascular drugs and genes across the endothelial barrier with only minimal side effects remains a significant obstacle in establishing effective therapies for many pathological conditions. Recent investigations have shown that contrast agent microbubbles, which are typically used for image enhancement in diagnostic ultrasound, may also be promising tools in emergent, ultrasound-based therapies. Explorations of the bioeffects generated by ultrasound-microbubble interactions indicate that these phenomena may be exploited for clinical utility such as in the targeted revascularization of flow-deficient tissues. Moreover, development of this treatment modality may also include using ultrasound-microbubble interactions to deliver therapeutic material to tissues, and reporter genes and therapeutic agents have been successfully transferred from the microcirculation to tissue in various animal models of normal and pathological function. This article reviews the recent studies aimed at using interactions between ultrasound and contrast agent microbubbles in the microcirculation for therapeutic purposes. Furthermore, the authors present investigations involving microspheres that are of a different design compared to current microbubble contrast agents, yet are acoustically active and demonstrate potential as tools for targeted delivery. Future directions necessary to address current challenges and advance these techniques to clinical practicality are also discussed.

Ultrasound: Mechanical gene transfer into plant cells by sonoporation

Development of nonviral gene transfer methods would be a valuable alternative of gene therapy or transformation. Ultrasound can produce a variety of nonthermal bioeffects via acoustic cavitation. Cavitation bubbles can induce cell death or transient membrane permeabilization (sonoporation) on cells. Application of sonoporation for gene transfer into cells or tissues develops quickly in recent years. Many studies have been performed in vitro exposure systems to a variety of cell lines transfected successfully. In vivo, cavitation initiation and control are more difficult, but can be enhanced by ultrasound contrast agents (microbubbles). The use of ultrasound for nonviral gene delivery has been applied for mammalian systems, which provides a fundamental basis and strong promise for development of new gene therapy methods for clinical medicine. In this paper, ultrasound applied to plant cell transformation or gene transfer is reviewed. Recently, most researches are focused on sonication-assisted Agrobacterium-mediated transformation (SAAT) in plant cells or tissues. Microbubbles are also proposed to apply to gene transfer in plant cells and tissues.

Contrast ultrasound perfusion imaging of lower extremities in peripheral arterial disease: a novel diagnostic method

AIMS

The purpose of this study was to establish contrast-enhanced ultrasound perfusion imaging (CUPI) of the lower extremities as a novel non-invasive diagnostic tool for patients with peripheral arterial disease (PAD).

METHODS AND RESULTS

Ultrasound contrast agent (SonoVue) was injected into a peripheral vein of 16 control subjects and 16 PAD patients and its appearance in the calf muscle was detected by low-energy harmonic ultrasound. Analysis of the wash-in curves revealed that PAD patients had a significantly longer time to peak intensity (TTP), i.e. duration of maximum contrast perfusion [37 s (19-79 s) in control subjects vs. 56 s (32-104 s) in PAD patients at rest, age-adjusted P=0.002]. Exercise stress test of the calf muscle resulted in a decrease of the TTP, maintaining the significant difference in TTP between the groups [19 s (8-37 s) in control subjects vs. 32 s (18-48 s) in PAD patients after exercise, age-adjusted P=0.004]. Neither ankle-brachial index and TTP nor age and TTP showed a significant correlation.

CONCLUSION

CUPI reflects the regional blood circulation of the calf muscle. In this pilot study, PAD patients show a significantly longer TTP than control subjects. The clinical relevance of CUPI is topic of ongoing studies.

Resonance frequency of microbubbles in small blood vessels: a numerical study

Microbubbles are currently used as ultrasound contrast agents. Their potential therapeutic applications are also under investigation. This work is designed to provide some insight into the mechanisms of energy absorption and deposition by a preformed gas bubble in the microvasculature to optimize its efficacy. In the linear regime, the most favourable condition for the transfer of energy from an ultrasonic field to a gas bubble occurs when the centre frequency of the ultrasonic field equals the resonance frequency of the bubble. The resonance frequency of gas microbubbles has been investigated up to now mainly in unbounded liquids; however when bubbles are confined in small regions, their resonance frequency is strongly affected by the surrounding boundaries. A parametric study on how the resonance frequency of microbubbles in blood vessels is affected by the bubble radius, vessel radius and the bubble position in the vessel is presented. The resonance frequency decreases below its free value with decreasing vessel radius for vessels smaller than 200-300 microm depending on the bubble size. This model suggests the possibility of using ultrasound in a range of frequencies that are, in general, lower than the ones used now for therapeutic and diagnostic applications of ultrasound (a few MHz). When microbubbles oscillate at their resonance frequency they absorb and therefore emit more energy. This energy may allow specific blood vessels to be targeted for both diagnostic and therapeutic applications of ultrasound.