Human biodistribution of an ultrasound contrast agent (Quantison) by radiolabelling and gamma scintigraphy

The biodistribution and kinetics of an air filled human serum albumin microcapsule formulation (Quantison) intended for use as an intravenous ultrasound contrast agent have been examined. 12 healthy subjects were administered with approximately 50 million microcapsules per kilogram body weight, radiolabelled with 50 MBq 123I. Imaging was performed over a period of 58 h using a large field-of-view gamma camera and the amount of labelled material present in the blood, urine and faeces measured. Imaging demonstrated that the liver was the organ with the highest uptake, with a mean uptake of 41.8% (SD 10.4%) of the administered dose 1 h following administration. The maximum uptake of the agent in the lungs was low, mean 4.0% (SD 3.4%). A small amount of uptake was visible in the bone marrow; however, this was not quantifiable. There was also evidence of minimal myocardial activity within 5 min of administration. No adverse events were observed and there were no changes in any of the individual post-study indices. The present study demonstrates the safety of Quantison. Gamma scintigraphy played a useful role in confirming the biodistribution of the agent with little lung uptake, high liver uptake and evidence of myocardial uptake.     

Acoustic backscatter properties of the particle/bubble ultrasound contrast agent

Bubble-based suspensions with diameters in the 1-5 microns range have been developed for use as ultrasound contrast agents. Bubbles of these dimensions have resonance frequencies in the diagnostic ultrasonic range, thus improving their backscatter enhancement capabilities. The durability of these bubbles in the blood stream has been found to be limited, providing impetus for a number of approaches to further stabilize them. One of the approaches has been the development of micrometer-size porous particles or 'nano-sponges' with properties suitable for the entrapment and stabilization of gas bubbles. However, the complex morphology and surface chemistry involved in the production of this type of agent makes it unfeasible to directly measure the volume of the entrained gas. A model based on acoustic scattering principles is proposed which indicates that only a small volume fraction of gas should be necessary to significantly enhance the echogenicity of this type of particle-based contrast agent. In the model, the effective scattering cross-section is evaluated as a function of the volume fraction of gas contained in the overall scatterer and the overall scatterer diameter. Initially, the volume fraction of gas is considered as a discrete entity of single bubble. Using common mixture rules, it is then shown that the gas can be considered to be distributed throughout the particle and still arrive at a result that is similar to that for a single, discrete volume of gas. The main contribution to the increased scattering cross-section is due to the compressibility difference between gas and water. The backscatter coefficient is computed as the product of the resulting differential scattering cross-section and the scatterer number density. This approach facilitates comparison with known backscatter coefficients of biological targets such as liver and blood. Simple experimental results are presented for comparison with the model, and the implications relevant to clinical use are suggested.     

Acoustically active lipospheres containing paclitaxel: a new therapeutic ultrasound contrast agent

RATIONALE AND OBJECTIVES: Paclitaxel-carrying lipospheres (MRX-552) were developed and evaluated as a new ultrasound contrast agent for chemotherapeutic drug delivery. METHODS: Paclitaxel was suspended in soybean oil and added to an aqueous suspension of phospholipids in vials. The headspace of the vials was replaced with perfluorobutane gas; the vials were sealed, and they were agitated at 4200 rpm on a shaking device. The resulting lipospheres containing paclitaxel were studied for concentration, size, acute toxicity in mice, and acoustic activity and drug release with ultrasound. Lipospheres containing sudan black dye were produced to demonstrate the acoustically active liposphere (AAL)-ultrasound release concept. RESULTS: Acoustically active lipospheres containing paclitaxel had a mean particle count of approximately 1 x 10(9) particles per mL and a mean size of 2.9 microns. Acute toxicity studies in mice showed a 10-fold reduction in toxicity for paclitaxel in AALs compared with free paclitaxel. The AALs reflected ultrasound as a contrast agent. Increasing amounts of ultrasound energy selectively ruptured the AALs and released the paclitaxel. CONCLUSIONS: Acoustically active lipospheres represent a new class of acoustically active drug delivery vehicles. Future studies will assess efficacy of AALs for ultrasound-mediated drug delivery.     

Gas-body-based contrast agent enhances vascular bioeffects of 1.09 MHz ultrasound on mouse intestine

Anesthetized hairless mice were exposed to continuous or pulsed 1.09-MHz ultrasound with or without prior injection of a gas-body-based ultrasound contrast agent. Albunex at a dose of 10 mL/kg increased the production of intestinal hyperemia, petechia and hemorrhages by continuous ultrasound. For pulsed ultrasound, with 10 micros pulses and 0.01 duty cycle, petechiae were produced for exposures as low as 1 MPa spatial peak pressure amplitude with added gas bodies. The enhancement of petechiae production was robust for pulsed exposure; for example, at 2.8 MPa, an average of 227 petechiae was obtained with added gas bodies, which was 30 times more than without the agent. The production of petechia was roughly proportional to the dosage of Albunex for pulsed exposure. Results did not appear to be strongly dependent on pulsing parameters, but long bursts (0.1 s) were somewhat more effective than pulses (10 micros). The observed vascular bioeffects appeared to involve both thermal and nonthermal mechanisms for continuous exposure, but to result primarily from gas-body activation for pulsed exposure.     

Three-dimensional MRI of coronary arteries using an intravascular contrast agent

OBJECTIVE: To clarify the influence of interindividual difference in the level of aldose reductase on the polyol pathway-related metabolism in diabetic patients. RESEARCH DESIGN AND METHODS: The enzyme protein content was determined by a two-site enzyme-linked immunosorbent assay using monoclonal and polyclonal antibodies to recombinant human aldose reductase in erythrocytes from 35 diabetic patients and 11 healthy volunteers. Patients were stratified into two groups by the median of aldose reductase content, and the erythrocyte sorbitol level, the fructose level, and the lactate-to-pyruvate ratio were compared between the two groups. We also examined the correlation of the enzyme content with these metabolic parameters. RESULTS: The group of patients whose enzyme content was above the median showed a significant increase in the levels of sorbitol (34.7 +/- 4.9 vs. 20.4 +/- 2.0 nmol/g Hb, P < 0.05) and fructose (99.8 +/- 17.2 vs. 45.9 +/- 4.6 nmol/g Hb, P < 0.05), along with an elevated lactate-to-pyruvate ratio (28.6 +/- 6.1 vs. 11.7 +/- 1.2, P < 0.05), compared with patients with low enzyme levels. The aldose reductase content in erythrocytes was well correlated with its activity, and there was a significant correlation between the enzyme content and the erythrocyte sorbitol (r = 0.58, P < 0.001) or fructose (r = 0.57, P < 0.001) levels as well as between the enzyme level and the lactate-to-pyruvate ratio (r = 0.38, P < 0.05). CONCLUSIONS: These results suggest that the interindividual variability of aldose reductase content may contribute tangibly to the polyol-pathway flux and cytoplasmic redox alteration in diabetic patients.     

Preparation and characterization of echogenic liposome as an ultrasound contrast agent: size-dependency and stabilizing effect of cholesterol on the echogenicity of gas-entrapping liposome

The liposome entrapping CO2 gas inside the vesicle, which is called the echogenic liposome, has been made and characterized in vitro as an ultrasound contrast agent. The small unilamellar vesicle (SUV), large unilamellar vesicle (LUV) and multilamellar vesicle (MLV) as echogenic liposomes were compared in their echogenic efficiency and stability, and the effect of size and acoustic property was tested. The acoustic reflectivity increased with the increase in size of the vesicle, largest for the gas filled MLV among the three liposome suspensions. The acoustic reflectivity obtained with the echogenic MLV was larger than that of the gas bubbles enclosed within a surfactant mixture. A half-lifetime of 39 min was observed for the MLV prepared from egg-yolk phosphatidylcholine liposomes. The duration of reflectivity was prolonged drastically to a half-lifetime of 866 min by incorporating cholesterol into the MLV, although the echogenicity was decreased by such incorporation. The stabilizing effect of cholesterol for the ordinary liposomal membrane was thus ascertained in the present case of the gas-entrapping liposome. Our findings encourage the future development of improved gas-entrapping liposomes for the clinical trials of ultrasound contrast agents.     

On the effect of lung filtering and cardiac pressure on the standard properties of ultrasound contrast agent

The goal standard of contrast echocardiography is the absolute measure of myocardial perfusion using a contrast agent. Actually, several contrast agents are developed. All these agents show left ventricular opacification after intravenous injection. However, none of these agents shows an acceptable enhancement of the myocardium yet using conventional imaging techniques. The explanation of this phenomenon should be easy by measuring the acoustic characteristics of the contrast agent and then making a comparison of these characteristics with those of the myocardium. In this study we present definitions of standard acoustic parameters of ultrasound contrast agent, the backscatter coefficient Bs and the scattering-to-attenuation ratio STAR. Afterwards, considering an intravenous injection of the contrast agent, and taking into account the effects of lung filtering and cardiac pressure, the standard properties of contrast agents are determined in different sites: right ventricle (before lung passage), left ventricle (after lung passage and taking into account the pressure effect) and in the coronary system. Calculations showed that the acoustic properties are considerably influenced by these two effects: lung filtering and cardiac pressure. Comparison of these properties with the tissue properties (myocardium) is then performed. This determines the contribution of the contrast agent to the enhancement of the tissue visualization. The simulations are performed on Albunex microspheres. The results reveal that the difference between scattering of the myocardium and scattering of intravenously injected Albunex is too slight to be visible on an echographic image.     

Standard properties of ultrasound contrast agents

A standardization procedure for in vitro acoustic characterization of ultrasound contrast agents is presented. One new acoustic parameter for particular importance is retained: This is STAR, scattering-to-attenuation ratio, for quantification of the effectiveness of the contrast agent. The STAR expresses the ability of the contrast agent to enhance the visualization of the tissue containing the contrast agent and, at the same time, represents the degree of its absorption. So, it is desirable to produce a contrast agent with high STAR, having good scattering properties to improve the image visualization, and low attenuation to image the underlying biological structures and to avoid shadowing. In this study, we present methods for calculations and measurements of the STAR and comparison between different contrast agents.     

Clinical applications of ultrasound contrast agents

Within the last decade safe and practical ultrasound contrast agents have been introduced. Most of these are based on gas-filled microbubbles, which markedly enhance Doppler signals and, in some cases, also gray-scale images. The clinical improvements expected from ultrasound contrast is reviewed. Tissue-specific contrast agents constitute another area of potential clinical significance. One particular agent is taken up by the reticulo-endothelial system and produces so-called acoustic emission signals when imaged. An introduction to the unique clinical applications of acoustic emission is given. Harmonic imaging is a new contrast-specific imaging modality, which utilizes the nonlinear properties of some agents in an attempt to alleviate current limitations of ultrasound contrast studies. Examples of harmonic images are presented.     

Cellulose as a gastrointestinal US contrast agent

Ultrasound (US) imaging of the abdomen often is compromised by artifacts due to adjacent bowel gas. In an attempt to decrease gas artifacts and improve US image quality, the authors evaluated the use of cellulose preparations as gastrointestinal US contrast agents. Optimal homogeneity and reflectivity were evaluated in phantom solutions, and two suitable agents were selected for clinical trial. Ten volunteers underwent abdominal US imaging before and after contrast agent administration on three separate occasions. The volunteers drank 800 mL of freshly degassed water and two different gastrointestinal US contrast agents. US images obtained before and after administration of contrast material were evaluated by five radiologists and scored for bowel marking, visualization of abdominal anatomy, and image degradation by bowel gas. Compared with water, the orally administered US contrast agents improved visualization of bowel and abdominal anatomy, with diminished gas artifact.     

Lesion contrast enhancement in medical ultrasound imaging

Methods for improving the contrast-to-noise ratio (CNR) of low-contrast lesions in medical ultrasound imaging are described. Differences in the frequency spectra and amplitude distributions of the lesion and its surroundings can be used to increase the CNR of the lesion relative to the background. Automated graylevel mapping is used in combination with a contrast-weighted form of frequency-diversity speckle reduction. In clinical studies, the techniques have yielded mean CNR improvements of 3.2 dB above ordinary frequency-diversity imaging and 5.6 dB over sharper conventional images, with no post-processing graylevel mapping.     

Experimental ultrasound contrast study of the myocardium]

The ultrasound contrast medium obtained by the original methods was administered into the left ventricular cavity and myocardium of 12 open-chest dogs by using a catheter. After its administration into the cavity there was its intensive contrast. When the ultrasound contrast was administered into the aortic root, the entire myocardium contrasted, on selective administrations of the contrast into the coronary arteries, the beds supplied by appropriate arteries contrasted. The ultrasound contrast study enabled the areas with impaired perfusion as echo-negative "filling defects" to be detected and mapped. The imaging of myocardial blood flow in tomographic sections and real time allows one to regard it promising for clinical use.     

Transmit aperture processing for nonlinear contrast agent imaging

Nonlinear contrast agents, such as bubbles, are used in ultrasound to enhance backscatter from blood. To increase contrast between these agents and tissue, nonlinear imaging methods, such as harmonic imaging or difference frequency imaging, can be used. For these, power is transmitted at one frequency and received at a different frequency. Nonlinear imaging methods, however, suffer from reduced contrast because of signal transmitted by the array at the receive frequency that linearly reflects off tissue. In this paper, transmit aperture processing is proposed to improve contrast in nonlinear imaging by suppressing signal transmission at the desired receive frequency. A specific method, known as alternate phasing, is presented as an example of this approach. Simulation results show that alternate phasing greatly improves contrast between tissue and bubbles.     

Intestinal and peritoneal bleeding: detection with an intravascular contrast agent and fast three-dimensional MR imaging--preliminary experience from an experimental study

We investigated transcobalamin II (TC) isoelectrofocusing (IEF) phenotype and codon 259 polymorphism, in Caco-2 and HT-29 cells and in blood drawn from 39 healthy Caucasians. Caco-2 cells expressed a single TC variant (259-Arg), while HT-29 cells expressed TC with either Arg or Pro at codon 259 and exhibited two isoproteins in IEF with urea, but only one in IEF without urea. Among the Caucasians, 7 subjects expressed the TC 259-Arg variant, 10 the 259-Pro variant, and 22 were heterozygous. The TC 259-Pro isoprotein issued from HT-29 cells and heterozygous caucasian sera, was, respectively, 2. 4-fold and 1.6-fold higher than the TC 259-Arg isoprotein. Apo-TC and vitamin B12 serum concentrations in 259-Pro homozygotes were, respectively, 1.7 and 1.4-fold higher than those in 259-Arg homozygotes (p<0.005 and p=0.05). In conclusion, the 259-Arg/Pro polymorphism yields two TC variants only titratable in denaturing conditions and affects the blood level of both Apo-TC and vitamin B12.     

Small vessels in the human brain: MR venography with deoxyhemoglobin as an intrinsic contrast agent

To assess a magnetic resonance (MR) imaging method for depicting small veins in the brain, a three-dimensional, long echo time, gradient-echo sequence that depended on the paramagnetic property of deoxyhemoglobin was used. Veins with diameters smaller than a pixel were depicted. This MR imaging method is easy to implement and may prove helpful in the evaluation of venous diseases.     

Use of an intravascular T1 contrast agent to improve MR cine myocardial-blood pool definition in man

The feasibility of improving myocardial/blood pool contrast in MR cine images through use of an intravascular contrast agent (Ferumoxtran, Advanced Magnetics, Inc., Cambridge, MA) was tested in four subjects. The contrast-to-noise ratio (CNR) demonstrated a trend toward improvement in the short axis and improved significantly in the long axis cine by an average of 128% (P < .05). Image intensity gradients at the myocardial/blood pool interface increased significantly in both the short and long axis (P < .01). It is expected that larger image intensity gradients at the endocardial border should improve the capabilities of automated segmentation algorithms, reducing the uncertainty and need for manual editing.     

Dopamine-containing gadolinium complex as magnetic resonance imaging contrast agent

Gadolinium diethylenetriamine-pentaacetic bisdopamide (Gd-DTPA-2DA) was synthesized by the incorporation of dopamine to diethylenetriaminepentaacetic acid (DTPA) and further reaction with gadolinium chloride. In vitro and in vivo properties were also evaluated. Gadolinium complex Gd-DTPA-2DA possessed higher relaxation effectiveness and less cytotoxicity to HeLa cells than that of Gd-DTPA. Moreover, Gd-DTPA-2DA greatly enhanced the contrast of MR images of the brains, provided prolonged intravascular duration, and produced highly contrasted visualization of the brain.     

Transcutaneous interruption of ultrasound contrast agents for blood flow evaluation

RATIONALE AND OBJECTIVES: The ability to create short boluses in targeted arteries with rapid rise times is limited by the transport of bubbles from the venous to arterial portion of the circulation. Acoustic interruption of contrast agent in arteries may create the short boluses necessary for simple wash-in/wash-out measures of blood flow. METHODS: An ultrasound contrast agent was used with spectral Doppler ultrasound to observe contrast interruption in femoral arteries and VX2 carcinoma in a rabbit model. At an upstream location in the femoral artery, single, sinusoidal ultrasound tone bursts at 1.8 MHz with durations of 0.25 to 1 seconds were applied to interrupt the flow of contrast agent injected intravenously. RESULTS: In VX2 carcinoma, bursts as short as 40 cycles produced contrast interruption lasting only one cardiac cycle within the tumor periphery and I(SPPA) <3 W/cm2 produced measurable interruptions. CONCLUSIONS: Acoustic fields applied transcutaneously interrupted flow of contrast agents to form temporally short negative boluses.     

Relation of contrast echo intensity and flow velocity to the amplification of contrast opacification produced by intermittent ultrasound transmission

Intermittent ultrasound transmission during contrast echocardiography, so-called transient response imaging (TRI), amplifies contrast intensity. This effect of TRI is attributed to decreased microbubble destruction by reduced exposure time to ultrasound energy. The present study examined the hypothesis that the signal amplification produced by TRI is related to the baseline intensity present in the image and the velocity of flow. We performed second harmonic (2.5/5.0 MHz) imaging during both continuous (frame rate 55 Hz) and electrocardiogram-triggered TRI mode. Contrast images produced by perfluorohexane microbubbles (AF0150) in a steady flow model were obtained every minute throughout the decay phase at transit velocities of 8.1, 6.2, 3.4, 1.9, and 0.7 cm/sec. The decay of videointensity over time could be fitted to a sigmoid curve for both imaging modes with r > 0.99 for individual velocities. The intensity with TRI was greater than that with continuous imaging (CI) at any time and velocity. The mean increase in intensity between modes throughout decay was 8.2 +/- 3.7, 12.8 +/- 4.2, 25.7 +/- 5.8, 49.5 +/- 8.0, and 64.0 +/- 14.4 gray levels for the respective velocity levels studied (p < 0.0001). Although varying with baseline intensity at early and late phases, the TRI amplification plateaued during middecay, and within the intensity range of 16 to 143 gray levels for CI and 67 to 186 gray levels for TRI, it showed no overlap among the different velocity levels. Thus the ability of TRI to enhance contrast opacification is much greater at low flow velocities, which has implications regarding the mechanism of TRI effect and preferential visualization of intramyocardial coronary arteries by this agent. Although this effect was influenced by the baseline intensity, it was relatively constant for each velocity level within an optimal intensity range during middecay, providing the basis for flow velocity measurement by contrast echo.