Destruction Of Contrast Microbubbles Research Articles

Update on ultrasound brain perfusion imaging

Several studies have demonstrated the value of ultrasound perfusion imaging to visualize the area of perfusion deficit in patients with acute ischemic stroke. Triggered high mechanical index (MI) imaging, which uses contrast microbubble destruction to analyze bolus contrast kinetics in the brain parenchyma, was used in these studies. Recently high sensitive, low MI imaging was introduced. With this new technology real-time bolus kinetics as well as refill kinetics could be analyzed without triggering. In the early phase of ischemic stroke, ultrasound perfusion imaging is useful in detecting the area of perfusion deficit and to assess outcome prognosis of the patient. This bedside technology is available for use in the stroke unit when patients with acute ischemic stroke undergo a color-coded duplex work-up to evaluate their vascular status.

Enhancement of subharmonic emission from encapsulated microbubbles by using a chirp excitation technique

Subharmonic contrast imaging promises to improve ultrasound-imaging quality by taking advantage of an increased contrast to tissue signal. However, acoustic pressures beyond the subharmonic generation threshold using common ultrasound pulses may induce significant contrast microbubble destruction. In this work, a chirp excitation technique is presented to enhance the subharmonic emission from encapsulated microbubbles. Chirp signals with a center frequency of 5 MHz, variable frequency range and duration time are employed to drive microbubbles in numerical simulation and experimental studies. We provide a theoretical evaluation of the chirp excitation pressure threshold and the acoustic pressure dependence of subharmonic based on Church's model and demonstrate that the amplitude and axial resolution of the subharmonic can be optimized by proper selection of the frequency range and time duration of the chirp signal. Measurements are qualitatively in agreement with the simulation. Moreover, we demonstrate that chirp excitation may be able to improve the amplitude of the subharmonic component up to 22 dB over the pulse excitation. The chirp excitation technique could potentially be used for improving the subharmonic contrast imaging quality.

Triggered Replenishment Imaging Reduces Variability of Quantitative Myocardial Contrast Echocardiography and Allows Assessment of Myocardial Blood Flow Reserve

Assessment of replenishment kinetics (RK) following ultrasound-induced destruction of contrast microbubbles allows quantification of myocardial blood flow reserve (MBFR) applying the model f (t) = A (1 - e(-betat)), with parameter beta describing mean flow velocity and parameter A representing blood volume. However, few data on the variability and reproducibility of RK in a clinical setting are available. Therefore, we examined 30 patients in a rest-adenosine protocol in one center. Off-line quantification of real-time perfusion imaging (RTPI) and triggered replenishment imaging (TRI) was performed at two sites and compared with coronary angiography and flow reserve measurements. Parameter A was found to be robust in all investigated segments (coefficient of variation (CV) < 7.2%+/- 5.1). Variability was lowest for parameter beta using TRI in apical segments (CV 6.5%+/- 5.2, P < 0.01). Highest CV was found with RTPI in lateral segments (CV : 39.8%+/- 40.6). Concerning day-to-day reproducibility both methods revealed similar results within range of heterogeneity of myocardial blood flow. Both sites obtained significantly lower MBFR in patients with flow-limiting CAD, compared to subjects without (P < 0.01). Correlation of both sites showed close relationship (y = 0.88x + 0.45, r = 0.83, P < 0.0001), without systematic bias. TRI significantly reduces variability of RK in quantitative MCE. Assessment of MBFR allows investigator-independent evaluation of CAD.

The influence of acoustic transmit parameters on the destruction of contrast microbubbles in vitro

In this study, the destruction of the contrast agent Sonazoid (GE Healthcare, Oslo, Norway) was measured in vitro as a function of centre frequency (2–3 MHz), acoustic amplitude (0.66–1.6 MPa), pulse length (2–16 cycles) and PRF (0.5–8.0 kHz). Up to 82% of microbubbles were destroyed after exposure to a single 1.6 MPa acoustic pulse (16 cycles, 2.5 MHz and PRF of 1.0 kHz), while at a low amplitude of 0.66 MPa, fractional destruction increased gradually from 0 to 40% after exposure to 9 (identical) pulses. Fractional destruction increased from approximately 8 to 66% as pulse length was changed from 2 to 16 cycles following exposure to a single 2.5 MHz, 1.3 MPa pulse. As the PRF was increased from 0.5 to 8.0 kHz, shorter exposure time intervals (from 4.8 to 1.2 ms) were needed to achieve the same fractional destruction of 80%. Conversely, as the transmit frequency was increased from 2 to 3 MHz the fractional destruction decreased (by more than half within the first 3 pulses). The influence of changes in acoustic pressure and duty cycle on the destruction of Sonazoid microbubbles was highly statistically significant (p ⩽ 0.01) with a threshold around 0.67 MPa for a duty cycle of 0.0064. In conclusion, the fractional destruction increases with the duty cycle and the acoustic pressure amplitude and decreases with ultrasonic transmit frequency. Better understanding of the influence of the ultrasound transmit parameters on the destruction of contrast microbubbles should help improve existing contrast-assisted imaging modalities and may help develop new techniques for better use of contrast agents.

Ultrasound-mediated destruction of contrast microbubbles used for medical imaging and drug delivery

Micron-size bubbles encapsulated by a stabilizing layer of surface-active materials are used in medical ultrasound imaging and drug delivery. Their destruction stimulated by ultrasound in vivo plays a critical role in both applications. We investigate the destruction process of microbubbles in a commercially available contrast agent by measuring the attenuation of ultrasound through it. The measurement is performed with single-cycle bursts from an unfocused transducer (with a center frequency of 5MHz) for varying pressure amplitudes at 50-, 100-, and 200-Hz pulse repetition frequencies (PRF) with duty cycles 0.001%, 0.002%, and 0.004%, respectively. At low excitation, the attenuation is found to increase with time. With increased excitation level, the attenuation level decreases with time, indicating destruction of microbubbles. There is a critical pressure amplitude (∼1.2MPa) for all three PRFs, below which there is no significant bubble destruction. Above the critical pressure amplitudes the rate of destruction depends on excitation levels. But at high-pressure amplitudes the destruction becomes independent of excitation pressure amplitude. The results are interpreted to identify two different mechanisms of bubble destruction by its signature in attenuation, namely, slow dissolution by diffusion and catastrophic shell rupture. The different modes are discussed in detail with their implications in medical applications.

Destruction of contrast microbubbles used for ultrasound imaging and drug delivery

Microbubble contrast agent destruction by ultrasound pulse is useful in real‐time blood‐flow velocity measurement, stimulating arteriogenesis, or targeted drug delivery. We investigated in vitro destruction of contrast agent Definity (Bristol Meyer‐Squibb Imaging, North Billerica, MA) by measuring attenuation of ultrasound through it. The measurement is performed with single‐cycle bursts for varying pressure amplitudes at 50‐, 100‐, and 200‐Hz pulse repetition frequencies (PRF). At low excitation levels, the attenuation increases with time, indicating an increase in bubble size due to ingress of dissolved air from the surrounding liquid. With increased excitation levels, the attenuation level decreases with time, indicating destruction of microbubbles. A critical pressure amplitude (1.2 MPa) was found for all three PRFs, below which there is no significant bubble destruction. Above the critical excitation level, the rate of destruction depends on excitation levels. But, at high pressure amplitudes destruc...

Targeted Delivery of Bone Marrow Mononuclear Cells by Ultrasound Destruction of Microbubbles Induces Both Angiogenesis and Arteriogenesis Response

Ultrasound (US)-mediated destruction of contrast microbubbles causes capillary rupturing that stimulates arteriogenesis, whereas intramuscular implantation (im) of bone marrow mononuclear cells (BM-MNCs) induces angiogenesis. We therefore studied whether US-targeted microbubble destruction combined with transplantation of BM-MNCs can enhance blood flow restoration by stimulating both angiogenesis and arteriogenesis. US-mediated destruction of phospholipid-coated microbubbles was applied onto ischemic hindlimb muscle and subsequently BM-MNCs were transfused. A significant enhancement in blood flow recovery after Bubble+US+BM-MNC infusion (34% increase, P<0.05) was observed compared with Bubble+US (25%). The ratio of capillary/muscle fiber increased by Bubble+US+BM-MNC-i.v (260%, P<0.01) than that in the Bubble+US group (172%), into which BM-MNCs were incorporated (angiogenesis). Smooth muscle alpha-actin-positive arterioles were also increased, and angiography showed augmented collateral vessel formation (arteriogenesis). Platelet-derived proinflammatory factors activated by Bubble+US induces the expression of adhesion molecules (P-selectin and ICAM-1), leading to the attachment of transplanted BM-MNCs on the endothelium. Flow assay confirmed that the platelet-derived factors cause the adhesion of BM-MNCs onto endothelium under laminar flow. This study demonstrates that the targeted delivery of BM-MNCs by US destruction of microbubbles enhances regional angiogenesis and arteriogenesis response, in which the release of platelet-derived proinflammatory factors activated by Bubble+US play a key role in the attachment of transplanted BM-MNCs onto the endothelial layer.

On the Usefulness of the Mechanical Index Displayed on Clinical Ultrasound Scanners for Predicting Contrast Microbubble Destruction

The purpose of this study was to evaluate the mechanical index (MI) displayed on clinical ultrasound scanners as a predictor of exposure conditions related to the destruction of sonographic microbubble contrast agents. Sonazoid (GE Healthcare, Oslo, Norway) and Optison (GE Healthcare, Princeton, NJ) microbubbles were injected into a tissue-mimicking flow phantom. Gray scale imaging was performed with 4 different scanners and 3 different transducers (3.5 MHz curved linear, 2.5 MHz convex, and 7.5 MHz linear array), and the MI displayed by the scanner was varied from 0.2 to 1.5 by changing the system output power. All other scanning parameters were kept constant. Downstream changes in echogenicity were monitored with a PowerVision 7000 scanner (Toshiba America Medical Systems, Tustin, CA) as an indirect measure of bubble destruction. Video intensity changes within the flow tube were determined as a function of MI for the different scanner/transducer combinations, and the best linear fit was determined. At a displayed MI of 0.7, different scanner/transducer combinations exhibited a range in video intensity from +16% to -3% of baseline for Sonazoid and from +8% to -71% for Optison. At an MI of 0.3, reductions in video intensity of up to 32% were produced. These results indicate a wide range in bubble destruction at identical MI values. Likewise, regression analysis found no linear fits for all scanner/transducer combinations (r2 < 0.046). The MI displayed on clinical ultrasound scanners does not predict the degree of microbubble destruction and should not be used by itself to define exposure conditions for destruction of microbubble contrast agents.

High-speed optical observations of contrast agent destruction

Ultrasound contrast agents are now available since a few years and used for diagnostic purposes. Improved diagnostic decisions have been made possible with new imaging methods that are mainly based on the nonlinear properties of gas microbubbles. Since it is well known that contrast agents are destroyed by ultrasound when the acoustic pressure exceeds a threshold, extremely low acoustic pressures were applied to achieve enhanced contrast image quality. However, destruction of contrast microbubbles is not necessarily undesirable, since it is beneficial in, for example, destruction/reperfusion imaging and recently in drug delivery. We investigate in this experimental study the destruction dynamics of a contrast agent consisting of nitrogen bubbles encapsulated in a double polymer/albumin wall shell. This is accomplished using an ultrafast camera Brandaris that operates at a frame rate of 25 MHz and records 128 frames. The measurements were performed with an ultrasound sine burst of 10 cycles at 1.7 MHz. Different acoustic pressures were applied and various microsphere sizes were examined. The results show three different zones depending on the applied pressure and bubble size: these are nondestruction zone, transient zone and destruction zone. The nondestruction zone is reached for either very small microspheres or low mechanical indices (MI) (<0.3). In the destruction zone lie either large microspheres (5 μm or higher) even when irradiated at low MIs or small microspheres (< 5 μm) when the MI is above 0.6. The optical observations revealed that the destruction of the microspheres is characterized by shell rupture and gas release. The release of the gas gives rise to new free microbubble that lasts for a few milliseconds and then disappears due to dissolution. In the transient zone, the microspheres are mainly compressed in the first few cycles but no expansion is induced. After intense compressions, the shell fissures and gas escapes in the last cycles of the burst or during a second burst depending on the initial size and MI. These optical recordings are important to investigate contrast bubble destruction and can help in amplifying or minimizing this process. Indeed, bubble disruption remains the basis of most current sensitive methods for detecting perfusion with contrast agents and is an essential component of perfusion quantification with microbubbles, in addition to drug delivery applications and pressure measurements. (E-mail: bouakaz@med.univ-tours.fr)

Characterization of focal liver lesions in real time using harmonic imaging with high mechanical index and contrast agent levovist.

The aim of this study was to characterize focal hepatic lesions using agent detection imaging and Levovist. Sixty-five patients (21 male and 44 female; age range, 8-82 years; mean +/- standard deviation, 58.1 +/- 14.5 years) were independently evaluated by two observers in a blinded manner using stored sonographic images. Seventy-five lesions were found: 15 hepatocellular carcinomas, nine focal nodular hyperplasias, two adenomas, 21 hemangiomas, 23 metastases, and five regenerative nodules. Nine patients were excluded (six because of technical failures, three with unproven diagnoses). New high-mechanical-index software was used to reveal power harmonic responses from contrast microbubble destruction. After a venous bolus injection of 4 g of Levovist at a strength of 400 mg/mL, delayed imaging was used to study lesion enhancement in the arterial, portal, and parenchymal phases. Two comparisons were made. The first was between the B-mode image and the first contrast-enhanced image after the flash. The second was between color Doppler sonograms and real-time contrast-enhanced perfusion images. Contrast-enhanced images after the flash and real-time contrast-enhanced images revealed more information for the characterization of the lesion than did gray-scale and color Doppler images (p < 0.0001, Wilcoxon's signed rank test). Different types of lesions showed statistically significant differences in enhancement during each of the three vascular phases (p < 0.005, Kruskal-Wallis test). Lesions with lower contrast enhancement were metastases and regenerating nodules. Good agreement was present between the two observers; differences were not statistically significant (p > 0.05). Agent detection imaging with Levovist increased diagnostic confidence in the characterization of focal hepatic lesions as compared with standard sonography.

Bioeffects of myocardial contrast microbubble destruction by echocardiography.

Microbubble destruction during contrast echocardiography is known to cause capillary leaks and red blood cell extravasation in skeletal muscle. This study evaluated the biological effects of microbubble destruction on cardiac muscle. Contrast echocardiography was performed in 36 rats randomized to receive either Definity or Optison at a mechanical index (MI) of 1.6, 1.2, or 0.8. Myocardial bioeffects were assessed by measuring left ventricular (LV) size and fractional area shortening and histopathology. In addition, blood samples for troponin T were drawn at baseline, postinfusion (30 minutes), day 1, day 4, and day 7. LV size and function were measured at baseline and immediately prior to euthanasia on day 7, after which the heart was removed and sectioned for histopathology. There was no statistical difference in LV size or function regardless of the contrast agent or MI, nor was there any histopathological evidence of myocardial damage. However, troponin T increased over time (F = 3.77, P = 0.012), peaking at 30 minutes and returning to normal by day 4. The difference between Definity and Optison was not statistically significant. However, troponin T values were higher at a higher MI (F = 5.01, P = 0.012). Of 12 rats imaged at a MI of 1.6, 9 (75%) had elevated troponin T as compared to 4 (33%) of 12 at a MI of 1.2. None of the 12 rats imaged at a MI of 0.8 had an elevated troponin T at any time point. Microbubble destruction at high acoustic power (MI 1.6) can cause mild troponin T elevations that are not associated with LV dysfunction or histopathological evidence of myocardial damage.

Destruction of contrast microbubbles by ultrasound: effects on myocardial function, coronary perfusion pressure, and microvascular integrity.

Recent experimental data indicate that ultrasound-induced destruction of ultrasound contrast microbubbles can cause immediate rupture of the microvessels in which these microbubbles are located. To examine the functional and morphological significance of these findings in the heart, isolated rabbit hearts were perfused retrogradely with buffer containing ultrasound contrast agents and were insolated at increasing levels of acoustic energy with a broadband transducer emitting at 1.8 MHz and receiving at 3.6 MHz and operated in the triggered mode (1 Hz). At the end of each experiment, the hearts were fixed in glutaraldehyde and examined with light microscopy. Neither exposure to ultrasound alone or to contrast alone affected left ventricular developed pressure. By contrast, simultaneous exposure to contrast and ultrasound resulted in a reversible, transient mechanical index (MI)-dependent decrease in left ventricular developed pressure (to 83+/-5% of baseline at an MI of 1.6) and a transient MI-dependent increase in coronary perfusion pressure (to 120+/-6% of baseline at an MI of 1.6). Myocardial lactate release also showed significant increases with increasing MIs. Macroscopically, areas of intramural hemorrhage were identified over the beam elevation in hearts exposed to both contrast and high-MI ultrasound. Light microscopy revealed the presence of capillary ruptures, erythrocyte extravasation, and endothelial cell damage. The mean percentage of capillaries ruptured at an MI of 1.6 was 3.6+/-1.4%. Simultaneous exposure of isolated rabbit hearts to ultrasound and contrast agents results in an MI-dependent, transient depression of left ventricular contractile function, a rise in coronary perfusion pressure, an increase in lactate production, and limited capillary ruptures.

Feasibility of the flash-replenishment concept in renal tissue: which parameters affect the assessment of the contrast replenishment?

The purpose of the study was to evaluate whether power pulse inversion (PPI) and pulse inversion (PI) techniques allow the measurement of indices of microcirculatory flow in real-time at low emission power using contrast microbubbles. PPI and PI imaging were performed in a kidney perfusion model during continuous infusion of Definity™ (0.12 mL/min). At steady state of tissue enhancement, contrast was destroyed by emission of echo bursts at high emission power (MI = 1.3). Consecutively, contrast replenishment was assessed at low emission power (MI = 0.09) in real-time imaging modes (PPI: 12 Hz; PI: 25 Hz). Regions-of-interest (ROI) of variable sizes were placed in the renal cortex and bigger arteries to compare replenishment of macro- and microcirculation. Nonlinear curve fitting was performed using the mathematical model y= s+ A(1− e − βt ), with A as the parameter describing blood volume and β as a parameter describing the speed of microbubble contrast replenishment. Replenishment curves could be visually appreciated and quantitatively analyzed in all renal segments. A was significantly higher in bigger arteries compared to renal cortex ( p < 0.001). β was found to be significantly higher in the arteries as compared to the cortex ( p < 0.001). The SD of β diminishes with increasing size of the ROI. The acquisition of replenishment curves following ultrasound (US)-induced destruction of contrast microbubbles is feasible at low power using PPI and PI. Assessment of replenishment kinetics allows the differentiation between macro- and microcirculation. Size and position of the ROI have an important impact on the generation of replenishment curves in both imaging modalities, which has to be taken into account. (E-mail:k-tiemann@Uni-Bonn.de)

Destruction of contrast microbubbles and the association with inertial cavitation

The destruction of insonified Sonazoid® microbubbles and its association with inertial cavitation in vitro utilizing an active acoustic detector was investigated. The experimental observation indicated that contrast microbubbles could be damaged at moderate acoustic pressures of 0.6–1.6 MPa (0.4–1.0 in mechanical index, MI). A damaged bubble could be dissolved into the medium on the order of 1 ms, implying that the destruction at moderate pressures is a relatively slow (relative to inertial bubble collapse), nonviolent dissolution process following the disruption of encapsulating surface materials. Inertial cavitation events in the presence of contrast microbubbles were observed using multiple highly intense ultrasound (US) pulses (>1.6 MPa). This observation suggested that intense US might disintegrate contrast microbubbles, and fragments of disintegrated microbubbles could be activated by an upcoming highly intense imaging pulse. The above results imply that inertial cavitation is unlikely to take place in the presence of Sonazoid® contrast microbubbles when exposed to diagnostic US with an MI <1.

Destruction of contrast microbubbles during ultrasound imaging at conventional power output

Inhomogeneous opacification of cardiac chambers has been frequently observed after intravenous administration of long-persisting echocardiographic contrast agents. We observed this phenomenon to be most pronounced at high acoustic powers with incomplete opacification of the left ventricular apex and left ventricular outflow tract. Reducing the acoustic energy to which the contrast was exposed by decreasing transmit power or intermittently suspending insonification resulted in homogenous opacification of the entire left ventricular cavity. We systematically examined the effect of varying insonification power on the persistence of three investigational ultrasound contrast agents in both in vitro and in vivo models. We found an inverse relationship between the insonifying power and the persistence of the contrast agents. Contrast intensity decay could be reduced either by decreasing exposure to ultrasound by minimizing the transmit power of the system or by intermittently suspending ultrasound generation (triggering). Minimization of ultrasound contrast exposure to ultrasound energy thus improves echocardiographic contrast duration and homogeneity. (J Am Soc Echocardiogr 1997;10:783-91.)