Bubble Physics, Pharmacology, Safety
Contrast on echocardiograms by injecting hand agitated saline solution has been recognized for over 30 years for its ability to opacify vascular structures . The primary mechanism by which injection of such fluids produces ultrasound contrast was determined to be increased backscatter from inclusions of microbubbles (MBB) within the injectant . This way MBB markedly enhanced the blood echo by introducing multiple liquid-gas interfaces. Shortly thereafter, smaller MBB were produced  and were rapidly adopted for intracoronary injections in animals and humans . Early attempts to encapsulate the bubbles resulted in agents with improved stability but of a size too large to traverse the pulmonary microvasculature. Therefore, early contrast echocardiography by intravenous injection was used primarily to detect cardiac shunts or examine right heart structures.
The first commercial contrast agents, developed by sonication of 5% human albumin solution, produced excellent myocardial opacification on intracoronary injection but poor left ventricular opacification (LVO). Those MBB of room air, which were small enough to pass through the microcirculation (red blood cell size; i.e. <8 µm), dissolved rapidly in blood. Consequently their size decreased and they lost their echogenicity. They became commercially available in Europe in 1991 first as Echovist (Berlex, Lachine, Quebec City, Canada). In 1996, Levovist with galactose microcrystals and with a trace of palmitic acid appeared on the market (Bayer Shering, Berlin Germany). In the U.S. in 1994, Albunex was introduced (Mallinckrodt, St. Louis, Missouri).
They were followed by second generation contrast agents in which the air in the bubble was changed to higher-molecular-weight gases (such as fluorocarbons) which resulted in more stable bubbles. Being insoluble in blood, the gas, even when it had escaped from the bubble, continued to produce effective ultrasound backscatter by acting as a free gas bubble . These new preparations were highly successful in opacifying the left ventricular (LV) cavity and the myocardium from a venous injection. Optison (GE Healthcare, Chalfont St Giles,UK), Definity (BMS, Billerica, Massachusetts) and Sonovue (Bracco, Milan, Italy) are the main representatives of this group. These MBB do not aggregate, are biologically inert and safe . They remain entirely within the vascular space  have an intravascular rheology that is very similar to that of erythrocytes and are eliminated from the body via the reticuloendothelial system with their gas escaping from the lungs.
Interaction contrast agents and ultrasound
When ultrasound waves encounter a microbubble, it alternately compresses and expands the microbubble, depending on the applied acoustic pressure. Therefore the MBB becomes symmetrically larger and smaller in response to the oscillations of the pressure caused by the incident wave. The volume expansion of a MBB is maximal at a specific frequency referred to as the natural resonant frequency and is inversely related to its size. At the resonant frequency the MBB scatters and absorbs ultrasound and can present nonlinear vibrations when the insonifying acoustic pressure is high enough. Consequently, the bubble vibration contains second and higher multiples of the transmitted frequency. Therefore, the backscattered signal from the MBB contains not only the fundamental frequency but also harmonic frequencies, most notably at twice the fundamental frequency (second harmonics). This non linear reflection is not shown by tissue, allowing the separation of response from the bubble from that of surrounding tissue. Finally, as the peak pressure becomes more intense, many of these MBB are disrupted, exhibiting an irreversible, transient and intense scattering depending on the type of gas released and its dissolution in the liquid. This scattered signal is also highly nonlinear. The parameter which expressed the energy of the ultrasound beam is the mechanical index (MI). It reflects the approximate exposure to ultrasound pressure at the focus of the beam in an average tissue.
|Acoustic Power (MI)||Pressure@1MHz||MBB behaviour||Acoustic Behaviour|
|Low(<0.1)||<100 Kpa||Linear Oscillation||Linear increase in signal|
|Medium (0.1 -1)||100 Kpa -1 MPa||Non Linear Oscillation||Signal increase detected with harmonic|
|High (>1.0)||>1MPa||Destruction||High intensity (Flash)|
The influence of the response of the MBB to ultrasound waves has led to different strategies to visualize the MBB in echo images. These techniques, exploiting the harmonics, have been implemented in commercial clinical ultrasound systems.
Imaging Strategies for contrast echocardiography
Continuous insonification with harmonic imaging
Processing only signals returned at the second harmonic of the fundamental frequency significantly improves left ventricular opacification. However, it offers little benefit for myocardial perfusion, with ongoing bubble destruction and sub-optimal differentiation between contrast and tissue, because both produce harmonics at high power.
High Power Techniques – Intermittent Imaging Triggered harmonic imaging
Allow a better myocardial opacification than continuous imaging, with the best opacification obtained using intermittent harmonic imaging. During intermittent high power imaging, high energy ultrasound is transmitted at specified intermittent intervals (PI), triggered to the ECG (eg once every 4 cardiac cycles; 1:4 triggering). The time between destructive pulses allows the MBB to replenish the myocardium. With each destructive pulse, high amplitude backscatter rich in harmonics is returned to the transducer, enabling static images of myocardial perfusion that can be measured as videointensity (VI). Moreover, by incrementally increasing the PI (continuous → 1:1 → 1:2 → 1:4 → 1:8 etc), the rate of replenishment of the ultrasound beam over time can be assessed both qualitatively and quantitatively. The qualitative assessment of perfusion using intermittent harmonic imaging can be improved by digital subtraction of the myocardial signal at baseline, and optimized by colour coding techniques to allow better extraction of bubble signals. However, this approach requires careful superimposition of each frame, and the processing has to be performed off-line.
Power Doppler technology
This technique is designed to detect motion of blood or tissue, and is used in myocardial contrast echo to overcome the need for off line digital subtraction. As with traditional Doppler, two or more pulses are sent successively along each scan line of the image and the pairs of echo trains are compared for the presence or absence of a frequency shift (indicating movement). However, unlike traditional Doppler, power Doppler ignores the direction and velocity of the moving structure. If a frequency shift is detected, indicating motion of the structure, then colour is displayed as an overlay whose saturation is related to the amplitude of the echo which has moved. If no frequency shift (movement) is detected, no colour is displayed. This method is ideally suited to high mechanical index destructive imaging, as the first pulse destroys the myocardial MBB, generating a brief, high amplitude echo rich in harmonics. The second pulse finds that the bubbles have 'moved', and thus colour is overlaid on the echo image over the areas of myocardium that contained MBB. In an area with no MBB, there is no 'movement' recognised and there is no colour overlay applied to that region. The technique is not detecting movement of the bubbles in the circulation, but rather their destruction.
This technique is another grey scale mode using high MI whereby two beam mode pulses are sent in rapid succession into the myocardium. The second pulse is a mirror image of the first (i.e. 180° phase shift). The scanner processes the consecutive returning pulses by adding them together. Tissue generates a linear echo, thus the addition of one pulse to the other should cancel out to zero and no signal is generated. MBB produce non-linear echo signals and the summation of returning pulses will not equal zero and a signal will be registered. Using this technique, processing can theoretically be limited only to signals generated by bubbles.
Low Power Techniques – Real Time Imaging
These are technologies developed to examine the non-linear responses of MBB. Even low amplitude MBB backscatter can be isolated from tissue signals for processing. This allows continuous low power imaging to be performed, with limited bubble destruction, enabling simultaneous assessment of wall motion and perfusion in real time (although frame-rate is minimized in order to reduce bubble destruction). Two types of techniques are actually available. Power Pulse Inversion imaging combines the non-linear detection performance of pulse inversion with the motion discrimination capability of power Doppler. Multiple transmit pulses of alternating polarity are used, and Doppler signal processing techniques are applied to distinguish between bubble backscatter and backscatter from tissue. In a typical configuration, echoes from a train of pulses are combined in such a way that signals from moving tissue are eliminated. Power Modulation Imaging uses the same signal subtraction principles, with the transmitted pulses identical in phase but of different amplitude or 'power', one impulse of full power, the other half that power. Echoes reflected from stationary tissue are linear, thus if we subtract two times the lower power from full power, the signals should cancel out whereas a non-linear oscillation of MBB will generate a signal. In power modulation, fundamental imaging is most suited, because the tissue subtraction technique is so effective that the best signal to noise ratio from contrast to tissue occurs at the fundamental frequency.
Safety concerns of contrast agents in echocardiography
In cardiac applications, the reporting of some adverse events after UCA has led to some restriction imposed by the European Medicines Agency (EMEA) and the U.S. Food and Drugs Administration (FDA). Due to clinical evidence showing that contrast echocardiography was safe in practice several contraindications were removed by the FDA in May 2008, and replaced by warnings on the use of UCA. FDA has continued to highlight the risk of serious cardiopulmonary reactions during or within 30 minutes following the administration of these products and recommended that high risk patients with pulmonary hypertension or unstable cardiopulmonary conditions be closely monitored during and for at least 30 minutes post administration of these contrast agents.
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- ↑ Klibanov AL, Hughes MS, Wojdyla JK, Wible JH, Jr., Brandenburger GH. Destruction of contrast agent microbubbles in the ultrasound field: the fate of the microbubble shell and the importance of the bubble gas content. Acad Radiol 2002;9 Suppl 1:S41-5.
- ↑ Postema M, van Wamel A, ten Cate FJ, de Jong N. High-speed photography during ultrasound illustrates potential therapeutic applications of microbubbles. Med Phys 2005;32:3707-11.
- ↑ Skyba DM, Camarano G, Goodman NC, Price RJ, Skalak TC, Kaul S. Hemodynamic characteristics, myocardial kinetics and microvascular rheology of FS-069, a second-generation echocardiographic contrast agent capable of producing myocardial opacification from a venous injection. J Am Coll Cardiol 1996;28:1292-300.
- ↑ Lindner JR, Ismail S, Spotnitz WD, Skyba DM, Jayaweera AR, Kaul S. Basic Science Reports - Albumin Microbubble Persistence During Myocardial Contrast Echocardiography Is Associated With Microvascular Endothelial Glycocalyx Damage. Circulation - Hagertown 1998;98:2187-2194.
- ↑ 10.0 10.1 Lindner JR, Song J, Jayaweera AR, Sklenar J, Kaul S. Microvascular rheology of Definity microbubbles after intra-arterial and intravenous administration. J Am Soc Echocardiogr 2002;15:396-403.
- ↑ Jayaweera AR, Edwards N, Glasheen WP, Villanueva FS, Abbott RD, Kaul S. In Vivo Myocardial Kinetics of Air-Filled Albumin Microbubbles During Myocardial Contrast Echocardiography: Comparison With Radiolabeled Red Blood Cells. Circulation Research 1994;74:1157-1165.
- ↑ Keller MW, Segal SS, Kaul S, Duling B. The behavior of sonicated albumin microbubbles within the microcirculation: a basis for their use during myocardial contrast echocardiography. Circ Res 1989;65:458-67.
- ↑ Medwin H. Counting bubbles acoustically: a review. Ultrasonics 1977;15:7-13.
- ↑ Porter TR, Xie F, Kricsfeld D, Armbruster RW. Improved myocardial contrast with second harmonic transient ultrasound response imaging in humans using intravenous perfluorocarbon-exposed sonicated dextrose albumin. J Am Coll Cardiol 1996;27:1497-501.
- ↑ Kaul S, Senior R, Dittrich H, Raval U, Khattar R, Lahiri A. Detection of coronary artery disease with myocardial contrast echocardiography: comparison with 99mTc-sestamibi single-photon emission computed tomography. Circulation 1997;96:785-92.
- ↑ Becher H, Tiemann K, Schlief R, Lüderitz B, Nanda NC. Harmonic Power Doppler Contrast Echocardiography: Preliminary Experimental Results. Echocardiography - A Journal of Cardiovascular Ultrasound and Allied Techniques 1997;14:637-642
- ↑ Heinle SK, Noblin J, Goree-Best P, Mello A, Ravad G, Mull S, Mammen P, Grayburn PA. Assessment of myocardial perfusion by harmonic power Doppler imaging at rest and during adenosine stress: comparison with (99m)Tc-sestamibi SPECT imaging. Circulation 2000;102:55-60.
- ↑ Porter TR, Xie F, Silver M, Kricsfeld D, Oleary E. Real-time perfusion imaging with low mechanical index pulse inversion Doppler imaging. J Am Coll Cardiol 2001;37:748-53.
- ↑ Dawson D, Rinkevich D, Belcik T, Jayaweera AR, Rafter P, Kaul S, Wei K. Measurement of myocardial blood flow velocity reserve with myocardial contrast echocardiography in patients with suspected coronary artery disease: comparison with quantitative gated Technetium 99m sestamibi single photon emission computed tomography. J Am Soc Echocardiogr 2003;16:1171-7.
- ↑ FDA. United States Food and Drug Administration Center for Drug Evaluation and Research alert for healthcare providers Available at http://www.fda.gov/cder/drug/infopage/microbubble/default.htm. (2008)