Contrast Agents for Ultrasound

Free Gas Bubbles

Gramiak and Shah first used bubbles to enhance the blood echo in 1968. They injected agitated saline into the left ventricle during an echocardiographic examination and saw strong echoes within the lumen of the aorta. It was subsequently shown that these echoes originated from free bubbles of air that came out of solution either during agitation or at the catheter tip during injection. Agitated solutions of compounds such as indocyanine green and Renografin—already approved for intraarterial injection—were also used. The application of free gas as a contrast agent was confined to the heart, including evaluation of valvular insufficiency, intracardiac shunts, and cavity dimensions. The fundamental limitation of bubbles produced in this way is that they are large, so they are effectively filtered by the lungs, and unstable, so they go back into solution on the order of a second. Apart from occasional use in the echocardiography laboratory to identify shunts, free bubbles are rarely used as a contrast agent today.

Encapsulated Air Bubbles

To overcome the natural instability of free gas bubbles, various shell coatings were developed to create a more stable particle ( Table 3.1 ). In 1980 Carroll and colleagues encapsulated nitrogen bubbles in gelatin and injected them into the femoral artery of rabbits with VX2 tumors in the thigh. Although echo enhancement of the tumor rim was identified, the large diameter of the coated bubbles (80 µm) precluded administration by an intravenous route. The challenge to produce a stable encapsulated microbubble of a comparable size to that of a red blood cell and that could survive passage through the heart and the pulmonary capillary network was first met by Feinstein and colleagues in 1984, who produced microbubbles by sonication of a solution of human serum albumin and showed that it could be detected in the left side of the heart after a peripheral venous injection. This agent was subsequently developed commercially as Albunex (Mallinckrodt Medical, Inc., St. Louis, MO).

Another approach to stabilizing an air bubble is to add a lipid shell upon dissolution of a dry powder. Levovist (Schering AG, Berlin, Germany), is a dry mixture comprising 99.9% microcrystalline galactose microparticles and 0.1% palmitic acid. On dissolving in sterile water, the galactose disaggregates into microparticles, which provide an irregular surface for the adherence of microbubbles 3 to 4 µm in size. Stabilization of the microbubbles takes place as they become coated with palmitic acid, which separates the gas/liquid interface and slows their dissolution. The resulting microbubbles have a median bubble diameter of about 3 µm with the 97th percentile at approximately 6 µm and are sufficiently stable for transit through the pulmonary circuit. The agent is chemically related to its predecessor Echovist (Schering), a galactose agent that forms larger bubbles and that has been used principally for visualization of nonvascular ductal structures such as the fallopian tubes. Numerous early studies with Levovist demonstrated its capacity to traverse the pulmonary bed in sufficient concentrations to enhance both color and spectral Doppler signals, as well as gray-scale examinations using nonlinear imaging modes such as pulse inversion imaging. Levovist remains approved for use in the European Union, Canada, Japan, and numerous other countries, although not the United States. Many clinical applications of intravenous contrast were pioneered using Levovist, which has now given way to the so-called “second-generation” agents and is no longer marketed.

Second-Generation Agents

Second-generation agents were designed both to increase backscatter enhancement and to last longer in the bloodstream by taking advantage of low-solubility gases such as perfluorocarbons. These heavier gases diffuse more slowly through the bubble shell and have much lower solubility in blood. Optison (GE Healthcare, Milwaukee, WI) (see Fig. 3.1A ) is a perfluoropropane-filled albumin shell with a size distribution similar to that of its predecessor, Albunex. It is currently approved for cardiology indications in the European Union, the United States, and Canada. SonoVue (Bracco, Milan, Italy) uses sulfur hexafluoride in a phospholipid shell and is available for cardiology and radiology indications in the European Union, China, and a number of other countries; it is approved in the United States under the name Lumason. Definity (Lantheus Medical Imaging, North Billerica, MA) (see Fig. 3.1B ) comprises a perfluoropropane microbubble coated with a flexible bilipid shell, which also showed improved stability and high enhancement at low doses. It is currently approved for cardiology and radiology indications in Canada, Australasia, and a number of Central and South American countries, and for cardiology indications in the United States. Finally, Sonazoid (Daiichi Sankyo, Tokyo, Japan and GE Healthcare, Milwaukee, WI) consists of a perfluorobutane bubble in a lipid shell and is currently approved for radiology indications in countries such as Japan and Korea. It should be noted that although these bubbles are very small, they are large when compared with the molecules and particles used as contrast agents for computed tomography (CT) and magnetic resonance imaging (MRI), which diffuse through the fenestrated endothelium of blood vessels into the interstitium. Thus x-ray and magnetic resonance contrast-enhanced images frequently show an “interstitial” or “parenchymal” phase of enhancement, which may be used to identify hyperpermeable vascular structures, such as those involved with tumor angiogenesis. Microbubbles, on the other hand, are of a size comparable to that of a red blood cell, so they go where a red blood cell goes (see Fig. 3.2 ) and, more important, do not go where a red blood cell does not go. They are clinical radiology's first pure blood pool contrast agent.

Selective Uptake Agents

An ideal blood pool agent displays the same flow dynamics as blood itself, and is ultimately metabolized from the blood pool. Agents such as Definity, SonoVue, and Optison are generally not detected outside the vascular system, and therefore come close to this ideal. Contrast preparations can be made, however, that are capable of providing ultrasound enhancement during their metabolism as well as while in the blood pool. Colloidal suspensions of liquids droplets such as perfluoroctylbromide and microbubble agents with certain shell properties are taken up by the reticuloendothelial system, from where they ultimately are excreted. There they may provide contrast from within the liver parenchyma, demarking the distribution of Kupffer cells. Agents such as Levovist and Sonazoid provide “late-phase” enhancement in the parenchyma of the liver and spleen after having cleared from the vascular system, allowing detection of Kupffer cell–poor lesions such as cancers.

A typical dose of an ultrasound contrast agent is on the order of 10s of microliters of bubble suspension per kilogram of body weight, so a whole-body dose might be on the order of 0.1 to 1 mL. Fig. 3.3 shows the enhancement of the echo from systemic arterial blood after a peripheral venous injection of a second-generation agent. A first-pass peak occurs, followed by recirculation and washout as the agent is eliminated over the next few minutes. By infusing the bubbles through a saline drip or pump, a steady enhancement lasting up to 20 minutes can be obtained. The small amount of perfluorocarbon gas goes into solution in the blood and is ultimately excreted by the lungs and liver. The trace amount of shell material is reduced to biocompatible elements that, in the case of the commonly used agents, are already present in the blood.