Ultrasound is often the preliminary investigation for many diseases affecting the kidney. The examination is safe, inexpensive, and widely available. Most investigators will refer the patient for CT or MRI if a focal abnormality is found. Contrast-enhanced ultrasound, however, may be a feasible alternative as a problem-solving tool when suspected renal pathology is found.
Contrast-enhanced ultrasound has become established in routine clinical practice for liver disease and its use is expanding into other areas. Published guidelines are available on the modality’s use for the characterization of focal liver lesions as well as its role in other intra-abdominal organs, including the kidney.1
This imaging method offers several practical advantages. Ultrasound contrast agents are simple to use and are well tolerated by patients.2 Real-time imaging often allows a diagnosis to be made promptly. No ionizing radiation is involved, there is no risk of patients suffering claustrophobia, and the cost of the examination is lower than that for CT or MRI. Contrast-enhanced ultrasound may also provide supplementary information not evident on other modalities.
Ultrasound contrast agents consist of air or perfluorocarbon gas microbubbles that are stabilized by a protein, lipid, or polymer shell. The microbubbles typically measure less than 7 ìm in diameter, making them small enough to cross capillary beds but too large to enter interstitial fluid. These properties enable the microbubbles to serve as blood pool agents.
Microbubble contrast agents survive passage through the cardiopulmonary circulation and typically remain in the vascular compartment for six to eight minutes after intravenous injection. This stability allows sufficient time for imaging. The principal agent used in Europe is SonoVue (Bracco), which is composed of a sulfur(Drug information on sulfur) hexafluoride gas with a phospholipid shell. The microbubbles are metabolized by the liver and the gas is exhaled via the lungs. Other available agents include Definity (Bristol-Myers Squibb), composed of octafluoropropane gas encapsulated in an outer lipid shell, and Optison (GE Healthcare), which has an albumin shell and an octafluoropropane gas core.
