Overview of echo-enhanced vascular ultrasound imaging for clinical diagnosis in neurosonology

Abstract

Small gas bubbles have been used as ultrasound signal enhancers for many years. However, only recently have microbubble preparations become available that provide useful and reproducible enhancement on ultrasound scans after i.v. administration. Levovist is a galactose/palmitic acid-based agent that, on dissolution and agitation in sterile water, generates air-filled microbubbles with a palmitic acid coating and a median diameter of 2 microns. These lipid-coated microbubbles are sufficiently small and stable to survive the cardiopulmonary circulation and to enhance the entire blood pool. In Doppler, these microbubbles increase the signal by 10 dB to 30 dB, enabling detection of flow in situations for which attenuation would otherwise render impossible, e.g., transcranially approached intracranial vessels. Echo enhancement can increase the technical success rate of the examination, reduce examination time, and allow color imaging and pulsed Doppler detection of a greater number of vessels. The low volume of flow in the preocclusive carotid lesion, whose unsatisfactory detection often necessitates angiography, is a second area in which these agents may make an impact. Other applications, such as using the agent as an indicator for bolus dilution estimates of flow and transit time, remain to be exploited. However, certain problems remain. The bolus character of the enhancement presents a challenge to the clinical practice of ultrasound, in which Doppler interrogation over tens of minutes is commonplace; perhaps infusions may be necessary in the future. Detection of smaller volumes of blood in smaller vessels is determined not only by strength of the enhanced Doppler signal from blood but also on interference from the much larger echo from slowly moving tissue. This forces use of a clutter or "thump" filter, which also eliminates the detected slow flow. Echo-enhancing agents combined with harmonic imaging-a new method that exploits the nonlinear oscillation of the bubbles in the ultrasound field-offers an entirely new way to suppress the effects of clutter and hence to detect flow in very small vessels, even in moving tissue.