Why Capillary Beds Remove Ultrasonic Contrast

Abstract

Ultrasonic contrast is an important part of ultrasonography because it improves identification of various interfaces. Complete definition without it is often difficult because only subtle dissimilarities in reflection characterictics exist. A good contrast agent has a vastly different acoustic impedance from the tissue with which it interfaces. Injectable contrast permits a clear partition between the contrast bearing fluid and its surroundings, and hence its diagnostic value. The acoustic impedance difference between injected ultrasonic contrast and surrounding tissue is due to microbubbles of gas [1–5]. Materials and substances other than bubbles have been studied [6, 7], but none have been found to be anywhere near as echogenic as microbubbles. This result may best be understood by inspecting the theoretical and experimental results of Lubbers and Van Den Berg [8] who examined the scattering from spheres for various excitation frequencies. For example, their results show that a 40 μm diameter microbubble excited by a 2 MHz frequency reflects 2× 10⁵ more signal than an equivalent size glass or steel sphere and nearly 4× 10⁵ times as much as an equivalent diameter red cell aggregate. This relative reflection ratio is even more pronounced for smaller bubbles. Microbubbles are thus superior ultrasonic contrast agents because they are superior reflectors: To say ultrasonic contrast is to say micobubbles. To ask the question, “Why do capillary beds remove ultrasonic contrast?” is really to ask the question, “Why do capillary beds remove micro-bubbles?” It is the latter question we wish to address in this chapter.