Fundamentals of contrast agent imaging

Abstract

When a gas bubble is hit by an ultrasound wave, it is forced into volume pulsation. The volume pulsation of the bubble is frequency and amplitude dependent. The acoustic response of the bubbles can be divided into three regimes: linear scattering, nonlinear scattering, and disruption. For small applied acoustic pressure, the bubble echoes are linearly related to the amplitudes of the ultrasound wave. For higher amplitudes, however, compression is smaller than expansion, and the bubble echoes contain second and higher harmonics of the transmitted frequency. If the amplitude of the acoustic wave is increased more, the bubbles are disrupted. Often, the scattering increases abruptly for a short time, and then the bubbles disappear over a short period of time. This is associated with bubble rupture and release of free gas bubbles. The nonlinear harmonics and transient nature of the echoes offer many opportunities and challenges to contrast agent imaging. Highly sensitive and specific imaging methods have been developed based on the behaviours of contrast bubbles in ultrasound fields. We will address the known behaviours of contrast bubbles as well as remaining quandaries in the context of imaging methods. As well, some attention will be given to potential applications in medical therapy.