The mechanical index and cavitation in tissue

Abstract

The mechanical index (MI) quantifies the likelihood that exposure to diagnostic ultrasound will produce an adverse biological effect by a nonthermal mechanism. The current formulation of the MI is based on inertial cavitation thresholds in two liquids, water and blood, as calculated by Apfel and Holland [Ultrasound Med. Biol. 17, 179–185 (1991)]. Although tissue contains a high proportion of water, it is not a liquid but a viscoelastic solid. The importance of this difference was studied by deriving a Keller-Miksis-like equation assuming a gas bubble in a linear Voigt solid and performing numerical computations similar to the analytical work underlying the MI. Thresholds for inertial cavitation were determined for pulse lengths of 1–14 acoustic periods, equilibrium bubble radii (Ro) of 0.1–10.0 mm, a frequency range of 0.5–15.0 MHz, 2 threshold (Pt) criteria: Rmax=2Ro, and Tmax=5000 K, and tissue elasticities and viscosities of G=0, 0.5, 1.0 or 1.5 MPa and μ=0.005 or 0.015 Pa⋅s; which span the range of values for soft tissue. It is found that thresholds in tissue are up to 10 times those in liquid, and that Pt increases nearly linearly with frequency. The relevance of these results to ultrasound safety will be discussed.