The interaction of pulsed ultrasound with mammalian lung

Abstract

The impedance difference for a water-air interface yields virtually complete reflection for ultrasound, which coincidentally delivers an acoustical radiation surface pressure (ARSP). ARSP may be a physical mechanism for pulmonary capillary hemorrhage induced by diagnostic ultrasound. Fundamental tissue parameters in biomedical acoustics are the density ρ, speed of sound c, and the resulting acoustical impedance z=ρc. Many tissues are similar to water (z = 1,628 krayl), but not lung. The gas content of lung imparts an extreme heterogeneity with air having z = 0.424 krayl. If the lung surface is modeled simply as a water-air interface, then the ARSP is maximal. During a diagnostic ultrasound pulse, ARSP can be comparable to the pulmonary capillary blood pressure, thus adding stress to capillaries. However, the bulk properties of the lung are different from air. One of the few reports on acoustical properties of lung was by Floyd Dunn (JASA 1986;80:1248). Consideration of these bulk properties of lung tissue yields a much higher z ≈ 336 krayl at 2.8 MHz, and indicates substantial transmission of ultrasound through the pleura (O’Brien et al. JASA 2000;108:1290). This model suggests a reduced ARSP, and the potential involvement of additional mechanisms for capillary injury within the pulmonary interior.