Tissue non-linearity

Abstract

The propagation of acoustic waves is a fundamentally non-linear process, and only waves with infinitesimally small amplitudes may be described by linear expressions. In practice, all ultrasound propagation is associated with a progressive distortion in the acoustic waveform and the generation of frequency harmonics. At the frequencies and amplitudes used for medical diagnostic scanning, the waveform distortion can result in the formation of acoustic shocks, excess deposition of energy, and acoustic saturation. These effects occur most strongly when ultrasound propagates within liquids with comparatively low acoustic attenuation, such as water, amniotic fluid, or urine. Attenuation by soft tissues limits but does not extinguish these non-linear effects. Harmonics may be used to create tissue harmonic images. These offer improvements over conventional B-mode images in spatial resolution and, more significantly, in the suppression of acoustic clutter and side-lobe artefacts. The quantity B/A has promise as a parameter for tissue characterization, but methods for imaging B/A have shown only limited success. Standard methods for the prediction of tissue in-situ exposure from acoustic measurements in water, whether for regulatory purposes, for safety assessment, or for planning therapeutic regimes, may be in error because of unaccounted non-linear losses. Biological effects mechanisms are altered by finite-amplitude effects.

Post-processing enhancement of reverberation-noise suppression in dual-frequency SURF imaging

“…For a recent review of tissue nonlinearity, second-harmonic tissue imaging, and related research, see Ref. 2.…”

Experimental investigation of chirp coded excitation in ultrasound superharmonic imaging

“…SHI relies on the third, fourth and fifth harmonic components of the nonlinear received signal. These higher order harmonic components are produced either due to the nonlinear propagation of ultrasound waves through biological tissue at high acoustic pressure [2] or by the nonlinear backscattering from ultrasound contrast agents insonated at their resonance frequency [3]. SHI provides improved axial and lateral resolution with reduced near-field and reverberation artifacts.…”

The collapse of single bubbles and approximation of the far-field acoustic emissions for cavitation induced by shock wave lithotripsy

“…This is very much less than the range seen in absorption between various tissues. Hence, in translating the nonlinearity propagation seen in water to that seen in tissue for the same beam, for most tissue types it is the change in absorption which is the overwhelming consideration, not the change in B/A (Duck 2010). The absorption coefficients for soft tissue in the literature are for lower signal amplitudes than the ∼1 GPa levels found in this paper, and vary considerably (Burley et al 1980;Goss, Johnston & Dunn 1980;Damianou, Sanghvi & Fry 1997;Bailey et al 2003;Zderic et al 2004;Liu et al 2006;Coussios & Roy 2008).…”

“…The authors could find no information for the clinical scenario they consider, of a spherically spreading blast wave of more than 1 GPa peak overpressure, the Gol'dberg numbers for tissue having been characterized for plane wave propagation of pressure fields with amplitudes typical of those generated directly by clinical diagnostic apparatus (Haran & Cook 1983;Duck 2010). In general, B/A varies only by a factor of around 2 between tissue types for a beam of given amplitude, being similar to water for most liquid tissues like urine, and ranging up to double that of water for fat.…”