title: Dynamic nonlinear focal shift in amplitude modulated moderately focused acoustic beams.
abstract: The phenomenon of the displacement of the position of the pressure, intensity and acoustic radiation force maxima along the axis of focused acoustic beams under increasing driving amplitudes (nonlinear focal shift) is studied for the case of a moderately focused beam excited with continuous and 25kHz amplitude modulated signals, both in water and tissue. We prove that in amplitude modulated beams the linear and nonlinear propagation effects coexist in a semi-period of modulation, giving place to a complex dynamic behavior, where the singular points of the beam (peak pressure, rarefaction, intensity and acoustic radiation force) locate at different points on axis as a function of time. These entire phenomena are explained in terms of harmonic generation and absorption during the propagation in a lossy nonlinear medium both for a continuous and an amplitude modulated beam. One of the possible applications of the acoustic radiation force displacement is the generation of shear waves at different locations by using a focused mono-element transducer excited by an amplitude modulated signal.
authors.affiliation: Laboratoire d'Acoustique de l'Université du Maine, UMR CNRS 6613, Av. O. Messiaen, 72085 Le Mans, France.
authors.forename: Noé
authors.initials: N
authors.lastname: Jiménez
authors.affiliation: Instituto de Instrumentación para Imagen Molecular (I3M), Centro Mixto Universitat Politècnica de València CSIC CIEMAT, Camino de Vera s/n, 46022 Valencia, Spain. Electronic address: fracafe@fis.upv.es.
authors.forename: Francisco
authors.initials: F
authors.lastname: Camarena
authors.affiliation: ITEFI, Spanish National Research Council (CSIC), Serrano 144, 28006 Madrid, Spain.
authors.forename: Nuria
authors.initials: N
authors.lastname: González-Salido
keywords: Acoustic radiation force; Biomedical ultrasound; Finite-amplitude acoustic beams; Nonlinear acoustics; Nonlinear focal shift