Intense waves for biomedical ultrasound applications

High intensity focused ultrasound fields are characterized in the focal area by the appareance of nonlinear acoustic effects such as harmonic generation and self-(de)focusing. Applications, in the weakly nonlinear regime, include the characterization of the acoustic field for ultrasound-induced Blood-Brain Barrier opening, a emerging technique for localized, transient, noninvasive and safe targeted drug delivery in the brain. In the highly nonlinear regime applications include the use of High Intensity Focused Ultrasound for thermal ablation and hypertermia applications.


Nonlinear acoustic beams

I'm studiying the phenomenon of the displacement of the position of the pressure, intensity and acoustic radiation force maxima along the axis of high intensity focused acoustic beams. Here, we study the case of a moderately focused beams excited with continuous and 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 behaviour, 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.

pdflinkdoi Dynamic Nonlinear Focal Shift in Amplitude Modulated Moderately Focused Acoustic Beams
N. Jiménez, F. Camarena, N. González-Salido,
Ultrasonics, 75, pp 106–114, (2016)

pdf link doi Nonlinear focal shift beyond the geometrical focus in moderately focused acoustic beams
F. Camarena, S. Adrián-Martinez, N. Jiménez, V. Sánchez-Morcillo,
The Journal of the Acoustical Society of America, 134 (2), pp 1463-1472, (2013)

pdf link doi Time-Domain Simulation of Ultrasound Propagation in a Tissue-Like Medium Based on the Resolution of the Nonlinear Acoustic Constitutive Relations
N. Jiménez, F. Camarena, J. Redondo, V. Sánchez-Morcillo, Y. Hou, E. E. Konofagou,
Acta Acustica united with Acustica, 102 (5), pp 876–892, (2016)

Transcranial propagation

Some neurodegenerative diseases as Alzheimer's or Parkinson's are supposed to be treated by using focused ultrasound therapy and microbubble injection, aiming the BBB (Blood Brain Barrier) opening to have access to concrete zones of brain and then to effectively deliver therapeutical drugs. In this line I'm working in collaboration with the Ultrasound Medical and Industrial Laboratory (UMIL)Universitat Politècnica de València (Spain) and Columbia University (USA) to study the acoustic aberrations produced in trasncranial propagation and methods to correct them. The work in this line include numerical simulations of acoustic focused fields into the skull in order to control the spatial characteristics of the focal area, i.e. a better control of the targeting.

In the right animation you can see an example of the transcranial beam propagation generated by a monoelement 500 kHz transducer, obtained by numerical simulation by our studient Sergio Jiménez Gambín who is working in this topic. The simulations, based on the k-space method, allow to evaluate the beam aberrations and predict preciselly the central nervous system area to be treated.

pdf link doi Estudio de la propagación no lineal de haces de ultrasonidos focalizados a través del cráneo para la apertura de la barrera hematoencefálica
N. Jiménez, F. Marquet, F. Camarena, E. E. Konofagou, J. Redondo, B. Roig, R. Picó,
Revista de Acustica, 43 (1), pp 5-11, (2012)

pdf link doi Transcranial Propagation with an Ultrasonic Mono-Element Focused Transducer
P. C. Iglesias, N. Jiménez, E. Konofagou, F. Camarena, J. Redondo
Physics Procedia., 63, pp 103-107, (2014)

Nonlinear acoustic radiation forces

I'm studing the finite-amplitude dependence of the acoustic radiation force (ARF) in soft tissue-like media. The amplitude dependent ARF and its spatial distribution is used to correct elasticity imaging techniques as Harmonic Motion Imaging to precisely estimate the elasticity of the tissue. The results obtained show that the nonlinear dependence of the acoustic radiation force highly depends on the tissue attenuation frequency dependence for both, weakly and strong nonlinear regime. We also show the underestimation of the momentum transfer in the nonlinear regime by calculating the ARF by using the wave intensity. Considerations due to nonlinear absorption are included and the relation between the momentum transfer and energy deposition is underlined.

pdflinkdoi Dynamic Nonlinear Focal Shift in Amplitude Modulated Moderately Focused Acoustic Beams
N. Jiménez, F. Camarena, N. González-Salido,
Ultrasonics, 75, pp 106–114, (2016)

Thermal patterns

The knowledge of the shape of thermal patterns is essential to acurately predict the result of ultrasound therapy techniques for HIFU or hyperthermia applications. Here, I'm working in numerical simulations of the thermal patterns produced by nonlinear acoustic focused beams in practical applications.

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