The nonlinear ultrasound needle pulse

Area of Science:

• Acoustics
• Nonlinear Optics
• Medical Imaging

Background:

• Needle fields are analytically formulated broadband wave fields that converge to a narrow axial line.
• Their unique properties stem from a constant forward propagating wavenumber across all frequencies.
• Applications in medical imaging are being explored due to their focusing capabilities.

Purpose of the Study:

• To investigate the nonlinear propagation of a 3 MHz needle field in a water medium.
• To compare the focal characteristics of the nonlinear needle field with a Gaussian beam.
• To assess the inheritance of linear needle field properties by higher harmonics in nonlinear propagation.

Main Methods:

• A finite amplitude distorted simulation was employed for the 3 MHz needle field.
• The simulation was conducted in a water path relevant to medical imaging.
• Focal features were compared against a comparable Gaussian beam.

Main Results:

• Nonlinear propagation of the needle field retains its narrow, extended axial convergence for higher harmonics.
• Short-duration focal pulses and asymptotic radial profile declines observed in linear needle fields are also present in higher harmonics.
• Needle fields exhibit unique and potentially advantageous features compared to Gaussian beams.

Conclusions:

• Needle fields demonstrate robust convergence properties under nonlinear propagation conditions.
• These fields offer distinct advantages over Gaussian beams for applications requiring precise focusing.
• The findings suggest significant potential for needle fields in advanced medical imaging techniques.