Phase and Amplitude Modulation Methods for Nonlinear Ultrasound Imaging With CMUTs

Area of Science:

• Ultrasound physics
• Biomedical engineering
• Acoustic imaging

Background:

• Conventional nonlinear ultrasound imaging relies on linear piezoelectric transducers.
• Capacitive micromachined ultrasonic transducers (CMUTs) exhibit inherent nonlinearity, especially at large signal amplitudes, limiting their use in standard nonlinear imaging.
• Existing methods are incompatible with CMUTs due to their nonlinear behavior.

Purpose of the Study:

• To develop novel pulse sequences for nonlinear ultrasound imaging applicable to both small- and large-signal CMUT operation.
• To compensate for the inherent nonlinearity of CMUTs during ultrasound imaging.
• To enable nonlinear imaging of tissues and ultrasound contrast agents using CMUTs.

Main Methods:

• Developed two-pulse amplitude and phase modulation methods for microbubble and tissue harmonic imaging in small-signal CMUT operation, using subharmonic excitation to compensate for nonlinearity.
• Derived a nonlinear model for large-signal CMUT operation, revealing a linear phase relationship between input signals and harmonic distortion components.
• Proposed an N+1 phase-modulated pulse sequence to extract N harmonic components of nonlinear contrast agent echoes, free from CMUT nonlinearity.

Main Results:

• Demonstrated compensation of CMUT nonlinearity via subharmonic excitation in small-signal operation.
• Validated a linear phase relationship between input signals and harmonic distortion in large-signal CMUT operation.
• Successfully extracted uncorrupted harmonic echoes from nonlinear contrast agents using the proposed phase modulation sequences.
• Experimental validation confirmed the efficacy of the phase modulated sequences for CMUTs in nonlinear collapse-snapback mode and for piezoelectric transducers.

Conclusions:

• The presented pulse sequences effectively enable nonlinear ultrasound imaging with CMUTs, overcoming their inherent nonlinearity.
• The methods are broadly applicable to any CMUT without requiring prior transducer information.
• The phase modulation technique is also adaptable to piezoelectric transducers and other nonlinear systems described by Taylor series expansions.