Image Formation in Vibro-acoustography
Glauber T. Silva, Ph.D. August 2003
In this work, the beamforming and image formation of vibro-acoustography systems are studied. Vibro-acoustography is an imaging technique that maps the mechanical response of an object (or biological tissue) to a localized dynamic ultrasound radiation force. This force is generated by a focused modulated ultrasound beam. Two schemes are used for this purpose: dual and amplitudemodulated single beam modes. The former uses two focused ultrasound beams at slightly different frequencies. The latter uses a focused modulated ultrasound beam whose modulation frequency is much smaller than the carrier frequency. In response to the dynamic radiation force, the object vibrates emitting an acoustic field which can be detected some distance way by a microphone or hydrophone. The detected signal is used to synthesize the image of the object. Dual beam mode presents superior results for vibro-acoustographic images. The point-spread function (PSF) of the system depends on the dynamic radiation force on a point-target in the focal zone of the transducer. Stemming from the Brillouin radiation-stress tensor, the ultrasound radiation force exerted by modulated waves on a point-target is calculated by means of acoustic scattering theory. This formulation generalizes the collimated quasi-plane wave model. Both transverse and axial components of the radiation force can be assessed in this formulation. It is shown that the the collimated quasi-plane wave model is still a good approximation for beamforming in vibro-acoustography. Effects of the transverse component of the radiation force in the spatial resolution of system is discussed. Three different transducers are analyzed with the radiation force formulation proposed here: confocal and sector spherical transducers and linear array transducers. The first two transducers are studied based on analytic results for the acoustic potential on the transducer focal plane. Measurements of the amplitude of the axial and transverse components of radiation force produced by the confocal transducer are in good agreement with theoretical results. Ultrasound waves radiated by linear array transducers are computationally simulated based on the spatial impulse method. More emphasis is given to linear array beamforming due to the potential use of these transducers for clinical applications.
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