Harmonic propagation of finite-amplitude sound beams: second harmonic imaging in ultrasonic reflection tomography

In biomedical ultrasound imaging, many studies show the advantages of harmonics to obtain a better quality of image; this is particularly true in echography and diffraction tomography. Recent work confirms that lateral resolution can be improved by the use of the harmonics generated by a nonlinear distortion in the medium. In this study, the enhancement made by the use of the second harmonic in ultrasound reflection tomography is shown. A focused transducer containing both concentric 2.5 and 5 MHz elements composes the experimental device. They are, respectively, the transmitter/receiver of the fundamental field and the receiver of the second harmonic field. The tests are carried out with PVC tubes. The first object is a single tube whereas the second one is an asymmetrical structure made up of two tubes. The image reconstruction is made from the measurements of the fundamental and second harmonics of the reflected fields using the back-projection method. This technique of relying on higher harmonics seems to provide a better resolution of tomographic images.

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