Non-linear propagation for medical imaging

Humphrey, V.F. (2003) Non-linear propagation for medical imaging. In Proceedings of the World Congress on Ultrasonics 2003. World Congress on Ultrasonics. pp. 73-80 .

Record type: Conference or Workshop Item (Paper)

Abstract

The propagation of high amplitude ultrasonic fields, such as those generated by some medical ultrasound systems, is not adequately described by the linear wave equation. Instead it is necessary to consider non-linear propagation if the drive levels are high enough to make non-linear effects significant. As a result of non-linear propagation the transmitted waveforms distort as they propagate, resulting in the generation of harmonics of the initial frequency components transmitted by the transducer. In the nearfield of medical transducers diffraction and focusing effects associated with the source complicate this process.
The basic physics of this non-linear propagation is reviewed and the complex characteristics of the finite amplitude fields generated by different sources are described. This is illustrated with both experimental results and numerical predictions obtained using a finite difference solution (the Bergen Code) to the Khokhlov-Zabolotskaya-Kuznetsov (or KZK) equation. The fields of both ideal sources and real medical systems are demonstrated. The use of harmonics to improve image quality is then considered, with the characteristics of the fields produced by harmonic imaging and pulse inversion systems being compared. The implications of non-linear propagation for medical ultrasound output regulation are also considered.

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