Tissue Harmonic Imaging: A

Nonlinear propagation creates additional high-frequency components in the ultrasound wave (called harmonics) as the wave is transmitted through tissue. When nonlinear propagation oc-curs, reflectors along the beam path generate echoes with both the fundamental and harmonic frequencies. Tissue harmonic imaging (THI) uses two methods, frequency band filtering and pulse phase inversion, to isolate the second har-monic frequency in the detected echo so that an image of harmonic reflection can be formed. The technique of THI is discussed with illustrative examples. Advantages and limitations of THI are presented. Key words: tissue harmonic imaging, nonlinear propagation, harmonic frequency, harmonic band filtering, pulse phase inversion Tissue harmonic imaging (THI) is a relatively new real-time imaging technique that relies on the detection of the harmonic frequencies created by nonlinear beam propagation through tissue. Non-linear propagation, and thus harmonic formation, depend on ultrasound beam intensity and path length. Reflection at interfaces following nonlinear propagation produces echoes with fundamental and harmonic frequency components. The echo-induced radiofrequency signals generated by the transducer will also contain fundamental and har-monic frequencies. Even though the detected har-monic signal is weaker than signals generated with traditional echo ranging, contrast is often improved by the suppression of interfering signals from clut-ter and multiangle scattering. In this article, nonlin-ear propagation, harmonic formation, and har-monic detection methods are reviewed before pre-senting illustrative examples of THI sonograms