Nonlinear propagation creates additional high-frequency components in the ultrasound wave (called harmonics) as the wave is transmitted through tissue. When nonlinear propagation occurs, 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 harmonic 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.
