Tissue harmonic imaging

Harmonics have much significance in music. A musical note has three characteristics. 1 Pitch (or frequency) 2 Loudness 3 Quality (or tone). Quality is the audible difference heard between two musical notes of the same pitch and loudness. Hence, a piano based C-note does not sound the same as a C-note played on a guitar, due to the existence of harmonic frequencies. The same note from various instruments has different qualities because the sounds are not pure notes i.e. of one frequency and these integral multiples of the fundamental frequency (overtones) give an instrument its characteristic sound. In an ultrasound context, tissue harmonic imaging is a signal processing technique also termed native harmonic imaging. An ultrasound beam insonates body tissues and generates such harmonic waves from nonlinear distortion during the transmit phase of the pulse-echo cycle. This imaging method was discovered by accident in USA when scientists working on the development of microbubbles for contrast in echocardiography found that a tissue image was still created when the receiver was tuned to receive at twice the transmitted frequency. This meant harmonics frequencies arose from soft tissue even without the injection of microbubble contrast agents. In fact some of the features of the image were superior to the fundamental image. The ultrasonic pulse gets altered with time as it traverses the tissues with non-linear motion. The peaks within the pulse waveform move faster than the troughs because the propagation speed is higher in compressed regions of tissue than in the areas which are expanded by the passing pressure wave. The degree of such acoustic signal distortion in tissue depends on the amplitude of the emitted pulse and the distance it has travelled in the tissue. For example, a 3.0 MHz transducer transmits a band of frequencies centered on the 3.0 MHz frequency. In traditional imaging, when tissue is insonated with a band of fundamental frequencies centered at 3.0 MHz, most of the returning echoes to the transducer will be linked to the same 3 MHz frequency band. Harmonics are multiples of the fundamental frequency, hence transmitting a band of frequencies centered at 3 MHz will result in the production of harmonic frequency bands centered at 6 MHz, 9 MHz, 12 MHz, etc.