With conventional imaging, the ultrasound system transmits and receives a sound pulse of a specific frequency. The difference between the transmitted and returned signal is that the returned signal is less intense, losing strength as it passes through tissue. The time it takes for the echo to return is proportional to the distance the sound wave traveled.
With Harmonic Imaging, on the other hand, the signal returned by the tissue includes not only the transmitted “fundamental” frequency, but also signals of other frequencies – most notably, the “harmonic” frequency, which is twice the fundamental frequency. Once this combined fundamental/harmonic signal is received, the ultrasound system separates out the two components and then processes the harmonic signal alone.
The harmonic signals used in this form of imaging do not come from the ultrasound system itself. These signals are generated in the body as a result of interactions with tissue or contrast agents. Thus, the harmonic signal only has to pass through the fat layer once.
Because the speed of sound is higher when the pressure is higher, the top of the waveform gets pulled forward as the wave passes through tissue. This distortion of the waveform causes harmonies to be generated. Note that the bottom of the wave doesn’t actually travel backwards, it travels forward at a slower speed. The arrows are for illustration only.
Harmonic imaging has a number of potential clinical benefits including improved spatial resolution to permit visualization of smaller objects, and improved contrast resolution to improve demonstration of increasingly subtle differences in grayscale. The benefits of harmonic imaging will be most apparent in the mid-field - center portion of the ultrasound image.
Near Field No Harmonics being generated Signal has not traveled enough to be distorted Near Mid Field Harmonics Increasing Harmonics beginning to be produced as signal travels through tissue Mid Field Harmonics Unchanging Additional harmonics generated and attenuated in equal proportion Far Mid Field Harmonics Decreasing Harmonics being attenuated faster than being produced Far Field Fundamental Frequency Only
The properties of tissue cause the primary ultrasound signal to distort in the body. The distortion of this signal causes harmonics to be generated in tissue and these harmonics can then be used to generate an ultrasound image. The properties of these harmonic signals are such that one can get significant improvements in spatial and contrast resolution, but these improvements are limited to the region of interest in which the acoustic energy is sufficiently high to cause the harmonics to be generated. The benefits of harmonic imaging are apparent in improved spatial resolution, increased contrast resolution and improved demonstration of subtle grayscale differences.