Gammex Phantom 404 LE User Manual - Page 14

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Harmonic Imaging

Harmonic imaging has become an important addition to the medical ultrasound

community. Harmonic imaging is when a pulse is sent from the transducer at a nominal

(fundamental) frequency, but the signal received by the transducer is twice that

frequency, which is the second harmonic. The result is that better resolution is attained

at any given depth than if the reception had been at the fundamental frequency, as in

conventional ultrasound.

There are three tissue properties that determine the effectiveness of harmonic imaging:

pulse propagation speed

attenuation (rate of pulse energy loss with depth)

the value of a thermodynamic parameter: B/A

In order for phantoms to present valid resolution results for harmonic imaging, these

three properties must adequately correspond to human tissue. Attenuation increases

with frequency and much of the propagation involves the fundamental frequency, so in

harmonic imaging, there is enhanced resolution without as much attenuation as there

would be if the higher frequency were used to generate the pulses at the transducer. So,

higher frequency resolution occurs for greater depths within the subject than if

conventional ultrasound was used.

The ratio of B/A quantifies the rate of transfer with respect to propagation distance of

ultrasonic fundamental frequency energy to harmonic frequencies. The greater the

amplitude, the greater the energy transfer rate; thus, the beam profile for the harmonic is

smaller than for the fundamental, which means better lateral and elevational resolution.

Tissue-mimicking phantoms will be appropriate for assessing harmonic imaging only if

B/A for the tissue-mimicking material in the phantom adequately approximates that of

soft tissues. Recently, we have developed the capacity to measure the value of B/A for

the tissue-mimicking materials in Gammex phantoms and have found it to lie in the

range for human soft tissue, meaning B/A is between 6 and 7

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Gong, X. F., Zhu, Z. M., Shi, T., Huang, J. H. (1989) Determination of the

acoustic nonlinearity parameter in biological media using FAIS and ITD methods, J.

Acoust. Soc. Am. 86 (1), pp 1-5.