Gammex 403 LE, Руководство пользователя

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14
Harmonic Imaging
Harmonic imaging has become an important addition to the medical ultrasound commu-
nity. Harmonic imaging is when a pulse is sent from the transducer at a nominal (funda-
mental) 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:
1. pulse propagation speed
2. attenuation (rate of pulse energy loss with depth)
3. the value of the nonlinearity 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 conven-
tional ultrasound was used.
The ratio of B/A quantifies the rate of transfer with respect to propagation distance of ul-
trasonic fundamental frequency energy to harmonic frequencies. The greater the ampli-
tude, 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
1
.
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1
Gong, X. F., Zhu, Z. M., Shi, T., Huang, J. H. (1989) Determination of the acoustic non-
linearity parameter in biological media using FAIS and ITD methods, J. Acoust. Soc. Am.
86 (1), pp 1-5.