Gammex 404GS LE, User Manual

15

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 (fun-
damental) 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     
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 resolu-
tion. 

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   

nonlinearity parameter in biological media using FAIS and ITD methods, J. Acoust. Soc. 
Am. 86 (1), pp 1-5.