Attenuation and tissue penetration

There is also an advantage to highest possible frequency, as both pulse length, i.e. radial resolution, and lateral resolution will then be optimal. This is only partially true as there is another phsyics restriction that has not been discussed, attenuation. The higher the frequency, the more the tissue attenuates the ultrasound energy. This limits the reach of the probe. An average value for attenuation is 0.5 dB per cm tissue per MHz center frequency. Therefore, in practice, low frequencies such as 2.5 - 3.5 MHz is used for cardiology in adults, 3.5 MHz for general abdominal imaging, 3.5 - 5 MHz for foster imaging, and 5 - 7.5 MHz for pediatric cardiology and imaging of peripheral blood vessels. 

In the ultrasound instruments, there is an automatic compensation for attenuation in "standard tissue". Another effect of attenuation can be seen when imaging cysts and calcifications. Fluid-filled cysts attenuate less than the surrounding tissue, the signal will therefore have a tendency to be stronger. This is shown as a brighter image. The opposite occurs for calcification that reflects and attenuates more than the surrounding tissue and gives a darker stripe.