For the sake of best possible resolution, it is an advantage with a high ultrasound frequency, as both pulse length, i.e. radial resolution, and lateral resolution increase with frequency. There is however another physical limitation that has not been discussed so far, namely attenuation. The higher the frequency, the more the tissue attenuates the ultrasound energy. This limits the penetration depth 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 fetal imaging, and 5 - 7.5 MHz for pediatric cardiology and imaging of peripheral blood vessels. 
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In ultrasound instruments, there is a built-in 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 at higher depths right underneath such a cyct will therefore have a tendency to be stronger. This shows up as a brighter image. The opposite occurs for calcification that reflects and attenuates more than the surrounding tissue and gives a darker stripe.