摘要：Now that you have persisted in understanding frequency, wavelength, resolution and depth, let us try and combine them in a meaning

Now that you have persisted in understanding frequency, wavelength, resolution and depth, let us try and combine them in a meaningful way.

For ultrasound to be clinically useful, we want sharp pictures. I.e. we want the best resolution.

Additionally, we want adequate depth of penetration to see deeper structures.

The physics of ultrasound is such that both these requirements cannot be fully met at the same time. For a good resolution, we need a short wavelength, which means we need a higher frequency. Unfortunately, an higher frequency also means a shorter penetration.

So when choosing the optimum frequency, we have to choose somewhere between the needs of a good depth of penetration and a good resolution.

If the frequency is too high, the depth of penetration of the ultrasound will be inadequate and you may not see the structure you want.

If the frequency is too low the ultrasound waves will penetrate deeper than necessary. The unnecessarily low frequency will give you a low resolution image that will be blurred.

The optimum frequency is one that will give you just the adequate amount of depth necessary to see the structures of interest. You don't allow unnecessary depth which would have unnecessarily lowered the frequency.

To summarise:

Correct depth = reasonable high frequency = reasonably short wave length = reasonably good resolution.

Too much depth = low frequency = long wavelength = poor resolution

Too little depth = Wont see structures of interest !

来源：总有无能为力的不愉