Perhaps you are too young to remember that REM song from the mid 1990s, but it does ask an important question: What's the frequency (of any given probe)? If each probe has a range of frequencies, how can you adjust or change the frequency within that range? On large and fancy ultrasound machines, the ability to change the frequency of a given probe is done by the press of a button. However, as our machine is simplified for the ease of use in the ED, it does not allow us to dial down the frequency at will. But do not fear, we can still adjust the frequency to optimize our imaging goal of deeper penetration vs. more detail:
Note that it states GEN on the screen, which denotes the General setting: a mid range frequency to give us the best of both worlds, detail and penetration. However, what about those times when you have a patient who is morbidly obese and you are unable to make out much detail? This is the time to lower the frequency to allow the sound beam to penetrate deeper into the body, past the subcutaneous fatty tissue and allow you to see intra abdominal organs.
By changing to PEN mode we are not selecting an exact frequency, but we are lowering the frequency enough to improve visualization but at a cost: the image looses some of the detail making it more grainy.
Now what if you just want more detail on a struture that is closer to the probe? Then changing to the RES mode will increase the frequency and make the image clearer, but at the cost that you will not be able to go as deep. You can also improve your resolution by increasing the frequency by changing the probe you use. By going from a low freqncy probe to a higher frequency probe, you will get more detail in your image.
Just remember: penatration and resolution are opposites. You cannot have it both ways, if you increase pentration, you will lose clarity. To improve image resolution, you will be limted by depth.
I guess we can't have it all now can we?
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