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Quick Concepts: Signal-to-Noise Ratio

Weak v Strong SNRThe concept of SIGNAL-TO-NOISE is foundational to understanding what represents a “quality” scan.  Yet, I find it ironic that very little time is spent explaining why the ratio is more relevant than either signal strength or noise level.

Consider the two extremes of:

(1) Shouting across a crowded restaurant

(2) Whispering in a public library

Which situation represents a more “audible” environment ?

Your answer would involve an assessment of the strength or weakness of the signal (your voice) relative to the level of background noise….hence, a ratio.

It is quite possible that the whisper in a library is more discernable than a shout across the crowded restaurant.


Shouting Across a Crowded Restaurant

Shouting Across a Crowded Restaurant

Whispering in the Library

Whispering in a Public Library

From this analogy, you can very quickly conclude:

Just because you have a large signal doesn’t mean you will have good signal-to-noise ratio AND just because you have low noise, doesn’t mean you will have good SNR.

For sonographers and ultrasound professionals, your job is to optimize signal-to-noise at all times.  Making sure that the signal is large enough relative to the noise will give us good clinical or diagnostic data.

In the video below, I discuss multiple strategies to improve SNR. It is also important to note that increasing the receiver gain does NOT improve signal-to-noise ratio.  While many novice sonographers associate a brighter image with good (or better) SNR, what is more important than the brightness of the image is the contrast between the signal and the noise levels. This will lead into a future discussion on “Apparent” versus “True” SNR.

This subject discussion intersects with a previous 2-part, Quick Concepts post on Transmit vs. Receive Gain.

This subject matter is discussed more thoroughly in Frank Miele’s  Ultrasound Physics and Instrumentation in Chapter 6: System Operation

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