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Quick Concepts: Apparent SNR

QuickConceptsSeries_Screen_FINAL (1)A previous ‘Quick Concepts’ discussed the significance of signal-to-noise ratio in ultrasound technology. Briefly summarized, sonographers and ultrasound professionals are trained to optimize signal strength relative to noise in order to produce good clinical or diagnostic data.

Poor signal-to-noise ratio results in an inferior image quality.  However, not all inferior ultrasound images are the result of poor signal-to-noise ratios.  This leads to the discussion of “apparent” versus “true” SNR.

Graphical Representation of Apparent SNR

Graphical Representation of Apparent SNR

The distinction behind apparent signal-to-noise involves situations in which the system adjustments may be set inappropriately. In these cases, the SNR may appear to be poor, even when the true SNR is relatively good.

The quality of an ultrasound scan is dependent on the equipment, the patient, and the power to observe the results.  Because of the inherent limitations of the human eye and ear, ultrasound systems employ “compression” techniques to map the larger dynamic range of an ultrasound device into the significantly smaller dynamic range of an operator (or interpreter).

Receive gain too high, creating poor apparent SNR

Receive gain too high, creating poor apparent SNR

In some circumstances, the signal is not mapped well into a range that we can see or hear.  This will create imaging that APPEARS poor, when in fact it is simply mapped outside the viewer’s detectable range.  This can happen in two ways.  Either the receive gain can be set too high, which maps noise into the visible range, OR the receive gain can be set too low, resulting in a darker, weak image.

In my outline of strategies to improve SNR, two of the strategies involved changing system settings, which is critical in discerning whether you are dealing with “apparent” or “true” signal-to-noise challenges.

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

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