How is CNR calculated, and what does it indicate?

• A. CNR = (mean_signal − mean_background) / standard deviation of background; it indicates the detectability of a lesion against background noise.
• B. CNR = (mean_signal − mean_background) / mean_background; it indicates color fidelity.
• C. CNR = standard deviation divided by mean signal; it indicates exposure.
• D. CNR = (mean_signal − mean_background) / standard deviation of the signal; it indicates noise level.

Answer: (A)