Which statement about Contrast-Noise-Ratio (CNR) is false?

• A. It is the ratio of the signal difference between areas of interest
• B. It is affected by SNR
• C. A high CNR is only possible with a low SNR
• D. If image noise is high, the CNR will be low

Answer: (C)

CNR measures how clearly two regions stand out in an image by comparing the difference in their signals to the image noise. It depends on both the contrast between the regions and the level of noise in the image, so higher SNR (less noise) generally helps increase CNR for a given level of contrast.

The statement that a high CNR is only possible with a low SNR is not correct. You can achieve a high CNR either by a large difference in signal between the regions or by reducing noise (which raises SNR). Conversely, high noise (low SNR) lowers CNR. So while low noise helps, high CNR does not require low SNR exclusively; a strong signal difference can yield high CNR even if the noise isn’t minimized.