What is SNR and CNR in DR and how do they influence image quality?

• A. SNR is the ratio of noise to signal; CNR is the contrast between tissues relative to signal; higher values decrease detectability.
• B. SNR is the ratio of signal to noise; CNR is the contrast between tissues relative to noise; higher SNR and CNR improve detectability of structures.
• C. SNR and CNR only affect image storage.
• D. SNR equals CNR.

Answer: (B)

In digital radiography, SNR and CNR describe how clearly the image represents anatomy. SNR is the signal-to-noise ratio: it compares the level of the useful signal coming from the tissue to the random noise introduced by quantum fluctuations and detector/electronic noise. A higher SNR means the signal rises well above the background noise, so fine details are more reliably seen.

CNR looks at the contrast between two different tissues and relates that difference to the noise. A higher CNR means the difference between tissue signals stands out more clearly against the noisy background, making boundaries and lesions easier to detect.

Both SNR and CNR improve detectability of structures when they are larger or where contrast is subtle, respectively. You can influence them with exposure choices (within safety limits), detector efficiency, and minimizing motion or scatter, all aiming to maximize signal relative to noise.

The other statements aren’t correct because SNR is not the ratio of noise to signal, CNR is not defined relative to the signal in that way, these metrics affect image quality rather than storage, and SNR does not equal CNR.