Final answer:
The correct CNR formula for contrast-to-noise ratio is A, which is the difference between the SNR of Tissue A and SNR of Tissue B, useful in differentiating tissues in MRI.
Step-by-step explanation:
The CNR formula is used to determine the contrast-to-noise ratio between two tissues in medical imaging, particularly in MRI (Magnetic Resonance Imaging). The correct formula for calculating CNR is A. CNR = SNR of Tissue A - SNR of Tissue B. This means the CNR is the difference between the Signal-to-Noise Ratio (SNR) of Tissue A and the Signal-to-Noise Ratio of Tissue B. It's important for distinguishing between different tissue types on MRI scans.