What are characteristics of different MR images?
Speaking from a medical imaging perspective, the SNR of MR images mainly depends on signal acquisition schemes, image reconstruction parameters, and the MR scanner hardware, such as, number of receive coils. To know about more about other parameters go here: http://wiki.ci.uchicago.edu/pub/HNL/DifficultQuestion/howdoesmriwork.pdf.

Parallel MR imaging is one of the acquisition technique which improves SNR. For example, I'm sharing with you the results from a paper published in journal of magnetic resonance imaging on "Measurement of Signal-to-Noise Ratios in MR
Images" by Olaf Dietrich and others below: 



PS: Take the reciprocal to get noise to signal ratio.