MR tools are used in many sectors such as medicine and mining. MR stands for magnetic resonance and MRI for magnetic resonance imaging. What are characteristics of different MR images? What are typical noise to signal ratios and other important decision factors?
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.
The origin of MR signal is vastly complicated; you'll need to read a few books to go to the bottom of it.
I guess that in a simple way, the signal from an MR image usually starts with the proton density of what is imaged, with SNR going typically going from below one to several tens, all modulated by the contrast techniques applied.
However, I think you'll need to be more specific about your aim if you want to get anything useful. For instance, I'm not sure what you mean by "characteristics" and "decision factor".