Thank you for your answer (and your welcome!)
Yes, I wasn’t sure about the correspondence between SNR calculation in MRIQC and MRIQC reference papers. I will keep the formulas as the “real” definition.
Even because this also happens for CNR: in Magnotta 2006 the denomitator is stdev of air, whereas in the formula the denominator is sqrt(stdev background^2+stdev white matter^2+stdev grey matter^2)
Surely, having the same definition is what is needed, this is why I asked 
I have to study a way to retrieve stdev of air and I will be ready to check my parameters