Hi @cbaronio, welcome to Neurostars!
Actually as you have noticed, there are 2 SNR definitions used in MRIQC:
- the one noted “SNR” is calculated as ratio between the mean of the foreground and the standard deviation of the same region.
- the second one noted SNRd (for Dietrich), is mentioned as ratio between the mean of the foreground and the standard deviation of air (=background).
Here is an example below, from a T1w group HTML report:
But there are even more definitions out there! You can look at this literature if you want to know more:
(1) Goerner, F. L.; Clarke, G. D. Measuring Signal-to-Noise Ratio in Partially Parallel Imaging MRI: Signal-to-Noise Ratio in Parallel Imaging MRI. Medical Physics 2011, 38 (9), 5049–5057. https://doi.org/10.1118/1.3618730.
(2) Kellman, P.; McVeigh, E. R. Image Reconstruction in SNR Units: A General Method for SNR Measurement. Magnetic Resonance in Medicine 2005, 54 (6), 1439–1447. https://doi.org/10.1002/mrm.20713.
(3) Dietrich, O.; Raya, J. G.; Reeder, S. B.; Reiser, M. F.; Schoenberg, S. O. Measurement of Signal-to-Noise Ratios in MR Images: Influence of Multichannel Coils, Parallel Imaging, and Reconstruction Filters. Journal of Magnetic Resonance Imaging 2007, 26 (2), 375–385. https://doi.org/10.1002/jmri.20969.
The situation gets even more complicated for fMRI:
(1) Welvaert, M.; Rosseel, Y. On the Definition of Signal-To-Noise Ratio and Contrast-To-Noise Ratio for FMRI Data. PLoS ONE 2013, 8 (11), e77089. On the Definition of Signal-To-Noise Ratio and Contrast-To-Noise Ratio for fMRI Data.
Depending on your acquisition settings and your image type, one definition might be more relevant than the other. Obviously it is important to stick to the same definition if you want to compare images.
I hope this helps!