nii.qc.snr: Regional Signal-to-Noise Ratio
In TKoscik/nifti.qc: Quality control metrics and reports for NIFTI data

Calculate the signal to noise ratio within the masked region.

SNR = μ_ROI/[σ_ROI√(n/(n-1))]

1	nii.qc.snr(img.nii, img.vol=1L, mask.nii, mask.vol=1L, mask.dir="gt", mask.thresh=0)

`img.nii`	Full directory listing to desired nifti image (unzipped)
`img.vol=1L`	For multi volume images; an integer indicating the volume to load (1..N)
`mask.nii`	Full directory listing to desired nifti image to use for masking (unzipped). Must be identical pixel dimensions to base image.
`mask.vol=1L`	For multi volume images; an integer indicating the volume to load (1..N)
`mask.dir="gt"`	Direction to threshold the mask image to generate the desired ROI. Options include ("gt") [>] Default, "ge" [>=], "lt" [<], "le" [<=], "eq" [==]
`mask.thresh=0`	Value at which to threshold the mask image to generate an ROI. E.g., for binary masks, where the value 1 indicates the ROI, default of >0, will generate the appropriate mask