That's really clever to use the t-stat Gang! Makes life considerably easier.
In my (now outdated) way of doing things, z-scoring the data would convert the b-value of the regression to a standardized regression coefficient (beta), which in simple regression is a correlation. So if you centered both of your datasets (MRI and covariates) the correlation value would appear in the "mean" column for the covariate. Whereas if you just convert the t-stat of the covariate from t --> R, you get the same value.