> I use the default operation, -center DIFF. I did not demean the covariates. Is it correct?
Since you have one group of subjects, that should be fine.
> There is not sub-brick that is correlation coefficient between image and blood-test after remove the age effect, right?
You already have the regression coefficient for blood test, right? The correlation coefficient between z-score and blood test is just a special coefficient when both z-scores and blood test values are standardized. And the t-statistic would be the same between the correlation coefficient and the regression coefficient. If you're desperately need the correlation coefficient, you can get it through standardizing those variables.
Gang