Paul,
One possible approach is to obtain the estimated inflammation values associated with the observed cytokine numbers when motion and VBM Jacobians are set at their respective average values. That is, use 3dcalc with the beta values from the 3dttest++ output, you can obtain for each subject
estimated inflammation = b1*cytokine + b2*(motion - motion_mean) + b3*(Jacobians - Jacobians_mean)
Then you can scatterplot the estimated inflammation vs. cytokine.
Gang