In $$$T_1$$$-weighted perfusion studies, the relationship between the contrast concentration and $$$T_1$$$ shortening effect is assumed to be linear within the clinical range. However, this assumption may not hold at higher concentration observed in the arterial input functions (AIFs). We set out to investigate this relationship by measuring the $$$R_1$$$ relaxation rate of various Gd-DTPA concentrations in saline. We find that the change in $$$R_1$$$ relaxation rate of Gd-DTPA is linear up to 4 mM at 9.4T and 11.7T, and up to 5 mM at 3T. At higher concentration, $$$R_1$$$ relaxation rate increases faster than a linear relationship.