OBJECTIVES: Calculation of accurate T1 relaxivity (r1) values for gadolinium-based magnetic resonance contrast agents (GBCAs) is a complex process. As such, often referenced r1 values for the GBCAs at 1.5 T, 3 T, and 7 T are based on measurements obtained in media that are not clinically relevant, derived from only a small number of concentrations, or available for only a limited number of GBCAs. This study derives the r1 values of the 8 commercially available GBCAs in human whole blood at 1.5 T, 3 T, and 7 T. MATERIALS AND METHODS: Eight GBCAs were serially diluted in human whole blood, at 7 concentrations from 0.0625 to 4 mM. A custom-built phantom held the dilutions in air-tight cylindrical tubes maintained at 37 ± 0.5°C by a heat-circulating system. Images were acquired using inversion recovery sequences with inversion times from 30 milliseconds to 10 seconds at 1.5 T and 3 T as well as 60 milliseconds to 5 seconds at 7 T. A custom MATLAB program was used to automate signal intensity measurements from the images acquired of the phantom. SigmaPlot was used to calculate T1 relaxation times and, finally, r1. RESULTS: Measured r1 values in units of s⁻¹·mM⁻¹ at 1.5 T (3 T/7 T) were 3.9 ± 0.2 (3.4 ± 0.4/2.8 ± 0.4) for Gd-DOTA, 4.6 ± 0.2 (4.5 ± 0.3/4.2 ± 0.3) for Gd-DO3A-butrol, 4.3 ± 0.4 (3.8 ± 0.2/3.1 ± 0.4) for Gd-DTPA, 6.2 ± 0.5 (5.4 ± 0.3/4.7 ± 0.1) for Gd-BOPTA, 4.5 ± 0.1 (3.9 ± 0.2/3.7 ± 0.2) for Gd-DTPA-BMA, 4.4 ± 0.2 (4.2 ± 0.2/4.3 ± 0.2) for Gd-DTPA-BMEA, 7.2 ± 0.2 (5.5 ± 0.3/4.9 ± 0.1) for Gd-EOB-DTPA, and 4.4 ± 0.6 (3.5 ± 0.6/3.4 ± 0.1) for Gd-HP-DO3A. The agents can be stratified by relaxivity, with a significant additional dependency on field strength. CONCLUSIONS: This report quantifies, for the first time, T1 relaxivity for all 8 gadolinium chelates in common clinical use worldwide, at current relevant field strengths, in human whole blood at physiological temperature (37°C). The measured r1 values differ to a small degree from previously published values, where such comparisons exist, with the current r1 measurements being that most relevant to clinical practice. The macrocyclic agents, with the exception of Gd-DO3A-butrol, have slightly lower r1 values when compared with the 2 much less stable linear agents, Gd-DTPA-BMA and Gd-DTPA-BMEA. The 2 agents with hepatobiliary excretion, Gd-EOB-DTPA and Gd-BOPTA, have, at 1.5 and 3 T, substantially higher r1 values than all other agents.