(A) T1 weighted spin–echo MR phantom images of DOTA-Gd, 4, 5, and 6 in Millipore water at equal molar concentration of each contrast agent. Images were acquired at 4.7 T at 25 °C (TR/TE = 300/18.4 ms). Concentrations (from left to right): 62, 51, 37, 21, 7, 0 μM. (B) T1 weighted spin–echo images of pelleted NIH-3T3 cells incubated with DOTA-Gd, 5, and 6 at approximately equal molar concentrations of Gd(III) (0, 7.60, 7.6, and 8.3 mM Gd(III), respectively). Images were acquired at 4.7 T at 25 °C (TR/TE = 500/12.5). Each sample diameter is 1 mm.