Variable field relaxometry of a Dy-DOTA-conjugated G = 5 PAMAM dendrimer and single Dy(III) chelates. Shown are the T2 relaxivities of (a) Dy-DOTA-PAMAM G = 5 dendrimers, (b) Dy-DOTA single chelates, and (c) Dy-DTPA single chelates as a function of magnetic field strength. Data are shown at 3 (▲), 10 (▼), 20 (●), and 37°C (+). Solid lines represent quadratic fits to the equation 1/T2 = a + bB0,2 with B0 being the external magnetic field strength. For comparison, the T1 relaxivities are negligible, shown as dashed lines at 3 (■) and 37 C (·). (Reprinted with permission form Ref 44 Copyright 1998 Wolters Kluwer Health)