Intracellular manganese ions provide strong T1 relaxation in rat myocardium.

The efficacy of manganese ions (Mn 2 + ) as intracellular (ic) contrast agents was assessed in rat myocardium. T 1 and T 2 and Mn content were measured in ventricular tissue excised from isolated perfused hearts in which a5-min wash-in with 0, 30, 100, 300, or 1000 μM of Mn dipyridoxyl diphosphate (MnDPDP) was followed by a 15-min wash-out to remove extracellular (ec) Mn 2 + . An inversion recovery (IR) analysis at 20 MHz revealed two T 1 components: an ic and short T 1 - 1 (650-251 ms), and an ec and longer T 1 - 2 (2712-1042 ms). Intensities were about 68% and 32%, respectively. Tissue Mn content correlated particularly well with ic R 1 - 1 . A two-site water-exchange analysis of T 1 data documented slow water exchange with ic and ec lifetimes of 11.3 s and 7.5 s, respectively, and no differences between apparent and intrinsic relaxation parameters. Ic relaxivity induced by Mn 2 + ions in ic water was as high as 56 (s mM) - 1 , about 8 times and 36 times higher than with Mn 2 + aqua ions and MnDPDP, respectively, in vitro. This value is as high as any reported to date for any synthetic protein-bound metal chelate. The increased rotational correlation time (τ R ) between proton and electron (Mn 2 + ) spins, and maintained inner-sphere water access, might make ic Mn 2 + ions and Mn 2 + -ion-releasing contrast media surprisingly effective for T 1 -weighted imaging.