Relaxation enhancement using liposomes carrying paramagnetic species

Studies were performed to investigate the effects upon the relaxation times of mouse organs of intravenously administered Mn-DTPA entrapped in multilamellar liposomes, Mn-DTPA, 0.9% NaCl entrapped in liposomes, 0.9% NaCl. Manganese concentrations in injectates and tissues were assessed with ⁵⁴Mn and atomic absorption. T₁ and T₂ of freshly excised tissues were measured in an NMR spectrometer at 20 MHz and 37%C with IR and CPMG sequences. Entrapment of Mn-DTPA in liposomes increased ⁵⁴Mn accumulation in liver by 207% and in spleen 1208% and reduced ⁵⁴Mn in heart by 20% and in kidney by 24% relative to free Mn-DTPA. Statistically significant increases in relaxation rates were produced. However, the increase in relaxation rate per unit concentration of Mn in tissue is reduced by 31% in liver and 62% in spleen when Mn is delivered inside liposomes. These observations have implications for the design of NMR contrast agents.