The relaxivity of Gd-EOB-DTPA and Gd-DTPA in liver and kidney of the Wistar rat

The NMR relaxivities of Gd-EOB-DTPA and Gd-DTPA were determined in the kidney and liver of intact male Wistar rats immediately following sacrifice and in vitro in solutions and gels, at 1.5 T using a clinical MR scanner. T₁ and T₂ values of tissue samples were derived from spin-echo image sequences. Tissue gadolinium concentrations were determined by radioassay of Gd¹⁵³. Gd-EOB-DTPA T₁ and T₂ relaxivities, R₁ and R₂ (s⁻¹ mmole⁻¹ kg), were found to be 10.7 ± 0.5 and 22.5 ± 3.2, respectively, for liver, 2.4 ± 0.2 and 12.1 ± 1.7 for kidney cortex, 2.7 ± 0.2 and 14.5 ± 1.9 for kidney outer medulla, 2.0 ± 0.2 and 11.4 ± 2.1 for kidney inner medulla. Gd-DTPA R₁ and R₂ were found to be 4.8 ± 0.4 and 14.5 ± 3.7 for liver, 1.2 ± 0.1 and 7.9 ± 0.8 for kidney cortex, 1.6 ± 0.1 and 10.2 ± 1.4 for kidney outer medulla, 1.3 ± 0.1 and 10.2 ± 1.2 for kidney inner medulla. Gd-EOB-DTPA and Gd-DTPA R₁ was increased in liver compared to agarose gels at 38°C (4.49 ± 0.03 and 3.47 ± 0.06), but reduced in kidney tissues. All R₂ were elevated compared to agarose gels at 38°C (5.72 ± 0.12 and 4.12 ± 0.03). Elevated R₂ and R₁ (expressed in terms of the concentration of gadolinium per kg of tissue) can be accounted for in part by the lower water content of tissues compared with gels or solutions, increased microviscosity and binding to macromolecules. In addition, susceptibility effects may give rise to further increases in R₂. By contract, the reduced R₁ observed in kidney may be the result of compartmentalization of the magnetopharmaceuticals. Statistically improved fits were obtained for T₁ recovery curves for liver in the presence of Gd-EOB-DTPA when a dual exponential model was used. Assuming in vitro values for the relaxivities of these artificial contrast agents will lead to inaccuracies when relating observed signal enhancement factors to tissue gadolinium concentration.