In vivo measurement of T1 and T2 relaxivity in the kidney cortex of the pig—based on a two-compartment steady-state model

A two-compartment system for approximating five successive steady-state levels of gadopentetate dimeglumine (Gd-DTPA) concentration in pigs was developed. The method of calculating Gd-DTPA concentration was based on a simultaneous reference determination of 99mTc-DTPA. Experimental and theoretical results showed a steady state after 25 min. The nuclear magnetic resonance (NMR) relaxivities of Gd-DTPA were determined in vivo in pig kidneys during steady-state levels. T1 relaxivity (R1) and T2 relaxivity (R2) in the kidney cortex were found to be 1.1 ± 0.03 and 2.0 ± 0.2 (s−1 mM−1), respectively. R1 and R2, in in vitro human plasma solutions at 25°C were 5.3 ± 0.02 and 5.8 ± 0.06 s−1 mM−1 and at 37°C 4.3 ± 0.04 and 4.9 ± 0.01 s−1 mM−1. Thus, the in vivo relaxivities were reduced 3.9 and 2.5 times for R1 and R2, respectively, compared to the in vitro relaxivities. This marked difference in relaxivities between tissue and plasma may be the result of the difference in steric relations. In plasma, the mobility and distribution of the Gd-DTPA complex are unrestricted, whereas they may be reduced in the tissues because of the close proximity to the cell wall, to the proteins and to other extracellular elements and compartments. ds: Gd-DTPA; 99mTc-DTPA; Relaxivity; Kidney.