STUDIES OF GD-DTPA RELAXIVITY AND PROTON-EXCHANGE RATES IN TISSUE

The image intensity in many contrast agent perfusion studies is design ed to be a function of bulk tissue T-1, which is, in turn, a function of the compartmental (vascular, interstitial, and cellular) T(1)s, and the rate of proton exchange between the compartments. The goal of thi s study was to characterize the compartmental tissue Gd-DTPA relaxivit ies and to determine the proton exchange rate between the compartments . Expressing [Gd-DTPA] as mmol/liter tissue water, the relaxivities at 8.45 T and room temperature were: saline, 3.87 +/- 0.06 (mM.s)(-1) (m ean +/- SE; n = 29); plasma, 3.98 +/- 0.05 (mM.s)(-1) (n = 6); and con trol cartilage (primarily an interstitium), 4.08 +/- 0.08 (mM.s)(-1) ( n = 17), none of which are significantly different. The relaxivity of cartilage did not change with compression, trypsinization, or equilibr ation in plasma, suggesting relaxivity is not influenced by interstiti al solid matrix density, charge, or the presence of plasma proteins. T -1 relaxation studies on isolated perfused hearts demonstrated that th e cellular-interstitial water exchange rate is between 8 and 27 Hz, wh ile the interstitial-vascular water exchange rate is less than 7 Hz. T hus, for Gd-DTPA concentrations, which would be used clinically, the T -1 relaxation rate behavior of intact hearts can be modeled as being i n the fast exchange regime for cellular-interstitial exchange but slow exchange for interstitial-vascular exchange. A measured relaxivity of 3.82 +/- 0.05 (mM.s)(-1) (n = 8) for whole blood (red blood cells and plasma) and 4.16 +/- 0.02 (mM.s)(-1) (n = 3) for frog heart tissue (c ells and interstitium) (with T-1 and Gd-DTPA concentration defined fro m the total tissue water volume) supports the conclusion of fast cellu lar-extracellular exchange. Knowledge of the Gd-DTPA relaxivity and ma intaining Gd-DTPA concentration in the range so as to maintain fast ce llular-interstitial exchange allows for calculation of bulk Gd-DTPA co ncentration from bulk tissue T-1 within a calculable error due to slow vascular exchange.