The effect of perfusion on T-1 after slice-selective spin inversion in the isolated cardioplegic rat heart: Measurement of a lower bound of intracapillary-extravascular water proton exchange rate

Many NMR measurements of cardiac microcirculation (perfusion, intramyocardial blood volume) depend on some kind of assumption of intracapillary-extravascular water exchange rate, e.g., fast exchange, The magnitude of this water exchange rate, however, is still unknown, The intention of this study was to determine a lower limit for this exchange rate by investigating the effect of perfusion on relaxation time, Studies were performed in the isolated perfused cardioplegic rat heart, After slice-selective inversion, the spin lattice relaxation rate of myocardium within the slice was studied as a function of perfusion and compared with a mathematical model which predicts relaxation rate as a function of perfusion and intracapillary-extravascular exchange rate, A linear relationship was found between relaxation rate T-(1) and perfusion P normalized by perfusate/tissue partition coefficient of water, lambda: Delta T-1 = m . Delta P/lambda with 0.82 less than or equal to m less than or equal to 1.06, Insertion of experimental data in the model revealed that a lower bound of the exchange rate from intra-to extravascular space is 6.6 s(-1) (4.5 s(-1), P < 0.05), i.e., the intracapillary lifetime of a water molecule is less than 150 ms (222 ms, P < 0.05), Based on this finding, the T-1 mapping after slice-selective inversion could become a valuable noncontrast NMR method to measure variations of perfusion.