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, intramyo cardial blood volume) depend on some kind of assumption of intracapill ary-extravascular water exchange rate, e.g., fast exchange, The magnit ude of this water exchange rate, however, is still unknown, The intent ion of this study was to determine a lower limit for this exchange rat e 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 myocar dium within the slice was studied as a function of perfusion and compa red with a mathematical model which predicts relaxation rate as a func tion of perfusion and intracapillary-extravascular exchange rate, A li near relationship was found between relaxation rate T-(1) and perfusio n P normalized by perfusate/tissue partition coefficient of water, lam bda: 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 mode l revealed that a lower bound of the exchange rate from intra-to extra vascular space is 6.6 s(-1) (4.5 s(-1), P < 0.05), i.e., the intracapi llary 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 inve rsion could become a valuable noncontrast NMR method to measure variat ions of perfusion.