The effect of perfusion on T1 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, λ: ΔT^?1 = m · ΔP/λ with 0.82 ≤ m ≤ 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 T₁ mapping after slice-selective inversion could become a valuable noncontrast NMR method to measure variations of perfusion.