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

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Abstract
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.