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

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, lambda: deltaT(-1) = m x deltaP/lambda with 0.82 < or = m < or = 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.

In vivo assessment of absolute perfusion and intracapillary blood volume in the murine myocardium by spin labeling magnetic resonance imaging

“…Furthermore, "fast exchange" of water between the intracapillary and extravascular compartments through the capillary wall was assumed. Some results from other studies speak in favor of fast exchange in tissues such as the myocardium (12). The last assumption was a homogeneous blood-tissue partition coefficient of 0.95 mL/g in the myocardium.…”

“…In an illustrative way, the parameter 1/k c can be considered to represent the mean time interval during which a spin is located within a capillary. Reference (12) provides that k c Ͼ 6.6 1/s (k c Ͼ 4.5 1/s with a probability of 95%) in the rat myocardium. Combining this result with Eq.…”

“…[9] of Ref. (12) shows that the relationship between the spin-lattice relaxation rate of the myocardium and the spin-lattice relaxation rate of capillary blood is linear over a wide range.…”

“…Details of this entire procedure are published in Refs. (11)(12)(13)18), so we give only a brief summary of the essential points.…”

“…This method has been well reported in the literature: the linear dependency between the spin lattice relaxation rate R1 ϭ 1/T 1 after slice-selective inversion and the coronary flow in the isolated cardioplegic rat heart was first described in Ref. (12). Subsequently, the method was applied to the in vivo rat heart for the determination of basal myocardial perfusion (13) and for the quantification of perfusion in murine skeletal muscle (14).…”