Measurement of spin-lattice relaxation times and chemical exchange rates in multiple-site systems using progressive saturation

Abstract

A new method for measuring spin-lattice relaxation times and chemical exchange (CE) rate constants in multiple-site exchanging systems is described. The method, chemical exchange and T1 measurement using progressive saturation (CUPS), was applied to determine T1s and analyze phosphorus exchange among phosphocreatine (PCr), ATP, and inorganic phosphate (Pi), mediated by creatine kinase (CK) and ATP synthase, using 31P-MRS. Two-site exchange was analyzed in vitro and in the rat leg, and three-site exchange was analyzed in the rat heart. Data were fitted to a model of progressive saturation incorporating T1 relaxation and CE. For the in vitro system at 8.45T, we found T1(PCr) = 2.86 s and T1(γ-ATP) = 1.72 s. For the rat gastrocnemius at 1.9T, we found T1(PCr) = 6.60 s and T1(γ-ATP) = 2.06 s. For the rat heart at 9.4T, we found T1(PCr) = 3.35 s, T1(γ- ATP) = 0.69 s, and T1(Pi) = 1.83 s. All of these values were within 20% of literature values. Similarly, the determined exchange rates were in the same range as published values. Using simulations, we compared CUPS with transient saturation transfer as a method for measuring T1s and rates. The two methods showed similar sensitivity to noise. We conclude that CUPS is a viable alternative for measuring T1s and CE rates in exchanging systems. © 2007 Wiley-Liss, Inc.