This article illustrates some problems and possible solutions to determine the apparent spin-lattice relaxation time (T 1 ) of the muscular 31 P metabolites at rest and during dynamic steadystate exercise using a clinical 1.5 T NMR scanner and a surface coil. T 1 was first estimated on a phosphates solution (phantom) using four different acquisition protocols, all based on the multiple-point "progressive saturation" method, and by fitting each data set with two different mathematical models. Subsequently, two of the four protocols and both models were used to estimate T 1 both at rest and during exercise on the calf muscles of 10 healthy volunteers. Experimental results obtained on the phantom showed that T 1 is greatly affected by the longest nominal explored repetition time (P < 0.001) and by the mathematical model (P < 0.001), ranging from 0. The availability of phosphorus magnetic resonance spectroscopy ( 31 P-MRS) on standard clinical units enabled researchers to assess high-energy phosphates noninvasively in human skeletal muscles under different experimental conditions (i.e., at rest, during steady-state exercise, or during rest-to-work or work-to-rest transients) (1-9). To maximize the signal-to-noise ratio per unit time of the experiment, data are usually collected under partially saturated conditions, applying the same acquisition protocol regardless of the different metabolic states. This is considered sufficient to guarantee that the change in the MRS peak intensity of the different metabolites is strictly proportional to their concentration changes only and thus no correction is made when the relative change in concentration is calculated. It also follows that when an absolute quantification must be performed, a single saturation factor, determined during the control state, is commonly applied to scale up the signal intensities to completely relaxed conditions. Different metabolic conditions of the tissue, however, may correspond to different apparent T 1 spin-lattice relaxation times of the species under consideration, which include the effects of the different chemical exchange and of the intrinsic T 1 . If this is true, a single proportionality constant between signal intensity and concentration no longer applies. Thus, to calculate relative changes in concentration and/or their absolute values, specific saturation factors would be needed for each different metabolic status. Alternatively, the knowledge of the apparent T 1 would allow an estimate of the saturation factors, provided the experimental acquisition parameters are also known.The apparent T 1 of 31 P metabolites have been investigated both in animal models (10 -13) and in humans, for which data are available mainly for resting conditions (14 -20). During dynamic exercise, the phosphocreatine (PC) T 1 values are sometimes a by-product of investigations concerning the mechanism of creatine phosphokinase reaction (21,22). These works indicate that, both in forearm and in calf muscles, PC T 1 does not change significantly during exercise. M...

On the phantom, the average T1 values for the various protocols and mathematical models used proved to differ widely, ranging from 0.61 s to 7.20 s. On volunteers, the T1 values obtained at rest and during exercise were not significantly different--0.91 s on average. Correction of this T1 value with the results obtained using the phantom provides a T1 value of 5.73 s, which is comparable with the value reported in the literature for resting conditions only.