Monitoring creatine and phosphocreatine by (13)C MR spectroscopic imaging during and after (13)C4 creatine loading: a feasibility study

Abstract

Creatine (Cr) supplementation to enhance muscle performance shows variable responses among individuals and different muscles. Direct monitoring of the supplied Cr in muscles would address these differences. In this feasibility study, we introduce in vivo 3D (13)C MR spectroscopic imaging (MRSI) of the leg with oral ingestion of (13)C4-creatine to observe simultaneously Cr and phosphocreatine (PCr) for assessing Cr uptake, turnover, and the ratio PCr over total Cr (TCr) in individual muscles. (13)C MRSI was performed of five muscles in the posterior thigh in seven subjects (two males and two females of ~20 years, one 82-year-old male, and two neuromuscular patients) with a (1)H/(13)C coil in a 3T MR system before, during and after intake of 15 % (13)C4-enriched Cr. Subjects ingested 20 g Cr/day for 4 days in four 5 g doses at equal time intervals. The PCr/TCr did not vary significantly during supplementation and was similar for all subjects and investigated muscles (average 0.71 ± 0.07), except for the adductor magnus (0.64 ± 0.03). The average Cr turnover rate, assessed in male muscles, was 2.1 ± 0.7 %/day. The linear uptake rates of Cr were variable between muscles, although not significantly different. This assessment was possible in all investigated muscles of young male volunteers, but less so in muscles of the other subjects due to lower signal-to-noise ratio. Improvements for future studies are discussed. In vivo (13)C MRSI after (13)C-Cr ingestion is demonstrated for longitudinal studies of Cr uptake, turnover, and PCr/TCr ratios of individual muscles in one exam.

Keywords: 13C; Creatine; Human muscle; MR spectroscopy.

Fig. 1

¹³C MRSI grid with voxels overlaid on T2-weighted MR image of the right thigh of a healthy young male after 4 days of creatine supplementation. Because of the k space sampling, the true voxel shape is more a sphere, which is approached in the image by a red circle. The ¹³C half-volume coil, positioned adjacent to the hamstring muscles, is indicated. The spectra are zoomed on the creatine signals. The muscles that are in the field of view of the coil include the biceps femoris (BF), semi-tendinosus (ST), semi-membranosus (SM), gracilis (G), and adductor magnus (AM), as delineated in the image (color figure online)

Fig. 2

¹³C MR spectra before, during and after creatine supplementation in the ST muscle of a healthy young male subject. a Full ¹³C MR spectrum of this muscles at day 7, 3 days after the creatine supplementation period. Cr and PCr resonate at 157.5 and 156.6 ppm, respectively. Even though creatine is 15 % ¹³C enriched the signal intensity is still much smaller than that of natural abundance ¹³C lipid resonances. The inset shows the fit of the PCr and Cr resonances with the residual below. b Starting with day 0 a clear increase in signal intensity is observed during the 4 days of 13C-4–Cr supplementation followed by a slower decrease; the signal is still elevated up to day 108. c The uptake rate is fitted with a linear function over the first 5 days, and the breakdown rate was fitted by a mono-exponential decay. Note that for the uptake part, the integral of the creatine signals at day 0 is not included as the fit of these signals did not pass the quality criteria

Fig. 3

Muscle specific creatine uptake rates (% per day) in four volunteers: two young healthy males (YHM; black bars) and two young healthy females (YHF; gray bars), from left to right: semi-tendinosus (ST), semi-membranosus (SM), adductor magnus (AM), biceps femoris (BF), and gracillis (G). Each uptake rate is the average of two muscles, except for the ST, BF, and G in YHF, for which the bars represent the uptake rate in one muscle. For the latter muscles, the error of the linear fit (see Fig. 2c) is given as SD. In the presented SD of the other muscles, these fitting errors have been considered

Fig. 4

Muscle specific Cr turnover rates (% per day) in young male volunteers. Only muscles in which the turnover signals were found to follow a mono-exponential decay were included. Turnover rate could not be determined if muscles showed relatively low Cr uptake. Bars indicate the mean uptake rate with standard error of the mean

Fig. 5

PCr/TCr ratios of the semi-tendinosus muscle before, during and after Cr supplementation of the four young healthy volunteers. PCr/TCr ratios were determined from ¹³C spectra acquired and analyzed as described in the materials and methods section. Bars indicate the mean values, and the error bars indicate the standard deviation

Fig. 6

PCr/TCr ratios. a Muscle specific ratios in young healthy volunteers (n = 4). One-way ANOVA did not reveal statistical significant muscle specific effect. Bars indicate the mean PCr/TCr per volunteer and the error bars indicate standard deviation. b Grouped data of the PCr/TCr ratio of the young healthy subjects, the FSHD patients, and the old volunteer. The PCr/TCr ratios of the FSHD and old subjects are within the range found in the young healthy subjects. YHmales/females young healthy subjects, FSHD FSHD patients, OH old healthy subject. Error bars indicate standard deviation