Carnitine plays an important role in fat metabolism, as it serves as a shuttle for acetyl groups from cytosol to mitochondrial matrix [1]. Therefore, it is crucial in glucose metabolism and pathogenesis of insulin resistance and of type 2 diabetes mellitus [2]. The main reservoir of carnitine is skeletal muscle. Acetylcarnitine (AC) could be observed at 2.13 and 3.17ppm chemical shifts in proton (1H) MR spectra. The fact, that in in vivo the AC line at 2.13ppm overlaps with lipid resonances and the line at 3.17ppm with trimethyl ammonium (TMA) peak, hinders a straight-forward detection and quantification of AC in skeletal muscle. Typically, the altered carnitine mechanism and increased concentration of AC after exercise enables the detection of AC [3] in difference spectra. Recently,the differences in T2-relaxation times of AC and lipids have been employed for detection of the 2.13ppm line of AC in long TE 1H MR spectroscopy (MRS) of quadriceps muscle at rest at 3T [4]. In this work we aimed to detect the AC resonance at 2.13ppm in calf (soleus) muscle and thigh muscle (vastus lateralis) in m. quadriceps femoris at rest, using long TE MRS at 7T; to determine itsT2– relaxation times; and to compare the AC concentrations in both muscle groups.

All measurements were performed on a 7T whole body Magnetom MR system (Siemens Healthcare, Erlangen, Germany). A 28 channel knee coil (QED, Mayfield Village, OH, USA) was used to acquire spectra from the calf and quadriceps muscles of the left leg. Four healthy, active, male volunteers (age 30.2±4.0 years, BMI 23.2±0.7 kg/m2) were recruited for this study. Spatial selection was achieved using a STEAM localization sequence (TR/TE=2000/350ms, spectral bandwidth=3kHz, number of averages=128, delta frequency=2.5ppm, preparation scans=4). The voxel of interest (40x35x15mm3) was placed in the soleus (SOL) or vastus lateralis (VL) muscles. Due to the long TE applied, water suppression was redundant. For the assessment of T2-relaxation time of AC peak at 2.13ppm at 7T a series of seven spectra with variable TE’s (100-450ms) were acquired in both muscles in all volunteers. Remaining sequence parameters were identical. All spectra were fitted in AMARES in the jMRUI software [5]. T2-relaxation time was calculated in MATLAB by fitting the data with the monoexponential function MTE=M0.e(-TE/T2). Using the residual water peak as an internal reference the absolute concentration of AC (in mmol/kg) was calculated according to the formula for molar concentration in wet weight:

CAC=(SAC/Sw)*(CFw/CFAC)*cw*nw*w%

where S are signals of metabolites (w-water, AC-acetylcarnitine), CF are correction factors for T2 relaxation, cw=55 mol/L is the molar concentration of the water,nw=2 is the number of protons in a water molecule and w% is the approximate water content of skeletal muscle tissue, i.e. 0.7 l/kg wet weight of the tissue.

Using long TE, we were able to detect AC resonance line at 2.13ppm at rest in both muscles (Fig. 1) in all four volunteers. The T2 of AC in VL muscle was found to be 137.8±47.7ms (Tab. 1) and the comparison of visibility of the AC signal at different TE’s is shown in Fig. 2. Unfortunately we were not able to resolve the AC peak in SOL muscle spectra with short TE. However, the T2 of TMA, lipids and water peak in SOL are almost equal as in VL muscle, so we can assume similar T2 of AC in SOL muscle as well. Concentrations of AC from SOL and VL muscles are shown in Tab. 2. Our results demonstrate effective detection of AC using long TE at rest at 7T in both muscle groups. The assessed concentration of AC from VL in healthy volunteers was found to be 1.69±0.21mmol/kg wet weight, which is in good agreement with the results of Lindeboom et al. who reported 1.58±0.30 mmol/kg in endurance-trained subjects [4]. The AC concentration in SOL at rest was found to be lower i.e., 0.54±0.19mmol/kg. Observed differences in skeletal muscle AC among different volunteers could be caused due to different training status, as was shown in study by Lindeboom et al. [4].The long-TE 1H MRS measurement at 7T represents successful method for the detection and quantitation of AC at rest. This method can be applied to clarify the role of AC levels in relation to impaired mitochondrial oxidation of fatty acids and insulin resistance in different skeletal muscle groups.