In vivo 1D and 2D correlation MR spectroscopy of the soleus muscle at 7T

Abstract

Aim: This study aims to (1) undertake and analyse 1D and 2D MR correlation spectroscopy from human soleus muscle in vivo at 7T, and (2) determine T1 and T2 relaxation time constants at 7T field strength due to their importance in sequence design and spectral quantitation.

Method: Six healthy, male volunteers were consented and scanned on a 7T whole-body scanner (Siemens AG, Erlangen, Germany). Experiments were undertaken using a 28cm diameter detunable birdcage coil for signal excitation and an 8.5cm diameter surface coil for signal reception. The relaxation time constants, T1 and T2 were recorded using a STEAM sequence, using the 'progressive saturation' method for the T1 and multiple echo times for T2. The 2D L-Correlated SpectroscopY (L-COSY) method was employed with 64 increments (0.4ms increment size) and eight averages per scan, with a total time of 17min.

Results: T1 and T2 values for the metabolites of interest were determined. The L-COSY spectra obtained from the soleus muscle provided information on lipid content and chemical structure not available, in vivo, at lower field strengths. All molecular fragments within multiple lipid compartments were chemically shifted by 0.20-0.26ppm at this field strength. 1D and 2D L-COSY spectra were assigned and proton connectivities were confirmed with the 2D method.

Conclusion: In vivo 1D and 2D spectroscopic examination of muscle can be successfully recorded at 7T and is now available to assess lipid alterations as well as other metabolites present with disease. T1 and T2 values were also determined in soleus muscle of male healthy volunteers.

Fig. 1

A localized 1D spectrum acquired from soleus muscle. Axial image of “a” of human soleus muscle from a 38 year old volunteer is shown inset and the voxel position from where the data was collected. About 3.3–4.3 ppm spectral region is expanded to show detail. See Table 1 for assignments. Acquisition parameters are: spectral width; 4000 Hz, vector size; 2048 points, voxel size of 6 × 6 × × 35 mm³, number of averages; 8, and repetition time; 2000 ms. The “WET” water suppression method [35] was applied before the acquisition sequence. The 1D spectrum was Fourier transformed after zero-filling to 4096 points and applying an exponential window function in MestReNova program [52]

Fig. 2

7T 2D localized ¹H L-COSY spectrum of human soleus muscle in vivo. See acquisition and processing parameters in text. Voxel position as shown in Fig. 1.

Fig. 3

Structures of the triglyceride molecule with the connectivities corresponding to each acyl chain cross-peak (A–F) as shown in Figs. 2 and 3. Cross-peaks, denoted A, E, F, G, and G′ are common to all triglyceride and diglyceride molecules. Resonance nomenclature according to [10]. Top: triglyceride molecule with the connectivities corresponding to each lipid cross-peak as shown in Figs. 2 and 3. Cross-peaks, denoted A, B, B′, C, D, E, F, G, and G′ are common to all triglyceride and diglyceride molecules. Bottom: chemical shifts values in IMCL in black color and EMCL in red color as derived from 2D L-COSY studies as shown in Fig. 2. Note the increase of 0.20–0.26 ppm for all EMCL moieties relative to IMCL. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

7T 2D localized ¹H L-COSY of human soleus muscle (data collected from voxel shown in Fig. 1). See Table 2 for assignments. Expansions reveal details in crowded spectral regions. The tCr methyl resonance at 3.02 ppm was used as an internal chemical shift reference. Horizontal (F2) and vertical (F1) axes have ppm units.

Fig. 5

Representative T1 (above) and T2 (below) array of spectra acquired from a healthy volunteer. The creatine methyl peak at 3.02 ppm was used as an internal chemical shift reference.