Investigating Regional Variations of Acetyl Carnitine In Thigh and Calf Muscles In Vivo using PRESS Localized Long TE-based MR Spectroscopy
Rajakumar Nagarajan1, Zohaib Iqbal1, Manoj K Sarma1, S. Sendhil Velan2, and M.Albert Thomas1

1Radiological Sciences, University of California Los Angeles, Los Angeles, CA, United States, 2Laboratory of Molecular Imaging, Singapore Bioimaging Consortium, Singapore, Singapore


Skeletal muscle plays a major role in the development of insulin resistance (IR) and progression to type 2 diabetes. A recent work has used long TE (350ms) based PRESS localized spectrum in the vastus lateralis region of thigh muscle without any exercise to investigate acetylcarnitine, a compound formed when acetyl-Coenzyme A exceeds use by the tricarboxylic cycle in the mitochondria. This work focused on examining regional variations of acetylcarnitine in the thigh and calf muscles using the long TE MRS. Our preliminary results show the unequivocal presence of acetylcarnitine in lean, young healthy thigh muscle regions and decreased level in one diabetic type 2 patient.


Even though cellular mechanisms producing insulin resistance (IR) remain unclear, the presence of IR is strongly predictive of the increased risk in development of type 2 diabetes (T2D) (1-4). Skeletal muscle plays a major role in the development of IR and progression to T2D (1, 3,4). Potential roles and mechanisms for alterations of intramyocellular lipids (IMCL) and extramyocellular lipids (EMCL) and lipid saturation index in muscles of different fiber type distributions in relation to the development of IR and T2D have been investigated by several researchers (1, 3-7). Using PRESS and STEAM-based single- and multi-voxel localized one-dimensional (1D) MR spectroscopy (MRS), previous research has focused mainly on investigating IMCL and EMCL levels (1, 5-7). It has been shown that acetylcarnitine is formed in conditions in which acetyl-CoA formation, either as an end product of glycolysis or b-oxidation, exceeds its entry into the tricarboxylic (TCA) cycle; free carnitine can act as a sink for excess acetyl groups in a reversible reaction catalyzed by the enzyme carnitine acetyltransferase (CRAT). Boss et al. and Ren et al. independently demonstrated the detection of carnitine and acetylcarnitine resonances after exercise using proton MRS (8-9). Recently, Lindeboom et al. reported that measuring acetylcarnitine concentrations using 1H-MRS is feasible in the skeletal muscle without exercise using long TE of 350ms on clinical MR scanners and their results demonstrated a reciprocal distribution with mean concentrations of acetylcarnitine in the vastus lateralis muscle correlating with mean insulin sensitivity in each of the four different groups including T2D, obese, endurance trained and lean healthy subjects (10). Hence, a major goal of this work was to investigate acetylcarnitine levels in different regions of human thigh and calf muscles using the long TE-based PRESS.

Materials and Methods

We have studied five healthy volunteers (mean age of 29.2 years) and one diabetic type 2 patient (61 years old) so far. A Siemens 3T Prisma MRI scanner equipped with a 2-channel body ‘transmit’ coil was used in combination with a 4-channel flexible phased-array combined with selected channels of a spine MRI ‘receive coil’ for the thigh muscle. A 15-channel transmit/receive coil was used for the calf muscle investigation. Right thigh and right calf muscles were chosen for the MRS scan. Axial T1-weighted MR images were used to select a volume of interest (VOI) from which the 1D MRS was acquired. The PRESS sequence (11) was used using the following acquisition parameters: TR/TE=6s/350ms, 64 averages, and a voxel volume of 4x4x3cm3 At this long TE, water suppression was not used. A larger voxel was used to compensate for the signal loss due to long TE. Manual shim was performed and the FWHM of water peak was approximately 25Hz. The muscle spectra were processed on TARQUIN software (12).

Results and Discussion

Fig.1 shows the axial T1 weighted MRI showing the PRESS VOI localization in the vastus muscle. Fig.2 shows a long TE spectrum with enhanced visibility of the acetylcarnitine peak at 2.13 ppm in the vastus muscle of a 25 year-old healthy male subject and also, the long TE MRS of a 61 year-old male diabetic patient in vastus muscle. The trimethylamine (TMA) peak of acetylcarnitine was overlapping with total choline groups. Compared to the lean young healthy control, the thigh muscle spectrum of the 61 year-old T2D patient showed decreased level of acetylcarnitine. There were other peaks due to EMCL and IMCL of poly-methylene protons and methyl protons of saturated/unsaturated lipid pool, Cr/PCr, residual water and olefenic/glycogen moieties. Fig.3 shows the long TE MR spectrum recorded in the mixed medial hip adductr and posterior knee flexor muscle regions of the same 25 year-old healthy subject whose vastus muscle data is presented in Figs.1-2. An axial T1-weighted calf MRI of a 38 year-old healthy male and the calf muscle long TE spectrum are presented in Fig.4. At short TE (30ms), the acetylcarnitine peak was hardly visible due to broad lipid resonances in the region of 2.0 to 2.5 ppm. Also very characteristic for the long-TE spectrum was the sharp, single, and symmetric appearance of the total creatine peak. Our pilot findings showing acetylcarnitine without exercise are in agreement with the previous study by Lindeboom et al. (10). However, there were varying levels of acetylcarnitine in different muscles of healthy subjects.


These results demonstrate our initial experience in evaluating PRESS-localized long TE spectra in different regions of the thigh and calf muscles monitoring acetylcarnitine, IMCL, EMCL, creatine, trimethylamines and carnosine. Future efforts will focus on evaluating the MRS technique in larger cohorts of endurance trained and lean young healthy, obese and T2D patients.


Authors acknowledge the partial support of an NIH R01 grant (DK090406) and also, scientific discussions with Dr. Theodore Hahn and Dr. Catherine Carpenter.


1. Petersen KF, Dufour S, Morino K, et al. Reveral of muscle insulin resistance by weight reduction in lean, insulin-resistant offspring of patients with type 2 diabetes. Proc Natl Acad Sci USA 2012; 109(21):8236-40. Epub 2012 Apr 30.

2. Shulman GI. Cellular mechanisms of insulin resistance. J Clin Invest 2000;106(2):171-6 3.

3. Petersen KF and Shulman GI. Etiology of Insulin Resistance. The Am J Med 2006;119:S10-6.

4. Lettner A and Roden M. Ectopic fat and insulin resistance. Cur Diabetes Reports 2008;8:185-91.

5. Boesch C, Slotboom J, Hoppeler H, Kreis R. In vivo determination of intramyocellular lipids in human skeletal muscle by means of localized 1H MR spectroscopy. Magn Reson Med 1997;37:484-493.

6. Steidle G, Machann J, Claussen CD, Schick F. Separation of intra- and extramyocellular lipid signals in proton MR spectra by determination of their magnetic field distribution. J Magn Reson 2002;154:228-235.

7. Torriani M, Thomas BJ, Halpern EF, et al. Intramyocellular lipid quantification: Repeatability with 1H MR Spectroscopy. Radiology 2005;236(2):609-14.

8. Boss A, Kreis R, Saillen P, et al. Skeletal muscle 1H MRSI before and after prolonged exercise. II. Visibility of free carnitine. Magn Reson Med 2012;68:1368-1375.

9. Ren J, Lakoski S, Haller RG, Sherry AD, Malloy CR. Dynamic monitoring of carnitine and acetylcarnitine in the trimethylamine signal after exercise in human skeletal muscle by 7T 1H MRS. Magn Reson Med 2013;69:7-17.

10. Lindeboom L, Nabuurs C, Hoeks J, et al. Long echo time MR Spectroscopy for skeletal muscle acetyl carnitine detection. J Clin Invest 2014;124:4915-25.

11. Bottomley P. Spatial localization in NMR spectroscopy in vivo. Ann N Y Acad Sci. 1987;508:333-48.

12.Wilson M, Reynolds G, Kauppinen RA, Arvanitis TN, Peet AC. A constrained least-squares approach to the automated quantitation of in vivo 1H magnetic resonance spectroscopy data. Magn Reson Med 2011;65:1-12.


Fig.1. T1-weighted axial thigh MRI of a 25-year-old lean young healthy volunteer with the white box (4x3x4cm3) indicating the PRESS VOI localization in the vastus muscle

Fig.2. PRESS-localized long TE spectra recorded in the vastus muscles of a 25 year-old healthy subject (A) and a 61 year-old T2D patient (B)

Fig.3. (A) PRESS localization (white box) shown on an axial thigh MRI of a 25 year-old healthy male subject. (B) A long TE MR spectrum recorded in the mixed medial hip adductor and posterior knee flexor muscle regions.

Fig.4. (A) PRESS localization (white box) shown on an axial calf MRI of a 38 year-old healthy male subject. (B) A long TE MR spectrum recorded in the soleus/gastronemic muscle.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)