Rajakumar Nagarajan1, Raissa Souza1, Edward Xu1, Manoj K Sarma1, S. Sendhil Velan2, Cathy C Lee3, Theodore Hahn3, Catherine Carpenter4, Vay-Liang Go5, 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, 3Geriatrics, VA Greater Los Angeles Healthcare System, Los Angeles, CA, United States, 4UCLA Schools of Nursing, Medicine, and Public Health, Los Angeles, CA, United States, 5UCLA Department of Medicine, Los Angeles, CA, United States
Synopsis
Obesity is a serious public health problem associated with
high rates of morbidity and mortality. One-dimensional MR spectroscopy suffers
from overlapping spectral resonances which can complicate metabolite
identification and quantitation. Two-dimensional spectroscopic techniques have
been demonstrated in calf muscle to reduce the problem of spectral overlap. In
this study, we used the four dimensional (4D) multi-echo echo planar correlated
spectroscopic imaging (ME-EPCOSI) technique to quantify the lipids and
metabolites in soleus, tibialis anterior and gastrocnemius
calf muscles of obese and normal healthy subjects. The 4D ME-EPCOSI acquired data enabled less ambiguous quantitation
of metabolites, unsaturated and saturated fatty acids in different calf muscle
regions using IMCL ratios and unsaturation indices.Purpose/Introduction
Obesity is a serious public health problem associated with high rates of morbidity and mortality in the U.S. has long term consequences including increased risk for coronary heart disease, cancer, stroke and diabetes (1). Obesity is the single most important risk factor for the development of type 2 diabetes mellitus, and current high rates of obesity are increasing the prevalence of this disorder (2). MRS has the capacity to quantitatively determine intramyocellular lipids (IMCL), extramyocellular lipids (EMCL) and metabolites in calf muscle in vivo (3). One-dimensional MRS is hindered by overlapping spectral resonances which can complicate assignment of metabolite resonances and quantitation. Two-dimensional MRS techniques have been demonstrated in calf muscle (4, 5) to minimizing the problem of spectral overlap by spreading resonances into a second (indirect) dimension, improving dispersion of spectral resonances. To investigate the utility of the MRSI technique in the assessment of obesity, we used the recently implemented four dimensional (4D) multi-echo echo planar correlated spectroscopic imaging (ME-EPCOSI) technique (6) to quantify the lipids and metabolites in soleus, tibialis anterior and gastrocnemius of calf muscles.
Materials and Methods
The 4D ME-EPCOSI sequence was tested in the calf
muscle of 31 obese subjects (mean age 28.0 years) and eleven normal healthy
subjects (mean age 32.2 years) in 3T MRI scanner (Siemens Medical Systems) using a 15 channel knee coil. The mean body mass indexes (BMI) for obese and
healthy subjects were 38.9 and 22.7, respectively. The parameters for the
ME-EPCOSI were: TR/TE/Average = 1.5s/30ms/1, 16 phase encoding steps, FOV=
16x16 cm
2, 256 complex points with an F
2 bandwidth of
1190Hz. For the second dimension (F
1), 50 increments with bandwidths
of 1250Hz were used. The individual voxel volume in calf was 2ml. A non-water-suppressed scan was
acquired with only one t
1
(Δt
1=0) and 16×16 spatial encoding as a reference to correct for
eddy current distortions created by the EPSI readout. The 4D ME-EPCOSI data were processed using home build GUI
based MATLAB program.
Results
Figs.1-3
show the IMCL/Cr, unsaturation index and choline/Cr recorded in obese and
normal healthy subjects. The MRI and 4D ME-EPCOSI voxel placement in a 25 year
old obese subject with extracted soleus muscle spectrum are shown in Fig.4. The
2D COSY spectra from the soleus, tibialis anterior and gastrocnemius muscle
spectra were extracted from 4D ME-EPCOSI data. Using 4D ME-EPCOSI, the
following resonances can be detected: creatine (Cr), choline , olefinic protons,
methyl and methylene resonances of IMCL and EMCL and carnosine. Because of the
overlap of methyl, methylene and olefinic protons from IMCL and EMCL resonances from saturated and unsaturated lipid pools in 1D MRS/MRSI, it is difficult to estimate the degree of unsaturation
in the two lipid pools. This provides a measure of double bonds, i.e. using
this ratio the unsaturation within IMCL and EMCL lipid pools. This can be
assessed using 4D ME-EPCOSI. In this study, we observed a significant elevation
of IMCL/Cr found in the soleus, tibialis anterior and gastrocnemius muscles in
obese (p<0.05) compared to normal healthy subjects. Significant
decrease of the EMCL unsaturation index was observed in tibialis anterior and
gastrocnemius muscle. Also a non-significant elevation trend of choline was observed
in obese compared to normal healthy subjects.
Discussion
Obesity is
a known risk factor for the development of insulin resistance, a key component
of the metabolic syndrome. Increased IMCL plays an etiologic role in the
development of insulin resistance (7). Most of the earlier studies are limited to single voxel of MRSI based approaches for investigating the lipids. By using 4D ME-EPCOSI, we were able to increase the spatial coverage in skeletal muscle within a shorter time (10 mins).Our pilot results demonstrated higher
IMCL and EMCL creatine ratios in obese compared to healthy subjects. We observed decreased unsaturation index in
obese subjects compared with normal healthy subjects, indicating a reduction of
desaturase activity in these subjects. A trimethylamine moiety is present in carnitine and acetylcarnitine, and both molecules play critical roles in muscle metabolism (8).The reduced degree of unsaturation in
obese subjects may result from increased lipid peroxidation (9). Previous
studies demonstrated similar results using bigger voxel size (27ml) with the
scan time of >15 mins in single location (10).
Conclusion
The
4D ME-EPCOSI acquired data enabled quantitation of metabolites, unsaturated and
saturated fatty acids in different calf muscle regions using IMCL ratios and
unsaturation indices. The ME-EPCOSI technique has a major potential to be
valuable in various clinical studies, such as diabetes and heart failure where
the levels of IMCLs in various muscles can be used as markers for differences
in metabolic activity.
Acknowledgements
Authors acknowledge the support by NIH/NIDDK: (R01DK090406). References
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