Published online Jan 28, 2019. doi: 10.4329/wjr.v11.i1.1
Peer-review started: August 7, 2018
First decision: October 16, 2018
Revised: November 14, 2018
Accepted: January 9, 2019
Article in press: January 10, 2019
Published online: January 28, 2019
Proton magnetic resonance spectroscopy (1H MRS) is a technique widely used for investigating metabolites in humans. Lipids that are stored outside the muscle cell are called extramyocellular lipids (EMCL), and lipids stored on the inside of muscle cells are called intramyocellular lipids (IMCL). The relationship between metabolic syndrome and IMCL has been extensively studied. However, muscle position in relation to the main magnetic field can affect spectra profiles, leading to inconsistency of metabolite quantification, which can then lead to misinterpretation.
There is no current study that has directly measured muscle alignment to the main magnetic field or how the muscle fibers are aligned between studies, as it is impossible exactly measure the angle of muscle relative to the main magnetic field in humans.
To determine the effects of the muscle fiber angle to the main magnetic field for obtaining spectrum profiles and muscle lipid quantification without the effects of muscle contraction. This study used extensor iliotibialis lateralis muscles taken from the thigh of a chicken as the muscle of interest. Since it is the uppermost muscle, it provides a clear visualization of the muscle fiber alignment related to the main magnetic field.
Chicken extensor iliotibialis lateralis muscles were used as the muscle of interest in this study. Magnetic resonance imaging (1.5 Tesla Philips Achieva) was used for the 1H MRS spectrum acquisition. The chicken extensor iliotibialis lateralis muscle fiber alignment was used as the reference and was place in the middle of the coil, positioned at 0˚, 30˚, 60˚, and 90˚ to the main magnetic field. Single voxel Point Resolved Spectroscopy pulse sequence was used for spectrum acquisition, having a voxel size of 8 mm × 8 mm × 20 mm. It was carefully placed on the iliotibialis lateralis muscle. Spectra acquisition was repeated 7 times for each angle. JMRUI version 6.0 β was used for metabolite peak assignment and analysis. Spectrum fitting was done by an AMARES algorithm with prior knowledge. The fitted spectrum showed various peaks of metabolites of interest in the following manner: IMCL (CH3) at 0.9 ppm, EMCL (CH3) at 1.1 ppm, IMCL (CH2) at 1.3 ppm, EMCL (CH2) at 1.5 ppm, and Cr at 3.02 ppm. IMCL and EMCL amplitudes fitted by AMARES were calculated into the ratio per signal intensity of Cr in each spectrum as the internal reference. The results of spectrum fitting at 0°, 30°, 60°, and 90° of muscle fiber orientation to the main magnetic field were compared using Wilcoxon signed-rank test.
The results showed that the metabolite profiles in each orientation of muscle fiber to the main magnetic field were different. The orientation at 90° was the most different compared to the other orientations. The quantity of muscle metabolites was statistically significantly changed at 30°, 60°, and 90° of muscle fiber relative to the main magnetic field when compared to 0° relative to the main magnetic field. Statistical analysis showed statistically significant differences for IMCL (CH3), EMCL (CH3), IMCL (CH2) at 30°, 60°, and 90° (P = 0.017, 0.018, and 0.018, respectively) and EMCL (CH2) at 30° and 60° (P = 0.017 and 0.042, respectively). EMCL (CH2) at 90° was unable to be measured in this study. Furthermore, the muscle lipids quantified at 30°, 60°, and 90° tended to be lower when compared to 0°. The metabolite profile changed due to the muscle fiber orientation, indicating that positioning potentially causes inaccuracies in 1H-MRS spectrum analysis.
This study has determined that the basic muscle orientations to the main magnetic field can and do affect 1HMRS spectrum profiles and quantification. Muscle orientation is often treated with less care in studies on 1H MRS. These metabolite profile changes are due to the muscle fiber orientation, which demonstrates that the positioning potentially causes inaccuracy in 1H-MRS spectrum analysis.
1H MRS practitioners and users need to be especially careful when positioning any muscles or any organs of interest in order to reduce error, to be able to compare spectrum results across various institutions and to ensure reproducibility and uniformity.