Chemical shift encoding techniques can quantify chemical species content and investigate metabolic changes in physiological and diseased conditions of multiple musculoskeletal tissues, including skeletal muscle, bone marrow, intervertebral disc, cartilage and bone. The present lecture aims to provide an overview of the most important technical aspects when applying chemical shift encoding techniques, including single-voxel magnetic resonance spectroscopy, chemical shift imaging and chemical shift encoding-based water-fat separation techniques, for targeting lipids, creatine, macromolecules, choline and phosphorous metabolites in musculoskeletal tissues.
Chemical shift is a fundamental MR property of the nucleus, corresponding to the resonant frequency of the nucleus relative to a standard in a magnetic field. The chemical shift is typically encoded using a magnetic resonance spectroscopy (MRS) experiment. The measurement of chemical shift properties can be used to quantify chemical species content and characterize tissue metabolism, using either protons (1H) or non-proton nuclei (31P, 13C) chemical shift encoding techniques.
Chemical shift encoding techniques can quantify chemical species content and study metabolic changes in most musculoskeletal tissues, including skeletal muscle, bone marrow, intervertebral disc, cartilage and bone. Specifically, chemical shift encoding techniques have been used to monitor muscle metabolism, quantify bone marrow composition, detect biochemical changes in cartilage and intervertebral disc, measure bone mineral content and metabolically characterize musculoskeletal tumors.
On the technical level, the development of chemical shift encoding techniques for musculoskeletal applications has been driven in the earlier years by the needs to non-invasively monitor muscle metabolism using MRS. More recently chemical shift encoding water-fat separation techniques have been emerging to non-invasively perform fat quantification in musculoskeletal tissues (e.g. skeletal muscle, bone marrow). The present lecture focuses on the technical aspects of the application of chemical shift encoding techniques in healthy and diseased musculoskeletal tissues. It would be practically impossible for the present lecture to cover all the technical aspects of the aforementioned musculoskeletal applications of chemical shift encoding techniques. Hydrogen and phosphorous are the two most commonly used nuclei in chemical shift encoding techniques for musculoskeletal applications. Therefore, the present lecture aims to provide an overview of the most important technical aspects when applying chemical shift encoding techniques for targeting lipids, creatine, macromolecules, choline and phosphorous metabolites in musculoskeletal tissues.
Lipids in skeletal muscle and bone marrow
Lipids constitute an important component of both skeletal muscle and bone marrow. The quantification of lipids using chemical shift techniques has been a central point of interest in studies of healthy and diseased skeletal muscle and bone marrow. Chemical shift encoding techniques can be used to separate different lipid compartments, quantify fat (using single-voxel MRS or imaging) and to measure fatty acid composition parameters.
Separation of lipid compartments
The doubling of the lipid resonances in skeletal muscle MRS was first observed by Schick et al. (1) and the two entities were then identified by Boesch et al. (2) as intra-myocellular lipids (IMCLs) and extra-myocellular lipids (EMCLs). EMCLs correspond to adipocytes in the interstitial space between the muscle fibers and IMCLs correspond to spherical droplets adjacent to mitochondria within the muscle fibers. The quantification of IMCLs concentration has gained a significant interest thanks to the association of IMCLs with insulin resistance. However, the separation of the EMCLs and IMCLs peaks in single-voxel MR spectra can be challenging in regions with strong EMCL component. High field strength MRS, long echo time MRS, CSI techniques and recently diffusion-weighted MRS techniques have been proposed to overcome the above challenge and robustly quantify IMCLs in severely fatty infiltrated skeletal muscles (3).
Single voxel MRS-based fat quantification
Single-voxel MRS can be used to integrate the area of the different fat peaks and measure tissue fat content. In order to measure the proton density fat fraction, T1 relaxation effects need to be reduced and T2 relaxation effects need to be corrected for (4). In addition, when applying single MRS for bone marrow fat quantification, constrained peak fitting routines should be used in order to robustly extract the water peak from the overlapping fat peaks in the spectrum (5).
Imaging-based fat quantification
To allow high spatial resolution fat quantification in clinically feasible scan times, fat selective imaging and chemical shift encoding-based water-fat separation separation techniques have been proposed. Fat selective imaging is inherently sensitive to B0 field inhomogeneity effects and requires appropriate correction steps to account for coil profile effects in the required calibration process using a reference signal (6). Chemical shift-encoding based water-fat separation techniques address the general issue of the sensitivity of chemical shift selective imaging to B0 field inhomogeneity effects. Chemical shift-encoding water-fat separation techniques excite both water and fat and acquire data at multiple echo times in a gradient-echo or an asymmetric spin-echo sequence. The separation of the total measured signal into water and fat components is based on the chemical shift difference between the two species.
Chemical shift-based water-fat separation has been emerging into becoming a quantitative tool for measuring proton-density weighted fat fraction maps in vivo (7,8). Specifically, quantitative water-fat imaging techniques have shown excellent agreement with single-voxel MRS in measuring fat content in different body parts, after consideration of multiple confounding factors, including main magnetic field inhomogeneity effects (9), the presence of multiple peaks in the fat spectrum (10,11), T2* effects (10,12), T1-bias effects (10,13) and eddy current effects (14,15).
In the context of quantitative water-fat imaging in bone marrow, previous works have addressed the need for modeling T2* effects (16). In the context of quantitative water-fat imaging in skeletal muscle, previous works have addressed the need for noise efficient correction of T1-bias effects (17) and have investigated the effect of susceptibility-induced EMCLs fat resonance shift on the measured fat fraction (18).
Quantification of fatty acid composition
Measurement of fatty acid composition requires the reliable extraction of the different fat peaks. The extraction of the olefinic fat peak, which is traditionally used to measure fat unsaturation, can become challenging in regions with low fat fraction (e.g. in skeletal muscle) or in regions with broad linewidths and strong water component (e.g. in vertebral bone marrow). Lipid J-coupling effects need to be also considered in the quantification of fatty acid composition based on 1H MRS. Long-TE PRESS and DW-STEAM MRS techniques have been recently emerging to overcome the above technical challenges in the quantification of fatty acid composition (19,20).
Other metabolites in skeletal muscle
In water-suppressed skeletal muscle MR spectra, in addition to lipids, contributions of methyl and methylene groups of creatine (Cr3, Cr2), trimethylammonium-containing compounds (TMA), including signals from carnitine (Ct), choline (Cho), and taurine (Tau) are also observable. An important feature of muscle spectra is the the splitting of the methylene and methyl signals of creatine (Cr2 and Cr3), and the frequency shifts of taurine (Tau). The above effects depend on the orientation of the muscle fibers relative to the main magnetic field and are caused by dipolar coupling effects, induced by the highly ordered structure of skeletal muscle (21).
Macromolecules in collagenous tissues
MRS techniques have been proposed to measure peaks which are related to proteoglycans and collagens for early detection in cartilage and intervertebral disc degeneration (22,23). The relative low SNR of such measurements and lipid contamination constitute the main challenges on the application of MRS in cartilage and intervertebral disc.
Choline in musculoskeletal tumors
MRS has been also applied in musculoskeletal tumor malignancy studies (24). These studies employed short‐ and intermediate‐TE 1H MRS to quantify the tCho concentration in malignant and benign skeletal tumors, and found that tCho may serve as a marker for malignancy in musculoskeletal tumors. A significant challenge when applying MRS in musculoskeletal tissues is again the lipid contamination.
In muscle, 31P-MRS has been established as the standard method for the in vivo study of skeletal muscle bioenergetics by measuring the post-exercise phosphocreatine (PCr) resynthesis rate (25). 31P MRS muscle work has used pulse-and-acquire methods and physical location by surface coil. Single-voxel localization has, until recent 7 T developments (26), required an unacceptable trade-off with time resolution. However, recent developments using spectrally selective chemical shift imaging data acquisition and accelerated reconstruction techniques have enabled high spatial and temporal resolution 31P metabolite mapping in skeletal muscles (27,28).
In bone, as the mineral component largely consists of calcium phosphates, 31P imaging had been proposed as a way to directly measure mineral content without ionizing radiation (29). However, the application of 31P imaging to examine bone is hampered by the ultra-short T2* relaxation time of the 31P signals in bone. Ultrashort echo time (UTE) techniques are therefore necessary for 31P bone imaging.
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