[1][2] Magnetic resonance spectroscopy (MRS), also known as nuclear magnetic resonance (NMR) spectroscopy, is a non-invasive, ionizing-radiation-free analytical technique that has been used to study metabolic changes in brain tumors, strokes, seizure disorders, Alzheimer's disease, depression, and other diseases affecting the brain.
MRS can be used to determine the relative concentrations and physical properties of a variety of biochemicals frequently referred to as "metabolites" due to their role in metabolism.
[5] Similar to MRI, MRS uses pulse sequences to acquire signal from several different molecules to generate a spectra instead of an image.
In MRS, STEAM (Stimulated Echo Acquisition Method) and PRESS (Point Resolved Spectroscopy) are the two primary pulse sequence techniques used.
Additionally, adiabatic pulses have substantially higher bandwidth, which reduces chemical shift displacement artefact, which is particularly important at high field strengths and when a large range of frequencies are desired to be measured (i.e., measuring both the signals upfield and downfield of water in proton MRS).
In PRESS, the two chief drawbacks are lengthy echo time (TE) and chemical shift displacement (CSD) artifacts.
[5] SPECIAL obtains spatial localization from only a single dimension with pre-excitation inversion pulses (cycled on and off every other repetition time [TR]), making it a two-cycle sequence.
[5] The largest drawback of SPECIAL and SPECIAL-sLASER is that they are two-cycle schemes, and systematic variations between cycles will manifest in their difference spectrum.
The second is not to set the amplitude of the pre-excitation inversion pulse to zero every other TR, but instead to shift the location of this ISIS plane such that the excited volume for the off condition is outside the object.
MRS equipment can be tuned (just like a radio receiver) to pick up signals from different chemical nuclei within the body.
The types of biochemicals (metabolites) which can be studied include choline-containing compounds (which are used to make cell membranes), creatine (a chemical involved in energy metabolism), inositol and glucose (both sugars), N-acetylaspartate, and alanine and lactate which are elevated in some tumors.
At present MRS is mainly used as a tool by scientists (e.g. medical physicists and biochemists) for medical research projects, but it is becoming clear that it also has the ability to give doctors useful clinical information, especially with the discovery that it can be used to probe the concentration of alpha-Hydroxyglutaric acid, which is only present in IDH1 and IDH2 mutated gliomas, which alters the prescribed treatment regimen.
The red box shows the volume of interest from which chemical information was obtained by MRS (a cube with 2 cm sides which produces a square when intersecting the 5 mm thick slice of the MRI scan).
Concentration marker Because of the overlap between GABA and Homocarnosine resonances, the GABA H-4 resonance at 3.01 ppm is the "total GABA" representing the sum of GABA and homocarnosine Short T2 relaxation time constants effectively eliminate macromolecular resonances from long-echo-time 1H NMR spectra Difference in T1 relaxations between metabolites and macromolecules is used to reduce contribution from extracranial lipid signal Water chemical shift used to detect temperature changes noninvasively in vivo In 1H Magnetic Resonance Spectroscopy each proton can be visualized at a specific chemical shift (peak position along x-axis) depending on its chemical environment.
Nevertheless, no alternate technique is able to quantify metabolism in vivo non-invasively and thus MRS remains a valuable tool for research and clinical scientists.
In addition, despite recent efforts toward international expert consensus on methodological details like shimming,[21] motion correction,[22] spectral editing,[23] spectroscopic neuroimaging,[24] other advanced acquisition methods,[25] data processing and quantification,[26] application to brain,[27] proton spectroscopy application to skeletal muscle,[28] phosphorus application to skeletal muscle,[29] methods description,[30] results reporting,[31] and other considerations, currently published implementations of in vivo magnetic resonance spectroscopy cluster into literatures exhibiting a broad variety of individualized acquisition, processing, quantification, and reporting techniques.
[32] This situation may contribute to a low sensitivity and specificity of, for example, in vivo proton magnetic resonance spectroscopy to disorders such as multiple sclerosis, that continue to fall below clinically beneficial thresholds for, e.g., diagnosis.
This is in large part because of the relatively high sensitivity of phosphorus NMR (7% of protons) combined with a 100% natural abundance.
Even at low field strengths, great spectra resolution is obtained because of the relatively large (~30 ppm) chemical shift dispersion for in vivo phosphates.
Clinically, phosphorus NMR excels because it detects all metabolites playing key roles in tissue energy metabolism and can indirectly deduce intracellular pH.
[19]: 96–102 However, interest in sodium NMR has been reinspired by recent significant gains in SNR at high magnetic fields, along with improved coil designs and optimized pulse sequences.
Because19F is not found in biological tissues, 19F does not have to deal with interference from background signals like in vivo 1H MRS does with water, making it especially powerful for pharmacokinetic studies.
[19]: 102–104 https://aclarion.com/ NOCISCAN (aclarion) – The first, evidence-supported, SaaS platform to leverage MR Spectroscopy to noninvasively help physicians distinguish between painful and nonpainful discs in the spine.