Basic Principles of MR Spectroscopy

 
 
Different MR techniques use similar principles to obtain information from a tissue sample. Although information can be obtained from any nuclei with an odd atomic number, 1H is most commonly used in medical applications because it is far more prevalent within the human body than other nuclei with magnetic moments such as 31P and 13C. MR imaging techniques use gradient magnetic fields to provide spatial information to generate an image within a slice of tissue. In contrast, MR spectroscopy uses these fields to localize a specific volume of tissue for spectral analysis. Mathematically, the free induction decay signal from this volume is transformed into specific components of different amplitudes and frequencies. Each frequency corresponds to nuclei in a distinct magnetic environment, with the amplitude related to the number of nuclei within the volume in that particular state.
 
Two different techniques are commonly used to localize the desired volume for analysis: STEAM (stimulated echo acquisition mode) and PRESS (point-resolved spectroscopy).[22] Typically, the STEAM pulse sequence uses relatively short echo times (20 to 30 ms) and permits metabolites with short T2 relaxation times, such as myoinositol, glutamine/glutamate, and mobile lipids, to be examined.[27] For longer echo times (> 135 ms), PRESS is the volume-localization method of choice. Compared to STEAM, PRESS provides a two-fold gain in signal intensity and is less sensitive to patient motion.[5] Primarily due to its advantageous signal-to-noise ratio (SNR), PRESS is the volume localization method most often used in clinical spectroscopy. Both techniques require that the magnetic field over the analyzed volume be as homogeneous as possible. Inhomogeneity of the magnetic field broadens the resonance peaks, often making it impossible to distinguish among smaller adjacent peaks.
 
Figure 1.  Proton magnetic resonance spectroscopy in a normal patient.  (A) Stimulated echo acquisition mode technique with short echo delay (35 ms) demonstrates normal choline (Cho), creatine (Cr), N-acetyl asparate (NAA), and myoinositol (Myo) peaks.  (B) Point-resolved spectroscopy (PRESS) technique with a long echo delay (288 ms) in the same location demonstrates only choline, creatine, and NAA peaks.  Due to the long echo time, the myoinositol peak is not seen with PRESS.

The spectra typically displayed are not expressed in absolute units of frequency (Hz) but rather in dimensionless units of parts per million (ppm). Spectra from different MR units can then be compared because the resonance frequencies are directly proportional to the strength of the magnetic field. Commonly encountered resonances in normal brain tissue include N-acetyl aspartate (NAA), choline, creatine, myoinositol, and glutamine/glutamate (Fig. 1). Lactic acid, lipids, acetate, succinate, and amino acids are other resonances often found in a variety of disease states.

NAA (2.0 ppm) is the largest peak in normal spectra. It is considered a marker of normal nerve cell body and axonal function. A decrease in its peak indicates axonal injury or neuronal loss.[27] Choline is a component of phospholipid metabolism. Increases in the choline peak (3.2 ppm) likely reflect an increase in membrane synthesis, an increase in the number of cells, or both.[5] Creatine (3.03 ppm) likely serves as a reserve for energy reservoirs and remains relatively stable even when disease is encountered. Myoinositol (3.56 ppm) is a metabolite involved in hormone-sensitive neuroreception; its peak increases in Alzheimer’s disease.[25] Glutamate and glutamine resonate together between 2.1 and 2.5 ppm. Glutamate is an excitatory neuro transmitter, and glutamine is in volved in the regulation of neurotransmitter activities.[24,32]

The lactate peak (1.32 ppm) consists of two distinct resonant peaks (a doublet), which reflect magnetic field interactions between adjacent protons (referred to as J-coupling). Typically, the lactate peak indicates the presence of carbohydrate catabolism.[30] A peak at 1.32 ppm can be confirmed as lactate by altering the echo time. At echo times of 35 and 288 ms, the lactate doublet is above baseline while it inverts below baseline when the echo time is 144 ms. Membrane lipids in the brain are seldom encountered unless echo times are short. When present, lipid resonances occur at 0.8, 1.2, 1.5, and 6.0 ppm. Acetate (1.9 ppm), succinate (2.4 ppm), and various amino acids (0.9 ppm) are bacterial metabolites that can be encountered in abscess cavities.

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