Physics Of Spectroscopy

Spectroscopy is based on chemical shifts. A proton (hydrogen nucleus) in a magnetic field (for example, 1.5 T) has a precession frequency governed by Larmor's equation (64 MHz at 1.5 T). In fact, the local magnetic field experienced by the proton is not exactly the same as the external magnetic field. This is because the adjacent orbiting charged particles (for example, electrons)

From: Minimally Invasive Neurosurgery, edited by: M.R. Proctor and P.M. Black © Humana Press Inc., Totowa, NJ

produce their own magnetic field, which add to, or subtract from, the external magnetic field. Thus the resonance frequency of a proton is different (shifted) from what is expected in the given external field. The degree of shift, expressed in parts per million (ppm), depends on the chemical structure of the material. For example, the chemical shift of a proton in the radical -CH3 is different from that of a proton in the radical -CH2-.

Like conventional MRI, imaging in MRS begins with the MR signal. Radio waves at an exact frequency will cause the protons in the magnetic field to go to higher energy levels. These higher energy levels are unstable, and protons give up the extra energy in the form of a signal, the amplitude of which diminishes with time (free induction decay). In order to produce spectra on which the differences in chemical shifts are recorded, the received MR signal is subjected to Fourier transform to change it from a plot of intensity vs time to a plot of intensity vs frequency (change from time domain to frequency domain). To ensure that the shifts in frequency reflect different chemical structures only, one has to ensure the homogeneity of the external magnetic field over the imaging region. Indeed, ensuring that the magnetic field is homogenous is key to obtaining good spectra (2).

To obtain spectra from important metabolites in the brain, it is essential to suppress signal generated by water. In biological specimens the concentration of water is 1000-10,000 times greater than that of these metabolites; thus its signal, if not suppressed, will overwhelm the spectrum. A variety of water suppression techniques are available.

From what has been said so far, it is clear that successful spectroscopy depends on good suppression of water as well as achieving external field homogeneity (2). The process of doing both has been simplified in most modern scanners by performing a prescan. At the end of this, the imager measures the degree of water suppression as well the homogeneity of the field. If these are satisfactory, the actual acquisition of spectra is allowed to begin.

Contrast enhancement is an essential part of MR evaluation of many lesions. Spectroscopy usually begins after this enhancement. Does this process change the spectra in a way detrimental to identification of the peaks? The effects of enhancement on spectra are subject to much debate, but overall it is believed that in most instances, these effects are negligible.

The important metabolites detected by MRS include N-acetyl aspartate (NAA), creatine, choline, lactate, myoinositol, and mobile lipids (3,4).

NAA is a marker of neuronal population and function, and this metabolite in adult brain is confined to neurons. Whenever neurons die (for example, in ischemic infarction) or are displaced by other elements (for example, in tumors), the NAA peak is diminished.

Creatine is in constant equilibrium with phosphocreatine and is a marker of oxidative metabolism of the cells. The concentration of creatine is tightly regulated and is not easily affected by disease processes. For this reason, it acts as a yardstick against which other peaks are measured. Brain does not produce its creatine; it is transported to the brain from liver and kidney.

A choline peak at 3.2 ppm is a combination of multiple metabolites (free choline, phosphocholine, phosphatidyl choline, and glycerophosphocholine). Some of these metabolites are degradation products of cell membranes; some others are metabolites used in the synthesis of membranes. Because of increases in cell membrane turnover, the choline peak is elevated in all brain tumors. Many investigators point to a high choline/creatine ratio as nonspecific spec-troscopic evidence of neoplasia.

Lactate is normally produced in small amounts. These small amounts are usually undetected by MRS. Whenever the glucose metabolism becomes essentially anaerobic, lactate in detectable amounts is produced. Lactate at 1.3 ppm is a doublet peak at TE of 140 ms. With a TE of 40 ms, the lactate peak is all entirely above the baseline. This change with different TEs is essential in positive identification of the lactate peak.

Plasma membrane lipids are seen in conditions associated with necrosis. The peak owing to these may extend over a large segment of the spectrum and overlaps the lactate peak. They do not have the characteristic doublet appearance of lactate and become far less noticeable with a longer TE (for example, 270 ms).

Glutamate is an abundant amino acid and a metabolite related to detoxification of ammonia. It is also an excitatory neurotransmitter.

Myoinositol at 3.6 ppm is a cerebral osmolyte. It may be a degradation product of myelin and possibly a marker for glial cells.

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