(MRS / MRSI -
Magnetic Resonance Spectroscopic Imaging) A method using the
NMR phenomenon to identify the chemical state
of various elements without destroying the sample. MRS therefore provides information about the chemical composition
of the tissues and the changes in chemical composition, which may occur with disease processes.
Although MRS is primarily employed as a research tool and has yet to achieve widespread acceptance in routine clinical practice, there is a growing realization that a noninvasive technique, which monitors disease biochemistry can provide important new information for the clinician.
The underlying principle
of MRS is that atomic nuclei are surrounded by a cloud
of electrons, which very slightly shield the
nucleus from any external
magnetic field. As the structure
of the electron cloud is specific to an individual
molecule or compound, then the
magnitude of this screening effect is also a characteristic
of the chemical environment
of individual nuclei.
In
view of the fact that the resonant
frequency is proportional to the
magnetic field that it experiences, it follows that the resonant
frequency will be determined not only by the external applied field, but also by the small field shift generated by the electron cloud.
This shift in frequency is called the chemical shift (see also
Chemical Shift). It should be noted that
chemical shift is a very small effect, usually expressed in ppm
of the main
frequency. In order to resolve the different chemical species, it is therefore necessary to achieve very high levels
of homogeneity of the main
magnetic field B0.
Spectra from humans usually require
shimming the
magnet to approximately one part in 100. High
resolution spectra of liquid samples demand a
homogeneity of about one part in 1000.
In addition to the effects
of factors such as relaxation times that can affect the
NMR signal, as seen in
magnetic resonance imaging, effects such as
J-modulation or the transfer
of magnetization after
selective excitation of particular spectral lines can affect the relative strengths
of spectral lines.
In the context
of human MRS, two nuclei are
of particular
interest - H-1 and P-31. (PMRS -
Proton Magnetic Resonance Spectroscopy)
PMRS is mainly employed in studies
of the
brain where prominent peaks arise from NAA, choline containing compounds, creatine and creatine phosphate, myo-inositol and, if present, lactate; phosphorus 31 MR
spectroscopy detects compounds involved in
energy metabolism (creatine phosphate, adenosine triphosphate and inorganic phosphate) and certain compounds related to membrane synthesis and degradation. The frequencies
of certain lines may also be affected by factors such as the local pH. It is also possible to determine intracellular pH because the inorganic phosphate
peak position is pH sensitive.
If the field is uniform over the
volume of the sample, "similar" nuclei will contribute a particular
frequency component to the detected response signal irrespective
of their individual positions in the sample. Since nuclei
of different elements resonate at different frequencies, each
element in the sample contributes a different
frequency component. A chemical analysis can then be conducted by analyzing the
MR response signal into its
frequency components.
See also
Spectroscopy.
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