Image Processing Reference
In-Depth Information
(H-MRS). If a spectroscopic examination is per-
formed under ideal conditions with
termed
proton spectroscopy
samples of human tissue, a great variety
of signals can be observed in H-MRS. Under the limitations of
in vitro
examinations,
signals from only a few metabolites can be clearly identified in the spectrum. The
metabolites with the signals that are easiest to evaluate are creatine, choline, and
in vivo
-
acetyl aspartate (NAA), which is a neurotransmitter observed predominantly in exam-
inations of brain and spinal cord. Signals from other metabolites such as glutamate,
glutamine, or citrate can be studied, if advanced measurement and evaluation tech-
niques are applied, but many other molecules remain invisible in spectroscopic exam-
inations due to their low concentration within the tissue, or due to short relaxation
times or strong coupling effects. The challenge of
N
spectroscopy is, therefore,
the interpretation of signals of those few molecules that can be identified within the
spectrum and that might give important additional diagnostic information.
In the case of H-MRS, no additional hardware is necessary to perform
spectroscopic measurements at an MR scanner. The available field gradients are
used for volume selection, and the radio frequency (RF) coils developed for MR
imaging can be used to apply RF pulses and to acquire the spectra. Only special
measurement sequences and evaluation procedures are necessary. In this chapter,
the most common measurement techniques are described, whereas appropriate
evaluation techniques are the topic of the following chapter. Because H-RMS
is the most frequently used type of MR spectroscopy in a clinical environment,
it is the chief topic of this chapter. Two possible measurement techniques for
H-RMS are described in two sections: single-voxel spectroscopy (SVS), which
can be used to obtain spectroscopic information from one specific selected voxel,
and chemical shift imaging (CSI), in which spectroscopic information is acquired
for several different locations with a single measurement.
in vivo
11.2
SVS
Spectroscopic measurements were the first type of measurement to use the mag-
netic resonance principle in the 1950s, and they were performed to obtain infor-
mation about the compounds of a chemical specimen in a probe within a test
tube. In these measurements, the spatial origin of the measured signals was
defined by the sensitivity of the used RF coil. In clinical patient examinations,
such a rough localization is insufficient. Spectroscopic measurements should
provide additional information about specific tissues within the brain that are
identified in conventional MR imaging. Therefore, a spatial selectivity with an
accuracy of at least 1 cm is necessary for a spectroscopic measurement.
In SVS, this selectivity is realized by a combination of slice-selective excitations.
Each of these excitations works as in MR imaging: an RF pulse with a specific
frequency bandwidth is applied while a field gradient is switched on. Thus, the
excitation of nuclei is restricted to a selected slice. By modifying the strength of the
field gradient and the center of the RF band, the thickness of the slice and the distance
of the center of the slice from that of the magnet can be modified. The orientation
of the selected slice can also be chosen freely by an appropriate weighted combination
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