Digital Signal Processing Reference
In-Depth Information
8.1.2.2
Nitrogen acetyl aspartate
Since NAA at∼2.0 ppm is a marker of number and viability of neurons, a
normal level of NAA is generally considered to be incompatible with tumor,
because most brain tumors are of nonneuronal origin [113, 211]. Sijens and
Oudkerk [219] reported significantly higher NAA in unaffected brain tissue
compared to sites of brain tumors in the abovementioned study. On the
other hand, the NAA level may be normal in small or low grade tumors. It
should also be noted that nitrogen acetyl aspartate can sometimes also be
normal with infiltrative brain tumors [211].
8.1.2.3
Ratios of choline to NAA or choline to creatine
Ratios of metabolites have also been used for identifying brain tumors. In
a comparison of MRIlocalized proton MR spectra from the tumor bed in
fiftyone patients with highgrade gliomas with those from thirty healthy vol
unteers, significantly higher choline to NAA as well as choline to creatine
ratios in the former were found by Tarnawski et al. [225].
Similarly, Lin et al. [226] reported that the choline to creatine ratio was
significantly higher among forty nine patients with biopsyproven brain tumors
compared with fourteen healthy persons. The mean choline to NAA ratios
from brain regions without tumor were 1.7±1.2 versus 5.3±2.1 to 6.8±3.7
in gliomas, depending upon the grade, in the study by McKnight et al. [215],
in which all findings were confirmed histopathologically. Increased choline
and decreased NAA were found by Utriainen et al. [220] to distinguish brain
tumors in twelve patients from corresponding regions in the normal brain
recorded in eleven healthy referents.
Hourani et al. [227] found that the NAA/Cho≤0.61 together with rel
ative cerebral blood flow≥1.50 distinguished sixtynine patients with pri
mary brain tumors of various grades from thirtysix patients with diverse non
malignant brain lesions with a sensitivity of 72.2% and specificity of 91.7%.
Choline to NAA ratios > 1.9 provided significant help for neuroradiologists to
distinguish tumorous from pseudotumorous lesions in a study of eightyfour
patients with solid brain masses [228].
8.1.2.4
Guidance for stereotactic biopsy
Guidance via MRSI based upon areas of increased choline to creatine ratios
has improved the yield of diagnostic tissue obtained with stereotactic biopsy
[113, 229, 230]. It has been suggested that by using multivoxel MRS the area
of highest choline levels or choline to NAA ratio can be identified, and that
would be the optimal site for biopsy [113, 211, 231]. Other metabolites, such
as lipid might also be instructive for locating the best biopsy site [232].
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