Biomedical Engineering Reference
In-Depth Information
14.3.5
Analysis of Brain Metabolism in Multiple Sclerosis
Metabolite images generated from magnetic resonance spectroscopy imag-
ing (MRSI) are often difficult to interpret because of low signal-to-noise ratio
(SNR) and imaging artifacts. SNR is increased with spatial standardization
and image averaging as is done with PET data,
24
and may thus reveal subtle
consistent changes when comparing images from two groups. Our strategy
has been to define the transformation that best maps a subject's MRI into the
standardized stereotaxic brain space, and then apply the same transforma-
tion to the inherently registered MRSI. Once in stereotaxic space, MRSIs from
different subjects can be directly compared on a voxel-to-voxel basis.
Proton MRSIs (90° - 180° - 180° sequence, TR 2 sec, TE 272 msec, 32
32 vox-
2 cm
volume of interest that was roughly parallel to the AC-PC line and centered on
the corpus callosum. Post processing was applied to correct for residual field
inhomogeneity, phase roll, and eddy currents.
99
Metabolite concentration
images for N-acetylaspartase (NAA), choline (Cho), and creatine (Cr) were
reconstructed by integration between automatically chosen frequency limits
surrounding each peak. To correct for remaining B
o
and radio frequency inho-
mogeneities, ratio images were also created. After registration, resampling, and
averaging in stereotaxic space, the average metabolite images for normal sub-
jects show a decrease in NAA signal intensity corresponding to the partial vol-
ume effect due to the lateral ventricles. Although not apparent on individual
MRSIs, comparison of average metabolite images in stereotaxic space enables
us to determine that Cho and Cr are not uniformally distributed in periventric-
ular region. This nonuniformity is due not only to the presence of the ventricles,
but also to metabolic specific differences in distribution in periventricular tissues.
Figure 14.12 shows an average ratio image of Naa
els, 25 cm field of view, 2 cm slice thickness) were acquired of a 10
10
(Cho
Cr).
FIGURE 14.12
Stereotanic average MRSI of NAA
(Cho
Cr) for seven normals; contours of scalp, cortex,
and ventricles from matched MRI.
