Principles of MR Spectroscopy and Chemical Shift Imaging - Advance Image Processing in Magnetic Resonance Imaging - page 395

can be displayed ( Figure 11.21b ). Because the matrix contains many spectra, the
size of the displayed spectra is often very small, preventing the operator from
resolving all the spectral details. To overcome this, only subsets of all spectra can
be displayed. The matrix of spectra provides an overview of spectral quality and
trends in signal distribution; however, spectra contain complex and often redundant
information. Some metabolites may be irrelevant or not well resolved for the study.
To summarize the metabolite distribution at a glance, metabolite images (called
also metabolite maps) can be computed. For each detectable metabolite in the
spectrum, a metabolite image can be computed when the intensity in the metabolite
image corresponds to the signal intensity of the selected metabolite in the given
voxel. The resulting image, usually in a color palette, is overlaid on the MR scout
image (Figure 11.22).
As pointed out earlier, due to sensitivity reasons, the voxel size is typically
of the order of 1 cm 3 . The image matrix of spectroscopic images is, therefore,
coarse. As previously mentioned, zero filling resulting in a finer matrix can be
performed. However, in this case, spectra from more voxels have to be pro-
cessed. To improve the appearance of the images, interpolation in the image
space (after the FT) can also be performed. For this purpose bilinear, cubic,
or other interpolation methods are used. Even if the apparent resolution of
images is improved and the images are better readable, similar to zero filling,
the image interpolation does not change the PSF and the true resolution of
images remains low.
Metabolite images are based on the results of spectra fitting and the assump-
tion that spectral quality is sufficient for the analysis. If the fitting routine is not
accurate or if spectra contain artifacts, the metabolic image may be misleading.
A typical example can be the incorrect computation of resonance areas due to
magnetic-field-inhomogeneity-induced frequency shifts of the spectra. If integration
FIGURE 11.22 The metabolite image of the sum of glutamine and glutamate signals.
The low signal values correspond to the blue color, whereas the high signal values
correspond to the red color.