th  - 1988 - 51st ENC Conference

I-- 1 2 8 IVOLUME-SELECTIVE SIGNAL SUPPRESSION IN SURFACE-COIL NrMR
SPECTROSCOPY: COMPARISON OF THREE METHODS. C.D. Smith, G.S. Thomas, S.L. Smith*,
Magnetic Resonance Center, University of Kentucky, Lexington, KY 40506
We compared the radio-frequency (RF) field profile of a 3.0cm 31p-tuned surface
coil, using three methods to suppress signal close to the coil while collecting
signal from deeper regions. These methods are: (I) Bendal's depth pulse using both
first order and second order elimination, (2) spatially-selective prpsaturation with
low-power pulses, and (3) Erst-angle optimization (in a surface coil, this amounts to
a second form of spatially-selective presaturation). The latter two methods may be a
useful "poor man's" alternative to gradient or multiple coil technique for collecting
in vivo 31p spectra of brain, with reduced contamination from overlying muscle, for
example. The basis of comparison is intensity ratio of the signal profile at
critical distances from the coil.
Experiments were performed on a Spectroscopy Imaging Systems VIS 4.7 tesla
system, oeprating at 81MHz for phosphorus. The surface coil was placed perpendicular
to a 0.5cm rectangular slab phantom containing 3M sodium dihydrogen phosphate
(T 1 = .Is); this form for the sample makes slice selection gradients unnecessary for
imaging. Images of the RF profiles and associated intensity traces, plotted to the
same scale and window settings to aid visual comparison, are highly instructive and
will be presented. Advantages and disadvantages of each method for in vivo
applications are discussed.
- - 12 9 I!N VIVO VOLUME LOCALIZED SURFACE COIL SPECTROSCOPY WITH ISIS AND
DRESS: THE CHEMICAL SHIFT DISPLACEmeNT. C.D. Smith, G.S. Thomas, and S.L. Smith*
Magnetic Resonance Center, University of Kentucky, Lexington, KY 40506
A problem in gradient volume selection spectroscopy, dependent directly on main
field strength, is the chemical shift effect, which can be summarized as follows:
the volumes from which signal is collected for individual lines in a spectrum are
displaced in space, the displacement proportional to their chemical shift. This
means, for e::ample, two peaks in a single rat brain 31p spectrum may originate from
opposite hemispheres. Some spectral lines may be attenuated or absent because the
corresponding volumes lie outside the brain entirely.
In DRESS the selected volumes consist of displaced parallel planes with centers
.,eparated by distance S, in centimeters, S = o • Bo/g " 102 , where O is the
separation in parts per million between spectral lines of interest, Bo is the main
field in Tesla and g the gradient strength in gauss per centimeter. The gradient
effect on chemical shift is negligible.
In the ISIS experiment, selected voiumes consist of a diagonally displaced stack
of cubes corresponding to each line; the overlap in volume between the cubes depends
both on gradient strength and pulse bandwidth. With some combinations of these, the
volumes may not overlap at all. For example, in a 4.7 tesla system at phorphorus
frequency with a nominal ma::imum gradient strength of 2.0 gauss/cm, the distance
between centers of cubes corresponding to opposite ends of a typical spectrum is
0.94cm. The effect is significant when considering localization to volumes of this
order= e.g., rat brain. Nuclei with a large chemical shift range, e.g., carbon,
su~er greatest with the phenomenon. Spatial variation of metabolic parameters, e.g.,
phosphorylation potential, over a range of centimeters must also be considered in
interpretation of spectra obtained using these methods; the assumption of tissue
homogeneity over this range should be included in such interpretations. We wi!7. sho~
in vivo spectra obtained using ISIS and DP~SS demonstrating the above considerations.
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