th  - 1987 - 51st ENC Conference

MF75
NMR IMAGING USING CIRCULAR SIGNALS IN THE SPATIAL FREQUENCY DOMAIN S
S. Matsul, K. Seklhara, and H. Kohno
Central Research Laboratory, Hitachi, Ltd.
P.O. Box 2, Kokubunji, Tokyo 185, Japan
We demonstrate experlmentally a new type of method of imaging, in
which siena1 measurements are taken during rotation of a field gradient.
Detailed aspects of the method will be discussed in connection with
unusual properties of the observed slgnal.
Generally, the slgnal measured during the gradient rotation traces
a circle in the spatlal frequency domain (so-called k space). I) Although
the clrcular signal contains some information for imaging, it does not
provide useful data that can be systematlcally and efficiently process-
ed to yield a spin image. This is because the circle is not symmetric
about the origin of the k space. However, if the center of the clrcle
is approprlately shifted to the k origin, useful data can be obtained.
The resultant signal directly provides symmetric circular phase infor-
mation. The shift can be achieved by applying, amon E many ways, a sta-
tionary fleld gradient prior to the rotation. The properties of such
a circular signal are completely different from those of a usual FID
slgnal. The unusual properties can be understood by expressing the
clrcular signal in terms of Bessel functions of the first kind.
The radius of the circle is given by 7G/WG, where Y is the gyromag-
netic ratio, G denotes the gradient amplitude, and ~G is the angular
frequency of the gradient rotation. Therefore, takin E measurements
after changing either the gradient amplitude or the angular frequency
permits a concentric set of clrcular signals with different radii to
be obtained. The signal set can be converted to a spin image by suit-
able data processing involving Fourier transformations along the radii
and back projections.
Reference
I) S. Ljunggren, J. Magn. Reson. 54, 338 (1983).
§ Published in part: S. Matsui and H. Kohno, J. Magn. Reson. 70, 157
(1986).