th  - 1987 - 51st ENC Conference

WF74
TAILORED EXCITATION IN THE ROTATING FRAME
G.S. Karczmar*, T. Lawry, M.W. Weiner,
J. Murphy-Boesch, and G.B. Matson
Veterans Administration Medical Center, and
University of California, San Francisco, California
Shaped RF pulses are currently used in magnetic
resonance imaging (MRI) to provide uniform excitation across
a selected frequency interval. The distribution of
frequencies across the sample is created by a gradient in
the B 0 field. It is also possible to utilize a gradient in
the RF (B 1) field for spatial discrimination. Here we
describe a new approach to localized spectroscopy based on
selective excitation of precessional frequencies about an
inhomogeneous B 1 field. Selective excitation is produced by
a second RF field, designated B2, which is orthogonal to the
inhomogeneous B 1 field. This approach derives from Hoult's
rotating frame selective pulses (1). However, it differs
in that the B 1 and B 2 fields are not on continuously.
Furthermore, the B 2 field is tailored to achieve uniform
excitation over a desired frequency interval.
In its simplest form, the experiment is initiated by
application of a B 1 field along the X axis of the rotating
frame, through an NMR probe which produces an inhomogeneous
RF field. At multiples of time tau the B 1 field is
momentarily removed and a brief B 2 pulse applied along the Y
axis. The B 2 pulses are selective for precessional
frequencies about B 1 which are multiples of 1/tau, and
continuation of the sequence produces a net tipping of
selected magnetization towards the negative Y axis. Halfway
through the sequence the phase of the B 1 field is reversed,
so that the experiment ends with selected magnetization
along the negative Y axis, and all other magnetization
aligned along the Z axis. The slice profile approaches a
sine shape. Improvement of the selection profile is
obtained through modulation of the duration of the
individual B 2 pulses to obtain tailored excitation
equivalent to that of shaped pulses such as the sine pulse.
Computer simulations are used to demonstrate a variety
of pulse sequences which achieve acceptable slice profiles
while minimizing RF power. Advantages over other B 1
localization experiments include improved uniformity of
excitation over the selected slice, and minimal perturbation
of magnetization outside of the selected slice. This makes
multiple slice experiments possible. Extension of the
method to the use of other shaped pulses is straightforward.
1. D.I. Hoult, J. Magn. Reson. 38, 369 (1980)