th  - 1987 - 51st ENC Conference

MF65
NMR IMAGING WITH EXTREMELY INHOMOGENEOUS B1 FIELDS
M.Garwood', K. UgurbU, Gray Freshwater Biological Institute,
UniverMty of Minnesota, Navarre, MN 55392.
M.R. Bendall, School of Science, Griffth University,
Nathan Queensland 4111, Australia.
D.L. Foxall, A.R. Rath, Varian Fremont Operations, 1120 Auburn Rd,
Fremont, CA 94538.
A critical requirement in many NMR experiments is the ability to induce
rotations of the magnetization vectors uniformly over the entire sample. This
requirement is especially severe in NMH imaging and localized spectroscopy. Con-
sequently, intense efforts are invested in designing RF coils that generate uniform
B 1 fields. An alternative approach to eliminating problems related to B 1 inhomo-
geneities is to generate pulses that are highly insensitive to B 1 variation. Two such
pulses that create transverse magnetization and carry out spin inversion have been
described [1,2]. These pulses, however, cannot execute a plane rotation with a well
defined phase. A pulse that provides a 180 ° plane rotation is needed for experi-
ments that require refocusing. We have recently introduced amplitude and
frequency/phase modulated pulses derived from the adiabatic passage principle
that can achieve 90 ° and 180 ° plane rotations [3,4,5]. When optimized, uniform
90 ° and 180 ° rotations can be obtained over a 50 to 100 fold variation in B 1 field
strength with these new pulses. This insensitivity to B 1 permits the execution of
experiments that require uniform RF fields, such as NMR imaging, with coils that
generate inhomogeneous fields such as the simple surface coil. We have success-
fully made NM~ images with these new adiabatic pulses using a surface coil for
both transmission and reception. The results to be presented include; a series of
images with slice planes selected either parallel or perpendicular to the plane of the
coil and the design of the pulse sequence used for this work.
REFERENCES:
[1] M.S. Silver, R.I. Joseph & D.I. Hoult (1984) J.Mag.Reson. 59,347.
[2] M.R. Bendall & D.T. Pegg (1986) J.Mag.Reson. 67,376.
[3] K. Ugurbil, M. Garwood & M.R. Bendall (1987) J.Mag.Reson (In press).
[4] M.R. Bendall, M. Garwood, K. Ugurbil & D.T. Pegg (1986) (Submitted).
{5] K. Ugurbil & M. Garwood (1987) (Submitted).