th  - 1988 - 51st ENC Conference

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SUPPRESSION OF ARTIFACTS
IN MULTIPLE ECHO NUCLEAR MAGNETIC RESONANCE
G.J.Barker*, T.H.Mareci.
Department of Radiology,
University of Florida, Gainesville, FL 32610.
Many techniques in both Magnetic Resonance Imaging (MRI) and
Magnetic Resonance Spectrosopy (MRS) use two or more rf pulses to ex-
cite the spin system and detect the echo signals which form between or
after the pulses. After the initial excitation the evolution of the
spin system depends upon relaxation times, exchange rates, diffusion
constants and other properties, with the dominant mechanisms being
determined by the details of the timing and tip angles of the pulses.
In general many different echoes form during each acquisition inter-
val, one of which carries the information required. The others lead to
distortion of peak heights and line shapes in MRS, and to ghost images
and similar artifacts in MRI.
The 'coherence transfer pathway' formalism (1) allows the evo-
lution of each echo to be studied and suggests methods of removing the
unwanted signals. Phase cycling schemes haye been investigated which
cause cancellation of the unwanted echoes, in certain cases during a~l
acquisition intervals of a multiple echo sequence. Such schemes re ~
qulre a large number of transients to be collected, however, so a
second method has been developed whereby the systematic application of
magnetic field gradients produce similar results within a single tran-
sient. Examples of the a~plication of both methods to the spin echo
and TART (2) sequences In Imaglng, and to RED NOES¥ (3) in spectros =
copy, show their success in remove artifacts.
This research was supported in part by NIH Biotechnology
Resource Grant (P41-RR-02278) and the Veteran Administration
Medical Research Service.
(1) G.Bodenhausen et al. , J. Magn. Reson. 58, 370, (1984)
(2) T.H.Mareci et al. , J. Magn. Reson. 67, 55, (1986)
(3) T.H.Mareci et a]. : 27th RNC_ R~]t~mn~A (lqR~%
--183 I
QUANTITATION OF EXCHANGE RATES
USING THE RED NOESY SEQUENCE
M.D. Cockman* and T.H. Mareci
Departments of Chemistry, Radiology, and Physics
University of Florida, Gainesville, FL 32610.
We have previously introduced the RED NOESY pulse sequence
for the simultaneous acquisition of several 2D NOESY spectra, each
with a different mixing time (i). The RED NOESY sequence is similar
to the NOESY sequence but the final 90 degree pulse of the latter is
replaced by a series of "read" pulses of tip angles less than 90 de-
grees to sample the exchanging longitudinal magnetization. This is
the same principle behind the TART and STEAM T1 imaging sequences
(2,3). Recently, Meyerhoff, et.al, have applied the sequence to a
small lactone and report good success in routlne use of the pulse se-
quence for the observation of NOEs (4). We have investigated the use
of RED NOESY for the quantitation of exchange rates for three small
N,N-dimeth[lamides. The most limiting aspects of RED NOESY are: i)
that the slgnal acquisition time imposes a lower bound on the mixing
times and 2) that the read pulses form unwanted echoes. We present a
homospoiling scheme designed to overcome the second factor and exam-
ine the implications of the first.
This research was supported in part by the NIH Biotechnology
Resource Grant (P41-RR-02278) and the Veterans Administration Medical
Research Service.
(I) T.H. Mareci, S. Donstrup, and M.D. Cockman, 27th ENC, Baltimore,
1986.
(2) T.H. Mareci, W. Sattin, K.N. Scott, and A. Bax, J. Magn. Reson,
67, 55 (1986).
(3) A. Haase and J. Frahm, J. Magn. Reson. 65, 481 (1986).
(4) D.J. Meyerhoff, R. Nunlist, and J.F. O'Connell, Magn. Reson.
Chem., 843, Oct (1987).
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