th  - 1987 - 51st ENC Conference

WF72
THE DESCI~IPTII]N AND ELIMII~TION OF SUBTRACTION ARTEFACTS IN :H
EDITING SCHEMES: A PHASE CYCLING SCHEME TO ELIMINATE ABSORPTIVE
AND DISPERSIVE ERRORS
H.P Hetherington °, D.L.Rothman, K.L. Behar, M.R. Bendall and R.G. Shulman
Dept of Molecular Biophysics and Biochemistry
Yale University New Haven C.T. 06510
In the past three years there has been a rapid growth in the
application of LH NMR to monitor in vivo metabolism. Due to the small
chemical shift dispersion of the JH spectra, many small metabolite
resonances are obscured by spectral overlap. To resolve these resonances
a variety of homonuclear editing schemes have been proposed which rely on
the use of a selective inversion pulse or CW decoupling to alter the 3-
modulation of the edited resonance(1). However, these methods
experimentally and theoretically fail to achieve exact cancellation of
non-modulating resonances at the frequency position of the e~ited
resonance, thereby resulting in quantitative errors.
By use of matrix methods(2) and the product operator formalism (3), we
have identified the source of these errors as arising fromthe editing
pulse creating: a) perturbations in the absorptive refocusing
magnetization due to off resonance spin inversion effects and b)
dispersive refocusing magnetization due primarily to the effective field
changes during the selective inversion pulse or ~ecoupling period.
HoNever, these errors can be theoretically and experimentally eliminated
by using the following sequence and appropriate cycling scheme,
where e and 2e form a standard spin echo sequence, ee.~ is a selective
inversion pulse, ~=I/23 for doublets, and ee.~*~e=~. The selective
inversion pulse, e~s~, is applied on alternate scans to the 3-coupled
spin of the resonance to be edited and to a position symmetrically
disposed to the edited resonance to eliminate errors in absorptive
refocusing magnetization. Incomplete cancellation due to dispersive
refocusing magnetization is eliminated by cycling the sequence of pulses
and delays (2e.~-~e-2e-~) according to the absorptive phase cycling
described by Hetherington and Rothman(2). This scheme has been applied to
the living cat brain at 4.?T to obtain an edited spectrum of lactate.
I) D.Rothman et al, Proc. Natl. Acda. Sci, 81, 6330, (19B4)
S. Williams et al, 3. Magn. Reson. bJ,40&, (19B5)
2) H.Hetherington and D.Rothman, 3. Magn. Reson.,b5t 348,(1985)
3) 0.Sorensen, Prog. Nucl. Magn. Reson. Spectrosc., 54,512,(1983)