Editing Pulse Research Articles

Measurements of human cerebral GABA at 4.1 T using numerically optimized editing pulses.

The goal of this work was to develop and evaluate a numerically optimized inversion pulse to be used with a homonuclear editing sequence to measure human cerebral GABA in vivo at 4.1 T in the occipital lobe. The optimized pulse was constructed using pallindromic symmetry with 30 pulses and 29 delays. The optimized pulse provided greater selectivity than the equivalent bandwidth matched DANTE pulse and sinc shaped DANTE. The improved selectivity reduced the co-editing of the macromolecule resonance, permitting the GABA edited doublet to be resolved in vivo. Using cerebral creatine as a reference, 7.1 mM, the measured GABA level was 1.15 +/- 0.13 mM in the occipital lobe.

A technique for detecting GABA in the human brain with PRESS localization and optimized refocusing spectral editing radiofrequency pulses.

A proton magnetic resonance spectral editing technique is presented that uses PRESS excitation to achieve spatially localized measurements of brain gamma-aminobutyric acid (GABA). The homonuclear difference spectroscopy technique employs a frequency selective inversion pulse to suppress the creatine resonance at 3.0 ppm. The timing of this pulse is optimized to maximize the suppression of creatine by minimizing the effect of the editing pulse on the 3.0 ppm resonances. The PRESS excitation achieves three dimensional spatial localization in a single acquisition making it less sensitive to patient motion than multiple acquisition techniques. The performance and utility of this technique were evaluated by phantom experiments and by in vivo measurements of brain GABA concentration in 10 normal subjects.

Spectral editing in 13C CP/MAS experiments at high magnetic field

We show that the spinning side-bands of protonated and non-protonated carbon atoms can be well separated by means of the standard SCP and LCPD experiments at a relatively slow sample spinning rate or at high magnetic field. These experiments offer a promising way of measuring the principal values of chemical shift anisotropies via spinning side-band analysis in a moderately complex system. General spectral editing in 13C cross-polarization magic-angle spinning (CP/MAS) experiments at high field is achieved by incorporating the total side-band suppression (TOSS) pulse sequence into the standard series of spectral editing pulse sequences. It is confirmed that the relative signal intensity for a certain kind of functional group obtained at different polarization, polarization-in-version and depolarization times is about the same as that obtained at low magnetic field, and that the signal intensity distortion introduced by the TOSS sequence for resonances having different chemical shift anisotropies does not interfere with the spectral editing process. However, quantitative results can only be expected in those cases where full restoration of the intensity of the central band can be achieved by the TOSS sequence. This new strategy at high field is demonstrated by using fumaric acid monoethyl ester as a model compound. A typical application to a Chinese resin is presented, where the relative ratio of each functional group in the aliphatic portion to the total number of aliphatic carbon atoms is determined from only three experimental spectra.

Heteronuclear spectral editing with adiabatic pulses

Spectral editing pulse sequences have greatly expanded the capabilities of NMR spectroscopy in many different areas of research. In the field of in vivo NMR, editing is frequently employed to extract spectral information which may otherwise be concealed by broad and overlapping resonances. Heteronuclear ( i3C‘H) spectral editing (1-6) provides the additional capability to monitor the metabolism of i3C-labeled compounds while exploiting the high sensitivity of ‘H observation. Recent studies have shown that heteronuclear editing of 13C signals has many advantages, including a more than lo-fold increase in signal-to-noise, over direct 13C observation with nuclear Overhauser enhancement ( 7). Existing spectral editing pulse sequences are based upon conventional radiofrequency pulses which induce flip angles that are directly proportional to the RF field strength, B, . This hip-angle dependence on Br can result in substantial signal losses when an inhomogeneous RF coil, such as a surface coil, is used for pulse transmission. Even with homogeneous RF coils, minor misadjustment of flip angles can cause significant signal losses in multiphase editing sequences since the net error accumulates in a multiplicative manner with each pulse. In this Communication, we describe a pulse which achieves uniform heteronuclear spectral editing above a threshold Br where the pulse operates adiabatically. This Brinsensitive spectral editing pulse (BISEP) selectively excites ‘H spins engaged in heteronuclear J modulation. Uncoupled spins are initially rotated into the transverse plane, but then are returned to the longitudinal axis by the end of the pulse. Theoretically, spectral editing is complete in a single acquisition. BISEP is based on our adiabatic pulse, BIR-4, which can induce any desired flip angle with compensation for Br inhomogeneity and resonance offset (8, 9). The flip angle generated by BIR-4 is determined by two discontinuous phase shifts, Ab, and A&, which take place at specific times during the pulse when the magnetization has been adiabatically rotated onto the transverse plane. For an arbitrary flip angle 8 to be induced at the completion of the pulse, the discontinuous phase shifts are A4, = ?r + 0 / 2 and A& = ( 7r + 6 / 2 ) . These expressions can be equated to give A4 1 A& = 2a + 0 = 8. If A& = A$,, this latter equation predicts that BIR4 will execute a B,-insensitive identity transformation (fl = 0) provided the pulse is operating adi-

Quantitation of metabolites by 1H NMR

In this report we describe the three factors that need to be measured when quantitating an edited resonance relative to an internal standard using a surface coil. These factors are necessary by virtue of the water, fat suppressing, and localization schemes used in studing a 1H metabolite. First, use of semi-selective pulses requires amplitude correction of the edited and reference resonances. Second, use of a single surface coil results in different sensitive volumes for different resonances due to the inhomogeneous B1 and therefore require separate acquisition of the resonances. Third, editing pulses alter the sensitive volumes and this correction must be made internally by applying the same editing pulse to the reference resonance. A rationalization of this correction is given in terms of rotation operators. We apply these corrections to quantitate edited lactate relative to total creatine in a MnCl2-doped phantom and find 91% rather than 145% of known concentration. In human skeletal muscle in vivo after exhaustive exercise, we measured the lactate after exercise and found it to be 27.2 mM in two experiments, in reasonable agreement with literature values for the given exercise protocol.

Inverse editing of protons in two‐dimensional homonuclear and heteronuclear relayed COSY spectra

AbstractSeveral 1D and 2D inverse proton editing pulse sequences have recently been introduced. One of these, the REPAY sequence, allows the individual editing of XH or XH2 groups. Some new 2D combinations of one‐ and two‐dimensional REPAY experiments with a COSY or TOCSY transfer are described. These combinations provide various possibilities for structural elucidation: the determination of heteronuclear long‐range coupling constants nJ(CH) (n = 2, 3), the editing of XH or XH2 groups in two‐dimensional H,H COSY spectra and the editing of XH or XH2 groups in X,H COSY spectra followed by a relayed transfer. In contrast to earlier versions of SHIPPED‐COSY experiments, the spectra show improved sensitivity and offer the possibility of selecting XH2 groups only, and contain no 1J(XH) coupling because of heteronuclear X decoupling.

Spectral editing of tumor 13C MRS in Situ

The editing pulse sequence DEPT (D.T. Pegg, D.M. Doddrell, and M.R. Bendall, J. Chem. Phys. 77, 2745 (1982)) was modified using a scheme of various composite pulses and a 16-step phase cycling to obtain proton-decoupled natural-abundance 13C edited subspectra of solid tumors. A solenoidal probe including a Faraday shield and an orthogonal saddle decoupling coil was built for this purpose.

Single-scan volume-selective editing of 1H signals for the indirect detection of coupled X nuclei

Volume-selective NMR spectroscopy is a promising tool for medical diagnosis of pathologic metabolism (e.g., ( 1-5)). For sensitivity reasons, applications are mainly expected with proton resonance. The information content of proton spectra, how- ever, is not fully exploitable by simple, straightforward Fourier-transform NMR. The huge differences in the concentrations of the manifold substances present in tissue and overlapping resonance lines can cause severe problems. The discussion and the comparison of volume selection methods therefore should never be restricted to the proper volume selection pulse sequences alone, The crucial point is rather how flexi- bly the method can be combined with other pulse sequences serving to edit the desired resonances. It was this philosophy which prompted us to develop the VOSY concept (4 5). In a previous paper (6) we reported on VOSY (volume selective spectroscopy) supplemented with a spectral editing sequence for the record of the methyl line of lactate. The volume selection and the spectral editing pulse sequences were subse- quently applied and signal subtraction was necessary in order to obtain the edited signal ( 7). In a later work (8) we showed that volume selection and spectral editing for which we use the acronym VOSING can also be combined in an integrated way similar to our original proposal (4). With this method, signal subtraction was again part of the editing procedure. The purpose of the present paper is to show that signal subtraction can be imple- mented in the pulse sequence so that each scan provides an edited signal. The subtrac- tion of the signals of subsequent scans is no longer necessary, and multiscan acquisi- tion serves merely accumulation and phase-cycling purposes. The method is based on the modified stimulated echo as described in Ref. ( 9). A further goal of this work is to demonstrate the applicability of the method to the heteronuclear case with the purpose of detecting X nuclei indirectly by signals of the ‘H nuclei coupled to them. In particular we will report on test experiments concerning the detection of i3C via ‘H nuclei which have been carried out partially under simulated in vivo conditions. It is clear that the signals from uncoupled protons simultaneously must be suppressed in any method serving this aim. The method to be described below is suppressive for uncoupled spins per se. Nevertheless we have taken additional measures for suppres- sion of water, which presents a particular problem in in vivo NMR. Figure 1 shows the new pulse sequence. The first three slice-selective pulses P, , P2,

Integrated volume-selective/spectral editing 1H NMR and postdetection signal processing for the sensitive determination of lactate.

A new volume selection/spectral editing pulse sequence (VOSING) is presented. The features specific to the technique are that the volume selection and the editing intervals coincide and that no decoupling is necessary. The pulse sequence can be applied under both homo- and heteronuclear conditions. Phantom experiments with lactate solutions and human serum led to water suppression factors of about 20,000. A postdetection signal processing method has been implemented. The final sensitivity for lactate determinations could thus be improved by a factor of more than 4. Ischemia-induced lactate could easily be detected in serum. At present, the lower detection limit of lactate is 1 mmol/liter for a (1.2 cm)3 voxel and 32 scans in a 4.7-T/40-cm magnet.