James Field

Two-dimensional 13C1H polarization transfer J spectroscopy

Abstract Modification of the INEPT sequence enables application of the sequence to two-dimensional NMR and provides a simplified form of two-dimensional 13 C J spectroscopy. The problem of phase correcting the final spectrum has been solved and the source of artifact signals has been identified and the problem alleviated. The uses and advantages of this form of two-dimensional spectroscopy are briefly described.

Water signal elimination in vivo, using suppression by mistimed echo and repetitive gradient episodes

A number of techniques (I) are available for the elimination of an unwanted signal (usually the ‘H signal from water) from a spectrum. Unless the unwanted signal is removed before the preamplifier and ADC are encountered in the signal detection pathway, difficulties are experienced in the observation of very weak signals because of dynamic range problems. O f the available techniques the most efficient appears to be the 1331 method developed by Hore (I, 2); however, as Hore points out, this technique is only of limited value if the water resonance is broad. This is the situation often encountered in vivo where very broad water signals are observed, the broadness usually being the result of magnetic field inhomogeneity. In general, for signal elimination the most successful methods have in the past relied on overall nonexcitation of the unwanted resonance, the aim being to leave the unwanted spin coherence along the z axis and away from the detection plane. The alternative procedure would be to destroy selectively the unwanted spin coherence and it is this strategy which has led to the application of reverse polarization transfer methods (3) as probably the most efficient water signal elimination method yet devised. In this paper we develop a method for selectively destroying the water signal, applicable to in vivo NMR studies where broad water signals are often encountered. We call the technique SUBMERGE. The method eliminates transverse water spin coherence over a wide but controllable frequency band. The significant point is that the water spin coherence is destroyed, thus eliminating the signal from the preamplifier and ADC, allowing the efficient detection of weak signals. As a test, we attempt to observe the methyl signal from lactic acid at a concentration of 10 mM in neat HzO. The water signal is thus about 3700 times more intense than the signal of interest. A 250 ml round-bottom flask was used as the sample container. A water signal having a half-height width of 10 Hz was typically detected in a single pulse experiment. In Fig. 1 is shown the experimentally determined excitation profile from a 10 ms 7r/2 sine pulse. The excitation profile from a 10 ms 7r/2 gaussian pulse is also shown for comparison. At a ‘H resonance frequency of 100 MHz (field strength 2.4 T) the chemical-shift separation between the two resonances of interest is about 345 Hz. Thus, by use of such shaped pulses, it is possible to excite the water signal but not the

Inverse DEPT sequence. Polarization transfer from a spin-12 nucleus to n Spin-12 heteronuclei via correlated motion in the doubly rotating reference frame

Abstract The “distortionless enhancement by polarization transfer sequence” (DEPT) is a heteronuclear multipulse sequence which provides for polarization transfer from n spin - 1 2 nuclei to a spin - 1 2 heteronucleus subsequent to a period of simultaneous free precession (correlated motion) of both types of nuclei in the transverse plane of the doubly rotating reference frame. A new multipulse sequence is described here which involves polarization transfer in the reverse direction, that is, from one spin - 1 2 heteronucleus to n spin - 1 2 heteronuclei again subsequent to a period of correlated motion. This inverse DEPT sequence has similar advantages to the DEPT sequence, over INEPT and related s sequences, in that it contains fewer pulses and the acquired signal is relatively insensitive to missetting the length of the free precession periods. The sequence is proved both theoretically and experimentally.

Application of volume-selected, two-dimensional multiple-quantum editing in vivo to observe cerebral metabolites.

The volume selection technique SPACE has been combined with a two‐dimensional multiple‐quantum editing sequence to uniquely detect certain J‐coupled cerebral metabolites. In vivo results demonstrating edited glutamate/glutamine and lactate from 0.4 ml of a rats brain at 4.7 T are presented. The sequence was optimized to balance multiple‐quantum generation and signal loss due to T2 relaxation. Without due regard to T2 relaxation little signal is observed.

Preliminary studies on the potential of in vivo deuterium NMR spectroscopy

Natural abundance deuterium NMR spectroscopy can be used to characterise in vivo 2H signals arising from water and fat in mice, with acquisition times of less than two minutes. Administration of D(2)0 (10% V/V) in the drinking water enhances these signals so that excellent spectra can be obtained with one scan. Using these procedures the in vivo turnover of 2H in water and fat in mice has been determined. This procedure may be of particular importance in studies of fat turnover in obesity.

Gradient-induced water-suppression techniques for high-resolution NMR spectroscopy

Water-suppression techniques can be divided into two types, those that result in zero net excitation of the water resonance and those in which the water magnetization is randomized prior to observation of the spectrum of interest. The randomization mechanism may involve selective irradiation, use of natural spin relaxation, or application of a homospoil gradient pulse to dephase selectively excited transverse spin coherence. Destruction techniques considered in this paper will involve the latter of these processes. Nonexcitation methods usually involve a binomial-type pulse cluster which results in off-resonance excitation but zero net rotation on resonance ( 1). Although the water-suppression factor of these methods is usually high ( > 1000)) they suffer from the major limitation that the spectral response is either not flat and/ or there is a large linear phase distortion of the spectrum. Destruction techniques are attractive because the final read pulse can be a hard 90” pulse which ensures uniform excitation of the spectrum with minimal phase distortion. The aim of these methods is the establishment of (S,) = (3,) = (&) = 0 over some selective band in frequency space. For those that utilize a pulsed field gradient, selectivity is achieved by the application of a shaped RF pulse which generates transverse spin coherence over a narrow band and the coherence is dephased by the field gradient. In a previous paper (2) we discussed the difficulties of maintaining high-resolution conditions (Au < 1 Hz) during data acquisition following a field gradient pulse. The major problems can be overcome by preemphasis of the gradient pulse and by applying a correction current through the Z0 coil during data acquisition to ensure that the static magnetic field is stable. Decoupling of the gradient coils from the magnet, shim set, and bore tubes by shielding and isolation may also alleviate these induced eddy current problems. In this paper we consider the nature of the shaped pulse to achieve the optimum water suppression. As well, we introduce a new mechanism to achieve water suppression using a pulsed field gradient and a selective noise pulse. All spectra were obtained on a Bruker MSL-200 spectrometer interfaced to an Oxford Instruments 4.7 T, 13 cm vertical bore magnet. Gradient pulses and Z0 correction were controlled by a Bruker MSL preemphasis unit and were applied to the

Reverse polarization transfer through maximum order multiple-quantum coherence: A reverse pommie sequence

A reverse POMMIE sequence [(randomize 1 H)-½π[C, x ] -(2 J ) −1 -½π[H, j ]½π [H, ψ] π[C]-(2 J ) −1 -½π[C, y ] π[H, k ]-(2 J ) −1 -½π[H, ±ψ], acquire 1 [H; (decouple 13 C)] is proven both theoretically and experimentally. This sequence, which utilises a selective multiple-quantum filter, will be useful for editing 1 H spectra based on 13 C- 1 H scalar coupling constants. Because 1 H H 2 O NMR signals can be effectively eliminated by application of an appropriate 1 H randomization process prior to the commencement of the pulse train, the major application of the method will be in metabolic studies of 13 C-labelled material. Suppression of H 2 O 1 H signals using this technique can exceed 10 5 : 1, a factor which should be adequate even for demanding applications.

In vivo determination of 31P spin relaxation times (T1, T2, T1ϱ) in rat leg muscle. Use of an off-axis solenoid coil

A probe using a solenoid coil tilted 45 degrees off-axis has been used to study the 31P NMR relaxation characteristics of the resonances arising from phosphorus metabolites in rats in vivo. T1, T1 rho and T2 values have been determined for phosphocreatine and ATP in leg muscle. The ratio of 31P T1(1700ms) to T2(12ms) for ATP was in excess of 200:1 compared with a ratio of 5:1 for 1H T1:T2. Of major significance was the observation that T2 values for phosphocreatine (230ms) were markedly longer than T2 values for ATP (12ms). Thus by use of appropriate delay times in spin echo sequences ATP signals can be nulled, and discrete 31P imaging of phosphocreatine in muscle may be possible provided the overall signal-to-noise is satisfactory.

In Vivo 13C nuclear magnetic resonance spectroscopy using a solenoid transmit/receiver coil

Topical nuclear magnetic resonance spectroscopy (NMR) using a solenoid transmit/receiver coil has been used to monitor the kinetics of intragastric utilization of [1-13C] glucose in mice in vivo. Using a double-tuned solenoid coil, signal to noise response was excellent and the natural abundance in vivo 13C spectra obtained with acquisition times of 10-15 minutes compares more than favourably with comparable studies using surface or saddle coils. This study clearly shows the potential of solenoid coils to monitor the kinetics of 13C-labelled metabolites in vivo.

The design of biplanar, shielded, minimum energy, or minimum power pulsed B0 coils

By extending the formalism previously developed for the design of unshielded, biplanar gradient coils, shielded biplanarB0 coils optimized for homogeneity and either minimum energy or minimum power may be designed. We present results from an integrated approach to shielded biplanar coil design, the results of which are also applicable to gradient coils, enabling the design of shielded coils with a concomitant decrease in total inductance of the coil. Length constraints are also included in the integrated minimization procedure. Results from a preliminary design indicate that high-homogeneity, low-impedance, well-shielded coils result from this design approach.