A.F. Mehlkopf

Homonuclear broadband-decoupled absorption spectra, with linewidths which are independent of the transverse relaxation rate

Recently Aue ef al. (I) presented a method for obtaining a homonuclear broadband-decoupled high-resolution NMR spectrum by means of diagonal projection of a 2D J-resolved absolute-value spectrum. Resolution in such a projection is severely lim ited by the fact that resonance lines have absolute-value l ineshapes (2). As has been shown by Nagayama et al. (3) it is impossible to obtain a homonuclear broadband-decoupled spectrum via projection of a phase-sensit ive spectrum which is obtained by using the method of Aue et al. (1). Until now it was impossible to eliminate the absolute-value character in the decoupled spectrum. A new method is introduced here by which it is possible io obtain a decoupled absorption spectrum with linewidths which are independent of the transverse relaxation rate. The basic schemes of the method described by Aue et al. (1) and our new method are the same: spin echoes are created by means of 90-180” pulse sequences, for different intervals t/2 between the 90 and 180” pulses (Fig. 1). In our method the magnetization is measured at a fixed time r after the initial 90” pulse, as a function of time t. Following the arguments of Aue et al. the observed magnetization, corresponding to a resonance line k of a set j of magnetically equivalent nuclei, is given in the case of weak coupling by

Absorption spectra from phase-modulated spin echoes

Abstract A method is presented by which pure absorption spectra can be derived from spin echoes with a symmetric envelope amplitude, but with arbitrary phases φ in the centers of the echoes. It is shown that without any phase correction, absorption spectra can be obtained in which the amplitudes of the spectral lines are either proportional to cos φ or sin φ, or independent of φ. As an example a T2 measurement of 1,1,2-trichloroethane is given.

A single-shot localization pulse sequence suited for coils with inhomogeneous RF fields using adiabatic slice-selective RF pulses

RF inhomogeneity affects the localization performance in localized MRS. To study these effects we will concentrate in this contribution on surface coils because they are widely used, provide a simple, easy-to-implement means of localization, and, due to a good filling factor, have a high signal-to-noise ratio (SNR) ( 1, 2). Their strong RF inhomogeneity prevents the use of amplitude-modulated (AM) localization RF pulses for accurate localization. Above a critical RF amplitude threshold adiabatic IZ ?r pulses (n = 1,2,. . .) have the property of rotating magnetization nearly independent of the RF amplitude (3-5 ), which makes them very useful in combination with surface coils. Most localization sequences based on the adiabatic principle [e.g., (6, 7)] are multishot methods. This means that the localized signal is obtained by adding and subtracting the signals originating from the total sample of several independent shots. This has the drawbacks that a large dynamic range of the receiver is needed and that the method is sensitive to motions of the sample and spectrometer instabilities. A different approach is the stimulated-echo sequence presented by Kunz (8)) based on adiabatic r/2 pulses, which allows a 1 D localization. A drawback of using stimulated echoes is the inherent 50% signal loss. In this contribution a single-shot adiabatic 3D localization pulse sequence is proposed which overcomes the above-mentioned problems. For convenience in the remainder of this contribution this method is denoted, SADLOVE (single-shot adiabatic localized volume excitation). With the aid of a two-compartment phantom and a single-turn circular surface coil ( C

A fast method for obtaining 2D J-resolved absorption spectra

= 10 mm), the localization performance of SADLOVE is tested for uncoupled spin systems. Figure 1 shows the SADLOVE pulse sequence. Directly after the adiabatic 7r/2 pulse, a gradient G, is switched on and a so-called slice-selective phase-compensated 2r pulse (9, 10) is applied for 1D localization. Extension to 2D or 3D localization is effected by adding one or two 27r pulses. If the time between the adiabatic pulses is negligible, acquisition of the FID-like localized signal can start directly after the last 27r pulse because the spins in the volume of interest (VOI) do not undergo a net phase change during the 27r pulses. If this time is not negligible, either a hard a refocusing pulse must be used after the last 27r pulse and the echo sampled, as we did, or the delayed FID is sampled and analyzed by time-domain fitting ( I1 ) to circumvent the problems of phase distortions and baseline roll. A central role in SADLOVE is played by the adiabatic phase-compensated 2a pulse (9, 10). This pulse consists of two identical sech/tanh a pulses B1 (t) = Q0 sech( 8t) ‘+&

Selective relaxation-time measurements in high-resolution NMR-spectra

Two-dimensional homonuclear J spectroscopy (1) is a useful method both for unraveling complicated spectra (2) and for the study of spin-spin coupling constants. The principle of the technique has been described by Aue er al. (I ). By means of one or more 180” pulses a spin echo is created at a time tr after an initial 90” pulse. The second half of this echo is acquired for a series of tl values. Two-dimensional Fourier transformation then generates a 2D frequency spectrum. This method needs a rather long measuring time and does not give the optimum sensitivity. Another disadvantage is that it is impossible to get 2D absorption spectra in an easy way (3,4), and therefore one usually calculates an absolute-value spectrum, which introduces line broadening, and decreases the obtainable resolution. As has been shown earlier (5), processing of complete spin echoes can remove this disadvantage. Slightly different measuring techniques are described, which allow a shorter measuring time and can also produce 2D J-resolved absorption spectra. The essence of J spectroscopy is that spin echoes are modulated by spin-spin coupling. The phase 4jk of the frequency component k of a set of magnetically equivalent nuclei j at the center of a spin echo, tl seconds after an initial 90” pulse, is given by

Multiple field modulation in N.M.R.

Abstract An analysis is given of the selectivity in the measurement of the transverse and longitudinal relaxation times of the separate absorption lines in a high-resolution NMR spectrum. The spin-echo signal of the line in resonance is selected from the spin-echo signals of the lines out of resonance by means of phase-sensitive detection in combination with a fourth-order filter. For T 2 measurements, use is made of a special compensation method. Theoretical calculations show that the resolution is strongly dependent on the form of the inhomogeneity of the H 0 field. Under certain conditions a resolution of 0.02 ppm can be obtained for T 2 measurement at a resonance frequency of 75 MHz and a Gaussian inhomogeneity of about 1:10 8 . For the longitudinal relaxation time a resolution of 0.03 ppm is obtainable under the same conditions. Measurements support the results of the theory.

Sources of t1 noise in two-dimensional NMR

The Bloch equations are solved in the case of multiple field modulation. The result is applied to the problem of homonuclear stabilization of the magnetic field. It is found that the stability of the field/frequency lock may be degraded by disturbance signals. A particular phase setting can be chosen to avoid disturbances.