Journal of Magnetic Resonance, Series B最新文献

A volume-selective NMR method is presented for very exact determination of the difference of the Larmor frequencies between the coupled resonances of homonuclear AX₃spin systems, as, for example, lactate. The frequency difference can be determined with an accuracy of 0.003 ppm even if only the doublet of the spin system can be detected. The method is based on the effects of homonuclear polarization transfer which occurs in localized double-spin-echo spectroscopy. It is used for determination of the chemical-shift difference of the lactate multiplets, which depends on the degree of dissociation and consequently on pH. Measurements were performed with a 1.5 T Siemens Magnetom SP 63 whole-body imager on solutions of lactate and acetic acid in physiological sodium chloride solution with pH from 1 to 11. As a consequence of these measurements, conclusions are possible for the optimum echo time for PRESS measurements which avoid signal losses from polarization transfer. Furthermore, the possibility of localized pH determination by this effect is discussed.

Material strain during the course of diffusion encoding by MRI will in general change the observed diffusional signal losses. These changes will occur even when the material returns cyclically to its initial location during the diffusion-evolution period. This effect derives from the modification of the local spatial modulation k of spin phase within a sample by a material deformation fontserif=cmss10 at 10pt[formula]as k →[formula]k, resulting in an observed diffusion tensor[formula]where[formula]is the material stretch tensor. For example, when a material is compressed during pulsed-gradient diffusion encoding, the compression acts to increase the attenuation due to diffusion just as if a larger gradient were used. By using a simple gelatin phantom, the existence of this effect is demonstrated, and an effective method for its correction based on an MRI mapping of the material strain is presented. This correction is particularly relevant for inferring myofiber structure in the beatingin vivohuman heart, since the measurement of[formula]is perturbed by the deformation of myocardium during the heart's contraction.

Coils with coaxial return paths are used to generate transverse magnetic field gradients. This paper describes optimization of such coils by the method of simulated annealing, a method known to be able to find the global minimum of a function. The adaptive simulated annealing (ASA) program has been analyzed and applied to optimization of a family of coils with 8–16 building blocks, each carrying equal current. Positions of the blocks along the longitudinal axis of the coils were optimized. A new subclass of coils is proposed; the diameter of return paths of this subclass of coils is not fixed but may be varied. The new coils provide greater gradient uniformity than those for which only positions of the blocks are optimized. All optimized coils should find applications in high-precision and high-resolution imaging and spectroscopic experiments.

Equations describing the multiple-quantum (MQ) signal produced by an MQ pulse sequence are systematically derived in both absence and presence of refocusing RF pulses. When the RF pulses in an MQ pulse sequence satisfy certain conditions, these equations may be arranged in a factorized form. The off-resonance effects on the MQ signal due to chemical shift can then be analyzed separately during the preparation and evolution times. Using the reformulated equations, the dependence of the amplitude of an MQ signal on the phase shift induced by the resonance offset during the preparation and evolution times is demonstrated. By use of the new equations, it is shown that the off-resonance effects, occurring during both the preparation and evolution times, may be described in terms of the same physical process, i.e., interference between echo and antiecho. In applying the off-resonance effects for the elimination of the MQ signal in the presence of chemical shift, it is possible to suppress the MQ signal over a wider off-resonance bandwidth by use of the nonrefocused preparation and evolution times than by use of a single time. Furthermore, by taking an alternative approach in deriving the equations, the interference between echo and antiecho due to the resonance offset is shown to be insensitive to the flip angle of the creation RF pulse (usually the second π/2 RF pulse). The theoretical findings were experimentally verified by use of a phantom containing sodium in agarose.

¹H NMR parameters were obtained for daunomycin in water solution in the free state as well as in the presence of dipalmitoylphosphatidylcholine model membranes. Spin–lattice relaxation rates were measured under nonselective, single-selective, and double-selective irradiation modes, and 2D NOESY spectra were obtained at several values of the mixing time. Proton–proton distances were calculated and the motional correlation time was evaluated in both the free and bound states. NMR parameters were used to show that ring A and the glucosamine moiety of daunomycin strongly interact with the external surface of the bilayer, while the rest of the molecule penetrates the membrane without crossing it. The structures of both free and bound daunomycin were obtained and compared by using molecular modeling.

New pulse sequences for two-dimensional phase-sensitive detection of heteronuclear zero- and double-quantum coherence are presented for use with and without pulsed field gradients. The magnetization is phase modulated duringt₁and then transferred to a proton for detection using a novel approach similar to the sensitivity-enhancement technique [Palmeret al. J. Magn. Reson.93, 151 (1991)]. These pulse sequences are useful for the measurement of size and relative sign of passiveJcouplings as well as for the measurement of the relaxation rates of zero- and double-quantum coherence. They can also be used as building blocks in multidimensional pulse sequences. The pulse sequences are tested on the peptide hormone motilin with a selectively¹³C-enriched α carbon.

A magnetic resonance imaging method for volume magnetic susceptibility estimate of materials immersed in a liquid containing resonant nuclei is proposed. The method uses either the standard spin-echo Fourier imaging technique or the projection-reconstruction technique. Image distortions of cylindrical macroscopic susceptibility inhomomogeneities are analyzed and a correlation is established between the susceptibility value and the size of image deformation along the read gradient. For measurements, the susceptibility of the liquid surrounding the sample (here deionized water) serves as a reference, and characteristic distances between particular points, usually highlights in the distorted image, are related to the susceptibility difference between the two media. Different samples, mainly prosthetic materials with a large susceptibility range from 5 to 200 ppm in absolute value, have been investigated, and the accuracy of susceptibility determination is discussed.