Spatially encoded spin echoes, J imaging

Abstract

Recently various methods of chemical-shift imaging were developed (I-5), in which the NMR spectra of different substances are obtained together with their spatial location. Unfortunately, due to the crude magnetic fields produced by whole-body magnets, a very poor resolution is obtained, and the fine details that make NMR spectroscopy so useful for chemical analysis are lost. Moreover, chemical-shift imaging experiments are usually performed only with high-field whole-body magnets, which are very expensive. We suggest an alternative experiment which may overcome these problems. In this experiment, which we call J imaging, the spectral information is given in the form of J spectra. The basic pulse sequence is seen in Fig. 1. The experiment starts with a creation of transverse magnetization, with an application of a 90” rf pulse. A magnetic field gradient is switched on in, say, the X direction. After some incremented time tl/2 a 180” pulse is applied. The phase of the different isochromates is then inverted. Then the gradient is switched on with an opposite sign, and is switched off after another tIj2 period. At the end of the evolution period t, the different components of the magnetization are dispersed only due to their location. The dispersions due to either field inhomogeneity or chemical shift are refocused. In the detection period a series of 180” rf pulses is applied, which creates an echo train. To get echoes with the same sign, the phase of the 180” pulse is shifted by 90” (6). At the top of each echo a single point is sampled. The envelope of the echoes is modulated due to the presence of J coupling. Fourier transformation of the spectra obtained in this way will yield J spectra in which the lines appear with their natural linewidths at frequencies determined by the coupling constant (7). The influence of the field inhomogeneities is eliminated also in the spectral dimension. Moreover since the J couplings are field independent, relatively low magnetic fields might be used. The phase of these lines will depend on the spatial encoding obtained in the evolution period. To express correctly the spatial information, quadrature detection must be used. In this way, the odd-numbered echoes are entered to one channel, and the evennumbered echoes are entered to the second one. Since the first channel is not sensitive to the inversion of the signal with respect to the Y axis, folding of the spatial information due to the opposite spatial encoding in odd and even numbered echoes is avoided. The experiment is done several times with incremented evolution time tl. Twodimensional Fourier transform is performed on the data. The twodimensional map