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A method for obtaining the three-dimensional distribution ofchemical shifts in a spatially inhomogeneous sample using Fourier transform NMR is presented. The method uses a sequence of pulsed field gradients to measure the Fourier transform ofthe desired distribution on a rectangular grid in (k,t) space. Simple Fourier inversion then recovers the original distribution. An estimated signal/noise ratio of 20 in 10 min is obtained for an "image" of the distribution ofa 10 mM phosphorylated metabolite in the human head at a field of 20 kG with 2-cm resolution.There has been considerable recent interest in obtaining images from animals and humans by using NMR spectroscopy (1-9). A recent review (10) summarizes and compares many of these methods. With few exceptions (11, 12), previous workers have used protons for NMR imaging because of signal/noise (S/N) considerations and because the proton signal from tissue comes predominantly from water and therefore is at a single resonant frequency. The latter condition is necessary because most imaging methods are unable to cope with a distribution ofresonant frequencies. We present here a method that determines the frequency (chemical shift) distribution at each spatial point with an optimum S/N ratio. As shown below, by suitably pulsing magnetic gradients across a specimen contained within a single pick-up coil, an "image" can be constructed consisting of highresolution NMR frequency distributions averaged over the resolution volume. This is possible because a pulsed gradient encodes positional information in the initial phases of the free induction decay but does not affect the resonant frequency distribution in space after the gradient has been turned off. Thus, by sampling the free induction decay after a gradient pulse, information about spatial variation can be separated from information about frequencies. The net effect is to measure the Fourier transform ofthe spatial and frequency distribution function of the spins. This is then inverted to obtain the spatial distribution of frequencies (chemical shifts) over the sample. THEORYWe wish to observe an object that has a spatially varying frequency distribution. Let p(x,8) be the distribution ofchemically shifted frequencies, 8, at the point x in such an object, as shown in Fig. 1. Ifwe apply a rfpulse in the presence ofa uniform static field, Ho0, the resultant free induction decay (FID) will be S(t) = f p(x,8)eidxd8, assuming the entire object is excited and detected uniformly.Obviously, in this case, there is no way to recover the original distribution p(x,86) because the spatial information is inextricably mixed with the frequency information. If, however, in addition to the static uniform field, HOZ, we add a slowly (compared with the resonant frequency of the spins) varying linear gradient, [G(t) x]z, as shown in Fig. 1, how Consider the case in which G(t) is merely a constant, G. Then S(t) = p(-yGt,t). So, by measuring the FID in the presence ofthe gradient G, we have measured the Fourier transform of the...

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A new kind of wiggler is proposed as an improved source of synchrotron radiation from high-energy electron storage rings. The electrons are made to travel in a short-period helix by a transverse helical magnetic field. The radiation spectrum produced is calculated and it is shown that the helical wiggler design could produce a total intensity (photons sec−1 per unit bandwidth) improvement of several hundred and a brightness (photons sec−1 per solid angle per unit bandwidth) improvement of 4×104 over the present state of the art in synchrotron radiation sources.

Although the phenomena of extended x=ray absorption fine structure (EXAFS) were observed as early as the 1930's, EXAFS has only recently, with the utilization of synchrotron radiation, been transformed into a powerful structural technique. The theory and experimental practice of the technique are described and illustrated with data on germanium. Applications to systems as diverse as hemoglobin, polymer-bound catalysts, ions in solution, amorphous solids, and adsorbate atoms on surfaces are reviewed. With the recent approval of funding for new, more powerful dedicated synchrotron sources, the future holds the possibility of a virtual revolution in structure determinations.