NMR chemical shift imaging in three dimensions

NMR chemical shift imaging in three dimensions

June 1982 | T. R. Brown, B. M. Kincaid, and K. Ugurbil
This paper presents a method for obtaining three-dimensional chemical shift imaging using Fourier transform NMR. The method involves applying a sequence of pulsed field gradients to measure the Fourier transform of the desired distribution on a rectangular grid in (k,t) space. Simple Fourier inversion then recovers the original distribution. The method allows for the reconstruction of high-resolution 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 of the spatial and frequency distribution function of the spins. This is then inverted to obtain the spatial distribution of frequencies (chemical shifts) over the sample. The method is demonstrated using a one-dimensional phantom consisting of two parallel cylinders, one containing Pi and the other containing fructose 6-phosphate (Fru-6-P). The results show that the method can resolve the two separate components even in the case in which the physical regions overlap. The method is also applied to a human head in a 20 kG static field, where it is estimated that images of the major phosphorylated metabolites can be obtained in tens of minutes with spatial resolutions of a few centimeters and S/N ratios of ≈10. The method is expected to produce an image of a phosphorylated metabolite of 10 mM concentration in a human head in a 20-kG static field in 10 min with a resolution of 2 cm and a S/N ratio of 10–20. The method is also expected to be applicable to other three-dimensional objects such as animals and humans. The method is considered to be of general applicability and is expected to produce high-quality images with good spatial resolution and S/N ratio. The method is also considered to be safe for human observations, as brief exposures to static fields of 20 kG cause no harmful effects. The method is also considered to be efficient for data acquisition, as the use of surface coils can reduce the number of different k values needed to recover an image and therefore result in faster data acquisition.This paper presents a method for obtaining three-dimensional chemical shift imaging using Fourier transform NMR. The method involves applying a sequence of pulsed field gradients to measure the Fourier transform of the desired distribution on a rectangular grid in (k,t) space. Simple Fourier inversion then recovers the original distribution. The method allows for the reconstruction of high-resolution 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 of the spatial and frequency distribution function of the spins. This is then inverted to obtain the spatial distribution of frequencies (chemical shifts) over the sample. The method is demonstrated using a one-dimensional phantom consisting of two parallel cylinders, one containing Pi and the other containing fructose 6-phosphate (Fru-6-P). The results show that the method can resolve the two separate components even in the case in which the physical regions overlap. The method is also applied to a human head in a 20 kG static field, where it is estimated that images of the major phosphorylated metabolites can be obtained in tens of minutes with spatial resolutions of a few centimeters and S/N ratios of ≈10. The method is expected to produce an image of a phosphorylated metabolite of 10 mM concentration in a human head in a 20-kG static field in 10 min with a resolution of 2 cm and a S/N ratio of 10–20. The method is also expected to be applicable to other three-dimensional objects such as animals and humans. The method is considered to be of general applicability and is expected to produce high-quality images with good spatial resolution and S/N ratio. The method is also considered to be safe for human observations, as brief exposures to static fields of 20 kG cause no harmful effects. The method is also considered to be efficient for data acquisition, as the use of surface coils can reduce the number of different k values needed to recover an image and therefore result in faster data acquisition.
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