Abstract :
It is proposed to use a modified form of Mansfield´s echo planar imaging to obtain NMR images which are entirely undistorted by background inhomogeneities of the magnetic field. In the proposed method, a train of 180?? pulses is applied in the presence of a periodically switched or sinusoidally modulated linear field gradient. The time-domain signal is sampled at half the distance between the 180?? pulses. At these points the magnetization will be modulated by the gradient, but will be independent of any mechanisms of inhomogeneous broadening, such as static field inhomogeneities, local susceptibility effects, or chemical shifts. A Fourier transform of the function comprising these points will therefore yield a faithful projection of the spin density, although the magnitude of the superimposed gradient need not be large compared to the inhomogeneous broadenings. This paper demonstrates the application of the proposed pulse sequence to a small-scale one-dimensional phantom. The major problem in upscaling this technique to human-scale dimensions lies in the limited available and allowed RF power, which in turn limits the maximal tolerable field inhomogeneities as well as the maximal practical field strength. An analysis of the tolerance of the proposed technique to these factors is presented, based on numerical simulation of its performance, using the Bloch equations. It is concluded that its use may be feasible on low-field systems, providing the advantages of increased signal-to-noise, lower required gradient strength, and drastically reduced sensitivity to the homogeneity and stability of the magnetic field, at the expense of larger RF power.
