Intensity artifacts in MRI caused by gradient switching in an animal-size NMR magnet.

The switching of magnetic field gradients in MRI gives rise to eddy currents in the structural components of superconducting magnet systems. The associated magnetic fields cause intensity artifacts which are particularly severe in some animal-size systems. We treat theoretically three mechanisms which cause intensity artifacts in one-dimensional projection images obtained by a spin-echo technique. The first is an off-resonance effect, caused by applying the refocusing pulse before the read compensation gradient pulse has decayed sufficiently. The other two mechanisms are caused by a spatial dependence of the phase accumulated by the spins at the time of formation of the echo, as a result of the eddy current fields. First, interference causes a loss of transverse magnetization because of a variation in the phase of spins which lie on the same isochromat during the read gradient pulse. Second, a variation of the phase of the spins in a direction orthogonal to the isochromats causes spins throughout the sample to refocus at different times. These two mechanisms are fundamentally different, since interference can occur even if the main magnetic field is homogeneous, whereas improper refocusing does not. It is shown that there is no loss of intensity by the interference mechanism if phase encoding is used to form two-dimensional images. This may well be a major reason why images obtained by 2DFT have been found to be generally superior to those obtained by projection reconstruction. Experimentally, the distribution of intensity in one-dimensional projection images of a square slice phantom is compared with theoretical intensities, estimated using eddy current field reported in the preceding paper.