Shimming method for NMR magnet using unshielded gradient systems

摘要

An method for determining the inhomogeneities of the magnetic field of the magnet of an NMR system makes use of unshielded coils while minimizing the effect of eddy currents induced in the magnet components by the gradient fields. A first dataset is acquired by ramping the level of a gradient magnetic field to a maximum value and generating an RF pulse to excite the spins. After a delay time t.sub.1, a single data point is sampled. While the spins are allowed to relax, the gradient field magnitude is changed to the next level. Another RF pulse is generated and, after time t.sub.1, the next data point is sampled. This process continues until a first complete dataset is acquired. The next gradient function is then applied in a similar manner, and the data points sampled after an RF pulse and a delay time of t.sub.2. During sampling of the second dataset, the gradient area for each spatial data point is equal to the gradient area for that spatial point during sampling of the first dataset. After the second dataset is acquired, a frequency distribution for the gradient direction is obtained using known techniques.