MAGNETIC RESONANCE IMAGING APPARATUS INCLUDING A GRADIENT COIL SYSTEM WITH A CORRECTION COIL

摘要

The design of a gradient system may be aimed at a high degree of linearity of the gradient field or a high speed during the generating of the gradient pulses, depending on the wishes of the user. These two wishes imply contradictory design criteria. In order to comply with both user wishes, the gradient system (32, 34, 36, 38) according to the invention is constructed so as to include a gradient coil (32) having a comparatively poor linearity, and a correction coil (36) which is intended to correct the linearity of the gradient coil; the linearity of the correction coil (36) itself thus is not important. The linearity of the system is enhanced, relative to that of the gradient coil alone, by addition of the fields of the two coils (32, 36). If a high speed is desired at the expense of the linearity, the gradient coil (32) alone may be activated; if a high linearity is desired at the expense of the speed, both coils (32, 36) can be switched on. The apparatus is provided with switching means (58, 60, 62, 64) for driving the gradient coil and the correction coil (36) mutually independently or not, as desired, and with a first and a second drive amplifier (54, 56) for driving the first coil (32) and the second coil (36), respectively.