U.S. Philips Corporation

Gerardus B J Mulder

Gerardus Nerius Peeren

Cornelis L G Ham

Nicolaas Bernardus Roozen

Aart J Mateboer

Johannes P M Verbunt

The MRI apparatus includes an actively shielded gradient system (,) with primary gradient coils and shielding coils (,). Despite the active shielding, such a system inevitably causes a magnetic field to some extent outside the shielding, so that eddy currents are generated in the conductive structures of the main magnet, said eddy currents causing heat dissipation and annoying noise. According to the invention, the gradient system also includes an eddy current shield (,) which consists of an electrically conductive, substantially closed plate; furthermore, the primary gradient coil (,) and the shielding coil (,) are arranged within the eddy current shield (,) and these three elements together constitute a mechanically rigid unit. As a result, eddy currents are avoided in the main magnet and annoying noise is strongly reduced.

MRI apparatus with a mechanically integrated eddy current shield in the gradient system

In an MRI apparatus for medical purposes it is desirable to minimize the length of the magnet system for generating the main field, inter alia in order to keep the accessibility of the region to be examined in the imaging volume as high as possible for the attending staff. In order to save costs it is also desirable to make the diameter of the magnet system as small as possible. According to the invention a short, actively shielded magnet system is obtained in that the turns of the magnet system are situated in an imaginary U-shaped space,with the opening of the U facing the axis,. The current component in the bottom,of the U has a value which is smaller than that of the current component in the limbs,of the U. The inner space,of the U does not contain a current for generating a contribution to the main field in the measuring space,of the apparatus. Consequently, a free inner space is formed in the magnet system; this free space can be used, for example to accommodate gradient coils and/or RF coils.

MRI apparatus having a short uniform field magnet with an internal space

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,according to the invention is constructed so as to include a gradient coil,having a comparatively poor linearity, and a correction coil,which is intended to correct the linearity of the gradient coil; the linearity of the correction coil,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,. If a high speed is desired at the expense of the linearity, the gradient coil,alone may be activated; if a high linearity is desired at the expense of the speed, both coils,can be switched on. The apparatus is provided with switching means,for driving the gradient coil and the correction coil,mutually independently or not, as desired, and with a first and a second drive amplifier,for driving the first coil,and the second coil,, respectively.

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Eindhoven, Netherlands

Gerardus B J Mulder