The present invention relates to a method of determining coil arrangements in a magnetic resonance magnet which provides active shielding to reduce the stray magnetic field surrounding the magnet, while maintaining field homogeneity in the magnet.
Magnetic resonance imaging magnets used in medical diagnostics employ uniform high intensity DC magnetic fields in the interior of the magnet. Variations in the DC magnetic field (inhomogeneities) are undesirable and result in imaging distortion. Designs to date have had inherent high levels of stray field surrounding the magnet. These stray fields adversely affect electronic equipment in the vicinity of the magnet unless the equipment is individually shielded or the stray fields are reduced. Pace-makers, for example, have been found to be affected by levels of the stray field as low as ten gauss, (the earth's magnetic field is 0.5 gauss), so that uncontrolled access to the areas surrounding the magnet have to be restricted.
A previous approach to the stray field problem has been to locate the magnet in a separate building reserved exclusively for the purpose of diagnostic imaging. This can be costly since it depends on providing additional real estate. At times, such as in crowded urban areas, it is not feasible to provide the extra space.
Another approach is to surround the room where the magnet is located with ferromagnetic sheets which act to inhibit the extent of the stray field around the room. The disadvantages to this approach are that the completed room is heavy, with the shielding adding up to one hundred tons, usually requiring structural modification to the building. Also, any dissymmetry in the ferromagnetic material surrounding the magnet can adversely affect the homogeneity in the working volume of the magnet.
Still another approach to the stray field problem is described in U.S. Pat. No. 4,646,045 in which the magnet is surrounded by enough ferromagnetic material to suppress the stray field. This approach has several potential disadvantages. The shield is designed for optimum operation at only one field level in the magnet and the proximity of the ferromagnetic material to the magnet, makes the homogeneity of the working volume of the magnet susceptible to any dissymmetry in the shield introduced during the manufacturing process.
Yet another approach to solving the stray field problem is to use an active shield, as described in U.S. Pat. No. 4,587,504 issued May 6, 1986 to Brown, et al. Brown et al. describes creating two homogeneous fields of different strengths using two different sets of coils, with the fields subtracting from one another thereby providing a homogeneous field in the bore of the magnet and reducing the stray field outside the magnet. It has been calculated by the present inventor that the stray field level at the walls of a typical hospital room (8 m.times.6 m) is in the range of 10-60 gauss using the active shield described in U.S. Pat. No. 4,587,504. Calculations further show that the level of theoretical homogeneity on the surface of a 50 cm sphere centered in the bore of the magnet (not taking manufacturing tolerances into account) is several hundred parts per million.
It is an object of the present invention to provide a method of determining coil arrangement in an actively shielded magnetic resonance magnet which results in a compact design with dimensions comparable to existing unshielded designs.
It is a further object of the present invention to provide a method of determining coil arrangement in an actively shielded magnetic resonance magnet which drastically reduces the stray field around the magnet and provides superior magnetic field homogeneity in the bore of the magnet.
It is a still further object of the present invention to provide a method of determining coil arrangement of an actively shield magnetic resonance magnet which provides reduced stray field and improved magnetic field homogeneity for different magnet operating fields by only linearly changing the current relative to the desired field.
It is yet another object of the present invention to provide a method of determining coil arrangement of an actively shielded magnetic resonance magnet in which each coil contributes to the overall homogeneity of the magnet with no distinction based upon the direction of current flow.