1. Field of the Invention
The present invention concerns a magnetic resonance apparatus and a method for conducting a magnetic resonance examination.
2. Description of the Prior Art
For the successful implementation of an imaging magnetic resonance examination (in the following “MR” stands for “magnetic resonance”), the basic magnetic field in an imaging or examination region (measurement volume) must be sufficiently strong and homogeneous in order to be able to take optimally exact measurements. A homogeneity with a maximum deviation of less than 3 ppm (in particular less than 1 ppm) (ppm: “parts per million”) is required in the measurement volume.
Basic magnetic fields of approximately 0.5 T and greater are generated with the use of a superconducting basic field magnet that conventionally is formed of multiple superconducting coils. In order to be able to achieve the required field strength and homogeneity in a predetermined measurement volume, a large effort must be made in the development of the basic field magnet.
The coils are typically arranged such that the homogeneous region that predetermines the maximum measurement volume has a spherical shape. The spherical shape results from the employed approach to bring to zero the coefficients of lower orders of a spherical function expansion of the magnetic field generated by the coils. The first coefficient not brought to zero typically describes the significant component of the remaining inhomogeneity. The goal of such a method is thus to bring as many coefficients of lower order to zero as possible.
Such methods for generation of a homogeneous magnetic field in a spherical region with superconducting coils go back to the start of MR technology. For example, J. R. Baker, “An improved three-coil system for producing a uniform magnetic field”, J. Sci. Instrum., vol. 27, pp. 197, 1950.
Today an optimally large, maximally adjustable measurement volume (and thus an optimally large homogeneity volume) is increasingly demanded, for example in order to enable MR angiographies of the peripheral vessels or MR tomography examinations of optimally large portions of the spinal column in the measurement volume.
The generation of an optimally large homogeneity region places high demands on the coil design. An increased number of superconducting coils for the basic field magnets and high technical effort are necessary in order to achieve such large regions with homogeneous basic magnetic field and sufficient basic magnetic field strength. The coil count has direct effects on the size and the cost of an MR apparatus and the available maximum measurement volume. The latter is often still smaller than an examination region of a patient to be examined.
In order to examine such a large examination subject, it is known (for example) from U.S. Pat. No. 5,928,148 to examine the examination subject step-by-step.
In chapter 3 in his dissertation “Magnet Optimization for Prepolarized Magnetic resonance Imaging”, Stanford University, October 2002, Hao Xu describes a method for the development of a magnet with an arbitrary predeterminable homogeneity volume with a low number of magnet coils and optimally small size and power. The magnetic field bm (m=1, 2, . . . , M) is predetermined at M target points on the edge of the homogeneity volume and the currents in (n=1, 2, . . . , N) required for the generation of this field are calculated in N possible magnet coils. bm=Amnin thereby applies. The matrix elements Amn depend on the radius of the n-th coil rn, the location of the n-th coil zn, the radius of the m-th coil ρm, and the location of the m-th target point ζm. ∥bm−B0∥≦εB0 is predetermined as a boundary homogeneity condition for bm, wherein B0 is the desired magnetic field strength and ε indicates the allowed deviation in ppm.
There are examples of special magnet forms for special applications. Among other things, a head and neck magnet for an examination of tobacco-caused cancer that has a cylindrical homogeneity volume in which the head and neck of a patient are precisely placed is described in chapter 3.3.3.