Patent Abstract:
A MRI shim element includes a plurality of thin wires extending substantially parallel to each other wherein each wire has cross sectional dimensions less than δ sk , wherein δ sk  is the wire&#39;s skin depth.

Full Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to methods and apparatus for magnetic resonance imaging (MRI) systems, and more particularly to methods and apparatus that facilitate making and using passive shim elements that provide for low eddy currents and an improved image stability. 
   Achieving a high final field homogeneity in MRI magnets typically requires the use of magnetic shimming, either a fully passive or a hybrid system that includes passive shims as an integral part. Shim elements made of magnetized steel, loaded strategically on shim rails, inserted inside the bore and saturated by the main field of the magnet, compensate for manufacturing tolerances and environmental inhomogeneities. These shim elements are exposed to pulsing fields from a plurality of gradient coils which generate heat in the shim elements, raising their temperature and thus affecting magnetic field stability (both B 0  and homogeneity) due to the temperature-dependent saturation B S (T) of the shims. Among major contributors to the shim heating are eddy currents induced in the conducting shims by the changing magnetic flux from the gradient coils. There is a need for efficient inexpensive passive shim elements with low eddy current heat generation. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In one aspect, a MRI shim element includes a plurality of thin wires extending substantially parallel to each other wherein each wire has cross sectional dimensions less than δ sk , wherein δ sk  is the wire&#39;s skin depth. 
   In another aspect, a method for making a plurality of shim elements is provided. The method includes placing a plurality of thin wires extending substantially parallel to each other to form either a flat sheet or an arced structure wherein each wire has cross sectional dimensions less than δ sk , wherein δ sk  is the wire&#39;s skin depth, and cutting or punching the flat sheet or arced structure to form a plurality of shim elements. 
   In still another aspect, an imaging apparatus for producing Magnetic Resonance (MR) images of a subject is provided. The apparatus has a magnet assembly for producing a static magnetic field B 0  and a gradient coil assembly disposed within the magnet assembly for generating a magnetic field gradient for use in producing MR images, the apparatus includes a MRI shim element including a plurality of thin wires extending substantially parallel to B 0 . 
   In still another aspect, an imaging apparatus for producing Magnetic Resonance (MR) images of a subject is provided. The apparatus has a magnet assembly for producing a static magnetic field B 0  and a gradient coil assembly disposed within the magnet assembly for generating a magnetic field gradient for use in producing MR images, the apparatus includes a MRI shim element including a plurality of thin wires extending substantially perpendicular to B 0   

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a magnetic resonance imaging (MRI) system. 
       FIG. 2  illustrates two shim elements. 
       FIG. 3  illustrates that shim elements of different sizes can be cut out or punched out from a prefabricated composite wire/epoxy sheet. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Herein described are methods and apparatus that provide a simple design to control the eddy currents generated in the shim elements during any gradient operational conditions. 
   As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     FIG. 1  illustrates a simplified block diagram of a system  10  for producing images to which embodiments of the below described shim elements of the present invention are applicable. Although a bore-type magnet is illustrated in  FIG. 1 , the present invention is equally applicable to open magnet systems and other known types of MRI scanners. The MRI system could be, for example, a GE Signa MR scanner available from GE Healthcare which is adapted as described herein, although other systems could be used as well. 
   The operation of the MR system  10  is controlled from an operator console  12 , which includes a keyboard and control panel and a display (not shown). The console  12  communicates with a separate computer system  14  that enables an operator to control the production and display of images. The computer system  14  includes a number of modules, which communicate with each other through a backplane. These include an image processor module, a CPU module, and a memory module, known in the art as a frame buffer for storing image data arrays. The computer system  14  is linked to a disk storage or optical drive for storage of image data and programs, and it communicates with a separate system control  16  through a high speed serial link. 
   The system control  16  includes a set of modules connected together by a backplane. These include a CPU module  18  and a pulse generator module  20 , which connects to the operator console  12  through a serial link. The system control  16  receives commands from the operator, which indicate the scan sequence that is to be performed. The pulse generator module  20  operates the system components to carry out the desired scan sequence. It produces data that indicate the timing, strength, and shape of the radio frequency (RF) pulses which are to be produced, and the timing of and length of the data acquisition window. The pulse generator module  20  connects to a set of gradient amplifiers  22  comprising of G x , G y , and G z  amplifiers (not shown) to indicate the timing and shape of the gradient pulses to be produced during the scan. 
   The gradient waveforms produced by the pulse generator module  20  are applied to the gradient amplifier system  22 . Each gradient amplifier excites a corresponding gradient coil in an assembly generally designated as  24  to produce the magnetic field gradients used for position encoding acquired signals. The gradient coil assembly forms part of a magnet assembly  26  which includes a polarizing magnet  28  and a whole-body RF coil  30 . A volume  32  is shown as the area within magnet assembly  26  for receiving a subject  34  and includes a patient bore. As used herein, the usable volume of a MRI scanner is defined generally as the volume within volume  32  that is a contiguous area inside the patient bore where the homogeneity of the main, the gradient and the RF fields are within known, acceptable ranges for imaging. 
   A transmitter module  36  in the system control  16  produces pulses that are amplified by an RF amplifier  38  coupled to RF coil  30  by a transmit/receive module  40 . The resulting signals radiated by the excited nuclei in the subject  34  may be sensed by the same RF coil  30  and coupled through the transmit/receive module  40  to a preamplifier  42 . The amplified MR signals are demodulated, filtered, and digitized in a receiver  44 . The transmit/receive switch  40 , is controlled by a signal from the pulse generator module  20  to electrically couple the transmitter  36  to the RF coil during the transmit mode and to connect the preamplifier  42  to the RF coil during the receive mode. 
   The MR signals picked up by RF coil  30  are digitized by the receiver module  44  and transferred to a memory module  46  in the system control  16 . When the scan is completed, an entire array of data has been acquired in the memory module  16 . An array processor (not shown) operates to Fourier transform the data into an array of image data. These image data are conveyed to the computer system  14  where they are stored. In response to commands received from the operator console  12 , these image data may be further processed by an image processor within computer system  14  and conveyed to the operator console  12  and subsequently displayed. 
   All commercial MRI systems employ passive shims to attain the desired final homogeneity of the uniform magnet field B 0 . A typical passive shim system includes a set of magnetized elements of predetermined different volumes/denominations, which allows one to create a desired magnetized moment on each shimming position. The degree of quantization in such a system is determined by the number of shim denominations and the volume of the smallest shim element. Passive shims are positioned inside the bore or within the gradient assembly, e.g. on the shim rails, in such a way that the multitude of available shimming positions would provide an adequate coverage of both axial and circumferential range, in order to enable compensation of both axial and radial harmonics representing B 0  inhomogeneity. In European patent application EP 0677751, the shim system employs shimming rings. Such rings due to their symmetry can be used to compensate axial harmonics only, while the need to compensate radial harmonics still remains. This need can be addressed by a separate set of discrete shim elements which unlike rings, do not have axisymmetric geometry and thus can allow variation of their circumferential distribution. The shim elements described below can be positioned on rails extending axially as well as circumferentially, and therefore provide full complete compensation for both axial and radial harmonics without resorting to additional rings, thus reducing complexity. In the radial direction, a stack-up of shim elements of different denominations within maximum allowable radial thickness, provides the desired total mass on each given position. System  10 , in some embodiments, includes shim elements as described below. 
   Herein described are shim elements that are composite and made of a parallel alignment of multiple long ferromagnetic steel wires held together by a filling material. The filling material can be epoxy, rubber, plastic, etc. When the shim elements are positioned on the rail, the wire orientation is typically parallel to the B 0  field of the magnet. However, the shim elements may be turned such that the wires extend perpendicular to B 0  and have a reduced magnetization. Additionally, the shim elements can be at any angle between being parallel and perpendicular to B 0  in order to achieve various shimming effects. Different sized shim elements would be employed to provide the desired quantization. 
     FIG. 2  illustrates two different shim element designs; a prior art shim  50  and a new shim element  60 . The main magnet field B 0  and gradient magnet field components are also shown in  FIG. 2 . By reducing wire cross-sectional dimensions a, and b to be below the skin depth δ sk , one can reduce eddy currents caused by either dB r   grad /dt, dB j   grad /dt, or dB z   grad /dt to any predetermined level. This is illustrated in  FIG. 2  portions a) and b). The skin depth of the shim material equals to δ sk =√{square root over (2ρ/μμ 0 ω)}, where ρ is the material resistivity, μ is the material relative permeability, μ 0  is the permeability of vacuum, and ω is the gradient circular or angular frequency. In a saturated state inside the magnet, μ is close to one. With 1006 steel, for example, δ sk =6.6 mm at 1 kHz gradient frequency and is 21 mm at 100 Hz. In the conventional shim piece  50  where dimensions are much greater than ask, (as in  2   a ) eddy currents are flowing around the outer perimeter. In the herein provided configuration, rectangular or round wires  62  have dimensions less than δ sk , and very limited or no contact with each other. In some embodiments, the dimensions are much less than δ sk . For example, some embodiments use dimensions of ¼ δ sk  and ⅓ δ sk . The eddy currents flow as indicated by an arrow in  FIG. 2   b . The resultant heat generation in each wire is proportional to d 3  where d is the dimension normal to the magnetic flux from the gradients. Since the number of wires per piece (in each direction) increases as 1/d with smaller d, the total heat generation per piece is decreasing with wire diameter as proportional to d 2 . A central opening  64  in the shim elements allow for easy mounting on a stud or screw. However, the central opening is not necessary, and the shim elements may be mounted in other ways. For example, the rail may include a pocket sized to receive a shim element. 
     FIG. 3  illustrates that shim elements  60  of different sizes can be cut out or punched out from a prefabricated composite wire/epoxy sheet  70 . In one example, the wires  62 , either round or rectangular, are laid out and impregnated with a non-conductive bonding agent or filler material, such as, for example, epoxy. Additionally, in another embodiment, rather than constructing a flat sheet, the wires are wound around a drum or other jig and then the non-conductive bonding agent is applied. The arced structure from the winding around the drum may be flattened (bent straight) or left in an arc resulting in an arced shim element. In the embodiment with round wire, one advantage and technical effect is lower cost. The wires are either bare (limited linear contact), or pre-coated with varnish (no contact) and than impregnated by epoxy. The maximum attainable volumetric ratio (bare wire) is π/2·{square root over (3)}, or 90.7%. 
   In the embodiment with rectangular wire, one advantage and technical effect is higher volumetric magnetization (filling factor). Wires can be separated (either sleeved or interlaid e.g. by glass cloth or other non conductive material) prior to the impregnation. Additionally, multiple shim elements may be stacked in multiple layers either aligned as shown in  FIG. 2   b , or offset as shown in  FIG. 3 . 
   Exemplary embodiments are described above in detail. The assemblies and methods are not limited to the specific embodiments described herein, but rather, components of each assembly and/or method may be utilized independently and separately from other components described herein. 
   While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.

Technology Classification (CPC): 6