JP2602513B2 - Method of performing passive shim action of magnet and passive shim assembly for magnet - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to the passive shim action of magnetic resonance magnets to achieve imaging quality homogeneity in the bore of the magnet.

To produce a highly uniform magnetic field using an array of electromagnets or permanent magnets, the magnets must be carefully assembled into a specified shape and efforts must be made to minimize deviations from the specified shape due to manufacturing variations. It is necessary to pay. However, in order to achieve the desired level of non-homogeneity, the resulting magnet may have to compensate for the magnetic field due to magnet deviations from the design or due to the presence of ferromagnetic material near the magnet. Is typically required.

Typically, a correction coil is used to improve the uniformity of the magnetic field. Such coils can generate different magnetic field shapes that can be tuned to the main magnetic field in a manner that superimposes on the main magnetic field with inhomogeneity and enhances overall magnetic field uniformity. . Unfortunately, typically such coils require several sets. Conventional magnetic resonance (MR) imaging magnets have 10 to 20 independent correction coils, each set having its own power supply to carry the correct current. Of course, such coils significantly increase the cost and complexity of the magnet.

One way to eliminate the need for a correction coil is to passively shim the magnet, using only iron pieces, to bring the initially inhomogeneous magnetic field within the specifications for homogeneity for imaging. It is to do. When the iron piece is placed inside the bore of the magnet, only a small increase in size and weight is required.
Passive shimmed magnets are less expensive and more reliable than typical compensation coil sets currently in use.

A major difficulty in implementing such a shim scheme is predicting the position and dimensions of the iron pieces required for the magnetic field shim action. In general, electromagnetic coils are designed to generate certain terms of the spherical harmonic expansion. These design criteria are difficult to implement using passive shims. This is because the magnetic permeability of iron cannot be reversed, but the current through the coil can be reversed to reverse the magnetic field of the correction coil. Furthermore,
This scheme is not feasible due to the size and complexity of the group of shim pieces required to generate one harmonic function. Since the magnetic coupling between shims is also a factor that complicates, if a large piece is used to perform the shim action, it is inevitable that these pieces physically come close to each other, so that the correct shim action of the magnet is It will be difficult to do.

Currently, passive shim action is used to correct for large deviations in the magnetic field that cannot be corrected with available correction coils alone. Passive shim action is achieved by placing an iron piece in a suitable location outside the magnet.
The desired level of magnetic field uniformity can then be achieved by the correction coil.

It is an object of the present invention to provide a method for passively shimming a magnetic resonance magnet without using a correction coil to achieve the level of heterogeneity required for magnetic resonance imaging. It is.

Another object of the invention is to use only a piece of ferromagnetic material,
It is to provide a method for determining the optimal axial and circumferential position of the shim so as to bring the homogeneity of the magnetic field to a level suitable for magnetic resonance imaging.

It is another object of the present invention to provide a method that minimizes the overall inhomogeneity of the magnetic field, rather than the selected harmonic function.

SUMMARY OF THE INVENTION In one aspect of the present invention, there is provided a method of performing shim passively on a magnet having a central bore using a shim disposed within the bore of the magnet. The method includes measuring the homogeneity of the initial magnetic field in the bore of the magnet. Inspect the magnetic field effects of the shim at each of the allowable predetermined shim locations in the bore of the magnet independently of each other to determine the required shim strength to improve the inhomogeneity of the magnetic field in the bore of the magnet. . The locations where a positive shim strength is found to be advantageous are selected and used to determine the shim strength at each selected location, considering all selected locations simultaneously. Remove the locations that were found to require negative shim strength and use the new selected locations to increase the shim strength again until all remaining selected locations require positive shim strength. decide. A shim having the predicted positive strength is placed at a selected location within the bore of the magnet.

While the gist of the present invention is specifically and clearly described in the appended claims, the objects and advantages of the present invention will be better understood from the following description of preferred embodiments thereof, taken in conjunction with the accompanying drawings.

Detailed description of the drawings Next, the drawings will be described. FIG. 1 shows a passive shim assembly composed of a non-magnetic thin-walled tube 11. This tube is made of glass fiber in a 1/8 inch thick tube. A plurality of longitudinally extending non-magnetic channel members 13 are circumferentially equally spaced along the inside of the tube 11. The channel extends the length of the tube and is secured by screws 14 in threaded engagement with the glass fiber tube. Each channel has two projecting edges 13a extending on both sides of the channel. These edges are parallel to and spaced from the tube. These edges extend over the longitudinal length of the tube. The channel can be manufactured by extruding aluminum into the desired shape, or if eddy currents are a concern, the channel can be pultruded from a composite material. Pultrusion is a method of drawing a continuous filament through an orifice, which removes encapsulating resins such as thermoplastics.

An arcuate support piece 15 of a non-magnetic material such as glass fiber is slidably mounted between two adjacent grooves.
Two adjacent groove-shaped projecting edges 13a prevent the support piece 15 from moving in the radial direction. Ferromagnetic strip with shim action 17
Is stacked on the support piece to the desired height. The strip is shorter in length than the length of the bow. The ferromagnetic strip is made of 0.010 inch thick low carbon steel and, for a 1 meter bore magnet, is cut to 2 cm in axial width and has a circumferential extent of 30 ° at its average radius. The strip is fixed to the bow-shaped support piece by a screw connection member 19 or the like.

The support piece is locked by the clamp 21 at its vertical position. This clamp can be made of aluminum. The clamp, shown in more detail in FIG. 2, captures a portion of the rim 13a of the channel 13 between the clamp and the arcuate support when tightened against the arcuate support by bolts 23. Fix the vertical position of the ferromagnetic strip.

The radial thickness of the entire assemblage is minimized so as not to interfere with the valuable space in the bore occupied by the gradient and RF coils and the patient table (not shown).

The axial position of the ferromagnetic strip is infinitely adjustable, and by changing the height of the stack of strips,
Very fine adjustment of shim strength. Thinner steel strips can be used to allow for finer adjustments in strength. The channels are arranged at 45 ° intervals along the inner circumference of the bore so that eight different ferromagnetic strips can be circumferentially located. In this embodiment, the selection of the circumferential position is chosen to perform a shim action on a spherical harmonic having a power of m = 2. The magnetic field at m = 2 changes sinusoidally at 2φ, where φ is the angle in the circumferential direction. Thus, such a magnetic field has a peak or node every 45 ° in the circumferential direction. It is clear that the ability to place shims every 45 ° allows shim action of a harmonic function of m = 2.

When a magnet is energized, the maximum axial force on a 1 cm thick shim is about 20 pounds for a 0.5 T (tesla) magnet. The shim support piece can be moved when the clamp is loosened while adjusting the axial position. It is easy to loosen the clamp and create a handle that allows easy control of the shim support.

FIG. 3 shows another embodiment. The passive shim assembly includes a thin-walled, non-ferromagnetic tube 31 which, in the preferred embodiment, is made of 1/8 inch thick glass fiber material. A plurality of channel members 33 are provided at equal intervals outside the tube. A channel extends over the length of the tube and is secured by screws in threaded engagement with the glass fiber tube 31. Several screws 35 located at both ends of the channel extend above the surface of the channel,
The tube is positioned concentrically within the bore of the magnet. This is more apparent in FIG. The channel members are at both ends of the channel,
It has a protruding edge 33a extending away from the channel. These edges are parallel to and spaced from the tube. These edges extend over the longitudinal length of the tube. The channel can be made by extruding aluminum into the desired shape, or if eddy currents are a problem, the channel can be made by pultrusion of a composite material. Increasing the number of positions in the circumferential direction increases flexibility in excluding equiaxed (axial periodic) harmonic functions.

As shown in FIG. 5, the edges where the bow-shaped drawers 37 are adjacent to each other
It fits between 33a and extends the length of the tube. A shimming ferromagnetic strip 41 is stacked on the puller at the desired height and secured to the puller at selected pre-drilled holes 42. The more holes provided in the axial direction, the greater the ability to finely adjust the axial magnetic field. The axial adjustability of the shim of the embodiment of FIG. 1 is infinitely adjustable, which may be undesirable in some cases. The number of ferromagnetic strips controls the intensity. The radial thickness of the shim is minimized so that the shim fits within the space defined by the adjustable height of the elongated screw 35. The puller can be slid out of the bore of the magnet to adjust the axial position and thickness of the ferromagnetic strip.
With the magnet energized and the position of the ferromagnetic strip adjusted, the retractor can be removed.

The position and height of the ferromagnetic strip in the bore of the magnet are used to create a magnetic field shape that corrects for the inhomogeneity of the magnetic field generated by the magnet. Therefore, the flexibility of positioning the steel strip so that it has all the necessary magnetic field shapes to oppose the magnetic field that hinders the homogeneity of the imaging quality is important. Since it is doubtful whether the initial prediction of the position of the ferromagnetic strip is perfect, flexibility in positioning is important. An arcuate shim at a particular location does not attempt to eliminate a particular harmonic. Rather, the combination of all shims together attempts to increase the homogeneity of the magnetic field. Where the required shim height impedes the space available for the bore, a wider shim can be used in either embodiment.

FIG. 6 shows a flowchart illustrating the steps for determining the correct position and thickness of the shim. The first step in block 45 is to determine the initial inhomogeneity in the bore of the shimed magnet. Energize the magnet,
The magnetic field is measured on the virtual grating 46. For a 0.5 Tesla superconducting magnet having a hole diameter of 1 meter, as shown in FIGS. 7 and 8, Z is located on the circumference of 13 circles and from the center of the hole.
A grid with 314 points at two points 20 cm on either side of the axis can be used. Five circles have a center point on the Z axis and a diameter of 44 cm, and six circles have a center point on the Z axis and a diameter of 20 cm. The remaining two circles are 28 cm in diameter. The larger circles are along the Z axis at 7.5 cm and 10 cm on either side of the center of the bore. The circle with the smaller diameter is 10c along the Z axis from both sides of the center point of the hole.
m, 15cm and 20cm. The middle circle is concentric with the small and large circles at 10 cm on either side along the Z axis from the center of the bore. Measurements are taken at 24 equally circumferentially spaced points along each circle. These circles lie at the boundaries of the volume of interest because the maximum and minimum values of the magnetic field should be there. The position of the circle is chosen close to the limit of the ideal magnet's magnetic field, and the actual inhomogeneity should also approach the value seen by sampling the points. Based on a comparison of the magnetic fields measured at each point, if the difference between each point exceeds 500 ppm, as determined at block 47, the location of the large shim to offset the gradient is determined at block 51, and this large With the shim in place, measure the magnetic field again. If the heterogeneity is less than 500 ppm, block 53 executes the PLAS3D code.

The PLAS3D code defines, for each acceptable shim position, a predetermined axial direction at each of the 314 magnetic field measurement points,
Determine the magnetic field effect of a bow steel shim having radial and circumferential dimensions. The axial and circumferential position of the bow steel shim is a variable in the shim procedure. For example, if the acceptable area of the arc is -90 cm to 90 cm along the Z axis, then Z
Magnetic field effects can be determined using arcs of every 10 cm along the axis. The higher the density, the longer the algorithm takes, but the more possible shim locations, and therefore the generally better the homogeneity.

The magnetic field of a magnetized material can be represented by a series of spherical harmonics developed around the origin of the magnet's coordinate system. The equation of the harmonic function representing the magnetic field is as follows.

Where the coefficient A (n, m) is the volume integral of the shim, a (n, m)
Is a conversion function defined by Schenk et al., P (n, m)
Is the associated Legendre polynomial. The number of terms required to accurately represent the magnetic field depends on the size of the volume of interest, but with current shim action, orders up to order and degree 8 are sufficient. The magnetization in the steel shim can be calculated or assumed.

The determination of the bow magnetic field effect need only be made for one circumferential position at each selected axial position, and to represent any of the 24 circumferential positions, , The arc field is indexed in 15 ° increments. Typical circumferential arcuate densities are only 8 to 12 per circle, so indexing gives accurate results.

Once the required data file containing the magnetic field effects at each of the 314 magnetic field measurement points has been created for all positions of a given arcuate shim, the individual effects of each arc position on the selected grid can be determined. Evaluate for optimal strength. This optimal intensity is defined as the intensity at which the non-homogeneity that occurs in the imaging volume is minimized. Such optimization can be performed using a least squares routine that solves the following equation:

Where ε is a chosen measure of magnetic field homogeneity, Bz
_m represents the magnetic field measured at point m, C _m is the magnetic field point m
Is a coefficient representing the magnetic field per unit thickness formed by the shim at the location in question. This equation is written for each arcuate location in the grid and solved to determine the thickness of the shim at each location to minimize magnetic field homogeneity. This optimization for thickness assumes that the bow field effect has a linear dependence on its thickness, ie, that the bow magnetization does not change with thickness. This assumption holds strictly for saturated arcs. If the thickness of the shim is negative (not physically a real solution), its position is ignored. The remaining set of positions is provided to the SHIMPSV code at block 55 as an initial estimate.

The SHIMPSV algorithm determines where to place the bow shim, as well as its thickness. In the PLAS3D program, there can be dozens of locations that require the positive strength of the shim,
You only need 20 to 25. Therefore, the algorithm must decide which to remove. The SHIMPSV algorithm initially uses all locations that require a positive strength shim, which is roughly half of the locations initially examined by the PLAS3D algorithm. afterwards,
A linear least squares optimization is performed simultaneously for all positive shim strengths. The result of this initial run includes shims of negative strength, but these locations are excluded from consideration. Negative strength occurs because the effect of all shim locations that have individually been found to have positive strength is not the same as the effect when these shims are considered simultaneously. Next, the remaining positions requiring a positive strength shim are taken and the least squares optimization is performed again. Until a solution having all positive intensities is obtained, the process of removing positions having negative intensities is repeated. The expected heterogeneity of the solution is compared at block 57 to the desired heterogeneity. The field homogeneity that can be achieved with a given set of positive strength shim positions is generally inversely proportional to the number of shim positions used in the operation, and therefore the more shims the better the results. . If a solution with all positive intensities is not obtained such that the predicted inhomogeneity falls within the specification, the parameters are changed in block 61 and the PLAS
Increase the number of acceptable shim positions used in 3D code. It is desirable to use a shim that is vertically approaching the center of the borehole. This is because a smaller shim closer to the center has a greater effect on non-homogeneity at the center of the magnet than a shim located vertically away from the center. If a solution is not obtained using a group of positions closer to the center, increase the number of allowable shim positions and execute the PLAS3D code again. A solution with all positive shim strengths should be physically obtained and the block
At 63, use it to initially place the shim within the borehole. Negative shim strength requires a material with negative magnetic permeability. The shim is placed at a predetermined position within the energized magnet, and at block 65 the magnetic field in the bore is measured again at 314 points on the grid. If the deviation from the expected heterogeneity is greater than desired, the position of the arc is fixed and the SHIMPSV algorithm is run again to determine the value of the magnetic field obtained with the shim in place. Next, the thickness of the arc is adjusted at block 67 using a least squares routine. These thickness changes should be a small fraction of the initial thickness, and if performed once, the inhomogeneity should be reduced to the desired range.

When implementing the SHIMPSV algorithm in block 55,
Several modifications are possible that may be advantageous in some cases. Due to the negative thickness of the shim, locations that were excluded from consideration in any step of repeating the SHIMPSV code can be introduced in later iterations to effectively try out more locations. As a result, if a solution with a large number of shims is obtained,
The expected inhomogeneity is generally less, which is desirable. The least squares determination assumes a linear relationship between the change in shim thickness and the effect on the magnetic field. The method described above for determining the position of a shim within the bore of a magnet can also be used for electromagnets, including superconducting magnets and permanent magnets.

Thus, a method of passively shimming an MR (magnetic resonance) magnet assembly that does not use a correction coil and achieves the level of magnetic field homogeneity required for magnetic resonance imaging. Was explained.

Although the invention has been described with reference to preferred embodiments, it will be understood that various modifications can be made within the scope of the invention. Therefore, it is to be understood that the appended claims are to cover all such modifications as possible within the scope of the invention.

[Brief description of the drawings]

1 is a perspective view of a passive shim assembly, FIG. 2 is an end view showing a cross section of a part of the shim assembly of FIG. 1 disposed in the bore of a magnetic resonance magnet, and FIG. FIG. 4 is an end view of a cross-section of a part of the shim assembly of FIG. 3 disposed in the bore of the magnetic resonance magnet, FIG. 5 is a passive end view of the passive shim assembly of FIG. FIG. 6 is a partial perspective view of the shim assembly, showing one detachable slide being slid to a predetermined position within the passive shim assembly. FIG. 6 is a perspective view of a magnetic resonance magnet according to the present invention. FIG. 7 is a flow chart showing a passive shim operation, FIG. 7 is a perspective view of a part of a hole in a magnetic resonance magnet, showing a position where a non-homogeneity of the magnet is inspected, and FIG. Fig. 3 is a partial side view with the dimensions shown, showing the position where the magnet is checked for non-homogeneity; Description of main reference numerals 11: tube, 13: grooved member, 15: arcuate support piece, 17: ferromagnetic strip, 19: screw coupling member, 21: clamp, 23: bolt, 31:
Tube, 33: channel, 35: screw, 37: bow, 41: ferromagnetic strip, 42: hole, 46: virtual lattice.

Claims (14)

(57) [Claims] 1. A correction coil for increasing the homogeneity of the magnetic field generated by a magnet having a central bore, using a shim (17) located in said bore. A method for passively shimming the magnet without: a) measuring the inhomogeneity of the initial magnetic field in the magnet bore; b) an acceptable shim position in the magnet bore. C) checking the field effect of the shim at each of the acceptable predetermined shim positions in the bore of the magnet independently of each other to improve the non-homogeneity of the magnetic field in the bore of the magnet. D) selecting locations where a positive shim strength is found to be advantageous; e) taking into account all selected locations simultaneously, and estimating the magnetic field at each selected location. Determining the required shim strength to improve the heterogeneity; f) Steps e) and f) are repeated until all remaining selected positions require a positive shim strength, removing the locations known to require a shim strength of Disposing a shim having a variable thickness at a selected location within the bore of the magnet. 2. Following step f), predicting the inhomogeneity of the magnetic field when a shim having a predicted thickness is placed at a selected location within the bore of the magnet; Comparing the obtained heterogeneity with the desired heterogeneity, and increasing the number of acceptable predetermined positions of step b) to reduce the difference between said predicted heterogeneity and said desired heterogeneity. 2. The method according to claim 1, comprising the step of repeating steps c), d), e) and f). 3. To help increase the number of selected locations that have been found to require a positive shim strength, step f) has already been removed in a previous iteration as requiring a negative shim strength. 2. The method of claim 1 further comprising the step of re-introducing the determined location. 4. A method according to claim 1, further comprising the step of: measuring the inhomogeneity of the magnetic field in the bore of the magnetic field with the shim in place and predicting the inhomogeneity of the magnetic field measured with the shim in place. Determining the incremental change in shim strength at the selected location to reduce the difference between the desired inhomogeneity and the desired inhomogeneity; and a thickness of the shim at the selected location within the bore of the magnet. 2. The method according to claim 1, further comprising the step of: incrementally changing by the amount determined in the previous step. 5. A passive shim assembly for a magnet having a central bore without the need to use a correction coil to enhance the homogeneity of the magnetic field generated by the magnet, the passive shim assembly comprising: A non-magnetic tube (1
A) a plurality of arcuate strips (17) removably secured to the tube and having a predetermined length of ferromagnetic material, the strips extending circumferentially around the tube; A passive shim assembly for the magnet, comprising a plurality of arcuate strips (17) located at different locations on the tube. 6. An assembly according to claim 5, wherein said strip (17) is fixed outside said tube (11). 7. The assembly according to claim 5, wherein said strip (17) is fixed inside said tube (11). 8. The assembly of claim 5, further comprising means for coaxially positioning said tube within said bore. 9. An assembly according to claim 6, wherein all of the strips (17) have the same length. 10. An assembly according to claim 9, wherein said strips (17) are fixed as a stack of strips of varying height. 11. Fixed inside said tube (11),
A plurality of channel members (13) provided at equal intervals in the circumferential direction and extending in the vertical direction, and are slidably mounted at selected positions between adjacent channel members. A plurality of arcuate support pieces (15); clamp means (21) for fixing the support pieces (15) to the groove-shaped member so as to prevent sliding; and detachably fixed to each of the support pieces. 6. The assembly of claim 5, further comprising a stack of said strips of ferromagnetic material. 12. The support piece (15) and the channel member (13).
An assembly according to claim 11, wherein each of said clamping means (21) comprises a non-magnetic material. 13. A tube fixed to the outside of the tube,
A plurality of longitudinally extending channel members provided at equal intervals in the circumferential direction; and a plurality of arcuate pulls slidably mounted between adjacent channel members. 6. The assembly of claim 5, further comprising a child (37) and a plurality of stacks of said strips (41) of ferromagnetic material removably secured to each of said drawers. . 14. The assembly according to claim 13, further comprising means (35) for coaxially positioning said tube within the bore of said magnet.