DE3782150T2 - PASSIVE ARRANGEMENT OF COMPENSATION BODIES AND METHOD FOR DETERMINING THE POSITION OF THE COMPENSATION BODIES FOR A MAGNET OF THE MAGNETIC RESONANCE. - Google Patents

Description

The invention relates to a passive field correction by inserting thin sheets (shimming) into magnetic resonance magnets in order to obtain a homogeneity in the bore of the magnet that ensures good imaging quality.

To induce a very uniform magnetic field with an electromagnet or an array of permanent magnets, it is necessary to build the magnet with a carefully specified shape and to strive to minimize deviations from the specified shape due to manufacturing variations. However, the resulting magnets usually require field corrections to achieve the desired degree of inhomogeneity due to deviations of the magnet from the design specification or due to the presence of ferromagnetic material in the vicinity of the magnet.

To improve field uniformity, correction coils are commonly used. These coils are capable of inducing different field shapes that can be superimposed on an inhomogeneous main magnetic field to influence the main magnetic field in a way that increases the uniformity of the overall field. Unfortunately, many sets of such coils are usually required. A common magnetic resonance (MR) imaging magnet has between 10 and 20 independent sets of correction coils, each with its own power supply to provide the correct current flow. Of course, these coils significantly increase the cost and complexity of the magnet.

One way to avoid correction coils is to provide passive field correction of the magnet, using only pieces of iron to bring an initially inhomogeneous field into homogeneity specifications for imaging. Placing the iron within the bore of a magnet would require a minimal increase in size and weight. A magnet with passive field correction would be cheaper and more reliable than a typical set of correction coils currently in use.

The main difficulty in implementing such a field correction solution lies in predicting the locations and sizes of iron gauges required to correct the field. Electromagnetic coils are generally designed to produce certain terms of spherical harmonic expansion. Such design criteria are difficult to implement with passive field correction gauges because the permeability of iron cannot be reversed, whereas a current reversal through a coil can be used to obtain a field reversal in a correction coil. In addition, the size and complexity of the groups of correction gauges that would be required to produce a single harmonic would make this solution inadvisable. Since magnetic coupling between correction gauges is also a complicating factor, error correction with large gauges that inevitably become physically close increases the difficulty in properly field correcting the magnet.

Currently, passive field correction (shimming) is used to correct large deviations in magnetic fields that cannot be corrected by the available correction coils alone. Passive error correction is achieved by placing a piece of iron in a suitable position outside the magnet. The desired degree of Field uniformity can then be achieved by the correction coils.

EP-A-0167059 describes in Fig. 3 a tube made of synthetic material on which magnetic correction gauges made of iron in the form of several iron plates can be fixed, the tube being arranged in the central bore of an MR magnet in order to compensate for an inhomogeneity in the bore. There is no disclosure that the iron plates are curved in the circumferential direction, that they are demountable and how their positions on the tube are determined.

It is an object of the present invention to provide a method for passive field correction of a magnetic resonance magnet in order to obtain a degree of field homogeneity required for magnetic resonance imaging without the use of correction coils.

It is a further object of the present invention to provide a method for determining the optimal locations of the correction gauges in the axial direction and in the circumferential direction in order to bring the field homogeneity to a value that is consistent with magnetic resonance imaging, using only gauges made of ferromagnetic material.

It is a further object of the invention to provide a method for minimizing the total field inhomogeneity and unselected harmonics.

According to the invention, a method is provided for passive field correction in a magnet with a central bore and using compensating bodies or gauges which are arranged in the bore of the magnet. The method comprises the steps of measuring the initial field inhomogeneity in the bore of the magnet. The magnetic field effect of a gauge on each of the predetermined allowable gauge positions in the magnet bore are independently tested to determine the gauge strength required to improve the magnetic field inhomogeneity in the magnet bore. The positions where positive gauge strengths are found to be advantageous are selected and used to determine the gauge strength at each selected position, considering all selected positions simultaneously. Positions where required negative gauge strengths are found are eliminated and gauge strengths are again determined using the new selected positions until all remaining selected positions require positive gauge strengths. The gauges of predetermined positive strengths are placed in their selected positions in the magnet bore.

More specifically, the present invention provides a method for passive field correction of a magnet and a passive field correction arrangement according to claims 1 and 5, respectively.

While the specification concludes with claims particularly pointing out and particularly claiming the present invention, objects and advantages of the invention can be more readily ascertained from the following description of preferred embodiments taken in conjunction with the drawings in which:

Fig. 1 is an isometric view of a passive field correction arrangement;

Fig. 2 is a partial sectional view of the field correction arrangement of Fig. 1 arranged in the bore of a magnetic resonance magnet;

Figure 3 is an end view of another passive field correction arrangement;

Fig. 4 is a partial sectional view of the field correction arrangement according to Fig. 3 arranged in the bore of a magnetic resonance magnet;

Fig. 5 is a partial isometric view of the passive field correction assembly of Fig. 3 showing one of the removable pull sections inserted into the passive field correction assembly;

Fig. 6 is a flow chart for a passive field correction of a magnetic resonance magnet according to the invention;

Fig. 7 is a cutaway isometric view of the bore of a magnetic resonance magnet showing the positions at which magnet inhomogeneity is checked; and

Fig. 8 is a partial side view with dimensions of Fig. 7 and shows the positions in which the magnetic inhomogeneity is checked.

In Fig. 1, a passive gauge or field correction assembly is shown which includes a non-magnetic thin-walled tube 11. The tube is made of fiberglass and forms a tube with a wall thickness of 3.175 mm (1/8 inch). A plurality of longitudinally extending non-magnetic channel pieces 13 are equidistantly spaced circumferentially around the interior of the tube 11. The channel pieces run along the length of the tube and are secured thereto by screws threaded into the fiberglass tube. The channel pieces each have two projecting rims 13a extending on each side of the rim. The rims are parallel to and extend away from the tube. The rims extend the longitudinal length of the tube. The ducts can be made by extruding aluminum into the desired shape or, if eddy currents are a problem, the ducts can be pultruded from a composite material. Pultrusion is a process in which continuous filaments are pulled through an orifice that also dispenses an enveloping resin, such as a thermoplastic.

Arc-shaped support pieces 15 made of non-magnetic material, such as fiberglass, are arranged between the slidably mounted in two adjacent channels, with the projecting edges 13a of two adjacent channels preventing radial movement of the carrier pieces 15. Ferromagnetic strips 17, which provide error correction (shimming), are stacked to the desired height on the carrier pieces, the strips having a length shorter than the length of the arcuate carrier pieces. The ferromagnetic strips may comprise 0.254 mm (0.010 inch) thick low carbon steel cut to an axial width of 2 cm and a circumferential extent at their central radius of 30 degrees for a magnet with a bore of 1 m. The strips are attached to the arcuate carrier pieces by, for example, screw fasteners 19.

The support pieces are anchored in their longitudinal position by clamps 21, which can be made of aluminum. The clamps, which can be seen better in Fig. 2, when fixed to the arcuate support piece by bolts 23, enclose a part of the edge 13a of the channel 13 between the clamps and the arcuate support piece, thereby fixing the longitudinal position of the ferromagnetic strips.

The radial thickness of the entire assembly is kept to a minimum to minimize any impact on the precious bore space occupied by gradient and RF coils and the patient table (not shown).

The axial position of the ferromagnetic strips is infinitely adjustable, and very fine adjustment of the gauge thickness is achievable by changing the stack height of the strips. Thinner steel strips can be used to provide finer thickness adjustment. The channels are arranged every 45 degrees around the inner circumference of the bore, providing eight discrete circumferential positions for the ferromagnetic strips.

The choice of circumferential positions in the present embodiment was chosen to obtain a field correction for spherical harmonics of degree m = 2. Fields of m = 2 vary sinusoidally with 2Φ, where Φ is the circumferential angle. Therefore, such fields have peaks or nodes every 45 degrees in the circumferential direction. By providing a possibility to arrange gauges every 45 degrees, a field correction of m = 2 harmonics is thus permitted.

The maximum axial force on a 1 cm thick gauge with the magnet energized is about 1 kg (20 pounds) in a 0.5 T magnet. The gauge carrier could be moved with the clamps released while adjustment of the axial positions is made. A handle could be easily fabricated that would allow easy control of the gauge carriers with the clamps released.

Another embodiment is shown in Fig. 3. A passive field correction arrangement comprises a non-ferromagnetic thin-walled tube 31, which in the preferred embodiment is made of fiberglass material having a thickness of 3.175 mm (1/8 inch). A plurality of channel pieces 33 are arranged equidistantly around the exterior of the tube. The channel pieces extend the length of the tube and are secured thereto by screws screwed into the fiberglass tube 31. Some of the screws 35 arranged at the ends of the channel extend beyond the surface of the channel pieces to position the tube concentrically in the magnet bore. This can be seen more clearly in Fig. 4. The channel pieces have projecting rims 33a on each side of the channel which extend away from the channel. The rims are arranged parallel to and spaced from the tube. The rims extend the longitudinal length of the tube. The channel pieces can be made by welding aluminum into the desired shape is extruded, or, if eddy currents are a problem, the channel can be pultruded from composite material. Increasing the number of circumferential positions provides greater flexibility in eliminating tesseral (axis-periodic) harmonics.

Arcuate shims 37, as shown in Fig. 5, fit between adjacent edges 33a and extend the length of the tube. Ferromagnetic strips 41, which serve as the shims, are stacked to the desired height on the shims and secured to the shims through selected pre-drilled holes 42. The more axial holes provided, the finer the adjustability of the axial field. The sliding axial displacement of the shims in the embodiment of Fig. 1 provides for stepless adjustability, which may be desirable in some situations. The number of ferromagnetic strips provides for adjustment of strength. The radial thickness of the shims is kept to a minimum so that the shims fit in the space formed by the adjustable height of the extended screws 35. The pullers can slide out of the bore of the magnet to allow adjustment of the axial position and thickness of the ferromagnetic strips. The pullers can be removed when the magnet is energized and the positions of the ferromagnetic strips are adjusted.

The position and height of the ferromagnetic strips in the bore of the magnet are used to create magnetic field shapes that correct for inhomogeneities in the field induced by the magnet. Flexibility in positioning the steel strips is therefore important so that all field shapes can be obtained that are required to counteract fields that could prevent homogeneity for imaging quality. Flexibility in positioning is also important because it is doubtful that the initial prediction of the positions of the ferromagnetic strips is perfect. Arc-shaped gauges in specific positions are not intended to eliminate specific harmonics. Instead, the combination of all gauges or shims together is intended to increase field homogeneity. In situations where a required gauge height would conflict with the available bore space, wider gauges can be used in both embodiments.

Fig. 6 shows a flow chart of the steps in determining the correct positions and thicknesses of the gauges or compensation bodies. The first step in block 45 is to determine the initial inhomogeneity in the bore of the magnet to be provided with a field correction. The magnetic field is measured in the fed magnet on an imaginary grid 46. For a 0.5 Tesla superconducting magnet with a bore of 1 m diameter, a grid with 314 points located on the circumference of 13 circles and at 2 points 20 cm on each side of the bore center on the z axis, as shown in Figs. 7 and 8, can be used. Five of the circles have their center on the z axis and a diameter of 44 cm, six of the circles have their center on the z axis and a diameter of 20 cm. The two remaining circles of the 13 circles have a diameter of 28 cm. The larger circles are located on each side of the center of the hole along the z axis at 7.5 and 10 cm. The smaller diameter circles are located on each side of the center of the hole along the z axis at 10, 15 and 20 cm. The intermediate circles are concentric with the smaller and larger circles at 10 cm on each side of the center of the hole along the z axis. The measurements were taken at 24 equally spaced points around the circumference along each circle. These circles follow the boundary of the volume of interest because the maximum and minimum field values must lie there. The circle positions are chosen to be near the ideal magnetic field extremities, and the actual inhomogeneity should be close to that found by sampling their points. Based on a comparison of the measured field at each of the points, if the difference among the points is over 500 ppm as determined in block 47, then the position of large gauges for gradient cancellation is determined in block 51 and the field is measured again with the large gauges in place. If the inhomogeneity is less than 500 ppm then PLAS3D code is run in block 53.

For each allowable gauge position, the PLAS3D code determines the field effect of an arc-shaped steel gauge with given axial, radial and circumferential dimensions at each of the 314 field measurement points. The axial and circumferential positions of the arc-shaped steel gauges are a variable in the field correction process. For example, if the allowable arc range extends from -90 to 90 cm along the z axis, the field effects can be determined with an arc every 10 cm along the z axis. With higher densities, the algorithm takes longer but results in more possible gauge positions and thus generally provides better homogeneity.

The magnetic field of magnetized material can be represented as a series of spherical harmonics propagating around the origin of the magnet's coordinate system. The equations for the magnetic field harmonics are:

where the coefficients A(n,m) represent the volume integral over the gauge, a(n,m) the transfer functions defined by Schenck et al., and P(n,m) the associated Legendre polynomial. The number of terms required to accurately represent the magnetic field depends on the size of the volume of interest; for the field correction purposes described here, an expansion of one order and degree eight is sufficient. The magnetization in the steel gauges can be calculated or assumed.

The determination of arc field effects need only be performed for one circumferential position at each selected axial position and the arc field indexed in fifteen degree increments to represent the field from each of twenty-four circumferential positions. Typical circumferential arc densities are only 8 to 12 per circle for the indexing to give accurate results.

Once the required data sets have been generated containing the field effect at each of the 314 field measurement points for all predetermined positions of the arcuate gauges, the effect of each arcuate position is individually evaluated on the chosen grid for its maximum strength, which is defined as the strength that induces the minimum inhomogeneity for the imaging volume. A Such optimization can be performed using a least squares routine that solves the following equations:

where ε is the chosen degree of field homogeneity, Bzm represents the measured field at point m and Cm is the coefficient representing the field per unit thickness induced by a gauge at the location in question at field point m. The equation is set up for each sheet position on the grid and solved to give the gauge thicknesses at each position that achieves the minimum field inhomogeneity. This optimization with respect to thickness is based on the assumption that the field effect on a sheet is linearly dependent on its thickness; i.e. that the magnetization of the sheet does not change with thickness. This assumption strictly speaking only applies to sheets that are saturated. If the gauge thickness is negative (not a physically realistic solution), the position is eliminated from consideration. The set of remaining positions is then entered into the SHIMPSV code in block 55 as an initial assumption.

The SHIMPSV algorithm determines where to place the arched gauges and what their thicknesses are. While the PLAS3D program can provide many dozens of positions that require a positive gauge thickness, only 20-25 of these positions are suitable for a Field correction is required. Therefore, the algorithm must decide which ones to eliminate. The SHIMPSV algorithm starts with all positions requiring positive strength gauges, which is about half of the positions initially examined by the PLAS3D algorithm. Then a linear least squares optimization is performed for all positive gauge strengths simultaneously. The result of the first pass will contain a gauge with a negative strength, and these positions are eliminated from consideration. Negative strengths arise because the effect of all gauge positions found with positive strengths individually is not the same as the effect of all of these gauges considered simultaneously. Then the remaining positions requiring positive strength gauges are taken and the least squares optimization is run again. The process of eliminating negative strength positions is repeated until a solution is obtained where all gauges have a positive strength. The predicted inhomogeneity of the solution is compared to the desired inhomogeneity in block 57. The field homogeneity achievable with a given group of gauge positions of positive magnitude is generally inversely related to the number of gauge positions being worked with, so the more gauges the better. If a solution with all magnitudes positive is not possible with a predicted inhomogeneity within specification, the parameters in block 61 are changed to increase the number of allowable gauge positions attempted by the PLAS3D code. It is desirable to use gauges that are longitudinally closer to the center of the bore, since a smaller gauge closer to the center will have a greater effect on the inhomogeneity in the center of the magnet than a gauge that is longitudinally offset from the center. If a solution with a group of positions closer to the center cannot be achieved can, the number of allowable gauge positions can be increased and the PlAS3D code rerun. A solution where all gauge thicknesses are positive is a physical possibility and is used for the initial placement of gauges in the bore in block 63. Negative gauge thicknesses would require a material with a negative permeability. With the gauges in place in the powered magnet, the field in the bore is again measured at the 314 positions on the grid in block 65. If the inhomogeneity deviates from the prediction more than desired, the SHIMPSV algorithm is run again with the arch positions fixed and with the field values obtained with the gauges in place. A least squares routine is then used to adjust the thicknesses of the arches in block 67. These thickness changes should be small fractions of the initial thicknesses and, once implemented, should reduce the inhomogeneity to the desired range.

There are several possible changes in the implementation of the SHIMPSV algorithm in block 55 that may prove advantageous in certain cases. The positions that were eliminated from consideration at any step in the iteration of the SHIMPSV code due to negative gauge thicknesses may be introduced in a later iteration, effectively providing more positions to try. If this results in a solution with more gauges, the predicted inhomogeneity will generally be smaller, making that solution desirable. The least squares estimates assume a linear relationship between changes in gauge thickness and the effect on the field. The method described above for determining gauge positions in the bore of a magnet may be used with electromagnets containing superconducting magnets and permanent magnets.

Above, a method has been described for passive field correction of an MR magnet arrangement that can achieve a degree of field homogeneity required for magnetic resonance imaging without the use of correction coils.

Claims (14)

1. Method for passively arranging compensating bodies in a magnet with a central bore using compensating bodies or gauges (17) arranged in the bore of the magnet, the method comprising the following steps: (a) Measuring the initial field inhomogeneity in the bore of the magnet, (b) Select acceptable gauge positions in the bore of the magnet, (c) testing the magnetic field effect of a gauge at each of the predetermined permissible gauge positions in the bore of the magnet independently to determine the required gauge strength to improve the magnetic field inhomogeneity in the magnet bore, (d) Selecting the positions where positive gauge strengths were found advantageous, (e) Determine the necessary gauge strengths to improve the magnetic field inhomogeneity at each selected position, considering all selected positions simultaneously, (f) eliminating positions where a required negative gauge strength was found, and repeating steps (e) and (f) until all remaining selected positions require positive gauge strengths, and Arranging gauges of predetermined thickness at their selected positions in the bore of the magnet. 2. The method according to claim 1, wherein after step (f) in claim 1, the following steps are carried out Predicting the field inhomogeneity using gauges of predetermined thickness at their chosen positions in the bore of the magnet, Comparing the predicted inhomogeneity with a desired inhomogeneity, Increasing the number of predetermined allowable positions in step (b) to reduce the difference between the predicted and desired inhomogeneity, and repeating steps (c), (d), (e) and (f). 3. The method of claim 1, further comprising the following step: Reintroduce a position in step (f) that was previously eliminated in a previous iteration because a negative gauge strength was required to help increase the number of selected positions found to be required for positive gauge strengths. 4. The method of claim 1, further comprising the steps: (h) Measuring the field inhomogeneity in the magnet bore with the gauges in place, (i) determining incremental changes in the gauge strength at the selected positions to reduce the difference between a desired inhomogeneity and a predicted inhomogeneity based on the field inhomogeneity measured with the gauges in position, and (j) incrementally changing the gauge thicknesses at the selected positions in the magnet bore by the amounts determined in the previous step. 5. Passive arrangement of gauges or compensating bodies for a magnet with a central bore, containing: a non-magnetic tube (11) arranged coaxially in the bore of the magnet, a plurality of arcuate strips (17) of predetermined length made of a ferromagnetic material, characterized in that the strips are releasably attached to the pipe, the strips extend circumferentially around the pipe and are arranged at different positions on the pipe selected using the method according to any one of claims 1 to 4. 6. Arrangement according to claim 5, wherein the strips (17) are attached to the outside of the tube (11). 7. Arrangement according to claim 5, wherein the strips (17) are attached to the inside of the tube (11). 8. An assembly according to claim 5, further comprising means for coaxially arranging the tube in the bore. 9. Arrangement according to claim 6 or 7, wherein all strips (17) have the same length. 10. Arrangement according to claim 9, wherein the strips (17) are attached as a stack of strips with different heights. 11. The arrangement of claim 5, further comprising: a plurality of channel parts (13) which are attached to the inside of the pipe (11) and which are arranged at equal distances around the circumference and extend in a longitudinal direction, several arched support pieces (15) which are slidably mounted between adjacent channel pieces at the selected positions, Clamping means (21) which secure the carrier (15) to the channels in order to prevent sliding movement, and a stack of strips (17) of ferromagnetic material releasably attached to each of the carrier pieces. 12. Arrangement according to claim 11, wherein the support pieces (15), the channel parts (13) and the clamping means (21) each comprise non-magnetic materials. 13. Arrangement according to claim 5, further comprising: a plurality of channel parts (33) which are attached to the outside of the pipe (31) and which are arranged at the same distance in the circumferential direction and extend in a longitudinal direction, several arched pull pieces (37) which are slidably mounted between adjacent channel pieces, and several stacks of the strips (41) of ferromagnetic material which are releasably attached to each of the pulling pieces. 14. An arrangement according to claim 13, further comprising means (35) for coaxially arranging the tube in the bore of the magnet.