DE102006034471B4 - Device for generating a graded homogeneous magnetic field - Google Patents

Abstract

Apparatus for subjecting a sample (3) to a graded homogeneous magnetic field, comprising at least one magnetic body (1) and an arrangement (2) with magnetized areas surrounding said body, characterized in that, on the surface which projects beyond the sample (3) at least one magnetic south pole (1a) of the magnetic body (1) is adjacent to a south magnetic pole (2a) of a magnetized region and at least one north magnetic pole (1b) of the magnetic body (1) is magnetized to a north magnetic pole (2b) Area bounded so that the same poles on the surface, which is passed over the sample (3), meet.

Description

The The invention relates to a device for producing a graduated homogeneous magnetic field.

State of the art

For magnetic resonance examinations it is necessary that the sample to be examined has a high static To expose magnetic field with defined characteristics. Especially becomes common a magnetic field is needed which in a plane as possible is homogeneous and perpendicular to this plane a defined gradient has (graded homogeneous magnetic field).

Out J. Mitchell, P. Blumler, P.J. McDonald, "Spatially resolved nuclear magnetic resonance studies of planar samples ", Progress in Nuclear Magnetic Resonance Spectroscopy 48, 161 (2006), it is known that for this for example, the stray field of a strong superconducting magnet exploit (Stray Field Imaging, STRAFI method, see also Samoilenko et al., JETP Letters 47, 417 (1998)). Disadvantageous are superconducting magnets big and not mobile, which makes the examination of extended samples difficult. In addition need They are a complex cooling.

Out P. Glover et al., "A Novel High-Gradient Permanent Magnet for the Profiling of Planar Films and Coatings ", Journal of Magnetic Resonance 139, 90 (1990), are permanent magnets known with special pole shoes (Gradient At Right Angles To Field, GARFIELD magnets). These magnets need neither cooling nor an energy source. The disadvantage, however, the sample between the Pole shoes are housed, so that the investigation more extensive Samples, such as a tree trunk, due to the then necessary size of the magnets not possible is.

Out Eidmann et al., "The NMR MOUSe, a mobile universal surface explorer ", Journal of Magnetic Resonance Series A 122 (1), 104-109, 1996), unilateral devices are associated with an iron yoke Permanent magnets known to be in a small area above their surface generate a magnetic field. These devices are so compact that she over be conducted an extensive sample can. With their help you can shallow Areas of fixed extended samples using magnetic resonance methods to be examined. The disadvantage is that the field is not sufficiently homogeneous and it leaves examine only a very small sample volume.

Out US 2006/0061445 A1 a permanent magnet is known, which is composed of segments of permanent magnets. Depending on the orientation of the magnetization direction of the individual permanent magnets, the desired magnetization direction can be set and optimized.

Task and solution

task The invention is therefore to a device available provide a graded homogeneous field in a larger sample volume, with a better homogeneity and with a more constant gradient than after State of the art.

These The object is achieved by a device according to the main claim. Further advantageous embodiments will be apparent from the following referred back Dependent claims.

Subject of the invention

in the The invention has a device for applying a Probe with a magnetic field that develops at least one magnetic body and this body surrounding arrangement with magnetized areas. Turns according to the invention the magnetization in the magnetized areas along at least one way around the body monotonous about the mean magnetization of the body.

in the special description part are embodiments of such Rotation specified. In these embodiments, the magnetization is in the magnetized areas substantially coplanar with the middle one Magnetization of the body: The magnetization directions of the body represented by arrows and the magnetized areas are each in the plane of the drawing.

Under the mean magnetization of the body becomes the vectorial arithmetic Means of magnetizations of all magnetic domains of the body Understood. As a rule, an extended body with a defined Magnetization direction not one-domed.

It was recognized why the prior art unilateral device produced only insufficiently homogeneous field in a small space area: the magnetic field lines running in this area parallel to the surface of the device must be returned to the permanent magnets. This causes magnetic field components that deviate from the desired graded homogeneous field. The inventively provided arrangement now surprisingly acts on the intended in her rotation of the local magnetization unwanted magnetic field components ent by the return of the field lines stand, contrary. The magnetic fields caused by the body and the arrangement add up vectorially. The result is a magnetic field which has in a significantly larger range the advantageous properties of a graded homogeneous magnetic field than the magnetic field generated by the unilateral device according to the prior art.

That's how it is US 2006/0061445 A1 Although a permanent magnet known, which is composed of segments of permanent magnets and depending on the orientation of the magnetization direction of the individual permanent magnets, the desired direction of magnetization can be adjusted and optimized. Due to the absence of the magnetic body according to the invention, which has an arrangement with magnetized regions surrounding this body, however, only a homogeneous magnetic field without a gradient can be set here.

to concrete design of the device can on the basis of the inventive solution that the magnetization in the magnetized areas along at least one way around the body monotonically rotating around the mean magnetization of the body, the common methods the magnetostatics are used. Among such common methods In particular, the solution is the Maxwell's equations, possibly by numerical optimization, Understood. However, these methods do not provide the solution according to the invention itself, but only the design parameters of special designs.

For one given body let yourself, For example, with finite element calculation and parameter optimization, a Create arrangement that the unwanted field components in the manner according to the invention counteracts. In such an invoice, further application-relevant boundary conditions incorporated. Such boundary conditions can For example, there are certain forms of magnets easier to make than others.

Becomes the device is applied to a sample, so is in a space area generates a graded homogeneous magnetic field in this sample. The exact extension of this field, its strength and its gradient depend on the concrete design of the body.

In a particularly advantageous embodiment of the invention, the body has an at least in a partial region not surrounded by the arrangement, substantially planar, preferably planar surface. This surface can be passed over the extended sample to be examined. This minimizes the body to sample distance, thus maximizing the strength of the graded homogeneous field generated in the sample. The field is homogeneous in a certain spatial area in a plane parallel to the plane surface and has a constant gradient perpendicular to this surface in this spatial area. If the body is magnetized parallel to its plane surface, then with a suitable arrangement over approximately half the extent of its plane surface and in a depth range in the sample which depends on the strength of its magnetization, a graded homogeneous field B with a relative deviation ΔB / B of Homogeneity of less than 10 ^{-5 are} generated. The magnetization of the body must be strictly parallel to the plane surface for such results.

Advantageous includes the arrangement flush with the flat surface of the body from. Then the device can be defined as a whole via a level sample passed without being able to tip over. In addition, simulations have revealed that with such an arrangement, the unwanted Field components of the body best compensated. As a result, the deviations of Magnetic field of the desired graded homogeneous form diminished.

The inventively increased quality and extent of the graded homogeneous magnetic field are particularly pronounced when the vector product from the mean magnetization vector of the body with the normal on the planar surface of the body in a rectangular, in particular right-handed, coordinate system forms the axis around which the location-dependent magnetization rotates in the magnetized areas. What is meant by this clarifies 1 in particular part of description. In a right-handed coordinate system, the location-dependent magnetization should rotate in a mathematically positive direction of rotation and in a left-handed coordinate system in a mathematically negative direction of rotation.

Ideally are the magnetization of the body and the direction of rotation of the location-dependent magnetization in the magnetized areas matched to one another such that at the flat surface at least one magnetic south pole of the body to a magnetic south pole a magnetized area is adjacent and at least one magnetic North pole of the body adjacent to a magnetic north pole of a magnetized region.

In particular, it is advantageous if the mean magnetization of the body is directed against the magnetic field caused by the arrangement on its planar surface, to a deviation of 10 degrees or less, in particular to a deviation of 5 degrees or less. To achieve this, one becomes in for a given body usually make a parameterized approach to the arrangement and determine the free parameters with an optimization method.

Around with alternating fields magnetic resonance in the sample to be examined to be able to stimulate is in a particularly advantageous embodiment of the invention the plane surface of the body or the arrangement arranged a coil. This points in particular a thickness of less than 2 mm, preferably less than 1 mm, so that the distance between body and sample is not unnecessarily increased. That through the body caused magnetic field decreases quadratically with this distance. Main application the device according to the invention are nuclear magnetic resonance (NMR). In these investigations is in the interest of a favorable signal-to-noise ratio a particularly strong and particularly homogeneous field needed.

Either for the body as well as for the arrangement have in the simulation cylindrical and ellipsoidal Shaping particularly proven, with the ellipsoidal form express also includes the circular shape. Alternatively, the arrangement can be configured cuboid be with a recess in which the body engages. In the simulation this is associated with no significant loss of quality. however let yourself Such an arrangement particularly simple and precise by mechanical processing produce. This is especially true when the body is one Has cuboid shape.

In a particularly advantageous embodiment of the invention, the arrangement and / or the body comprise permanent magnets. These need neither energy source nor cooling to maintain the magnetic field and are therefore mobile to use. Particularly suitable materials for the permanent magnets are ferromagnetic materials from the group ferrite, NdFeB, SmCo ₅ , Sm ₂ Co ₁₇ or AlNiCo _n (n = 2 - 9).

The Production of the device is particularly simplified when the Arrangement comprises a number of nominally identical permanent magnets. Under nominally identical magnets are understood to mean magnets which are in their magnetic properties only to manufacturing tolerances of deviate from or spread around a common value. In this case lets Improve the accuracy by making a larger number of magnets becomes. This will be for the arrangement hit a selection of those magnets whose properties closest to the nominal values.

In a particularly advantageous embodiment of the invention, the magnetic field caused by the arrangement is lower than the local coercive field of the body at any location in the body. Then the arrangement provides only the opposing field which compensates for the unwanted field components of the body without affecting the magnetization of the body itself. The magnetic field B _{0 of} the array and the coercive field B _{C of} the body are typically in a ratio B ₀ / B _C between 0.2 and 0.6.

In a further advantageous embodiment of the invention the device has several bodies with different mean magnetizations. Of the Strength the magnetization depends at what depth in the sample to be examined is the usable range with the graded homogeneous field forms. Become the different bodies arranged adjacent, information can be from different Interrogate depths in the sample by moving the device relative to the Sample is moved.

Special description part

following the object of the invention is explained in more detail with reference to figures, without that the subject of the invention is limited thereby. It is shown:

1 : Embodiment of the device according to the invention.

2 : Through the device 1 caused magnetic field distribution.

3 : Course of the z-component B _z (z) of the magnetic field as a function of z at different heights x over the device 1 ,

4 : Dependence of the properties of the graded homogeneous field on the concrete design of the device 1 ,

5 : Cuboid embodiments of the device according to the invention.

6 : Embodiment of the coil.

1 shows a section through an embodiment of the device according to the invention. The magnetic body 1 is a permanent magnet in the form of a half-cylinder with a magnetic south pole 1a and a magnetic north pole 1b , The order 2 also has the shape of a half-cylinder and contains identical permanent magnets 2c each with a magnetic south pole 2a and a magnetic north pole 2 B , The order 2 has a recess into which the magnetic body 1 is fitted; thus surrounds the arrangement 2 the body 1 , The body 1 does not have one of the arrangement 2 surrounded flat surface with which the arrangement is flush. Thus, the entire device has a flat surface over the sample to be examined 3 can be performed.

The Arrows indicate the local directions of magnetic flux density (Magnetic field) B on. Here as well as in the following figures is within of magnetized matter (body or magnetized areas) only by the local magnetization M caused local magnetic field B drawn. The following will be in the context of matter instead of the local magnetic field B the proportional local magnetization M is discussed: A Magnetic field can basically also from the outside imprinted be while magnetization is a property inherent in matter.

The body 1 is homogeneously magnetized; in it, the local magnetization points to every location of the South Pole 1a to the north pole 1b , Also in the permanent magnets 2c the arrangement 2 indicates the local magnetization from the south pole 2a to the north pole 2 B , It stands each perpendicular to the dividing plane between the South Pole 2a and North Pole 2 B , Because this parting plane is at the transition from a permanent magnet 2c Turning to the next in space, the location-dependent magnetization also rotates at this transition. In the whole arrangement 2 The location-dependent magnetization monotonically rotates around the mean magnetization of the body 1 ,

Right in 1 the coordinate system is shown, which will be used in this special description section below. The vector product from the mean magnetization direction of the body 1 with the normal on the plane surface of the body 1 is obviously on the negative y-axis and thus forms the axis about which the location-dependent magnetization rotates in the magnetized areas in the positive direction of rotation.

The magnetization of the body and the direction of rotation of the location-dependent magnetization are coordinated in the magnetized areas such that at the edge of the planar surface of the magnetic south pole 1a of the body to a magnetic south pole 2a a permanent magnet 2c borders. The magnetic north pole 1b of the body is adjacent to a magnetic north pole 2 B a permanent magnet 2c ,

The magnetizations of the magnetized regions are obviously tangent vectors. These are perpendicular to the mid-perpendicular, which in the magnetized areas of the magnetic south pole 2a from the north pole 2 B separate.

2 shows the magnetic field distribution passing through the device 1 is effected. Over the body 1 is clearly an area with a graded homogeneous field to recognize. The arrows each point in the direction of the homogeneous magnetic field and indicate its strength by its length. Obviously, the mean magnetization of the body 1 that on its plane surface through the arrangement 2 caused magnetic field up to a deviation of less than 10 degrees counteracted.

In 3 the z-component B _z (z) of the magnetic field is shown as a function of z. Shown is the range of values between z = -r / 4 and z = + r / 4, where r is the radius of the cylindrical array 2 is and the zero point of z in the middle of the plane surface of the body 1 lies. The curves drawn on top of each other correspond to courses of B _z (z) in different equidistant heights x over the device. The top and bottom curves show first deviations from the homogeneity in the form of a flat extremum at z = 0. Thus, the range shown for the height x approximately indicates the extent of the usable graded homogeneous region in the x-direction. Deviations from the homogeneity also show up at the edge of the area shown in the z-direction (z = +/- r / 4). Thus, the range shown from z = -r / 4 to z = + r / 4 approximately indicates the extent of the usable graded homogeneous region in the z-direction. It is also noticeable that the homogeneous level of B _z (z) changes linearly with height x. The gradient of B _z in the x-direction is therefore constant.

4 shows how the properties of the graded homogeneous magnetic field of the specific embodiment of the device 1 depend. In sub-image a, the ratio r ₁ / r ₂ between the radius r _{1 of} the cylindrical body was determined on the ordinate axis 1 and the radius r _{2 of} the cylindrical arrangement 2 varied. The left axis of abscissa indicates the ratio d / r ₁ , where d is the distance from the planar surface to the center of the usable graded homogeneous region. The right axis of abscissa indicates the magnitude of the graded homogeneous magnetic field in the unit Tesla in the middle of the graded homogeneous area. The arrows indicate in each case which curve is assigned to which axis of abscissa. The specification of ratios d / r ₁ and r ₁ / r ₂ illustrates the scalability of the device.

In field b, the remanence field B _{R of} the body became on the ordinate axis 1 varied. The left axis of abscissa indicates the distance d of the graded homogeneous area from the plane surface. The right axis of abscissa indicates the strength of the graded homogeneous magnetic field. The arrows indicate in each case which curve is assigned to which axis of abscissa. The ratio r ₁ / r ₂ here is 0.4 and the remanence field B _{R of} the arrangement 2 be carries 1.4 tesla.

5 shows cuboidal configurations of the device according to the invention. In sub-picture a, the arrangement exists 2 from essentially trapezoidal blocks 2c made of magnetic material, each with a magnetic south pole 2a and a magnetic north pole 2 B exhibit. The blocks form a common recess into which the cuboid body 1 intervenes. This body 1 is a permanent magnet with a magnetic south pole 1a and a magnetic north pole 1b , The arrows give as in 1 each magnetization directions. In the arrangement 2 The magnetization rotates monotonically around the mean magnetization of the body 1 , The magnetizations of the magnetized areas 2c are also here, as with the device 1 , Tangential vectors. These are perpendicular to the perpendicular bisectors, which are in the magnetized areas (blocks) 2c the magnetic south pole 2a from the north pole 2 B separate.

In place of a cuboid body 1 can also be a cylindrical body 1 to get voted. Then, however, are the trapezoidal blocks 2c that make up the arrangement 2 exists, more difficult to manufacture. But it is also possible to use cuboid instead of trapezoidal blocks in the interest of easier production. However, this is associated with quality losses.

In field b, all blocks have 2c that make up the arrangement 2 is composed of a cuboid shape. This construction is advantageous since permanent magnets can be produced in cuboid shape particularly simply and precisely by mechanical processing.

6 shows an embodiment of a flat coil, which is used for magnetic resonance examinations on the body 1 can be arranged. The gray line in the center of the picture symbolizes an electrical connection that crosses the coil windings without being conductively connected to them at the crossing points. The coil consists of serially connected counter-wound meanders, making them insensitive to external electromagnetic interference: each external disturbance induces oppositely directed currents in the two coils which cancel each other out. The coil has an extension of about 14 mm × 26 mm. It is designed as a printed circuit on a commercially available carrier material, which is available for example in thicknesses of 1.5 mm, but also 0.75 mm.

Claims (16)

Device for applying a sample ( 3 ) with a graded homogeneous magnetic field comprising at least one magnetic body ( 1 ) and an arrangement surrounding this body ( 2 ) with magnetized areas, characterized in that at the surface, which over the sample ( 3 ), at least one magnetic south pole ( 1a ) of the magnetic body ( 1 ) to a magnetic south pole ( 2a ) of a magnetized region and at least one magnetic north pole ( 1b ) of the magnetic body ( 1 ) to a magnetic north pole ( 2 B ) of a magnetized region, so that the respective same poles on the surface, the above the sample ( 3 ), meet. Device according to claim 1, characterized by a body ( 1 ) with at least a partial area not of the arrangement ( 2 ) surrounded plan surface. Device according to claim 2, characterized by an arrangement ( 2 ), which are flush with the plane surface of the body ( 1 ) completes. Device according to one of claims 2 to 3, characterized in that the vector product from the mean magnetization vector of the body ( 1 ) with the normal on the plane surface of the body ( 1 ) in a right-angled, in particular right-handed, coordinate system forms the axis about which the location-dependent magnetization rotates in the magnetized regions. Device according to one of claims 2 to 4, characterized in that the mean magnetization of the body ( 1 ) on its plane surface through the arrangement ( 2 ) is directed counter to a deviation of 10 degrees or less, in particular to a deviation of 5 degrees or less. Device according to one of claims 2 to 5, characterized by a on the flat surface of the body ( 1 ) or the arrangement ( 2 ) arranged coil. Apparatus according to claim 6, characterized by a coil having a thickness of less than 2 mm, preferably less than 1 mm. Device according to one of claims 1 to 7, characterized by a cylindrical or ellipsoidal body ( 1 ). Device according to one of claims 1 to 8, characterized by a cylindrical or ellipsoidal arrangement ( 2 ). Device according to one of claims 1 to 7, characterized by a cuboidal arrangement ( 2 ) with a recess into which the body ( 1 ) intervenes. Apparatus according to claim 10, characterized by a cuboid body ( 1 ). Device according to one of claims 1 to 11, characterized by an arrangement ( 2 ) and / or a body ( 1 ) comprising permanent magnets ( 2c ). Device according to claim 12, characterized by permanent magnets ( 2c ) comprising a ferromagnetic material from the group ferrite, NdFeB, SmCo ₅ , Sm ₂ Co ₁₇ or AlNiCo _n (n = 2 - 9). Device according to one of claims 12 to 13, characterized by an arrangement ( 2 ) comprising nominally identical permanent magnets ( 2c ). Device according to one of claims 12 to 14, characterized in that at each location in the body ( 1 ) that by the arrangement ( 2 ) caused magnetic field is lower than the local coercive field of the body ( 1 ). Device according to one of claims 1 to 15, characterized by a plurality of bodies ( 1 ) with different mean magnetizations.