WO2009135264A1 - Arrangement of coils for mri apparatus - Google Patents

Abstract

A method of determining a magnet arrangement for use in magnetic resonance imaging apparatus, the method including, determining a function representing current densities required within a magnet region to generate a field, determining a current density distribution required to generate a desired field, using the function and determining the magnet arrangement using the current density distribution, the magnet arrangement including a number of current carrying coils arranged within the magnet region.

Description

ARRANGEMENT OF COILS FOR MRI APPARATUS

Background of the Invention

The present invention relates to a magnet arrangement and a method of determining a magnet arrangement, and in particular, a magnet arrangement for use in MRI (magnetic resonance imaging) or other similar applications.

Description of the Prior Art

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

The design of superconducting magnets has been widely investigated in the past using different optimization strategies to obtain coil layouts that have certain magnetic field properties.

Typically one desirable characteristic of an MRI magnet system is the ability to generate a strong and homogenous axial magnetic field over an imaging region, which is commonly referred to as the diameter sensitive volume (DSV). It is also desirable to restrict the peak magnetic field produced inside the superconductors themselves, and minimize the stray field external to the assembly.

The minimization of stray fields is a process generally referred to as shielding, and is historically achieved by placing coils as part of the assembly on the exteriors of the field producing magnet, to essentially reduce the stray field of the magnet and to minimize the size of the fringe field. Usually, active shielding is a process whereby coils with reverse current directions are placed on top of the main winding to reduce stray magnetic fields exterior to the magnet chamber. Other factors that influence the design and manufacturing of magnets are related to geometrical constraints, that is, to reduce the magnet size, weight and associated wire cost, and the time taken to wind the individual coils that collectively comprise a magnet.

Previously investigated optimization methods to design superconducting magnets typically fall into two categories. The first involves searching a broad parameter space for an optimal coil layout, as described for example in Shaw, N.R. and R.E. Ansorge, Genetic algorithms for

MRI magnet design. IEEE Transactions on Applied Superconductivity, 2002. 12: p. 733-736.

The second places initial constraints on the optimization strategy to enable the attainment of a solution in either less time, or in a more convergent and stable manner, as described for example in Cheng, Y.C.N., et al., Design of actively shielded main magnets: An improved functional method. Magn. Res. Mat. Phys. Bio. Med., 2003. 16(2): p. 57-67.

The former technique tends to be associated with optimization strategies that require large computational resources, and the latter tends to achieve magnet coil layouts faster, given a good initial coil layout approximation or seed data. However, as described in Cheng, Y.C.N., et al., A Comparison of Two Design Methods for MRI Magnets. IEEE Transactions on Applied Superconductivity, 2004. 14(3): p. 2008-2014, both optimization categories tend to provide very similar results, irrespective of the search space.

US2005/0146332 describes a magnetic field generator for producing a homogenous magnetic field region and a method of designing an MRI system that produces a low fringe field region. The method comprises defining a solution space, defining a field of view, a centre field and homogeneity requirements, defining fringe field requirements, and running an optimization algorithm to determine coil positions. However, the optimisation algorithm does not yield a single global optimum solution, and consequently leads to localised minima, making the determination of an optimum layout complex.

US-6,255,929 describes a method of making optimized electromagnets. Whilst the described technique allows the magnet design optimization problem to be cast as an Ll -norm minimization linear programming calculation, for which a global solution can always be found, limitations still exist. In one example, the magnets are optimized to require the least amount of dissipated power for the given predetermined axial magnetic field, which can only be used for resistive and not superconducting magnets. In another example, superconducting magnet designs are produced by minimising the length of superconducting wire used. Thus, different analysis is required for superconducting and resistive magnets.

The minimisation process is based on a constant current density coil configuration, which does not necessarily result in an optimum magnet design. Furthermore, the global solution arises from casting the problem as a simpler Ll -norm minimisation calculation allowing a global solution within the constrains of the specified functions and does not therefore represent a optimum magnet configuration.

The approach also employs a virtual coil comprising an infinitesimally thin conductor. Each iteration of the design process requires an update of the virtual coil, which makes the process inappropriate for superconducting coil design. This is highlighted by the fact that the method only caters for unshielded long magnets in which each of the coils are wound in the positive sense.

US-5,760,582 describes gradient coil assemblies and shim coil assemblies for magnetic resonance imaging ("MRI") devices, wherein the coil assemblies comprise a coil support and a conductive wire having a locus described by a solution of a current continuity equation over a finite interval for which certain terms of the magnetic field expansion are equal to zero, to generate non-uniform magnetic fields. The gradient coil produces gradient magnetic fields and linear magnetic fields which cancel non-uniformities in the magnetic field of the magnet of the MRI device. Shim coils can be used to cancel such non-uniformities, as well.

It will be appreciated that as these techniques are described for use in gradient and shim coil assemblies, these would not be considered as suitable for use in designing a primary magnet arrangement.

US-5, 382,904 describes superconducting electromagnets suitable for use in the NMR tomography of human organs. Each of the disclosed electromagnets are constructed according to a methodology for structured coils, where the desired field at locations within - A -

the volume of interest and, optionally, outside of the location of the coils is selected; the current magnitude and polarity for a plurality of coil element locations are then optimized, by way of a computer program, to provide the desired field magnitude at the locations. The magnet construction results in a plurality of coils of varying current polarity, and of irregular shape and size, optimized to provide the uniform field within the DSV. However, the irregular coil shapes are difficult to produce in practice, thereby limiting the applicability of this technique.

US-5,818,319 describes procedures for designing magnets, including superconducting magnets, shim magnets, and gradient magnets for magnetic resonance systems. The procedures involve the use of a simulated annealing procedure in which weighted spherical harmonics are included in the procedure's error function. The procedure has resulted in the development of previously unknown magnet designs. In particular, superconducting magnets have been designed that include at least one coil in which the current flow is opposite to that in adjoining coils. Such reversed flow in combination with a relatively large number of coils, e.g., more than 6 coils, have enabled the development of short, yet homogeneous, whole body magnets for use in magnetic resonance imaging (MRI).

However, such magnet systems, and methods of designing the magnet systems still include some limitations. For example, the use of simulated annealing processes is computationally expensive. Furthermore, the minimization function used in the simulated annealing process has multiple local minima, meaning that when a minima is found, there is no guarantee that this is the optimum, or the global solution. Consequently, using these techniques can result in non-optimal magnet designs.

Summary of the Present Invention

It is an object of the present invention to substantially overcome, or at least ameliorate, one or more disadvantages of existing arrangements.

In a first broad form the present invention provides a method of determining a magnet arrangement for use in magnetic resonance imaging apparatus, the method including: a) determining a function representing current densities required within a magnet region to generate a field; b) determining a current density distribution required to generate a desired field, using the function; and, c) determining the magnet arrangement using the current density distribution, the magnet arrangement including a number of current carrying coils arranged within the magnet region.

Typically the method includes: a) dividing the magnet region into a number of elements; and, b) determining a current density for each element using the function to thereby determine the current density distribution.

Typically the method includes determining the current density distribution by optimizing the function.

Typically the function has a global minimum.

Typically the function is quadratic with respect to the current density within the magnet region.

Typically the function is based on the energy stored within the magnet region.

Typically the function is of the form:

F oc LI²

where:

(a) F is the cost function;

(b) / is the current density within the magnet region; and

(c) L is related to an inductance.

Typically the method includes defining the magnet region.

Typically the magnet region has an arbitrary shape positioned relative to at least one of: a) a bore; and, b) an imaging region.

Typically the magnet region substantially surrounds at least one of: a) a bore; and, b) an imaging region.

Typically the magnet region is rotationally symmetric with respect to a field axis.

Typically the field axis is aligned with a bore axis.

Typically magnet region has a substantially cylindrical shape.

Typically the method includes, defining a desired field within at least one of: a) an imaging region; and, b) a stray field region.

Typically the method includes defining the imaging region to have a substantially predefined shape positioned with respect to the magnet region.

Typically the method includes defining an imaging region having a substantially spherical shape positioned on a field axis radially inwardly of the magnet region.

Typically the method includes defining a desired field including a substantially homogeneous magnetic field within an imaging region.

Typically the desired field has a homogeneity having a variation in the imaging region of less than at least one of: a) 100 ppm; b) 20 ppm; and, c) 10 ppm.

Typically the method includes defining a stray field region substantially surrounding the magnet region. Typically the stray field region is positioned on the bore axis radially outwardly of the magnet region.

Typically the method includes defining a desired field including a field strength of less than a selected amount within the stray field region.

Typically the selected amount is less than, at least one of: a) 20 Gauss; b) 10 Gauss; and, c) 5 Gauss.

Typically a current density distribution has a series of extremities lying around a perimeter of the magnet region.

Typically the direction of current flow in the coils depends on the polarity of the extremities within the current density distribution.

Typically the method includes using the current density distribution to determine at least one of: a) coil locations within the magnet region; b) coil sizes; c) a current density within coils; d) a total current for coils; e) current flow directions within coils; and, f) coil winding directions.

Typically the method includes determining the coil arrangement based on extremities within the current density distribution.

Typically the coils are located on local positive maxima and local negative minima of the current density distribution.

Typically the current carrying coils are arranged substantially around a perimeter of the magnet region. Typically, for current carrying coils on a bore perimeter of the magnet region, each current carrying coil carries a current in an opposing direction to each adjacent coil.

Typically each current carrying coil carries a current in an opposing direction to each adjacent coil.

Typically the magnet region has a substantially cylindrical shape surrounding a field axis, each current carrying coil being arranged within the magnet region with a coil axis substantially aligned with the field axis.

Typically the method further includes optimising the coil arrangement.

Typically the method includes: a) defining a second function; and, b) optimising the second function to thereby optimise the coil arrangement.

Typically the method includes optimising the second function such that the coil arrangement has optimal coil current densities

Typically the second function has a global solution.

Typically the second function is quadratic with respect to the current density within the magnet region.

Typically the second function is a function of the energy stored within the coils.

Typically the method includes: a) defining coil constraints; and, b) optimising the second function using the coil constraints to thereby determine an optimised magnet arrangement.

Typically the coil constraints include at least one of: a) a minimum separation between coils to prevent coil overlap; and, b) an optimal current density flow for each coil.

Typically the optimal current density flow is smaller than a maximum current density. Typically method includes selecting the maximum current density in accordance with a magnetic field strength and properties of superconducting material used in the current carrying coils.

Typically the method is performed at least in part using a processing system.

In a second broad form the present invention provides a magnet arrangement for use in magnetic resonance imaging apparatus, the magnet arrangement including a number of current carrying coils arranged substantially around a perimeter of a magnet region, and wherein, for current carrying coils on a bore perimeter of the magnet region, each current carrying coil carries a current in an opposing direction to each adjacent coil on the bore perimeter.

Typically the current carrying coils are located substantially on extremities of a current density distribution determined for the magnet region.

Typically each coil carries a current in a direction defined by a polarity of the extremity.

Typically each coil carries a current in an opposing direction to each adjacent coil.

Typically each coil includes a number of current carrying windings.

Typically the number of current carrying windings is determined so that the coil generates a required magnetic field.

Typically the magnet region has an arbitrary shape positioned relative to at least one of: a) a bore; and, b) an imaging region.

Typically the magnet region is rotationally symmetric with respect to a field axis.

Typically the field axis is aligned with a bore axis.

Typically magnet region has a substantially cylindrical shape.

Typically the magnet generates a desired field within at least one of: a) an imaging region; and, b) a stray field region.

Typically the imaging region has a substantially predefined shape positioned with respect to the magnet region.

Typically the imaging region has a substantially spherical shape positioned on a field axis radially inwardly of the magnet region.

Typically the imaging region has a substantially homogeneous magnetic field.

Typically the homogeneity has a variation of less than at least one of: a) 100 ppm; b) 20 ppm; and, c) 10 ppm.

Typically the stray field region substantially surrounds the magnet region.

Typically the stray field region is positioned on the field axis radially outwardly of the magnet region.

Typically the desired field has a field strength of less than a selected amount within the stray field region.

Typically the selected amount is less than, at least one of: a) 20 Gauss; b) 10 Gauss; and, c) 5 Gauss.

Brief Description of the Drawings

An example of the present invention will now be described with reference to the accompanying drawings, in which: -

Figure 1 is a flow chart of an example of a process for determining a magnet arrangement; Figure 2A is a schematic side view of an example magnet arrangement generated using the process of Figure 1;

Figure 2B is a schematic end view of the magnet arrangement of Figure 2 A;

Figure 2C is a schematic side view of a second example magnet arrangement generated using the process of Figure 1;

Figure 2D is a schematic side view of a third example magnet arrangement generated using the process of Figure 1;

Figure 2E is a schematic perspective view of a fourth example magnet arrangement generated using the process of Figure 1; Figure 3 is a schematic diagram of an example processing system;

Figures 4A and 4B are a flow chart of a second example of a process for determining a magnet arrangement;

Figure 5 is a schematic representation of a coordinate system used in determining a magnet arrangement; Figure 6 is a schematic representation of regions used in determining a magnet arrangement;

Figure 7 is a schematic representation of the magnet geometries used in optimising a magnet arrangement;

Figures 8A and 8B are 3D and 2D representations of examples of minimum stored energy

(MSE) current density profiles for 3 m long magnet domains for unshielded order 10 degree 0 configurations;

Figures 9A and 9B are 3D and 2D representations of examples of MSE current density profiles for 3m long magnet domains for shielded order 10 degree 2 configurations;

Figures 1OA and 1OB are 3D and 2D representations of examples of MSE current density profiles for unshielded order 14 degree 0 2m length magnet configurations; Figures HA and HB are 3D and 2D representations of examples of MSE current density profiles for unshielded order 14 degree 0 \m length magnet configurations;

Figures 12A and 12B are 3D and 2D representations of examples of MSE current density profiles for shielded order 16 degree 4 2m length magnet configurations;

Figures 13A and 13B are 3D and 2D representations of examples of MSE current density profiles for shielded order 16 degree 4 Im length magnet configurations; Figures 14A and 14B are 3D and 2D representations of examples of MSE current density profiles for unshielded order 16 degree 0 \m length magnet configurations;

Figures 15A and 15B are representation of the resulting DSV magnetic field with final coil arrangement and stray fields for the initial magnet configuration of Figure 14B; Figures 16A and 16B are 3D and 2D representations of examples of MSE current density profiles for shielded order 14 degree 4 1m length magnet configurations;

Figures 17A and 17B are representations of the resulting DSV magnetic field with final coil arrangement and stray fields for the initial magnet configuration of Figure 16B;

Figures 18A and 18B are representations of the resulting DSV magnetic field with final coil arrangement and stray fields for a short bore clinical magnet;

Figures 19A and 19B are representations of the resulting DSV magnetic field with final coil arrangement and stray fields for a short bore 3.0T clinical magnet;

Figures 2OA and 2OB are representations of the resulting DSV magnetic field with final coil arrangement and stray fields for a high field clinical magnet; and, Figures 21 A and 21B are representations of the resulting DSV magnetic field with final coil arrangement and stray fields for an open bore clinical magnet.

Detailed Description of the Preferred Embodiments

An example of a process for designing a magnet arrangement for use in MRI or other similar applications will now be described with reference to Figure 1.

In this example, at step 100, a function is determined representing the current densities required within a magnet region in order to generate a given field.

The nature of the function may vary depending on the preferred implementation and an example function will be described in more detail below. However, in one example, the function is based on the stored energy within the magnet coils, and hence can be quadratic with respect to the current density, when combined with a term representing a coil inductance. The use of a function that is quadratic with respect to the current density means that the function has a single global solution for a given desired field. At step 110 a current density distribution required in order to generate a desired field is determined using the function.

The desired field may be any suitable field required for performing MRI, and will typically therefore require the presence of a suitably homogenous field in an imaging region, and a low external stray field magnitude. The desired field will therefore depend on factors such as the nature of the MRI process being performed. The current density is typically determined in any one of a number of ways depending on the nature of the function used, but in one example, this involves finding a minimum or maximum of the function for the desired field, as will be described in more detail below.

In one example, the resulting current density distribution has a series of extremities lying around the perimeter of the magnet region. For larger magnet regions, the current density may have a relatively large number of maxima and minima of a common polarity, so that for example, positive maxima are adjacent to positive minima, or vice versa. As the size of the magnet region is reduced, the number of such extremities becomes smaller, with a further reduction in size resulting in the adjacent extremities having opposite polarities, such that positive maxima are located adjacent to negative minima.

Once a current density distribution within the magnet region has been determined, this can be used to determine the magnet arrangement at step 120.

The magnet arrangement will typically specify one or more magnet parameters, such as a number and location of current carrying coils within the magnet region, the respective current density and/or total current required for each coil, the direction of current flow within the coils, or the like. In one example, the arrangement is at least partially based on the location and/or magnitude of extremities, such as local positive maxima and local negative minima, in the ideal current density distribution obtained for the specified magnet parameters.

At step 130 the magnet arrangement determined in step 120 can be optimised further, using an optimisation strategy. This can be performed in any suitable manner, and may be used to refine the magnet arrangement, for example to ensure that the current density for each current carrying coil is feasible, and to ensure the peak field on the coil is within acceptable ranges.

An example of a coil arrangement determined utilising the above described methodology will now be described in more detail with reference to Figures 2 A and 2B.

In this example, a cylindrical magnet region 200 is provided surrounding a cylindrical bore 220, defining a bore axis 225. The magnet region 200 contains a number of current carrying coils 201, having their axes aligned with the bore axis 225, which are used to generate a desired magnetic field within an imaging region 230. This configuration results in a magnetic field that is rotationally symmetric about a magnetic field axis (not shown) that is aligned with the bore axis 225. A coil region 210 is also typically provided between the magnet region 200 and the bore 220 for containing RF coils, shim coils and/or gradient coils, as required for the respective MRI process.

In this example, the coils 201 are generally spaced apart around the perimeter of a magnet region 200, which results from performing the methodology described above. Furthermore, the number, size and shape of the coils is not intended to be limiting and is merely for illustrative purposes, as will become apparent from the specific examples described in more detail below.

The direction of current flow in each of the coils is indicated by the "+" and "-" symbols, showing that in at least some of the resulting magnet arrangements, the current flow in each adjacent coil is in an opposite direction to that in each other adjacent coil. This is not essential, and arrangements with current flows in adjacent coils in the same direction are described. However, in general the opposing current flow arrangement results in a more optimal and compact design.

Accordingly, current flow in at least the coils along a bore perimeter boundary 202 adjacent the bore 220 are typically in opposing directions. For optimum performance current flow in each coil is opposite the current flow in each adjacent coil for the entire perimeter. Current flow in opposing directions may be achieved in any one of a number of ways, such as by reverse winding adjacent coils, or by controlling the current flow in each coil.

The magnet arrangement described above, and in particular, the positioning of coils around the perimeter of the magnet region, and the reverse current directions for adjacent coils results in a significantly improved field generating capability. In particular, for an imaging region of a given homogeneity and size, this can generally be produced by a magnet arrangement having a shorter axial length than can be achieved using prior art techniques and arrangements. Furthermore, the arrangement typically results in improved homogeneity within the imaging region, and a reduction in stray field (or fringe field) levels outside the magnet region.

It will therefore be appreciated that this represents a significant improvement over prior art magnet arrangements, and magnet arrangement protocols.

The techniques can also be applied to a wide range of different magnet region configurations, examples of which are shown in Figures 2C, 2D and 2E.

In the example of Figure 2C, an alternative design of magnet region 240 is provided. In this example, the magnet region 240 is still rotationally symmetrical about the bore axis 225, but has a non-rectangular cross section, resulting in a non-circular imaging region 245. It is apparent from this, that a magnet region having any desired cross sectional shape can be used.

In the example of Figure 2D, a further alternative design of magnet region 250 is provided. In this example, the magnet region 250 is rotationally symmetrical about a field axis 251, which is substantially perpendicular to the bore axis 225. It is apparent from this, that the field axis .251 need not be coincident with the bore axis 225.

In the example of Figure 2E, two magnet regions 260 are provided, situated on either side of an effective bore axis 225. In this example, the magnet region 260 is not rotationally symmetrical, but does define a field axis that is coincident with the effective bore axis 225. It will be appreciated from the above, that virtually any configuration of magnet region can be defined. In one example, the magnet region has any arbitrary shape that is optionally rotationally symmetric with respect to a bore or field axis. However, this is not necessary and for example, non rotationally symmetric arrangement can be used. Additionally, whilst the magnet region surrounds the imaging region in the above examples, the imaging region can instead be offset from the magnet regions, as occurs for example in open magnet configurations.

In one example, the above described process is performed at least in part utilising a processing system, an example of which will now be described with reference to Figure 3.

In this example the processing system 300 includes a processor 310, a memory 311, an input/output device, such as a keyboard and mouse 312, and an optional external interface 313 coupled together via a bus 314. The optional external interface may be coupled to a database 315, allowing the processing system 300 to store data and/or access previously stored data.

In use, the processor 310 typically executes applications software stored in the memory 311, to allow the processor 310 to perform required calculations and/or display results. This can include, for example, performing analysis of the function and desired field in order to generate the current density distribution, displaying the current density distribution to a user, determining the magnet arrangement and performing further optimisation. It will be appreciated that these processes can be performed automatically, but typically involve at least some input or other control by the user.

It will therefore be appreciated that the processing system 300 may be a suitably programmed computer system, such as a laptop, desktop, PDA, computer server, or the like, although alternatively the processing system 300 may be formed from specialised hardware.

An example of the magnet design process will now be described in more detail with reference to Figures 4 A and 4B.

In this example, at step 400 a magnet region, stray field region and imaging region are defined. The regions will typically be defined based on a combination of the MRI imaging requirements and the required physical size of the resulting arrangement. Thus, for example, the imaging region is typically defined based on the need to encompass a certain physical volume depending on required imaging purposes. It will be appreciated that a number of different standard configurations are known, such as short bore configuration or the like, and these can therefore be generated by appropriate configuration of the regions.

Defining the regions will typically be achieved by allowing the user to input parameters into the processing system 300 representing the respective regions. This may be achieved in any suitable manner and can include the use of a graphical user interface for displaying a representation of the defined field regions, or the like.

At step 405 a cost function is defined based on the current density within the magnet region. The cost function is typically proportional to the energy stored within the magnet region, which in turn depends on the current density and an inductance within the region. This leads to an equation of the form:

F x Z/² where:

F is the cost function;

/ is the current density within the magnet region; and

L is related to an inductance.

The inductance and current density are typically summed over finite elements defined throughout the magnet region, as will be described in more detail below. Accordingly, in this instance, the inductance corresponds to the inductance that a coil within the region may have and can include a self inductance and/or a mutual inductance.

By basing the cost function on the square of the current density, the function has a single global maximum or minimum (depending on whether the function is negative or positive), thereby leading to a single optimum configuration for a desired field. Furthermore, as the cost function is indicative of the energy stored in current carrying coils located within the magnet region, finding the minimum therefore minimises the total stored energy, which in turn minimises quenching strains and the amount of superconducting wire used in the design. The cost function can be defined in a case-by-case basis, but in one example is pre-defined in applications software executed by the processing system 300.

At step 410, a desired field configuration is determined. The desired field is determined based on imaging field and stray field requirements, which will typically require that the magnetic field is substantially homogenous across the entire volume of the imaging region, and typically has homogeneity of at least 100 ppm, typically better than 20 ppm, and preferably better than 10 ppm. Similarly, the stray field typically needs to be minimised to avoid undue interference with equipment external to the magnet configuration, and therefore preferably has a strength below 20 Gauss, and preferably below 15, 10 or even 5 Gauss.

The field requirements can be defined manually by having a user input appropriate parameters. Additionally, and/or alternatively, appropriate parameters may be pre-defined in the applications software executed by the processing system 300.

At step 415, the cost function is optimised for the desired field based on the definitions provided in step 410. The manner in which the cost function is optimised will vary depending on the preferred implementation. However, as mentioned above, by using the above described form of cost function, this leads to a single global minima for a given desired field, which in turn allows a single solution to be derived. Accordingly, this allows the processing system 300 to perform appropriate calculations, allowing the minima to be determined.

At step 420 the processing system 300 generates a current density map which can then be optionally displayed to a user via the I/O device 312, using a suitable user interface. The current density map represents the current densities required within the magnet region in order to generate the desired field, and particular example current density maps will be described in more detail below.

At step 425 coil parameters are determined based on the current density distribution. This may be achieved in any one of a number of ways such as by manual definition by a user, or through automatic definition in the processing system 300. Thus, for example, the user can be presented with a representation of the current density map, allowing the user to indicate preferred coil parameters using the I/O device 312, such as a mouse.

The parameters determined can include any one or more of coil locations, coil sizes, coil shapes, current directions, coil winding directions, or the like. Coil locations are typically defined to correspond to the locations of extremities in the current density distribution, whilst coil sizes and shapes depend on magnitude of the extremities.

The coil winding direction then depends on whether the current density for a given maxima or minima is positive or negative. As discussed above, the coil winding direction may represent a physical direction for the coil winding or alternatively may represent a direction of current flow through the coil, depending on the preferred implementation.

At this stage, the coil arrangement represents an unoptimised coil layout. Accordingly, in one example, further optimisation may be performed.

In this example, at step 430 a second cost function is determined. This is typically of a similar form to the cost function described above, and a specific example will be described in more detail below. Again, the cost function can be pre-defined in the applications software executed by the processing system 300.

At step 435 coil constraints are defined. The coil constraints will typically include limitations such as providing at least a minimum separation between coils to prevent coil overlap, as well as providing a maximum current density flow for each coil. These represent physical constraints on coils which are practically implementable, and are typically selected based on a required magnetic field strength and properties of superconducting material used in the current carrying coils.

At step 440 the coil layout cost function can be optimised to allow an optimised coil configuration to be determined. The optimisation is typically performed using quadratic sub- problems given constraints, as will be described in more detail below, although any suitable technique may be used. It will be appreciated that in this instance, instead of optimising for current density throughout the magnet region, the optimisation is performed specifically with respect to the coils and their current densities, thereby differing from the previous optimisation problem.

At step 445 the obtained coil parameters for each coil are then compared to allowable criteria to ensure that the coil location and peak field constraints are met. Thus, for example, this can include ensuring the coils do not overlap, and that the maximum peak field on the coil is within an acceptable range. The optimisation can be repeated with different coil constraints, if this is required at step 450. Otherwise, the resulting coil arrangement can be provided as an output by the processing system 300 at step 455.

A specific example will now be described in more detail.

For the purpose of this example, the magnetic field produced by an arbitrarily shaped electric current carrying conductor is expressed as the sum of an infinite series of spherical harmonics. The amplitude and sign of each spherical harmonic expansion term depends on the coil geometry, current strength, winding direction and relative positions of coils in a particular magnet configuration.

A collective set of coils can be organised in space to meet the needs of a specific application by appropriate choice of size, current magnitude and direction, and spacing, to emphasise certain spherical harmonic expansion terms that define the magnetic field produced by the current carrying coils themselves.

An example of the geometry and reference frame for a cylindrical coil in space is shown in Figure 5, which defines the magnetic field at point P(r,θ) produced by a superconducting coil, having cross section 500 located in the illustrated yz-plane. For the purpose of this example, appropriate symmetry is assumed to characterize the cylindrical nature of the problem, and the problem is simplified to 2D due to the inherent symmetry.

The axial component of the magnetic field at a point (r < r_Q,θ) in an axisymmetric configuration comprising current carrying circular filaments coaxial with the z-axis is given by:

where

μ₀ is the permeability of free space; / is the filament current;

(r_o,θ_o) defines the coil geometry with respect to an arbitrary origin;

(r, θ) is the field point with respect to the origin; and

P™ (cos θ) is the associated Legendre polynomial evaluated at the appropriate location in space.

The corresponding field expansion for points lying in the external region defined for r > r_Q is given by:

where

Further details of such equations are described in Edminister, J. A., Theory and problems of Electromagnetics. 1979: McGraw-Hill Book Company.

The equations (1) and (2) can be extended to calculate the magnetic field generated by a rectangular cross-section solenoid conductor as:

where flfjtffe.

In equation (3) coordinates (^₁, Z₁) and (y₂,z₂) define a rectangular conductor cross-section in the ^z-plane as shown in Figure 5. The harmonic coefficients a_n and β_n can be computed analytically or numerically. In one example, the analytic expressions for the coefficients are obtained by solving the given integral for a_n and β_n , although alternatively Gaussian quadrature can be used to obtain the individual values of a_n and β_n , and typically this procedure is both accurate and efficient, when compared to the analytic expressions. Nevertheless, the first two harmonic coefficients for the inner region given in analytical form are:

and for the outer region:

For the purpose of this example, the process is performed in two primary stages. This first stage is to determine an initial layout of the superconducting coils and the associated current densities in a predefined domain subject to constraints, such as the homogeneity of the DSV and the size of the magnet stray field. This is achieved by changing the size of the magnet domain and by adjusting the number of internal and external harmonic coefficients to be vanished.

The second stage is the refinement of coil geometries to enhance the field homogeneity, to decrease the stray field (or fringe field) of the magnet and to restrict peak field values at the coils to an acceptable range.

To allow for the calculation of the magnetic field using the spherical harmonic method, the regions are typically configured as shown in Figure 6.

In this example, the magnet region 600 lies within a region 610 centred at an origin 620. For this example, the region 610 has substantially spherical shell shape. However, as will be apparent from the remaining description, this is not essential, and any desired shape of region and magnet region can be employed using this method.

The stray field region 630 is positioned externally to an outer boundary 611 of region 610, while the imaging region 640 is positioned internally within an inner boundary 612 of region 610.

The domain of interest is divided into K small elements 601, as shown in the upper representation of the magnet region 600 in Figure 6. The current densities I_k associated with individual elements are then determined by minimizing the cost function F_mt , which is formulated as a stored energy sum:

subject to: K-I

∑^ak, k=0 A = B₀,

where

and Bo is the desired magnetic field strength at the iso-centre; a.k,_n andβicm are the spherical harmonic terms of element k;

M specifies the number of external harmonic coefficients;

N specifies the number of internal harmonic coefficients, given that K≥M+N;

L_k is the inductance of element & with its dimensions shown in Figure 6; and A_k is the cross-sectional area of a single wire element.

The values chosen for M and JV help define the size of the stray field and the DSV, respectively. M and N are increased to reduce the size of the stray field and to increase the size of the DSV. If a magnet design has N-I internal coefficients and M external coefficients vanished, it is referred to as an N order M degree magnet design.

For magnet configurations in which coils are coaxial and symmetric about the illustrated xy- plane, the spherical harmonic expansion results in the elimination of all even order terms within the expansion. To further reduce computational complexity, the strategy employed here considers only one quarter of the magnet domain embedded within the computations and the constraint (5) is simplified as follows: K-I β k=0 ^

min "^ ^■* — -" max '

where, iC is the number of elements in one quarter of the magnet domain.

The cost function defined in equation (4) is formulated to ensure that the stored energy within the coils is minimized, and consequently magnet training and quenching strains will be kept to a minimum. The formulated cost function also provides the benefit of minimizing the amount of superconducting wire used in the design, since the coil inductance is proportional to the conductor volume when restrictions on the length of the magnet domain are enforced.

In one example, the cost function F_m is solved using the general quadratic program (QP)

Nocedal, J. and SJ. Wright, Numerical Optimization. 2nd ed. 2006, New York: Springer. The problem is stated as a strictly convex QP, since L_k > 0 and hence, the solution obtained using this procedure yields the global minimum, or the derived total stored energy has the smallest value. However, any other suitable optimization technique may also be used.

As previously described, this allows designs to be obtained that are unique and cannot be improved using other methodologies for this particular stored energy formulation. This stage of the magnet design process enables the generation of unique current density maps, named the minimum stored energy (MSE) current density map, in which the coils are embedded.

At the end of the initial layout stage, a current density map is obtained that has several local maxima and minima within the magnet domain referred to as extremities. The number of extremities is proportional to the number eliminated spherical harmonics, and notably these extreme points are distributed along the boundary or perimeter of the magnet domain.

The next step is to establish and refine the superconducting coil geometries to enhance the magnetic field homogeneity in the inner field, as well as, to satisfy other constraint requirements, including the peak magnetic field condition on the superconducting coils and the stray magnetic field strength.

Superconducting coils are placed in the magnet domain, based on the positioning of extremities in the form of local positive maxima and local negative minima. The coils are initially overlapped on the extreme point locations with their initial cross sectional areas being proportional to the value of the overlapping current densities. This process of identifying and locating coils will be described in more detail below.

After this step, the constant current coil geometries are refined by using a non-linear optimization method that minimizes the cost function F_refine defined similarly as F_mi :

subject to:

K-I

A-=0

⁷∑«,,_« = 0₅ n = 2...N,

4=0 K-I (9)

where, K is the number of the superconducting coils in the magnet domain.

As coils are coaxial and symmetric about the xy-plane, the computational complexity is reduced, and only one quarter of the magnet domain is considered. Constraint (9) in this case therefore becomes:

^Kz\ N

*⁼⁰ (10)

'∑/W, = 0, m = 0...— -1,

Jt=O ^ o < / ≤ /_max,

where, Z is the number of the superconducting coils in one quarter of the magnet domain.

It should be noted that for the purpose of this example, within the optimization process the current densities are positive and are the same for all elements, and hence for the coils, However, if required, the optimization process can also be used when different current densities on different coils are needed. The sign of the current in a particular coil is implied by the order of the y_λ and y₂ coordinates, as shown in Figure 5. lfy_x > y₂ , then the current has a positive sign, otherwise it has a negative sign. The need for negative currents within the computations is eliminated by using such an approach.

Additionally, similar results can be obtained taking into account mutual inductance. In this instance, this can be modelled by substituting equation (8) above for the following equation:

,₌₀ where M ¹U₁₁ is either self or mutual inductance.

In the initialisation of the coils, or to obtain the initial layouts using F_lml , linear constraints were used. In the refinement stage, F_reβm is a constrained non-linear optimization problem that requires the use of an appropriate non-linear optimization algorithm to obtain the coil layout. One of the most effective methods of obtaining solutions to nonlinearly constrained optimization problems is to generate and solve quadratic sub-problems. For this reason, sequential quadratic programming (SQP) Lawrence, C, JX. Zhou, and A.L. Tits, User's Guide for CFSQP Version 2.5: A C Code for Solving (Large Scale) Constrained Nonlinear (Minimax) Optimization Problems, Generating Iterates Satisfying All Inequality Constraints. 1997, University of Maryland) is implemented to solve the problem. The SQP method is a local optimization algorithm, which can give a globally optimal solution given suitable initial starting values. However, it will be appreciated that other suitable optimization techniques can be used.

Through the optimization process the individual coil dimensions and spatial locations are altered to achieve a better minimized solution, since L_k is a function of coil geometry.

During the calculation iterations coil overlap is avoided by dividing the magnet domain into several layers and extra geometrical constraints based on the current density map are introduced to limit the movement of the coils. This step is fundamentally correct, since the current density maps do not suggest that the coils should be overlapped. In particular, limits on the ^-coordinate of the individual coils are imposed by breaking the domain into layers, and large axial movements are restricted by imposing z-coordinate bounds.

An example of a particular layout of coils in the optimization stage is shown in Figure 7.

In this example, a three layer configuration is shown, although this is for the purpose of example only, and any suitable number of coil layers can be provided. Each of the layers has respective boundaries provided at distances Yo, Yi, Y ₂, Y ₃ from the origin. The coils are generally designated b_\φ, where / is a layer identifier, and p is a coil number. The separation between the coils within each layer are generally designated α_/,_p associated with coil p for a particular layer /. The values for a, and b_l must be non-negative, and in any given layer the sum of them must not be larger than the half length of the domain.

As discussed above, the extreme current densities are distributed over the domain boundary, hence, the upper and lower layers can have multiple coils and any layers between them consist of a single coil.

After convergence has been achieved the magnetic field at the superconducting coils is calculated using available methods, such as outlined in Forbes, L.K., S. Crozier, and D.M. Doddrell, Rapid Computation of Static Fields Produced by Thick Circular Solenoids. IEEE Transactions on Magnetics, 1997. 33(5): p. 4405-4410. The magnetic fields are then tested for peak field criteria. If within the coils the peak field is greater than the allowable limit, then the individual layers are adjusted, such that the spacing between the coils is increased. For example as shown in Figure 7, for the depicted three layer configuration, Y₁ and Y₂ are changed such that |ζ ~ ^| is increased. The optimization process is then repeated to obtain the new coil layouts according to the new layer constraints. It may happen that this process of iterative refinement needs to be repeated two or three times to ensure that the best coil layout can be obtained, for a given maximum level of peak magnetic field for the coils.

Example resulting configurations for different size and configurations of MRI systems will now be described.

Both unshielded and shielded magnets are designed using the outlined strategy to generate the inner field, and also, to gauge the stray field produced by the magnets. The results are also used to illustrate the location of the MSE current density map extremities and alterations to these extremities as the size of the magnet domain is varied.

Figures 8 A and 8B show 3D and 2D MSE current density profiles with contours for 3m long magnet domains for unshielded order 10 degree 0 configurations, whilst equivalent shielded order 10 and degree 2 configurations are shown in Figures 9 A and 9B.

Comparison of Figures 8 and 9 show that as the shielding is incorporated into the design, the extremities or the second stage coil locations wrap around the perimeter of the magnet domain to achieve a desired level of stray field. Both designs of Figures 8 and 9 aim to achieve the same DSV size, but the magnet of Figure 9 would have a significantly smaller practical footprint due to the significantly smaller stray field.

It is notable that the current density maps of Figure 8 do not have any negative values, whereas in Figures 9 a negative minima is incorporated, which assists in providing the reduced footprint. This finding explains the reason why in lengthy, small DSV magnets, the winding directions are only in the positive sense, and negative winding coils are added if shielding is considered. As the dimensions of the magnet domain are decreased, the current density minima start to take on negative values, which imply that negative winding direction coils are required to reduce the magnet length, as shown in Figures 10 and 11.

In this regard, Figures 1OA and 1OB show 3D and 2D MSE current density profiles with contours for unshielded order 14 degree 0 2m length magnet domains, with \m length domains being shown in Figures 1 IA and 1 IB.

As shown in Figures 1OA and 1OB, if the magnet domain is sufficiently long, then the extremities may only be positioned on the lower domain boundary. The effect of decreasing the domain length is shown in Figure 11. As can be seen by comparing Figures 10 and 11, the extremities within the magnet domain still alternate in current direction, but wrap around the perimeter of the domain. As a result for the unshielded magnet design, the individual coils are not restricted to the inner domain boundary, or bore perimeter, as it is generally referred to above.

Figures 12A and 12B show 3D and 2D MSE current density profiles with contours for shielded order 16 degree 4 2m length magnet domains, with Im length domains being shown in Figures 13A and 13B.

Similarly to Figure 11, Figure 13 provides the current density map for the shortened design. As can be seen in Figures 12 and 13 and according to the theory, the predicted coil layout due to the extreme current density values indicates alternating currents within the individual magnet coils. Once again, the current density map, as was in the case for the unshielded design, allocates the coils on the perimeter of the domain.

To obtain good stray field reduction in the design, it is necessary to increase the degree of the magnet, whereas to increase the DSV size, the order has to be increased. This fact will be more evident in the following sections, whereby the unshielded and shielded magnet arrangements for particular cases are outlined in more detail.

Short Bore Unshielded Magnet Design In the following cases, the domain is limited in length to Xm with bore diameter of Im, hence qualifying the designs as short MRI magnets. The superconducting wires used in the designs are taken from Sciver, S. W. V. and K.R. Marken, Superconducting Magnets Above 20 Tesla. Physics Today, 2002(August): p. 37-42. All of the coils are formed using \mm x\mm NbTi wires, by limiting the current to a level below 180_^4 and the peak magnetic field on the superconducting coils to be less than 9T .

An example of a resulting unshielded example configuration is shown in Figures 14A and 14B, which are 3D and 2D MSE current density profiles with contours for order 16 degree 0 magnet arrangements.

In this example, the resulting coil layout defined by the current density profile is shown in Figure 14B. The allocation of the individual coils aligns with the current density map peaks, and the size of the coils is proportional to the magnitude of the current density at that location. In this manner, the initialised coils are reflective of the results obtained through the first initial layout stage.

The second optional optimisation stage is then seeded using the coil allocation of shown in Figure 14B, and rapid convergence is achieved, due to the fact that the initial seed is highly accurate. Once the optimization of stage two has converged, by changing the location and size of the blocks, the resultant magnetic field is calculated.

An example of the final optimised magnet configuration and the resulting magnetic field is shown for the DSV in Figure 15A and for the stray fields in Figure 15B. In this example, the innermost contour corresponds to lppm , followed by the IQppm contour, and the stray field with 5, 10, 15 and 20 gauss contour lines.

In this example, the DSV is 50cm in diameter at the lppm contour line, and approximately 56cm in diameter to the 10 ppm contour line. The footprint of this unshielded magnet is quite large, as can be seen in Figure 15B.

The final coil configuration of Figure 15 A and the corresponding coil numbering of Figure 14B are given in Table 1 below. In the table the (y_c, z_c) centre coordinate of the individual coils is provided, with the number of coil windings defined as (N_y, N_z) in the radial and axial directions, respectively. As can be seen in the table, the current in the coils is below the NbTi superconductor limit. The peak magnetic field on the superconducting coil was calculated to be 7.07417" , which is also well below the required limit.

Table 1

As was highlighted earlier, the final configuration for the unshielded case has an alternating current direction coil layout around the perimeter of the magnet domain. It should also be noted that the largest coil is actually not on the inner layer of the magnet (i.e. near the bore perimeter 202), but rather on an outer perimeter of the magnet region 200.

An increase in magnet domain length would see the outer layer disappear, and the upper coil would be inline with the other inner coils. Since both cases have the same order, their DSV dimensions are the same. However, the length of the magnet is shorter when outer coils are used in the design.

Short Bore Shielded Magnet Design

An example of an equivalent shielded case is shown in Figures 16 A, 16B, 17 A, 17B.

For the shielded magnet design, the external magnetic field harmonic coefficients are used to reduce the stray field, which means that the number of coils has to be increased to allow for appropriate magnetic field definition.

In the initial phase one stage, eighteen harmonics (order 14 degree 4) were included in the design to obtain a current density map as shown in Figure 16A. The corresponding coil configuration for phase two of the optimization is illustrated in Figure 16B₅ whereby the coils themselves are allocated on the perimeter of the magnet domain, and the size of the coils are proportional to the current density for that particular extreme current density. Independent of the design strategy, that is whether unshielded or shielded, the current within the coils alternates as highlighted earlier.

Table 2 provides the dimensions and locations of the final coil layout, after optimisation. The current in the NbTi wire is below the current carrying capability of the wire, and the peak field was calculated to be 8.98867¹ , which is also below the peak magnetic field requirement.

The DSV and stray magnetic fields for the final coil layouts following optimisation are shown in Figures 17A and 17B respectively. In Figure 17 A, the innermost contour corresponds to \ppm, followed by the lOppm contour, whilst the stray field in Figure 17B shows 5, 10, 15 and 20 gauss contour lines.

It can be seen from Figure 17A that the DSV has a 40cm diameter to the lppm contour line and a diameter of 50cm to the \0ppm contour line. An important observation in this design is that the most outer coils do not necessarily have to be opposing current coils to reduce the stray field of the magnet, but rather, these coils are used in conjunction with the rest of the coils to obtain an overall effect satisfying both DSV and stray field requirements. Additionally, it can be seen from Figure 17B that the stray field at the 5 gauss level extends no more than Am from the centre of the magnet.

Table 2

A number of further designs are shown in Figures 18 to 21. ShortBore Clinical 1.5T Magnet

Figures 18A and 18B show DSV fields with contours at 0.25, 1 and 10 ppm lines, and stray fields with contours at 5, 10, 15 and 25 gauss for a short bore clinical magnet. The properties of the resulting configuration are as follows: Bore diameter: Im Bore length: 1.28m

DSV diameter at lppm homogeneity: 50cm Stray Field (5G) : 2.8m (axial) 2.6m (radial) Symmetrical Magnet: BO = 1.5 T, Bpeak = 7.5938T, I = 175MA/m2 Zmax = 0.64m, Ymax = 0.95m, Ymin = 0.5m

Short Bore Clinical 3.0T Magnet

Figures 19A and 19B show DSV fields with contours at 0.25, 1 and 10 ppm, and stray fields with contours at 5, 10, 15 and 25 gauss for a short bore 3.0T clinical magnet. The properties of the resulting configuration are as follows: Bore diameter: Im

Bore length: 1.44m

DSV diameter at lppm homogeneity: 50cm

Stray Field (5G) : 3.1m (axial) 2.8m (radial)

Symmetrical Magnet: BO = 3 T, Bpeak = 8.2580T, I = 175MA/m2 Zmax = 0.72m, Ymax = 1.01m, Ymin = 0.5m

Active Shield 7.0T Magnet

Figures 2OA and 2OB show DSV fields with contours at 0.25, 1 and 10 ppm, and stray fields with contours at 5, 10, 15 and 25 gauss for a high field clinical magnet. The properties of the resulting configuration are as follows: Bore diameter: Im Bore length: 1.8m

DSV diameter at lppm homogeneity: 50cm Stray Field (5G) : 4.45m (axial) 3.85m (radial) Symmetrical Magnet: BO = 7 T, Bpeak = 10.5124T (Nb3Sn), I = 175MA/m2 Zmax = 0.9m, Ymax = 1.Om, Ymin = 0.5m

Open 1.0T Magnet

Figures 21 A and 21B show DSV fields with contours at 0.25, 1 and 10 ppm, and stray fields with contours at 5, 10, 15 and 25 gauss for an open bore clinical magnet. The properties of the resulting configuration are as follows: Gap: 60cm Pole length: 0.4m Total length: 1.44m

DSV diameter at lppm homogeneity: 50cm Stray Field (5G) : 3.5m (axial) 3.1m (radial)

Open Magnet: BO = 1 T, Bpeak = 7.9319T, I = 175MA/m2 Zmax = 0.7m, Zmin = 0.3m, Ymax = 1.Om, Ymin = 0.5m

Accordingly, the above described processes provide a method of designing magnet arrangements, and in particular for designing superconducting magnet arrangements suitable for use in MRI imaging apparatus. The processes have the ability to arrange coils in a manner that ensures that the overall dimensions and stored magnetic energy are minimized with peak current and peak magnetic field in acceptable ranges.

In one example, the process uses a magnet region or domain that is treated as current density maps, in which, superconducting coils are embedded. The current density maps from which the coils themselves are derived are unique, and allow a minimum stored energy configuration to be derived.

In one example, a second optimisation phase may also then be performed to generate final coil layouts that take into account coil dimensions for required field linearity, and peak and stray field minimization.

The results suggest that coils should be placed around the perimeter of the domain with adjacent coils having alternating winding directions for best design performance. The techniques can be used to derive unshielded and shielded designs, both of which yield magnet configurations that allocate coils around the perimeter of the magnet domain. For the unshielded case different order and zero degree implementations can be used, and the degree is varied to achieve certain shielding requirements for the designs. Irrespective of the shielding requirements, the coils themselves tend to be placed on the boundaries of the current density map domain, with the current direction alternating between adjacent coils.

The stray field of the shielded magnet designs are very small whilst maintaining a large DSV, when compared to the overall magnet dimensions.

Whilst the above examples have focused on spherical DSV magnets, this is not essential, and similar techniques can be used for any desired field configuration.

Persons skilled in the art will appreciate that numerous variations and modifications will become apparent. All such variations and modifications which become apparent to persons skilled in the art, should be considered to fall within the spirit and scope that the invention broadly appearing before described.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: I) A method of determining a magnet arrangement for use in magnetic resonance imaging apparatus, the method including: a) determining a function representing current densities required within a magnet region to generate a field; b) determining a current density distribution required to generate a desired field, using the function; and, c) determining the magnet arrangement using the current density distribution, the magnet arrangement including a number of current carrying coils arranged within the magnet region.

2) A method according to claim 1, wherein the method includes: a) dividing the magnet region into a number of elements; and, b) determining a current density for each element using the function to thereby determine the current density distribution. 3) A method according to claim 1 or claim 2, wherein the method includes determining the current density distribution by optimizing the function.

4) A method according to any one of the claims 1 to 3, wherein the function has a global minimum.

5) A method according to any one of the claims 1 to 4, wherein the function is quadratic with respect to the current density within the magnet region.

6) A method according to any one of the claims 1 to 5, wherein the function is based on the energy stored within the magnet region.

7) A method according to claim 5 or claim 6, wherein the function is of the form:

F oc LI² where:

F is the cost function;

/ is the current density within the magnet region; and

L is related to an inductance.

8) A method according to any one of the claims 1 to 7, wherein the method includes defining the magnet region. 9) A method according to any one of the claims 1 to 8, wherein the magnet region has an arbitrary shape positioned relative to at least one of: a) a bore; and, b) an imaging region. 1O) A method according to any one of the claims 1 to 9, wherein the magnet region substantially surrounds at least one of: a) a bore; and, b) an imaging region.

H) A method according to claim 10, wherein the magnet region is rotationally symmetric with respect to a field axis.

12) A method according to claim 11, wherein the field axis is aligned with a bore axis.

13) A method according to any one of the claims 1 to 12, wherein magnet region has a substantially cylindrical shape.

14) A method according to any one of the claims 1 to 13, wherein the method includes, defining a desired field within at least one of: a) an imaging region; and, b) a stray field region.

15) A method according to claim 14, wherein the method includes defining the imaging region to have a substantially predefined shape positioned with respect to the magnet region.

16) A method according to claim 15, wherein the method includes defining an imaging region having a substantially spherical shape positioned on a field axis radially inwardly of the magnet region.

17) A method according to any one of the claims 1 to 16, wherein the method includes defining a desired field including a substantially homogeneous magnetic field within an imaging region.

18) A method according to claim 17, wherein the desired field has a homogeneity having a variation in the imaging region of less than at least one of: a) 100 ppm; b) 20 ppm; and, c) 10 ppm. 19) A method according to any one of the claims 1 to 18, wherein the method includes defining a stray field region substantially surrounding the magnet region.

20) A method according to claim 19, wherein the stray field region is positioned on the bore axis radially outwardly of the magnet region. 2I)A method according to claim 19 or claim 20, wherein the method includes defining a desired field including a field strength of less than a selected amount within the stray field region.

22) A method according to claim 21, wherein the selected amount is less than, at least one of: a) 20 Gauss; b) 10 Gauss; and, c) 5 Gauss.

23) A method according to claims 1 to 22, wherein a current density distribution has a series of extremities lying around a perimeter of the magnet region.

24) A method according to claim 23, wherein the direction of current flow in the coils depends on the polarity of the extremities within the current density distribution.

25) A method according to any one of the claims 1 to 24, wherein the method includes using the current density distribution to determine at least one of: a) coil locations within the magnet region; b) coil sizes; c) a current density within coils; d) a total current for coils; e) current flow directions within coils; and, f) coil winding directions.

26) A method according to any one of the claims 1 to 25, wherein the method includes determining the coil arrangement based on extremities within the current density distribution.

27) A method according to claim 26, wherein the coils are located on local positive maxima and local negative minima of the current density distribution.

28) A method according to any one of the claims 1 to 27, wherein the current carrying coils are arranged substantially around a perimeter of the magnet region. 29) A method according to any one of the claims 1 to 28, wherein, for current carrying coils on a bore perimeter of the magnet region, each current carrying coil carries a current in an opposing direction to each adjacent coil.

30) A method according to any one of the claims 1 to 29, wherein each current carrying coil carries a current in an opposing direction to each adjacent coil.

3I)A method according to any one of the claims 1 to 30, wherein the magnet region has a substantially cylindrical shape surrounding a field axis, each current carrying coil being arranged within the magnet region with a coil axis substantially aligned with the field axis. 32) A method according to any one of the claims 1 to 31, wherein the method further includes optimising the coil arrangement. 33) A method according to claim 32, wherein the method includes: a) defining a second function; and, b) optimising the second function to thereby optimise the coil arrangement. 34) A method according to claim 33, wherein the method includes optimising the second function such that the coil arrangement has optimal coil current densities

35) A method according to claim 33 or claim 34, wherein the second function has a global solution.

36) A method according to any one of the claims 33 to 35, wherein the second function is quadratic with respect to the current density within the magnet region.

37) A method according to any one of the claims 33 to 36, wherein the second function is a function of the energy stored within the coils.

38) A method according to any one of the claims 33 to 37, wherein the method includes: a) defining coil constraints; and, b) optimising the second function using the coil constraints to thereby determine an optimised magnet arrangement.

39) A method according to claim 38, wherein the coil constraints include at least one of: a) a minimum separation between coils to prevent coil overlap; and, b) an optimal current density flow for each coil. 40) A method according to claim 39, wherein the optimal current density flow is smaller than a maximum current density. 4I)A method according to claim 40, wherein method includes selecting the maximum current density in accordance with a magnetic field strength and properties of superconducting material used in the current carrying coils.

42) A method according to any one of the claims 1 to 41, wherein the method is performed at least in part using a processing system.

43) A magnet arrangement for use in magnetic resonance imaging apparatus, the magnet arrangement including a number of current carrying coils arranged substantially around a perimeter of a magnet region, and wherein, for current carrying coils on a bore perimeter of the magnet region, each current carrying coil carries a current in an opposing direction to each adjacent coil on the bore perimeter.

44) A magnet arrangement according to claim 43, wherein the current carrying coils are located substantially on extremities of a current density distribution determined for the magnet region.

45) A magnet arrangement according to claim 44, wherein each coil carries a current in a direction defined by a polarity of the extremity.

46) A magnet arrangement according to any one of the claims 43 to 45, wherein each coil carries a current in an opposing direction to each adjacent coil.

47) A magnet arrangement according to any one of the claims 43 to 46, wherein each coil includes a number of current carrying windings. 48) A magnet arrangement according to any one of the claims 43 to 47, wherein the number of current carrying windings is determined so that the coil generates a required magnetic field. 49) A magnet arrangement according to any one of the claims 43 to 48, wherein the magnet region has an arbitrary shape positioned relative to at least one of: a) a bore; and, b) an imaging region. 5O)A method according to claim 49, wherein the magnet region is rotationally symmetric with respect to a field axis.

51) A method according to claim 50, wherein the field axis is aligned with a bore axis. 52) A magnet arrangement according to claim 50 or claim 51, wherein magnet region has a substantially cylindrical shape. 53) A magnet arrangement according to any one of the claims 43 to 52, wherein the magnet generates a desired field within at least one of: a) an imaging region; and, b) a stray field region. 54) A magnet arrangement according to claim 50, wherein the imaging region has a substantially predefined shape positioned with respect to the magnet region. 55) A magnet arrangement according to claim 51, wherein the imaging region has a substantially spherical shape positioned on a field axis radially inwardly of the magnet region. 56) A magnet arrangement according to claim 51 or claim 52, wherein the imaging region has a substantially homogeneous magnetic field.

57) A magnet arrangement according to claim 53, wherein the homogeneity has a variation of less than at least one of: a) 100 ppm; b) 20 ppm; and, c) 10 ppm.

58) A magnet arrangement according to any one of the claims 51 to 54, wherein the stray field region substantially surrounds the magnet region.

59) A magnet arrangement according to claim 55, wherein the stray field region is positioned on the field axis radially outwardly of the magnet region.

60) A magnet arrangement according to claim 55 or claim 56, wherein the desired field has a field strength of less than a selected amount within the stray field region.

61) A magnet arrangement according to claim 57, wherein the selected amount is less than, at least one of: a) 20 Gauss; b) 10 Gauss; and, c) 5 Gauss.