Magnetic resonance imaging scanner with booster iron

A magnetic resonance imaging scanner includes a magnet (20) generating a temporally constant magnetic field, magnetic field gradient-generating structures (30) superimposing selected magnetic field gradients on the temporally constant magnetic field, and a radio frequency coil (32) producing a radio frequency field. A magnetic field-modifying structure (60) disposed inside a radio frequency shield (64) includes dispersed particles of magnetic material (701, 702, 703, 704) that enhance the temporally constant magnetic field. The particles are generally smaller in at least one dimension than a skin depth of the radio frequency field in the magnetic material. The magnetic field-modifying structure has a longitudinal demagnetization factor (Nz) parallel to the temporally constant magnetic field and a tangential demagnetization factor (NT) in a tangential direction transverse to the temporally constant magnetic field. The longitudinal demagnetization factor is larger than the tangential demagnetization factor to produce tangential flux guiding.

DESCRIPTION

The following relates to the magnetic resonance arts. It finds particular application in magnetic resonance imaging, and will be described with particular reference thereto. However, it also finds application in magnetic resonance spectroscopy and other techniques that benefit from a substantially uniform main B0magnetic field.

Magnetic resonance imaging scanners with magnet bores that are short in the axial or z-direction reduce patient claustrophobia and can provide improved access to the patient for interventional procedures. A short bore magnet may, for example, have bore length of less than 1.5 meters, or less than 1 meter. Short-bore magnets, however, typically have degraded static main B0magnetic field spatial uniformity as compared with longer bore magnets, due to field bending at the ends of the short bore.

One approach for improving field uniformity is the use of “booster” iron. In this approach, a magnetic field-modifying structure includes iron or another ferromagnetic material that is coupled with the main B0magnetic field. The magnet coils are designed in conjunction with the magnetic field-modifying structure, such that the main magnet and the magnetic field-modifying structure together produce a substantially spatially uniform static main B0main magnetic field. The booster iron stretches the magnetic field to compensate for the reduced bore length. Moreover, as the booster iron is typically in saturation at typical main B0magnetic field magnitudes (e.g., at 1.5 T or higher), the effect of the booster iron on the static main B0magnetic field is substantially independent of magnetic field gradients. Instead of designing the booster iron concurrently with the magnet, the booster iron can be designed empirically after magnet manufacture by adding iron to improve uniformity, stretch the field, or otherwise improve the main B0magnetic field in the already-manufactured magnet.

In some designs, the magnetic field-modifying structure is placed outside of the radio frequency coil and the radio frequency shield that shields surrounding structures from the radio frequency signals. This approach substantially reduces interaction between the booster iron and the radio frequency fields. However, placing the booster iron outside the radio frequency shield has certain disadvantages. Space constraints in the scanner can make placement of the booster iron outside the radio frequency shield difficult. Moreover, the booster iron is less effective at modifying the main B0magnetic field in the imaging region as the booster iron is moved further away from the imaging region. Thus, more booster iron is required, which occupies additional space in the bore.

Moving the booster iron closer to the imaging volume by placing it inside of the radio frequency shield is problematic. Inside the shield, the booster iron interacts with and can degrade the radio frequency B1magnetic field. The skin depth in iron of the radio frequency B1magnetic field at typical magnetic resonance imaging frequencies is small, being typically of order 10-20 microns or less. Consequently, the radio frequency B1magnetic field is substantially expelled from the interior of the booster iron. This field expulsion causes the radio frequency coil to operate less efficiently, and can cause non-uniformities in radio frequency fields. Moreover, eddy currents induced in the booster steel by the radio frequency fields can produce magnetic field distortions, image artifacts, and detrimental heating inside the scanner.

The present invention contemplates an improved apparatus and method that overcomes the aforementioned limitations and others.

According to one aspect, a magnetic resonance imaging scanner is disclosed. A magnet generates a temporally constant magnetic field. One or more magnetic field gradient-generating structures superimpose selected magnetic field gradients on the temporally constant magnetic field. A radio frequency coil is disposed inside of a radio frequency shield and selectively produces a radio frequency field. A magnetic field-modifying structure is designed to enhance the temporally constant magnetic field. The magnetic field-modifying structure is disposed inside of the radio frequency shield, and includes particles of magnetic material generally smaller in at least one dimension than a skin depth of the radio frequency field in the magnetic material dispersed in an insulating binder.

According to another aspect, a magnetic resonance imaging scanner is disclosed. A magnet generates a temporally constant magnetic field. One or more magnetic field gradient-generating structures superimpose selected magnetic field gradients on the temporally constant magnetic field. A radio frequency coil selectively produces a radio frequency field. A magnetic field-modifying structure is designed to enhance the temporally constant magnetic field. The magnetic field-modifying structure has a longitudinal demagnetization factor parallel to the temporally constant magnetic field and a tangential demagnetization factor in a tangential direction transverse to the temporally constant magnetic field. The longitudinal demagnetization factor is larger than the tangential demagnetization factor to produce tangential flux guiding.

One advantage resides in reducing space consumption in a magnetic resonance imaging scanner.

Another advantage resides in improved radio frequency coil operating efficiency.

Yet another advantage resides in providing preferential flux guiding in the tangential direction.

Still yet another advantage resides in reduced eddy current losses.

Numerous additional advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments.

With reference toFIG. 1, a magnetic resonance imaging scanner10includes a housing12defining a generally cylindrical scanner bore14inside of which an associated imaging subject16is disposed. Main magnetic field coils20are disposed inside the housing12, and produce a temporally constant main B0magnetic field directed generally along and parallel to a central axis22of the scanner bore14. The central axis22lies parallel to the z-direction in the reference x-y-z Cartesian coordinate system indicated inFIG. 1; however, other coordinate systems can be used. For example, a vertical magnet can be used, in which the temporally constant B0field is vertically oriented in the y-direction. The main magnetic field coils20are typically superconducting coils disposed inside cryoshrouding24, although resistive permanent magnetic main magnets can also be used.

The housing12also houses or supports magnetic field gradient-generating structures, such as magnetic field gradient coils30, for selectively producing magnetic field gradients parallel to the central axis22of the bore14, along in-plane directions transverse to the central axis22, or along other selected directions. The housing12further houses or supports a radio frequency body coil32for selectively exciting magnetic resonances. Specifically, the radio frequency body coil32produces a radio frequency B1magnetic field transverse to the static main B0magnetic field. The radio frequency B1magnetic field is generated at the Larmor frequency for exciting nuclear resonances. For exciting1H proton nuclei, the magnetic resonance Larmor frequency fresgenerally corresponds to fres=γB0where γ=42.58 MHz/Tesla is the gyrometric ratio for the1H nuclei and B0is the static main B0magnetic field. Thus, for example, at B0=3 T, fres=128 MHz. While1H proton nuclei exist in high concentrations in the human body and are commonly used for magnetic resonance imaging, other nuclear magnetic resonances can be similarly excited and imaged.

In the illustrated embodiment, the coil32is a birdcage coil. A coil array34is optionally disposed inside the bore14to receive magnetic resonance signals. The coil array34includes a plurality of coils, specifically four coils in the illustrated example coil array34, although other numbers of coils can be used, including a single surface coil. Moreover, the optional coil array34can be omitted altogether, and the body coil32used for receiving magnetic resonance signals. The housing12typically includes a cosmetic inner liner36inside the birdcage coil32defining the scanner bore14.

The main magnetic field coils20produce the main B0magnetic field parallel to the z-direction in the bore14. A magnetic resonance imaging controller40operates magnetic field gradient controllers42to selectively energize the magnetic field gradient coils30, and operates a radio frequency transmitter44coupled to the radio frequency coil32to selectively energize the radio frequency coil32. By selectively operating the magnetic field gradient coils30and the radio frequency coil32, magnetic resonance is generated and spatially encoded in at least a portion of a region of interest of the imaging subject16. By applying selected magnetic field gradients via the gradient coils30, a selected k-space trajectory is traversed during acquisition of magnetic resonance signals, such as a Cartesian trajectory, a plurality of radial trajectories, or a spiral trajectory. Alternatively, imaging data can be acquired as projections along selected magnetic field gradient directions. During imaging data acquisition, the magnetic resonance imaging controller40operates a radio frequency receiver46coupled to the coils array34, as shown, or coupled to the whole body coil32, to acquire magnetic resonance samples that are stored in a magnetic resonance data memory50.

The imaging data are reconstructed by a reconstruction processor52into an image representation. In the case of k-space sampling data, a Fourier transform-based reconstruction algorithm can be employed. Other reconstruction algorithms, such as a filtered backprojection-based reconstruction, can also be used depending upon the format of the acquired magnetic resonance imaging data. The reconstructed image generated by the reconstruction processor52is stored in an image memory54, and can be displayed on a user interface56, stored in non-volatile memory, transmitted over a local intranet or the Internet, viewed, stored, manipulated, or so forth. The user interface56can also enable a radiologist, technician, or other operator of the magnetic resonance imaging scanner10to communicate with the magnetic resonance imaging controller40to select, modify, and execute magnetic resonance imaging sequences.

To stretch the static main B0magnetic field, to improve uniformity of the main B0magnetic field, or to otherwise modify or configure the main B0magnetic field, a magnetic field-modifying structure60which includes a plurality of ferromagnetic annular rings, specifically eight annular rings62in the embodiment illustrated inFIG. 1, are disposed in and designed to enhance the static main B0magnetic field. While eight annular rings62are illustrated, other numbers and/or placements of rings can be used. Moreover, a magnetic field-modifying structure including partial rings, rods, or other ferromagnetic structures may be employed. Typically, the number, distribution, shape, and other geometrical characteristics of the rings62of the magnetic field-modifying structure60are selected during concurrent design of the magnet20. For example, these characteristics of the magnetic field-modifying structure60are suitably optimized during a finite element modeling optimization of the static magnetic field produced by the magnet20in conjunction with the magnetic field-modifying structure60.

InFIG. 1, the magnetic field-modifying structure60is disposed between the radio frequency coil32and a radio frequency shield64of the radio frequency coil32. In this position, it overlaps and interacts with the radio frequency B1magnetic field generated by the radio frequency coil32. At typical magnetic resonance frequencies, such as the example 128 MHz resonance frequency for1H protons in a 3T static magnetic field, penetration of the radio frequency B1magnetic field into ferromagnetic materials is limited to a skin depth of typically about 10-20 microns or less. For example, using fres=128 MHz, relative magnetic permeability μr˜1 since the ferromagnetic material is in saturation, and a conductivity σ˜1×107S/m, the skin depth δ(fres) is approximately:

δ⁡(fres)≅1π⁢⁢fres⁢μr⁢μo⁢σ=14⁢⁢microns,(1)
where μo=4π×10−7H/m is the magnetic permeability of free space and the product μrμois the absolute magnetic permeability of the ferromagnetic material. The magnetic field in a ferromagnetic particle having dimensions substantially larger than the skin depth is substantially expelled from the interior of the ferromagnetic particle. Such magnetic flux expulsion can adversely affect the performance of the radio frequency coil32.

With reference toFIGS. 2A and 2B, in a first embodiment the magnetic field-modifying structure601(where the subscript “1” on certain reference numbers ofFIGS. 2A and 2Bindicate components specific to the first described embodiment of the magnetic field-modifying structure60) has ferromagnetic annular rings621which are made up of ferromagnetic particles701dispersed in an insulating binder72. In one example embodiment, the ferromagnetic particles701are pure iron particles, iron alloy particles such as iron-cobalt alloy particles, or the like, and the binder72is an electrically insulating non-magnetic material such as a polymer, resin or the like.

To substantially reduce flux expulsion of the radio frequency B1magnetic field from the ferromagnetic particles701, the particles are generally smaller in at least one dimension (for example, at least one of length, width, and depth, or an annular cross-sectional dimension of ring-shaped particles) than a skin depth of the radio frequency B1magnetic field in the ferromagnetic material, to allow the radio frequency B1magnetic field to enter the ferromagnetic particles701. The phrase “generally smaller than” the skin depth recognizes that the particles701may have a statistical size distribution in which some particles may be larger than the skin depth. In such cases, the statistical distribution is such that most of the particles are smaller in the at least one dimension than the skin depth, so that flux expulsion is substantially reduced.

The ferromagnetic annular rings621include ferromagnetic particles701that generally do not have a direction of elongation, and are thus generally smaller than the skin depth of the radio frequency B1magnetic field in the ferromagnetic material in all dimensions. The flux expulsion decreases as the size of the ferromagnetic particles701decreases. In one specific embodiment, the ferromagnetic particles701are generally smaller than about one-tenth of the skin depth of the radio frequency field. In another specific embodiment, the ferromagnetic particles701are generally smaller than about 10 microns, which corresponds to the skin depth of typical ferromagnetic materials at typical magnetic resonance frequencies. In yet another specific embodiment, the ferromagnetic particles701are generally smaller than about 4 microns, which corresponds to about one-third of the skin depth of typical ferromagnetic materials at typical magnetic resonance frequencies.

The fill factor of the ferromagnetic particles701dispersed in the binder72should be high enough to provide the desired magnetic field modification of the static B0magnetic field. In one embodiment, the fill factor is at least about 50% by volume. The fill factor of a specific embodiment determines the ferromagnetic properties of the annular rings621. The fill factor is, in turn, used in designing the magnetic field-modifying structure. The magnetic field-modifying structure601is designed to enhance the main B0magnetic field. This design can be performed concurrently with design of the main magnetic field coils20, for instance by a finite element model optimization incorporating both the magnetic field coils20and the magnetic field-modifying structure601. Alternatively or in addition, the structure601or portions thereof can be designed empirically, for example by empirical shimming of the manufactured magnet to correct the static B0magnetic field for manufacturing flaws. Regardless of how and when the design is performed, the design of the magnetic field modifying structure601incorporates the specific ferromagnetic properties of the ferromagnetic particles701dispersed in the binder72.

With reference toFIGS. 3A and 3B, in a second embodiment, the magnetic field-modifying structure602(where the subscript “2” on certain reference numbers ofFIGS. 3A and 3Bindicate components specific to the second described embodiment of the magnetic field-modifying structure60) has ferromagnetic annular rings622which are made up of elongated ferromagnetic particles702, such as rods, cigar-shaped particles, or wires of ferromagnetic material, dispersed in an insulating binder72. The elongated ferromagnetic particles702can be, for example, iron filings or iron whiskers. The magnetic field-modifying structure602ofFIGS. 3A and 3Bdiffers from the magnetic field-modifying structure601ofFIGS. 2A and 2Bin that the geometrically isotropic ferromagnetic particles701ofFIGS. 2A and 2Bhave been replaced by elongated ferromagnetic particles702shown inFIGS. 3A and 3B.

To substantially reduce flux expulsion of the radio frequency B1magnetic field from the ferromagnetic particles702, a cross-sectional dimension (for example, the wire diameter in the case of round wires) of the elongated ferromagnetic particles702are generally smaller than the skin depth of the radio frequency B1magnetic field in the ferromagnetic material. In one specific embodiment, the elongated ferromagnetic particles702have cross-sectional dimensions that are generally smaller than about one-tenth of the skin depth of the radio frequency field. In another specific embodiment, the cross-sections are generally smaller than about 10 microns. In yet another specific embodiment, the cross-sections of the particles702are generally smaller than about 4 microns. The fill factor of the ferromagnetic particles702dispersed in the binder72is at least about 50% by volume.

InFIGS. 3A and 3B, the elongated ferromagnetic particles702are shown substantially aligned with the tangential direction (designated by a curved arrow labeled “T” in the drawings). The tangential direction is spatially dependent. The tangential direction is everywhere transverse to the z-direction. The tangential direction is also at each point in space transverse to a radial direction parallel to the x-y plane and directed from the central axis22to that point in space.

The tangential alignment of the elongated ferromagnetic particles702can be achieved, for example, by dispersing the elongated ferromagnetic particles702in the binder with the binder in a liquid form, and applying an aligning magnetic field while the binder is cured or otherwise solidified. As will be discussed later, the tangential orientation of the elongated ferromagnetic particles702shown inFIGS. 3A and 3Bcan provide advantageous magnetic flux guiding. However, in other contemplated embodiments, the orientation of the elongated ferromagnetic particles702is substantially random.

Moreover,FIG. 3Aillustrates that the annular rings62can be discontinuous. For example,FIG. 3Ashows a gap66in the rings622. Although gaps such as the gap66can be included, for flux-guiding embodiments such gaps should be relatively few, and each gap should be narrow.

With reference toFIGS. 4A and 4B, in a third embodiment, the magnetic field-modifying structure603(where the subscript “3” on certain reference numbers ofFIGS. 4A and 4Bindicate components specific to the third described embodiment of the magnetic field-modifying structure60) has ferromagnetic annular rings623which are made up of generally planar ferromagnetic particles703, such as plates or disks of ferromagnetic material, dispersed in an insulating binder72. It will be appreciated that the magnetic field-modifying structure603ofFIGS. 4A and 4Bdiffers from the magnetic field-modifying structure601ofFIGS. 2A and 2Bin that the geometrically isotropic particles701ofFIGS. 2A and 2Bhave been replaced by the generally planar particles703shown inFIGS. 4A and 4B.

To substantially reduce flux expulsion of the radio frequency B1magnetic field from the generally planar ferromagnetic particles703, the thickness of the generally planar ferromagnetic particles703are generally smaller than the skin depth of the radio frequency B, magnetic field in the ferromagnetic material. In one specific embodiment, the generally planar ferromagnetic particles703have thicknesses that are generally less than about one-tenth of the skin depth of the radio frequency field. In another specific embodiment, the thicknesses are generally less than about 10 microns. In yet another specific embodiment, the thicknesses of the particles703are generally less than about 4 microns. The fill factor of the ferromagnetic particles702dispersed in the binder72is at least about 50% by volume.

InFIGS. 4A and 4B, the generally planar ferromagnetic particles703are shown with planar normals substantially aligned parallel with the z-direction which corresponds to the direction of the main B0magnetic field, and transverse to the tangential direction. Such alignment can be achieved, for example, by dispersing the generally planar ferromagnetic particles703in the binder with the binder in a liquid form, and applying an aligning magnetic field while the binder is cured or otherwise solidified. As will be discussed later, the orientation of the generally planar ferromagnetic particles703shown inFIGS. 4A and 4Bcan provide advantageous magnetic flux guiding. However, in other contemplated embodiments, the orientation of the generally planar ferromagnetic particles703is substantially random. Random orientation of the generally planar ferromagnetic particles703is particularly suitable when the cross-sectional area of the generally planar ferromagnetic particles703is relatively small.

With reference toFIGS. 5A and 5B, in a fourth embodiment, the magnetic field-modifying structure604(where the subscript “4” on certain reference numbers ofFIGS. 5A and 5Bindicate components specific to the fourth described embodiment of the magnetic field-modifying structure60) has ferromagnetic annular rings624which are made up of generally geometrically isotropic planar ferromagnetic particles704, such as particles similar to the particles701of the first embodiment ofFIGS. 2A and 2B, dispersed in an insulating binder72. The ferromagnetic annular rings624differ from the annular rings621of the first embodiment in that the annular rings624are disposed at about the same radial position (respective to the central axis22) as the rungs of the birdcage coil32. To accommodate the overlap between the annular rings624and the rungs of the birdcage coil32, the ferromagnetic annular rings624each include gaps68in which the rungs are disposed. In other words, the ferromagnetic material of the annular rings624is disposed between the rungs of the birdcage coil32. This arrangement enables the gap between the rungs of the birdcage coil32and the radio frequency screen64to be narrowed versus the first embodiment. Alternatively, continuous annular ring portions can extend around the rungs of the birdcage coil radially inward of the rungs, outward of the rungs, or both.

With reference toFIGS. 6A,6B, and6C, in a fifth embodiment, the magnetic field-modifying structure605(where the subscript “5” on certain reference numbers ofFIGS. 6A,6B, and6C indicate components specific to the fifth described embodiment of the magnetic field-modifying structure60) has ferromagnetic annular rings625which are made up of ferromagnetic material not dispersed as particles in a binder. For example, the ferromagnetic annular rings625may be solid or laminated iron rings, iron-alloy rings, or rings of another ferromagnetic material. The rings625preferably form complete circuits in the tangential direction; however, one or a few narrow gaps such as the gap66may be included.

The magnetic field-modifying structure605includes annular rings625that promote tangential flux guiding. The magnetic field Hobjin an ferromagnetic object responsive to an applied external field Hextis given by:
Hobj=Hext−NMsat(2),
where Msatis the saturation magnetization and N is the demagnetization factor. The term NMsatis called the demagnetization field, and for ferromagnetic materials is directed opposite to the applied external field Hext. The saturation magnetization Msatis a characteristic of the material, and the demagnetization factor N is a characteristic of the physical geometry of the object.

For example, a spherical object has an isotropic demagnetization factor N which is independent of direction. A wire- or rod-shaped object has a demagnetization factor component of about zero for applied external field directed parallel to the wire or rod, and a non-zero demagnetization factor component for applied external field directed parallel to the wire or rod. A generally planar object has a demagnetization factor component of about zero for in-plane directions, and a non-zero demagnetization factor component in the direction of the planar normal, that is, transverse to the plane. In general, the demagnetization factor N has larger components in directions of small spatial extent, and smaller components in directions of large spatial extent.

With particular reference toFIG. 6C, the ferromagnetic annular rings625have a thickness dzin the z-direction which is thin relative to the width drof the annular ring625in a radial direction transverse to the tangential direction. The extent of the annular ring625in the tangential direction for a continuous ring or for a ring with one or a few narrow gaps66is larger than either the thickness dzor the width dr. Because of the small thickness dzrelative to the extended nature of the annular rings625in the tangential direction, the demagnetization factor component Nzin the z-direction is substantially larger than the demagnetization factor component NTin the tangential direction. That is, Nz>>NT. This is indicated inFIG. 6Cby using a thin short arrow to designate NT and a long thick arrow to indicate Nz. For an isotropic saturation magnetization Msat, therefore, the magnetic flux in the annular rings625is preferentially guided in the tangential direction. In the z-direction, the relatively large Nzsubtracts from the flux producing reduced magnetic flux in the z-direction.

To provide a numerical example, for NT≅0 due to the large tangential extent, a relatively large demagnetization factor component Nz=0.5 in the z-direction due to the small thickness dz, and a ferromagnetic material having a saturation magnetization Msat=2 T/μo, applying Equation (2) in the z-direction gives:
Hobj,z=Hext,z−NzMsat=Hext,z−1 T/μo(3).
Applying Equation (2) in the tangential direction gives:
Hobj,T=Hext,T−NTMsat=Hext,T(4).
Equations (3) and (4) show that the z-component of the magnetic field in the annular rings625is suppressed by the subtractive factor 1 T/μo, whereas the tangential component of the magnetic field is not suppressed, producing preferential flux guiding in the tangential direction. If the result of Equation (3) drops below 1 T/μo, then the material is not in saturation in the z-direction.

The annular rings625of the flux-guiding fifth embodiment are made up of ferromagnetic material not dispersed as particles in a binder. In other contemplated flux-guiding embodiments, the ferromagnetic rings may be made of ferromagnetic particles dispersed in a binder. For example, the annular rings621,622,623of the first, second, and third embodiments, respectively, can provide flux guiding if the annular rings are made thin in the z-direction compared with the width of the rings in the radial direction. In contemplated embodiments, the rings are less than a few centimeters thick in the z-direction, and more preferably are a few millimeters thick in the z-direction, to provide a substantial demagnetization factor component in the z-direction.

The second embodiment602has elongated ferromagnetic particles702that are advantageously oriented to promote the tangential flux guiding. The elongated direction of the elongated ferromagnetic particles702is parallel to the tangential direction, which results in a small tangential demagnetization factor component. In the z-direction, the tangentially oriented elongated ferromagnetic particles702present a thin dimension which enhances the demagnetization factor component in the z-direction, thus suppressing the z-component of the magnetic field in the particles702. Thus, if the annular rings622are designed to be thin in the z-direction relative to a width of the rings in the radial direction, the annular rings622typically provide tangential flux guiding.

Similarly, the third embodiment603has generally planar ferromagnetic particles703that are advantageously oriented to promote the tangential flux guiding. The tangential direction lies in the plane of the generally planar ferromagnetic particles703, which results in a small tangential demagnetization factor component. The planar normal of the generally planar ferromagnetic particles703lies along the z-direction, so that the particles703are thin in the z-direction which enhances the demagnetization factor component in the z-direction, thus suppressing the z-component of the magnetic field in the particles703. Thus, if the annular rings623are designed to be thin in the z-direction relative to a width of the rings in the radial direction, the annular rings623typically provide tangential flux guiding.

The annular rings624of the fourth embodiment generally provide limited tangential flux guiding since the rings are broken by the gaps68. If the segments of the annular rings624between the gaps68are extended in the tangential and radial directions compared with the thickness of the annular rings624in the z-direction, some tangential flux guiding may be achieved.

Embodiments of the magnetic field-modifying structure60which promote tangential flux guiding of the radio frequency B1field will also produce some preferential tangential flux guiding of the magnetic field gradients produced by the magnetic field gradient coils30. Since the magnetic field gradients are imposed on the main B0magnetic field which is directed in the z-direction, the magnetic field gradients typically have small or non-existent components in the tangential direction at the position of the magnetic field-modifying structure60. Moreover, tangential flux guiding of the gradient fields can be further suppressed by including the magnetic field-modifying structure60in the design of the gradient coils30. For example, the magnetic field-modifying structure60can be incorporated into a finite element model optimization of the gradient coils geometry.

Although the example magnetic field-modifying structures60have been described with reference to a horizontal closed cylindrical magnet20, the described embodiments are readily adapted to other magnetic resonance imaging scanners such as vertical magnet scanners, asymmetric scanners, open scanner geometries, and the like.