JPH09238919A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable properly shielding of a high frequency magnetic field between an inclined magnetic field coil and an RF coil by arranging a ferromagnetic body between the inclined magnetic field coil and the RF coil and a shielding means to shield the high frequency magnetic field. SOLUTION: A magnetostatic field magnet 1 is driven by a magnetostatic field controller 4 and a transmitting/receiving coil 3 is driven by a transmitter 5 in exciting of a magnetic resonance while being coupled to receiver 6 in the detection of a magnetic resonance signal. An X axis/Y axis/Z axis inclined magnetic field coil 2 is driven by X, Y and Z axis inclined magnetic field power sources 7, 8 and 9 and the X, Y and Z axis inclined magnetic field power sources 7, 8 and 9 and the transmitter 5 are driven by a sequencer 10 according to a specified sequence to generate X, Y and Z axis inclined magnetic fields and a high frequency (RF) pulse according to a specified pulse sequence. At this point, a cylindrical RF shield 40 is provided between the inclined magnetic field coil 2 and the RF coil 3 and has a ferromagnetic layer comprising thin soft iron stuck on a thin insulation base material.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging apparatus (hereinafter sometimes abbreviated as MRI apparatus).

[0002]

2. Description of the Related Art Conventionally, in this type of MRI apparatus, a so-called RF shield for shielding a high frequency magnetic field is generally provided between the gradient magnetic field coil and the RF coil. Various types of RF shields are already known as a conventional technique, and a typical arrangement structure thereof is, for example, FIG.
become that way. This is a view assuming an MRI apparatus using a superconducting magnet (not shown) as a static magnetic field generation source. In this case, the gradient coil 2 is a cylindrical assembly and is arranged inside the superconducting magnet. Although not shown, the gradient magnetic field coil 2 is composed of a multi-layer winding and a resin impregnating the winding.

The RF shield 25 is attached inside the gradient magnetic field coil 2, and an RF coil (not shown) is arranged inside the RF shield 25. The RF coil is for generating a high-frequency RF excitation magnetic field, transmitting the RF excitation magnetic field to the subject, and receiving an MR signal from the subject. Such an RF coil has an R
There are one in which the F coil and the RF coil dedicated to reception are configured as separate bodies, or one in which these are integrated.

As a concrete structure of the RF shield 25, as shown in FIG. 9, a layer 30 of an insulating base material is adhered to the layer 30 of the insulating base material via an adhesive layer 31, which is typically And a copper layer 32 having a thickness on the order of 35 microns. The thickness of this copper layer 32 depends on the RF used.
It is generally selected to be several times the skin depth at the frequency of the excitation magnetic field. If this thickness is too thin, the performance as an RF shield cannot be obtained.

FIG. 10 is a view showing the appearance of the RF shield 25. As shown in the figure, the RF shield 25 has a seam 251 due to its structure. This seam 25
As shown in FIG. 11, a copper foil tape 35 is soldered to 1 so that the RF coil is completely isolated from the outside of the RF shield 25.

At the earliest stage, the RF shield 25 as described above was attached to the inner peripheral surface of the gradient magnetic field coil 2 as shown in FIG. By the way, with the high-speed switching of the gradient magnetic field by the gradient magnetic field coil 2,
There is a problem that an eddy current flows through the RF shield layer, and the magnetic field created by this eddy current disturbs the gradient magnetic field, although temporarily. Since the RF shield layer is a good conductor and has a certain thickness, the eddy current formed in the shield layer is not immediately attenuated. Therefore, the conventional RF shield has a problem in that it is not suitable for the MRI imaging method in which the gradient magnetic field is switched rapidly.

In order to avoid such a problem of eddy current, it is known that the RF shield 26 having a cylindrical shape has a slit 36 on the peripheral surface as shown in FIG. According to this, the eddy current due to the gradient magnetic field does not flow across the slit 36 and can be relatively rapidly attenuated. The slit 36 has an overlapping structure as shown in FIG. 13 (a diagram in which the cylindrical RF shield layer 26 is developed) so as to be short-circuited at the frequency of the RF excitation magnetic field. In this case, the copper layers sandwiching the insulating base material form a capacitor, and are short-circuited at a high frequency. Alternatively, as shown in FIG. 14, a large number of discrete capacitors 27 may be arranged across the slit.

However, even the RF shield having such a slit has the following problems.
So-called echo planar imaging, EPI (Ech
High-speed imaging methods such as o Planar Imaging) are needed for imaging the heart, for example, but extremely rapid gradient magnetic field response is essential. For this reason, it becomes necessary to provide a large number of slits at fine intervals (intervals). However, when a large number of slits are provided, there is a problem in that high-frequency short circuits in individual slits are incomplete, and the slits do not function perfectly, that is, high-frequency magnetic field interruption is incomplete. The inventors have
As a result, external noise is mixed into the RF coil arranged inside the shield, RF pulses are radiated to the outside of the shield, and high-frequency loss in the slit portion becomes remarkable and the performance of the RF coil deteriorates. Are experiencing problems such as.

Further, as a conventional example of an RF shield which is further made into a slit, Roemer, P., Edelstein, W. et al., US Pat. No. 4,879,515 "Double-Sided RF Shi"
eldfor RF Coil Contained within Gradient Coils of
NMR Imaging Device ". Also in the RF shield described in this specification, again, it is necessary to provide a large number of slits with a large number of fine steps in order to respond to extremely rapid switching of the gradient magnetic field. The interruption of the high-frequency magnetic field is incomplete, and the above problems cannot be solved.

As another conventional example of the RF shield having no slit as described above, there are Frederick, P., and Roeme.
r, P. et al., "An RF Shield Design for NMR High-Spe"
ed and Echo Planar Imaging ", Proceedings of the SMR
1994, p.1094. This has a pattern of a good conductor (eg copper) that provides a suitable RF shield for the RF coil of a particular embodiment, and almost no eddy currents in the gradient field flow. Therefore, it responds well to high-speed switching control of the gradient magnetic field. However, R other than that particular RF coil
The F coil has a problem that it does not function as an RF shield at all. As a result, when an RF coil other than the specific RF coil is used, the RF coil picks up external noise and the assembly of the gradient magnetic field coil outside the RF shield is not isolated from the RF coil. There is a problem that the performance of the RF coil deteriorates.

[0011]

SUMMARY OF THE INVENTION The present invention has been made in order to cope with the above-mentioned circumstances, and rapidly attenuates an eddy current due to switching of a gradient magnetic field to such an extent that it can be applied to a high-speed imaging method such as EPI, Also, a specific aspect of RF
It is not necessary to use a pattern of a good conductor which is an appropriate shield only for the coil, and can be applied to any RF coil, and it is possible to appropriately shield the high frequency magnetic field between the gradient magnetic field coil and the RF coil. An object is to provide a magnetic resonance imaging apparatus.

[0012]

A magnetic resonance imaging apparatus according to the present invention comprises a ferromagnetic material provided between a gradient magnetic field coil and an RF coil, and is provided with a shield means for shielding a high frequency magnetic field. To do.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION A first embodiment of a magnetic resonance imaging apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of the first embodiment. Inside the gantry 20, a static magnetic field magnet 1, an X-axis / Y-axis / Z-axis gradient magnetic field coil 2, and a transmission / reception coil (RF coil) 3 are provided.

The static magnetic field magnet 1 as the static magnetic field generator is
For example, a superconducting magnet is used. X axis, Y axis, Z
The axial gradient magnetic field coil 2 is arranged inside the static magnetic field magnet 1 to form a cylindrical assembly, and has X-axis gradient magnetic fields Gx and Y.
The axis gradient magnetic field Gy and the Z axis gradient magnetic field Gz are generated. The transmission / reception coil 3 is arranged inside the gradient magnetic field coil 2, has a cylindrical shape, generates a radio frequency (RF) pulse as a selective excitation pulse for selecting a slice, and generates a magnetic resonance signal by magnetic resonance. (MR signal) is detected. The transmission / reception coil 3 is not embedded in the gantry but is arranged near the subject or directly attached to the subject. Also, separate coils dedicated to transmission and reception may be used instead of the transmission and reception coils.

The subject P placed on the tabletop of the bed 13 is an imageable region in the gantry 20 (a spherical region where an imaging magnetic field is formed, and diagnosis is possible only in this region). Inserted in.

The static magnetic field magnet 1 is driven by the static magnetic field controller 4. The transmission / reception coil 3 is driven by the transmitter 5 when magnetic resonance is excited, and is coupled to the receiver 6 when detecting a magnetic resonance signal. The X-axis / Y-axis / Z-axis gradient magnetic field coil 2 is driven by an X-axis gradient magnetic field power source 7, a Y-axis gradient magnetic field power source 8 and a Z-axis gradient magnetic field power source 9.

The X-axis gradient magnetic field power supply 7, the Y-axis gradient magnetic field power supply 8, the Z-axis gradient magnetic field power supply 9 and the transmitter 5 are driven by a sequencer 10 according to a predetermined sequence, and an X-axis gradient magnetic field Gx, a Y-axis gradient magnetic field Gy, A Z-axis gradient magnetic field Gz and a radio frequency (RF) pulse are generated in a predetermined pulse sequence described later. In this case, the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy, and the Z-axis gradient magnetic field Gz are mainly used as the phase encoding gradient magnetic field Ge, the reading gradient magnetic field Gr, and the slice gradient magnetic field Gs, respectively. The computer system 11 drives and controls the sequencer 10, captures the magnetic resonance signal received by the receiver 6 and performs predetermined signal processing to generate a tomographic image of the subject, and displays it on the display unit 12.

FIG. 2 is a view showing a sectional structure of the RF shield of the magnetic resonance imaging apparatus according to the first embodiment of the present invention. The RF shield 40 has a cylindrical shape and is provided between the gradient magnetic field coil 2 and the RF coil 3. The RF shield 40 is formed by attaching a ferromagnetic layer 33 made of soft iron having a thickness of about 10 microns to a thin insulating substrate 30 made of polyester and having a thickness of about 10 microns via an adhesive layer 31. In order to prevent rust from being generated on the surface of the body layer 33, the insulating base material 30 is attached again via the adhesive layer 31. The RF shield 40 is provided by being attached to the inner peripheral surface of the gradient magnetic field coil 2 as shown in FIG. Also in the RF shield of this embodiment, one seam is formed as shown in FIG. Therefore, as in the case of FIG. 11 described above, a copper foil tape is soldered to the joint portion. The layer of the insulating base material is peeled off only in the portion where the copper foil tape is soldered. If it can be manufactured, it is desirable that it is seamless (seamless).

The RF shield of this embodiment is constructed as described in more detail below. That is,
The ferromagnetic layer 33 made of soft iron, which is provided as an RF shield material for shielding a high frequency magnetic field between the gradient magnetic field coil 2 and the RF coil 3, has a thickness (thickness of the RF shield layer) of 30 μm or less. Is configured to be.

The magnetic resonance frequency of protons in a static magnetic field of 1.5 Tesla is about 64 MHz. At this frequency, the copper skin depth is less than 10 microns. As described above, the thickness of the RF shield layer is generally several times the skin depth. Therefore, in the conventional example in which copper is used as the RF shield material, the thickness is several times the skin depth (here, It is necessary to set it to about 30 microns (about 3 times). However, the electric conductivity of the soft iron used in this embodiment is about 1/7 that of copper, and the relative magnetic permeability is approximately 5%.
It is about 00. As is well known, the skin depth is inversely proportional to the square root of the product of conductivity and relative permeability. According to this, the skin depth of soft iron is about 1 / 8.5 of copper (about 1.17 microns). Therefore, if the thickness of the RF shield layer of this embodiment is 3.5 μm, which is several times the skin depth, a sufficient RF shield function can be achieved. In reality, this thinness is difficult to manufacture, so in this embodiment, the thickness of the RF shield layer is larger than the skin depth value and does not exceed 10 times the skin depth (11.76). That is, about 10 microns.

Such a soft iron RF shield is sufficiently thin for the frequency component of the gradient magnetic field. here,
Looking at the eddy currents associated with the switching of the gradient magnetic field,
The decay time constant is proportional to the thickness and conductivity of the conductive layer. Therefore, in the case of the soft iron layer, the time constant of decay of the eddy current is (10/30) × compared with the case of copper having a thickness of 30 μm.
(1/7) = 1/21, which is short. That is, the eddy current generated in the RF shield of soft iron is attenuated 21 times faster than that of the RF shield of copper. As a result, there is no need to provide a slit even in the case where the gradient magnetic field is controlled much faster than a normal RF shield. If the slits are not used, it is possible to provide a complete shield environment for the RF coil without causing the above-described problem of high frequency magnetic field interruption in the slit portion. Incidentally, when the static magnetic field strength becomes lower, the frequency of the RF excitation magnetic field lowers accordingly, and the skin depth increases, but the soft iron layer of 10 microns still functions as a sufficiently thick RF shield.

Regarding soft iron, which is a ferromagnetic material, installed inside the static magnetic field magnet 1 of the MRI apparatus, since the magnetic saturation of soft iron is about 2 tesla, MRI with a static magnetic field of 1.5 tesla is used. It is fully applicable to devices. Further, although soft iron disturbs the uniformity of the static magnetic field, in the RF shield of this embodiment, a ferromagnetic material made of soft iron is thinly arranged in a cylindrical shape, and the ferromagnetic material is locally concentrated. Therefore, the static magnetic field is disturbed in a small manner, and even if it is disturbed to some extent, it is a symmetry-rich disturbance, so that the adjustment of the magnetic field uniformity correction is easy.

Next, various modifications (1) to (8) of the RF shield of this embodiment will be described. (1) As shown in FIG. 3, one surface of the ferromagnetic layer 33 may be covered with a surface protective conductive layer 34 made of a metal other than soft iron. This also prevents the rust of soft iron. Specifically, for example, nickel may be plated. Nickel is also a magnetic substance like soft iron, but since it is magnetically saturated in a high magnetic field of 1.5 Tesla, in this case, the effect of thinning the skin depth by using the magnetic substance as described above is not effective. I can't expect. Therefore, it is desirable that the nickel-plated layer be as thin as possible and have a thickness of about several microns. In a low magnetic field, magnetic saturation does not occur, so even if the nickel plating layer itself is thin like soft iron, it contributes as an effective RF shield layer. In this case, it is desirable to reduce the thickness of the soft iron layer by the effect of the nickel layer in order to reduce the eddy current due to the gradient magnetic field switching.

Of course, a non-magnetic metal may be used for coating. In this case, in order to reduce the resistance loss of the RF induced current flowing in the surface protective conductive layer 34, it is desirable to use a metal having a resistivity as low as possible and to make the thickness as thin as possible. (2) As shown in FIG. 4, both sides of the ferromagnetic layer 33 may be coated with a metal other than soft iron. Also in this case, as in the modified example (1), a thin layer of a good conductor of a non-magnetic material, or
When used as a magnetic substance in a region where magnetic saturation does not occur, it is desirable to make the soft iron layer thin accordingly, or when used as a magnetic substance in a region where magnetic saturation occurs, it is desirable to make the layer as thin as possible. (3) When the RF shield according to the present embodiment is attached to the inside of the gradient magnetic field coil 2, slits as shown in FIGS. 12 to 14 may be provided to reduce the eddy current more efficiently. . Similar to the case of copper, the high-frequency connection (short circuit) between the slits can be made by the overlapping arrangement as shown in FIG. 13 also in the RF shield layer made of a magnetic material. When the insulating base material is not provided as in the modified example (2), it is necessary to interpose a thin insulating layer in order to form a capacitor in the overlapping portion. (4) Although the shape of the RF shield of this embodiment is cylindrical, the shape is not limited to this and may be non-cylindrical. (5) As the ferromagnetic material included in the RF shield of this embodiment, soft iron has been described as an example, but nickel or cobalt may be used. Alternatively, it may be another magnetic alloy. These are RF due to resistance loss of the ferromagnetic material.
In order to prevent the performance of the coil from deteriorating, one having a high conductivity is preferable. In terms of this high conductivity, cobalt is preferable although it is expensive. As long as the magnetic material does not rust or corrode, it is not necessary to cover the surface as shown in FIGS. 2 to 4 with another material (insulating base material 30 or surface protective conductive layer 33). (6) As a ferromagnetic material included in the RF shield of the present embodiment, a material having a large hysteresis loss (for example, a hard magnetic material) is not suitable because the high frequency loss of the RF coil is large. It is possible to suppress the eddy current and to obtain high RF shield performance. (7) The RF shield of this embodiment may be composed of fibers and a magnetic material coating the fibers. Alternatively, a wire containing a magnetic material may be woven.

According to this embodiment as described above,
Since the gradient magnetic field can be controlled at high speed, high-speed image capturing becomes possible and patient throughput is improved. Further, the RF shield of the present embodiment has a small loss, so that the S / N of the image is improved, and since the small field of view can be taken by the strong gradient magnetic field, the resolution of the image is improved. Furthermore, it is possible to capture an easy-to-see image in which the artifacts due to the eddy current are reduced, and the diagnostic ability is improved.

Next, another embodiment of the present invention will be described.
Here, a more specific configuration regarding the ferromagnetic material described in the above embodiment will be described. Depending on the material, the ferromagnetic substance is magnetically saturated at a considerably low magnetic field, so that the selection of the material is limited depending on the magnetic field strength assumed by the MRI apparatus. For example, cobalt is a material suitable as an RF shield material because of its excellent conductivity, but it is not suitable for a high magnetic field MRI apparatus because it is magnetically saturated in a magnetic field of relatively low strength. In addition, among various magnetic materials, there are cases in which they cannot be used as an RF shield material due to magnetic saturation even though they have preferable characteristics.

In addition, the hysteresis loss of the selected ferromagnetic material may not be the minimum. It is impossible to know where to use the minimum hysteresis loss in the magnetization curve unless the characteristics are actually measured. Even if the actual measurement is performed, once the magnetic field strength and the material are determined, there is no further choice.

FIG. 5 is a view showing an example of a ferromagnetic sheet forming the RF shield of this embodiment. The ferromagnetic sheet 40 forming the RF shield has magnetic anisotropy when viewed macroscopically, and the direction of the hard axis thereof is substantially equal to the static magnetic field direction B0 as shown in FIG. Has been done.

In general, a ferromagnetic crystal has magnetic anisotropy. FIG. 6 is a graph showing magnetization characteristics. When exposed to a magnetic field in which magnetization is easy, as shown by the magnetization curve A, magnetic saturation easily occurs even in a weak magnetic field. Further, when exposed to a magnetic field in a direction in which magnetization is difficult, as shown by the magnetization curve B, magnetic saturation does not occur easily even in a considerably strong magnetic field.
Furthermore, a material in which crystals are aggregated at a high density does not necessarily have magnetic anisotropy macroscopically and has an intermediate magnetization characteristic as shown by a magnetization curve C.

It is well known that heat treatment in a magnetic field is sufficient to give a magnetic material macroscopic magnetic anisotropy. When the sheets of ferromagnetic material having magnetic anisotropy are arranged and used as shown in FIG. 5, magnetic saturation does not occur easily even in a high magnetic field. Therefore, a considerably high magnetic permeability can be maintained. That is, it is possible to obtain a thin RF shield having a skin depth even in a high magnetic field.
By the way, when magnetically saturated, the relative permeability becomes very low,
It becomes a value close to 1 and no longer has a property as a ferromagnetic material.

FIG. 7 is a diagram showing another example of the ferromagnetic sheet forming the RF shield of this embodiment. The ferromagnetic sheet 40 forming the RF shield has magnetic anisotropy when viewed macroscopically, and the direction of the easy axis of magnetization thereof is substantially equal to the static magnetic field direction B0 as shown in FIG. Has been done.

Where in the magnetization curve is the loss due to the RF shield (for example, hysteresis loss mentioned above) the minimum?
The relationship between the orientation of the ferromagnetic sheet and the orientation of the static magnetic field is examined, and if the state in which the easy axis of magnetization is aligned with the direction of the static magnetic field is good, for example, a low-field MRI equipped with a normal conducting magnet.
As long as the device is assumed and there is a sufficient margin for magnetic saturation, the configuration as in this example may be adopted.

[0033]

As described above, according to the present invention, the eddy current due to the switching of the gradient magnetic field is rapidly attenuated to the extent that it can be applied to a high-speed imaging method such as EPI, and
It does not use a pattern of a good conductor that is an appropriate shield only for the RF coil of a specific mode, and can be applied to any RF coil.
A magnetic resonance imaging apparatus capable of appropriately shielding a high frequency magnetic field with a coil can be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of a magnetic resonance imaging apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing a sectional structure of an RF shield according to the embodiment.

FIG. 3 is a diagram showing another configuration example of the RF shield according to the embodiment.

FIG. 4 is a diagram showing still another configuration example of the RF shield according to the embodiment.

FIG. 5 is a diagram showing an example of a ferromagnetic sheet forming an RF shield according to another embodiment of the present invention.

FIG. 6 is a graph showing magnetization characteristics according to the above embodiment.

FIG. 7 is a diagram showing another example of a ferromagnetic sheet forming an RF shield according to the above embodiment.

FIG. 8: RF of a magnetic resonance imaging apparatus according to a conventional example
The figure which shows the typical arrangement structure of a shield.

FIG. 9: RF of a magnetic resonance imaging apparatus according to a conventional example
Sectional drawing which shows a shield structure.

FIG. 10 shows R of a magnetic resonance imaging apparatus according to a conventional example.
The figure which shows the external appearance of the F shield 25.

FIG. 11 shows R of a magnetic resonance imaging apparatus according to a conventional example.
Sectional drawing which shows the structure of the F shield 25.

FIG. 12 shows R of a magnetic resonance imaging apparatus according to a conventional example.
The figure which shows the slit of the peripheral surface of the F shield 26.

FIG. 13 shows R of a magnetic resonance imaging apparatus according to a conventional example.
The figure which expanded the F shield layer 26.

FIG. 14 shows R of a magnetic resonance imaging apparatus according to a conventional example.
FIG. 6 is a developed view of the F shield layer 26, showing the arrangement of discrete capacitors 27.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Static magnetic field magnet, 2 ... X-axis / Y-axis / Z-axis gradient magnetic field coil, 3 ... Transmission / reception coil (RF coil), 40 ... RF shield, 4 ... Static magnetic field control device, 5 ... Transmitter, 6 ... Receiver , 7 ... X-axis gradient magnetic field amplifier, 8 ... Y-axis gradient magnetic field amplifier, 9 ... Z-axis gradient magnetic field amplifier, 10 ... Sequencer, 11 ... Computer system, 12 ... Display unit.

Claims (11)

[Claims] 1. A magnetic resonance imaging apparatus comprising a shield member made of a ferromagnetic material provided between a gradient magnetic field coil and an RF coil and shielding a high frequency magnetic field. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the shield means is made of the ferromagnetic material layer and has a thickness of 30 μm or less. 3. The shield means is made of a layer of the ferromagnetic material, and has a thickness of the RF of the RF coil.
The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is larger than the skin depth of the ferromagnetic material at an excitation frequency and does not exceed 10 times the skin depth. 4. The magnetic resonance imaging apparatus according to claim 1, wherein the ferromagnetic material has a conductivity equal to or higher than that of soft iron. 5. The magnetic resonance imaging apparatus according to claim 1, wherein the ferromagnetic body is made of soft iron, nickel, cobalt, or a magnetic alloy. 6. The magnetic resonance imaging apparatus according to claim 1, wherein the shield means further includes a coating member that prevents the surface of the ferromagnetic material from rusting. 7. The magnetic resonance imaging apparatus according to claim 6, wherein the covering member is made of an insulating member or a non-magnetic material having a conductivity equal to or higher than that of soft iron. 8. The magnetic resonance imaging apparatus according to claim 6, wherein the coating member is made of an insulating member or a magnetic material having a conductivity equal to or higher than that of soft iron. 9. The magnetic resonance imaging apparatus according to claim 1, wherein the shield means has a cylindrical shape. 10. The ferromagnetic material according to claim 1, wherein the ferromagnetic material has magnetic anisotropy such that the direction of hard magnetization is substantially equal to the direction of the magnetic field applied to the ferromagnetic material. Magnetic resonance imaging system. 11. The ferromagnetic material according to claim 1, wherein the ferromagnetic material has magnetic anisotropy such that a direction of easy magnetization is substantially equal to a direction of a magnetic field applied to the ferromagnetic material. Magnetic resonance imaging system.