Magnetic field generating apparatus and MRI apparatus

To achieve a good balance among the magnetic field intensity, region of homogeneous magnetic field intensity and cost, a magnetic field generating apparatus for generating a magnetic field in a vertical direction comprises: a pair of magnets having respective magnetic poles with mutually opposite polarities disposed to face each other in a vertical direction, for generating a static magnetic field having different dimensions of its region of homogeneous magnetic field intensity in two mutually orthogonal directions in a horizontal plane; a pair of planar magnetic elements, each element supporting respective one of the pair of the magnets; and a vertical columnar magnetic element lying in a direction different from the two directions, each end of which is joined to respective one of the pair of the planar magnetic elements.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No. 200610160589.7 filed Oct. 13, 2006.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic field generating apparatus and an MRI (Magnetic Resonance Imaging) apparatus, and particularly to a magnetic field generating apparatus for generating a magnetic field in a vertical direction, and an MRI apparatus using the magnetic field in the vertical direction.

An MRI apparatus is configured to reconstruct an image based on magnetic resonance signals collected by applying a static magnetic field, a gradient magnetic field, and an RF (radio frequency) magnetic field to a subject. In an MRI apparatus of a vertical magnetic field type, a pair of magnets disposed to face each other in a vertical direction generate a static magnetic field. The static magnetic field is generated such that its region of homogeneous magnetic field intensity has a spherical shape (see Patent Document 1, for example).

[Patent Document 1] Japanese Patent Application Laid Open No. 2001-070284.

Although a static magnetic field desirably has magnetic field intensity as high as possible and a region of homogeneous magnetic field intensity as large as possible, a magnet whose magnetic pole has a large surface area and a commensurately large magnetic circuit are required accordingly, unavoidably leading to increased cost.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a magnetic field generating apparatus capable of achieving a good balance among the magnetic field intensity, region of homogeneous magnetic field intensity and cost, and also provide an MRI apparatus comprising such a magnetic field generating apparatus.

In one aspect of the invention for solving the problem, there is provided a magnetic field generating apparatus for generating a magnetic field in a vertical direction, comprising: a pair of magnets having respective magnetic poles with mutually opposite polarities disposed to face each other in a vertical direction, for generating a static magnetic field having different dimensions of its region of homogeneous magnetic Field intensity in two mutually orthogonal directions in a horizontal plane; a pair of planar magnetic elements, each element supporting respective one of the pair of the magnets; and a vertical columnar magnetic element lying in a direction different from the two directions, each end of which is joined to respective one of the pair of the planar magnetic elements.

In another aspect of the invention for solving the problem, there is provided an MRI apparatus for reconstructing an image based on magnetic resonance signals collected by applying a static magnetic field, a gradient magnetic field, and an RF magnetic field to a subject, comprising: a pair of magnets having respective magnetic poles with mutually opposite polarities disposed to face each other in a vertical direction, for generating a static magnetic field having different dimensions of its region of homogeneous magnetic field intensity in two orthogonal directions in a horizontal plane; a pair of planar magnetic elements, each element supporting respective one of the pair of the magnets; a vertical columnar magnetic element lying in a direction different from the two directions, each end of which is joined to respective one of the pair of the planar magnetic elements; a cradle movable in one axis direction with the subject laid thereon; and a table for supporting the cradle, capable of aligning the direction of its movement axis with any of the two directions.

Preferably, the region of homogeneous magnetic field intensity has a horizontal cross section of an elliptical shape, in that a good balance is obtained between the magnetic field intensity and region of homogeneous magnetic field intensity.

Preferably, a magnetic pole surface of each one of the pair of the magnets has different dimensions in the two directions, in that a magnetic field is provided with a good balance between the magnetic field intensity and region of homogeneous magnetic field intensity.

Preferably, the magnetic pole surfaces of the pair of the magnets have an elliptical shape, in that the region of homogeneous magnetic field intensity has a horizontal cross section of an elliptical shape.

Preferably, the magnetic pole surfaces of the pair of the magnets have a rectangular shape, in that the region of homogeneous magnetic field intensity has a horizontal cross section of an elliptical shape.

Preferably, the number of the columnar magnetic elements is two, in that good support for the planar magnetic elements is obtained.

According to the invention, a magnetic field generating apparatus for generating a magnetic field in a vertical direction comprises a pair of magnets for generating a static magnetic field having different dimensions of its region of homogeneous magnetic field intensity in two mutually orthogonal directions in a horizontal plane, a pair of planar magnetic elements, each element supporting respective one of the pair of the magnets, and a vertical columnar magnetic element lying in a direction different from the two directions, each end of which is joined to respective one of the pair of the planar magnetic elements; thus, there are provided a magnetic field generating apparatus capable of achieving a good balance among the magnetic field intensity, region of homogeneous magnetic field intensity and cost, and an MRI apparatus comprising such a magnetic field generating apparatus.

DETAILED DESCRIPTION OF THE INVENTION

Now the best mode for carrying out the invention will be described in detail with reference to the accompanying drawings. It should be noted that the invention is not limited to the best mode for carrying out the invention.FIG. 1shows a block diagram of an MRI apparatus. The apparatus is an example of the best mode for carrying out the invention. The configuration of the apparatus represents an example of the best mode for carrying out the invention of the MRI apparatus.

As shown inFIG. 1, the present apparatus comprises a magnetic field generating apparatus100. The magnetic field generating apparatus100comprises a main magnetic field magnet section102, a gradient coil section106and an RF coil section108.

The main magnetic field magnet section102, gradient coil section106and RF coil section108each comprise a pair of members disposed to face each other across a space between them. They have a generally planar shape and disposed to have a common center axis.

An internal space (bore) of the magnetic field generating apparatus100receives a subject1laid on a table500and carried into/out of the bore. The table500is driven by a table drive section120.

The main magnetic field magnet section102generates a static magnetic field in the internal space of the magnetic field generating apparatus100. The direction of the static magnetic field is generally orthogonal to the body axis direction of the subject1. That is, what is generally called a “vertical magnetic field” is generated. The main magnetic Field magnet section102is made using a magnet. The main magnetic field magnet section102will be sometimes referred to as magnet102hereinbelow.

The gradient coil section106generates three gradient magnetic fields for creating gradients in the static magnetic field intensity in directions along three mutually orthogonal axes, i.e., a slice axis, a phase axis and a frequency axis. The gradient coil section106comprises three gradient coils (not shown) corresponding to the three gradient magnetic fields.

The RF coil section108transmits an RF pulse (radio frequency pulse) to the static magnetic field space for exciting spins within the subject1. The RF coil section108also receives magnetic resonance signals generated by the excited spins. The RF coil section108may comprises a single coil or separate coils for transmission and reception.

The gradient coil section106is connected with a gradient drive section130. The gradient drive section130supplies a drive signal to the gradient coil section106to generate the gradient magnetic fields. The gradient drive section130comprises three drive circuits (not shown) corresponding to the three gradient coils in the gradient coil section106.

The RF coil section108is connected with an RF drive section140. The RF drive section140supplies a drive signal to the RF coil section108to transmit an RF pulse for exciting spins within the subject1.

The RF coil section108is also connected with a data collecting section150. The data collecting section150takes in signals received by the RF coil section108by sampling them, and collects them as digital data.

The table drive section120, gradient drive section130, RF drive section140and data collecting section150are connected with a control section160. The control section160controls these sections120,130,140and150to carry out imaging.

The control section160is made up using a computer, for example. The control section160comprises a memory. The memory stores a program and several kinds of data for use by the control section160. The function of the control section160is implemented by the computer executing the program stored in the memory.

The data collecting section150is connected on its output side to a data processing section170. Data collected by the data collecting section150are input to the data processing section170. The data processing section170is made up using a computer, for example. The data processing section170comprises a memory. The memory stores a program and several kinds of data for use by the data processing section170.

The data processing section170is connected to the control section160. The data processing section170is above the control section160and controls it. The function of the present apparatus is implemented by the data processing section170executing the program stored in the memory.

The data processing section170stores the data collected by the data collecting section150into the memory. A data space is established in the memory. The data space constitutes a Fourier space. The Fourier space is sometimes referred to as k-space. The data processing section170performs inverse Fourier transformation on the data in the k-space to reconstruct an image of the subject1.

The data processing section170is connected with a display section180and an operating section190. The display section180comprises a graphic display, etc. The operating section190comprises a keyboard provided with a pointing device, etc.

The display section180displays a reconstructed image output from the data processing section170, and several kinds of information. The operating section190is operated by a user to input several kinds of commands, information and so forth to the data processing section170. The user is capable of interactively operating the present apparatus via the display section180and operating section190.

FIG. 2schematically shows the configuration of an example of the magnetic field generating apparatus100. InFIG. 2, (a) represents a front elevation view, and (b) represents a cross-sectional view taken along a line A-A. The magnetic field generating apparatus100is an example of the best mode for carrying out the invention. The configuration of the magnetic field generating apparatus100represents an example of the best mode for carrying out the invention of the magnetic field generating apparatus.

As shown inFIG. 2, the magnetic field generating apparatus100comprises a pair of magnets102supported by a yoke200. The magnet102is an example of the magnet of the invention. The magnet102has a magnetic pole surface of a generally elliptical shape.

The yoke200is provided for forming a return path for the pair of magnets102. The yoke200is formed generally in a C shape of a pair of planar magnetic elements202for supporting the pair of magnets, and a vertical columnar magnetic element204with each end joined to respective one of the pair of the planar magnetic elements202, positioned in an obliquely rear direction. The planar magnetic elements202and columnar magnetic element204are made of a ferromagnetic material such as soft iron, for example.

The pair of magnets102are supported by the pair of planar magnetic elements202in parallel and concentrically such that their respective magnetic poles with mutually opposite polarities are disposed to face each other at a predetermined distance. A vertical magnetic field is thus generated between the magnetic poles. In the following description, the direction of the magnetic field is defined as Z direction. The direction from the front of the magnetic field generating apparatus100is defined as Y direction, and a lateral direction is defined as X direction.

The vertical magnetic field has a region of homogeneous magnetic field intensity300. The region of homogeneous magnetic field intensity300will be sometimes referred to simply as homogeneous region300hereinbelow. The shape of the homogeneous region300is ellipsoidal. That is, the dimensions of the homogeneous region300are different in the three directions X, Y and Z.

As shown inFIG. 3, radii of the ellipsoid in the three directions are represented as Rz, Ry and Rz, and then, these values are all different and an inequality is represented as Rx>Rz>Ry. Actual values are, for example, Rx=20 cm, Rz=15 cm, and Ry=10 cm.

The magnetic field having such an ellipsoidal homogeneous region300can be generated by a magnet with a smaller magnetic pole surface area than that for a spherical homogeneous region with radii Rx, Ry and Rz in three directions of 20 cm. The yoke can also be made smaller commensurately with the smaller size of the magnet. Thus, the magnetic field generating apparatus100can be manufactured at low cost. Alternatively, if a cost as much as that for the conventional magnetic Field generating apparatus is affordable, a magnetic field generating apparatus with higher magnetic field intensity can be manufactured using a stronger magnet.

50% or more of imaging procedures by MRI are applied to the head, 20-30% to the spine, 10% to the extremities, and the balance to the abdomen. That is, imaging procedures on the head, spine and extremities account for 90% or more, and those on the abdomen account for only 10% or less.

FIG. 4shows a case in which the head, spine and extremities are to be imaged. As shown inFIG. 4, the cradle502is positioned to align the body axis of the subject laid thereon with the X direction of the homogeneous region300. It should be noted that the cradle502is a movable top plate provided on the table500, and the position of the cradle502is changeable in the X direction. Since the columnar magnetic element204of the magnetic field generating apparatus100is positioned in the obliquely rear direction, the cradle502can be unrestrictedly moved in the X direction.

When the cradle502is thus positioned, the dimensions of the homogeneous region300are 40 cm in the body axis direction, 30 cm in the body thickness direction, and 20 cm in the body width direction. The homogeneous region having such dimensions is sufficient to perform axial, sagittal and coronal imaging on the head, spine and extremities. That is, 90% or more of imaging procedures can be achieved in such a position.

FIG. 5shows a case in which the abdomen is to be imaged. As shown inFIG. 5, the cradle502is positioned to align the body axis of the subject laid thereon with the Y direction of the homogeneous region300. The position of the cradle502is changeable in the Y direction. Since the columnar magnetic element204of the magnetic field generating apparatus100is positioned in the obliquely rear direction, the cradle502can be unrestrictedly moved in the Y direction.

When the cradle502is thus positioned, the dimensions of the homogeneous region300are 40 cm in the body width direction, 30 cm in the body thickness direction, and 20 cm in the body axis direction. The homogeneous region having such dimensions is sufficient to perform axial imaging on the abdomen. That is, the remaining imaging procedures by MRI less than 10% can be achieved in such a position. Imaging procedures on all regions can be thus achieved.

FIG. 6shows a configuration of the table500. As shown inFIG. 6, the table500is comprised of the cradle502and a support base504for supporting the cradle502movable in the longitudinal direction. The support base504is open in its lower portion so as to straddle the lower structure of the magnetic field generating apparatus100, and the support base504has wheels506as well.

FIG. 7schematically shows the configuration of another example of the magnetic field generating apparatus100. InFIG. 7, (a) represents a front elevation view, and (b) represents a cross-sectional view taken along a line A-A. The magnetic field generating apparatus100is an example of the best mode for carrying out the invention. The configuration of the magnetic field generating apparatus100represents an example of the best mode for carrying out the invention of the magnetic field generating apparatus.

As shown inFIG. 7, the magnetic field generating apparatus100comprises a pair of magnets102supported by a yoke200. The magnet102is an example of the magnet of the invention. The magnet102has a magnetic pole surface of a generally rectangular shape. The shape of the homogeneous region300of the vertical magnetic field generated by the magnetic field generating apparatus is again ellipsoidal. It should be noted that the shape of the magnetic pole surface of the magnet102is not limited to an elliptical or rectangular shape, and any other appropriate shape is applicable.

FIG. 8schematically shows the configuration of an example of the magnetic field generating apparatus100. InFIG. 8, (a) represents a front elevation view, and (b) represents a cross-sectional view taken along a line A-A. The magnetic field generating apparatus100is an example of the best mode for carrying out the invention. The configuration of the magnetic field generating apparatus100represents an example of the best mode for carrying out the invention of the magnetic field generating apparatus.

As shown inFIG. 8, the magnetic field generating apparatus100comprises a pair of magnets102supported by a yoke200. The magnet102is an example of the magnet of the invention. The magnet102has a magnetic pole surface of a generally elliptical shape.

The yoke200is provided for forming a return path for the pair of magnets102. The yoke200is made up with a pair of planar magnetic elements202for supporting the pair of magnets, and two vertical columnar magnetic elements204with each end joined to respective one of the pair of the planar magnetic elements202, provided at two positions in an obliquely rear direction. Since the two columnar magnetic elements204are employed, supportiveness for the planar magnetic elements202is improved. The planar magnetic elements202and columnar magnetic element204are made of a ferromagnetic material such as soft iron, for example. The pair of magnets102form a magnetic field whose homogeneous region300has an ellipsoidal shape elongated in the X direction.

FIG. 9shows a configuration of the table500adapted for such a magnetic field generating apparatus100. As shown inFIG. 9, the table500is comprised of a cradle502and a support base504for supporting the cradle502movable in the longitudinal direction. The support base504has wheels506.

FIG. 10shows a case in which the head, spine and extremities are to be imaged using such magnetic field generating apparatus100and table500. As shown inFIG. 10, the table500is disposed to have its support base504abutting against the lateral side of the magnetic field generating apparatus100, and the longitudinal direction of the cradle502aligned with the X direction of the homogeneous region300. The position of the cradle502is changeable in the X direction. Since the two columnar magnetic elements204of the magnetic field generating apparatus100are positioned in the obliquely rear direction, the cradle502can be unrestrictedly moved in the X direction.

Although hidden inFIG. 10(visible inFIG. 11), an electric connector600is provided at the position where the support base504abuts against the magnetic field generating apparatus100, and a control signal for controlling movement of the cradle502is supplied from the magnetic field generating apparatus100to the table500.

FIG. 11shows a case in which the abdomen is to be imaged. As shown inFIG. 11, the table500is disposed to have its support base504abutting against the front side of the magnetic field generating apparatus100, and the longitudinal direction of the cradle502aligned with the Y direction of the homogeneous region300. The position of the cradle502is changeable in the Y direction. Since the columnar magnetic elements204of the magnetic field generating apparatus100are positioned in the obliquely rear direction, the cradle502can be unrestrictedly moved in the Y direction.

Although hidden inFIG. 11(visible inFIG. 10), an electric connector600is provided at the position where the support base504abuts against the magnetic field generating apparatus100, and a control signal for controlling movement of the cradle502is supplied from the magnetic field generating apparatus100to the table500.