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United States Patent |
5,266,898
|
Konishi
|
November 30, 1993
|
Magnetic resonance imaging apparatus
Abstract
In a magnetic resonance imaging apparatus, a radio-frequency shielding
body, which is disposed between the gradient magnetic field generating
coils and the radio-frequency coil for interrupting electromagnetic
coupling between the gradient magnetic field generating coils and the
radio-frequency coil due to the radio-frequency pulse applied to the
radio-frequency coil, is formed of a conductive material having a
thickness less than the skin depth defined by
##EQU1##
where .pi. stands for the ratio of the circumference of a circle to its
diameter, fo stand for the Larmor frequency of atomic nuclei of an imaging
object, .pi. stands for the conductivity of the conductive material and
.sigma. stands for the magnetic permeability of the conductive material.
Inventors:
|
Konishi; Mineyuki (Ootawara, JP)
|
Assignee:
|
Kabushiki Kaisha Toshiba (Kawasaki, JP)
|
Appl. No.:
|
618386 |
Filed:
|
November 27, 1990 |
Foreign Application Priority Data
Intern'l Class: |
G01V 003/00 |
Field of Search: |
324/300,318,322,307,319
336/84 C,84 H,84 R
|
References Cited
U.S. Patent Documents
4642569 | Feb., 1987 | Hayes et al. | 324/318.
|
4871969 | Oct., 1989 | Roemer et al. | 324/300.
|
4879515 | Nov., 1989 | Roemer et al. | 324/318.
|
4924184 | May., 1990 | Yoda | 324/318.
|
4980641 | Dec., 1990 | Breneman et al. | 324/318.
|
Foreign Patent Documents |
0151726 | Aug., 1985 | EP.
| |
3445724 | Jun., 1985 | DE.
| |
3621107 | Jan., 1987 | DE.
| |
63-290554 | Nov., 1988 | JP.
| |
Primary Examiner: Arana; Louis
Attorney, Agent or Firm: Limbach & Limbach
Claims
What is claimed is:
1. In a magnetic resonance imaging apparatus comprising:
static magnetic field generating means for generating a static magnetic
field along the direction of a first axis;
gradient magnetic field generating coil means for generating gradient
magnetic fields along the direction of said first axis and the directions
of second and third axes orthogonal to said first axis;
a radio-frequency coil means for transmitting or receiving electromagnetic
waves from a direction other than the direction of said first axis; and
a radio-frequency shielding body disposed between said gradient magnetic
field generating coil means and said radio-frequency coil means and having
a thickness along the direction of said second and third axes,
the improvement wherein said radio-frequency shielding body is formed of a
conductive material having a thickness less than approximately
##EQU5##
where: .pi. is a constant equal to the ratio of the circumference of a
circle to its diameter;
fo is the Larmor frequency of the atomic nuclei of an imaging object;
.sigma.]is the conductivity of the conductive material; and
.mu.]is the magnetic permeability of the conductive material; and is
arranged to interrupt electromagnetic coupling between said gradient
magnetic field generating coil means and said radio-frequency coil means
resulting from a radio-frequency pulse applied to said radio-frequency
coil means and reduce a time constant of eddy currents generated in the
surface of said gradient magnetic field coil means.
2. The magnetic resonance imaging apparatus according to claim 1, in which
said radio-frequency shielding body is disposed close to the
radio-frequency coil side of said gradient magnetic field generating coil
means.
3. A magnetic resonance imaging apparatus according to claim 2, in which
said radio-frequency shielding body has a shape similar to that of said
gradient magnetic field generating coil means.
4. A magnetic resonance imaging apparatus according to claim 1, in which
said radio-frequency shielding body has a sectional structure in which a
conductive layer is formed on a sheet of insulating material.
5. A magnetic resonance imaging apparatus according to claim 1, in which
said radio-frequency shielding body has a sectional structure in which an
organic fiber textile is plated with a metal.
6. A magnetic resonance imaging apparatus according to claim 5, in which
said organic fiber is made of polyester.
7. A magnetic resonance imaging apparatus according to claim 5, in which
said metal is copper.
8. A magnetic resonance imaging apparatus according to claim 1, in which
said radio-frequency coil means is a whole-body transmit and receive coil
which is disposed coaxially with said gradient magnetic field generating
coil means.
9. A magnetic resonance imaging apparatus according to claim 1, in which
said radio-frequency coil means is a transmit and/or receive coil which is
disposed inside said gradient magnetic field coil means with a space
between them.
10. A magnetic resonance imaging apparatus according to claim 1, in which
said radio-frequency coil means comprises a whole-body transmit and
receive coil which is disposed coaxially with said gradient magnetic field
generating coil means and a transmit and/or receive coil which is disposed
inside said gradient magnetic field coil means with a space between them.
11. A magnetic resonance imaging apparatus according to claim 1, in which
said static magnetic field generating means comprises an electromagnet for
generating a static magnetic field horizontally.
12. A magnetic resonance imaging apparatus according to claim 1, in which
said static magnetic field generating means comprises an electromagnet for
generating a static magnetic field vertically.
13. A magnetic resonance imaging apparatus according to claim 1, in which
said static magnetic field generating means comprises a permanent magnet
for generating a static magnetic field horizontally.
14. A magnetic resonance imaging apparatus according to claim 1, in which
said static magnetic field generating means comprises a permanent magnet
for generating a static magnetic field vertically.
15. In a magnetic resonance imaging apparatus comprising a static magnetic
field generating coil for generating a static magnetic field which is
applied to a subject under examination, gradient magnetic field generating
coils for generating gradient magnetic fields used to obtain information
on the position of a body portion of the subject in which magnetic
resonance signals are induced, a radio-frequency coil responsive to
application of a radio-frequency pulse thereto for transmitting a
radio-frequency magnetic field adapted to induce the magnetic resonance
signals and for detecting the induced magnetic resonance signals and a
radio-frequency shielding body disposed between said gradient magnetic
field generating coils and said radio-frequency coil for interrupting
electromagnetic coupling between said gradient magnetic field generating
coils and said radio-frequency coil due to said radio-frequency pulse
applied to said radio-frequency coil,
the improvement wherein said radio-frequency shielding body is formed of a
conductive material having a thickness less than the skin depth defined by
##EQU6##
where .pi. stands for the ratio of the circumference of a circle to its
diameter, fo stands for the Lamor frequency of atomic nuclei of an imaging
object, .sigma. stands for the conductivity of said conductive material
and .mu. stands for the magnetic permeability of said conductive material.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus
employing nuclear magnetic resonance (NMR) phenomena and, more
particularly, to a magnetic resonance imaging apparatus with an improved
radio-frequency shielding body.
2. Description of the Related Art
The nuclear magnetic resonance is a phenomenon in which atomic nuclei
placed in magnetic fields absorb electromagnetic energy at specific
frequencies and then emit the energy as electromagnetic waves. A
diagnostic apparatus employing the phenomenon senses electromagnetic waves
emitted by the atomic nuclei, protons in particular, and processes
received signals to obtain diagnostic information of a subject under
examination, such as a tomographic image, which contains the atomic
nucleus density (the proton density in particular), the longitudinal
spin-lattice relaxation time T1, the transversal spin-lattice relaxation
time T2, flow, chemical shifts and so on.
The magnetic resonance imaging apparatus for obtaining cross-sectional NMR
images of a subject under examination is provided with gradient magnetic
field forming coils for forming gradient magnetic fields which serve to
obtain position information of a body portion in which magnetic resonance
signals are induced and a radio-frequency coil responsive to application
of a radio-frequency pulse thereto for radiating a radio-frequency
magnetic field serving to induce the magnetic resonance signals in the
body portion and detecting the induced magnetic resonance signals. Between
the gradient magnetic field forming coils and the radio-frequency coil is
disposed a radio-frequency shielding body for interrupting electromagnetic
coupling between the gradient magnetic field forming coils and the
radio-frequency coil due to the radio-frequency pulse applied to the
radio-frequency coil.
The radio-frequency shielding body is generally formed of metallic foil
made of a good conductor such as copper. However, eddy currents will be
induced in the surface of the radio-frequency shielding body by
time-varying gradient magnetic fields formed by the gradient magnetic
field forming coils. A problem arises due to the eddy currents in that the
rising and falling characteristics of the gradient magnetic fields are
deteriorated. As a result, resulting cross-sectional NMR images will have
poor quality.
In order to solve the problem with the radio-frequency shielding body, an
approach has been proposed in Japanese Unexamined Patent Publication No.
60-177249 according to which metallic foil consisting of first and second
conductive regions separated by a relatively narrow nonconductive region,
such as a slit, is used to form the radio-frequency shielding body.
Other techniques have been proposed in Japanese Unexamined Patent
Publication No. 62-334. That is to say, a metal cylinder, which forms a
radio-frequency shielding body, is provided with a lengthwise slit so as
to decrease eddy currents by confining them locally or a metal cylinder is
formed of plural pieces of metallic foil connected by insulating materials
or dielectric materials so as to reduce impedance of a radio-frequency
shielding body to a radio-frequency pulse and to thereby reduce
undesirable electromagnetic coupling between the gradient magnetic field
coils and the radio-frequency coil.
However, such a radio-frequency shielding body is complex in structure and
costly because of the provision of the metallic foil with a slit or
interposition of a insulating material or dielectric material between
pieces of metallic foil.
According to still another proposal in Japanese Unexamined Patent
Publication No. 63-290554, the radio-frequency shielding body is formed of
a metal cylinder made of copper foil having a thickens not less than the
skin depth. The skin depth is the depth in the direction to the center of
the cylinder at which the amplitude of electromagnetic waves decays to 1/e
(e=2.718 . . . , the base of the natural logarithm). More specifically,
when electromagnetic waves with the Larmor frequency of protons penetrate
into a cylinder made of some metal by a depth of .delta. in the direction
to the center of the cylinder, if the amplitude of all the electromagnetic
waves decays to 1/e at the depth .delta., then the skin depth is defined
as .delta..
In the case of a radio-frequency shielding body with a thickness less than
the skin depth, the time constant of eddy currents can be made small so
that the time it takes for eddy currents to dissipate is effectively
shortened. However, the technique disclosed in the Publication, which is
directed to a radio-frequency shielding body made of a metal cylinder,
implies that it is impossible to make a radio-frequency shielding body
having a thickness less than the skin depth.
As described above, the conventional radio-frequency shielding bodies are
complex in structure and costly.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a magnetic
resonance imaging apparatus which is provided with a radio-frequency
shielding body which is simple in construction and sufficiently interrupts
electromagnetic coupling between gradient magnetic field forming coils and
a radio-frequency coil due to a radio-frequency pulse applied to the
radio-frequency coil so as not to deteriorate the rising and falling
characteristics of gradient magnetic fields.
The object of the present invention is attained by the following magnetic
resonance imaging apparatus.
In a magnetic resonance imaging apparatus comprising:
static magnetic field generating means for generating a static magnetic
field along the direction of a first axis;
gradient magnetic field generating coil means for generating gradient
magnetic fields along the direction of said first axis and the directions
of second and third axes orthogonal to said first axis;
a radio-frequency coil means for transmitting or receiving electromagnetic
waves from a direction other than the direction of said first axis; and
a radio-frequency shielding body disposed between said gradient magnetic
field generating coil means and said radio-frequency coil means and having
a thickness along the directions of said second and third axes,
the improvement wherein said radio-frequency shielding body being arranged
to interrupt electromagnetic coupling between said gradient magnetic field
generating coil means and said radio-frequency coil means resulting from a
radio-frequency pulse applied to said radio-frequency coil means and
reduce a time constant of eddy currents generated in the surface of said
gradient magnetic field coil means.
Also, the object of the present invention is attained by the following
magnetic resonance imaging apparatus.
In a magnetic resonance imaging apparatus comprising a static magnetic
field generating coil for generating a static magnetic field which is
applied to a subject under examination, gradient magnetic field generating
coils for generating gradient magnetic fields used to obtain information
on the position of a body portion of the subject in which magnetic
resonance signals are induced, a radio-frequency coil responsive to
application of a radio-frequency pulse thereto for transmitting a
radio-frequency magnetic field adapted to induce the magnetic resonance
signals and for detecting the induced magnetic resonance signals and a
radio-frequency shielding body disposed between said gradient magnetic
field generating coils and said radio-frequency coil for interrupting
electromagnetic coupling between said gradient magnetic field generating
coils and said radio-frequency coil due to said radio-frequency pulse
applied to said radio-frequency coil,
the improvement wherein said radio-frequency shielding body is formed of a
conductive material having a thickness less than the skin depth defined by
##EQU2##
where .pi. stands for the ratio of the circumference of a circle to its
diameter, fo stands for the Larmor frequency of atomic nuclei of an
imaging object, .sigma. stands for the conductivity of said conductive
material and .mu. stands for the magnetic permeability of said conductive
material.
With the magnetic resonance imaging apparatus of the present invention,
since the radio-frequency shielding body is formed of a conductive
material having a thickness less than
##EQU3##
the time constant of eddy currents induced in the surface of the
radio-frequency shielding body in accordance with variations in the
gradient magnetic fields with time can be made small and thus the eddy
currents decay quickly. As a result, the rising and falling
characteristics of the gradient magnetic fields will become abrupt,
whereby good tomographic images are obtained. More specifically, the time
constant of the eddy currents induced in the radio-frequency shielding
body is in inverse proportion to the resistance of the radio-frequency
shielding body, while the resistance is in proportion to the thickness of
the radio-frequency shielding body. Therefore, the time constant of the
eddy currents can be made small by decreasing the thickness of the
radio-frequency shielding body.
Additional objects and advantages of the invention will be set forth in the
description which follows, and in part will be obvious from the
description, or may be learned by practice of the invention. The objects
and advantages of the invention may be realized and obtained by means of
the instrumentalities and combinations particularly pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part
of the specification, illustrate presently preferred embodiments of the
invention, and together with the general description given above and the
detailed description of the preferred embodiments given below, serve to
explain the principles of the invention.
FIG. 1 is a diagrammatic representation of a magnetic resonance imaging
apparatus of the present invention;
FIG. 2 is a cross-sectional view illustrating a radio-frequency shielding
coil of the present invention;
FIG. 3 is a sectional view taken along the line III--III of FIG. 2;
FIG. 4 is a sectional view of a radio-frequency shielding body according to
another embodiment of the present invention; and
FIG. 5 is a sectional view of a radio-frequency shielding body according to
still another embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter the embodiments of the present invention will be described with
reference to the drawings. FIG. 1 is a graphical representation of a
nuclear magnetic resonance imaging apparatus embodying the present
invention.
As illustrated in FIG. 1, the magnetic resonance imaging apparatus, which
is generally indicated at 10, includes gradient magnetic field forming
coils 12 for forming gradient magnetic fields adapted to acquire
information on the position of a body portion of a subject under
examination 100 in which magnetic resonance signals are induced and a
radio-frequency coil 13 responsive to application of a radio-frequency
pulse thereto for radiating to the subject a radio-frequency magnetic
field adapted to induce the magnetic resonance signals and for detecting
the induced magnetic resonance signals. More specifically, with the
lengthwise axis of the subject under examination 100 taken as the Z axis
and the axes orthogonal to the Z axis taken as the X and Y axes, the
gradient magnetic field forming coils 12 comprise a X-axis gradient
magnetic field forming coil 12a for forming a gradient magnetic field in
the X-axis direction, a Y-axis gradient magnetic field forming coil 12b
for forming a gradient magnetic field in the Y-axis direction and a Z-axis
gradient magnetic field forming coil 12c for forming a gradient magnetic
field in the Z-axis direction. The gradient magnetic field forming coils
12a, 12b and 12c are connected to an X-axis gradient magnetic field power
supply 14a, a Y-axis gradient magnetic field power supply 14b and a Z-axis
gradient magnetic field power supply 14c, respectively. The
radio-frequency coil 13 is connected to a transmit circuit system 15 and a
receive circuit system 16.
The apparatus 10 is further provided with a sequencer 17 for practicing an
imaging pulse sequence and a computer system 18 for controlling all of the
power supplies 14a, 14b and 14c, the transmit circuit system 15, the
receive circuit system 16 and the sequencer and processing the detected
NMR signals. Signals processed by the computer system 18 are displayed on
a display 19. The apparatus is provided with a static magnetic field
forming coil (refer to FIG. 2) for applying a static magnetic field to the
subject 100 in the Z-axis direction and a power supply (not shown) for
supplying the static magnetic field forming coil with current as well.
As illustrated in FIG. 2, on the inside of the gradient magnetic field
forming coils 12 of the apparatus 10 is mounted a cylindrical
radio-frequency shielding body 20 adapted for interrupting electromagnetic
coupling between the gradient magnetic field forming coils 12 and the
radio-frequency coil 13 due to a radio-frequency pulse applied to the
radio-frequency coil 13. In FIG. 2, like reference numerals are used to
designate corresponding parts to those in FIG. 1. Reference numeral 21
designates the static magnetic field coil adapted to form a static
magnetic field along the Z-axis direction of the subject 100 of FIG. 1.
The radio-frequency shielding body 20 for use in the apparatus consists of
copper-plated metal which is about 10 .mu.m in thickness and formed on the
surface of a base consisting of an organic fiber textile made of, for
example, polyester cloth. Since the radio-frequency shielding body 20 is
thinner than the skin depth of 30 .mu.m which is represented by
##EQU4##
(where fo=63.874 MHz, the Larmor frequency of protons), the rising and
falling characteristics of the gradient magnetic field forming coils 12
will not be deteriorated.
As illustrated in FIG. 3, the radio-frequency shielding body 20 consists of
copper layers 20B1 and 20B2 plated on both sides of a base 20A. This
structure permits the radio-frequency shielding body 20 of a thickness
less than the skin depth to be obtained.
Besides the above embodiment, the radio-frequency shielding body may be
formed of a conductive layer formed on the surface of a sheet of
insulating material. Also, an evaporated film may be used as the
radio-frequency shielding body. In addition, it is desirable that the
radio-frequency shielding body 20 have a thickness from 5 to 10 .mu.m
which can be manufactured.
Next, another embodiment of the magnetic resonance imaging apparatus of the
present invention will be described with reference to FIG. 4. The
apparatus shown in FIG. 4 includes a transversal magnetic field forming
electromagnet 30 for use as the static magnetic field forming device. As
the transversal magnetic field forming electromagnet 30 a desired one may
be selected from among various types of electromagnets such as a solenoid
coil type magnet, a solenoid coil type superconducting magnet, a Helmholtz
type electromagnet, etc. The electromagnet 30 has cylindrical space
therein. A static magnetic field Bo is produced in the cylindrical space.
A cylindrical gradient magnetic field forming coil 12 is disposed in the
cylindrical space. A whole-body transmit and receive radio-frequency coil
31 is disposed inside the cylindrical gradient magnetic field coil 12.
Furthermore, a radio-frequency shielding body 32 formed in the shape of a
cylinder is disposed between the cylindrical gradient magnetic field coil
12 and the whole-body transmit and receive radio-frequency coil 31. This
radio-frequency shielding body 32 is substantially the same as that
illustrated in FIG. 2.
Next, still another embodiment of the magnetic resonance imaging apparatus
of the present invention will be described with reference to FIG. 5. The
apparatus shown in FIG. 5 includes a longitudinal magnetic field forming
magnet 40 as the static magnetic field forming device. As the magnet 40 a
desired one may be selected from among various types of magnets such as a
permanent magnet, an ordinary electromagnet, a superconducting magnet,
etc. The magnet 40 has first and second magnetic poles 41 and 42 which
face each other and are attached to a yoke 40A. A static magnetic field Bo
is formed in the space between the first and second magnetic poles 41 and
42. A gradient magnetic field coil 43 is disposed in the space between the
first and second magnetic poles. A whole-body transmit and receive
radio-frequency coil 44 is disposed inside the gradient magnetic field
coil 43. Furthermore, a radio-frequency shielding body 45 formed in the
shape of a cylinder is disposed between the gradient magnetic field coil
43 and the whole-body transmit and receive radio-frequency coil 44. This
radio-frequency shielding body 45 is substantially the same as that
illustrated in FIG. 2.
As can be seen from the foregoing, according to the present invention, a
magnetic resonance imaging apparatus can be provided which is provided
with a radio-frequency shielding body that is simple in construction and
permits electromagnetic coupling between a gradient magnetic field coil
and a radio-frequency coil due to a radio-frequency pulse applied to the
radio-frequency coil to be interrupted sufficiently so as not to
deteriorate the rising and falling characteristics of gradient magnetic
fields.
Additional advantages and modifications will readily occur to those skilled
in the art. Therefore, the invention in its broader aspects is not limited
to the specific details, and representative devices, shown and described
herein. Accordingly, various modifications may be without departing from
the spirit or scope of the general inventive concept as defined by the
appended claims and their equivalents.
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