Title: RF coil and magnetic resonance imaging apparatus
Abstract: A magnetic resonance imaging apparatus acquires magnetic resonance signals by the PI method using an RF coil unit having basic coils serving as surface coils which are arrayed with at least two coils along a static magnetic field direction (z direction) and at least two coils alone each of two orthogonal x, y directions. The coils are divided into an upper unit and a lower unit. The upper unit and lower unit are fixed by a band or the like to allow them to be mounted on an object to be examined. The signals detected by the respective surface coils are sent to a data processing system through independent receiver units and formed into a magnetic resonance image.
Patent Number: 6,998,843 Issued on 02/14/2006 to Okamoto, et al.
| Inventors:
|
Okamoto; Kazuya (Saitama, JP);
Hamamura; Yoshinori (Otawara, JP);
Machida; Yoshio (Nasu-gun, JP)
|
| Assignee:
|
Kabushiki Kaisha Toshiba (Tokyo, JP)
|
| Appl. No.:
|
034949 |
| Filed:
|
January 14, 2005 |
Foreign Application Priority Data
| Nov 22, 2001[JP] | 2001-358372 |
| Current U.S. Class: |
324/318; 600/421; 600/422; 324/307; 324/309 |
| Current Intern'l Class: |
G01V 3/00 (20060101); A61B 5/05.5 (20060101) |
| Field of Search: |
324/311,318,322,309,307
600/407,411
|
References Cited [Referenced By]
U.S. Patent Documents
| 4825162 | Apr., 1989 | Roemer et al.
| |
| 4857846 | Aug., 1989 | Carlson.
| |
| 5208534 | May., 1993 | Okamoto et al.
| |
| 5256971 | Oct., 1993 | Boskamp.
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| 5370118 | Dec., 1994 | Vij et al.
| |
| 5389880 | Feb., 1995 | Mori.
| |
| 5594337 | Jan., 1997 | Boskamp.
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| 5621323 | Apr., 1997 | Larsen.
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| 5664568 | Sep., 1997 | Srinivasan et al.
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| 5682098 | Oct., 1997 | Vij.
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| 5777474 | Jul., 1998 | Srinivasan.
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| 6097186 | Aug., 2000 | Nabetani.
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| 6137291 | Oct., 2000 | Szumowski et al.
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| 6300761 | Oct., 2001 | Hagen et al.
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| 6323648 | Nov., 2001 | Belt et al.
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| 6498489 | Dec., 2002 | Vij.
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| 6591128 | Jul., 2003 | Wu et al.
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| 6639406 | Oct., 2003 | Boskamp et al.
| |
| 6798202 | Sep., 2004 | Savelainen.
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| 6930481 | Aug., 2005 | Okamoto et al.
| |
| 2002/0169374 | Nov., 2002 | Jevtic.
| |
| 2002/0180442 | Dec., 2002 | Vij.
| |
| 2003/0100826 | May., 2003 | Savelainen.
| |
| 2003/0132750 | Jul., 2003 | Machida et al.
| |
| 2003/0210049 | Nov., 2003 | Boskamp et al.
| |
| 2004/0061498 | Apr., 2004 | Ochi et al.
| |
| 2004/0183534 | Sep., 2004 | Chan et al.
| |
| 2005/0122113 | Jun., 2005 | Okamoto et al.
| |
| Foreign Patent Documents |
| 195 05 062 | Oct., 1996 | DE.
| |
| 6-14901 | Jan., 1994 | JP.
| |
| 10-66683 | Mar., 1998 | JP.
| |
| 2000/-166896 | Jun., 2000 | JP.
| |
| 2000/-254109 | Sep., 2000 | JP.
| |
Primary Examiner: Gutierrez; Diego
Assistant Examiner: Fetzner; Tiffany A.
Attorney, Agent or Firm: Nixon & Vanderhye P.C.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional application of our commonly assigned application
Ser. No. 10/234,242 filed Sep. 5
th 2002 (nor U.S. Pat. No. 6,930,481
issued Aug. 16
th 1005), the entire content of which is hereby incorporated
by reference. This application is also based upon and claims the benefit of Priority
from the prior Japanese Patent Application No. 2001-358372, filed Nov. 22, 2001,
the entire contents of which are incorporated herein by reference.
Claims
What is claimed is:
1. An RF coil unit used in a magnetic resonance imaging apparatus which receives
magnetic resonance signals generated from a common imaged volume of an object to
be examined which is placed in a static magnetic field and generates a magnetic
resonance image, said RF coil unit comprising:
a first unit including a plurality of surface coils which are arrayed, at least
two of said surface coils being disposed along each of a first direction and a
second direction perpendicular to the first direction;
a second unit which is spaced apart from and arranged symmetrically opposite
to the first unit in a third direction which is perpendicular to both of the first
and second directions;
said first and second units also including a plurality of at least two surface
coils arranged in the third direction whereby there are at least two coils of the
RF coil unit located in each of the first, second, and third directions, that each
receive magnetic resonance signals; and
a first cable configured to transmit the magnetic resonance signals received
by said surface coils of said first unit to said second unit.
2. An RF coil unit as in claim 1 further comprising a second cable configured
to transmit the magnetic resonance signals transmitted by said first transmit unit
and the magnetic resonance signals received by said surface coils of said second
unit to outside of the RF coil.
3. An RF coil unit as in claim 1 further comprising a fixing device that fixes
said plurality of surface coils with respect to the object.
4. An RF coil unit as in claim 1 further comprising an alignment portion configured
to serve as a reference for alignment of respectively corresponding surface coils
in one group with respect to another group.
5. An RF coil unit as in claim 1 further comprising a shape maintaining portion
configured to maintain at least some of said plurality of surface coils in a predetermined shape.
6. An RF coil unit as in claim 1 wherein at least one of said plurality of surface
coils is a QD coil which detects substantially orthogonal magnetic resonance signals.
7. An RF coil unit as in claim 6 wherein said QD coil includes a looped coil
element and an 8-shaped coil element.
8. An RF coil unit as in claim 6 wherein said QD coil has a plurality of 8-shaped
coil elements which are overlapped so as to be orthogonal to each other.
9. An RF coil unit as in claim 1 wherein
each of said surface coils includes a looped coil element and an 8-shaped coil
element which detect substantially orthogonal magnetic resonance signals;
said looped coil element being partly overlapped on another looped coil element
of an adjacent surface coil in at least one of three directions; and
said 8-shaped coil element being partly overlapped on another 8-shaped coil element
of an adjacent surface coil in at least one of the three directions.
10. An RF coil unit claim 1, wherein
each of said surface coils has two 8-shaped coil elements which detect substantially
orthogonal magnetic resonance signals, and
said plurality of 8-shaped coil elements are arrayed in at least one of the three
directions such that 8-shaped coil elements of adjacent surface coils are partly overlapped.
11. An RF coil unit as in claim 1 wherein at least one of said plurality of surface
coils is removably connected to the others.
12. An RF coil unit used in a magnetic resonance imaging apparatus which receives
magnetic resonance signals generated from a common imaged volume of an object to
be examined which is placed in a static magnetic field and generates a magnetic
resonance image, said RF coil unit comprising:
a first unit including a plurality of surface coils which are arrayed, at least
two of said surface coils disposed along each of a first direction and a second
direction perpendicular to the first direction,
a second unit which is spaced apart from and arranged symmetrically opposite
to the first unit in a third direction which is perpendicular to both of the first
and second directions;
said first and second units also including a plurality of at least two surface
coils arranged in the third direction whereby there are at least two coils of the
RF coil unit located in each of the first, second, and third directions, that each
receive magnetic resonance signals;
a first cable configured to transmit the magnetic resonance signals received
by said surface coils of said first unit to said second unit; and
a plurality of second cables electrically connecting those surface coils which
are disposed to oppose each other, or are adjacent to each other, to a ground potential
thereby making at least one of said surface coils which oppose each other, or are
adjacent to each other, acquire magnetic resonance signals having substantially
the same phase.
13. An RF coil unit as in claim 12 further comprising a third cable configured
to transmit the magnetic resonance signals transmitted by said first transmit unit
and the magnetic resonance signals received by each said coil included by said
second unit itself to said magnetic resonance imaging apparatus.
14. An RF coil unit as in claim 12 wherein
each of said plurality of surface coils includes a looped coil element and an
8-shaped coil element which detect substantially orthogonal magnetic resonance
signals, and
the plurality of said second cables which electrically connect said looped coil
elements to ground to make at least one of said looped coil elements which oppose
each other and said looped coil elements which are adjacent to each other acquire
magnetic resonance signals having substantially the same phase.
15. An RF coil unit as in claim 12 wherein
each of said second plurality of surface coils has a looped coil element and
an 8-shape coil element which detect substantially orthogonal magnetic resonance
signals, and
the plurality of said cables which electrically connect said 8-shaped coil elements
to the ground side to make at least one of said 8-shaped coil elements which oppose
each other and said 8-shaped coil elements which are adjacent to each other acquire
magnetic resonance signals having substantially the same phase.
16. An RF coil unit as in claim 12 wherein
each of said plurality of surface coils has two 8-shaped coil elements which
detect substantially orthogonal magnetic resonance signals, and
the plurality of said second cables which electrically connect said 8-shaped
coil elements to ground to make at least one of said 8-shaped coil elements which
oppose each other and said 8-shaped coils which are adjacent to each other acquire
magnetic resonance signals having substantially the same phase.
17. An RF coil unit as in claim 12 wherein at least one of said plurality of
surface coils is replaceable removably connected to the others.
18. A magnetic resonance imaging apparatus which receives magnetic resonance
signals generated from a common imaged volume of an object to be examined which
is placed in a static magnetic field and generates a magnetic resonance image,
said apparatus comprising:
a first unit including a plurality of surface coils which are arrayed, at least
two of said surface coils disposed along each of a first direction and a second
direction perpendicular to the first direction;
a second unit which is spaced apart from and arranged symmetrically opposite
to the first unit in a third direction which is perpendicular to both of the first
and second directions;
said first and second units also including a plurality of at least two surface
coils arranged in the third direction whereby there are at least two coils of the
RF coil unit located in each of the first, second, and third directions, that each
receive magnetic resonance signals;
a first cable configured to transmit the magnetic resonance signals received
by said surface coils of said first unit to said second unit;
a receiver which receives the magnetic resonance signals from said second unit; and
an image generating unit configured to generate a magnetic resonance image utilizing
magnetic resonance signals received by said receiver.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a signal detection RF coil which generates a
magnetic resonance signal by applying RF pulses to an object to be examined in
a static magnetic field and at the same time, acquires a magnetic resonance signal,
and a magnetic resonance imaging apparatus which using the RF coil.
2. Description of the Related Art
A magnetic resonance imaging (MRI) apparatus is an apparatus which images the
chemical
and physical microscopic information of a substance by using a phenomenon in which
when a group of nuclei having a unique magnetic moment is placed in a uniform static
magnetic field, they resonantly absorb energy of an RF magnetic field that rotates
at a specific frequency. Among recent techniques associated with such a magnetic
resonance imaging apparatus, a phased array technique is available, in which a
plurality of surface coils are arranged with respect to a region of interest, and
an image with a high S/N ratio is acquired by receiving a magnetic resonance signal.
For example, a magnetic resonance imaging apparatus is disclosed in Jpn. Pat.
Appln. KOKAI Publication No. 4-42937, in which a plurality of surface coils (e.g.,
looped coils) are arranged in a desired region of an object to be examined which
is to be imaged, and magnetic resonance signals from the object are detected through
these surface coils, respectively. The detected magnetic resonance signals are
converted into a plurality of series of image data by imaging processing. Data
corresponding to the same spatial position are multiplied by predetermined weighting
functions (function determined in advance on the basis of the distribution of RF
magnetic fields generated by the respective surface coils), and the resultant data
are added together. The respective pixel data obtained in this manner are combined
to provide an image with a high S/N ratio of an overall desired region of the object.
A parallel imaging method (to be referred to as a "PI method" hereinafter) which
is a high-speed imaging method using multiple surface coils is proposed in Magnetic
Resonance in Medicine, Vol. 29, pp. 681 to 688 (1993) or Magnetic Resonance in
Medicine, Vol. 30, pp. 142 to 145 (1993). The contents of the former are also disclosed
in "Rapid MRI using multiple receivers producing multiple phase-encoded data derived
from a single NMR response" (U.S. Pat. No. 4,857,846). The phased array technique
is also introduced as a noteworthy technique in Magnetic Resonance in Medicine,
Vol. 42, pp. 952-962 (1999). According to the techniques disclosed in these references,
when a plurality of surface coils are arranged around a region of interest, the
data amount of raw MRI data in the encoding direction can be reduced by almost
the reciprocal of the number of coils arrayed in the direction. Assume that a 256×256
matrix axial image is to be acquired. In this case, if the X and Y directions correspond
to the reading and encoding directions, respectively, 256 data are generally sampled
while a gradient field is applied in the X direction. This operation is repeated
256 times while the gradient filed pulse intensity in the Y direction is changed
in predetermined steps, thereby obtaining 256×256 raw data. By performing
a Fourier transform of the raw data, an axial image can be obtained. Assume that
two surface coils are so arranged as to sandwich the patient in the vertical direction,
and the PI method is used. In this case, even if the number of times data acquisition
is done while the gradient field pulse intensity in the Y direction is changed
in predetermined steps, a 256×256 matrix image can be reproduced properly.
In this manner, a data acquisition time T is reduced to 1/n, and the S/N ratio
is reduced to 1/n
1/2. By acquiring data using a plurality of surface
coils with a high S/N ratio, a decrease in S/N ration due to a decrease in data
acquisition time can be compensated for. In addition, if surface coils are arrayed
in the X direction or Z direction (static magnetic field direction), the number
of times of encoding can be decreased in accordance with the number of coils arrayed
in encoding in the X or Z direction. This makes it possible to shorten the data
acquisition time. That is, high-speed imaging can be done.
However, conventional magnetic resonance imaging apparatuses are not designed
to arrange RF coils in the three directions, i.e., the X, Y, and Z directions,
but are designed to arrange RF coils in the two directions, i.e., the X and Y directions
or the Y and Z directions. When, therefore, an abdominal region is to be imaged
by using the PI method, the number of times of encoding in the Z or X direction
cannot be decreased. Furthermore, when a slice in an arbitrary direction (oblique
imaging) is selected, the PI method is difficult to apply.
BRIEF SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above situation,
and has as its object to provide a magnetic resonance imaging apparatus which can
freely apply the PI method to a slice in an arbitrary direction in the imaging
method using a plurality of surface coils, thereby obtaining a magnetic resonance
image with a high S/N ratio or at high speed, and an RF coil used in the apparatus.
The present invention may provide an RF coil used in a magnetic resonance imaging
apparatus which receives magnetic resonance signals generated from an object to
be examined which is placed in a static magnetic field and generates a magnetic
resonance image, the RF coil comprising: a plurality of surface coils which are
arrayed at least in twos in a direction of the static magnetic field and two directions
perpendicular thereto and receive the magnetic resonance signals.
The present invention may provide an RF coil used in a magnetic resonance imaging
apparatus which receives magnetic resonance signals generated from an object to
be examined which is placed in a static magnetic field and generates a magnetic
resonance image, the RF coil comprising: a plurality of surface coils which are
arrayed at least in twos in a direction of the static magnetic field and two directions
perpendicular thereto and receive the magnetic resonance signals; and a plurality
of cable which electrically connect the surface coils which oppose each other or
are adjacent to each other to a ground side to make at least one of the surface
coils which oppose each other or are adjacent to each other acquire magnetic resonance
signals having substantially the same phase.
The present invention may provide a magnetic resonance imaging apparatus which
receives magnetic resonance signals generated from an object to be examined which
is placed in a static magnetic field and generates a magnetic resonance image,
the magnetic resonance imaging apparatus comprising: an RF coil having a plurality
of surface coils which are arrayed at least in twos in a direction of the static
magnetic field and two directions perpendicular thereto and receive the magnetic
resonance signals; a receiver which is placed for each of the surface coils independently
and receives each magnetic resonance signal from the RF coil; and an image generating
unit configured to generate a magnetic resonance image on the basis of the magnetic
resonance signals received by the receiver.
The present invention may provide a magnetic resonance imaging apparatus which
receives magnetic resonance signals generated from an object to be examined which
is placed in a static magnetic field and generates a magnetic resonance image,
the magnetic resonance imaging apparatus comprising: an RF coil having a plurality
of surface coils which are arrayed at least in twos in a direction of the static
magnetic field and two directions perpendicular thereto and receive the magnetic
resonance signals; a plurality of cable which electrically connect the surface
coils which oppose each other or are adjacent to each other to a ground side to
make at least one of the surface coils which oppose each other or are adjacent
to each other acquire magnetic resonance signals having substantially the same
phase; a receiver which is placed for each of the surface coils independently and
receives each magnetic resonance signal from the RF coil; and an image generating
unit configured to generate a magnetic resonance image on the basis of the magnetic
resonance signals received by the receiver.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
FIG. 1 is a block diagram showing the arrangement of a magnetic resonance imaging
apparatus according to an embodiment;
FIG. 2 is a view showing an receiving RF coil 14
b constituted
by upper and lower units 140 and 141 and used for the diagnosis of
an abdominal region or the like;
FIG. 3 is a block diagram of the receiving RF coil 14
b;
FIG. 4 is a perspective view showing an example of the receiving RF coil 14
b
which is constituted by the upper and lower units 140 and 141
and fixed to an object to be examined when it is used;
FIG. 5 is a perspective view showing another example of the receiving RF coil
14
b which is constituted by the upper and lower units 140
and 141 and fixed to an object to be examined when it is used;
FIG. 6 is a perspective view for explaining an embodiment of the receiving RF
coil 14
b having a form 50;
FIG. 7 is a view showing a surface QD coil constituted by a looped coil 53
and 8-shaped coil 55;
FIG. 8 is a view showing a second surface QD coil 57 serving as a basic
coil 142 which is constituted by 8-shaped coils 55
a and 55
b;
FIG. 9 is a view showing an example of how a method of suppressing coupling
is applied to this receiving RF coil 14
b;
FIG. 10 is a view showing a wiring method used for a looped coil 53U
of the upper unit 140 and a looped coil 53L of the lower unit 141
to prevent a deterioration in sensitivity in a region of interest;
FIG. 11 is a view showing a wiring method used for an 8-shaped coil 55U
of the upper unit 140 and an 8-shaped coil 55L of the lower unit
141 to prevent a deterioration in sensitivity in a region of interest;
FIG. 12 is a view for explaining the effect of this magnetic resonance imaging apparatus;
FIG. 13 is a view for explaining the effect of this magnetic resonance imaging apparatus;
FIG. 14 is a graph showing the result of the above computer simulation;
FIG. 15 is a view showing an example of the upper unit 140 used for chest
imaging; and
FIG. 16 is a view showing another example of the upper unit 140 used
for chest imaging.
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention will be described below with reference
to the views of the accompanying drawing. Note that the same reference numerals
denote constituent elements having substantially the same functions and arrangements,
and a repetitive description will be made only when required.
FIG. 1 is a block diagram showing the arrangement of a magnetic resonance imaging
apparatus according to this embodiment. Referring to FIG. 1, a magnetic resonance
imaging apparatus
10 includes a static magnetic field magnet
11,
gradient coil
12, shim coil
13, RF coil
14, gradient coil
power supply
16, shim coil power supply
17, transmitting section
18, receiving section
19, data acquisition section
20, sequence
control section
21, computer system
22, console
23, and display
24.
The static magnetic field magnet
11 is a magnet for generating a static
magnetic field. This magnet generates a uniform static magnetic field. As the static
magnetic field magnet
11, a permanent magnet, superconductive magnet, or
the like is used.
The gradient coil
12 is placed inside the static magnetic field magnet
11 and converts the pulse current supplied from the gradient coil power
supply
16 into a gradient field. A signal generating region (position) is
specified by the gradient field generated by the gradient coil
12.
The shim coil
13 is placed inside the static magnetic field magnet
11
and serves to improve the homogeneity of a magnetic field. The shim coil
13
is driven by the shim coil power supply
17.
The shim coil
13 and gradient coil
12 apply a uniform static magnetic
field to an object to be examined (not shown) and gradient fields having linear
gradient field distributions in three directions, i.e., X, Y, and Z directions,
which are perpendicular to each other. Assume that the Z-axis direction coincides
with the static magnetic field direction in this embodiment.
An RF coil unit is comprised of a transmitting RF coil unit
14a for
applying RF pulses to an imaging region of the object to generate a magnetic resonance
signal and the receiving RF coil unit
14b which is placed near the
object, more preferably placed to sandwich the object in tight contact and receives
a magnetic resonance signal from the object. The receiving RF coil unit
14b
generally has a shape specialized for each region.
The receiving RF coil unit
14b has a plurality of surface coils
arrayed in the X, Y, and Z directions which are perpendicular to each other. These
surface coils have been treated to prevent magnetic coupling. The contents of this
will be described in detail later.
The transmitting section
18 has an oscillating section, phase selecting
section, frequency conversion section, amplitude modulating section, and RF power
amplifying section (none of which are shown), and transmits RF pulses corresponding
to a Larmor frequency to the transmitting RF coil
14a. The magnetization
of a predetermined nucleus of the object is excited by the is RF pulses generated
from the transmitting RF coil
14a upon this transmission.
The receiving section
19 has an amplifying section, intermediate frequency
converting section, phase detecting section, filter, and A/D converter (none of
which are shown). The receiving section
19 performs amplification processing,
intermediate frequency conversion processing using an oscillation frequency, phase
detection processing, filter processing, and A/D conversion processing for the
magnetic resonance signal (RF signal) which is emitted when the magnetization of
the nucleus relaxes from the excited state to the ground state and received from
the receiving RF coil
14b.
The data acquisition section
20 acquires the digital signals sampled by
the receiving section
19.
The sequence control section
21 controls the operations of the gradient
coil power supply
16, shim coil power supply
17, transmitting section
18, receiving section
19, and data acquisition section
20.
The computer system
22 controls the sequence control section
21
on the basis of the commands input from the console
23. The computer system
22 executes post-processing, i.e., reconstruction such as a Fourier transform,
for the magnetic resonance signal input from the data acquisition section
20
to obtain the spectrum data or image data of a desired nuclear spin inside the object.
The console
23 has an input unit (e.g., a mouse, trackball, mode switch,
and keyboard) for inputting various commands, instructions, and information from
the operator.
The display
24 is an output means for displaying the spectrum data, image
data, or the like input from the computer system
22.
(RF Coil)
The arrangement of the RF coil will be described in detail next.
FIG. 2 shows the receiving RF coil unit
14b which is constituted
by an upper sub-unit
140 and lower sub-unit
141 and used to diagnose,
for example, the abdomen. As shown in FIG. 2, each of the upper and lower sub-units
140 and
141 has 2 (X direction) X 3 (Z direction)=6 basic coils
142.
The upper and lower sub-units
140 and
141 are arranged to oppose
each other along the Y direction. The receiving RF coil unit
14b therefore
has the two basic coils
142 (in each sub-unit
140,
141) as
surface coils in the X direction, three basic coils
142 as surface coils
in the Z direction, and pairs of opposed basic coils distributed over the upper
and lower units
140 and
141 as surface coils in the Y direction.
The receiving RF coil unit
14b therefore has a total of
12
surface coils arranged in the above manner. When the PI method is executed by using
this RF coil, the number of data acquisitions used for encoding of gradient field
pulses typically can be reduced to ½ in the X direction, ½ in the Y direction,
and ⅓ in the Z direction.
FIG. 3 is a block diagram of the receiving RF coil unit
14b. As
shown in FIGS. 3 and 4 the upper and lower sub-units
140 and
141
are coupled to each other through a first cable
40 and first connector
41.
The magnetic resonance signals detected by the surface coils of the upper unit
140 are temporarily received by the lower unit
141 through the cable
40 having the connector
41 and loaded altogether into a signal processing
system (incorporated in the main body of the magnetic resonance imaging apparatus
10) through a cable
42 and connector
43.
As described above, each of the upper and lower sub-units
140 and
141
has a plurality of six basic coils
142 arranged in each of the X and Z directions.
FIG. 3 shows each basic coil
142 constituted by a circular coil element
and 8-shaped coil element. This basic coil
142 will be described in detail
later. The lower sub-unit
141 has preamplifiers
44 and hybrid circuits
45. The preamplifiers
44 are connected to the respective coils to
amplify signals. The hybrid circuit
45 changes the phase of a signal from
one of the circular and 8-shaped coil elements constituting the basic coil
142
and combines the resultant signal with a signal from the other coil element. Note
that the preamplifiers
44 and hybrid circuits
45 may be arranged
in the magnetic resonance imaging apparatus
10.
In actual imaging, an object to be examined is placed on the lower sub-unit
141,
and the upper sub-unit
140 is placed on the object so as to oppose the lower
sub-unit
141 (see FIGS. 4 and 5). This receiving RF coil unit
14b
is designed to minimize the number of components included in the upper sub-unit
140 by connecting the upper sub-unit
140 and lower sub-unit
141
through connectors. The upper sub-unit
140 is sufficiently reduced in weight,
and hence the load on the object can be reduced.
The receiving RF coil unit
14b constituted by the upper and lower
sub-units
140 and
141 in FIGS. 2 and 3 is generally fixed to the
object when it is used. This usage of the coil unit will be described with reference
to FIGS. 4 and 5.
FIGS. 4 and 5 show an example of the receiving RF coil unit
14b constituted
by the upper and lower sub-units
140 and
141 and fixed to the object
when it is used. The receiving RF unit coil
14b constituted by the
upper and lower sub-units
140 and
141 shown in FIGS. 4 and 5 is suitable
for imaging a thoracicoabdominal region, in particular. Fro example, this receiving
RF coil unit
14b is used in magnetic resonance imaging in the following
manner. Referring to FIG. 4, the object is laid down on the lower sub-unit
141,
and the upper sub-unit
140 is placed on the object. The lower sub-unit
141
is mounted on a bed (not shown). The upper unit
140 is fixed to the lower
sub-unit
141 with a band
47.
It is preferable that the upper and lower sub-units
140 and
141
be arranged to maintain a predetermined positional relationship, and oppose to
each other along the Y direction, in particular. For this purpose, this receiving
RF coil unit
14b has a mark
46 to be used as a reference for
the positioning of the upper and lower sub-units
140 and
141. The
operator places the upper sub-unit
140 such that the mark on the upper sub-unit
140 opposes the lower sub-unit
141. This facilitates alignment of
the receiving RF coil unit
14b. As shown in FIG. 5, in place of the
mark
46, grooves
48 may be formed in the upper sub-unit
140
to allow positioning of the upper and lower sub-units when they are fixed with
the band
47.
Alternatively, the receiving RF coil unit
14b may have
a form to allow the upper sub-unit
140 to be place don the object by utilizing
a predetermined shape.
FIG. 6 is a perspective view for explaining an embodiment of the receiving RF
coil unit
14b having a form
50. As shown in FIG. 6, when the
upper sub-unit
140 is placed on the object above and in registered opposition
to the lower sub-unit
141, the form
50 is placed between the upper
sub-unit
140 and the object. This form
50 serves to stabilize the
shape of the upper sub-unit
140.
In general, in the PI method, a pre-scan is executed to obtain an RF magnetic
field on each surface coil before the acquisition of main data. The position of
the receiving RF coil unit
14b in this pre-scan preferably coincides
with the position of the receiving RF coil unit
14b in acquisition
of main data. According to the above receiving RF coil
14b, the presence
of the form
50 can stabilize the shape and prevent a change in coil position,
thus realizing excellent main data acquisition.
(Electromagnetic Coupling Preventing Function)
The function of preventing the effect of electromagnetic coupling of the receiving
RF coil
14b, which this magnetic resonance imaging apparatus
10
has, will be described next. This function is realized by one of the two techniques
described next or a combination thereof.
The first technique of preventing electromagnetic coupling of the receiving RF
coil
14b is a technique which is to be applied to a case wherein
the basic coil
142 is a QD coil and devises the arrangement of two coils
constituting the QD coil.
FIG. 7 shows a first surface QD coil
51 serving a s the basic coil
142,
which is constitute by a looped coil element
53 and 8-shaped coil element
55. As shown in FIG. 7, this first surface QD coil
51 has the looped
coil element
53 placed in the center of the 8-shaped coil element
55.
This arrangement can suppress electric coupling between the looped coil element
53 and the 8-shaped coil element
55. The first surface QD coil
51
is especially suitable for a case wherein a static magnetic field direction is
a lateral direction (Z direction) with respect to the shape of "8" of the 8-shaped
coil element
55 (i.e., the body axis direction of the lying object coincides
with the static magnetic field direction).
FIG. 8 shows a second surface QD coil
57 serving as the basic coil
142,
which is constituted by 8-shaped coil elements
55a and
55b.
As shown in FIG. 8, in the second surface QD coil
57, the 8-shaped coil
elements
55a and
55b are so overlapped as to be perpendicular
to each other. With this arrangement, similar to the first surface QD coil
51,
electric coupling between the 8-shaped coil elements
55a and
55b
can be suppressed. This second surface QD coil
57 is suitable for a
case wherein a static magnetic field direction is perpendicular to the plane formed
by the 8-shaped coil elements (i.e., the static magnetic field direction coincides
with the Y direction).
Note that in this receiving RF coil
14b, for example, the first
or second surface QD coils are arrayed as the basic coils
142 in the X or
Z direction. When a plurality of surface coils adjusted to the same resonance frequency
in this manner are arrayed to simultaneously acquire data, electric coupling between
the coils is suppressed by one of the following two methods or a combination thereof.
One method is a method of suppressing coupling by adjusting a spatial arrangement.
The other method is a method of suppressing coupling by using preamplifiers having
a low input impedance as preamplifiers which are coupled to the coils to amplify
signals, as disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 4-42937.
FIG. 9 shows an example of how the method of suppressing coupling by adjusting
a spatial arrangement is applied to this receiving RF coil
14b. In
the case shown in FIG. 9, the spatial arrangement in which the first surface QD
coils
51 shown in FIG. 7 are arrayed in twos two-dimensionally (X-Z plane)
is adjusted to suppress coupling between the coils. More specifically, when 8-shaped
coils
55A,
55B,
55C, and
55D are adjacent to each other
in the X or Z direction, the adjacent coils are partly overlapped to suppress electric
coupling. In addition, looped coils
53A and
53C and looped coils
53B and
53D are partly overlapped to suppress electric coupling.
Referring to FIG. 9, coupling between the diagonally opposite 8-shaped
coils, the looped coils arrayed in the X direction, the diagonally opposite looped
coils, and the looped coils which are adjacent and opposite to the 8-shaped coils
cannot be sufficiently suppressed by adjusting the spatial arrangement. Even if
decoupling can be theoretically done by overlapping coils, a manufacture error
may be caused. As a consequence, coupling may be left unsuppressed. In such a case,
sufficient coupling suppression can be attained by also using the above method
of suppressing coupling by using amplifiers with low input impedance.
The second technique of preventing the effect of electromagnetic coupling of
the RF coil
14 will be described next. In the technique, for example, the
positional relationship or connection to the output cable of the upper unit
140
and the lower unit
141 is devised to prevent a deterioration in sensitivity
in a region of interest when some electromagnetic coupling remain. This technique
is sufficiently effective by itself. If, however, the technique is combined with
the first technique, a deterioration in sensitivity in the center of the object
can be prevented even in the presence of residual coupling.
In general, when coils couple to each other, two types of modes are produced,
i.e., a mode in which RF currents flow in directions to cancel out generated RF
magnetic fields produced by the respective coils and a mode in which RF currents
flow to generate RF magnetic fields in the same direction so as to enhance the
magnetic fields. In the former case, since the RF magnetic fields generated by
the coils cancel each other, the sensitivity in the center of the object sandwiched
by the coils deteriorates. In the latter case, the RF magnetic fields do not cancel
each other, and hence no deterioration in sensitivity occurs.
Even if residual coupling remain, a deterioration in the sensitivity in a region
of interest can be prevented by devising the wiring of signal cables or the arrangement
of coils so as to allow selection of the latter mode. The magnetic resonance imaging
apparatus
10 selects the latter mode by devising the positional relationship
or wiring between the upper and lower units
140 and
141.
FIG. 10 shows a wiring method to be applied to a looped coil
53U of the
upper unit
140 and a looped coil
53L of the lower unit
141
to prevent a deterioration in sensitivity in a region of interest. Referring to
FIG. 10, a portion of a signal cable which is connected to the ground side is referred
to as a C side (cold side), and a portion which is not connected to the ground
side is referred to as an H side (hot side). In this case, the arrangements of
the H and C sides coincide between the looped coil
53U and the looped coil
53L, detected signals have the same phase. Therefore, RF currents flow in
the same direction, and RF magnetic fields are generated in the same direction,
thereby preventing a deterioration in sensitivity due to coupling.
FIG. 11 shows wiring done for an 8-shaped coil
55U of the upper unit
140 and an 8-shaped coil
55L of the lower unit
141. As shown
in FIG. 11, the arrangement of the H and C sides of the upper coil may be reversed
with respect to those of the lower coil to match the directions of the RF magnetic
fields generated by the 8-shaped coils.
The effects obtained by the above RF coil and magnetic resonance imaging apparatus
will be described next.
This magnetic resonance imaging apparatus has a plurality of surface coils arrayed
in the X, Y, and Z directions which are perpendicular to each other. In the imaging
method using the PI method, therefore, an imaging time T can be greatly shortened.
In addition, owing to the high sensitivity characteristics of the surface coils,
a decrease in S/N ratio with the shortening of the imaging time T can be compensated
for. This makes it possible to acquire high-precision magnetic resonance images.
This increase in S/N ratio will be described in detail below with consideration
given to the arrangement of the RF coil in the Z direction.
The magnetic resonance imaging apparatus
10 has the receiving RF coil
14b having a plurality of basic coils
142 arrayed in the Z
direction. In contrast to this, a conventional RF coil has only integral-type coil
in the Z direction. An increase in S/N ratio owing to the difference between the
arrangements in the Z direction can be checked in the following manner. For example,
the sensitivity (S/N ratio) on the central body axis in a case wherein a phantom
61 is imaged by an integral-type long-axis volume coil
60 as shown
in FIG. 12 is compared with that in a case wherein the phantom
61 is imaged
by two short-axis volume coils
63 and
64 arrayed in the Z direction
as shown in FIG. 13 by computer simulation.
FIG. 14 is a graph showing the above computer simulation result. As shown in
FIG. 14, the short-axis volume coils
63 and
64 are higher in sensitivity
than the long-axis volume coil
60.
The model shown in FIG. 13 using the short-axis volume coils
63 and
64
is equivalent to this magnetic resonance imaging apparatus having a plurality of
surface coils arrayed in the Z direction. The model shown in FIG. 12 using the
long-axis volume coil is equivalent to the conventional magnetic resonance imaging
apparatus in which a plurality of surface coils are not arrayed in the Z direction
(i.e., the apparatus using a long integral-type coil in the Z direction). Therefore,
this apparatus can acquire signals with higher S/N ratios than the conventional apparatus.
In addition, the RF coil
14 has a band for fixing the coil to the object
and a reference for positioning. Each QD coil serving as each surface coil exhibits
the maximum S/N ratio characteristic. By devising a wiring method, a deterioration
in sensitivity in a central region is prevented even with residual coupling. Each
arrangement described above can realize a high S/N ratio.
The present invention has been described on the basis of the embodiment. However,
those who skilled in the art can make various modifications and corrections of
the embodiment within the spirit and scope of the invention, and hence it should
be understood that such modifications and corrections fall within the range of
the present invention. For example, the embodiment can be variously modified within
the spirit and scope of the invention as follows.
The receiving RF coil
14b takes different shapes depending on the
regions to be imaged. FIG. 15 shows an example of the upper unit
140 suitable
for chest imaging. As shown in FIG. 15, the upper unit
140 has looped coils
53G,
53H,
53I, and
53J serving as surface coils arrayed
in the X and Z directions. In addition, the adjacent looped coils are partly overlapped
to reduce coupling between the coils.
The shape of each surface coil element is not limited to a circular shape. For
example, as shown in FIG. 16, a looped coil element
70 having another shape
may be used (e.g., in conjunction with circular coil elements
53G and
53H).
In addition, the respective embodiments may be practiced upon being properly
combined
as much as possible. In this case, an effect corresponding to the combination can
be obtained. The above embodiment includes inventions of various stages, and various
inventions can be extracted by proper combinations of a plurality of disclosed
constituent elements. When, for example, the problem described in "BACKGROUND OF
THE INVENTION" can be solved and at least one of the effects described in "BRIEF
SUMMARY OF THE INVENTION" can be obtained even if several constituent elements
are omitted from the all the constituent elements in each embodiment, the arrangement
from which these constituent elements are omitted can be extracted as an invention.
As has been described above, according to this embodiment, there is provided a
magnetic resonance imaging apparatus which can freely apply the PI method to a
slice in an arbitrary direction in the imaging method using a plurality of surface
coils, thereby obtaining a magnetic resonance image with a high S/N ratio or at
high speed, and an RF coil used in the apparatus.
*
