RF coil, shield and magnetic resonance imaging apparatus

Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation

Reexamination Certificate

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Reexamination Certificate

active

06618610

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an RF (radio frequency) coil, shield and a magnetic resonance imaging apparatus, and particularly relates to a TEM (transverse electromagnetic mode) resonator RF coil, a shield used to adjust TEM RF coils, and a magnetic resonance imaging apparatus using the TEM resonator RF coil.
In magnetic resonance imaging apparatus using for example a high magnetic field of approximately 3 T (tesla) as the static magnetic field, TEM resonator RF coils are utilized on account of a high RF (radio frequency) signal transmit/receive efficiency to receive magnetic resonance signals occurring from spin excitation and RF excitation of the spin of the imaging target.
The TEM resonator RF coil has a cylindrical tube
700
as shown in FIG.
1
. Both ends of the tube
700
have orifices
702
,
702
′. The orifices
702
,
702
′ have openings
704
,
704
′ concentrically formed with a smaller inner diameter than the inner diameter of the tube
700
.
The tube
700
and the orifices
702
,
702
′ are formed with consecutive conductive elements. Tube bodies having a tube
700
and orifice
702
,
702
′ of this kinds are called shields or cavities.
A plurality of line elements
802
are disposed on the inner side of the shield in parallel with the shield axis. The line elements
802
form an LC series circuit. Both ends of the plural line elements
802
are respectively coupled mechanically and electrically to the orifices
702
,
702
′, and arranged concentrically along the periphery of the openings
704
,
704
′. The line elements
802
are separated from the inner side of the tube
700
.
A rotating RF magnetic field is generated within the surface axially perpendicular to the columnar space enclosed by the line elements
802
, by supplying an RF signal to specified locations of an RF coil of this structure. The RF signal (magnetic resonance signal) generated by the rotating spin on the same plane is extracted from specified positions of this RF coil.
In actual magnetic resonance imaging, the shield is separated into a plurality of slits
706
in the axial and radial directions as shown for example in FIG.
2
. Each of the shield segments separated by the slits
706
are electrically isolated, preventing excess current from flowing along the outer periphery of the shield during application of a magnetic field gradient, and preventing disturbances in the static magnetic field during excess current flow.
In the RF coil having the shield separated by slits, the shield effect is decreased because both ends of the orifices
702
,
702
′ are separated into individual segments by the slits
706
. When this kind of RF coil is used as the head coil for capturing images of the cranium, the neck section of the target image is added as an external load to the other orifice so that the electrical characteristics are asymmetrical along the coil axis and the RF coil operation tends to be unstable.
To achieve stable operation, capacitors
122
are disposed to connect the adjoining shield segments on both ends of the orifices
702
,
702
′ of the shield as shown in FIG.
3
.
Capacitors used as the capacitor
122
have for example, sufficient high impedance in a frequency region of 1 kilohertz to 10 kilohertz and for example, sufficiently low impedance in the frequency range of approximately 128 megahertz.
The orifices
702
,
702
′ connected by such capacitors are equivalent to an electrical short in the RF region across the slit
706
. A plurality of the line elements
802
therefore have a value equivalent to conductive elements each jointly connected on both ends, so the shield effect in the RF region is improved, and operation is stabilized regardless of whether a load is present or absent in the vicinity of the orifices
702
,
702
′.
In the 1 kilohertz to 10 kilohertz frequency region on the other hand, the capacitor
122
has a sufficiently high impedance so that excess current does not flow to the outer periphery during application of a magnetic field gradient forming the signal in the same frequency range. Therefore the effect of excess current on the static magnetic field can be prevented.
The RF coil imaging object can be easily input and output so that a two-segment structure along the axis can be formed as shown for example in
FIG. 4
, and both segments joined by a connector. In such a case, the width of the slit
706
in the connector joint must be wider than the other sections so that the static electrical coupling in that section is weak and the shield effect deteriorates.
In the joint of the connector
124
, the slits
706
,
706
′ are respectively rotated into both shield segment sides as shown for example in
FIG. 5
, conductive foil lands
126
,
126
′ are respectively formed, and electrically connected to the connector
124
, and the slits
706
,
706
′ respectively bridged to the capacitors
122
,
122
′. In this way a sufficient static electrical connection can be maintained in the segments.
In the above segmented TEM resonator RF coil, the conductive pattern for electrical connection to the connectors is different from the conductive pattern of other RF coils so that the circuits in the segments have poor uniformity, and the operating characteristics as an RF coil deteriorate.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a TEM resonator RF coil with excellent operating characteristics, a shield for adjusting this type of RF coil, and a magnetic resonance imaging apparatus using this TEM resonator RF coil.
(1) To solve the above problems, in a first aspect of the invention, a TEM resonator RF coil is comprised of a cylindrical shield having a ring-shaped orifice at both ends, a plurality of line elements connected at both ends to the orifice and arranged at equal intervals along an opening of the orifice, a plurality of slits segmenting the shield into two equally portioned positions at line element intervals in parallel along the axis and forming a plurality of ring segments on the orifice, and the plural ring segments are comprised of a first conducting pattern connected to the line elements, a second conducting pattern disposed symmetrically for the line elements in two directions from the line elements towards the slit, capacitors connecting the first conducting pattern with the second conducting pattern, and a connection means to electrically connect the second conducting patterns spanning the adjacent ring segments.
In this aspect of the invention, the circuits between the segments are uniform (equivalent) because the plural ring sections made from an orifice segmented by slits, have a first conducting pattern connected to the line elements, a second conducting pattern disposed symmetrically to the first conducting pattern for the line elements, in two directions from the line elements towards the slit.
(2) In another aspect of the invention to solve the above problems, the RF coil according to (1) is characterized in that the shield can be divided into two portions at the slit locations.
In this aspect of the invention, the shield can be segmented into two portions at the slit locations so that the uniformity of the circuits is continually maintained between segments, and the imaging object can be easily inserted and removed.
(3) In another aspect of the invention to solve the above problems, the RF coil according to (1) is characterized in that the shield can be disassembled into a plurality of cylinder segments per the slit locations.
In this aspect of the invention, the shield can be disassembled into a plurality of cylinder segments at the slit locations so that the uniformity of the circuits is continually maintained between segments, and frequency alignment to the same frequency can be performed in each segment.
(4) In another aspect of the invention to solve the above problems, the TM resonator RF coil comprises a cylindrical shield having a ring-shaped orifice at both ends, a plurality of line elemen

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