JPH03251227A - Mri device - Google Patents

Abstract

PURPOSE:To stabilize the receiving sensitivity characteristic at the time of photographing by placing a receiving RF coil in a position being eccentrically from the center of a photographing hole by holding the RF coil in each position in which an electromagnetic interaction of the RF coil and a conductive member is equivalent to each other. CONSTITUTION:A conductive member 3 is provided with a shield member 3a for shielding an unnecessary RF wave placed in the periphery of a photographing hole 2, a whole body RF coil 3b which can excite an atomic nucleus of a photographing part of a body to be examined, a gradient magnetic field coil 3c for generating gradient magnetic field and giving coordinate information to an MR signal from the body to be examined, and a static magnetic field coil 3d for generating a static magnetic field. The shield member 3a, and the coils 3b to 3d are both formed cylindrically, and have a geometrical symmetric property in which symmetry axes exist infinitely in a cross section being orthogonal to the center axis direction Z of the photographing hole 2. By holding the RF coil by a coil moving mechanism 7 which can hold it in a position an electromagnetic interaction of a QD coil 6 and a conductive member 3 of an MR rack base 4 is equivalent to each other, the receiving sensitivity characteristic of the RF coil is stabilized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Objective of the Invention (Field of Industrial Application) The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an "rMRr apparatus"), and more particularly to stabilization of reception sensitivity characteristics of an RF coil.

(Prior art) An MR mount in an MRI apparatus includes an imaging hole in which a subject is placed, a static magnetic field coil that generates a static magnetic field around the imaging hole, a gradient magnetic field coil that generates a gradient magnetic field, and transmits and receives RF waves. It is equipped with various conductive members necessary for imaging, such as a whole-body RF coil that can generate waves, and a shield member that blocks unnecessary RF waves.

Furthermore, when performing local imaging of the upper and lower limbs of a human body, the top plate on which the subject is placed is moved into the imaging hole, and the subject is placed within the imaging hole. Then, a receiving RF coil formed in a cylindrical shape is inserted into the cylinder, and the limb to be photographed is inserted into the cylinder.

Next, when the whole body RF coil irradiates an excitation pulse to excite the atomic nucleus of the limb inserted into the cylinder of the receiving RF coil, the receiving RF coil receives the MR multiplied signal generated from the excited atomic nucleus of the limb. To detect. This detected MR multiplied signal is taken into the data processing unit, reconstructed and CR
An MR image of the limb to be photographed is displayed on the T-display.

The receiving RF coil is placed inside the imaging hole during imaging,
It is placed close to the conductive member of the MR frame. When the receiving RF coil approaches the conductive member, the receiving RF coil receives electromagnetic interaction with the conductive member, and the receiving sensitivity characteristics of the MR multiplier signal receiving system change. Therefore, the receiving RF coil is adjusted to have the best receiving sensitivity characteristics on the premise that it is always placed at the center of the imaging hole.

However, when performing local imaging of a part of the human body, such as a limb, which is eccentric from the human body, the receiving RF coil is positioned away from the center of the imaging hole. For this reason, the positional relationship between the conductive member around the imaging hole and the receiving RF coil changes, and the electromagnetic interaction between the conductive member and the receiving RF coil changes, causing the reception sensitivity of the receiving RF coil to change. Characteristics deteriorate.

The test data at this time is shown in FIG. Figure 10 shows
Measurements were taken for two channels (Ca, Cb) of a 90 degree phase shift coil (hereinafter referred to as rQD (qlIdrature) coil), with the horizontal axis representing frequency and the vertical axis representing received intensity. As a QD coil, each channel Ca and Cb is formed in a direction in which the reception sensitivity direction (phase) is different from each other by 90 degrees, and the elliptical coils are made to intersect with each other to form a cylindrical shape with a diameter of 20 cm and a length of 20 cm. (referred to as a cross-elliptic QD coil) was used. Further, the upper part of the figure shows the received strength Da of channel Ca, and the lower part of the figure shows the received strength Db of channel Cb. In addition, the curve shown by the dotted line in the same figure represents the reception strength measurement results when the QD coil was placed at the center of the imaging hole and the tuning circuit of the QD coil was adjusted to a resonance frequency of 21.3 MH2.
The curve shown by the solid line represents the reception strength measurement result when the QD coil was moved 12 cm in the lateral direction X from the center of the imaging hole.

In addition, the method of measuring the reception strength is to place a loop antenna for measurement inside the cylinder of the QD coil, and to
By transmitting radio waves in the same direction as the receiving sensitivity direction and measuring the receiving strength of each channel Ca and Cb, channel independence, that is, the sensitivity difference between the receiving sensitivity of channel Ca and the receiving sensitivity of channel cb is determined. The larger the difference in sensitivity, the more excellent the QD coil is. The measurement results are the 10th
As shown in the figure, the resonance frequency of channel Ca is 21.
4 MH2 and 100 KH2, the received strength of channel Ca at 21.3 MH2 decreased by about 7 dB, and the received strength difference between channels Ca and Cb decreased from 34 dB to 18 dB.
dB, and the receiving sensitivity characteristics have deteriorated.

If the MR multiplied signal is collected and reconstructed while the receiving sensitivity characteristics are degraded in this way, the S/N ratio will decrease and the image quality will deteriorate.

Also, if readjustment is performed to prevent the S/N ratio from decreasing, the automatic adjustment of the tuning circuit will correct the shift in the resonant frequency in about 1 minute, but the difference in reception strength between channels Ca and Cb will be corrected in about 1 minute. Adjusting the sensitivity difference to a predetermined value takes an hour or more even for an experienced person, resulting in poor imaging efficiency.

(Problems to be Solved by the Invention) As described above, in the conventional device, when the receiving RF coil is arranged at a position eccentric from the center of the imaging hole to perform imaging, the receiving sensitivity characteristics deteriorate and the S/N There was a problem that the ratio decreased.

Therefore, the present invention has been made in view of the above-mentioned circumstances, and provides an MRI apparatus that can stabilize reception sensitivity characteristics when performing imaging with a receiving RF coil disposed at a position eccentric from the center of the imaging hole. It is intended to.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention has an imaging hole in which a subject is placed and a geometric symmetry provided around the imaging hole. an MR frame configured with a conductive member;
In an MRI apparatus having an RF coil disposed in the imaging hole and capable of transmitting and receiving RF waves to and from the subject, the RF coil is arranged in a manner that the electromagnetic interaction between the RF coil and the conductive member is equivalent to each other. It is characterized by comprising a holding means capable of holding it in position.

(Function) The function of the apparatus having the above configuration will be explained with reference to FIGS. 6 to 8.

If the conductive member in the MR frame of this apparatus is formed into a circular shape as shown in FIGS. 6 to 8, the number of geometric symmetry axes of the conductive member 3 will be infinite. Therefore, there are infinitely many positions where the electromagnetic interaction between the conductive member 3 and the RF coil C is equivalent to each other.

The positions where this electromagnetic interaction is mutually equivalent may be positions equidistant S from the center 0 of the conductive member 3 as shown in FIG.
In some cases, the position is higher and equidistant S from the axis of symmetry passing through the center O. Furthermore, as shown in FIG. 8, the RF coils C may be at positions rotated by an angle θ with respect to the center O.

Even when the conductive member 3 has a shape other than a circle, for example, a rectangular shape, there are positions where the electromagnetic interaction is equivalent to each other.

In this way, at each position where the electromagnetic interactions are equivalent to each other,
When the RF coil C is held by the holding means, the receiving sensitivity characteristics of the RF coil C are stabilized.

(Example) An example device 1 of the present invention will be described below with reference to the drawings.

FIG. 1 shows a block diagram of a device 1 according to a first embodiment of the present invention.

The present device 1 includes an MR mount 4 that includes a cylindrical imaging hole 2 in which a subject is placed, and a conductive member 3 with geometric symmetry provided around the imaging hole 2. , a top plate 5 as a holding means on which a subject is placed and moved along the central axis direction 2 of the imaging hole 2; A cross-elliptic QD coil 6 as a coil and this QD coil 6 can be moved in the direction It has a coil moving mechanism 7 as a holding means.

The conductive member 3 is connected to an unnecessary R disposed around the imaging hole 2.
A shield member 3a that shields F waves, a whole-body RF coil 3b that can transmit and receive RF waves to excite the atomic nucleus in the imaging region of the object, and generate a gradient magnetic field to obtain MR multiplied signal coordinate information from the object. It has a gradient magnetic field coil 3C that generates a static magnetic field, and a static magnetic field coil 3d that generates a static magnetic field. The shield member 3a.

The coils 3b to 3d are both formed in a cylindrical shape,
Therefore, in a cross section perpendicular to the central axis direction Z of the imaging hole 2, it has geometric symmetry with an infinite number of symmetry axes.

This device 1 also includes a top plate moving unit 8 for moving the top plate 5 in the direction Z, an RF wave transmitting unit 9 for outputting a pulse signal for transmitting RF waves to the whole body RF coil 3b, and a tilt A magnetic field power supply section 10 for supplying power to the magnetic field coil 3c and the static magnetic field coil 3d, and the RF detected by the QD coil 6.
It has an RF wave receiving section 11 that receives waves, and a system controller 12 that controls each section of this device 1.

The structure of the coil moving mechanism 7 will be explained with reference to FIGS. 2 to 4.

2 is a perspective view of this coil moving mechanism 7, FIG. 3 is a bottom view of this mechanism 7, and FIG. 4 is a sectional view taken along line AA in FIG. 3.

This coil moving mechanism 7 includes a guide hole 7a that allows the QD coil 6 to move in the direction X shown in FIGS.
It has a pin 7b connected to the bottom 6a of the D coil 6, and a spring member 7c that holds the QD coil 6 at both ends of the movable range of the QD coil 6. By manually moving the QD coil 6, the coil 6 can be easily held at both ends of the movable range in the direction A as shown in FIG. 1 or 2.

FIG. 9 shows the measurement results of the receiving sensitivity characteristics when the QD coil 6 is moved as shown in FIG. 6 in this device 1.

This measurement result is obtained when the whole body RF coil 3b is used for transmitting RF waves, and the QD coil 6 is used for receiving RF waves. In addition, the coil used is a cross elliptical QD coil similar to that used in the measurement with the conventional device, and the center O
Move the resonance frequency by 8 an to one side in the X direction to increase the resonance frequency by 21.
After adjusting to 4 MH2 and measuring the receiving sensitivity characteristics (shown by the dotted curve in the figure), move this QD coil to the other side in the X direction by 8
The reception sensitivity characteristics were similarly measured (indicated by the solid curve in the figure) after moving by cm. As a result, there is almost no change in the resonance frequency and the reception strength difference of 30 dB between channels Ca and Cb, which not only eliminates the need for conventional frequency adjustment and sensitivity difference adjustment, but also allows the reception sensitivity characteristics to remain unchanged. QD coils can be easily arranged.

This makes it possible to provide an MRI apparatus having stable reception sensitivity characteristics.

Particularly when performing local imaging of positions eccentric from the center of the human body, such as upper and lower extremities, there is little change in reception sensitivity characteristics between, for example, when imaging the right lower limb and when imaging the left lower limb. MR images can be obtained.

In addition, when using a coil with multiple channels such as a QD coil, a signal with a high S/N ratio can be obtained, and
MR with stable receiving sensitivity characteristics and no adjustment required
I equipment can be provided.

FIG. 5 is a perspective view showing another example 17 of the coil moving mechanism 7 shown in FIGS. 1 to 4.

This coil moving mechanism 17 moves along an arcuate rail 17a placed on the top plate 5, and this arcuate rail 17a, and holds a surface coil 16 as an RF coil so as to be movable in the normal direction. It has a holder 17b that
Move the surface coil 16 to the desired position and turn the knob 17c.
17d so that it can be fixed. As shown in FIG. 8, this coil moving mechanism 17 is capable of continuously moving and fixing the surface coil 16 to a position where the receiving sensitivity characteristics are constant.

According to the MRr device 1 using this coil moving mechanism 17, the receiving sensitivity characteristics are stable even if the surface coil 16 is moved along the arcuate rail 17a.

Although the embodiments have been described above, the present invention is not limited thereto and can be modified in various ways.

For example, although the coil moving mechanism is disposed on the top plate, it may be fixedly disposed within the imaging hole. Further, the position where the electromagnetic interaction is mutually equivalent exists also in the direction Z along the central axis of the imaging hole, and the RF coil is placed and maintained at the position where the electromagnetic interaction is mutually equivalent in this direction as well. You can do it like this.

[Effects of the Invention] According to the present invention described in detail above, when the RF coil is arranged at a position eccentric from the center of the imaging hole and the RF coil is taken, the RF coil is held by the holding means, and the electromagnetic force between the RF coil and the conductive member is Since the mutual interaction can be maintained at each position equivalent to each other, it is possible to provide an MRI apparatus in which receiving sensitivity characteristics can be stabilized.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a device according to an embodiment of the present invention, FIGS. 2 to 4 are diagrams showing a coil moving mechanism of this device, and FIG. 5 is a diagram showing a coil moving mechanism other than those shown in FIGS. 2 to 4. 6 to 8 are explanatory diagrams illustrating the principle of the present invention, FIG. 9 is a diagram showing the measurement results of the reception strength of the QD coil in this device, and FIG. 10 is a diagram showing the QD in the conventional device. FIG. 3 is a diagram showing the measurement results of the received strength of the coil. DESCRIPTION OF SYMBOLS 1... MRr apparatus, 2... Photographing hole, 3... Conductive member, 3a... Shield member (conductive member), 3b.
... RF coil for whole body (conductive member), 3c... gradient magnetic field coil (conductive member), 3d... static magnetic field coil (
conductive member), 4...MR mount, 5...top plate (holding means), 6...cross elliptical QD coil (RF coil)
, 7.17... Coil moving mechanism (holding means), 16.
...Surface coil (RF coil), Fig. 3 Fig.

Claims (4)

[Claims] (1) An MR mount configured with an imaging hole in which a subject is placed and a conductive member having geometric symmetry provided around the imaging hole, and An MRI apparatus having an RF coil capable of transmitting and receiving RF waves to and from a specimen, comprising a holding means capable of holding the RF coil at each position where electromagnetic interaction between the RF coil and the conductive member is equivalent to each other. M characterized by having done
RI device. (2) The holding means includes a top plate on which the subject is placed and moves within the imaging hole, and a coil moving mechanism arranged on the top plate and capable of moving the RF coil. MRI machine. (3) The MRI apparatus according to claim 1, wherein the holding means is fixedly arranged within the imaging hole. (4) The MRI apparatus according to claim 1, 2 or 3, wherein the RF coil has a plurality of channels.