JPH07308304A - Rf coil for use - Google Patents

Abstract

PURPOSE:To realize an RF coil for a bird cafe coil-type MRI use wherein, while uniformity of magnetic field is ensured in a region being necessary for photographing, the strength of magnetic field can be decreased in a region being unnecessary for photographing. CONSTITUTION:This RF coil for MRI use with a bird cage structure is constituted of the first and the second ring parts 2a and 2b being arranged in parallel each other each with approximately circular shapes, a plurality of elements 3 connecting points being apart at a specified distance along the peripheral edges of the first and the second ring parts 2a and 2b and above one intermediate ring parts 5a and 5b being arranged on a plurality of elements 3 between the first and the second ring parts 2a and 2b and approximately circular.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an RF coil used in a magnetic resonance imaging (MRI) apparatus, and more particularly to an RF coil for MRI having a birdcage coil structure.

[0002]

2. Description of the Related Art An MRI apparatus measures the density distribution, relaxation time distribution, etc. of nuclear spins at a desired examination site in a subject by utilizing the nuclear magnetic resonance phenomenon, and the cross section of the subject is determined from the measured data. The image is displayed.

Nuclear spins of a subject placed in a uniform and strong static magnetic field generator perform precession around the direction of the static magnetic field at a frequency (Larmor frequency) determined by the strength of the static magnetic field. Therefore, when a high frequency pulse having a frequency equal to this Larmor frequency is irradiated from the outside, spins are excited and transition to a high energy state. This is called a nuclear magnetic resonance phenomenon. When the irradiation of this high-frequency pulse is stopped, the spin returns to the original low energy state with a time constant corresponding to each state, and at this time, the electromagnetic wave is emitted to the outside. A high-frequency receiver coil (R
F coil) to detect. At this time, a triaxial gradient magnetic field is applied to the static magnetic field space for the purpose of adding position information to the space. As a result, position information in the space can be captured as frequency information.

In such an MRI apparatus, an RF coil used for applying a high-frequency rotating magnetic field to a subject accommodates the subject therein, and a high-frequency current is applied to a portion of a wire ring (element) around the subject. Is flowing.

As one type of the structure of this RF coil, there is a shape called a birdcage structure. FIG. 6 shows an example of a high-pass type RF coil of the birdcage coil structure (hereinafter, simply referred to as a birdcage coil).

The first ring portion 2a and the second ring portion 2
b (hereinafter, also simply referred to as a ring portion) forms a conductive loop and has a substantially circular shape, and the two ring portions 2a and 2b are arranged such that their surfaces are parallel to each other. The element 3 is formed by connecting the two ring portions 2a, 2b orthogonally to the planes of the ring portions 2a, 2b at the points spaced at equal intervals along the periphery of each of them. It is configured. The capacitor 4 is inserted in series between the element connection points of the ring part 2 and the element 3
Is selected so as to resonate at the Larmor frequency with the inductance and the capacitance of the capacitor 4. Since such a birdcage coil can simultaneously combine both in-phase and quadrature components and has a sufficiently high Q, it is often used.

[0007]

The birdcage coil shown in FIG. 6 is of a high-pass type, in which the element 3 mainly has an inductance component and the ring portion has a capacitance component. Further, in the low-pass type (not shown), the inductance and the capacitance are arranged in reverse. In such a bird cage coil, when the total length of the coil (= element length) and the cross section of the coil (cross section perpendicular to the element) are determined, the magnitude of the inductance of the element and the ring portion is determined. In order to make this birdcage coil resonate at a required frequency (= Larmor frequency), the required capacitance is uniquely determined. That is, by determining the shape of the birdcage coil, the constants of the elements in each part are uniquely determined. Further, by determining the shape of the birdcage coil, the magnetic field distributions in the surroundings and inside are also almost determined.

FIG. 7 is a characteristic diagram showing a magnetic field distribution of the birdcage coil 1 of this type. In this FIG. 7, the horizontal axis represents the position in the element direction, and the vertical axis field represents the intensity of the magnetic field distribution. Here, it is assumed that the range from a to b is an area where imaging is performed. In such a case, it is conceivable to increase the total length of the coil in order to improve the homogeneity of a to b (flatness of the magnetic field distribution strength) as much as possible. Normally, the characteristics of the magnetic field distribution as shown in FIG. 7 are used in consideration of the homogeneity, but there is an unnecessary magnetic field distribution (sensitivity) also in the regions outside a and b. Since this unnecessary magnetic field distribution also has a certain intensity, unnecessary power consumption occurs if the subject is present in that portion during transmission. Further, at the time of reception, since a signal from a wide area is received, a signal component (that is, noise) from outside the desired area increases. As a result, the SN ratio decreases.

On the contrary, in order to reduce the magnetic field distribution (or sensitivity) in the regions outside a and b, it is conceivable to reduce the total length of the birdcage coil to obtain the characteristics shown in FIG. . However, in this case, the homogeneity of the magnetic field strength in the necessary regions (a to b) is lowered in exchange for the reduction in the magnetic field distribution and sensitivity in the unnecessary regions. As a result, the imaging performance is degraded.

The present invention has been made in view of the above points, and it is an object of the present invention to reduce the magnetic field strength in an area not required for imaging while ensuring the homogeneity of the magnetic field in the area required for imaging. It is to realize a bird cage coil type RF coil for MRI.

[0011]

Means for Solving the Problems The above-mentioned problems are determined along first and second ring portions which are substantially circular and are arranged so as to be parallel to each other, and along the peripheral edges of the first and second ring portions. A plurality of elements that connect points separated by a distance of, and one or more substantially circular intermediate ring portions arranged so as to be connected to the plurality of elements between the first and second ring portions. It is achieved by a birdcage structure RF coil for MRI.

Further, the above problem is that the first and second ring portions, which are substantially circular, are arranged so as to be parallel to each other, and the first and second ring portions are separated from each other by a predetermined distance along the periphery of the first and second ring portions. A plurality of elements that connect only points to each other;
And a second ring portion, and a plurality of substantially circular intermediate ring portions arranged on the plurality of elements so as to sandwich a region for imaging, the birdcage structure. This is achieved by the RF coil for MRI.

[0013]

The RF for MRI which is the first means for solving the problems
The coil includes one or more substantially circular intermediate ring portions arranged so as to be connected to the plurality of elements between the first and second ring portions, and the intermediate ring portions cause the current direction of the elements to flow. Changes. As a result, the homogeneity of the magnetic field is ensured in the area required for imaging, and the magnetic field strength in the area not required for imaging is reduced.

A second means for solving the problem, M
The RF coil for RI has two or more substantially circular intermediate ring portions arranged so as to be connected to the plurality of elements between the first and second ring portions, and the intermediate ring portions form the elements. The current direction of changes alternately.
As a result, the homogeneity of the magnetic field is ensured in the region sandwiched by two or more intermediate ring portions and required for imaging, and the magnetic field strength is reduced in the regions located outside the intermediate ring portion and not required for imaging. .

[0015]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a configuration diagram showing a configuration example of a birdcage coil type RF coil for MRI according to an embodiment of the present invention. Here, a high-pass birdcage coil will be described as an example. In FIG. 1, the first ring portion 2a and the second ring portion 2b (hereinafter, also simply referred to as ring portions) form a conductive loop and have a substantially circular shape, and the surfaces of the ring portions 2a and 2b are They are arranged in parallel. Element 3 consists of these two ring parts 2
a and 2b are connected orthogonally to the surfaces of the ring portions 2a and 2b at points spaced at equal intervals along their respective peripheral edges, and are composed of a plurality of elements. The capacitor 4 is inserted in series between the element connection points of the ring portion 2, and is selected to have a value that causes the inductance of the element 3 and the capacitance of the capacitor 4 to resonate at the Larmor frequency. The intermediate ring portions 5a and 5b are arranged on the plurality of elements 3 between the ring portions 2a and 2b.

FIG. 2 shows an equivalent circuit at the resonance frequency of the birdcage coil. That is, the birdcage coil shown in FIG. 1 can be represented by the equivalent circuit of FIG. 2, and the capacitance of the capacitor at each position can be expressed by the following equation. Cm = Cm '/ (1-cos (2 mπ / n)) where m is a number corresponding to the position of the capacitor (1 to
4) and n mean the number of elements.

Therefore, after obtaining the capacitance C1 'of the capacitor in the equivalent circuit of FIG. 2, the actual capacitance C1 of the capacitor of the ring portion 2a can be determined in consideration of the number of elements. Capacitance C2 of the intermediate ring portion 5a, capacitor C3 of the intermediate ring portion 5b, ring portion 2b
The same applies to the capacitance C4 of the capacitor.

FIG. 3 is an explanatory view showing a current state and a magnetic field distribution of the birdcage coil of this embodiment in a resonance state at a certain moment. As is clear from FIG. 3A, the birdcage coil in the resonance state can be treated as a resonance circuit. In the birdcage coil having two intermediate rings shown in FIG. 3, it can be considered that three circuits are connected. in this case,
For convenience, consider the three circuits as
The circuit where the capacitors in the middle ring are adjacent (and
And) are common, so the direction of the current in the adjacent circuit is opposite due to the direction of the voltage in each capacitor. That is, if C2 'is − in the upper side of the figure and + in the lower side, a clockwise current flows in the circuit and a counterclockwise current flows in the circuit. Similarly, when a counterclockwise current flows in the circuit, the capacitor C3 'has + on the upper side and − on the lower side, so that a clockwise current flows in the circuit. In this way, the directions of the currents are alternately inverted in adjacent circuits.

In such a current distribution, the magnetic field distribution generated from each current is as shown in FIG. 3 (b). Here, FIG. 3B shows a magnetic field generated by the current shown in FIG. Further, FIG. 3B shows a magnetic field generated by the current shown in FIG. And FIG.3 (c) is a magnetic field produced by the above-mentioned electric current of FIG.3 (a). As described above, the direction of the electric current is reversed in each adjacent circuit, so that the direction of the generated magnetic field is also reversed.

Therefore, when the respective magnetic fields are combined, FIG.
The distribution is shown in. This magnetic field distribution is such that both ends of the magnetic field distribution generated in the central circuit are canceled by the magnetic field distribution of the circuit. As a result, the homogeneity of the magnetic field distribution is increased in the area where the circuit is located, and the magnetic field distribution is almost zero in the unnecessary area outside the area.

Therefore, the positions of the region for enhancing the uniformity and the region for canceling the magnetic field distribution are determined by the positions of the intermediate ring portions 5a, 5b. In addition, since the magnitude (ratio) of each magnetic field strength in FIG. 3B can be changed by adjusting the ratio of the inductance of the element and the capacity of the capacitor in the circuit, the uniformity can be adjusted and the unnecessary magnetic field strength can be reduced. The level can be adjusted freely. That is, it is possible to simultaneously increase the homogeneity of the magnetic field strength in the imaging area and reduce the magnetic field strength outside the imaging area. In addition, by reducing the magnetic field strength outside the imaging region, power consumption by the subject does not occur during transmission, which can contribute to reduction in power consumption of the MRI apparatus, and can reduce the power consumption of the MRI apparatus from unnecessary regions during reception. Since there is no reception of, the SN ratio is improved.

Further, as shown in FIG. 4, the ring portion 2a (2b)
Transmitter / receiver terminals 6a and 6b provided at different positions by 90 °
Quadrature transmission / reception for transmission / reception can also be performed by the birdcage coil of this embodiment. In that case, transmission and reception can be performed from the outer ring portions 2a and 2b as in the conventional birdcage coil, and as shown in the development view of FIG. 5, the outer ring portions 2a and 2b and the intermediate ring portions 5a and 5b. It is also possible to send and receive from all of. Further, in addition to the transmission / reception from the ring portion only on the outer side and the transmission / reception from all the ring portions, the transmission / reception can be performed by the ring portion at any position.

In the above description, the case where the number of the intermediate ring portions is two has been described, but other embodiments can be considered. For example, if the number of the intermediate rings is one, it is possible to increase the homogeneity of either one when viewed from the center and reduce the magnetic field intensity outside one of the imaging regions. Further, by using three or more intermediate rings, it is possible to finely adjust the homogeneity within the imaging region and the reduction of the magnetic field strength outside the imaging region. The shape of the magnetic field distribution shown in FIG. 3C can be arbitrarily changed by setting the arrangement of the intermediate ring to an arbitrary position. For example, it is possible to create a magnetic field distribution that is not of interest when viewed from the center position.

Although the high-pass birdcage coil has been described in the above embodiments, the above-described actions and effects can be achieved by providing the intermediate ring even in the low-pass birdcage coil. is there. Further, in the above embodiments, the individual elements and the single capacitors are provided at the respective positions, but in addition to this, the element and the intermediate ring part may be configured by an element group in which a plurality of coils and capacitors are combined in each part. Is also possible. It is also possible to arrange a switch on the intermediate ring portion or element so that the magnetic field distribution can be changed as necessary. With this configuration, in the case of reception, it is possible to switch between a case where the sensitivity is wide in a low SN ratio and a case where the sensitivity is constant in a narrow range with a high SN ratio, as needed.

As described above, the substantially circular first and second ring portions arranged in parallel to each other and the first and second ring portions are separated from each other by a predetermined distance along the peripheral edges of the first and second ring portions. A plurality of elements connecting the points, and the first
And the second ring portion, the bird's-cage structure MRI RF coil comprising one or more substantially circular intermediate ring portions arranged to be connected to a plurality of elements. The current direction of the element changes because the portion is arranged in the middle of the element.
As a result, the homogeneity of the magnetic field is ensured in the area required for imaging, and the magnetic field strength in the area not required for imaging is reduced.

Further, the substantially circular first and second ring portions arranged so as to be parallel to each other are connected to the points separated by a predetermined distance along the peripheral edges of the first and second ring portions. A plurality of elements, and two or more substantially circular intermediate ring portions arranged between the first and second ring portions so as to sandwich a region for imaging on the plurality of elements. According to the MRI RF coil having the birdcage structure characterized in that the current direction of the element is alternately changed by disposing the intermediate ring portion in the middle of the element. As a result, the homogeneity of the magnetic field is ensured in the region sandwiched by two or more intermediate ring portions and required for imaging, and the magnetic field strength is reduced in the regions located outside the intermediate ring portion and not required for imaging. .

[0027]

As described in detail above, according to the present invention,
By disposing the intermediate ring portion on the plurality of elements between the first and second ring portions, the current direction of the element changes, so that the homogeneity of the magnetic field is ensured in the region necessary for imaging. A birdcage coil type MRI RF coil capable of reducing the magnetic field strength in a region not required for imaging can be realized.

[Brief description of drawings]

FIG. 1 is a birdcage coil type M according to an embodiment of the present invention.
It is a block diagram which shows the detail of the structural example of RF coil for RI.

FIG. 2 is a birdcage coil type M according to an embodiment of the present invention.
It is a circuit diagram which shows the equivalent circuit of RF coil for RI.

FIG. 3 is a birdcage coil type M according to an embodiment of the present invention.
It is explanatory drawing which shows the electric current distribution and magnetic field distribution of RF coil for RI.

FIG. 4 is a birdcage coil type M according to an embodiment of the present invention.
It is explanatory drawing which shows the detail of quadrature transmission / reception of RF coil for RI.

FIG. 5 is a birdcage coil type M according to an embodiment of the present invention.
It is explanatory drawing which shows the detail of quadrature transmission / reception of RF coil for RI.

FIG. 6 is a configuration diagram showing a configuration example of a conventional birdcage coil type RF coil for MRI.

FIG. 7 is a characteristic diagram showing a distribution of magnetic field strength of a birdcage coil.

[Explanation of symbols]

 1 Bird cage coil type RF coil for MRI 2a, 2b Ring part 3 Element 4 Capacitor 5a, 5b Intermediate ring part

Claims (2)

[Claims] 1. A substantially circular first and second ring portion arranged in parallel with each other, and connecting points separated by a predetermined distance along the peripheral edges of the first and second ring portions. A plurality of elements, and one or more substantially circular intermediate ring portions arranged so as to be connected to the plurality of elements between the first and second ring portions. Bird cage structure M
RF coil for RI. 2. A substantially circular first and second ring portion arranged so as to be parallel to each other, and points which are separated from each other along a peripheral edge of the first and second ring portion by a predetermined distance. A plurality of elements, and two or more substantially circular intermediate ring portions that are arranged between the first and second ring portions so as to sandwich the imaging region on the plurality of elements. Bird cage structure M characterized by
RF coil for RI.