JPH01238849A - Rf coil for head part - Google Patents

Abstract

PURPOSE:To provide an RF coil for a head part which improves SNR (Signal to Noise Ratio) of an apex during photographing of a head part and also improves operability, by a method wherein a title coil is formed with a conduction loop approximately in the shape of a circle, a plurality of conduction elements approximately in an arcuate shape, and volume elements, and is formed approximately in a semispherical shape. CONSTITUTION:An RF coil for a heat part is formed in a semispherical shape and comprises a circular conduction loop 8, 1/4 arcuate conduction elements 10 having the one end connected to the peripheral edge of a loop and the other end connected to a semispherical apex, and capacitors 11, each connected in series between element connection parts on the peripheral edge of the conduction loop. A feed point at a lowermost point seen from a direction -Z is formed, and an RF source is connected to both ends of the capacitor. When an RF voltage of a given frequency is applied, in the conduction element 10, resonance current distribution extending at an angle of theta circumferentially counterclockwisely starting from + of an Y-axis and proportioning costheta is formed. An RF magnetic field and receiving signal sensitivity being uniform on an axial surface can be provided. As a result, during photographing of a head part, the intensity of a receiving signal is increased, an SNR at an apex can be improved, mounting is facilitated, and operability is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention is directed to RF (Radio-F) of NHR-CT equipment.
The present invention relates to a head RF coil that transmits and receives (request) signals. For more details, see SNR (Sig
The present invention relates to a local coil for the head that has improved natural to Nolze RatiO).

(Prior Art) As is well known, the NHR-CT apparatus uses nuclear magnetic resonance (NHR).
) phenomenon to measure the density distribution and relaxation time of specific atomic nuclei such as protons to obtain a cross-section image of the human body, etc. This NHR31J! The method for measuring the image will be explained using FIG. 6. Figure 6 shows a conventional NHR-C
FIG. 2 is a schematic diagram of the T device. First, a static magnetic field H uniform in the Z direction
The subject 2 is inserted into the main magnetic field coil 1 which is generating . The gradient magnetic field coil 4 superimposes a linear gradient magnetic field Gz on the static magnetic field H, and generates a slice (
Select the 0th order (axial = RF rotating with
For example, in the spin warp method, a 90° pulse is first applied to induce the NOR phenomenon by transmitting a pulsed magnetic field.
After obtaining the signal, a 180° pulse is applied after τ seconds, and the RF coil 3 receives the NHR (echo) signal again. At this time, in the 2D Fourier method, the position information in the X direction is assigned to frequency (readout time) and collected using the readout gradient magnetic field Gx, and the NOR signal is repeated while regularly changing the magnitude of the gradient magnetic field Gy. By collecting the information, position information in the Y direction is provided as phase information. The collected NHR signals are reconstructed, and a density distribution image (tomographic image) of hydrogen nuclei and biochemical information about the pond are displayed on a CRT.

There are various shapes of the RF coil of such an 11t4R-CT apparatus as shown in FIG. Figure 7(a) shows a saddle-type coil with a high resonant frequency ω(ωX([C)-1
72), the inductance is configured to be small. However, with such a saddle-shaped coil, the uniformity of the RF magnetic field during transmission and the signal sensitivity during reception is not very good, and therefore the resolution of the obtained tomographic image cannot be improved. b) is a birdcage-type coil in which two conductive loops 51, 52 are separated by a predetermined distance in the direction of the static magnetic field, and these are interconnected by a plurality of axial conductive elements 6 having a specific inductance. A capacitive element 7 is connected in series to each conductive element. When an RF power source (not shown) is connected to both ends of one of the capacitive elements 7 and an RF voltage of a predetermined frequency is applied, the conductive loops 51 and 52 have a sinusoidal current distribution with θ in the circumferential direction. Each conductive element 6 has a sinusoidal current distribution with a phase delay of 90' from the loop current, and a uniform RF magnetic field and reception signal sensitivity can be obtained in the axial plane inside the coil (Japanese Patent Laid-Open No. 60-132547). , Japanese Patent Publication No. 61-9523
No. 4) and Fig. 7 (C) are surface type coils, which are used when locally receiving NHR signals and performing partial imaging, and the coils are placed closely to the imaging site. The SNR can be improved within a limited area.

(Problem to be Solved by the Invention) However, in the birdcage-type coil as described above, in the sagittal plane (YZ plane), the received signal sensitivity, that is, the SNR (Signal
to No.1ze Ratio) decreases. In addition, when photographing the head, the subject's head is placed in the center of a cylindrical RF coil, so there is no heat generated in areas other than the photographed area (head), such as the face and neck. Noise due to movement is received and the SNR deteriorates. In addition, although the surface type coil has good adhesion to the subject, the imaging area is narrow;
There is also a problem in that the operation of attaching it to the imaging site is cumbersome.

Therefore, an object of the present invention is to provide an RF coil for the head that improves the SNR of the top of the head when photographing the head and also improves operability.

(Means for Solving the Problems) In order to achieve the above object, the head RF coil of the present invention has the following configuration. That is, each conductive loop has a substantially circular shape, one end of which is connected to the periphery of the loop, and the other end of which is connected to a connecting portion that passes through the center of the loop and is placed at a predetermined distance on a line perpendicular to the loop plane. It is composed of a plurality of substantially arc-shaped conductive elements and a capacitive element connected in series between each of the conductive element connecting portions on the periphery of the conductive loop or to each of the conductive elements, and has a substantially hemispherical shape. It is characterized by

(Function) The surface current distribution of the coil is in an ideal state, creating a uniform magnetic field on the axial plane, and the coil shape is approximately hemispherical. In addition, due to the proximity of the coil surface to the head imaging surface, the received signal strength increases.
SNR improves.

(Example) FIG. 1 is a diagram showing an RF coil for a head according to an example of the present invention. As shown in FIG. 1(a), the head RF coil has a hemispherical shape, and includes a circular conductive loop 8, one end connected to the periphery of the loop, and the other end connected to the apex of the hemisphere.
That is, it is composed of a 174-arc conductive element 10 connected to the connecting portion 9, and a capacitor 11 connected in series between each of the electric conductive connecting portions on the periphery of the conductive loop 8, and has a hemispherical shape. The conductive loop 8 and the conductive element 10 are made of a material with good electrical conductivity, such as copper, silver, or copper plated with silver, and are shaped like a rod.
Constructed of tube or foil. Figure 1(b) is the same as Figure 1(b).
It is a figure which looked at the RF coil 5 of a) from the Z direction. As shown in FIG. 1(b), each conductive element 10 connects one end to the connecting portion 9 so that the connecting portion 9 passes through the center of the conductive loop 8 and is on a line perpendicular to the loop plane. , the other end is connected to the loop periphery at equal intervals. The number of conductive elements 10 is 16, and is 4n (
n is a natural number), the capacitors 11 at both ends of the conductive loop 8 are separated by 90 inches in the circumferential direction.
``Quadrature (Quadrature)' performs RE transmission with a phase shift, and when receiving, shifts the phase by 90 degrees and adds the double sign.
re) mode can be used. The advantage of using the orthogonal mode is that during transmission, the rotation mode of the magnetization vector is limited to only the rotation direction of the spins within the imaged area, reducing the transmission power to 172, and during reception, the NHR signal is doubled and the correlation is Since the noise signal without this signal is only 15@, the receiving sensitivity can be improved by four times.

For transmission and reception of RF signals, terminals are provided at both ends of one of the capacitors 11 or two capacitors separated by 90 degrees from each other, and an RF power source and a receiving circuit are connected. In addition, an impedance conversion circuit for preventing reflection, which is used in -a, is connected to the input/output part of the coil at an arbitrary position.
Rose for balanced unbalanced transformation: y (balanced
unbalanced transformer)
A circuit (both not shown) is connected between the capacitor and the terminal.

FIG. 2 is an equivalent circuit diagram of the head RF coil of this embodiment for reception only. In FIG. 2, 12, to 12,6 are the inherent inductances of the conductive element 10;
13 is a decoupling circuit that prevents magnetic coupling between the receiving head RF coil and the transmitting body coil (not shown) disposed outside the receiving head RF coil; 14 is connected in parallel to the capacitor 114; Variable capacitor (ba) with variable capacity
As shown in Figure 0, which is liabl13condenser), the overall structure is a bypass filter, and the resonance angular frequency of the coil is ω = γH (γ: gyromagnetic ratio, H: static magnetic field strength). Capacitor 11.
Determine the capacity of ~11.6 and adjust the value using a variable capacitor of 14 cm. The decoupling circuit 13 is also provided at a position of 90° in the circumferential direction of the loop in order to suppress the two orthogonal rotation modes of the magnetization vector, and at 180° and 270″ to more reliably prevent coupling. The decoupling circuit 13 is installed at each of a total of four capacitors at separate locations.
, 15b and an inductance 16, and during body coil transmission, the induced electromotive force generated in the inductances 12 to 12.6 creates a potential difference across the cross diodes 15a and 15b, and these are turned on alternately. As a result, the inductance 16 and the capacitor 11+ become a resonant circuit, and the resonant frequency of the head RE coil shifts, which prevents magnetic coupling between the head RF coil and the body RF coil. When receiving with the RF coil, the inductance is 121 because the NHR signal is weak.
~12.4 The induced electromotive force generated in the diodes 15a, 1
5b is not large enough to turn on, the decoupling circuit 13 does not operate.

Next, the operation of the head RF coil of this embodiment configured as described above will be explained. FIG. 3 shows the head R[
It is a current distribution diagram of a coil. As shown in Figure 3(a), -
A power supply point at the lowest point when viewed from two directions is provided, an RF' source (not shown) is connected to both ends of this capacitor, and an RF lightning voltage of a predetermined frequency is applied.

As a result, as shown in FIG. 3(b), the conductive element 10 has C
The resonant current distribution is proportional to OSθ.

This is an ideal current distribution on the coil surface, and it is possible to obtain a uniform RF magnetic field and reception signal sensitivity on the axial plane (CXY plane) inside the coil. FIG. 4 is a graph showing the 2 dependence of the SNR of the head R[coil] in this example. FIG. 4(a) is a diagram of the cheek RF coil viewed from a direction perpendicular to the Z-axis, with the point where the conductive loop surface intersects with the Z-axis as 0, and the distance in the coil in the Z-direction as dim. shall be. FIG. 4(b) shows the signal on the axial plane of dTi11M when the head RF coil of this example was measured using the single (Sin!11113> mode (one receiving AID)). Ratio of intensity to noise intensity (S14R)
(If the orthogonal mode is used, the SNR will be further p
In FIG. 4(b), the dotted line is the result of a comparative experiment using a conventional bird gauge coil. As shown in the graph, in the head RF coil of this embodiment, 5I4R exceeds the conventional birdcage coil where d is large. Therefore, in head imaging, the strength of the received signal at the top of the head is high, the SNR is improved, and the image quality of the top of the head can be improved. In this way, in the head RF coil of this embodiment, since the coil shape is hemispherical, only the head enters the coil when photographing the head, and everything other than the head is outside the coil. Therefore, reception of noise caused by thermal motion from outside the imaging area can be reduced. Furthermore, since the coil surface and the top of the head are in close contact, the received signal strength increases and the SNR at the top of the head improves. Furthermore, since the coil does not surround the area other than the head, visibility can be secured and the patient's feeling of pressure can be reduced.

Note that the present M invention is not limited to the above embodiments, and various modifications can be made within the scope of the claims. FIG. 5 is a schematic diagram showing a head RF coil according to another embodiment of the present invention. Symbols in FIG. 5 are used in the same way as in FIG. 1, so their explanations will be omitted here. In the above embodiment,
Although the capacitive element was provided in series with the conductive loop 8, it may also be provided in series with the conductive element 10 as shown in FIG. 5(a). In this case, one equivalent circuit becomes a low-pass filter type. FIG. 5(b) is a view of the RF=z-il 5 in FIG. 5(a) seen from two directions. In the embodiment described above, the connecting portion 9 was configured so that the conductive element 10 is connected at one point, but the area near the connecting portion 9 has a large angle with the Z axis and does not have an effective coil surface. , Figure 5(b)
The conductive loop 8 may be configured as a small conductive loop 17 with a small radius arranged in parallel with the conductive loop 8, or may have a metal plate structure, or the conductive loop 8 may have a shape other than a perfect circle. By adjusting the capacitance of the capacitive element, it is also possible to form an elliptical shape with the longitudinal direction as the major axis to match the shape of a human head. Similarly, the shape of the conductive element 10 is not a perfect 174-arc shape, but may have a curved shape.
1114, an element with a small current or a capacitive element may be removed.Furthermore, the capacitor 11 is connected in series both between the connection portions of the conductive elements 10 on the periphery of the conductive loop 8 and to each of the conductive elements 10. In this case, the equivalent circuit will be a bandpass filter type. Furthermore, although it was used only for reception in the above embodiment, it may also be used for both transmission and reception.
The step 16 may be omitted.

(Effects of the Invention) As described above, the head RF coil of the present invention provides the following effects.

(1) The surface current distribution of the coil is in an ideal state, creating a uniform magnetic field on the axial plane, and the coil shape is hemispherical, so when photographing the head, only the head enters the coil. Therefore, reception of noise caused by thermal motion from sources other than the head (imaging area) can be reduced. Furthermore, since the coil surface and the top of the head are in close contact, the strength of the received signal increases, and the SIR at the top of the head can be improved. Therefore, when photographing the head, the image quality of the top of the head can be improved.

(2) Since the coil shape is hemispherical, it is easy to attach the coil to the imaging site by positioning the upper hemisphere with the conventional surface coil, improving operability. In addition, since the coils surround the area other than the head, a good field of view can be ensured, and the patient's feeling of pressure during imaging can be reduced.

[Brief explanation of the drawing]

FIGS. 1(a) and 1(b) are schematic diagrams showing a head RF coil according to an embodiment of the present invention, and FIG. 2 is an equivalent circuit diagram of a head RF coil according to an embodiment of the present invention. , FIG. 3 is a current distribution diagram of one embodiment of the present invention, and FIGS. 4(a) and (b) are:
FIG. 5(a) is a graph showing the Z dependence of the SNR of the head RF coil of this example. (b) is a schematic diagram showing a head RF coil according to another embodiment of the present invention, FIG. 6 is a schematic diagram showing a conventional NHR-CT wave device, and FIG. 7 (a>, (b), (c) is a schematic diagram showing a conventional RF coil. 1... Static magnetic field magnet, 2... Subject, 3...
RF coil, 4... Gradient magnetic field coil, 5.8
... Conductive loop, 6.10... Conductive element, 7... Capacitive element, 9... Connection part, 11.110 to 111. ... Capacitor, 121-1
216... Inductance, 13... Decoupling circuit, 14... Variable capacitor, 15a, 15b... Cross diode, 16... Inductance, 17...
・Small loop.

Claims (4)

[Claims] (1) Approximately circular conductive loops, one end of which is connected to the periphery of the loop, and the other end connected to a connection part that passes through the center of the loop and is placed at a predetermined distance on a line perpendicular to the loop surface. a plurality of substantially arc-shaped conductive elements; and a capacitive element connected in series between each of the conductive element connection portions on the periphery of the conductive loop or to each of the conductive elements, and has a substantially hemispherical shape. An RF coil for the head characterized by the following. (2) The head RF coil according to claim (1), wherein at least one of the capacitive elements includes a variable capacitor in parallel. (3) The head RF coil according to claim (1), wherein the number of conductive elements is 4n (n is a natural number). (4) Claim (1) characterized in that a decoupling circuit is provided in parallel with at least one of the capacitive elements.
The described head RF coil.