JPH05109U - Receiver coil for magnetic resonance imaging - Google Patents

Abstract

(57) [Summary] [Object] To flatten the frequency characteristics of a receiving coil of a magnetic resonance imaging apparatus in use and eliminate the need for tuning operation. [Structure] A preamplifier 23 whose input impedance 7 is set low is connected to the receiving coil 2 shown in FIG. 1, and the input impedance 7 suppresses the selectivity of the receiving coil 2 forming a resonance circuit and flattens the characteristics over a wide frequency range. Therefore, it is not necessary to tune even when the constant of the coil element changes. [Advantageous Effects] Since the resonance frequency, which has been finely adjusted every time of imaging by using the variable capacitance element, can be eliminated by the present invention, the preparation time for imaging can be shortened, and the receiving coil having a high Q value especially in use condition. It is effective in stabilizing. Moreover, it becomes possible to easily realize a flexible coil using a flexible conductor.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a receiving coil in a magnetic resonance imaging apparatus that uses magnetic resonance to image a desired portion of a subject.

[0002]

[Prior Art]

 A magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) uses a nuclear magnetic resonance phenomenon to measure the density distribution of nuclear spins, relaxation time distribution, etc. at a desired examination site in a subject, and the measured data The image of the cross section of the subject is displayed. The nuclear spins of the subject placed in the 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 with a frequency equal to this Larmor frequency is externally irradiated, spins are excited and transit to a high energy state (nuclear magnetic resonance phenomenon). When this irradiation is stopped, the spin returns to the original low energy state with a time constant corresponding to each state, and at this time, an electromagnetic wave (NMR signal) is emitted to the outside. This is detected by the high frequency receiving coil tuned to the frequency. 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, it is possible to capture position information in space as frequency information.

The receiving coil is tuned to the Larmor frequency by the inductance of the conductor loop 1 and the resonance capacitors 3 and 4 as shown in FIG. 2, and this impedance characteristic is tuned to the Larmor frequency according to FIG. This impedance characteristic becomes a parallel resonance characteristic as shown in Fig. 3, and shows the maximum impedance of pure resistance at the resonance point. The sharpness (Q value) of the resonance characteristic is determined by the resistance 5 (consisting of the resistance of the elements forming the receiving coil and the equivalent resistance generated by the coupling loss with the human body) that can be regarded as being connected in series with the coil circuit. However, the smaller this value is, the larger the Q value is, resulting in a sharp resonance characteristic. The impedance at resonance can be adjusted by changing the ratio of the resonance capacitances 3 and 4, and if the capacitance 4 connected in parallel with the output terminal 9 is reduced, the impedance will be adjusted within the range determined by the Q value. Can be large. Moreover, this capacity 4 is called a matching capacity.

[0004]

[Problems to be solved by the device]

 The receive coil is used in tune, but this does not take advantage of the receive coil's frequency selectivity. This is because the NMR signal has a much narrower band. The purpose of tuning is due to the characteristics of the connected preamplifier.

When considering the noise of the amplifier, as shown in FIG. 4, the amplifier A that does not generate noise at all is considered, and the voltage noise En and the current noise In generated inside the amplifier are input to this input. It is assumed that they are connected (Ri is the input resistance of the amplifier). Although a signal source is connected to this, assuming that the signal source resistance is Rs and the thermal noise generated by this is Et, the total noise voltage EN input to the amplifier changes with the signal source resistance Rs as shown in FIG. , There is a point where the thermal noise Et and the total noise voltage EN are closest to each other, that is, a point where the inherent noise of the amplifier is the smallest. Therefore, it is necessary to adjust the signal source resistance Rs to such a value and use it in order to improve the SN ratio, so that the resonance impedance of the receiving coil matches the noise minimum signal source resistance Rs of the preamplifier. It is adjusted by the matching capacitance 4. Of course, as the impedance increases, the thermal noise Et also increases in proportion to its square root, but since the signal voltage itself can also be taken out in proportion to the square root of the signal source resistance Rs, the impedance at resonance of the receiving coil alone should be changed. Also, the SN ratio is constant. If the tuning is lost, the impedance of the receiving coil is lowered and the noise generated by the preamplifier is increased, resulting in a lower SN ratio.

Conventionally, this tuning operation has been dealt with by connecting a variable capacitance element to the receiving coil circuit so that tuning can be performed from the outside, but the conductor loop is largely deformed like a flexible coil and the inductance is significantly increased. There was a problem in tuning operation because the receiving coil that changes and the receiving coil that is weakly coupled to the human body and has a high Q value when used require a large variable range and fine adjustment. An object of the present invention is to provide a receiver coil for an MRI apparatus that solves the problem of tuning the receiver coil by the conventional means and can obtain a good image.

[0007]

[Means for Solving the Problems]

 The above problem is caused by a large change in the impedance characteristic with respect to the frequency of the receiving coil. To solve this, the Q value of the resonance characteristic should be suppressed. Although the Q value can be lowered by increasing the series resistance of the receiving coil circuit, increasing the resistance value of the element lowers the receiving sensitivity and is not desirable. Also, the coupling loss with the human body is determined by the shape of the coil and cannot be adjusted arbitrarily. Therefore, the effect of reducing the Q value can be obtained by setting the human-power impedance of the preamplifier connected to the receiving coil sufficiently small. The input impedance of the preamplifier can be set arbitrarily according to the feedback conditions, and it is usually used with a high impedance so as not to disturb the signal source, or with matching with the transmission line. By intentionally using this with a low impedance, a low resistance value is connected in parallel to the receiving coil circuit, so that the Q value can be suppressed. Also, the human-power impedance of the preamplifier is an active resistance, so it does not cause a decrease in coil sensitivity.

In the receiving coil according to the present invention, since the resonance characteristic is suppressed by the low input resistance of the preamplifier, the impedance becomes almost constant over a wide frequency range, and the optimum signal source resistance that does not decrease the SN ratio can be maintained. You don't have to.

[0009]

[Action]

 According to the present invention, the impedance characteristic of the receiving coil is flattened by using the input impedance of the preamplifier and the tuning operation is unnecessary. Therefore, the problem caused by the poor tuning is solved and a good MRI image is obtained. Be done.

[0010]

【Example】

 Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. FIG. 6 is a configuration diagram showing an example of the overall configuration of the MRI apparatus according to the present invention. This MRI apparatus obtains a tomographic image of the subject 6 using a nuclear magnetic resonance (NMR) phenomenon, and includes a static magnetic field generating magnet 10, a central processing unit (hereinafter referred to as CPU) 11, a sequencer 12, and a transmitter. The system 13 includes a gradient magnetic field generation system 14, a reception system 15, and a signal processing system 16. The static magnetic field generating magnet 10 is for generating a strong and uniform static magnetic field around the subject 6, and a permanent magnet system, a normal conducting system, or a superconducting system is provided in a space having a certain space around the subject 6. The magnetic field generating means is arranged.

The sequencer 12 operates under the control of the CPU 11 and sends various commands necessary for data acquisition of tomographic images of the subject 6 to the transmission system 13, the gradient magnetic field generation system 14, and the reception system 15. is there. The transmission system 13 includes a high frequency generator 17, a modulator 18, a high frequency amplifier 19, and an irradiation coil 20 on the transmission side. The high frequency pulse output from the high frequency generator 17 is modulated according to a command from the sequencer 12. The electromagnetic wave is irradiated to the subject 6 by modulating the irradiation pulse 18 and amplifying the modulated irradiation pulse by the high frequency amplifier 19 and then supplying the amplified irradiation pulse to the irradiation coil 20 arranged close to the subject 6. ing. The gradient magnetic field generation system 14 is composed of a gradient magnetic field coil 21 wound in three directions of X, Y, and Z, and a gradient magnetic field power source 22 for driving each coil. By driving the gradient magnetic field power source 22 of the coils, gradient magnetic fields Gx, Gy, and Gz in the X, Y, and Z triaxial directions are applied to the subject 6. The slice plane for the subject 6 can be set by the method of applying this gradient magnetic field.

The receiving system 15 includes a receiving coil 2, a preamplifier 23, a quadrature phase detector 24, and an A / D converter 25, and the subject 6 is irradiated with electromagnetic waves emitted from the transmitting side irradiation coil 20. The response electromagnetic wave (NMR signal) is detected by the receiving coil 2 arranged close to the subject 6 and input to the A / D converter 25 through the preamplifier 23 and the quadrature phase detector 24 to be converted into a digital amount. Further, the collected data of the two systems sampled by the quadrature phase detector 24 at the timing according to the command from the sequencer 12 and the signals thereof are sent to the signal processing system 16. The signal processing system 16 includes a CPU 11, a recording device such as a magnetic disk 26 and an optical disk 27, and a display 28 such as a CRT. The CPU 11 performs processing such as Fourier transform, correction coefficient calculation, and image reconstruction. The signal intensity distribution of an arbitrary section or the distribution obtained by performing an appropriate calculation on a plurality of signals is imaged and displayed on the display 28. In this figure, the irradiation coil 20, the reception coil 2 and the gradient magnetic field coil 21 are arranged in the magnetic field space of the static magnetic field generating magnet 10 arranged in the space around the subject 6.

The structure of the receiving coil 2 according to the present invention is shown in FIG. The receiving coil 2 is composed of a conductor loop 1 enclosing the subject 6 and resonance capacitors 3 and 4, and a constant is set so that the resonance frequency becomes substantially the signal frequency. The resistors 5a and 5b are connected to the receiving coil circuit, but this is not the resistance element that is actually connected, but 5a is a coil component such as the conductor resistance of the conductive loop 1 and the loss of the capacitive elements 3 and 4. The existing resistance, and 5b, shows the induced loss with respect to the magnetic flux 8 passing through the subject 6 as an equivalent resistance. The Q value in the resonance characteristic of the receiving coil alone is determined by the resistors 5a and 5b, but the loss 5a of the element affects the receiving sensitivity and must be made sufficiently small. On the other hand, the induced loss 5b of the human body varies depending on the wearing condition, but is relatively large, and the Q value of the receiving coil in the used condition is almost determined by this loss. Therefore, the Q value becomes large in the receiving coil which is less coupled to the human body, and fine tuning operation is required. Further, in a flexible coil in which the shape of the conductor loop 1 is changed and used, it is not easy to realize it because it is necessary to perform tuning accurately with a change in inductance over a wide frequency range.

In the present invention, in order to solve this problem, the Q value is reduced by using the input impedance 7 of the preamplifier 23 connected to the output 9 of the receiving coil 2 for use. At this time, the Q value can be suppressed to be lower as the input impedance 7 is smaller. Therefore, the current impedance of the preamplifier 23 is greatly increased to reduce the input impedance 7 as much as possible. At this time, the input impedance 7 is an active resistance generated by the feedback of the preamplifier, and it is important that it does not become a noise source. As a result, as shown in FIG. 7, the Q value of the receiving coil in the actual use condition can be sufficiently lowered regardless of the coupling condition with the human body, and the fluctuation of the impedance with respect to the frequency can be suppressed, It is possible to make synchronization unnecessary. Further, as a result, it becomes possible to easily realize a flexible coil using the conductor loop 1 having high plasticity.

[0015]

[Effect of the device]

 As described above, in the receiving coil according to the present invention, the Q value can always be kept low regardless of the use condition, so that the impedance variation with respect to the frequency is small, and the tuning operation is unnecessary or simple. It will be possible. In addition, a flexible coil that requires a wide tuning range can be easily realized. As a result, there is an effect that the receiving operation is stabilized, the maximum sensitivity is realized, and a high quality image can be obtained.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a Q-value reduced receiving coil.

FIG. 2 is a receiving coil circuit diagram.

FIG. 3 is an impedance characteristic diagram.

FIG. 4 is a noise analysis diagram of an amplifier.

FIG. 5 is a noise characteristic diagram of an amplifier.

FIG. 6 is an overall configuration diagram of an MRI apparatus.

FIG. 7 is a Q value reduction characteristic diagram.

[Explanation of symbols]

 1 Conductive Loop 2 Receiver Coil 3 Resonance Capacitance 4 Resonance Capacitance 5 Resistor 6 Subject 7 Input Impedance 8 Magnetic Flux 9 Receiver Coil Output 23 Preamplifier

Claims (1)

Claims for utility model registration: 1. A magnetic circuit for applying a uniform static magnetic field to a subject, and a gradient magnetic field coil for applying a slice gradient magnetic field, a readout gradient magnetic field and a phase encoding gradient magnetic field to the subject. , An irradiation coil that repeatedly applies a high-frequency pulse that causes magnetic resonance to atomic nuclei of the atoms that form the tissue of the subject in a predetermined pulse sequence, a receiving coil that detects a magnetic resonance signal, and the detection signal In a magnetic resonance imaging apparatus including an image reconstructing unit that obtains an image representing the physical properties of a target object, the input impedance of a preamplifier connected to the receiving coil is actively set to be low, and this is used as a load. This reduces the selection characteristics of the receiving coil that forms the resonance circuit and simplifies or eliminates the tuning operation. A receiving coil for a magnetic resonance imaging apparatus, comprising: