JPH0231737A - Nuclear magnetic resonance apparatus - Google Patents

Abstract

PURPOSE:To prevent an erroneous diagnosis in medical treatment by suppressing the artifact due to the cyclical body motion of an examinee by a method wherein a computer inputs the output signal of an impedance change detector to control the transmission point of time of the high frequency pulse due to a high frequency transmitter-receiver in synchronous relation to the cyclical change of impedance. CONSTITUTION:When a high frequency current is allowed to flow to a high frequency coil 11 through an impedance bridge 13 and a matching circuit 12 by utilizing the function of a high frequency transmitter-receiver 14 as a high frequency power supply, the output signal of the impedance bridge 13 is zero in an equilibrium state and, at a time shifted from the equilibrium state, the high frequency signal proportional to the shift quantity is outputted. An impedance change detector 15 amplifies the change quantity of said signal to apply A/D conversion thereto and inputs the converted digital value to a computer 16. The computer 16 receives said digital value to calculate the cycle of the body motion of an examinee and further calculates the point of time becoming a state of the same slice surface on the basis of said cycle to control the transmission point of time of the high frequency pulse of the corresponding high frequency transmitter-receiver 14. By this method, the artifact generated by respiration can be reduced to a large extent.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention utilizes a nuclear magnetic resonance apparatus (hereinafter referred to as MHI) for imaging a cross section of a human body and performing medical diagnosis based on this image. equipment).

[Conventional technology]

The MRI apparatus places the subject (patient) in a highly uniform static magnetic field generated by a uniform magnetic field coil, and transmits high-frequency pulses at a frequency corresponding to the resonance frequency of the atomic nucleus, which is proportional to the strength of this static magnetic field, as shown in Figure 2. It is transmitted by the pulse transmitting device 102, irradiated to the subject by the high frequency transmitting coil 103 to cause resonance in the atomic nucleus, and after the irradiation of this high frequency pulse is stopped, a free induced S decay signal, abbreviated as FID signal, is generated. The electromagnetic waves emitted by the resonating atomic nuclei are received by the high-frequency receiving coil 105, amplified by the high-frequency receiving device 104, converted into digital signals, and inputted to the main body of the computer, CPU10I, which is then sent to the CPU.
After arithmetic processing is performed using L0I to create an image signal, the captured cross-sectional image is displayed on the display device 108.

When irradiating high-frequency pulses, first, the space in which nuclear magnetic resonance occurs is determined to be a thin plate-shaped space by superimposing a gradient magnetic field whose magnetic field strength changes linearly in a direction perpendicular to the imaging cross section. Set the imaging section by. Then,
By superimposing a gradient magnetic field in which the strength of the magnetic field changes linearly in one direction parallel to this imaging cross section on a uniform magnetic field, the resonance frequency is changed in this direction, and the difference in the frequency included in the FED signal is reflected in the imaging cross section. Therefore, by changing this direction and repeatedly receiving signals within the imaging section, it is possible to obtain the signals necessary to construct the imaging section image.

FIG. 3 is a partially cutaway perspective view of the MRI magnet, in which a tomographic image is obtained with a patient (subject 7) lying on his back at the center of the uniform magnetic field formed inside the uniform magnetic field coil 1. placed in the desired position. In this figure, if the direction from the head to the feet is the two axes, the direction the head is facing is the y-axis, and the direction parallel to both shoulders is the y-axis, then the head is the center of the uniform magnetic field.

The head is shown being placed at the origin position of the three-dimensional coordinates of ``y+'', and a high-frequency coil 5 that generates a high-frequency electromagnetic field in the X-axis direction is placed to surround this head, and further generates a gradient magnetic field outside of the high-frequency coil 5. Three coils are arranged to generate gradient magnetic fields in three directions: a two-direction gradient magnetic field coil 3Z, an X-direction gradient magnetic field coil 3Y, and an x-direction gradient magnetic field coil 3X. It is possible to generate a gradient magnetic field in which the strength of the magnetic field changes linearly in the direction of .

Figure 4 is a chart showing an example of the time series of high-frequency pulse irradiation and gradient magnetic field application; this time series of electric pulses is called a pulse series, and various methods have been devised. . This diagram shows a pulse sequence called the Fourier transform zoom chromatography method, which is the most basic pulse sequence of the MHI device. In this figure, the imaging cross section is a cross section that is the xy cross section of FIG. 3, and therefore, it is the two-directional gradient magnetic field that sets this imaging cross section. A two-directional gradient magnetic field is applied during the time period (■) shown in this figure, and a high-frequency pulse is irradiated for about 150 μsec during this time to rotate the direction of the spin of the atomic nucleus by 90 degrees from the direction of the uniform magnetic field. is called a 90 degree pulse, and at this time, the resonance is 1 perpendicular to the two directions.
limited to only one plane. At time (2), an FID signal is received after applying an X-direction gradient magnetic field and an X-direction gradient magnetic field. During the time (2), the nuclear resonance attenuates and becomes substantially zero by the time the next step (2) is reached.

In step (2), the same two-directional magnetic field gradient as in step (2) is applied and a high-frequency pulse is irradiated to cause the nuclei in the same cross section as in (2) to resonate, and in step (2), an X-direction gradient magnetic field and an X-direction gradient magnetic field are applied. receive the FID signal. The only difference from case (2) is that the strength of the X-direction gradient magnetic field is different. Image data on one line parallel to the y-axis of the imaging cross section is obtained by the strength of one X-direction gradient magnetic field. X
By changing the directional gradient magnetic field in predetermined steps from the intensity corresponding to the top of the imaging section to the intensity corresponding to the bottom, and receiving the FED signal in the pulse series as described above, 1.
Information for creating two cross-sectional images can be obtained.

Since the number of steps of the strength of the X-direction gradient magnetic field required to construct one cross-sectional image is approximately 200, the above-mentioned FID
The reception of the signal will be repeated 200 times.

[Problem to be solved by the invention]

By the way, the time interval T shown in Fig. 4, which is the interval between repetitions, needs to be about a fraction of a second to one second, so it takes about a few minutes to obtain one cross-sectional image. When performing head cross-sectional images, which require the patient to be restrained in the imaging state during this period, it is relatively easy to hold the head immobile using an appropriate fixation method; When targeting the subject's body, the influence of the subject's body movement due to breathing cannot be avoided, and as described above, by changing the strength of the y-direction gradient magnetic field, image information on a straight line parallel to the X-axis can be converted to a position in the y-direction. When data is taken sequentially while being shifted, each of the information on different Y locus IjI4t1 is no longer the same cross-sectional image, and the cross-sectional image constructed by computer processing based on the obtained information may differ from the actual one. This results in a distorted image. Image distortion in an MHI device is generally referred to as an artifact, but there is a problem in that the occurrence of such an artifact causes misdiagnosis during medical diagnosis.

An object of the present invention is to provide a nuclear magnetic resonance apparatus that suppresses artifacts caused by periodic body movements of a subject and eliminates the risk of misdiagnosis during medical diagnosis.

(Means for Solving the Problems) In order to solve the above problems, according to the present invention, a method is provided for obtaining a 1i image of a cross section of a subject using nuclear magnetic resonance phenomena and performing medical diagnosis based on this image. A nuclear magnetic resonance apparatus that serves as a transmitting antenna for irradiating a subject with high-frequency pulses and a receiving antenna for receiving resonance signals emitted by atomic nuclei that have generated nuclear magnetic resonance due to the irradiated high-frequency pulses. A coil, a high frequency transmitting/receiving device that serves as a transmitting device that generates the high frequency pulse, and a high frequency receiving device that processes the received signal, and a tomographic image is generated by a predetermined calculation using the received signal processed by the high frequency transmitting/receiving device as an input signal. A nuclear magnetic resonance apparatus comprising: a computer for generating and displaying a signal and controlling a transmission point of the high-frequency transmitting/receiving device; The impedance bridge includes an impedance bridge that includes a matching circuit provided as one impedance circuit, and an impedance change detector that converts the output signal of the impedance bridge into a digital signal proportional to the change in the input signal as a human input signal. It is assumed that the computer uses the output signal of the change detector as an input signal and controls the time point at which the high frequency pulse is transmitted by the high frequency transmitting/receiving device in the same amount as the periodic change in impedance.

[Effect]

In the configuration of this invention, an impedance bridge is provided between the high frequency transmitting/receiving device and the high frequency coil by utilizing the functions of the high frequency power sources of the high frequency transmitting/receiving device (three devices), and a parallel circuit of the high frequency coil and the matching circuit is connected to the balance of this impedance bridge. By using one impedance of the circuit, it is possible to detect a slight change in the impedance of the high-frequency coil with high sensitivity.The slight change in the output signal of the detected impedance bridge is detected by an impedance change detector suitable for the input signal of the combiator. The computer calculates the period of body movement based on this input signal, and controls the transmission point of the high-frequency pulse of the high-frequency transmitting/receiving device based on this period. It is possible to obtain image data at a time when the condition of the tomographic plane of the subject is the same.

〔Example〕

The present invention will be explained below based on examples. FIG. 1 is a block diagram showing an embodiment of the present invention, in which a high frequency coil 11
A matching circuit 12 consisting of two variable capacitors in series and parallel is connected to the high-frequency transmitter/receiver! The two variable capacitors of this matching circuit 12 are adjusted so that the impedance seen from the matching circuit 14 becomes a pure resistance. computer 16
controls the time point at which the high-frequency pulse is transmitted by the high-frequency transmitting/receiving device 14, processes the received signal as an input signal, generates a tomographic image, and displays it on a display device attached to the computer 16 or stores it in a storage device as necessary. or These are part of the configuration of the MR [device shown in FIG.

An impedance bridge 13 is provided between the high frequency transmitting/receiving device 14 and the matching circuit 12, so that the high frequency coil 11 and the matching circuit 12 serve as one impedance of the bridge circuit. This impedance bridge 13 is set to a balanced state with the subject placed within the high-frequency coil. The impedance change detector 15 which receives the output signal of the impedance bridge 13 as an input signal is connected to the impedance bridge 1
An amount proportional to the change in the output signal No. 3 is output as a digital signal.

When a high-frequency current is passed through the high-frequency coil 11 through the impedance bridge 13 and the matching circuit 12 by utilizing the function as a high-frequency power source of the high-frequency transmitting device 14 that can apply high-frequency pulses to the high-frequency coil 11, the above-mentioned equilibrium state is achieved. At certain times, the output signal of the impedance bridge 13 is zero, and when it deviates from the equilibrium state, a high frequency signal proportional to the amount of deviation is output.

When the patient, the subject, breathes, the distance between the high-frequency coil 11 and the surface of the subject changes, and the stray capacitance between them changes slightly, so the impedance bridge 13
The impedance of the high frequency coil 11 as seen from above changes. The impedance bridge 13 is this high frequency coil 11
Outputs a high frequency voltage proportional to the impedance change.

Since the value of this output signal changes periodically in synchronization with the patient's breathing, the amount of change in this signal is amplified by the impedance change detector 15 which uses this signal as a human input signal.
After performing D conversion, the signal is input to the combiator 16. Computer! 6 receives this input signal and determines the cycle of the subject's body movement, and based on this cycle, determines the point in time when the state of the same tomographic plane is reached, and transmits the high-frequency transmitter/receiver 1 corresponding to this point in time.
By controlling the time point at which the high-frequency pulse is transmitted in step 4, the state of the cross section at the time of obtaining image data can be always set to a constant state, which greatly reduces artifacts caused by periodic body movements such as breathing. can.

When a change in the impedance of the high-frequency coil 11 occurs due to a body movement of the patient, who is the subject, for example, moving an arm, when such a non-periodic signal is input from the impedance change detector 15, it is detected by the computer 16. By determining that the signal is not a signal and excluding it, the calculation of the period based on the signal related to the original periodic body movement is not disturbed.

〔Effect of the invention〕

As mentioned above, this invention utilizes the function of the high frequency power supply of the high frequency transmitter to provide an impedance bridge between the high frequency transmitter and the high frequency coil.The high frequency coil and matching circuit are connected to one of the balanced circuits of the impedance bridge. By using impedance, a slight change in impedance of the high frequency coil can be detected with high sensitivity. The output signal of the detected impedance bridge is converted into a signal suitable for the input signal of the computer using an impedance change detector, and then inputted to the computer, and the computer calculates the period of body movement based on this input signal. By controlling the transmission time point of the high-frequency pulse of the high-frequency transmitter based on the period, image data can always be obtained at a time point when the state of the tomographic plane of the subject is the same.

As a result, artifacts caused by periodic body movements of the subject are significantly reduced, resulting in an MRI apparatus that can obtain excellent tomographic images.

[Brief explanation of the drawing]

Fig. 1 is a block diagram including a partial circuit diagram showing an embodiment of the present invention, Fig. 2 is a block diagram for explaining the operation of the MHI device, Fig. 3 is a perspective view of an MRI magnet, and Fig. 4 is a block diagram including a partial circuit diagram. It is a chart figure showing an example of a pulse sequence. 11... High frequency combinator, 12... Matching circuit,
13... Impedance bridge, 14... High frequency transmitter, 15... Impedance change detector, first field

Claims (1)

[Claims] 1) A nuclear magnetic resonance apparatus for imaging a cross-section of a subject using nuclear magnetic resonance phenomena and performing medical diagnosis based on this image, which includes a transmitting antenna and irradiation for irradiating the subject with high-frequency pulses. a high-frequency coil that also serves as a receiving antenna for receiving a resonance signal emitted by an atomic nucleus that has caused nuclear magnetic resonance due to the high-frequency pulse, a transmitter that generates the high-frequency pulse, and a high-frequency receiver that processes the received signal. a computer that generates and displays a tomographic image through predetermined calculations using the received signal processed by the high-frequency transmitter-receiver as an input signal, and controls the transmission time point of the high-frequency transmitter-receiver; In a magnetic resonance apparatus, an impedance bridge provided between the high-frequency coil and the high-frequency transmitting/receiving device and having the high-frequency coil and a matching circuit provided in parallel with the high-frequency coil as one impedance circuit;
and an impedance change detector that converts the output signal of the impedance bridge into a digital signal proportional to the change in the input signal. A nuclear magnetic resonance apparatus characterized in that the time point at which the high-frequency pulse is transmitted by the high-frequency transmitting/receiving device is controlled in synchronization with the change.