JP2921115B2 - Nuclear magnetic resonance inspection system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

The present invention relates to a nuclear magnetic resonance inspection apparatus such as an MRI apparatus that performs imaging using nuclear magnetic resonance (NMR) and an MRS apparatus that performs spectroscopy. The present invention relates to a nuclear magnetic resonance inspection apparatus for collecting signal data.

[Prior art]

In a nuclear magnetic resonance inspection apparatus, a specific region of a subject is selectively excited, an echo signal from the specific region is received, spectroscopy is performed, or a specific slice plane is selectively excited, and one axis direction in the slice plane is selected. Is encoded into the frequency of the echo signal and the other axial position information is encoded into the phase of the echo signal, and the received echo signal is subjected to two-dimensional Fourier transform to decode the above-described two-axis position information. Then, imaging is performed to obtain a tomographic image on the slice plane. When performing an examination of the vicinity of the heart, the examination site largely moves due to the pulsation of the heart, and it is necessary to collect resonance signal data in synchronization with the pulsation. Therefore, an electrocardiographic waveform is obtained using a so-called electrocardiograph. That is, an electrode is attached to the body surface of the subject to capture body potential fluctuations of the subject, and this is amplified to obtain an electrocardiograph. A trigger signal is generated based on an arbitrary point (generally an R wave) of the electrocardiographic waveform, and a resonance signal data acquisition sequence is started in response thereto.

[Problems to be solved by the invention]

However, in a nuclear magnetic resonance inspection apparatus, a gradient magnetic field may be generated in a pulsed manner to perform a resonance signal data acquisition sequence, so that noise is mixed into the electrode attached to the body surface under the influence of the alternating magnetic field, There is a problem that only an electrocardiographic waveform having a poor S / N ratio is obtained, and a resonance signal data acquisition sequence accurately synchronized with a specific phase of the electrocardiographic waveform cannot be performed. In view of the above, the present invention obtains an electrocardiographic waveform having an increased S / N ratio by effectively removing noise due to a gradient magnetic field or the like, and can perform a resonance signal data acquisition sequence that is accurately synchronized with a heart beat. It is an object of the present invention to provide a nuclear magnetic resonance inspection apparatus improved as described above.

[Means for Solving the Problems]

In order to achieve the above object, according to the present invention, a means for generating a static magnetic field, a means for generating a gradient magnetic field in each direction, and an RF signal to a subject placed in the static magnetic field and the gradient magnetic field Means for irradiating and exciting, means for receiving a resonance signal from the subject, and means for controlling a resonance signal data collection sequence consisting of a series of sequences for gradient magnetic field generation, RF signal irradiation and signal reception. In the nuclear magnetic resonance examination apparatus, an earth electrode attached to a substantially central portion of the subject's body, two signal detection electrodes attached to positions substantially symmetrical to the left and right with respect to the earth electrode,
A noise detection coil having a conductor loop wound around two signal detection electrodes; a difference between signals appearing at the two signal detection electrodes; and a noise detection coil for the difference signal obtained thereby. An electrocardiographic waveform detecting means comprising means for subtracting a signal appearing in the coil for use, and controlling the control means based on the electrocardiographic waveform obtained from the electrocardiographic waveform detecting means to start the resonance signal data acquisition sequence. It is characterized in that means for determining are provided.

[Action]

The two signal detecting electrodes of the electrocardiographic waveform detecting means are mounted at positions substantially symmetrical to the left and right with respect to the grounding electrode mounted substantially at the center of the subject's body. Therefore, an electrocardiographic waveform appears in opposite phases on each of the two signal detection electrodes, and noise is superimposed on the electrocardiographic waveform. When the difference between the signals generated at the two signal detection electrodes is calculated, the electrocardiographic waveforms are added to each other, but noises appearing in phase at the two signal detection electrodes are mutually subtracted and canceled. In-phase noise includes noise caused by a gradient magnetic field whose magnetic field strength is inclined in the left-right direction of the body. On the other hand, noises appearing in opposite phases on the two signal detecting electrodes are added to each other and become rather large. The anti-phase noise may be a gradient magnetic field having no left-right symmetry, for example, a noise caused by a gradient magnetic field whose magnetic field strength is inclined in the body axis direction. Noise that cannot be removed in this process is captured by the noise detection coil. That is, since the noise detection coil has a conductor loop wound around the two signal detection electrodes, the same type of noise as the noise appearing in opposite phases on the two signal detection electrodes can be detected. . Since the noise captured by the noise detection coil is subtracted from the difference signal between the signals generated at the two signal detection electrodes, noise that cannot be removed in the above process can also be removed. Therefore, noise due to the gradient magnetic field can be effectively removed from the electrocardiographic waveform, and the S / N ratio of the electrocardiographic waveform can be increased. Since the S / N ratio of the electrocardiographic waveform is increased in this way, the phase of the timing determined based on the electrocardiographic waveform is accurate, and a resonance signal data acquisition sequence accurately synchronized with the heart beat can be performed. .

【Example】

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 shows an MR imaging apparatus according to an embodiment of the present invention.
The transmission coil 12 and the reception coil 13 are attached to the subject 11, and these are arranged in a static magnetic field formed by the main magnet 15 and the gradient coil 14 and a gradient magnetic field formed by being superimposed on the static magnetic field. The gradient coil 14 is configured so as to be able to independently generate gradient magnetic fields in which the magnetic field strength is inclined in each of the three orthogonal axes. The gradient magnetic fields of the three orthogonal axes are a gradient magnetic field Gs for slice selection, a gradient magnetic field Gr for reading (frequency encoding), and a gradient magnetic field Gp for phase encoding. The gradient coil 14 has gradient magnetic fields Gs, Gr,
Current is supplied from each of the power supplies 21, 22, and 23 of Gp, and a gradient magnetic field in each direction is formed. The gradient magnetic field power supplies 21 to 23 control the supply current waveforms of the gradient magnetic field power supplies 21 to 23 so that the gradient magnetic field pulses having a predetermined waveform are formed by the gradient coil 14. On the other hand, the transmission coil 12 is sent from the high-frequency power supply 33.
An RF pulse is provided. This RF pulse is
In 32, a signal obtained by AM-modulating a sine waveform from the RF waveform generator 31 using a RF sine wave signal from the synthesizer 34 as a carrier signal is amplified by a high frequency power supply 33. An NMR signal generated after irradiating the subject 11 with an RF pulse from the transmission coil 12 to excite its nuclear spin is received by the reception coil 13.
Received at. Note that the transmission coil 12 and the reception coil 13 can be shared, and the high-frequency power source 33 on the transmission side and the preamplifier 35 on the reception side can be switched using a signal switch (not shown). The received NMR signal is amplified by a preamplifier 35, detected by a quadrature phase detector 36, and then detected by an A / D converter 3
Is converted into digital data in 7 and the host computer 41
It is taken in. This quadrature phase detector 36 is a PSD (Phase S
ensitive Detector), a synthesizer 3
A circuit is used that outputs the difference between the frequencies of the two signals by mixing the reference signal sent from 4 with the received signal. The sequence controller 42 gives the gradient magnetic field control device 24 the waveform information and the generation timing information of each gradient magnetic field pulse under the control of the host computer 41, and sends the RF waveform generator 31
In addition to providing the sinc waveform information and the generation timing information of the pulse, the A / D converter 3 sends information on the frequency of the carrier signal (corresponding to the resonance frequency) to the synthesizer 34.
Control the sample timing of 7. The console 43 having a display device and an input device such as a keyboard device is connected to the host computer 41. The data taken into the host computer 41 is 2
The image is reconstructed by performing the dimensional Fourier transform,
The image is displayed on the display device of the console 43. As a pulse sequence for imaging, a normal spin echo method, a saturation recovery method, an inversion recovery method, or the like can be used.The start timing of the pulse sequence in each view is synchronized with the movement of the heart. And do it. Therefore, the electrocardiogram waveform from the electrocardiograph 51 is guided to the timing generation circuit 44, and a timing signal based on an arbitrary point of the electrocardiogram waveform, for example, an R wave is obtained. I am trying to apply. Here, the electrocardiograph 51, as shown in FIG.
The electrode 52 for grounding and the electrode 5 for signal detection
3, 54, a noise detecting coil 55, and differential amplifiers 56 to 58. The ground electrode 52 is attached to the body epidermis almost in the center of the body of the subject 11, and is used as a signal detection electrode.
The reference numerals 53 and 54 are attached at substantially symmetric positions with respect to the ground electrode 52. The noise detection coil 55 is disposed on the left and right sides of the body of the subject 11 and has two signal detection electrodes.
It is made of a conductor that is looped between 53 and 54. The signal waveforms at the respective parts a to g in FIG. 2 are shown in FIGS.
G. That is, the electrode for signal detection
Electrocardiographic waveforms 61 appear at 53 and 54 in opposite phases as shown in FIGS. 3A and 3B. However, these two electrodes 53 and 54 have noises 62 and 63 respectively.
Also overlap. The noise 62 is in phase and the noise 63 is out of phase. For example, a gradient magnetic field for readout (frequency encoding)
Since the magnetic field strength of the Gr is inclined so as to have the opposite polarity with respect to the center point in the imaging space, the Gr is applied in the opposite direction symmetrically in the left-right direction with respect to the body center axis of the subject 11. Therefore, the magnetic field due to the gradient magnetic field Gr is generated in the right and left directions of the subject body 11 in opposite directions. Then, the induced current generated by the fluctuation of the magnetic field flows as shown by an arrow through a resistance (shown by a dotted line in the figure) in the body between the earth electrode 52 and the signal detection electrodes 53 and 54. That is, it flows symmetrically with respect to the ground electrode 52. In other words, in-phase noise currents flow through the signal detection electrodes 53 and 54. Therefore, the noise due to the gradient magnetic field having the left and right symmetric components appears as the in-phase noise 62. Such in-phase noise 62 is removed by passing through the differential amplifier 56, and a signal as shown in FIG. 3C is obtained from the differential amplifier 56. That is, the in-phase noise 62 is removed, but the out-of-phase noise 63 is added and becomes rather large. This opposite phase noise 63 is two signal detection electrodes
It flows through the body between 53 and 54, that is, flows in series through two resistors indicated by dotted lines. Such noise 63
Is caused by a change in the magnetic field that is not symmetrical with respect to the body of the subject 11. That is, for example, this is caused by pulse-like application of the slice selection gradient magnetic field Gs whose magnetic field strength is inclined in the body axis direction. On the other hand, the noise detection coil 55 has two signal detection electrodes 5.
It has a loop-shaped conductor spanned between 3, 54. Therefore, this conductor loop forms the same loop as that formed by the series resistance in the body between the electrodes 53 and 54. Therefore, the same noise current as the noise current flowing through the loop due to the series resistance in the body between the electrodes 53 and 54 flows through the loop-shaped conductor of the detection coil 55.
That is, the same noise as the noise appearing in the opposite phase on the signal detection electrodes 53 and 54 can be detected by the noise detection coil 55. As shown in FIGS. 3D and 3E, noise 63 appears at opposite ends of the noise detecting coil 55 in opposite polarities, and is amplified by the differential amplifier 57, as shown in FIG. 3F. Two differential amplifiers 56,
Since the output of 57 is input to the differential amplifier 58, the differential amplifier
The noise 63 remaining in the output 56 (FIG. 3C) will be canceled. Thus, the electrocardiographic waveform 61 from which the in-phase noise 62 and the anti-phase noise 63 have been removed from the differential amplifier 58 is the third waveform.
It is obtained as shown in FIG. The signal g representing the electrocardiographic waveform 61 is supplied to the timing generation circuit 44.
(FIG. 1), from which a timing signal synchronized with a specific phase of the electrocardiographic waveform is generated.
Sent to Therefore, a pulse sequence for acquiring resonance signal data accurately synchronized with the heart beat can be performed.

【The invention's effect】

According to the nuclear magnetic resonance inspection apparatus of the present invention, it is possible to obtain an electrocardiographic waveform having an increased S / N ratio by effectively removing noise due to a gradient magnetic field and the like, thereby accurately synchronizing with the heart beat. The obtained resonance signal data acquisition sequence can be performed.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a block diagram showing a part of the electrocardiograph of the embodiment in detail, and FIG. 3 is a waveform diagram showing waveforms of respective parts in FIG. It is. 11 subject, 12 transmission coil, 13 reception coil, 14 gradient coil, 15 main magnet, 21 gradient power supply for slice selection, 22 gradient power supply for reading, 23 gradient magnetic field for phase encoding Power supply, 24 ... gradient magnetic field controller, 31 ... RF
Waveform generator, 32… Frequency converter, 33… High frequency power supply, 34…
Synthesizer, 35 ... Preamplifier, 36 ... Quadrature detector,
37 ... A / D converter, 41 ... Host computer, 42 ... Sequence controller, 43 ... Console, 44 ... Timing generation circuit, 51 ... Electrocardiograph, 52 ... Electrode for ground, 53, 54 ... Electrode for signal detection, 55 ... Noise detection coil, 56-58 ... Differential amplifier, 61 ... Electrical waveform, 62, 63 ... Noise.

Claims (1)

(57) [Claims] A means for generating a static magnetic field; a means for generating a gradient magnetic field in each direction; a means for irradiating a subject placed in the static magnetic field and the gradient magnetic field with an RF signal to excite the subject; In a nuclear magnetic resonance inspection apparatus comprising: means for receiving a resonance signal from a subject; and means for controlling a resonance signal data acquisition sequence consisting of a series of sequences of gradient magnetic field generation, RF signal irradiation, and signal reception. An earth electrode attached to a substantially central portion of the examiner's body, two signal detection electrodes attached to positions substantially symmetrical to the left and right with respect to the ground electrode, and a signal electrode is wound around the two signal detection electrodes. A noise detection coil having a conductor loop, and a difference between signals appearing on the two signal detection electrodes, and a signal appearing on the noise detection coil with respect to a difference signal obtained thereby. An electrocardiographic waveform detecting means comprising a subtracting means, and means for controlling the control means based on the electrocardiographic waveform obtained from the electrocardiographic waveform detecting means and determining a start timing of the resonance signal data acquisition sequence. A nuclear magnetic resonance inspection apparatus characterized by the above-mentioned.