JPH04200533A - Nuclear magnetic resonance examination device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] [Industrial application field]

This invention is applicable to MHI equipment that performs imaging using nuclear magnetic resonance (NMR) and MRS that performs spectroscopy.
The present invention relates to a nuclear magnetic resonance examination apparatus such as a nuclear magnetic resonance examination apparatus, and particularly relates to a nuclear magnetic resonance examination apparatus that collects resonance signal data from a subject in synchronization with heart beats.

[Conventional technology]

Nuclear magnetic resonance examination equipment selectively excites a specific region of the subject, receives echo signals from there, and performs spectroscopy, or selectively excites a specific slice plane and performs spectroscopy in one axial direction within that slice plane. The position information of the above two axes is decoded by encoding the position information in the frequency of the echo signal and the position information in other axial directions into the phase of the echo signal, respectively, and performing two-dimensional Fourier transform on the received echo signal. 8. When performing an examination near the heart, the examination area moves significantly due to the heartbeat, so resonance signal data is collected in synchronization with the heartbeat. Therefore, an electrocardiographic waveform is obtained using a so-called electrocardiograph. That is, electrodes are attached to the subject's body surface to capture the subject's body potential fluctuations, and this is amplified to obtain an electrocardiograph. A trigger signal is created based on an arbitrary point in the electrocardiographic waveform (generally an R wave), and a resonance signal data acquisition sequence is started in response to this.

[Problem to be solved by the invention]

However, in a nuclear magnetic resonance examination system, a gradient magnetic field is generated in a pulsed manner to perform a resonance signal data acquisition sequence, so the @, f! There is a problem in that noise is mixed into the electrocardiogram, and only an electrocardiogram waveform with a poor S/N ratio can be obtained, making it impossible to perform a resonance signal data collection sequence accurately synchronized with a specific phase of the electrocardiogram waveform. In view of the above, the present invention obtains an electrocardiogram waveform with a high S/N ratio by effectively removing noise caused by gradient magnetic fields, etc., and performs a resonance signal data acquisition sequence accurately synchronized with the heart beat. It is an object of the present invention to provide a nuclear magnetic resonance examination apparatus that is improved as described above. [Means for Solving the Problems] In order to achieve the above object, the present invention provides means for generating a static magnetic field, means for generating gradient magnetic fields in each direction, and a means for generating a static magnetic field and a gradient magnetic field in the static magnetic field and the gradient magnetic field. A resonance signal consisting of a means for irradiating and exciting a subject with an RF signal, a means for receiving a resonance signal from the subject, and a series of sequences for generating a gradient magnetic field, irradiating an RF signal, and receiving the signal. In a nuclear magnetic resonance examination apparatus comprising means for controlling a data acquisition sequence, a grounding electrode is attached to the approximate center of the subject's body; A noise detection coil having two signal detection electrodes and a conductor loop passed around the two signal detection electrodes, and the difference between the signals appearing at the two signal detection electrodes, and an electrocardiographic waveform detection means comprising means for subtracting a signal appearing in the noise detection coil from the difference signal; The present invention is characterized in that it includes means for determining the start timing of the resonance signal data acquisition sequence.

[For production]

The two signal detection electrodes of the electrocardiographic waveform detection means are attached at positions substantially symmetrical in the left and right directions with respect to the grounding electrode attached to the approximate center of the subject's body. Therefore, at each of these two signal detection poles, an electrocardiographic waveform appears in opposite phase, and noise is superimposed on this electrocardiographic waveform. When the difference between the signals generated at these two signal detection electrodes is taken, the electrocardiographic waveforms are added together, but the noises appearing in the same phase at the two signal detection electrodes are subtracted from each other and canceled. In-phase noise includes noise caused by a gradient magnetic field whose magnetic field strength is tilted in the left-right direction of the body. On the other hand, noises appearing in opposite phases on the two signal detection electrodes are added together and become even larger. The anti-phase noise may be caused by a gradient magnetic field without left-right symmetry, for example, a gradient magnetic field whose magnetic field strength is tilted in the body axis direction. Noise that cannot be removed in this process is captured by a noise detection coil. In other words, this noise detection coil is
Since the conductor loop is provided near the two signal detection electrodes, it is possible to detect the same type of noise as the noise that appears in the opposite phase to the two signal detection electrodes. The noise detected by this noise detection coil is
Since this is subtracted from the difference signal between the signals generated at the two signal detection electrodes, noise that could not be removed in the above process can also be removed. Therefore, the noise caused by the gradient magnetic field can be effectively removed from the electrocardiogram waveform, thereby increasing the S/N ratio of the electrocardiogram waveform.3 Because the S/N ratio of the electrocardiogram waveform increases in this way,
The phase of the timing determined based on this electrocardiographic waveform becomes accurate, and it is possible to perform a resonance signal data acquisition sequence accurately synchronized with the heartbeat.

【Example】

Hereinafter, one 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, and in this figure,
A transmitting coil 12 and a receiving coil 13 are attached to the subject 11, and these are attached to the main magnet 15 and the gradient coil 1.
4 and a gradient magnetic field superimposed thereon. The gradient coil 14 has three orthogonal
It is configured to be able to independently generate gradient magnetic fields whose magnetic field strengths are tilted in each direction of the axis. The gradient magnetic fields of the three orthogonal axes are a gradient magnetic field Gs for slice selection and a gradient magnetic field Gr for readout (frequency encoding).
, a phase encoding gradient magnetic field Gp. Gradient coil 1
4 has gradient magnetic field Gs, Gr, Gp power supplies 21, 22.
23, current is supplied to form gradient magnetic fields in each direction. The waveforms of currents supplied by the gradient magnetic field power supplies 21 to 23 are controlled by a gradient magnetic field controller 24 so that the gradient coil 14 forms gradient magnetic field pulses with predetermined waveforms. On the other hand, the transmitting coil 12 is supplied with an RF pulse sent from a high frequency power source 33. This RF pulse is AM-modulated in the frequency converter 32 with a 5-inch waveform from the RF waveform generator 31 using the RF sine wave signal from the synthesizer 34 as a carrier signal, and then
It is amplified by An NMR multiplied signal generated after the subject 11 is irradiated with an RF pulse from the transmitting coil 12 to excite its nuclear spins is received by the receiving coil 13. It is also possible to use the transmitting coil 12 and the receiving coil 13 as both, and to switch between the high frequency power supply 33 on the transmitting side and the preamplifier 35 on the receiving side using a signal switcher (not shown). This received NMR signal is amplified by a preamplifier 35, then detected by a quadrature phase detector 36, and then converted into digital data by an A/D converter 37 and taken into the host computer 41. The device 36 is PSD (PhaseSen
Detector) type detection circuit,
A circuit is used that mixes the reference signal sent from the synthesizer 34 and the received signal and outputs the difference in frequency between the two signals. The sequence controller 42 is the host computer 4]
Under the control of Information about the frequency (corresponding to the resonant frequency) is sent to control the sampling timing of the A/D converter 37, etc. A 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 subjected to two-dimensional Fourier transformation to reconstruct an image, and the resulting image is displayed on the display device of the console 43. Pulse sequences for imaging include the usual spin echo method, saturation recovery method,
If an inversion recovery method or the like can be used, the start timing of the pulse sequence for each view should be synchronized with the movement of the heart. Therefore, the electrocardiogram waveform from the electrocardiograph 51 is calculated at the timing F! @
44, a timing signal is obtained based on the R wave at any point of the electrocardiographic waveform, and this timing signal is sent to the post computer 41 to apply a trigger. Here, the electrocardiograph 51 is connected to the subject 1 as shown in FIG.
], a signal detection voltage 5B, 54, a noise detection coil 55, and differential amplifiers 56 to 58. The grounding electrode 52 is attached to the skin of the subject 11 at the approximate center of the body, and the signal detection electrodes 53 and 54 are attached at positions that are approximately symmetrical with respect to the electrode 52. . The noise detection coil 55 is arranged across the left and right sides of the body of the subject 11, and is made of a conductor that is looped between the two signal detection electrodes 53 and 54. The signal waveforms at each part from 8 to g in Fig. 2 are shown in Fig. 3 A to G.
are shown respectively. That is, the signal detection electrode 5
3.54, the electrocardiographic waveform 61 appears in reverse phase as shown in FIGS. 3A and 3B. However, noise 62 and 63 is also superimposed on these two electrodes 53 and 54. The noise 62 is in phase, and the noise 63 is out of phase. For example, gradient magnetic field G for readout (frequency encoding)
Since the magnetic field strength of r is inclined so as to have opposite polarity around the center point in the imaging space, it is applied symmetrically in the left-right direction with respect to the body center axis of the subject 11 with opposite polarity. Therefore, the magnetic field due to the gradient magnetic field Gr is generated in opposite directions on the left and right sides of the subject's body 11. Therefore, the induced current generated by this variation in the magnetic field flows through the resistance (indicated by dotted lines in the figure) in the body between the earthing electrode 52 and the signal detection electrodes 53 and 54 as shown by the arrow. In other words, the grounding voltage [! It flows symmetrically with respect to 52. In other words, an in-phase noise current a flows through the signal detection electric currents fi53 and fi54. Therefore, the noise caused by the gradient magnetic field having left-right symmetrical components appears as in-phase noise 62. Such common-mode 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, although the in-phase noise 62 is removed, the anti-phase noise 63 is added and becomes even larger. The second opposite phase noise 63 is caused by the two signal detection voltages.
i! 5B, 54, which flows in series through the two resistances shown by dotted lines.3 Such noise 63 is caused by fluctuations in the magnetic field that are asymmetrical with respect to the body of the subject 11. brought about by. That is, it is generated, for example, by applying a pulsed 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 extending between 3.54 and 54. Therefore, this conductor loop allows the electrodes 53,54
The same loop would have been formed due to the series resistance in the body between. Therefore, 'K 'lf! 53.5
The same noise current as the noise current flowing in the loop due to the series resistance in the body between 4 and 4 will flow in the loop-shaped conductor of this detection coil 55. In other words, iJ for signal detection, 11i
This noise detection coil 55 can detect the same noise that appears in opposite phases in 5B and 54. A third pattern is provided at both ends of this noise detection coil 55. As shown in FIG. 3E, noise 63 appears with opposite polarity, and is amplified by the differential amplifier 57, resulting in the noise shown in FIG. 3F. Since the outputs of the two differential amplifiers 56 and 57 are input to the differential amplifier 58, the noise 63 remaining in the output of the differential amplifier 56 (FIG. 3C) is canceled.In this way,
From the differential amplifier 58, in-phase noise 62 and anti-phase noise 63 are generated.
An electrocardiographic waveform 61 from which the electromagnetic waves are removed is obtained as shown in FIG. 3G. The signal g representing this electrocardiographic waveform 61 is generated by the timing generation circuit 4.
4 (FIG. 1), from which a timing signal synchronized with a specific phase of the electrocardiographic waveform is generated and sent to the host computer 41. Therefore, it is possible to perform a pulse sequence for collecting resonance signal data that is accurately synchronized with the heartbeat.

【Effect of the invention】

According to the nuclear magnetic resonance examination apparatus of the present invention, it is possible to obtain an electrocardiogram waveform with a high S/'N ratio by effectively removing noise caused by gradient magnetic fields, etc., thereby accurately detecting the heart beat. A synchronized resonance signal data acquisition sequence can be performed.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is a block diagram showing details of the electrocardiograph section of the same embodiment, and Fig. 3 is a waveform diagram showing waveforms of each part of Fig. 2. It is. 11... Subject, 12... Transmission coil, 13...
Receiving coil, 14... Gradient coil, 15... Main magnet, 21... Gradient magnetic field power supply for slice selection, 22
...Gradient magnetic field power supply for readout, 23... Gradient magnetic field power supply for phase encoding, 24... Gradient magnetic field control device, 3
1. RF waveform generator, 32...frequency converter, 33
...High frequency power supply, 34...Synthesizer, 35...
・Preamplifier, 36...Quadrature phase detector, 37...
A/D converter, 4101. host computer, 42.
...Sequence controller, 43...Console,
44... Timing generation circuit, 51... Electrocardiograph, 5
2... Earth electrode, 53.54... Signal detection electrode, 55... Noise detection coil, 56-58... Differential amplifier, 61... Electrocardiographic waveform, 62.63... ··noise.

Claims (1)

[Claims] (1) 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, and In a nuclear magnetic resonance examination apparatus comprising means for receiving resonance signals from a patient 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, A grounding electrode attached to approximately the center of the body, two signal detection electrodes attached at positions substantially symmetrical to the left and right with respect to the grounding electrode, and a conductor extending around the two signal detection electrodes. It consists of a noise detection coil having a loop, and means for calculating the difference between the signals appearing on the two signal detection electrodes, and subtracting the signal appearing on the noise detection coil from the difference signal obtained thereby. It is characterized by comprising an electrocardiographic waveform detection means, and a means for controlling the control means based on the electrocardiogram waveform obtained from the electrocardiogram waveform detection means and determining the start timing of the resonance signal data acquisition sequence. Nuclear magnetic resonance examination equipment.