JPH05305063A - Mr imaging device - Google Patents

Abstract

PURPOSE:To mitigate the effect of sound generation to the utmost and prevent a subject from being frightened or applied with uneasiness by generating the inclined magnetic field increased gradually in the period preceding the image pickup period repeated with image pickup sequences. CONSTITUTION:A computer 20 controls the wave-form and timing of the inclined magnetic field generated by a wave-form generator 21, controls the wave-form and timing of the RF pulse from a wave-form generator 24, controls a signal generator 23 to generate the resonance-frequency signal, and controls the total sequences. The inclined magnetic field pulse is applied to be gradually increased in the period preceding the image pickup scan at the same cycle and duty cycle as those in the image pickup period, and it is applied to be gradually decreased in the period immediately after the image pickup period. The change of the sound in magnitude is smoothly continued, and a subject is prevented from being frightened or applied with uneasiness in the image pickup period.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention relates to nuclear magnetic resonance (NM).
The present invention relates to an MR imaging device that performs imaging using R).

[0002]

2. Description of the Related Art In an MR imaging apparatus, a subject is inserted in a static magnetic field and three-dimensional gradient magnetic field pulses in each direction are generated.
By adding a high-frequency pulse having a frequency corresponding to the resonance frequency to the subject while controlling its waveform and exciting the subject, and repeating a pulse sequence of receiving a resonance signal generated thereafter. , Perform an imaging scan to collect data for imaging.

In such an MR imaging apparatus, a high-speed imaging method has been developed in recent years, and the imaging scan is completed in a very short period (tens of milliseconds to several seconds).

[0004]

However, this M
In the R-imaging device, a gradient magnetic field or a high-frequency excitation pulse is suddenly applied, and in particular, the application of the gradient magnetic field pulse generates a large sound, which burdens the heart of the subject and surprises the subject. There is a problem that sometimes it causes an inadvertent movement.

That is, the gradient magnetic field is generated by passing a current through the gradient magnetic field generating coil, and the gradient magnetic field generating coil is housed in the gantry together with the main magnet for generating the static magnetic field. When a pulse current is applied to this coil, the Fleming's force is suddenly generated, and this coil hits the coil bobbin to generate a sound. The generated sound becomes louder as the magnitude of the pulse current flowing through the coil (strength of the generated gradient magnetic field) increases.

Since the high-speed imaging method uses a strong gradient magnetic field, this sound also becomes loud and reaches a level that is unbearable for the subject. Therefore, even if the imaging time is as short as a few milliseconds to a few seconds, a loud sound is suddenly generated, and the subject is surprised. The subject is given caution in advance through a microphone / speaker, etc. However, the intensity of the sound cannot be predicted by the subject and only gives anxiety.

Therefore, it has been considered to block this sound by using an earphone or the like, but this is not sufficient. It is also possible to generate a sound having a phase opposite to that of the generated sound to cancel the generated sound, but this requires special hardware and cannot be easily realized.

In view of the above, the present invention provides an MR imaging apparatus which is improved so as to mitigate the influence of sound generation as much as possible so as not to surprise or anxiety a subject. To aim.

[0009]

In order to achieve the above object, the MR imaging apparatus according to the present invention performs a gradient magnetic field generation sequence unrelated to the imaging sequence prior to the imaging scan, and the gradient magnetic field generation sequence is performed. It is characterized by generating a gradually increasing gradient magnetic field.

[0010]

Since the gradually increasing gradient magnetic field is applied prior to the imaging scan, the gradually increasing acoustic sound is generated by the gradient magnetic field. As a result, rather than sudden loud sound being generated by the start of the imaging scan, a small sound is generated at first before the imaging scan starts, and gradually becomes louder after that, and then a loud sound is generated by the imaging scan. You can As a result, the subject can be prepared and is not caught by unnecessary anxiety in the process of hearing a sound that gradually increases from a small sound, and the burden on the auditory sound is reduced.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. 1 is a time chart showing a pulse sequence used in an MR imaging apparatus according to an embodiment of the present invention, and FIG. 2 is a block diagram showing a configuration of the MR imaging apparatus according to the embodiment.

In this embodiment, as shown in FIG. 1, during the imaging period, a pulse sequence is repeated in which an RF excitation pulse is applied, a Gz pulse is applied, and a Gy pulse and a Gx pulse are further applied. In this example, the pulse sequence shown in this figure is based on the field echo method, but may be based on other spin echo methods or the like. The Gz pulse is a gradient magnetic field pulse in the Z direction, and is used as a gradient magnetic field for slice selection in this example. The Gy pulse is a gradient magnetic field pulse in the Y direction and is used as a gradient magnetic field for phase-encoding the position information in the Y direction. The Gx pulse, which is a gradient magnetic field pulse in the X direction, is used here for frequency encoding position information in the X direction and for aligning spin phases.

In this embodiment, the period T immediately before the imaging period is
In No. 1, a gradually increasing Gz pulse is applied. Further, in the period T2 immediately after the imaging period, the gradually decreasing Gz pulse is applied.

These three gradient magnetic fields Gz, Gy, G
x is generated by passing a current through the Gz coil 12, the Gy coil 13, and the Gx coil 14 arranged in the main magnet 11 as shown in FIG. The main magnet 11 is for generating a static magnetic field in which the magnetic flux is oriented in the Z direction. These Gz coils 1
A current is supplied from the gradient magnetic field power source 22 to the 2, Gy coil 13, and Gx coil 14. Those current waveforms are provided by the waveform generator 21.

A subject (not shown) is inserted into the space to which the static magnetic field and the gradient magnetic field are applied, and a transmitter antenna and a receive antenna (not shown) are attached to the subject. An RF excitation pulse is supplied from the transmission power amplifier 26 to the transmission antenna. This RF excitation pulse is applied to the modulation circuit 25.
In the above, the RF signal from the signal generator 23 is modulated by the signal from the waveform generator 24. The NMR signal received by the receiving antenna is sent to the detection circuit 28 through the preamplifier 27, the signal from the signal generator 23 is phase-detected as a reference signal, and further is sampled by the A / D converter 29 and converted into digital data. Being a computer 2
It is taken into 0.

The computer 20 processes this data to reconstruct an image and obtain an MR image. The computer 20 controls the waveform of each gradient magnetic field generated from the waveform generator 21 and its timing, controls the RF pulse waveform from the waveform generator 24 and its timing, and further controls the signal generator 23. Then, the entire sequence including the imaging sequence is controlled by generating a signal of the resonance frequency.

The computer 20 controls the Gz pulse generation at T1 and T2 in FIG. As a result, in the period T1 as shown in FIG. 1, the Gz pulse is applied so as to gradually increase with the same cycle and duty cycle as in the imaging period. Therefore, the sound due to the Gz pulse is initially small and gradually becomes louder, and the change in the loudness smoothly leads to the sound during the imaging period. As a result, the subject is not surprised or anxious about the sound during the imaging period.

Further, the period T2 immediately after the end of the imaging period
Then, a Gz pulse is applied with a period and a duty cycle similar to those in the imaging period so that the Gz pulse becomes gradually smaller.
The sound produced by the z pulse is loud at first and then gradually diminishes. Therefore, a loud sound is not suddenly cut off, and no auditory residual sound is present.

Further, since it is only necessary to add a sequence for applying the gradient magnetic field in the periods T1 and T2 before and after the imaging period, it is only necessary to change the program held in the computer 20, and no additional hardware is required. Therefore, it can be easily realized.

When the high-speed imaging method is used in clinical medicine, the same imaging scan may be repeated, but in such a case, the subject must keep his body still after each imaging scan. In this way, the sound automatically increases gradually before the start of the imaging period, and the sound automatically decreases when the imaging period ends. Can know the timing of the imaging period, and the body can be made stationary according to the timing. Therefore, it is not necessary for the operator to instruct the examinee's body to be stationary for each imaging scan, and a voice transmission device using a microphone / speaker or an auto voice device for that purpose is not required.

By making the cycle and duty cycle of the gradient magnetic field Gz pulse applied in the above periods T1 and T2 similar to those in the imaging period, the eddy current due to the gradient magnetic field pulse is gradually increased in the period T1 until the imaging period. It can be made to be in a steady state, and can be smoothly eliminated in a period T2 after the imaging period.

The change in the magnitude of the Gz pulse during the periods T1 and T2 is preferably such that the resulting change in the sound is the smoothest to human hearing and does not give more discomfort. In the above, one gradient magnetic field G
sound is generated by z, but other gradient magnetic fields Gy, G
x, etc., or two or three of these may be combined.

[0023]

As described above, according to the MR imaging apparatus of the present invention, the sound that gradually becomes louder is generated prior to the imaging scan, so that it is possible to avoid sudden occurrence of the loud sound by the imaging scan. The subject can be prepared, does not feel anxious, and the burden of sound generation is reduced.

[Brief description of drawings]

FIG. 1 is a time chart showing a pulse sequence in an embodiment of the present invention.

FIG. 2 is a block diagram showing the configuration of the embodiment.

[Explanation of symbols]

 RF high-frequency excitation pulse Gz Z-direction gradient magnetic field Gy Y-direction gradient magnetic field Gx X-direction gradient magnetic field 11 Main magnet 12-14 Gradient magnetic field generating coil 20 Computer 21 Gradient magnetic field waveform generator 22 Gradient magnetic field power supply 23 Signal generator 24 RF excitation Waveform generator 25 Modulation circuit 26 Transmission power amplifier 27 Preamplifier 28 Detection circuit 29 A / D converter

Claims (1)

[Claims] 1. A static magnetic field generating means for applying a static magnetic field to a subject, and a gradient magnetic field generating means for applying a gradient magnetic field to the subject,
A transmitting means for irradiating the subject with a high frequency of a resonance frequency, a receiving means for receiving an NMR signal from the subject, and an imaging scan for collecting the NMR data by repeating the sequence of the gradient magnetic field, the high frequency irradiation and the reception. An MR imaging apparatus comprising: a control unit that controls and performs a gradient magnetic field generation sequence that is independent of the imaging sequence and that generates a gradually increasing gradient magnetic field prior to the imaging scan.