JP2007268028A - Magnetic resonance imaging device - Google Patents

Abstract

Provided is a magnetic resonance imaging apparatus capable of accurately presenting a stimulus sound to a subject while minimizing noise burdened on the subject as much as possible even during MRI imaging in which large noise occurs.
An MRI apparatus (1000) includes a static magnetic field coil (100), a gradient magnetic field coil (102) and an RF coil (106) for applying a magnetic field to a subject (10) to detect a detection signal resulting from nuclear magnetic resonance, and an oscillating magnetic field as an RF coil. And a tomography control unit 200 that generates a tomographic image from a detection signal from the RF coil 106. The stimulus sound processing unit 230 adjusts the magnitude of the stimulus sound for each frequency band.
[Selection] Figure 1

Description

  The present invention relates to a configuration of a magnetic resonance imaging (MRI) device for performing tomographic imaging of a living body, and more particularly to a configuration for a subject and a measurer / operator to interact during measurement.

  As a method of imaging a cross-section of the brain and whole body of a living body, magnetic resonance imaging using a nuclear magnetic resonance phenomenon with respect to atoms in the living body, particularly nuclei of hydrogen atoms, is used for human clinical imaging diagnosis and the like. .

  When applying it to the human body, the magnetic resonance imaging has the following features, for example, compared to “X-ray CT” which is a similar tomographic image of the human body.

  (1) An image having a density corresponding to the distribution of hydrogen atoms and the signal relaxation time (reflecting the strength of atomic bonding) can be obtained. For this reason, the lightness and darkness corresponding to the difference in tissue properties is exhibited, and the difference in tissue is easily observed.

  (2) The magnetic field is not absorbed by the bone. For this reason, it is easy to observe the part (intracranial, spinal cord, etc.) surrounded by bones.

(3) Since it is not harmful to the human body like X-rays, it can be used in a wide range.
Such magnetic resonance imaging utilizes the magnetic properties of hydrogen nuclei (protons) that are most abundant in each cell of the human body and have the greatest magnetism.

  The movement in the magnetic field of the spin angular momentum responsible for the magnetism of the hydrogen nucleus is classically compared to the precession of the coma. The direction of the spin angular momentum of the hydrogen nuclei (the direction of the rotation axis of the coma) as described above is directed in a random direction in an environment without a magnetic field, but is directed in the direction of the lines of magnetic force when a static magnetic field is applied.

When an oscillating magnetic field is further superimposed in this state, the frequency of the oscillating magnetic field is a resonance frequency f ₀ = γB ₀ / 2π (γ: a coefficient specific to a substance) determined by the strength of the static magnetic field. The energy moves to the side and the direction of the magnetization vector changes (precession increases). When the oscillating magnetic field is cut in this state, the precession returns to the direction in the static magnetic field while returning the tilt angle. An NMR (Nuclear Magnetic Resonance) signal can be obtained by detecting this process from the outside with an antenna coil.

Such a resonance frequency f ₀ is 42.6 × B ₀ (MHz) for hydrogen atoms when the strength of the static magnetic field is B ₀ (T).

  Furthermore, in the nuclear magnetic resonance imaging method, it is possible to visualize the active site of the brain with respect to an external stimulus or the like by using the change in the detected signal according to the change in the blood flow. Such a nuclear magnetic resonance imaging method is particularly called fMRI (functional MRI).

  In fMRI, an ordinary MRI apparatus equipped with hardware and software necessary for fMRI measurement is used.

  Here, the change in blood flow brings about a change in the NMR signal intensity utilizing the fact that oxygenated and deoxygenated hemoglobin in blood has different magnetic properties. Oxygenated hemoglobin has a diamagnetic property and does not affect the relaxation time of hydrogen atoms present in the surrounding area, whereas deoxygenated hemoglobin is a paramagnetic material and changes the surrounding magnetic field. Therefore, when the brain is stimulated, local blood flow increases, and oxygenated hemoglobin increases, the change can be detected as an MRI signal. As the stimulus to the subject, for example, a visual stimulus or an auditory stimulus is used.

  In order to take an image of the brain function, a sequence called echo planar imaging (EPI) is employed.

  FIG. 6 is a timing chart showing an outline of such an fMRI measurement sequence.

  The subject is repeatedly given the same stimulation task at a predetermined interval every predetermined period. Here, as the “stimulation task”, there is a task of presenting auditory stimulation (stimulation sound) to the subject. At this time, while the tomography is continued in time series for the same cross section in the brain, a change in the EPI signal within the stimulation task period is detected in the cross section.

  Such a change in signal intensity is caused by a change in blood flow in the brain as described above. However, such a change in signal strength is on the order of several percent, and in order to obtain a sufficient signal-to-noise ratio, the same stimulation task is repeated a plurality of times, and the change pattern of each EPI signal is determined at the start of the task. Average. By such processing, the response in the brain to the stimulus is visualized. In other words, it is possible to identify a site where activity is activated in the brain when performing a specific task.

  The principle of image generation by such echo planar imaging, clinical application, and the like are described in Non-Patent Documents 1 and 2.

  However, in the MRI apparatus, noise is generated due to vibration of a coil for generating a magnetic field during measurement. For this reason, when applying auditory stimuli to a subject during measurement, although the noise is reduced with earplugs or earmuffs for the purpose of protecting the subject, the stimulating sound may be masked by the noise and hinder hearing. There was a problem of having sex.

  In addition, there is a problem that not only the brain activity due to the original stimulation sound but also the possibility of mixing the brain activity due to the exposure of noise is not denied.

For such problems, a method of presenting auditory stimulation using bone conduction sound has been proposed (see Non-Patent Document 3).
Junichi Oshio, “EPI Revisited”, Journal of the Magnetic Resonance Medical Society of Japan, Vol. 19, No. 1 (1999) p. 1-5 Suzuki Kiyotaka, “Echo Planar Imaging”, Journal of the Magnetic Resonance Medical Society of Japan, Vol. 19, No. 1 (1999) p. 7-18 Yukiko Noda, Tatsuya Kitamura, Hiroyuki Hirata, Kiyoshi Honda, “Method of Presenting Auditory Stimulus by Bone Conduction in MRI System”, Proceedings of the Acoustical Society of Japan, March 2005, p. 385-386

  However, when bone conduction sound is used, although effective in reducing noise, there is a problem that vibration from a bone conduction speaker peculiar to bone conduction sound may affect the brain activity of the subject.

  The present invention has been made in order to solve the above-described problems, and its purpose is to reduce noise that is a burden on the subject as much as possible even during MRI imaging in which large noise is generated. An object of the present invention is to provide a magnetic resonance imaging apparatus capable of accurately presenting a stimulus sound to a subject.

  According to one aspect of the present invention, a magnetic resonance imaging apparatus for detecting a detection signal resulting from nuclear magnetic resonance from a subject and generating a tomographic image of the subject, for applying a static magnetic field to the subject Static magnetic field applying means, a gradient magnetic field applying means for applying to the subject a magnetic field modulated so that the detection signal has position information of the nucleus that emits the detection signal within the cross section selected by the subject, The variable magnetic field transmitting / receiving means for detecting the detection signal from the subject by applying the variable magnetic field, the bone conduction speaker for applying the stimulation sound to the subject, the variable magnetic field being supplied to the variable magnetic field transmitting / receiving means and receiving the detection signal And tomography control means for generating a tomographic image, the tomography control means includes stimulation sound processing means for adjusting the magnitude of the stimulation sound for each frequency band.

  Preferably, the stimulation sound processing means includes amplification degree adjustment means for adjusting the amplification degree for the stimulation sound so as not to exceed a vibration sensation generation threshold of the subject measured in advance.

  According to the magnetic resonance imaging apparatus of the present invention, it is possible to give a stimulating sound by voice without giving a sense of incongruity to the subject even during MRI imaging in which a large noise occurs.

Embodiments of the present invention will be described below with reference to the drawings.
[Configuration and operation of magnetic resonance imaging apparatus]
FIG. 1 is a functional block diagram showing a configuration of a magnetic resonance imaging apparatus (hereinafter referred to as “MRI apparatus”) 1000 according to the present invention.

  Referring to FIG. 1, an MRI apparatus 1000 includes a platform 12 for supporting a subject 10, a static magnetic field coil 100 for generating a static magnetic field, and an observation cross section (slice) of the subject as will be described later. From the gradient magnetic field coil 102 for assigning information of the position and the position in the slice to the observation signal, the RF coil 104 for outputting an electromagnetic wave for applying a variable magnetic field to the observation target nucleus, and the observation target nucleus And a tomography control unit 200 for controlling the coils 100 to 104 and generating a tomographic image based on the signal received by the RF coil 106.

  Further, the tomography control unit 200 is controlled by the input unit 210 for inputting an instruction from the user, the control unit 220 for controlling the measurement operation, and the control unit 220 to store in advance. The sound signal generated based on the stimulus sound data stored in (not shown) or the like is subjected to sound processing as described later, and then the stimulus sound is sent to the subject 10 via the bone conduction speaker 232. A stimulation sound processing unit 230 to output, an RF pulse transmission unit 240 for applying an RF pulse to the RF coil 104 under the control of the control unit 220, and a signal amplification unit 250 for amplifying a signal from the RF coil 106 An image reconstruction unit 260 for reconstructing a cross-sectional image to be observed by performing a Fourier transform process based on the detection signal from the signal amplification unit 250; And a display unit 270 for displaying the reconstructed cross-sectional image based on information from the image reconstruction unit 260.

The subject 10 wears earplugs and earmuffs 234 to shield noise.
Here, more specifically, the static magnetic field coil 100 is composed of, for example, four air-core coils, and a combination thereof creates a uniform magnetic field to give orientation to the spins of hydrogen nuclei in the body of the subject 10. .

  The RF coil 104 emits a high frequency to excite nuclei in the body of the subject 10, and the RF coil 106 detects a detection signal (echo signal) caused by the generated nuclear magnetic resonance.

  The gradient magnetic field coil 102 includes three sets of gradient coils of X, Y, and Z (not shown). The Z coil tilts the magnetic field intensity in the Z direction during excitation to limit the resonance surface. Immediately after the magnetic field is applied, a short-time gradient is added to the detection signal, and phase modulation proportional to the Y coordinate is added (phase encoding). The X coil subsequently adds a gradient when collecting data, and the detection signal is proportional to the X coordinate. Applied frequency modulation (frequency encoding).

  That is, when a high frequency electromagnetic field having a resonance frequency is applied to the subject 10 in a state in which a Z-axis gradient magnetic field is added to the static magnetic field through the RF coil 104, the hydrogen nuclei in the portion where the strength of the magnetic field is in the resonance condition are generated. , Selectively excited to begin to resonate. Hydrogen nuclei in a portion that matches the resonance condition (for example, a tomography of a predetermined thickness of the subject 10) are excited, and spins rotate together. When the excitation pulse is stopped, an electromagnetic wave radiated from the rotating spins induces a signal in the RF coil 106, and this signal is detected for a while. By this signal, a tissue containing water in the body of the subject 10 is observed. And in order to know the transmission position of a signal, it is the structure of adding a gradient magnetic field of X and Y, and detecting a signal.

The control unit 220 measures the detection signal while repeatedly applying the excitation signal, and the image reconstruction unit 260 reduces the resonance frequency to the X coordinate by the first Fourier transform calculation, and the Y coordinate by the second Fourier transform. Is restored to obtain an image, and an image corresponding to the display unit 270 is displayed.
(Noise generation in MRI equipment)
First, a mechanism of noise generation in the MRI apparatus 1000 shown in FIG. 1 will be described.

  FIG. 2 is a diagram showing a generation time chart of electromagnetic waves and various gradient magnetic fields in the MRI apparatus 1000. FIG. 2 shows a sequence for three MRI imaging.

  An electromagnetic wave (RF pulse) is generated simultaneously with the slice selective gradient magnetic field, and this is repeated at a constant period. Within each period, a signal emitted from the observation object is detected and processed to obtain an MR image.

  At that time, it is necessary to generate a phase encoding gradient magnetic field and a frequency encoding gradient magnetic field at a certain timing and sequence after electromagnetic wave irradiation. In order to generate the gradient magnetic field, the gradient magnetic field coil is repeatedly increased and decreased and intermittently periodically. At that time, the coil itself “moves” according to Fleming's left-hand rule, but because this is a high-speed alternating current, the entire coil vibrates, resulting in a loud sound.

The frequency of current interruption varies depending on the imaging conditions, but the range is between several tens of Hz to several thousand Hz. At this time, the increase / decrease and interruption of the current that generates the oblique magnetic field occur in the same timing sequence between the periods of the electromagnetic pulse, and therefore the generated noise repeats the same sound between the periods of the electromagnetic pulse (periodic sound) It becomes.
(Operation of the present invention)
FIG. 3 is a conceptual diagram for explaining the appearance of the bone conduction speaker 232 attached to the subject 10 and the attachment method.

  The bone conduction speaker 232 as shown in FIG. 3A is a piezoelectric ceramic speaker made of a nonmagnetic material.

  As shown in FIG. 3B, the subject 10 is shielded from noise caused by air conduction sound by wearing a measurement earplug (not shown) and the ear muff 234. The bone conduction speaker 232 is mounted by adjusting a rubber belt so that a pressure of about 0.25 kgf is applied to the center of the forehead portion of the subject 10.

FIG. 4 is a functional block diagram illustrating the configuration of the stimulus sound processing unit 230.
Referring to FIG. 4, stimulation sound processing unit 230 receives the audio signal given from control unit 220 in parallel, and passes the audio signal in a band obtained by dividing the low frequency side audio band into predetermined frequency bands. Band pass filters 2302.1 to 2302. N and these bandpass filters 2302.1-2302. A high-pass filter 2304 for passing an audio signal on a higher frequency side than N and band-pass filters 2302.1 to 2302. Auto gain control (AGC) 2306.1 to 2306. for performing amplification processing with a predetermined gain (amplification degree) for each of the N outputs. N and AGC 2306.1 to 2306. An adder 2308 for combining the output of N and the output of the high pass filter 2304 and an amplifier 2310 for amplifying the output of the adder 2308 are included. The output of the amplifier 2310 is supplied to the bone conduction speaker 232.

  AGC 2306.1 to 2306. The gain of N is determined as described below.

  Since the noise when observing brain activity using fMRI exceeds 100 dB (A), it is necessary to present sound information at a large volume, which is a burden on the subject. Therefore, in the present invention, as described above, the noise that enters the ear is blocked by the earplug and the earmuff, and then the bone conduction speaker 232 is fixed to the forehead portion of the subject, and the sound information is presented by bone conduction.

  However, when a sound including a lot of low frequency regions is presented, vibration is felt at the forehead portion. Accordingly, the relationship between the upper limit frequency and acceleration at which vibration is felt is obtained by experiment, and when a sound exceeding this is input, AGC 2306.1 to 2306. N adjusts the gain.

  FIG. 5 is a diagram showing the frequency dependence of the subject's minimum audible threshold value and vibration sensation threshold value for the audio signal from the bone conduction speaker 232.

  In FIG. 5, in order to examine the frequency response of the bone conduction speaker 232, an acceleration sensor is attached to the vibration transmission surface to measure the acceleration, and based on the response from the subject, it is audible to the subject, or It measures whether to perceive vibration.

  When a pure tone from 100 Hz to 1 kHz is presented, the acceleration of the speaker vibration surface at the time of minimum audibility and vibration perception is measured in a state where the noise is shielded by wearing the measurement earplug and the earmuff 234. The input voltage to the speaker was increased, and the acceleration of the speaker vibration surface was measured when the sound began to be heard and when a vibration sensation occurred in the forehead. Next, the input voltage was lowered and the acceleration was measured when the vibration sensation disappeared and when the old days were no longer heard. These values were measured for each pure tone frequency.

  If the sound heard before the vibration sensation was too loud for the subject to withstand, the experiment was interrupted and the acceleration was set to no value.

  Referring to FIG. 5, as the frequency increases, the difference between the minimum audible sound threshold and the vibration sensation threshold tends to increase.

  Therefore, for example, as an example, an audio signal of 800 Hz or higher passes through the high-pass filter 2304, and an audio signal of 100 to 800 Hz is divided into N bands and each bandpass filter 2302.1 to 2302. N is configured to pass through. Then, in each frequency band, AGC 2306.1 to 2306. so as not to exceed the vibration sensation generation threshold (or to be lower than the vibration sensation generation threshold by a predetermined level). Adjust the gain of N.

  In this way, it is possible to suppress the possibility that the activity of the bone conduction speaker 232 is mixed into the brain activity of the subject 10.

  By presenting sound information by the above method, brain activity was observed for three men and four women. As a result, the burden on the subject was greatly reduced, and brain activity with the same strength was observed in the same part as the conventional sound information presentation method using headphones.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a functional block diagram which shows the structure of the MRI apparatus 1000 which concerns on this invention. It is a figure which shows the generation | occurrence | production time chart of the electromagnetic waves in the MRI apparatus 1000, and various gradient magnetic fields. It is a conceptual diagram for demonstrating the external appearance and mounting method of the bone conduction speaker 232 with which the test subject 10 is mounted | worn. 3 is a functional block diagram illustrating a configuration of a stimulation sound processing unit 230. FIG. It is a figure which shows the frequency dependence of a test subject's minimum audible threshold value with respect to the audio | voice signal from the bone conduction speaker 232, and a vibration sensation generation | occurrence | production threshold value. It is a timing chart which shows the outline | summary of the measurement sequence of fMRI.

Explanation of symbols

  10 subjects, 12 units, 100 static magnetic field coils, 102 gradient magnetic field coils, 106 RF coils, 120 microphones, 122 sound data storage units, 124 sound data calculation units, 130 speakers, 200 tomography control units, 210 input units, 220 Control unit, 230 stimulation sound processing unit, 232 bone conduction speaker, 234 earmuff, 240 RF pulse transmission unit, 250 signal amplification unit, 260 image reconstruction unit, 270 display unit, 1000 MRI apparatus.

Claims (2)

A magnetic resonance imaging apparatus for detecting a detection signal resulting from nuclear magnetic resonance from a subject and generating a tomographic image of the subject,
A static magnetic field applying means for applying a static magnetic field to the subject;
In the selected cross section of the subject, gradient magnetic field applying means for applying to the subject a magnetic field that is modulated so that the detection signal has positional information of the nucleus that emits the detection signal;
A varying magnetic field transmitting / receiving means for applying a varying magnetic field to the subject and detecting the detection signal from the subject;
A bone conduction speaker for providing a stimulating sound to the subject;
Providing the fluctuating magnetic field to the fluctuating magnetic field transmission / reception means, and comprising the tomography control means for receiving the detection signal and generating the tomographic image,
The tomography control means includes
A magnetic resonance imaging apparatus comprising stimulation sound processing means for adjusting the magnitude of the stimulation sound for each frequency band.   The magnetic resonance imaging according to claim 1, wherein the stimulation sound processing means includes amplification degree adjustment means for adjusting an amplification degree for the stimulation sound so as not to exceed a vibration sensation generation threshold of the subject measured in advance. apparatus.

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