JP3512115B2 - Inspection device using magnetic resonance - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inspection apparatus (hereinafter referred to as MR) for measuring biological functions such as brain function measurement using magnetic resonance.
Abbreviated as I device. 3) relates to an apparatus and method for improving image quality, such as improvement in spatial resolution and reduction of false images. 2. Description of the Related Art Conventionally, as a device for nondestructively inspecting the internal structure of a human body, an X-ray CT or an ultrasonic diagnostic device has been widely used. However, X-ray CT is only applicable to a limited number of people from the viewpoint of safety, and there is a problem that a sufficient density resolution cannot be obtained with an ultrasonic diagnostic apparatus. recent years,
2. Description of the Related Art Magnetic resonance imaging apparatuses for examining the structure inside a living body by using nuclear magnetic resonance phenomena, which have been widely used in the field of chemical analysis for structural analysis and identification of substances, have come to be used. The nuclei to be measured are nuclei such as hydrogen and phosphorus contained in various parts of the living body. By using this apparatus, it has become possible to acquire various biological-related information that cannot be obtained with an X-ray CT or an ultrasonic diagnostic apparatus. In an inspection apparatus using such magnetic resonance, it is necessary to separate and identify a signal from an inspection object in correspondence with each part of the object. As a method for this, for example, a method is known in which a gradient magnetic field is applied to an inspection object so that static magnetic fields applied to respective parts of the object are different from each other, and thereby positional information is given. That is, in magnetic resonance imaging, an atomic nucleus to be measured is placed under a uniform and strong static magnetic field H ₀ , and a high-frequency magnetic field H ₁ is applied thereto. When the frequency of the high frequency magnetic field H ₁ is equal to the resonant frequency of the nuclei, resulting nuclear magnetic resonance phenomenon, the signal is generated. Since this resonance frequency is equal to the strength of the static magnetic field where the nuclei are located,
If a different static magnetic field strength is formed for each part by the gradient magnetic field, it is possible to identify and separate signals for each part from a difference in frequency. However, since the signal obtained in this way is a projection image of a nucleus existing in a two-dimensional or three-dimensional cross section, it is necessary to calculate the original distribution from a plurality of projection images by image reconstruction calculation processing. . At present, the Fourier transform method is most widely used for image reconstruction calculation. The basic principle of this type of device is described in, for example, “Journal of Magnetic Resonance”.
Journal, Volume 18 (1975), Page 69 (J. Magn. Re
son., vol. 18, 1975, pp. 69).
Recently, using such a device, FLASH (FastLow
Angle Single Shot) method and Echo Planar
Attempts to visualize brain activation sites and observe the dynamics of the heart have been attempted by high-speed imaging methods such as the) method. That is, for example, in the case of depiction of an activated site of the brain, a stimulus such as light, sound, smell, or heat, or a stimulus for activating a taste or a somatic sensation is applied to the test object from the outside. This activates the brain and slightly increases oxygen consumption at specific sites. Therefore, in the corresponding site, the amount of reduced hemoglobin that has lost oxygen increases. Then, to compensate for the lack of consumed oxygen, the fresh blood flow to a particular site is excessively increased. Only a 5% increase in oxygen consumption increases blood flow by 50%. The increased blood flow is rich in oxygenated hemoglobin combined with oxygen, which flows from arteries to the veins. Now, reduced hemoglobin is a paramagnetic substance and strongly disturbs the magnetic field around it. If the magnetic field is disturbed, the phases of the precessing nuclei existing there will be different for each nucleus. Since the signal detected by magnetic resonance is the sum of nuclear magnetizations of these nuclei, if the phase difference between the nuclei increases with time, the signal intensity decreases. On the other hand, oxyhemoglobin combined with oxygen is a diamagnetic substance and does not disturb the magnetic field around it. Therefore, the signal intensity increases in proportion to the blood flow. Therefore, an image is acquired when the stimulus is applied and when the stimulus is not applied,
By comparing the two, it is possible to extract, as an image, only the part that has changed due to the application of the stimulus. By examining the correspondence between the type of stimulus and the activation site using the method described above, information on the function of the brain can be obtained. For more information on this, see Proceedings of National Academies of Science USA, Vol. 89 (1991), p.
Acad. Sci. USA: 5951-5595, 1992). A general feature of the biological function measuring method using the high-speed imaging method is that the spatial resolution is low. The reason is described below. In order to perform high-speed imaging, it is necessary to acquire many signals in a short time. However, it is difficult to measure signals continuously at high speed by a method such as the normal SE (Spin Echo) method. In the SE method, a nucleus is excited by application of a high-frequency magnetic field, and a signal generated from nuclear magnetization generated by the excitation is measured. Thereafter, excitation and signal measurement are repeated as before. However, the time from the end of the first signal measurement to the next signal measurement cannot be extremely short. This is because it takes a second order to recover the nuclear magnetization lost by the first signal measurement, and if the recovery time is extremely shortened, the signal intensity is significantly reduced. The echo planar method has been proposed to solve this. In this method, 1
This is a method of acquiring a plurality of signals necessary to create one image by applying (exciting) a high frequency magnetic field twice. In order to acquire a plurality of signals in a short time, it is necessary to rapidly switch a high-intensity gradient magnetic field. However, the intensity of the gradient magnetic field that can be generated and the switching time are limited, and the number of signals that can be acquired by one excitation is limited.
Therefore, it has been difficult to further increase the spatial resolution. In addition, since the gradient magnetic field is switched at a high speed, the amount of eddy current generated in the metal portion of the magnet that generates the static magnetic field also increases, which increases the number of false images and degrades image quality. Was. An object of the present invention is to solve these problems by measuring a nuclear magnetic resonance signal from the brain in synchronization with the application or removal of a stimulus. Another object of the present invention is to achieve a further improvement in image quality by dividing and measuring a frequency space in synchronization with a stimulus. An inspection apparatus using magnetic resonance according to the present invention comprises a magnetic field generating means for generating a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field, and a signal for detecting a magnetic resonance signal from an inspection object. It comprises a detecting means, a computer for performing an operation on the detection signal of the signal detecting means, an output means of the operation result by the computer, and a stimulus applying means for applying a stimulus to the test object. Measuring the magnetic resonance signal from the brain by performing (2) starting measurement of the magnetic resonance signal from the brain after a predetermined time after the application or removal of the stimulus, 3) starting the measurement of the magnetic resonance signal from the brain after a predetermined time after applying or removing the stimulus a plurality of times;
(4) Applying or removing the stimulus a plurality of times, and measuring the magnetic resonance signal from the brain in synchronization with the stimulus or removal of the last applied. (5) While performing the stimulus applying or removing a plurality of times, Starting measurement of magnetic resonance signals from the brain after a predetermined time from the first application of the stimulus, or after a predetermined number of times of application or removal of the stimulus,
(6) An inspection apparatus using magnetic resonance characterized in that a nuclear magnetic resonance signal from a brain is measured and a nuclear magnetic resonance signal is added in synchronization with application or removal of a stimulus.
Further, in this device, the magnetic resonance signal is measured by dividing it into a plurality of times in the frequency space, and the magnetic resonance signals corresponding to a plurality of phase encodings are continuously obtained in synchronization with one stimulation application or removal. Elmi of the measured magnetic resonance signal
It is characterized in that the frequency space to be measured is reduced to about half of the entire frequency space by utilizing the characteristics. The measurement of the magnetic resonance signal is performed by dividing the trajectory of the measurement point in the frequency space so as to draw a zigzag along the direction of the phase encoding from the vicinity of the center of the phase encoding in the frequency space to the outside. There is also a feature that the space is divided and drawn in a spiral from the vicinity of the center to the outside. The applied stimulus may be any of sound, light, smell, heat, drugs, or a stimulus that activates taste and somatic sensation.
Or a combined stimulus. After the change of the test object due to the stimulus has almost disappeared, the next stimulus can be applied and the application of the stimulus can be synchronized with any of the heartbeat, respiration, or EEG change. The change is measured during a period in which no gradient magnetic field and a high-frequency magnetic field for measuring a magnetic resonance signal are applied. Furthermore, the intensity of the applied stimulus is periodically changed, or the applied stimulus is applied at substantially constant time intervals, and the nuclear magnetic resonance signal is measured to determine the change in the function of the test object. Explore. By measuring magnetic resonance signals from the brain in synchronization with the application or removal of a stimulus, a large number of measurement data can be obtained in the same state, and the spatial resolution or SN ratio can be increased. Further, since the division measurement of the frequency space is performed, the measurement data can be further increased, which can contribute to the improvement of the spatial resolution or the SN ratio.
In addition, since the measurement of nuclear magnetic resonance signals from the brain is performed by applying various stimuli and applying or removing the stimulus, the physiological activity function in the brain generated in the brain by the application or removal of the stimulus is performed. You can explore changes in detail. Furthermore, in the present invention, the measurement of the magnetic resonance signal is efficiently performed with the emphasis on the target frequency region, so that the measurement is performed under the same gradient magnetic field as the conventional one without using a strong gradient magnetic field. Thus, information on functions related to a living body can be obtained at high speed. FIG. 2 shows a configuration diagram of an MRI apparatus to which the present invention is applied. In FIG. 2, 1 is a control device, 2 is a high-frequency pulse generator, 3 is a power amplifier, 4 is an RF that detects a signal generated from the inspection object 12 and generates a high-frequency magnetic field.
Reference numeral 5 denotes a signal detection system, 6 denotes an A / D converter, 7 denotes a signal processing device, and 8 denotes a display device. Reference numeral 9 denotes a gradient coil for generating gradient magnetic fields in three orthogonal directions, 10
Indicates a power supply unit for driving a gradient magnetic field coil 9 for generating a gradient magnetic field in three directions. The operation of each device is as follows. The control device 1 has a function of outputting various control commands to each device at a fixed timing. This timing is determined by a photographing sequence, and is recorded in advance in a storage medium of the control device before photographing. The frequency of the high frequency pulse generated by the high frequency pulse
Is determined in proportion to the strength of the uniform static magnetic field generated at the place where the inspection object 12 is placed. For example, when the nucleus to be measured is a hydrogen nucleus, when the static magnetic field strength is 1.5 T, the frequency of the high-frequency pulse is 63.8 MHz. The output of the high-frequency pulse generator 2 is amplified by a power amplifier 3 and supplied to a high-frequency coil 4 called a probe.
This power amplifier can generate high frequency power of up to several kw,
A high frequency current sufficient to excite the nuclei is passed through the high frequency coil. On the other hand, the coil 9 has three directions orthogonal to each other,
A gradient magnetic field that changes linearly is generated independently. Coil 9
The magnetic field distribution in the space where the inspection object 12 is placed is set to a distribution having a desired gradient by the gradient magnetic field generated by the above.
15 is a stimulus generator for generating a stimulus such as light or sound;
Is a power supply for driving the stimulus generator. High frequency coil 4
Are passed through the signal detection system 5, amplified and detected, and then A / D converted by the A / D converter 6. The converted digital signal undergoes image reconstruction processing in a signal processing device 7 and is converted into an image.
Displayed with. The human body 12 to be inspected is a bed 13
The bed 13 is mounted on the support base 14 so as to be movable on the support base 14. (First Embodiment) FIG. 1 shows a first embodiment of the present invention. The signal measurement 101 starts at time t ₀ ,
Stimulus is applied from the time of the time t _1. Specifically, at time t ₁ , the control device 1 drives the stimulus generator driving power source 1.
A stimulus generation trigger 211 is sent to 6, and the stimulus generation device 15 generates a stimulus such as sound or light for a predetermined time based on the trigger. Thereafter, the signal measurement 102, the stimulus application trigger 212, the signal measurement 103, the stimulus application trigger 213, and the signal measurement 104 are repeated. Here, signal measurement is not necessarily 1
This includes not only acquiring signals necessary for composing a plurality of images, but also acquiring signals corresponding to a plurality of images. The example of FIG.
A case is shown in which the signals acquired in step 04 are added and averaged to improve the image quality by improving the low S / N ratio, which is a problem in high-speed imaging. (Second Embodiment) FIG. 3 shows a second embodiment of the present invention. The control device 1 includes a drive power source 1 for the stimulus generator.
A stimulus generation trigger 211 is sent to 6, and the stimulus generation device 15 generates a stimulus such as sound or light for a predetermined time based on the trigger. Subsequently, the high-frequency pulse generator 2 generates a high-frequency pulse 22, amplifies it with the power amplifier 3, and applies it to the high-frequency coil 4. When applying the high-frequency pulse, the gradient magnetic field 23 for slice selection is also applied at the same time. next,
Phase encoding gradient magnetic field 25 and readout gradient magnetic field 2
6 is applied by a gradient magnetic field coil 9 to generate a signal 2
7 is detected by the high-frequency coil 4. As described above, this signal is amplified and detected by the signal detection system 5, and then the A / D converter 6
Is converted from an analog signal to a digital signal, and finally converted into an image by the signal processing device 7. Now, in the high-speed imaging method, as shown in FIG. 3, since a plurality of signals 27 having different phase encodings are obtained by one application of a high-frequency magnetic field, the imaging time can be reduced to about several tens of ms. But,
The number of signals that can be obtained at one time is at most about several tens because the signal attenuates with time. In the conventional example, the spatial resolution is restricted by the number. In order to increase the number of acquired signals, it is necessary to increase the gradient magnetic field strength and shorten the switching time. However, along with that, the demand for the gradient magnetic field power supply 10 has become extremely high, and realization has not been easy. Therefore, as shown in FIG. 3, signals necessary for reconstructing one image are generated by a stimulus generation trigger 211,
In synchronization with 212, the measurement was performed in two divided times. As a result, the number of acquired signals can be doubled, and the spatial resolution can be improved. On the other hand, it is also possible to make the number of acquired signals the same and extend the measurement time of one signal instead. In this case, the spatial resolution is the same, but since the rise time of the gradient magnetic field can be lengthened,
The generation of the eddy current can be suppressed. As a result, the occurrence of false images can be reduced, and the image quality can be improved. When this method is applied to brain function measurement, the method of applying phase encoding is important. That is, as shown in the locus of the frequency space in FIG.
Two measurements are performed along the trajectory 31 and the trajectory 32. The coordinates of the frequency space shown in FIG. 4 are obtained by taking the horizontal axis (kr) as the time integration value of the readout gradient magnetic field and the vertical axis (kp) as the time integration value of the phase encoding gradient magnetic field. By performing a Fourier transform on the data in the frequency space, an MR image is obtained. Since the central portion of the frequency space corresponds to the low-frequency component when the MR image is formed, the signal is attenuated by performing the signal measurement from the central portion toward the periphery to determine the low-frequency component that determines the contrast of the image. It will be possible to measure with high sensitivity before. By the way, the degree of activation of the brain by the external stimulus changes with the application of the stimulus, and the state of this change is substantially the same in both measurements even when the measurement is divided into two. Therefore, when the measurement is divided into two times, simply dividing the frequency space in the phase encoding direction into two halves means that the effects of brain activation are assigned to different spatial frequencies, and the obtained image has an error. Will be bigger. However, if the division shown in FIG. 4 is performed, the effect of activation is assigned to almost the same spatial frequency, so that the error can be minimized. The selection of the two trajectories is performed by changing the size of the first application portion 24 of the phase encoding gradient magnetic field and the polarity of the phase encoding gradient magnetic field 25 shown in FIG. FIG. 4 shows an example in which the frequency space is divided into two parts.
Even in the case of division, the measurement may be started from a line near kp = 0 as in the case of FIG. (Third Embodiment) FIG. 5 shows a third embodiment of the present invention, in which the entire upper half (kp.gtoreq.0) and only a part of the lower half (kp <0) of the frequency space are used. This shows the case of measurement. Almost the lower half, which is the frequency space that is not measured, is predicted from the upper half using Hermite properties. That is, a method of assigning the complex conjugate of the corresponding value of the upper half to the value of an unmeasured point located at a position symmetrical with respect to the center of the frequency space with respect to the measurement point of the upper half. This method is generally called a half Fourier method. In this case, as shown in FIG. 5, by dividing the measurement along the trajectory 33 and the trajectory 34 in the frequency space, it is possible to allocate the effects of the activation of the brain to almost equal spatial frequencies. (Fourth Embodiment) FIG. 6 shows a fourth embodiment of the present invention, in which the locus of a measurement point in a frequency space is spiral. Since the image reconstruction requires a measurement value on the rectangular coordinate point, a value on the rectangular coordinate point is obtained by interpolation from the measurement value on the spiral. The example shown here is 1
This is the case of dividing into two, and the locus 35
The measurement is performed from the center of the frequency space to the periphery along the locus 46. As in the previous example, the effects of brain activation can be assigned to approximately equal spatial frequency regions. Although FIGS. 5 and 6 show the case where the frequency space is divided into two or twelve, the application of the present invention is not limited to this. The half Fourier method can be applied to the example shown in FIG. (Fifth Embodiment) FIG. 7 shows a fifth embodiment of the present invention in which the stimulation application cycle 400 is longer than the duration 401 of the brain activation state caused by the stimulation. If the change caused by the stimulus generated in synchronization with the first stimulus generation trigger 211 remains even at the time of signal measurement performed in synchronization with the next stimulus generation trigger 212,
The measurement is a measurement influenced by the previous stimulus, and does not reflect the original activation. To prevent this, after the activation state 401 by the first stimulus, a period 411 in which the activation state almost disappears is provided, and the next stimulus generation trigger 2
At the time point of 12, it is sufficient to return to the steady state. Thereafter, as before, an extinction period 412 is provided after the activation state 402, and the next trigger 213 is generated. (Sixth Embodiment) FIG. 8 shows a sixth embodiment of the present invention in which the application of a stimulus is synchronized with body movement such as respiration or heartbeat, or movement of blood flow, electrocardiogram, or electroencephalogram. Show. The movement of the body due to breathing can be detected by a strain gauge attached to the body. Time change curve 50 of signal from strain gauge
, The rate of change of the curve is obtained, the rising point is detected, and the trigger 51 is generated. This trigger causes the stimulus generation trigger 21
Is generated, and a signal is measured based on the stimulus generation trigger. This operation is repeated as many times as necessary, as in the other embodiments. In the embodiment described above, when measuring the electrocardiogram or the electroencephalogram, it is preferable to use a period in which no gradient magnetic field or high-frequency magnetic field is applied, from the viewpoint of removing noise. Further, in the embodiments described so far, the signal-to-noise ratio can be improved by integrating the signal in synchronization with the stimulus, so that it can be used as needed. Furthermore, in the above-described embodiment, it is needless to say that after repeating the applied stimulus a plurality of times, the signal measurement may be performed in synchronization with the last applied stimulus.
It goes without saying that signal measurement may be performed after a predetermined time has elapsed after one or more stimuli are applied. The plurality of stimuli to be applied may be applied at intervals of time, or may be applied a plurality of times so that the intensity of the stimulus changes with time. For example, the time interval may be a fixed time interval such as 0.1 second, or 0.1 second, 0.2 second, 0.3 second, or 0.3 second.
It may be a time interval having a regularity of seconds. When the intensity of the stimulus initially applied at time t = 0 is S, the intensity of the stimulus to be applied a plurality of times may be St in proportion to time t, or may be periodic like sin (St). The intensity of the stimulus may be changed at the same time. According to the present invention, since signal measurement is performed in synchronization with application of a stimulus, a high spatial resolution or a high SN
An image of the ratio can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a first embodiment of the present invention. FIG. 2 is a diagram showing a configuration of an MRI apparatus used in the present invention. FIG. 3 is a diagram showing a second embodiment of the present invention. FIG. 4 is a diagram showing a locus in a frequency space of two measurements in a second embodiment. FIG. 5 is a diagram showing a third embodiment of the present invention. FIG. 6 is a diagram showing a fourth embodiment of the present invention. FIG. 7 is a diagram showing a fifth embodiment of the present invention. FIG. 8 is a diagram showing a sixth embodiment of the present invention. [Description of Signs] 1 ... Control device, 2 ... High frequency pulse generator, 3 ... Power amplifier, 4 ... High frequency coil, 5 ... Signal detection system, 6 ... A / D converter, 7 ... Signal processing device, 8 ... Display Apparatus, 9: gradient magnetic field coil, 10: gradient magnetic field coil drive power supply unit, 11: magnet, 12: inspection object, 13: bed, 14: support base, 1
5 stimulus generator, 16 stimulus generator drive power supply, 10
1, 102, 103, 104 ... signal measurement, 21, 21
1, 212, 213: stimulus generation trigger, 22: high frequency pulse, 23: gradient magnetic field for slice selection, 25: phase encoding gradient magnetic field, 26: readout gradient magnetic field, 27 ...
Signals, 31, 32, 33, 34, 35 to 46: locus of measurement points in the frequency space; 24, the first applied portion of the phase encoding gradient magnetic field; 50, the time change curve of the body motion signal;
51: trigger signal, 400: stimulus application cycle, 401,
402: duration of brain activation state, 411, 412: duration of disappearance of activation state.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroyuki Itagaki 1-280 Higashi Koigakubo, Kokubunji-shi, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd. (72) Inventor Yo Yo Taniguchi 1-280 Higashi Koigakubo, Kokubunji-shi, Tokyo Hitachi, Ltd. In the Central Research Laboratory (56) References JP-A-62-231642 (JP, A) JP-A-1-91842 (JP, A) JP-A-1-107750 (JP, A) JP-A-1-131649 (JP, A A) JP-A-1-209905 (JP, A) JP-A-4-117942 (JP, A) JP-A-5-49611 (JP, A) JP-A-5-176912 (JP, A) JP-A-5 -207987 (JP, A) JP-A-6-269423 (JP, A) JP-A-7-23920 (JP, A) Seiji Ogawa, Tso-Ming Lee, Magnetic Resonan ce in Medicine, 1990, vol. 16, no. 1 pp. 9-18 Hirotake Kamei, Human Sensation by Head NMR Measurement of Information Processing Site and Amount, Journal of Biomechanism Society, Japan, Biomechanism Society, 1991, vol. 15, no. 3, pp. 111-117 John W.S. Belliveau, FUNCTIONAL BRAIN MAPPING USING MAG NETIC RESONANCE IM AGING, Annual International Conference of the IEEE Engineering in Medicine in Medicine in Medicine in Medicine in Medicine in Medicine in Medicine in Medicine in Medicine in Medicine. 13, no. 1 pp. 73-74 (58) Field surveyed (Int. Cl. ⁷ , DB name) A61B 5/055

Claims (1)

(57) [Claims 1] A magnetic field generating means for each of a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field, a signal detecting means for detecting a magnetic resonance signal from a test object, and a detection signal of the signal detecting means. And a means for outputting an operation result by the computer,
A stimulation means for applying a stimulus to the test object, the puncturing
The magnetic resonance signal is measured in synchronization with intense application or removal.
Inspection apparatus using magnetic resonance having
The synchronous measurement is divided into a plurality of times in the frequency space.
Is performed, and multiple phase encodings are performed in each divided region.
Measuring the corresponding magnetic resonance signal,
Changes in the response to the stimulus are observed in each divided area.
Measurement order that can be assigned to almost equal spatial frequencies
An inspection device using magnetic resonance, which is measured in the beginning .