JP3690874B2 - Magnetic resonance imaging system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for magnetic resonance imaging (hereinafter referred to as “MRI”) that obtains a tomographic image of a subject by utilizing a nuclear magnetic resonance (hereinafter referred to as “NMR”) phenomenon. The present invention relates to an MRI apparatus capable of simultaneously visualizing a flow and a brain surface structure.
[0002]
[Prior art]
The MRI technique is used not only to obtain a living body tomographic image but also to image living body functions. Such a technique forms an image using a change in magnetic behavior accompanying a change in the oxidation state or the like of a substance in a living body.
[0003]
Imaging of the brain activation region by MRI forms an image using the fact that the intensity of the NMR signal changes due to the oxidation and reduction reactions of hemoglobin in the bloodstream that accompany the brain activation. That is, it utilizes the fact that the paramagnetic substance hemoglobin is reduced by oxygen consumption accompanying brain activation, changes to diamagnetism when it becomes deoxyhemoglobin, and the NMR signal is enhanced. Specific examples of such imaging of the brain activation region can be found in, for example, Magnetic Resonance in Medicine, 33, 736-743 (1995).
[0004]
[Problems to be solved by the invention]
However, when imaging the brain activation area, it is difficult to distinguish the change in the intensity of the signal from the change in the intensity of the signal due to the pulsation of blood flow in the main veins and the increase and decrease of the blood flow. . Therefore, when trying to obtain an accurate image of the brain activation area, an image of the brain activation area and another blood flow image showing the blood flow of the brain are taken, and the brain activity is determined from their positional relationship. It is necessary to distinguish between the blood flow region and the pulsation of blood flow and increase / decrease in blood flow. In addition, in order to know where the brain activation region is in the brain, it is necessary to separately capture images representing the positions of blood vessels and brain surface structure images.
[0005]
Therefore, imaging must be performed a plurality of times for imaging the brain activation area, which is complicated and time consuming.
[0006]
By the way, conventionally, when a high frequency pulse is irradiated with an extremely short repetition time (TR) with respect to the relaxation time of a subject to be photographed, a steady precession state (SSFP, Steady-State Free Precession) occurs in the photographing region, as shown in FIG. In addition, it is known that two types of echo signals 202 and 201 are obtained immediately before and immediately after the high-frequency pulse. The signal 201 obtained immediately after the high frequency pulse is called a free induction decay (FID) signal, and the signal 202 obtained just before the high frequency pulse is called a time-reversed decay (FID) signal. . Since this time-reversed FID signal has the same properties as the preceding high-frequency pulse and the echo signal generated by the previous high-frequency pulse, it is strongly subjected to lateral relaxation. In addition, this signal has a double repetition time as an echo time. A method of visualizing a signal generated in such a steady precession state is discussed in detail in, for example, Magnetic Resonance in Medicine, 7, 35-42 (1988).
[0007]
The present invention can simultaneously image the active region of the brain, the cerebral blood flow, and the brain surface structure using two types of echo signals obtained by using the high-frequency pulse that gives the SSFP state as described above. An object is to provide an MRI apparatus.
[0008]
[Means for Solving the Problems]
That is, the magnetic resonance imaging apparatus of the present invention applies a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field to the space in which the subject is placed, and outputs a nuclear magnetic resonance signal to the atomic nucleus constituting the tissue of the subject imaging region. Each magnetic field generating means to be generated, a receiving means for receiving a nuclear magnetic resonance signal generated from the imaging target region, each magnetic field generating means and the receiving means are controlled to generate a predetermined magnetic field pulse in a predetermined pulse sequence and each magnetic field A pulse executed by the sequence control means, comprising a sequence control means for measuring the resonance signal and an image processing means for performing an image reconstruction calculation using the signal detected by the reception means and displaying the obtained image The sequence includes 1) a step of repeatedly applying a high-frequency magnetic field pulse with a repetition time shorter than the relaxation time of the imaging target region, and 2) each repetition time. And measuring two echo signals generated immediately before and immediately after the application of the high-frequency magnetic field pulse, and the image processing means obtains a functional measurement image of the subject using one of the two echo signals. The blood flow image of the imaging target region is obtained using the other. Here, when the pulse sequence of the sequence control unit includes a step of acquiring three-dimensional image data of the imaging target region, the image processing unit may select a signal for obtaining a function measurement image from the two echo signals or The surface structure of the imaging target region is also drawn using either one of the signals for obtaining the blood flow image.
[0009]
In the magnetic resonance imaging apparatus of the present invention, the pulse sequence executed by the sequence control means is different between when the imaging target region is active and when it is at rest, and the first pulse sequence is the measurement of the echo signal immediately after the application of the high-frequency magnetic field pulse. And applying a gradient magnetic field that enhances the signal from the bloodstream in the second pulse sequence applies a gradient magnetic field that lowers the signal from the bloodstream in the measurement of the echo signal immediately after the application of the high-frequency magnetic field pulse. Including the steps of: The image processing means calculates the difference between the two images formed from the two echo signals obtained in the first pulse sequence and the two images formed from the two echo signals obtained in the second pulse sequence. A functional image and a blood flow image are drawn by taking the images. In this case, when a signal for three-dimensional drawing is acquired, one of the two echo signals is used to draw the surface structure of the imaging target region.
[0010]
Here, the first pulse sequence is executed, for example, when the subject's brain is activated, and a gradient magnetic field for removing a phase rotation generated in the fluid portion in the subject when measuring a signal generated immediately after the high frequency pulse. Is applied. The second pulse sequence for lowering the signal from the blood flow is executed when the subject's brain is resting, and the phase of the fluid portion in the subject is measured when measuring the signal generated immediately after the high frequency pulse. A gradient magnetic field for applying rotation is applied.
[0011]
The MRI apparatus of the present invention is particularly suitably applied to brain function measurement. In the preferred embodiment, the image of the brain activation area, the image of the brain dish flow, and the image of the brain surface structure are synthesized. , Draw the positional relationship of each.
[0012]
According to the MRI apparatus of the present invention, a high-frequency pulse of an arbitrary angle is applied at a repetitive time extremely short with respect to the relaxation time in the imaging target region at the time of brain activation and rest, causing a steady precession state, and the high frequency generated at that time Measure the FID signal immediately after the pulse and the time-reverse FID signal immediately before the pulse. For example, when measuring the FID signal immediately after the high frequency pulse, a gradient magnetic field is applied so as to remove the phase rotation that occurs in the fluid part when the brain is activated, and the gradient magnetic field is generated so that the fluid part is rotated around the phase when the brain is resting. Apply and create a differential image of the resulting image, image the brain active region from one side, and blood flow from the brain from the other, and use either signal to image the brain surface structure.
[0013]
Therefore, according to the MRI apparatus of the present invention, the brain activation area, cerebral blood flow, and brain surface structure can be visualized simultaneously, and the imaged brain activation area image, brain dish flow image, and brain It is possible to synthesize images of the surface structures and draw the positional relationships from the synthesized images.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a block diagram showing an outline of the configuration of an MRI apparatus that implements the magnetic resonance imaging method of the present invention. The MRI apparatus shown in FIG. 1 generates and irradiates a magnet 101 which is a static magnetic field generating means for applying a static magnetic field to a space where a subject is placed, and a high frequency magnetic field for generating nuclear magnetic resonance in the subject. After receiving and detecting the signal generated from the subject, the excitation system 102 which is a system, the receiving system 103 for A / D conversion, and the magnetic field strength are independently and linearly changed in the X, Y and Z directions, respectively. Image processing system 105 that performs various operations necessary for image formation and image display based on measurement data from the gradient magnetic field generation system 104 and the reception system 103, and a sequence that controls the operation timing of each system in the above configuration A control system 106, a probe 107 used for high-frequency transmission / reception, and a console 108 for operation are provided.
[0015]
As described above, the configuration of the MRI apparatus for performing the magnetic resonance imaging method of the present invention is based on the sequence control system for executing the pulse sequence for performing the method of the present invention as described below and the method of the present invention. The configuration may be the same as that of a conventional MRI apparatus except that a control system for processing an echo signal is provided.
[0016]
In the MRI apparatus of the present invention, a signal obtained in a steady precession state (SSFP) generated by applying a high-frequency pulse having a repetitive time extremely short with respect to the relaxation time of an imaging target, for example, brain tissue, as shown in FIG. (SSFP signal) is used. The SSFP signal includes an FID signal 201 that occurs immediately after the high-frequency pulse and a time-reversed FID signal 202 that occurs immediately before the high-frequency pulse. In the MRI apparatus of the present invention, these two types of signals are measured simultaneously. On the other hand, for example, the FID signal is used for blood flow measurement and the time-reversed FID is used for functional measurement.
[0017]
Next, an example of the brain function measuring method implemented in the MRI apparatus of the present invention will be described using the time chart shown in FIG. In order to measure brain function, it is necessary to create a state where the brain is activated and a state of rest, and the time for applying a motion stimulus such as light stimulus or finger tapping to the subject and their time The resting time without applying the stimulus is alternately repeated to alternately form a state where the brain is activated and a resting state. Then, signal measurement is performed several times during each period.
[0018]
In FIG. 3, the period during which the activation stimulation is performed is indicated as ON, and the period during which the activation stimulation is not performed is indicated as OFF. Circle numbers {circle over (1)}, {circle over (2)}, {circle over (3)} respectively represent repetition of the pulse sequence, and the application of the high frequency pulse as shown in FIG. For example, when signal measurement is performed with a repetition time (TR) of 20 ms and image formation is performed in a 128 × 128 matrix, the time required for measurement for image formation is 20 ms × 128 = 2560 ms. Accordingly, as shown in the figure, if the time for applying a stimulus and the rest time for not applying a stimulus are 30 seconds, signal measurement can be performed 11 times during those times.
[0019]
As described above, in the present invention, two echo signals of the FID signal and the time-reversed FID signal are measured in one sequence. In order to draw the activated region of the brain, the time-reversed FID signal is drawn. Is used. For this reason, first, as shown in FIG. 6, an image 601 is formed using a time-reversed FID signal measured when the stimulus is turned on. Similarly, an average value image 602 is formed from an image using a time-reversed FID signal measured when the stimulus is OFF. A subtraction image 603 is obtained by calculating a difference between these two images 601 and 602. In this subtracted image 603, a place where the signal intensity at the time of stimulus ON and stimulus OFF is different remains, and this is the place activated by the stimulus. Further, statistical processing such as t-test can be performed as other processing for drawing the activated region.
[0020]
By the way, the functional image 603 obtained in this way may contain information due to changes in blood flow, and needs to be related to the blood flow image. For this reason, in the present invention, a blood flow image is formed using the remaining signal, that is, the FID signal, of the two signals measured in one signal measurement. The blood flow drawing method will be described below.
[0021]
In general, blood flow can be visualized by MRI using the Time of Flight (FOT) method, which uses the inflow effect of spins that newly flow into the slice plane, and the presence or absence of spin phase diffusion due to blood flow. The PS (Phase Sensitive) method, which obtains a blood flow image by performing a difference, and the PC (Phase Contrast) method, which obtains a blood flow image by inverting the polarity of phase diffusion due to blood flow, are mainly known. In the embodiment, the case where the PS method is used will be described.
[0022]
In the PS method, a pulse sequence (rephase sequence) for increasing the blood flow signal to reduce the signal between the image obtained by increasing the blood flow signal and the image obtained by reducing the signal is used. Two pulse sequences (dephase sequence) of the pulse sequence are adopted.
[0023]
FIG. 4 shows an example of a pulse sequence in which the PS method is applied to a pulse sequence for obtaining two signals in one measurement. FIG. 4A shows a rephase sequence (FIG. 4B )) Shows the dephasing sequence.
[0024]
First, in the rephasing sequence of FIG. 4A, a target region to be imaged is slice-selected with a slice gradient magnetic field pulse 403 and excited with a high-frequency pulse 402 of an arbitrary angle. The FID signals generated at this time are collected as echo signals by the frequency encoding gradient magnetic field pulses 410 and 411, and AD sampling is performed in a section 413. At this time, in the rephasing sequence (a), the echo signal is encoded in the phase encoding direction by the phase encoding gradient magnetic field pulse 407, and the gradient magnetic field to be flow rephased so that the phase rotation of the fluid part is zero. Applied. The gradient magnetic field pulses 409, 410, and 411 form a gradient magnetic field pattern that rephases in the frequency encoding direction. The gradient magnetic field pulses 404 and 405 return the phase disturbed in the slice direction by the pulse 403, and the pulses 403, 404, and 405 form a gradient magnetic field pattern of the flow phase in the slice direction. By such rephasing, a signal having the same intensity as the signal intensity obtained in the later-described phase phase sequence is obtained for the stationary part, and the loss of signal due to phase diffusion is suppressed in the moving magnetization existing part, that is, the blood flow part, A signal higher than the phase sequence can be obtained.
[0025]
In this pulse sequence, since the high frequency pulse 402 is applied with an extremely short repetition time TR (401) as shown in FIG. 2, a time-reversed FID signal is generated immediately before the high frequency pulse. This time-reversed FID signal is used for the function measurement already described, and is collected as an echo signal by the frequency encode pulses 411 and 412 and A / D-sampled in the section 414. After this A / D sampling, in order to restore the phase rotation by the phase encoding gradient magnetic field pulse 407, the pulse 408 is applied as an amount having the same absolute value and the opposite sign as the pulse 407. This phase encoding gradient magnetic field pulse 408 has exactly the same magnitude and application time as the phase encoding pulse 407. As a result, the process of one iteration is measured within the same phase encoding projection.
[0026]
The dephasing sequence in FIG. 4B is exactly the same as the rephasing sequence in FIG. 4A for the time-reversed FID signal measurement, but the slope in the slice direction and the frequency encoding direction after applying the high frequency magnetic field pulse. Each magnetic field pattern is different. That is, the slice selection pulses 415 and 416 cause phase rotation in the slice direction with respect to the fluid part, and the frequency encode pulses 417 and 418 cause phase rotation in the frequency direction. As a result, the FID signal obtained by the dephasing sequence is a signal in which the signal from the fluid part is suppressed by phase diffusion as compared with the FID signal measured by the above rephasing sequence.
[0027]
A rephase image and a dephase image can be obtained by repeating these two types of sequences for each required number of times, for example, 128 times. In the present embodiment, as shown in the time chart of FIG. 3, the rephasing sequence and the dephasing sequence are switched in accordance with the switching of the stimulus ON and the stimulus OFF in the function measurement. That is, the signal measurement by the rephase sequence (128 repetitions of the sequence shown in FIG. 4) is performed a plurality of times during the stimulus ON time period (for example, 30 seconds), and the signal measurement by the phase sequence is performed during the stimulus OFF time period. I try to do it multiple times.
[0028]
Two images obtained by repeating these two sequences are subtracted as shown in FIG. 5, and a difference image 503 is obtained by taking the difference between the rephase image 501 and the phase image 502. In this image 503, the static part is erased, and for example, a subtraction angio image obtained by imaging only a moving part such as a blood flow in a cerebral blood vessel with a high signal can be obtained. In this case, complex subtraction between signals may be performed in the subtraction processing.
[0029]
As described above, by executing the pulse sequence according to the present invention, two types of images, ie, a functional image of the brain activation region and a cerebral blood flow image can be obtained at a time. Moreover, these functional images and blood flow images can be obtained as two-dimensional or three-dimensional images, similarly to known functional image measurement and blood flow rendering methods.
[0030]
Furthermore, in a preferred aspect of the present invention, an image of the brain surface structure is obtained using either the FID signal or the time-reversed FID signal obtained by executing the pulse sequence described above. In this case, it is necessary to perform measurement by a three-dimensional or multi-slice method. The FID signal and time-reversed FID signal used to form an image of the brain surface structure may be either those obtained by the rephasing sequence or those obtained by the dephasing sequence. Drawing is performed by a known rendering method using the three-dimensional data formed by the FID signal. For example, when using image data created by an FID signal, volume rendering is performed from brain extraction. When using a time-reversed FID signal image, the image itself is a T2-weighted image with an emphasis on cerebrospinal fluid. Can be drawn. FIG. 7 shows a process of synthesizing a brain surface structure image in the method implemented by the MRI apparatus of the present invention. Here, it is shown that the brain surface structure image 703 is formed from one of the image 701 of the FID signal and the image 702 of the time-reversed FID signal.
[0031]
Furthermore, in the MRI apparatus of the present invention, the brain function image, the cerebral blood flow image, and the brain surface structure image obtained as described above can be synthesized by addition processing, and the respective positional relationships can be drawn. FIG. 8 shows a process of synthesizing the brain function image, the brain blood flow image, and the brain surface structure image in the method implemented by the MRI apparatus of the present invention.
[0032]
With such a composite display, it is possible to identify and observe an active region of the brain, an abnormality in cerebral blood flow, and the like, and an image with high diagnostic value can be obtained.
[0033]
In the embodiment of FIG. 3 described above, stimulation ON and rephasing are performed simultaneously. However, a dephasing sequence is performed when the stimulation is ON, and a rephasing sequence is performed when the stimulation is OFF. Good. The numbers given in the present embodiment, such as the number of sequence repetitions and the number of matrices, are embodiments of the present invention and are not limited thereto.
[0034]
Further, although the PS method is adopted as the blood flow rendering method, other blood flow rendering methods such as the PC method may be employed.
[0035]
【The invention's effect】
According to the MRI apparatus of the present invention, by simultaneously using the FID signal generated in the SSFP state and the time-reversed FID signal, the brain activation region, cerebral blood flow, and brain surface structure are simultaneously imaged in one measurement. It is possible to synthesize the image of the brain activation area, the image of the cerebral dish flow, and the image of the brain surface structure, and depict the positional relationship from the synthesized image. Thereby, an image with high diagnostic value can be obtained in a short time.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of an MRI apparatus of the present invention.
FIG. 2 is a diagram showing high-frequency pulses used in the MRI apparatus of the present invention, and FID signals and time-reversed FID signals generated in the SSFP generated thereby.
FIG. 3 is a diagram showing an example of a time chart of a pulse sequence executed by the MRI apparatus of the present invention.
4A and 4B are diagrams showing examples of pulse sequences for measuring FID signals and time-reversed FID signals used in the MRI apparatus of the present invention, respectively.
FIG. 5 is a diagram showing a process of synthesizing a cerebral blood flow image in the method performed by the MRI apparatus of the present invention.
FIG. 6 is a diagram showing a process of synthesizing a brain function image in the method performed by the MRI apparatus of the present invention.
FIG. 7 is a diagram showing a process of synthesizing a brain surface structure image in the method performed by the MRI apparatus of the present invention.
FIG. 8 is a diagram showing a process of synthesizing a brain function image, a brain blood flow image, and a brain surface structure image in the method implemented by the MRI apparatus of the present invention.
[Explanation of symbols]
101 Magnet (Measuring means for static magnetic field)
102 Excitation system (high-frequency magnetic field generation means)
103 Receiving system (receiving means)
104 Gradient magnetic field generation system (gradient magnetic field generation means)
105 Image processing system (image processing means)
106 Sequence control system (sequence control means)
107 Probe (receiving means).

Claims (2)

Each magnetic field generating means for applying a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field to the space where the subject is placed, and causing a nuclear magnetic resonance signal to be generated in the atomic nucleus constituting the tissue of the imaging target region of the subject, The receiving means for receiving the nuclear magnetic resonance signal generated from the imaging target region, the magnetic field generating means and the receiving means are controlled to generate a predetermined magnetic field pulse in a predetermined pulse sequence and measure the magnetic resonance signals. A magnetic resonance imaging apparatus comprising: a sequence control unit for performing an image reconstruction calculation using a signal detected by the receiving unit; and an image processing unit for displaying the obtained image.
The pulse sequence executed by the sequence control means is different between when the imaging target area is active and at rest, and a gradient magnetic field that enhances a signal from the bloodstream in measurement of an echo signal immediately after application of the high-frequency magnetic field pulse is used. A first pulse sequence including an applying step, and a second pulse sequence including a step of applying a gradient magnetic field that lowers a signal from a blood flow in measurement of an echo signal immediately after the application of the high-frequency magnetic field pulse. Each pulse sequence consists of
1) repeatedly applying the high-frequency magnetic field pulse with a repetition time shorter than the relaxation time of the imaging target region;
2) measuring two echo signals generated immediately before and immediately after the application of the high-frequency magnetic field pulse within each repetition time;
The image processing means obtains the functional image and the blood flow image by taking a difference between images obtained from the echo signals acquired in the first and first pulse sequences, and among the two echo signals, A magnetic resonance imaging apparatus characterized in that a function measurement image of the subject is obtained using one and a blood flow image of the imaging target region is obtained using the other. Each magnetic field generating means for applying a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field to the space where the subject is placed, and causing a nuclear magnetic resonance signal to be generated in the atomic nucleus constituting the tissue of the imaging target region of the subject, The receiving means for receiving the nuclear magnetic resonance signal generated from the imaging target region, the magnetic field generating means and the receiving means are controlled to generate a predetermined magnetic field pulse in a predetermined pulse sequence and measure the magnetic resonance signals. A magnetic resonance imaging apparatus comprising: a sequence control unit for performing an image reconstruction calculation using a signal detected by the receiving unit; and an image processing unit for displaying the obtained image.
The pulse sequence executed by the sequence control means is different between when the imaging target area is active and at rest, and a gradient magnetic field that enhances a signal from the bloodstream in measurement of an echo signal immediately after application of the high-frequency magnetic field pulse is used. A first pulse sequence including an applying step, and a second pulse sequence including a step of applying a gradient magnetic field that lowers a signal from a blood flow in measurement of an echo signal immediately after the application of the high-frequency magnetic field pulse. Each pulse sequence consists of
1) repeatedly applying the high-frequency magnetic field pulse with a repetition time shorter than the relaxation time of the imaging target region;
2) measuring two echo signals generated immediately before and immediately after the application of the high-frequency magnetic field pulse within each repetition time, and obtaining three-dimensional image data of the imaging target region,
The image processing means obtains the functional image and the blood flow image by taking a difference between images obtained from the echo signals acquired in the first and first pulse sequences, and among the two echo signals, A function measurement image of the subject is obtained using one, a blood flow image of the imaging target region is obtained using the other, and a surface structure of the imaging target region is drawn using either one Magnetic resonance imaging device.

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