JP3977067B2 - Magnetic resonance imaging system - Google Patents

Description

[0001]
[Technical field to which the invention belongs]
The present invention relates to a magnetic resonance imaging (MRI) apparatus that measures nuclear magnetic resonance (hereinafter referred to as NMR) signals from hydrogen, phosphorus, etc. in a subject and visualizes nuclear density distribution, relaxation time distribution, etc. In particular, the present invention relates to an MRI apparatus having a function of continuously imaging while arbitrarily changing an imaging section.
[0002]
[Prior art]
One of the imaging functions of the MRI apparatus is dynamic imaging that continuously images a subject. In dynamic imaging, for example, an NMR signal is measured using a high-speed imaging method such as an echo planar method (EPI method) to obtain time-series image data. Such dynamic imaging makes it possible to visualize the temporal change of the target part when studying the function of a specific part of the subject or performing surgery or treatment on the subject.
[0003]
High-speed imaging methods such as the EPI method employed for dynamic imaging are generally susceptible to eddy currents due to gradient magnetic fields and static magnetic field inhomogeneity, and therefore phase correction using correction data is performed. The correction data is acquired by performing pre-scanning under the condition of the phase encoding gradient magnetic field 0 separately from imaging (main scanning) for acquiring image data, and normally uses correction data obtained by one pre-scanning. All the data obtained in the main scan are corrected. In the EPI method, there is a divided EPI method (multi-shot EPI) in which a signal necessary for image reconstruction is divided into a plurality of pieces, and in this case, data obtained by a plurality of scans is the same pre-scan data. It is corrected by.
[0004]
By the way, in recent years, various techniques using MRI as a monitor for surgery and treatment have been developed, and one of them has been put into practical use a technique called an interactive scan for imaging while arbitrarily changing the cross section during continuous imaging. . In the interactive scan, a desired section is indicated using a section setting device called a pointer, and the section is imaged.
[0005]
[Problems to be solved by the invention]
When performing continuous imaging while arbitrarily changing the cross section in the interactive scan, there is a problem that accurate phase correction cannot be performed even if a signal acquired by the EPI method or the divided EPI method is corrected by pre-scan data. This is due in part to the fact that the imaging position at the time of pre-scan data acquisition does not necessarily match the imaging position at the time of main scan data acquisition. This is because the shape of the target part changes at the time of acquiring this scan data.
[0006]
Accordingly, the present invention provides an MRI apparatus capable of effectively performing phase correction when performing continuous imaging while changing the imaging section, and thereby performing continuous imaging of an arbitrary section with good image quality. With the goal. It is another object of the present invention to provide an MRI apparatus that can obtain an image with a constant contrast while minimizing an increase in imaging time due to prescan and without causing destruction of a steady state due to prescan.
[0007]
[Means for Solving the Problems]
The MRI apparatus of the present invention that achieves the above object comprises a magnetic field generating means for causing nuclear magnetic resonance in a predetermined cross section of a subject, a detecting means for detecting a nuclear magnetic resonance signal generated from the subject, and the subject A cross-section setting means for setting a predetermined cross-section, a control means for controlling the magnetic field generation means and the detection means, and a nuclear magnetic resonance signal detected by the detection means to create a cross-sectional image of the subject In the MRI apparatus provided with a calculation means and a means for displaying an image, the control means sets a gradient magnetic field for selecting a predetermined section based on a signal from the section setting means, and performs phase correction for the predetermined section Control for executing acquisition of image data and subsequent acquisition of image forming data.
[0008]
According to this MRI apparatus, even in imaging in which the imaging section is changed at any time by setting by the section setting means, the phase of the data for image formation can be accurately corrected, and the influence of eddy current and static magnetic field inhomogeneity is eliminated. Good images can be obtained. Also, according to this MRI apparatus, new data can be used as correction data, so that accurate correction can be performed even on a moving subject, and an image with stable image quality can be obtained.
[0009]
In the MRI apparatus of the present invention, preferably, the control unit executes the step for acquiring the data for phase correction and the step for acquiring the data for image formation at the same repetition time. As a result, a steady precession state is maintained, and a good image with a constant contrast can be obtained.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing the configuration of an MRI apparatus to which the present invention is applied. This MRI apparatus has a magnet 102 for generating a static magnetic field in a space around a subject 101 as a magnetic field generating means, and a gradient magnetic field in this space. An RF coil 104 that generates a high-frequency magnetic field in a predetermined region of the subject, and an RF probe 105 as detection means for detecting an NMR signal (echo) generated by the subject 101 Yes. Further, a signal processing unit 107 that performs signal processing on the detected NMR signal and converts it into an image signal, and a display unit 108 that displays an image representing the form, function, and spectrum of the subject based on the image signal from the signal processing unit 107, The control unit 111, a bed 112 for the subject to lie down, and a cross-section setting device 113 for instructing a predetermined cross-section of the subject.
[0011]
The gradient magnetic field coil 103 is constituted by gradient magnetic field coils in three directions of X, Y, and Z, and each generates a gradient magnetic field in response to a signal from the gradient magnetic field power supply 109. The subject cross section to be imaged is determined by the application direction of the gradient magnetic field, and position information in the phase encoding direction and the readout direction can be given to the generated echo.
[0012]
The RF coil 104 generates a high-frequency magnetic field according to the signal from the RF transmission unit 110. The signal of the RF probe 105 is detected by the signal detection unit 106 and processed by the signal processing unit 107. The gradient magnetic field power source 109, the RF transmission unit 110, and the signal detection unit 106 are controlled by the control unit 111 in accordance with a control time chart called a pulse sequence. In the MRI apparatus according to the present embodiment, the control unit 111 executes high-speed imaging sequence by multi-shot EPI as shown in FIG. 2 to form image formation data (main scan data) for imaging a predetermined cross section of the subject. ) And correction data for phase correction of the main scan data are acquired.
[0013]
The signal processing unit 107 has a function of correcting main scan data using correction data in addition to processing necessary for normal image reconstruction. The display unit 108 displays an image reconstructed with the main scan data corrected by the correction data.
[0014]
The cross-section setting device 113 includes a three-dimensional position detection device 113a and a device 113b (called a pointer) whose position and direction can be detected by the three-dimensional position detection device 113a, and the pointer 113b detected by the three-dimensional position detection device 113a. Position information (six-dimensional data consisting of coordinates and directions) is sent to the control unit 111. As the three-dimensional position detection device, a known type such as a mechanical type, an ultrasonic type, or an infrared type can be used. For example, a combination of a light emitting diode and an infrared camera can be used as an infrared type. . As the pointer, a pointer in which a reflecting sphere that reflects light generated by the light emitting diode is fixed to at least three portions of a rod-shaped member can be used. By using such a position detection device, for example, real-time position information can be sent to the control unit 111 at 20 to 60 Hz.
[0015]
The control unit 111 determines the imaging section of the subject based on the signal from the three-dimensional position detection device 113a, and controls the RF pulse and the gradient magnetic field during imaging. The three-dimensional position detection device detects coordinates in a measurement space (real space) where the subject is laid down, and in order to determine an imaging cross section from the coordinates of the real space detected by the three-dimensional position detection device, Conversion to the coordinate system of the MRI apparatus is required. Therefore, a reference marker (not shown) is fixed in the vicinity of the static magnetic field magnet of the MRI apparatus, and coordinates can be converted by detecting the position of the reference marker in real space in advance.
[0016]
Next, an imaging method using the MRI apparatus having such a configuration will be described with reference to FIG.
[0017]
First, the subject is placed in the imaging space with the subject lying on the bed, and imaging is started. The imaging method is based on the pulse sequence by the multi-shot EPI method with pre-scan as described above. That is, as shown in FIG. 2, a gradient magnetic field pulse 202 for selecting a slice is applied at the same time as irradiation with a high frequency pulse 201 to selectively excite a slice to be imaged. Next, a pulse 203 giving a phase encoding offset and a pulse 205 giving a readout gradient magnetic field offset are applied. After that, the readout gradient magnetic field pulse 206 that is continuously inverted is applied, and the phase encoding gradient magnetic field pulse 204 is discretely applied in synchronization with the readout gradient magnetic field pulse 206. Since the echo signal 207 of each phase encoding is generated in time series within each period of the readout gradient magnetic field 206 to be inverted, it is sampled during the time range 208 to obtain time series data. Such a sequence 209 is repeated until echo signals having the number of phase encoding necessary for reconstruction of one image are obtained.
[0018]
In the prescan, the same number of echo signals are measured without applying the phase encoding gradient magnetic fields Ge203 and 204 in the sequence shown in FIG.
[0019]
The first imaging section can be determined by displaying a cross-sectional image of the subject acquired in advance on the display unit 108 and setting it with a cursor, for example, as in normal imaging. First, the above-described pre-scan is performed on the cross section determined in this way, and pre-scan data is acquired (step 301). The acquired prescan data is arranged in k-space. At this time, the signals acquired when the polarity of the readout gradient magnetic field pulse is positive are arranged, for example, from the left of the k space to the right, and the signals acquired when the polarity is negative are conversely from the right of the k space. The data array is inverted such that it is arranged on the left (step 302). Next, the prescan data is Fourier-transformed in the reading direction to obtain correction data (303).
[0020]
Next, a main scan is performed on the same imaging section, and main scan data is obtained (step 304). Similarly to the pre-scan data, the main scan data is inverted in the data array according to the polarity of the read gradient magnetic field pulse and arranged in the k space (step 305), and then subjected to Fourier transform in the read direction (step 306). The phase of the data after the Fourier transform is corrected with the previously obtained correction data (step 307), and further Fourier transformed in the phase encoding direction to obtain an image of the cross section (step 308). The image is displayed on the display (step 309).
[0021]
While imaging is continued for the same section, steps 304 to 309 are sequentially repeated using the correction data. In step 309, the display is updated to the newly acquired image.
[0022]
Next, when a new cross-section is set by the cross-section setting device, the CPU waits for the end of imaging of the previous cross-section, changes the condition of the slice selection gradient magnetic field (step 310), proceeds to step 301, and Pre-scan is performed on a simple cross section to create correction data. Thereafter, steps 304 to 309 are repeated until the cross section is changed, and phase correction is performed using correction data for the cross section, and images are sequentially updated and displayed.
[0023]
FIG. 4 is a diagram schematically showing the relationship between the pre-scan and the main scan and the processing of these data in the embodiment. In the figure, 40 indicates the timing at which the CPU sends a section change instruction based on information from the section setting device, 41 indicates a pre-scan, and 42 indicates a main scan.
[0024]
As shown in the figure, the pre-scan data obtained by the pre-scan 41 and the main scan data 42 (4210 to 4219) obtained by the main scan 42 are Fourier-transformed (43) in the reading direction, respectively, and then the main data using the correction data. Correct the phase of the scan data. In the example shown in the figure, the main scan obtains an echo signal for reconstructing one image in 10 repetitions, and performs phase correction (44) for each time of data. The phase-corrected data 45 (4510 to 4519) is further subjected to Fourier transform (46) in the phase encoding direction to obtain one image 471.
[0025]
If there is a section change instruction 40 here, pre-scan 41 and main scan 42 are performed on the set new section after the completion of imaging of one image, and one image 472 is displayed in the same manner as described above. obtain. At this time, the pre-scan 41 ensures that the time interval between the main scans performed before and after that is exactly the repeat time TR of the main scans. As a result, the Steady State Free Precession continues and the contrast of the image is maintained constant. If there is no instruction to change the cross section, the next main scan is performed on the same cross section as it is, and an image 473 is obtained.
[0026]
As described above, according to the present embodiment, every time the imaging section set by the section setting device changes, pre-scanning is performed, correction data about the section after change is created, and phase correction is performed. Accurate phase correction can be performed. Even when the pre-scan is inserted, the steady time precession state is maintained by keeping the repetition time TR constant, so that a good image with a constant contrast can be obtained. In this embodiment, since pre-scanning is performed only when the imaging cross section changes, it is possible to minimize the extension of imaging time due to pre-scanning.
[0027]
In this embodiment, the pre-scan is performed only when the cross section is changed. However, the pre-scan may be performed for each main scan as shown in FIG. In this case, the pre-scan 41 is always performed between the main scan 42 and the next main scan 42 regardless of whether or not the cross section is changed. In FIG. 5, the same data and processing as those in FIG. 4 are denoted by the same reference numerals. In the case of the embodiment shown in FIG. 5, the repetition rate is slower than in the embodiment of FIG. 4, but since it can always be corrected using the latest correction data that follows the movement of the subject, the heart, etc. It is possible to obtain an image with stable image quality in measurement of a part with a certain movement.
[0028]
In the embodiment shown in FIG. 4, the case where an image is reconstructed every time a series of measurements for obtaining one piece of image data is completed is shown. The present invention can also be applied when updating to the above.
[0029]
Such an embodiment is shown in FIGS. In this embodiment, as shown in FIG. 6, the k-space is divided into, for example, three areas 601 to 603, and three echoes arranged in these three areas are measured within one repetition time. For example, the data 6011, 6021, 6031 is measured in the execution of the main scan 72 for the first time, and then the data 6013, 6023, 6032 is measured for the third time. These data are subjected to Fourier transform (73) in the reading direction, and then phase correction (74) is performed using the correction data, and then one image 751 is reconstructed. In the fourth time, the data 6011, 6021, 6031 are measured as in the first time, and the second image 752 is reconstructed using the data 6012, 6022, 6032, data 6013, 6023, 6033 measured before that. To do. In this way, images are reconstructed sequentially while updating only a part of the k-space data, and time-series images are updated at high speed. When the cross-section is changed by the cross-section setting device, one image is reconstructed using the data for three times obtained after the cross-section change, and similarly, the image is reconstructed while partially updating the data. .
[0030]
In this case, the pre-scan 71 may be performed every time as shown in FIG. 7, or may be performed when the cross section is changed. In the former case, the phase can be corrected with the latest correction data for each image reconstruction, so that accurate correction can be performed even for a subject having movement. Since the pre-scan is inserted, the repetition rate is slowed down, but the image update speed is fast, so that it is not a problem to extend the imaging time. In this embodiment, the interval between the pre-scan 71 and the main scan 72 is always constant so as to maintain a steady precession state.
[0031]
In the present embodiment, the method and the number of divisions of the k space are not limited to those shown in FIG. 6. For example, data measured in one measurement is divided into high frequency data and low frequency data of k space. The high frequency data and the low frequency data may be measured and updated sequentially, or only the low frequency data may be updated.
[0032]
【The invention's effect】
According to the present invention, in the interactive scan, the prescan is automatically executed in accordance with the change of the slice position, so that the image at each slice position can be accurately performed. Further, since new data can always be used as the correction data, accurate correction can be performed even on a moving subject, and an image with stable image quality can be obtained.
[Brief description of the drawings]
FIG. 1 is a diagram showing an overall configuration of an MRI apparatus to which the present invention is applied. FIG. 2 is a diagram showing a pulse sequence by an EPI method executed by the MRI apparatus of the present invention. FIG. 4 is a diagram showing an embodiment of an imaging method in the MRI apparatus of the present invention. FIG. 5 is a diagram showing another embodiment of the imaging method in the MRI apparatus of the present invention. FIG. 7 is a diagram illustrating another embodiment of the imaging method using the MRI apparatus of the present invention. FIG. 7 is a diagram illustrating another embodiment of the imaging method of the MRI apparatus of the present invention.
101 ... subject, 102 ... static magnetic field generating magnet, 103 ... gradient coil 103, 104 ... RF coil, 105 ... RF probe (detection means), 107 ... signal processor 107, 108 ... display unit, 111 ... control unit, 113 ... cross-section setting device

Claims (4)

A magnetic field generating means for causing nuclear magnetic resonance in a predetermined cross section of the subject, a detecting means for detecting a nuclear magnetic resonance signal generated from the subject, a cross section setting means for setting the predetermined cross section of the subject, Control means for controlling the magnetic field generation means and detection means; calculation means for creating an image of a cross section of the subject using the nuclear magnetic resonance signal detected by the signal detection means; and means for displaying the image In the magnetic resonance imaging apparatus provided,
The control means sets a gradient magnetic field for selecting a predetermined cross-section based on a signal from the cross-section setting means, and each nucleus constituting the cross-sectional image every time the imaging cross-section set by the cross-section setting means changes. A magnetic resonance imaging apparatus that performs control to execute acquisition of phase correction data for correcting the phase of a magnetic resonance signal and subsequent acquisition of image formation data.   The cross-section setting unit includes an instruction unit that indicates a desired position of the subject in real space, and a position detection unit that detects the position of the instruction unit and sends the position information to the control unit as a signal indicating position information. The magnetic resonance imaging apparatus according to claim 1.   3. The magnetic resonance imaging apparatus according to claim 1, wherein the control unit executes the step for acquiring the data for phase correction and the step for acquiring the data for image formation at the same repetition time. 4.   The control means measures and divides a k-space in which data for reconstructing one image is divided into a plurality of areas, and acquires phase correction data each time data in each area is measured. The magnetic resonance imaging apparatus according to claim 1, wherein the steps are executed.

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