JP4497686B2 - Magnetic resonance imaging system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging apparatus that images the inside of a subject based on the magnetic resonance phenomenon of the subject.
[0002]
[Prior art]
In magnetic resonance imaging (MRI), a nuclear spin of a subject placed in a static magnetic field is magnetically excited with a high-frequency signal of its Larmor frequency, and an MR signal generated with this excitation is reconstructed as an image. This is an imaging method.
[0003]
In this magnetic resonance imaging, an ultra high-speed imaging method represented by an echo planar method has been put into practical use. As is well known, in the case of a spin echo type, this echo planar method applies a 90 ° RF pulse for excitation, and after a half of the echo time TE has elapsed, a 180 ° RF pulse for phase inversion is applied. Apply. Then, the gradient magnetic field is switched at high speed for de-phase and in-phase around the time point of the echo time TE from the 90 ° RF pulse, thereby continuously echoing. Generate and sample. In the case of the field echo type, after applying a 90 ° RF pulse for excitation, the gradient magnetic field is switched at high speed for de-phase and in-phase, thereby continuously Echo is generated and sampled.
[0004]
As described above, in the echo planar method, the gradient magnetic field is switched at a high speed, so that the temporal variation (dB / dt) of the magnetic field strength becomes very high. In general, when high-speed switching of a gradient magnetic field is performed in a direction from the front to the back of the human body (Y-axis direction), an eddy current loop is likely to be formed in the subject. Therefore, irritation may be felt particularly in the peripheral nerve of the subject.
[0005]
It is known that the gradient magnetic field intensity becomes maximum on the central axis of the gradient coil that is not used for imaging. For example, in the case of abdominal radiography, when a gradient magnetic field is applied in the left-right direction (upward and X-axis direction) In particular, it becomes easy to feel a stimulus in the vicinity of the eyebrows where the intensity fluctuation becomes strong.
[0006]
[Problems to be solved by the invention]
An object of the present invention is to provide a magnetic resonance imaging apparatus capable of reducing stimulation of a subject due to strong magnetic field time fluctuation when performing an imaging method using high-speed switching of a gradient magnetic field in order to continuously generate echoes. There is to do.
[0007]
[Means for Solving the Problems]
The present invention provides a magnetic resonance imaging apparatus for imaging a subject by an imaging method using a high-speed switching of the gradient magnetic field in order to continuously generate an echo, the orientation of the gradient magnetic field from the reference direction, the left and right of the subject It is characterized in that the angle is inclined according to a threshold value with respect to time variation of the magnetic field strength of the gradient magnetic field with respect to the direction and a threshold value with respect to time variation of the magnetic field strength of the gradient magnetic field with respect to the front-back direction of the subject .
[0008]
The present invention also provides a magnetic resonance imaging apparatus for imaging a subject by an imaging method that uses high-speed switching of a gradient magnetic field in order to continuously generate echoes, in order to display various information on the subject. An indicator provided inside, and a control unit that controls display of the indicator, wherein the control unit causes the indicator to display an indicator for tilting the head or the entire body of the subject, After the display is completed, the remaining time of photographing is displayed on the indicator.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a magnetic resonance imaging apparatus according to the present invention will be described with reference to the drawings according to preferred embodiments.
[0010]
FIG. 1 is a configuration diagram of a magnetic resonance imaging apparatus according to the present embodiment. A substantially cylindrical static magnetic field magnet 1 installed in the gantry 20 and a gradient magnetic field coil unit 2, a shim coil 3 and an RF coil 4 disposed in the magnet 1 are provided. The gradient magnetic field coil unit 2 includes three sets (types) of X, Y, and Z coils to generate three types of gradient magnetic fields that are orthogonal to each other and whose directions in which the magnetic field strength changes are X, Y, and Z axis directions. ing. Usually, the subject is inserted into the gantry 20 so that its body axis substantially coincides with the Z axis, the left and right direction of the subject substantially coincides with the X axis, and the front and back direction of the subject substantially coincides with the Y axis. The
[0011]
The gradient coil power supply 5 is equipped with three power supply portions corresponding to the XYZ coils. Under the control of the sequence controller 8, the gradient magnetic fields in the X and Y directions are synthesized by changing the ratio of the pulse current supplied from the gradient coil power supply 5 to the X coil and the Y coil, and the phase encoding direction The gradient magnetic field Ge and the readout direction (frequency encoding direction) gradient magnetic field Gr can be arbitrarily tilted (oblique) about the Z axis.
[0012]
The transmitter 6 has a Larmor frequency for exciting magnetic resonance (MR) under the control of the sequence controller 8 in the RF coil 4 disposed in the vicinity of the subject P in the imaging space in the magnet 1. Supply RF current pulses. The receiving unit 7 receives an MR signal (high frequency signal) via the RF coil 4 and performs various signal processing such as detection to form a corresponding digital signal (MR data). The MR data is sent to the image generation processing unit 12 via the data collection unit 11. The image generation processing unit 12 applies MR data to 2DFT (two-dimensional Fourier transform processing) to generate MR image data such as a T1-weighted image, a T2-weighted image, and a density image. The MR image data is sent to the image display unit 14 after being subjected to rotation processing according to the direction and angle of the axial rotation of the gradient magnetic field supplied from the operation panel 10 by the image rotation processing unit 13 as appropriate.
[0013]
The above is the basic configuration of a typical magnetic resonance imaging apparatus. In the following, a configuration for reducing stimulation of a subject due to strong magnetic field time fluctuation when executing an imaging method using high-speed switching of a gradient magnetic field to continuously generate echoes will be described.
[0014]
As described above, it is known that, for example, a subject feels a strong stimulus particularly between the eyebrows of the head during abdominal radiography. In order to reduce the stimulation between the eyebrows, two types of countermeasures (two types of operation modes) are proposed here. Both of these two operation modes are basically installed in the apparatus, and are selectively used by the operator via the operation panel 10. However, the present invention is not limited to this, and only one of them may be equipped.
[0015]
First, as shown in FIG. 2, the first operation mode includes a phase encoding gradient magnetic field PE (initially coincides with the Y-axis direction substantially parallel to the front-back direction of the subject) and a frequency encoding gradient magnetic field RO. By rotating the axis (oblique, tilting) by an arbitrary angle around the Z axis of the slice selection gradient magnetic field (initially coincides with the X-axis direction substantially parallel to the left-right direction of the subject), the magnetic field is magnetically The magnetic stimulation felt by the subject is effectively reduced by shifting the place where the temporal fluctuation (dB / dt) of the intensity is very high from the eyebrow between the head of the subject.
[0016]
First, when the stimulus reduction mode button is pressed by the operator via the operation panel 10, a selection menu for the first operation mode and the second operation mode is displayed. When the first operation mode is selected here, a screen for specifying the oblique angle θ is displayed. Here, the oblique angle θ is SRxth is a threshold value of dB / dt at which the subject receives peripheral nerve stimulation in the X direction (left and right direction of the subject), and SRyth is the subject in the Y direction (front and back direction of the subject). As a threshold of dB / dt to receive peripheral nerve stimulation,
θ = tan ⁻¹ (SRyth / SRxth)
Is given by The oblique angle is set according to this equation. Here, a GUI (graphical user interface) is employed to simplify the setting operation.
[0017]
Basically, the threshold values SRxth and SRyth are determined according to an imaging rule and a physique (height) of the subject by an empirical rule or simulation. These threshold values SRxth and SRyth are obtained in advance for each combination of the imaging region and the physique of the subject, and the oblique angle θ corresponding to these finally required threshold values is determined for the physique of the imaging region and the subject. In association with the combination, the information is given to the operation panel 10, and a pop-up menu for selecting the imaging region and the physique of the subject and inputting numerical values is constructed in the operation panel 10. When the operator selects or numerically inputs the physique of the imaging region and the subject via the operation panel 10 on the menu, the oblique angle θ associated therewith is set. In this menu, the oblique angle is initially set to 30 °, and in most cases, it is assumed that the initial angle (30 °) may be maintained. In particular, the imaging region and the physique of the subject are considered. Is considered to be limited to the case where conditions such as the physique of the subject are not general.
[0018]
When the setting is completed as described above, the sequence data generation unit 9 updates standard sequence data according to the oblique angle θ. In the standard sequence data, the phase encoding gradient magnetic field PE coincides with the Y-axis direction substantially parallel to the front-rear direction of the subject, and the frequency encoding gradient magnetic field RO is substantially parallel to the left-right direction of the subject. The frequency encoding gradient magnetic field RO is rotated from the X axis in a predetermined direction by the oblique angle θ by the update process, that is, the rotation process using the rotation matrix using the oblique angle θ. In order to maintain an orthogonal relationship with the frequency encoding gradient magnetic field RO, the phase encoding gradient magnetic field PE is also rotated in a predetermined direction by an oblique angle θ with respect to the Y-axis direction.
[0019]
In the updated sequence data, as shown in FIG. 2, the magnetic field gradient direction of the frequency encoding gradient magnetic field RO is oblique by the oblique angle θ from the X axis, and the magnetic field gradient direction of the phase encoding gradient magnetic field PE is also the Y axis. Is oblique by the oblique angle θ.
[0020]
The sequence control unit 8 controls the transmission unit 6, the reception unit 7 and the gradient magnetic field power supply 5 according to the updated sequence data from the sequence data generation unit 9 to convert the RF pulse (high frequency magnetic field pulse) and the gradient magnetic field pulse into sequence data. A magnetic resonance signal generated by the indicated procedure and continuously generated by high-speed switching of the frequency encoding gradient magnetic field RO is sampled.
[0021]
Thus, according to the first operation mode, the phase encoding gradient magnetic field PE (initially coincides with the Y-axis direction substantially parallel to the front-back direction of the subject) and the frequency encoding gradient magnetic field RO (initially (Which coincides with the X-axis direction substantially parallel to the left-right direction of the subject) is oblique by an arbitrary angle around the Z-axis of the gradient magnetic field for slice selection, so that the magnetic field strength temporal variation (dB / dt) is magnetically The place where the height becomes very high deviates from between the eyebrows of the subject's head, and the magnetic stimulation felt by the subject is effectively reduced.
[0022]
Here, regarding the image display, initially, the image is displayed in a state where the X-axis direction is aligned with the left-right direction of the screen and the Y-axis direction is aligned with the vertical direction of the screen. However, when the oblique is applied as described above in this initial state, the subject is displayed in a posture inclined with respect to the screen. In order to solve this problem, an image rotation processing unit 13 is provided, and the image rotation processing unit 13 performs rotation processing on the image based on the information about the oblique angle θ supplied from the operation panel 10, that is, the object to be processed. This time, rotation processing is performed on the image at (−θ) so that the rotation angle θ is rewound so that the vertical and horizontal directions of the specimen substantially coincide with the vertical and horizontal directions of the screen. Thereby, the vertical and horizontal directions of the subject are substantially matched with the vertical and horizontal directions of the screen, so that the difficulty in viewing due to the tilting of the image can be eliminated.
[0023]
Next, the second operation mode will be described. In the first operation mode, the gradient magnetic field is oblique as described above. However, in the second operation mode, no gradient is applied to the gradient magnetic field, that is, the sequence data is not updated, and the RF pulse is not generated. In accordance with the standard sequence data, the phase encoding gradient magnetic field PE is made to coincide with the Y-axis direction substantially parallel to the front-rear direction of the subject, and the frequency encoding gradient magnetic field RO is set in the left-right direction of the subject. It is generated as it is with the standard matched to the substantially parallel X-axis direction. In this way, the gradient magnetic field remains standard, and the stimulus is reduced by tilting the subject's posture entirely or only at the imaging part. However, the second operation mode tilts the subject to a certain degree. The index is given to the subject.
[0024]
For example, as shown in FIG. 3, a plurality of indicators 16 are attached to an inner surface 30 of a cylinder into which the subject is inserted, and the upper half of the cylinder where the subject inserted therein can be visually recognized. . These indicators 16 are composed of a plurality of optical fibers 17 so as not to affect the magnetic field. These optical fibers 17 are arranged in a circular arc shape with the Z axis as a center (for example, 10 °) and are limited within a range of + 90 ° to −90 ° with the highest position being 0 °.
[0025]
The indicator control unit 15 receives information on the oblique angle θ from the operation panel 10 and outputs light from one optical fiber 17 attached at an angular position corresponding to the oblique angle θ. The subject may be tilted so that the light from the single optical fiber 17 is viewed in front. Note that the image rotation processing of the image rotation processing unit 13 when the posture of the imaging part is tilted is the same as in the first mode, and thus the description thereof is omitted. In the second mode, the same effect as in the first mode can be obtained.
[0026]
Here, the indicator 16 is not limited to the optical fiber 17, and a mark such as a light spot may be projected onto the inner surface 30 of the cylinder from a liquid crystal projector arranged outside the cylinder, or for each angle. A color-coded tape (for example, 0 ° is a white tape, ± 10 ° is a yellow tape, ± 20 ° is a blue tape) is affixed to the inner surface 30 of the cylinder. You may be encouraged to ask.
[0027]
The indicator 16 basically has its role completed after the subject changes his / her posture according to the indicator 16 and can be used for other purposes thereafter. For example, the present invention can be used for providing imaging remaining time information to a subject. For example, when the remaining shooting time is 5 minutes or more, the optical fiber 17 for the oblique index is continuously turned on, and when the remaining time is 5 minutes, the optical fiber 17 is alternately turned on for 5 seconds and turned off for 1 second, for example. When one minute has passed, the lighting time of the optical fiber 17 is shortened to 3 seconds, and when the photographing is completed, it is completely turned off. Further, the emission color may be changed according to the decrease in the remaining photographing time.
[0028]
Further, immediately before the indicator 16 executes a pulse sequence involving high-speed switching of a gradient magnetic field such as EPI (echo planar imaging method), the output light from the optical fiber 17 for the oblique index is changed to, for example, the emission color to red. The subject may be informed that “the pulse sequence that feels some stimulation will be executed from now on” to encourage mental preparation and have a mental attitude to the stimulation. In addition, as an application for informing the state of contrast medium injection, the output light from the optical fiber 17 for the oblique index is changed to, for example, the emission color to blue, yellow during the injection, and red after the injection is completed. You may do it.
[0029]
Of course, the information regarding the remaining time, contrast medium injection, and the like may be displayed as information on a liquid crystal projector instead of being provided by changing the blinking mode or display color.
[0030]
The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the invention at the stage of implementation. Furthermore, the above embodiment includes various stages, and various inventions can be extracted by appropriately combining a plurality of disclosed constituent elements. For example, some constituent requirements may be deleted from all the constituent requirements shown in the embodiment.
[0031]
【The invention's effect】
According to the present invention, the stimulus of the subject can be reduced by shifting the direction of the gradient magnetic field or the direction of the body of the subject to feel the stimulus strongly, for example, by shifting the eyebrows from the location where the magnetic field time fluctuation is strong.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a magnetic resonance imaging apparatus according to an embodiment of the present invention.
2 is a diagram schematically showing a gradient magnetic field rotated by a sequence data generation unit in FIG. 1. FIG.
FIG. 3 is a diagram illustrating a configuration example of an indicator in FIG. 1;
[Explanation of symbols]
1 ... Static magnetic field magnet,
2. Gradient magnetic field coil unit,
3 ... shim coil,
4 ... RF coil,
5 ... Gradient magnetic field power supply,
6 ... Transmitter,
7: Reception unit 8 ... Sequence control unit,
9 ... Sequence data generation unit 10 ... Operation panel,
11 ... Data collection unit,
12 ... Image generation processing unit,
13: Image rotation processing unit,
14 ... image display part,
15 ... Indicator control unit,
16 ... Indicator.

Claims (7)

In a magnetic resonance imaging apparatus that images a subject by an imaging method using high-speed switching of a gradient magnetic field to continuously generate echoes,
The direction of the gradient magnetic field from the reference direction, the threshold for the time variation of the magnetic field strength of the gradient magnetic field in the horizontal direction of the subject, and the time variation of the magnetic field strength of the gradient magnetic field in the front-back direction of the subject A magnetic resonance imaging apparatus characterized by tilting at an angle corresponding to a threshold value . Before Symbol magnetic resonance imaging apparatus of claim 1, wherein the subject is an indicator as an indicator of tilting the entire head or body, characterized in that provided inside the gantry.   3. The magnetic resonance imaging apparatus according to claim 1, wherein an image generated according to the imaging method is rotated and displayed with respect to a display screen according to the tilt.   The magnetic resonance imaging apparatus according to claim 2, wherein the remaining time of the imaging is displayed using the indicator. Magnetic resonance imaging apparatus according to claim 1, wherein the indicator for indicating the remaining time before Symbol photographing, characterized in that provided inside the gantry. The threshold concerning the left-right direction of the subject and the threshold concerning the front-rear direction of the subject are determined according to at least one of the imaging region and the physique of the subject. Item 2. The magnetic resonance imaging apparatus according to Item 1. A control unit for controlling display of the indicator, causing the indicator to display an indicator for tilting a head or the whole body of the subject, and causing the indicator to display the remaining time of imaging after the display of the indicator is completed. The magnetic resonance imaging apparatus according to claim 5, further comprising:

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