JP3967210B2 - Magnetic resonance imaging system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
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 that realizes reduction of artifacts due to body movement.
[0002]
[Prior art]
In an MRI apparatus, an object placed in a static magnetic field is irradiated with an RF pulse to excite atomic nuclei constituting the tissue of the object, thereby detecting the NMR signal generated from the object as an echo signal, A desired tomographic image of the subject is reconstructed by signal processing. At this time, in order to give position information to the echo signal, different phase encoding is given depending on the gradient magnetic field. Values such as 128, 256, and 512 are usually adopted as the number of phase encodings necessary for reconstructing one MR image. In MRI, a phase-encoded echo signal is two-dimensionally Fourier transformed to reconstruct one image.
[0003]
By the way, it is known that if the subject moves while acquiring echo signals for one MR image, a large artifact is generated in the image. This is called body movement artifact. The body motion artifact occurs because a predetermined phase encoding amount is to be given to a predetermined measurement point, but an image is created by performing Fourier transform in a state where the phase encoding amount is applied to another measurement point by movement. A typical example of a body motion artifact is a body motion artifact due to breathing.
[0004]
As a method of removing body motion artifacts due to respiration, there is a synchronous imaging method that matches the time phase of respiration. In this method, respiratory motion is monitored by a respiration sensor attached to a subject, and an imaging trigger is set at a specific time phase to collect data. Thereby, only the data of the specific displacement in respiratory motion is collected, and the image in which the body motion artifact is reduced is acquired. However, since this method does not acquire data until the trigger is detected, the data collection efficiency is low and the imaging time is extended.
[0005]
Another method for suppressing body motion artifacts due to breathing is to control the phase encoding order (Respiratory Ordered Phase Encoding (ROPE): A Method for Reducing Respiratory Motion Artifacts in MR Imaging, Journal of Computer Assisted Tomography 9 (4 ); 835-838 1985, DR Bailes et al). For example, as shown in FIG. 8, this method is caused by respiratory motion by monitoring the respiratory fluctuation of the abdominal wall or the chest wall with a respiratory sensor 82 attached to the subject 80 and controlling the phase encoding order in accordance with the displacement. Suppresses artifacts in the phase encoding direction. By appropriately selecting the correspondence between the respiratory time phase and the phase encoding, it is possible to reduce the body motion artifact without significantly extending the imaging time.
[0006]
On the other hand, a method for reducing body motion artifacts without using an external respiratory sensor has also been proposed (Japanese Patent Laid-Open No. 2000-296120). In this method, navigation echoes are used to monitor respiratory motion and control phase encoding. That is, as shown in FIG. 9, when the imaging section 91 is excited and the echo signal is measured, the navigation echo is acquired separately from the echo signal for image formation.
[0007]
A pulse sequence used in this method is shown in FIG. As shown in the figure, first, an RF pulse 1011 is applied at the head of the repetition time 1001, and simultaneously a slice selective gradient magnetic field 1021 is applied. Next, after applying the phase encoding gradient magnetic field 1031, a plurality of main measurement echoes 1081 are generated and acquired by a series of reversed readout gradient magnetic fields 1071. After acquiring the main measurement echo, the navigation echo gradient magnetic field 1051 is applied to generate and acquire the navigation echo 1091. The acquired navigation echo 1091 is subjected to a one-dimensional Fourier transform to create a projection image of the subject (1100). By comparing this projection image with the projection image of the standard navigation echo, the displacement of the subject is calculated (1101), referring to the correspondence table of displacement and phase encoding prepared in advance, and the phase corresponding to the current displacement The encoding amount is determined (1102). The phase encoding amount determined in step 1102 is used when applying the phase encoding gradient magnetic field 1032 within the next repetition time 1002 (1103). At this time, if the main measurement data at the encoding amount has already been acquired, the next phase encoding amount is selected at 1103. As a result, an image with reduced body motion artifacts is obtained (FIG. 9).
[0008]
[Problems to be solved by the invention]
As shown in FIG. 9, the above-described method enables the most stable correction when the surface direction of the imaging section 91 of the main measurement coincides with the direction in which the subject moves due to respiratory motion. In other words, it is possible to stably detect respiratory fluctuations in this measurement imaging plane and control the phase encoding amount, but depending on the navigation echo, if there is a large amount of respiratory movement perpendicular to the imaging plane, The movement may not be monitored, and the control may not be performed accurately.
[0009]
Therefore, an object of the present invention is to obtain MR images in which phase encoding control using navigation echo can always be realized regardless of the relationship between the imaging section and the body motion direction, and body motion artifacts are suppressed. .
[0010]
[Means for Solving the Problems]
In order to achieve the above object, in the MRI apparatus of the present invention, an area to be monitored for body motion is selected independently from the cross section to be imaged, and the object calculated using the navigation echo obtained from the area is selected. Based on the correspondence table of the displacement and the preset displacement and phase encoding, the phase encoding in the subsequent measurement sequence to be executed is controlled.
[0011]
That is, the MRI apparatus of the present invention includes a magnetic field generating unit that applies a high-frequency magnetic field and a gradient magnetic field to a subject, a detection unit that detects an NMR signal from the subject, the application of the high-frequency magnetic field and the gradient magnetic field, and an NMR In an MRI apparatus comprising control means for controlling signal detection according to a predetermined sequence, and arithmetic means for reconstructing a cross-sectional image of the subject using the detected NMR signal,
The control means includes a main measurement sequence for generating an NMR signal for obtaining a cross-sectional image of the subject , and a navigation sequence for measuring a navigation echo from a partial region of the subject. The displacement of the region is calculated, and the phase encoding in the main measurement sequence executed after the navigation sequence is controlled based on the calculated displacement and the correspondence table between the preset displacement and the phase encoding.
Preferably, the area measured in the navigation sequence is an area different from the cross section selected in the main measurement sequence.
[0012]
According to the MRI apparatus of the present invention, the phase encoding at the time of the main measurement is controlled based on the correspondence table between the displacement and the phase encoding, so that the control becomes easy. In addition, by selecting the area for detecting the navigation echo for calculating the displacement of the subject independently of the imaging cross section to be measured, a signal from the area most reflecting the body movement of the subject of interest is obtained. It is possible to appropriately control the phase encoding amount by detecting not only the displacement in the measurement imaging section but also the displacement amount of the body movement in an arbitrary direction.
[0013]
In addition, the MRI apparatus of the present invention includes first detection means for detecting an echo signal generated in this measurement sequence and second detection means for detecting a navigation echo as detection means for detecting an NMR signal. It is characterized by.
According to the MRI apparatus of the present invention having such a configuration, the navigation echo is highly sensitive even when the region to be imaged is separated from the region to be monitored for the body movement of the subject. The displacement of the subject can be accurately monitored and the body motion artifact can be reduced.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below.
FIG. 1 is a diagram showing a configuration of a typical MRI apparatus to which the present invention is applied. This MRI apparatus generates a high-frequency magnetic field in a region to be imaged of the subject 101, a magnet 102 that generates a static magnetic field in the imaging space where the subject 101 is placed, a gradient magnetic field coil 103 that generates a gradient magnetic field in this space An RF coil 104, an RF probe 105 for detecting an MR signal generated by the subject 101, and a bed 112 for laying the subject 101 into the imaging space. Further, drive systems 109 and 110 for the gradient magnetic field coil 103 and the RF coil 104, signal processing systems 106 to 108 for processing signals from the RF probe 105, and a control unit 111 are provided at positions away from the imaging space.
[0015]
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. By applying this gradient magnetic field, a region to be imaged of the subject 101 can be selected, and position information (phase encoding) can be given to an echo signal generated from the subject 101.
[0016]
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, processed by the signal processing unit 107, and converted into an image signal by calculation. The image is displayed on the display unit 108.
The control unit 111 controls the gradient magnetic field power supply 109, the RF transmission unit 110, and the signal detection unit 106 according to a control time chart called a pulse sequence, and irradiates RF pulses, applies a gradient magnetic field, and echo signals at a predetermined timing. Is to be detected. There are various imaging sequences depending on the imaging method, and these imaging sequences are stored in the memory of the control unit 111. As will be described in detail later, the MRI apparatus of the present invention includes an imaging sequence including a sequence (navigation sequence) for measuring navigation echoes for monitoring body motion of a subject.
[0017]
The control unit 111 calculates the displacement amount of the subject 101 using the navigation echo obtained in this imaging sequence, and controls the phase encoding in the main measurement sequence combined with the navigation sequence based on the displacement amount. For this control, the memory of the control unit 111 stores a correspondence table in which correspondence between the displacement amount of the subject and the phase encoding is set in advance. How to obtain the correspondence table will be described later.
[0018]
Hereinafter, an embodiment of an imaging method using the MRI apparatus having the above configuration will be described. FIG. 2 is a diagram illustrating an example of an imaging sequence executed in the present embodiment. In this embodiment, the navigation sequence 2011 is executed prior to the main measurement, and the phase encoding amount of the main measurement sequence 2121 is determined from the acquired navigation echo. A case where multi-shot EPI is adopted as the main measurement sequence 2121 will be described. However, the imaging sequence is not limited to the multi-shot EPI, and various imaging sequences can be employed.
[0019]
The navigation sequence 2011 is a spin echo type sequence in this embodiment, and excites the subject by applying a 90 ° pulse 2021 and a 180 ° pulse 2031. When applying the 90 ° pulse and the 180 ° pulse, slice selective gradient magnetic fields 2091 and 2071 are applied, respectively, to locally select the region of the subject to be monitored. Here, the slice selection gradient magnetic fields 2091 and 2071 are applied to two orthogonal axes, and the portions to be monitored are selected by making the excitation surfaces of the excitation pulses orthogonal. No phase encoding is performed. By applying the read-out gradient magnetic fields 2041 and 2051, the navigation echo 2111 is acquired from the excited site. Thereafter, gradient magnetic fields 2061, 2081, and 2101 are applied to return the spin to the state before excitation. The navigation echo 2111 acquired in this way is used to calculate the displacement of the subject.
[0020]
In this measurement sequence 2121, the slice selection gradient magnetic field 2141 is applied together with the excitation pulse 2131, the phase encoding offset 2151 is given, the inversion of the readout gradient magnetic field 2191 is repeated, and the phase encoding gradient magnetic field 2161 is applied in a blip shape. Thus, a plurality of phase-encoded echo signals 2201 are acquired from the selected region of the subject 101.
[0021]
As described above, since the navigation sequence 2011 and the main measurement sequence 2121 are executed independently from excitation by RF pulse to data acquisition, the imaging positions can be arbitrarily selected.
[0022]
The above-described navigation sequence 2011 and main measurement sequence 2121 are repeated to measure the number of echo signals 2201 necessary for one image. At this time, the gradient magnetic field offset 2151 applied in the repetition of this measurement sequence 2121 is controlled based on the displacement of the subject calculated from the immediately preceding navigation echo.
[0023]
Next, control of the phase encoding amount using navigation echo will be described.
[0024]
First, the control unit 111 performs one-dimensional Fourier transform on the acquired navigation echo 2111 to create a projection image of the subject (step 200). The displacement of the region of interest is obtained from this projection image (step 201). For example, as described in Japanese Patent Laid-Open No. 2000-296120, the displacement detection method may set a reference echo and compare it with the projection image, or the signal of the projection image itself of the navigation echo 2111. A method of detecting an edge from an intensity profile may be used. If the displacement is obtained, the phase encoding amount corresponding to the calculated displacement is determined based on the correspondence table of the displacement and phase encoding amount stored in the memory of the control unit 111 (step 202).
[0025]
The correspondence table between the displacement and the phase encoding amount can be created as follows. First, as shown in FIG. 3, a plurality of continuous navigation sequences 301 are executed, and a plurality of navigation echoes 302 are acquired in time series. From these navigation echoes 302, the displacement of the subject when they are acquired is obtained. The body motion of the subject in question is a periodic motion such as a respiratory motion, and when the obtained displacement is plotted in time series, as shown in FIG. Period 401 is obtained. A correspondence table 402 of displacement and phase encoding amount (FIG. 5B) is determined so that all phase encodings are covered during one period of the body motion. In the example shown in the figure, the vicinity of phase encoding 0 (low frequency region) is assigned to the vicinity of the maximum and minimum of the displacement with the smallest change in motion, and the high frequency region of phase encoding is assigned to the place where the motion is large. In the illustrated example, a simple linear correspondence example is shown, but the correspondence relationship is not limited to this.
[0026]
Next, after determining the phase encoding amount using such a correspondence table, the control unit 111 uses the determined phase encoding amount to perform the phase encoding amount of the main measurement sequence to be executed next, that is, the phase encoding direction. The gradient magnetic field 2151 is set (step 203). At this time, if the main measurement data with the set phase encoding amount has already been acquired, the phase encoding amount that is close to the set phase encoding amount and has not been acquired with the main measurement data is set.
[0027]
After completing the main measurement sequence 2121 with the phase encode 2151 set in this way, the next navigation sequence 2012 is executed, and the phase encode amount of the subsequent main measurement sequence is similarly determined. Thereafter, the navigation sequence and the main measurement sequence are alternately repeated until all echoes necessary for image reconstruction are acquired.
[0028]
As described above, in the present embodiment, the navigation sequence 2011 is executed independently of the main measurement sequence 2121. Therefore, the signal from the part that should be monitored most is obtained regardless of the imaging region in the main measurement. Can always suppress body movement artifacts.
[0029]
A specific example of use of this embodiment is shown in FIG. FIG. 5 shows a case where cardiac imaging is performed while monitoring the respiratory motion of the diaphragm using navigation echoes. As shown in FIG. 5A, when imaging a cross-section 506 perpendicular to the body axis direction (x direction in the figure) with respect to the heart 501, the respiratory motion of the heart is a body axis perpendicular to the imaging surface rather than within the imaging surface The direction is big. In such a case, if the position where the navigation echo is acquired and the cross section of this measurement imaging are the same, the displacement in the x direction with the largest respiratory motion cannot be monitored.
[0030]
Therefore, the navigation echo is obtained from the diaphragm 503 at the boundary between the lung 502 and the liver 504 and most reflecting the respiratory motion in the body axis direction, and the displacement in the x direction is monitored. Therefore, in the navigation sequence 2012 of FIG. 2, the surface 507 including the diaphragm 503 (FIG. 5B) is excited by the 90 ° RF pulse 2021, and the surface including the diaphragm 503 and orthogonal to the surface 507 is excited by the 180 ° RF pulse 2031. 508 is excited and a navigation echo is obtained from a region 505 that vertically traverses the diaphragm 503.
[0031]
The obtained raw data of navigation echoes are converted into absolute values after one-dimensional Fourier transform in the x direction. Since the navigation echo does not apply phase encoding, the echo converted into an absolute value is a projected image of the excitation site. From the signal intensity profile of the projection image, the displacement of the diaphragm moving in the x direction in the respiratory motion can be detected. From the displacement thus obtained, a phase encoding amount based on a preset displacement-phase encoding amount correspondence table is obtained and applied in the main measurement sequence for cardiac imaging.
[0032]
The navigation echo is not limited to the illustrated example, and may be acquired from a region inclined with respect to the diaphragm as indicated by a dotted line 51 in the figure. Further, not limited to the diaphragm, it may be acquired from other parts such as the boundary of the heart as indicated by a dotted line 52 in the figure.
[0033]
Next, as another embodiment of the present invention, an MRI apparatus including two systems of an RF probe 105 for detecting MR signals and a signal processing system will be described. The configuration of the receiving system of this MRI apparatus is shown in FIG. Other configurations are the same as those of the MRI apparatus of FIG.
[0034]
As shown in the figure, this MRI apparatus includes two RF probes, that is, an RF probe 601 for main measurement and an RF probe 604 for navigation echo, and two corresponding signal detection units and signal processing units. . This embodiment is suitable when the navigation echo acquisition position 603 and the imaging region 602 are separated from each other. For example, as shown in the figure, the respiratory echo is monitored by the movement of the diaphragm, and the diffusion image of the head is obtained. It can be performed under respiratory synchronization.
[0035]
As the imaging sequence, a sequence in which a navigation sequence similar to FIG. 2 and the main measurement sequence are combined is executed. The echo detected by the navigation echo RF probe 604 every time the navigation sequence is executed is subjected to a one-dimensional Fourier transform by the signal processing unit to which the RF probe 604 is connected, and the displacement of the monitor part is calculated. The phase encoding amount corresponding to is determined. The next main measurement is performed with this phase encoding amount, and signals detected by the main measurement RF probe 601 are collected to finally obtain one image data. This is subjected to two-dimensional Fourier transform, converted into an image signal, and displayed on the display unit.
[0036]
According to the present embodiment, navigation echoes can be acquired using an RF probe that is optimal for the part to be monitored regardless of the position of the imaging part and the cross-sectional direction, so that accurate body movement monitoring can be performed. As a result, the body motion artifact can be effectively suppressed.
[0037]
In the embodiment described above, a sequence by the multi-shot EPI method is exemplified as the main measurement sequence. However, the present invention is a phase encoding loop such as a spin echo method, a fast spin echo method, a gradient echo method, and a three-dimensional EPI method. It can be applied to various pulse sequences having
[0038]
In addition, as an example of the correspondence table between displacement and phase encoding amount, an example in which the vicinity of phase encoding 0 (low frequency region) is assigned near the maximum and minimum of displacement is shown, but the correspondence table can be changed in various ways. Selected to reduce body motion artifacts. For example, as shown in FIG. 7A, the phase encoding amount can be changed monotonously with respect to the displacement, or the minimum displacement can be assigned to zero encoding as shown in FIG. 7B.
[0039]
【The invention's effect】
Since the present invention monitors the respiratory motion of the region of interest from the navigation echo acquired in the navigation sequence independent of the main measurement sequence, even with a displacement orthogonal to the main measurement imaging section, that is, the displacement of the body motion in an arbitrary direction It is possible to control the amount of phase encoding by detecting the amount.
[Brief description of the drawings]
FIG. 1 is a diagram showing an outline of an MRI apparatus to which the present invention is applied.
FIG. 2 is a diagram showing an example of an imaging sequence executed by the MRI apparatus of the present invention.
FIG. 3 is a diagram for explaining a method for obtaining correspondence between displacement and phase encoding amount;
FIG. 4 is a schematic diagram showing a correspondence between a displacement period and a phase encoding amount.
FIG. 5 is a diagram for explaining an application example of the imaging sequence of FIG. 2;
FIG. 6 is a view showing another embodiment of the MRI apparatus of the present invention.
FIG. 7 is a schematic diagram showing a correspondence example between a displacement and a phase encoding amount.
FIG. 8 is a diagram for explaining a conventional method for suppressing body motion artifacts.
FIG. 9 is a diagram for explaining a conventional method for suppressing body motion artifacts.
10 is a diagram showing a pulse sequence used in the body motion artifact suppression method of FIG. 9. FIG.
[Explanation of symbols]
101 ... subject, 102 ... static magnetic field magnet, 103 ... gradient coil, 104 ... RF coil, 105 ... RF probe, 106 ... signal detector, 107 ... signal Processing unit 108 ... Display unit 109 ... Gradient magnetic field power supply 110 ... RF transmitter 111 ... Control unit

Claims (2)

Magnetic field generation means for applying a high-frequency magnetic field and a gradient magnetic field to the subject, detection means for detecting an NMR signal from the subject, and application of the high-frequency magnetic field and the gradient magnetic field and NMR signal detection are controlled in accordance with a predetermined sequence. In an MRI apparatus comprising control means for performing, and arithmetic means for reconstructing a cross-sectional image of the subject using the detected NMR signal,
The control means includes a main measurement sequence for generating an NMR signal for obtaining a cross-sectional image of the subject, and a navigation sequence for measuring a navigation echo from a region selected independently from the cross section selected in the main measurement sequence. And
The detection means includes first detection means for detecting an echo signal in the main measurement sequence and second detection means for detecting the navigation echo,
The control means calculates the displacement of the region from the navigation echo, and based on the calculated displacement and a correspondence table of a preset displacement and phase encoding, the phase in the main measurement sequence executed after the navigation sequence A magnetic resonance imaging apparatus characterized by controlling encoding. The magnetic resonance imaging apparatus according to claim 1,
An area to be measured in the navigation sequence is an area different from a cross section selected in the main measurement sequence.

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