JP2805405B2 - Magnetic resonance imaging equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging apparatus (hereinafter, referred to as an MRI apparatus) for obtaining a cross-sectional image of a subject by utilizing a nuclear magnetic resonance (NMR) phenomenon.

[0002]

2. Description of the Related Art An MRI apparatus includes not only anatomical information such as an image obtained by visualizing the distribution of an X-ray absorption coefficient of a tissue in a subject obtained by an X-ray CT apparatus, but also biochemical information and chemical shift. Since information and even blood flow information can be obtained, they have been attracting attention in recent years and are rapidly spreading.

[0003] In MRI apparatuses, in order to visualize a signal based on the NMR phenomenon of nuclear spins in a subject placed in a magnetic field, it is common to use an image reconstruction technique called a two-dimensional Fourier transform method. Are coming. In the MRI apparatus using the two-dimensional Fourier transform method, an imaging part of a subject is placed in a uniform and strong static magnetic field space (a spherical space of 40 cm to 50 cm), and a specific cross section (hereinafter, slice plane) having a predetermined thickness of the subject is provided. ), Two-dimensional position information is given to the excited nuclear spins, and a signal from the nuclear spin to which the two-dimensional position information is added is extracted. The two-dimensional position information given to the nuclear spin is given as phase information and frequency information corresponding to the position of the nuclear spin in two directions orthogonal to each other on the slice plane. And M
The imaging field of view of the RI device is set to a square or rectangle of a predetermined size including a slice plane. In this case, the direction of one side of the square or rectangular imaging field is the phase information direction, and the direction of the other side is It is made to correspond to the frequency information direction.

For this purpose, in addition to the static magnetic field generating magnet, the MRI apparatus generates a gradient magnetic field in each of three orthogonal directions within a uniform and strong static magnetic field space generated by the static magnetic field generating magnet. Three sets of coils are provided. These gradient coils are used for positioning the slice plane,
For phase encoding, which assigns phase information according to the position of nuclear spins to excited nuclear spins on the slice plane, and for frequency encoding, which also assigns frequency information according to the position of nuclear spins to excited nuclear spins on the slice plane Can be used.

[0005] Now, suppose that the imaging visual field is set in the measurement area.
D _{p in the} phase encoding direction, D in the frequency encoding direction
_f, i.e., a rectangular field of view comprising D _p × D _f, M _p the number of pixels in the phase encoding direction of the image to be displayed, the frequency encoding direction to N _f, i.e., a tomographic image at M _p × N number _f pixels Shall be configured. In this case, in the phase encoding direction,
It is necessary to perform M _p times of phase encoding, and the phase of the nuclear spin at both ends of the visual field D _{p in} the phase encoding direction is γ · G _p · D _p · T _p = M _p · π (1) where , Γ: magnetic rotation ratio of the target nucleus G _p : phase encoding direction gradient magnetic field strength T _p : phase encoding direction gradient magnetic field and application time are set so as to be shifted by the following.

On the other hand, if the field of view is D _f in the frequency encoding direction, the phase rotation of nuclear spins at both ends of the field of view D _f is γ · G _f · D _f · T _f = N _f · π (2) Here, the gradient magnetic field in the frequency encoding direction and its application time are set so that G _f : gradient magnetic field intensity in the frequency encoding direction T _f : application time of the gradient magnetic field in the frequency encoding direction. As described above, the nuclear spin in the slice plane of the subject corresponds to the predetermined imaging designated field of view.
The NMR signal is read out with dimensional positional information added.

[0007]

However, when the above two-dimensional position information is applied to an MRI apparatus, as shown in FIG. 6, the visual field is smaller than the subject, that is, the subject protrudes from the visual field. When the field of view is set as described above, a signal from a portion outside the field of view is also detected, and an aliasing superimposition phenomenon of an image called aliasing artifact shown in FIG. 7 occurs. This aliasing artifact is caused by the inability to identify the position of the nuclear spin, and occurs in both the phase encoding direction and the frequency encoding direction. The aliasing artifacts that occur in the frequency encoding direction are not due to the NMR signal itself, but to the NMR signal sampling method. This can be dealt with by the sampling method.

However, aliasing artifacts occurring in the phase encoding direction are caused by the fact that the NMR signal itself, that is, the phase of the nuclear spin is the same,
It cannot be removed by a signal sampling method or an image reconstruction method. Conventionally, as a method of removing this aliasing artifact, a method of saturating and exciting nuclear spins outside the visual field designated for imaging before signal measurement has been proposed and implemented. However, in this saturation excitation method, since it is necessary to perform a pulse sequence for saturation excitation in addition to the pulse sequence for imaging, as a result, the imaging time becomes longer, and the burden on the subject increases, There is a problem that the diagnostic throughput is reduced. When aliasing artifacts occur, it has been difficult to provide images for diagnosis.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has as its object to improve the imaging time without prolonging the imaging in the phase encoding direction without causing a reduction in throughput. An object of the present invention is to provide an MRI apparatus capable of obtaining an image.

[0010]

According to the present invention, in order to achieve the above object, a subject is placed in a static magnetic field, and a high frequency magnetic field and a slice position setting, phase encoding, and frequency encoding are applied to the subject. Each gradient magnetic field is applied in accordance with a predetermined pulse sequence, an NMR signal from a first predetermined measurement visual field including the subject is detected, and an image is reconstructed by applying a Fourier transform method to the NMR signal. In a magnetic resonance imaging apparatus capable of displaying an image of the entire first predetermined measurement visual field on a display visual field of a display device, when the size of the subject protrudes from the first predetermined measurement visual field in a phase encoding direction. The step width of application of the gradient magnetic field for phase encoding is 1 / n times and the number of steps is n times as large as the time of imaging of the first predetermined measurement visual field. n the first magnitude of 2 or more integer) 2
A measurement field-of-view variable setting means capable of setting the measurement field of view; and, when the second measurement field of view is set, the number of times of addition and averaging of NMR signals having the same phase encoding is taken as an image of the first measurement field of view. Means for changing the number of additions set to 1 / n times the time; extracting an image in the display field of view of the display device from the image data obtained from the second measurement field of view and displaying the image on the display device; Image processing means for removing, from the display image, aliasing artifacts in the phase encoding direction that occur in the image when the sample protrudes from the first predetermined measurement field of view.

Further, according to the present invention, when the number of additions at the time of imaging of the first predetermined measurement visual field is N and the number of phase encoding steps is M, the measurement visual field is changed to n times the first predetermined measurement visual field. Also in this case, there is provided means for imaging while maintaining the value of the product of the number of additions and the number of phase encoding steps at N × M.

[0012]

In the MRI apparatus using the two-dimensional Fourier transform method, the field of view consists of two directions, a phase encoding direction and a frequency encoding direction. The field of view in the phase encoding direction is determined by the step width and the number of steps of the phase encoding. Therefore, the measurement field of view can be variably set by variably setting the step width and the number of steps of the phase encoding by the measurement field variable setting means. The ability to variably set the measurement field of view in the phase encoding direction means that the range in which nuclear spin phases can be identified can be widened. Can be put into the possible range. Therefore, no aliasing artifacts appear. When the measurement visual field is variably set, particularly when the measurement visual field is set to be larger than the normal predetermined imaging visual field, the image processing means displays only the image within the predetermined imaging visual field. This makes it possible to display the image of the diagnosis site in an easily viewable manner, and to eliminate the influence of a case where a signal is generated so as to cause a folding artifact even when the measurement visual field is enlarged. Further, in a method in which the normal imaging field of view and the measurement field of view correspond to 1: 1 and the number of additions is N and the number of phase encode steps is M, even if the measurement field of view is changed, the number of additions and the number of phase encode steps are different. By setting the product value of N × M to N × M, the measurement time does not increase, and the S / N and spatial resolution of the image can be maintained.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. FIG. 5 is a block diagram showing a schematic configuration of the MRI apparatus. In FIG. 5, reference numeral 10 denotes a static magnetic field generating magnet, 1
1 is a central processing unit (hereinafter referred to as a CPU), 12 is a sequencer, 13 is a transmission system, 14 is a magnetic field gradient generation system, 15
Denotes a receiving system, and 16 denotes a signal processing system.

The static magnetic field generating magnet 10 applies a strong and uniform static magnetic field in a predetermined direction, for example, in the body axis direction of the subject 1 or in a direction orthogonal to the body axis, in a predetermined area of the space in which the subject 1 can be accommodated. A permanent magnet or a magnet such as a normal or superconducting magnet is arranged so as to surround the space. The sequencer 12 operates under the control of the CPU 11 and sends various commands necessary for data collection of tomographic images of the subject 1 to the transmission system 13, the magnetic field gradient generation system 14, and the reception system 15.

The transmission system 13 includes a high-frequency oscillator 17, a modulator 18, a high-frequency amplifier 19, and a high-frequency coil 20a on the transmission side, and transmits a high-frequency pulse signal output from the high-frequency oscillator 17 according to a command from the sequencer 12. The amplitude-modulated signal is modulated by the modulator 18, the amplitude-modulated signal is amplified by the high-frequency amplifier 19, and the amplified high-frequency pulse is supplied to the high-frequency coil 20 a arranged close to the subject 1, and The object 1 is irradiated with electromagnetic waves. The magnetic field gradient generation system 14 includes orthogonal X,
Gradient magnetic field coil 21 wound in each of the three axial directions of Y and Z
And the gradient magnetic field power supply 22 for driving each gradient magnetic field coil. The static magnetic field generating magnet 10 is formed by driving the gradient magnetic field power supply 22 for each gradient magnetic field coil in accordance with an instruction from the sequencer 12. The gradient magnetic field generated by each gradient magnetic field coil is superimposed on the static magnetic field to generate a gradient magnetic field in a uniform static magnetic field region.

The details of the gradient magnetic field are required for setting the position of the slice plane of the tomographic image of the subject 1 and for giving position information to nuclear spins, which will be described in detail later. The receiving system 15 includes a receiving-side high-frequency coil 20b and an amplifier 23.
, A quadrature detector 24, and an A / D converter 25, and a signal of the behavior of nuclear spins in the subject 1 generated by the electromagnetic wave irradiated from the high-frequency coil 20a on the transmitting side (an electromagnetic wave, which is an NMR signal). Signal) is detected by the high-frequency coil 20b on the receiving side arranged close to the subject 1, and the detected signal is amplified by the amplifier 23 and then input to the quadrature phase detector 24, where the quadrature phase detector 24 Performs detection under the control of the output of the high-frequency oscillator 17 and separates the signals into two systems of a sine component and a cos component.
The D converter 25 performs sampling under the instruction of the sequencer 12 and outputs it to the signal processing system 16 as a digital signal.

The signal processing system 16 includes a CPU 11
And a recording device such as a magnetic disk device 27 or a magnetic tape device 28, and a display device 26 such as a CRT. The CPU 11 performs a Fourier transform, a correction coefficient calculation, an image reconstruction, and the like on the signal from the receiving system 15 by the CPU 11. Do the processing,
Image of nuclei density distribution in the slice plane of the subject 1, for example hydrogen nuclei (protons) density distribution image and the nuclear spin behavior (relaxation time) the image display apparatus such as a T ₁ weighted images and T ₂ weighted images showing the 26, and image data is recorded on a recording device.

Next, in the MRI apparatus configured as described above, the present invention for eliminating aliasing artifacts in the phase encoding direction is applied to a spin echo method (hereinafter, referred to as SE method) which is a typical example of a pulse sequence for MR imaging. Apply and explain. FIG. 4 schematically shows a pulse sequence of the SE method, in which the horizontal axis represents time, and the vertical axis represents magnetostatic strength or signal strength. Then, those RF of Figure 4 showing the envelope for excitation irradiation timing and nuclear spins of a radio frequency signal, G _s indicates the timing of applying a gradient magnetic field in the slice direction, G _p is the phase encoding direction gradient magnetic field indicates that performed by changing the inclination timing and each time to repeat this pulse sequence is applied to, G _f represents the timing of applying a frequency encoding gradient magnetic field, signal is measured NM
The R signal (spin echo signal) is shown. The bottom row in FIG. 4 shows the time sequence divided into sections 1 to 6. Hereinafter, the sections 1 to 6 will be described in order.

First, the subject 1 is positioned in a uniform static magnetic field region generated by the static magnetic field generating magnet 10, and the imaging site is brought to a predetermined position. Then, in section 1, while applying a slice-direction gradient magnetic field 101, a 90 ° RF pulse 102 having a frequency corresponding to the imaging site, ie, the slice plane, is irradiated. The slice direction gradient magnetic field 101 is generated by a gradient direction coil set to the slice direction by driving the slice direction gradient magnetic field power supply,
The slice-direction gradient magnetic field 101 is superimposed on the static magnetic field to form a gradient magnetic field. The 90 ° RF pulse 102 is a high-frequency pulse of a frequency corresponding to the intensity of the slice plane position of the gradient magnetic field and has a bandwidth for setting the slice thickness, and reduces the nuclear spin in the slice plane by 90 °. To excite.

Next, in section 2, one of the positional information is given to the nuclear spins in the slice plane excited by 90 °.
This is given by the phase encoding direction gradient magnetic field 103 generated from the gradient coil set to the phase encoding direction by driving the phase encoding direction gradient magnetic field power supply with the above-mentioned phase information. Further, the gradient of the gradient magnetic field 103 in the phase encoding direction is changed stepwise every time the sequence of FIG. 4 is repeated. Further, in this section 2, the gradient magnetic field 104 in the frequency encoding direction is applied. The gradient magnetic field 104 in the frequency encoding direction is a gradient magnetic field set and applied in a direction orthogonal to the phase encoding direction in the slice plane. The gradient magnetic field 104 is generated from the gradient magnetic field coil in the frequency encoding direction by driving the gradient magnetic field power supply in the frequency encoding direction. Then, a phase shift corresponding to the position is given to the nuclear spin excited by 90 ° in the slice plane so that a signal peak occurs at a certain point in signal reading.

Here, the phase encoding method which is one of the feature points of the present invention in the section 2 will be described in detail. As mentioned in the prior art, the imaging visual field of the MRI apparatus, the phase length Enkoto magnetic field gradient G _p, is directly related to the applied. That is, assuming that the number of pixels in the phase encoding direction is M, when the maximum intensity of the gradient magnetic field in the phase encoding direction is applied, the nuclear spin is −Mπ / at both ends.
The part where the phase shift between 2 and Mπ / 2 corresponds to the imaging visual field. This is a conventional method. In this method, as shown in FIGS. 6 and 7, a signal in a portion beyond the imaging field of view appears as an aliasing artifact on the opposite side of the reconstructed image.

The MRI apparatus is limited by the size of the static magnetic field space formed by the static magnetic field generating magnet.
A square visual field of about 40 to 50 cm in a phase encoding direction × frequency encoding direction is set as a normal imaging visual field. This value is such that when the phase encoding direction is a direction crossing the body axis of the subject, the body width of the adult just fits or slightly protrudes. Then, when it protrudes, a folding artifact occurs. This is because the measurement visual field of the NMR signal and the imaging visual field of the reconstructed image correspond to each other in a ratio of 1: 1.

Therefore, in the present invention, the ratio between the field of view of the NMR signal and the field of view of the reconstructed image is set to a predetermined value so that aliasing artifacts are substantially eliminated. That is, one of the features of the present invention is that the gradient magnetic field in the phase encoding direction is set so that the measurement visual field is twice, three times, or the like of the imaging visual field, and the number of phase encodings is increased accordingly. is there. Hereinafter, a case where the measurement visual field is twice the imaging visual field will be described as an example.

FIG. 1 shows the relationship between the measurement visual field and the imaging visual field. In the figure, D is the field of view for imaging, and 2D twice that is the field of view for measuring NMR signals. In order to display the imaging field of view D with the number of pixels of M, the phase shift of the nuclear spin at both ends of the 2D measurement field is shifted by at most 2Mπ (between -Mπ to Mπ or Mπ to -Mπ) ( Accordingly, the phase shift of the nuclear spins at both ends of the imaging field of D becomes Mπ at the maximum.) During that time, 2M phase encoding steps that increase or decrease by π are performed. That is, in the present invention, as shown in FIG. 3B, the phase encoding method (FIG.
(A)), the step width G _po of the inclination is set to _の , and the number of phase encoding steps is doubled. Accordingly, the gradient magnetic field applied in the section 2 in the phase encoding direction performs a gradient magnetic field in which the phase of the nuclear spin shifts by π at both ends with respect to the distance 2D in the phase encoding direction in a plurality of steps corresponding to the repetition of the sequence. The number of steps is 25, the number of pixels in the phase encoding direction with respect to the field of view.
If the number is 6, it is doubled to 512.

An image pickup time when the above-described phase encoding method is adopted will be described. In the MRI apparatus, a signal is measured and added a plurality of times with respect to the same phase encoding in order to improve S / N because the detection signal is weak. Therefore, if the number of times of addition is the same as the conventional one, the imaging time will increase. However, according to the present invention, when the number of phase encodings is doubled, the number of additions is reduced to １ ／, thereby preventing an increase in the imaging time. The reason is that if the product of the number of phase encodings and the number of additions is the same, the S / N ratio of the image is the same. Therefore, when the normal imaging field of view is used as the measurement field of view and the number of additions is N, in order to increase the measurement field of view by n times, the number of phase encodes is increased by n times and the number of additions is set to N / n, and based on the number of additions N, it is appropriately factorized and an appropriate prime number m (m <n) is set. Even if it is at least m times or more, if the product of the number of additions and the number of phase encodes is kept constant, the imaging time will not be extended, the S / N of the image will not change, and the spatial resolution will be the same. Can be maintained. Returning to the SE method again, following the section 2, neither the RF pulse nor the gradient magnetic field is applied in the section 3. Next, in section 4, while applying the slice-direction gradient magnetic field 105,
The F-pulse 106 is applied to further excite the nuclear spins on the slice plane selectively excited by 90 ° in section 1 by 180 °.
Subsequently, in the section 5, neither the RF pulse nor the gradient magnetic field is applied.

Next, in the section 6, an NMR signal (spin echo signal) is detected while applying the gradient magnetic field 107 in the frequency encoding direction. As described in the equation (2), the frequency encoding direction gradient magnetic field 107 is for adding position information corresponding to the position to the nuclear spin in a direction orthogonal to the phase encoding direction. That is, if the number of pixels is 256 between both ends of the visual field, for example, the number of pixels is 256, rotation is given to the nuclear spin so that the rotation speed of 256 pixels can be discriminated. The spin echo signal shown in the section 6 is detected by the high-frequency coil 20b on the receiving side in FIG. 5, amplified by the amplifier 23, and input to the quadrature detector 24. The quadrature phase detector 24 is controlled in synchronization with the output signal of the high-frequency oscillator 17, shapes the input high-frequency signal, and separates and outputs the two signals of a sin component and a cos component. These two signals are supplied to the A / D converter 2
Enter 5 The A / D converter 25 samples the input two-system signals in accordance with a command from the sequencer 12,
The signal is converted into two digital signals and output. These two output digital signals are input to the CPU 11 and used as real part data and imaginary part data of the Fourier transform by the CPU 11, but are temporarily stored in a memory device (not shown) in the CPU 11.

The one pulse sequence of the SE method shown in FIG. 4 has been described above.
Is executed. This repetition is performed as many times as the product of the number of additions and the number of phase encodings. Then, by performing a two-dimensional Fourier transform using the measurement data for one image, reconstructed image data of a tomographic image on a slice plane can be obtained.

Next, a method of displaying the reconstructed image data will be described. If the reconstructed image data is displayed as it is, a rectangular image having a ratio of 2: 1 in the phase encoding direction to the frequency encoding direction can be displayed according to the above embodiment, but it is inconvenient to display on a CRT. Therefore, in order to display an image having a square view as in the related art, only a necessary portion of the reconstructed image data is cut out and provided for display. The configuration for this is possible if the reconstructed image data is stored in the memory and only necessary parts are read out by address control, and it is not necessary to particularly explain in detail. .

Next, an embodiment in which the present invention is applied to an actual apparatus will be described. If an apparatus having only the configuration of the above embodiment is used, it can not be denied that the reconstruction operation takes a long time. Therefore, for a portion where the folding artifact does not occur, image display is delayed, which may cause a problem in use. Therefore, it is desirable to allow the operator to switch from the conventional method to the method of the present invention when imaging an area where a folding artifact occurs. The imaging mode switching operation unit 30 in FIG. 5 is for that purpose. The imaging method switching operation device 30 is, for example,
A conventional operation device 31 having a reconstructed image field of view in which a conventional measurement field of view in the phase encoding direction is displayed as it is, and an operation device 32 of the method of the above-described embodiment are provided. If it is stored in advance in the memory, it can be easily realized.

Next, a method for actually implementing the present invention will be described. As described above, the current commercialized MRI apparatus has a uniform magnetic field space of 40 to 50 static magnetic fields.
It is a spherical space of cm. For this reason, the gradient magnetic field coils in the three orthogonal axes also generate a gradient magnetic field having linearity substantially in the uniform static magnetic field space region. Therefore, the space in which the phase encoding can be completely performed also has the size of the substantially uniform static magnetic field space, and the phase encoding is incomplete in a region beyond the space. Therefore, in the above embodiment, the phase encoding in the section 2 in FIG. 4 has been described such that the imaging visual field is D and the measurement visual field is 2D, and the phase of the nuclear spin is shifted up to 2Mπ at both ends of the measurement visual field. In a real device, this cannot be adopted. Because the field of view of the current device is 40 to 50 cm in the phase encoding direction,
In order to be able to completely perform the phase encoding for the field of view of 80 to 100 cm, the cost may be too high, including the fact that the magnet for generating the static magnetic field must be large. . Therefore, as a method that can be practically adopted, for example, a gradient step of one half of the conventional phase encoding method in the case where the visual field is D without changing the static magnetic field generating magnet and the gradient magnetic field coil. Thus, the phase encoding may be performed by doubling the number of steps. Thus, the NMR signal is measured as if the measurement field of view was 2D. Then, the measurement signal is used for displaying only the image data of the necessary imaging field of view after image reconstruction as in the above-described embodiment, or the image is obtained by using only the signal corresponding to the required imaging field of view of the measurement signal. A method of reconstructing and displaying an image can be adopted.

Finally, the reason why aliasing artifacts in the phase encoding direction can be substantially eliminated according to the present invention will be described. As described above, the MRI apparatus has an imaging field of view of 40 to 50 cm in the phase encoding direction,
The phase encoding direction is set with respect to the body width direction of the subject. For this reason, if the body width of the subject is wider than the imaging field of view (= measurement field of view) determined by the phase encoding, a signal from a portion protruding from the field of view appears as a folding artifact. Therefore, if the measurement visual field is set to be wider than the body width of the subject, the aliasing artifact does not occur. In principle, no matter how wide the measurement field of view, if the object is larger than that, a signal that causes aliasing artifacts may be generated.However, in actual products, the sensitivity of the receiving coil to that signal is involved, In general, artifacts in the field of view are eliminated.

It should be noted that the present invention not only eliminates the effects of the aliasing artifacts, but also has an effective effect of reducing body motion artifacts in the case of imaging an abdominal region. That is, when an area more than twice the field of view to be originally measured is measured, a signal which is an artifact of non-stationary motion is diffused in the phase encoding direction, that is, the influence of the motion artifact is scattered in all the phase encoding steps. Therefore, this is Fourier-transformed and the image is reconstructed, and a half area of the measurement field of view is cut out and used as an image. Unnecessary discarded image areas also have an artifact component evenly. Artifact components are reduced.

[0033]

As described above, according to the present invention, the following effects can be obtained. (1) Since the measurement field of view can be changed according to the size of the subject, the signal of the portion which has been a folding artifact in the conventional method can also be position-identified in the phase encoding direction. Can not occur. In addition, even when the return artifact is generated even when the measurement visual field is changed, the image of the predetermined imaging visual field is displayed, so that the diagnosis can be performed using the image without the return artifact. (2) Since the value of the product of the number of phase encodings and the number of additions when the measurement visual field is changed is the same as that when the predetermined imaging visual field and the measurement visual field are the same, the measurement time may be prolonged. A good image can be obtained without deteriorating the S / N and spatial resolution of the image.
Throughput can be improved compared to conventional devices.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of setting a measurement visual field according to the present invention.

FIG. 2 is a diagram showing an image in which a measurement signal in a measurement visual field shown in FIG. 1 is image-processed and displayed.

FIG. 3 is a diagram showing a phase encoding method when a measurement visual field is changed.

FIG. 4 is a schematic diagram of a pulse sequence of a spin echo method.

FIG. 5 is a block diagram showing a schematic configuration of an MRI apparatus of the present invention.

FIG. 6 is a diagram showing a measurement field of view of a conventional device.

FIG. 7 is a view showing an image in which aliasing artifacts have occurred in the measurement visual field shown in FIG. 6;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Subject 10 Static magnetic field generating magnet 11 Central processing unit 21 Gradient magnetic field coil 30 Imaging system switching operator

Claims (2)

(57) [Claims] An object is placed in a static magnetic field, and a high-frequency magnetic field and a gradient magnetic field for slice position setting, phase encoding, and frequency encoding are applied to the object in accordance with a predetermined pulse sequence. An NMR signal from a first predetermined measurement field including the subject is detected, an image is reconstructed by applying a Fourier transform method to the NMR signal, and the entire first predetermined measurement field is displayed on a display device. the magnetic resonance imaging apparatus and the image can be displayed to the viewing, the size of the subject when protrudes from said first predetermined measurement visual field in the phase encode direction, of the first predetermined measurement visual field
When imaging, the application of the gradient magnetic field for phase encoding
The step width is 1 / n times, the number of steps is n times, and the second time is n times (n is an integer of 2 or more) in the phase encoding direction .
A measurement field variable setting means for may set measurement field, the
If the second measurement field of view is set, the same phase encoding
The number of times of addition and averaging of NMR signals having
Change the number of additions set to 1 / n times that of the image of the 1 measurement field of view
Means for extracting an image in the display field of view of the display device from the image data obtained from the second measurement field of view, displaying the image on the display device, and the subject protruding from the first predetermined measurement field of view. A magnetic resonance imaging apparatus comprising: image processing means for removing, from a display image, aliasing artifacts in a phase encoding direction that occur in the image. 2. The method according to claim 1, further comprising the step of :
When the number of operations is N and the number of phase encoding steps is M
In this case, the measurement visual field is changed to n times the first predetermined measurement visual field.
The number of additions and the number of phase encoding steps
Means for imaging while maintaining the product value of N × M at N × M
The magnetic resonance imager according to claim 1, wherein:
Device.