JPH05123314A - Multislice image pick-up method in magnetic resonance imaging device - Google Patents

Abstract

PURPOSE:To take respective tomographic images under imaging conditions differed depending on each slice position of a subject in multislice image picked-up method in a magnetic resonance imaging device. CONSTITUTION:According to the control command from a control means in the signal processing system of a magnetic resonance imaging device, a high frequency pulse applied to collect the echo signal of the tomographic image of each slice position within the repeating time of one imaging to a subject and a three-axial inclined magnetic field pattern are added as those according to different imaging conditions corresponding to each slice position, and a plural number of tomographic images are taken in the imaging directions differed depending on each slice position by one imaging. Thus, even when the images of plural positions are taken for one subject, one imaging is sufficient.

Description

Detailed Description of the Invention

[0001]

The present invention relates to nuclear magnetic resonance (hereinafter referred to as "N
A multi-slice imaging method for obtaining a plurality of tomographic images by one imaging in a magnetic resonance imaging apparatus that obtains a tomographic image of a desired site of a subject (human body) by utilizing the phenomenon "MR". The present invention relates to a multi-slice imaging method in a magnetic resonance imaging apparatus capable of capturing a tomographic image under different imaging conditions depending on the position.

[0002]

2. Description of the Related Art A magnetic resonance imaging apparatus uses an NMR phenomenon to measure the density distribution, relaxation time distribution, etc. of nuclear spins at a desired examination site in an object, and uses the measured data to measure an arbitrary cross section of the object. Is displayed as an image. The conventional magnetic resonance imaging apparatus is shown in FIG.
As shown in, magnetic field generating means (2, 3) for applying a static magnetic field and a gradient magnetic field to the subject 1 and a high-frequency signal for causing nuclear magnetic resonance in the atomic nuclei of the atoms constituting the biological tissue of the subject 1. A transmission system 4 for irradiating a laser beam, a reception system 5 for detecting an echo signal emitted by the nuclear magnetic resonance, and a signal processing system 6 for performing an image reconstruction calculation using the echo signal detected by the reception system 5. In order to obtain a tomographic image, a sequence for measuring an echo signal emitted by nuclear magnetic resonance is repeatedly performed.

In order to determine a slice position and image a tomographic image of the subject 1 in such a magnetic resonance imaging apparatus, first, a scanogram image for determining a slice position is imaged, and then an imaging sequence, slice thickness,
Imaging conditions such as the size of the effective field of view, the number of multi-slices, the number of multi-echoes, the echo time, the reversal time, the high frequency flip angle, the signal measurement repetition time (TR), the number of additions and the number of projections were set, and then the above-mentioned imaging was performed. A slice position was set on the scanogram image and a tomographic image was taken.

An example of capturing the tomographic image will be described with reference to FIG. First, a scanogram image 21 for determining the slice position of the subject 1 is imaged, and as the first imaging, for example, an imaging condition for obtaining a tomographic image of the subject's head is set, and the imaged scanogram. Slice positions S ₁₁ and S ₁₂ are set on the head 23 of the subject image 22 of the image 21 and the image is captured. Here, S ₁₁ indicates the slice position of the first slice in the first imaging of the subject, and S ₁₂ similarly indicates the slice position of the second slice in the first imaging. Then, the imaging conditions at these slice positions S ₁₁ and S ₁₂ are all the same except the condition regarding the position of the slice. Next, after the completion of the first imaging, as the second imaging, for example, imaging conditions for obtaining a tomographic image of the jaw of the subject are set, and the jaw of the subject image 22 of the scanogram image 21 is set. The slice positions S ₂₁ , S ₂₂ , and S ₂₃ are set to 24 and an image is taken. Here, S ₂₁ represents the slice position of the first slice in the second imaging of the subject, S ₂₂ represents the slice position of the second slice in the second imaging, and S ₂₃ represents the second position of the second slice. The slice position of the 3rd slice in the imaging of the 3rd time is shown. The imaging conditions at these slice positions S ₂₁ , S ₂₂ , and S ₂₃ are all the same except for the condition regarding the position of the slice. Further, after the second imaging is completed, as the third imaging, for example, imaging conditions for obtaining a tomographic image of the subject's neck are set, and the neck 25 of the subject image 22 of the scanogram image 21 is set. Slice positions S ₃₁ and S ₃₂ are set to and the image is taken.
Here, S ₃₁ indicates the slice position of the first slice in the third imaging of the subject, and S ₃₂ similarly indicates the slice position of the second slice in the third imaging. The imaging conditions at these slice positions S ₃₁ and S ₃₂ are all the same except for the condition regarding the position of the slice.

As described above, in the conventional tomographic image pickup, the image pickup conditions suitable for the image pickup region of the subject 1 are set for each image pickup region. For example, in FIG. 4, the neck portion 2 in which the size of the imaging region is smaller than that of the head portion 23
For 5, the effective field of view when capturing the tomographic image is
The imaging condition is set to be smaller than the effective visual field when the head 23 is imaged. Then, for example, in the first imaging, only the head 23 is imaged, then in the second imaging, only the jaw 24 is imaged, and in the third imaging, only the neck 25 is imaged. Was there.

Next, an example of a sequence of multi-slice imaging for obtaining a plurality of tomographic images of the imaged site in one imaging will be described with reference to FIG. In FIG. 5, the horizontal axis represents time and the vertical axis represents amplitude, which represents the n-th projection in the sequence of m multi-slice imaging by the spin echo method. Figure 5
The high frequency pulse shown in (a), the slice direction gradient magnetic field Gs shown in (b), the phase encoding direction gradient magnetic field Gp shown in (c), and the frequency encoding direction gradient magnetic field Gf shown in (d) are shown in FIG. 5, respectively. The echo signals emitted from the subject are collected as shown in FIG. 7E, and the collected echo signals are Fourier-transformed by the signal processing system 6 shown in FIG. Obtain a tomographic image of. Here, the frequency of the high frequency pulse shown in FIG. 5A was changed according to each slice position to be imaged.
As an example, the slice position of the i-th slice is Xi, the intensity of the static magnetic field is Ho, the nuclear gyromagnetic ratio of hydrogen atoms is γ, and the frequency of the high-frequency pulse for data acquisition of the i-th slice is If fi, Becomes That is, the frequency fi of the high frequency pulse applied in each slice is changed according to each slice position Xi. However, as shown in FIGS. 5B to 5D, the patterns of the gradient magnetic fields Gs, Gp, and Gf in the three-axis directions applied for data acquisition of each slice are the same in the entire range of the nth projection. Was becoming. In addition,
There are various imaging sequences such as a spin echo method, a spin echo method with rephase for reducing artifacts due to blood flow, an inversion recovery method, and a gradient echo method. The amplitude of the phase-encoding-direction gradient magnetic field Gp shown in FIG. 5C is changed for each projection.

[0007]

However, in such a conventional multi-slice imaging method, even if tomographic images are taken at a plurality of slice positions in one imaging, the imaging conditions for a plurality of tomographic images are Since they are all the same, it is impossible to image a plurality of tomographic images under different imaging conditions in one imaging. Therefore, as shown in FIG. 4, in order to image the same subject with different imaging regions, for example, the head 2
3, different imaging conditions suitable for each imaging site such as the jaw 24, the neck 25, etc. have to be set, and imaging must be performed three times in total, once for each site. From this, when the imaging of a plurality of parts is performed on one subject, the imaging time becomes long and the restraint time for the subject becomes long.

Therefore, the present invention addresses such a problem and provides a multi-slice imaging method in a magnetic resonance imaging apparatus capable of capturing a tomographic image under different imaging conditions depending on each slice position in one imaging. The purpose is to provide.

[0009]

In order to achieve the above object, a multi-slice imaging method in a magnetic resonance imaging apparatus according to the present invention comprises a magnetic field generating means for applying a static magnetic field and a gradient magnetic field to a subject, and the above-mentioned subject A transmission system that irradiates a high-frequency signal to cause nuclear magnetic resonance in the atomic nuclei that make up the biological tissue, a reception system that detects the echo signal emitted by the above-mentioned nuclear magnetic resonance, and this reception system In a magnetic resonance imaging apparatus including a signal processing system for performing an image reconstruction calculation using an echo signal, a control command from a control means in the signal processing system causes a repeat time of one imaging of the subject to be performed. At each slice position, the pattern of the high-frequency pulse and the gradient magnetic field in the three-axis direction applied to collect the echo signal of the tomographic image at each slice position at In addition as in accordance with different imaging conditions corresponding Re respectively, it is intended to capture a plurality of tomographic images at different imaging conditions by each slice position at one imaging.

[0010]

According to the multi-slice imaging method configured as described above, the echo signal of the tomographic image at each slice position within the repetition time of one imaging with respect to the subject by the control command from the control means in the signal processing system. The patterns of the high-frequency pulse and the gradient magnetic field in the three-axis directions applied to collect the pulse are added according to different imaging conditions corresponding to the respective slice positions. This makes it possible to capture a plurality of tomographic images under one imaging condition under different imaging conditions depending on each slice position.

[0011]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. FIG. 1 is a block diagram showing the overall configuration of a magnetic resonance imaging apparatus to which the multi-slice imaging method according to the present invention is applied. This magnetic resonance imaging apparatus obtains a tomographic image of a subject by utilizing a nuclear magnetic resonance (NMR) phenomenon, and as shown in FIG. 1, a static magnetic field generating magnetic circuit 2, a gradient magnetic field generating system 3, It comprises a transmission system 4, a reception system 5, a signal processing system 6, a sequencer 7, and a central processing unit (CPU) 8.

The static magnetic field generating magnetic circuit 2 generates a uniform static magnetic field around the subject 1 in the body axis direction or in the direction orthogonal to the body axis. A magnetic field generating means of permanent magnet type, normal conducting type or superconducting type is arranged in the space provided. The gradient magnetic field generation system 3 is composed of a gradient magnetic field coil 9 wound in three axial directions of X, Y, and Z, and a gradient magnetic field power supply 10 for driving each coil, and each of them is instructed by a sequencer 7 which will be described later. By driving the gradient magnetic field power supply 10 of the coil, the gradient magnetic fields Gs, G in the three axial directions of X, Y, Z are obtained.
P and Gf are applied to the subject 1. The slice plane for the subject 1 can be set by the method of applying the gradient magnetic field.

The transmission system 4 irradiates a high-frequency signal in order to cause nuclear magnetic resonance in the atomic nuclei of the atoms constituting the biological tissue of the subject 1 by the high-frequency magnetic field pulse sent from the sequencer 7 described later. The oscillator 11, the modulator 12, the high frequency amplifier 13, and the high frequency coil 14a on the transmission side.
The high frequency pulse output from the high frequency oscillator 11 is amplitude-modulated by the modulator 12 according to the instruction of the sequencer 7, and the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 13 and then close to the subject 1. Electromagnetic waves are radiated to the subject 1 by being supplied to the arranged high frequency coil 14a.

The receiving system 5 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of atomic nuclei of a living tissue of the subject 1, and includes a high frequency coil 14b on the receiving side, an amplifier 15 and a quadrature phase detector. 16 and an A / D converter 17, the electromagnetic wave (NMR signal) of the response of the subject 1 due to the electromagnetic wave emitted from the high frequency coil 14a on the transmitting side is placed in the high frequency coil 14 close to the subject 1.
detected by b, input to the A / D converter 17 via the amplifier 15 and the quadrature detector 16, converted into a digital amount, and further sampled by the quadrature detector 16 at the timing according to the instruction from the sequencer 7. Two series of collected data are provided, and the signals thereof are sent to the signal processing system 6.

The signal processing system 6 includes a CPU 8, a recording device such as a magnetic disk 18 and a magnetic tape 19, and a CRT.
And the like, and the CPU 20 performs processing such as Fourier transform, correction coefficient calculation, and image reconstruction,
The signal intensity distribution of an arbitrary section or the distribution obtained by performing an appropriate calculation on a plurality of signals is imaged and displayed on the display 20.
It is designed to be displayed as a tomographic image.

The sequencer 7 serves as a control means for repeatedly applying a high frequency magnetic field pulse for causing nuclear magnetic resonance to the atomic nuclei of the atoms constituting the biological tissue of the subject 1 in a predetermined pulse sequence, and controls the CPU 8. And sends various commands necessary for collecting data of a tomographic image of the subject 1 to the transmission system 4, the gradient magnetic field generation system 3 and the reception system 5.

Here, the multi-slice imaging method of the present invention uses the magnetic resonance imaging apparatus having the above-mentioned configuration,
An echo signal of a tomographic image at each slice position within one image capturing repetition time is collected by a command from a sequencer 7 which operates according to a control command from a CPU 8 in a signal processing system 6 shown in FIG. The high-frequency pulse applied for this purpose and the gradient magnetic field pattern in the three-axis directions are added according to the different imaging conditions corresponding to the respective slice positions, and are added under the different imaging conditions depending on the slice positions in one imaging. A plurality of tomographic images are taken.

An example of such a multi-slice imaging method will be described with reference to FIG. First, a scanogram image 21 for determining the slice position of the subject 1 shown in FIG. 1 is captured. In FIG. 2, reference numeral 22
Shows the subject image displayed in the scanogram image 21. In this multi-slice imaging method,
For each imaged region of the subject image 22 shown in FIG. 2, each region is sequentially imaged by one image capturing. First, for example, in order to obtain a tomographic image of the subject's head, the scanogram image 21 is imaged. Slice position S on the head 23 of the sample image 22
_1. Then, for example, in order to obtain a tomographic image of the jaw of the subject, the jaw 2 of the subject image 22 of the scanogram image 21 is also set.
4, the slice position S ₂ is set, and for example, in order to obtain a tomographic image of the subject's neck, the slice position Sm is also set on the neck 25 of the subject image 22 of the scanogram image 21.
Images are taken one after another. Here, S ₁ indicates the slice position of the first slice in one imaging of the subject, the size of the effective visual field thereof is L, and the slice thickness is T. Further, S ₂ indicates the slice position of the second slice in the above-mentioned one-time imaging, its effective visual field L ′ is, for example, L / 2, and the slice thickness T ′ is, for example, T / 2. Further, Sm indicates the slice position of the m-th slice in the above-mentioned one-time imaging, its effective visual field is the same as L ', and the slice thickness is the same as T'. In addition,
For the slice positions S ₁ and S ₂ , for example, a spin echo method may be used as an imaging sequence, and for the slice position Sm, for example, a spin echo method with rephase may be used to reduce artifacts due to blood flow in the neck 25.

Next, FIG. 3 shows an example of an imaging sequence for multi-slice imaging in which a plurality of tomographic images are imaged under different imaging conditions depending on the slice positions S ₁ , S ₂ , ... It will be described with reference to FIG. In FIG.
The horizontal axis represents time, the vertical axis represents amplitude, and represents the n-th projection in the sequence of m multi-slice imaging. Then, the application of the high frequency pulse of the first slice and the second slice (see FIG. 3A) and the application pattern of the gradient magnetic fields Gs, Gp, Gf (see FIGS. 3B to 3D) are spin echo method. And the application pattern of the high frequency pulse of the mth slice and the gradient magnetic fields Gs, Gp, Gf is
This is a spin echo method with rephase.

Here, the strength Gs of the gradient magnetic field in the slice direction
Is gsmT / m when the slice thickness is 10 mm,
The strength Gs (T) of the gradient magnetic field in the slice direction when the slice thickness is Tmm is Becomes

If the intensity Gp of the gradient magnetic field in the phase encoding direction is gp mT / m when the effective visual field is 200 mm, the intensity Gp (L) of the gradient magnetic field in the phase encoding direction when the effective visual field is L mm is Becomes

Further, assuming that the intensity Gf of the gradient magnetic field in the frequency encode direction is gf mT / m when the effective visual field is 200 mm, the intensity Gf (L) of the gradient magnetic field in the frequency encode direction when the effective visual field is L mm is Becomes

The relationship between the flip angle θ of the high frequency pulse, the amplitude Arf of the high frequency pulse, and the high frequency pulse application time Trf is expressed by the following equation. θ = γ · Arf · Trf (5) where γ is the nuclear gyromagnetic ratio of hydrogen atoms.

Thus, the above equations (2), (3),
As can be seen from (4), the gradient magnetic fields Gs, Gp, and Gf in the three-axis directions can be determined in accordance with the slice thickness T of the imaging region of the subject and the value of the effective visual field L in FIG. Therefore, the slice positions S ₁ , S ₂ ,
... even if the imaging conditions such as the slice thickness, the size of the effective field of view, and the flip angle of the high-frequency pulse differ depending on Sm,
The amplitude or the application time of the gradient magnetic fields Gs, Gp, Gf can be obtained and applied using the equations (2) to (5) for each slice position, and a tomographic image can be taken. For example, in FIG. 2, the slice thickness T ′ and the effective visual field L ′ of the second slice S ₂ are 1 / th of the respective amounts T and L of the first slice S _1.
2, the gradient magnetic fields Gs, Gp, and Gf at the time of data acquisition of the second slice S ₂ are twice as large as those at the time of data acquisition of the first slice S ₁ from the above equations (2) to (4). .. The frequency of the high frequency pulse of each slice S ₁ and S ₂ at this time may be determined by the above-mentioned formula (1). In this way, within the repetition time TR shown in FIG. 3, the application pattern of the high frequency pulse and the gradient magnetic fields Gs, Gp, Gf in the three-axis directions for collecting the echo signal of the tomographic image at each slice position is set at each slice position. It is possible to form a pattern according to the imaging conditions of S ₁ , S ₂ , ... Sm.

[0025]

Since the present invention is constructed as described above,
By a control command from the control means (8, 7) in the signal processing system 6, echoes of tomographic images at the slice positions S ₁ , S ₂ , ... Sm within the repetition time TR of one imaging of the subject 1. It is assumed that the high frequency pulse applied to collect the signal and the patterns of the gradient magnetic fields Gs, Gp, Gf in the three-axis directions are according to different imaging conditions corresponding to the respective slice positions S ₁ , S ₂ , ... Sm. Add. As a result, a plurality of tomographic images can be picked up in one pick-up under different picking conditions depending on the slice positions S ₁ , S ₂ , ... Sm. Therefore, even when imaging the same subject 1 with different imaging regions, it is possible to set different imaging conditions suitable for each imaging region and perform imaging once. From this, even when imaging is performed on a plurality of regions of one subject, the imaging time can be shortened as compared with the conventional technique, and the restraint time for the subject can be shortened.

[Brief description of drawings]

FIG. 1 is a block diagram showing the overall configuration of a magnetic resonance imaging apparatus to which the present invention and a conventional multi-slice imaging method are applied.

FIG. 2 is an explanatory diagram showing a plurality of slice positions on a scanogram image in the multi-slice imaging method of the present invention.

FIG. 3 is a timing diagram showing an example of a multi-slice imaging sequence in which data is acquired by imaging once for a plurality of slice positions shown in FIG.

FIG. 4 is an explanatory diagram showing a plurality of slice positions on a scanogram image in a conventional multi-slice imaging method.

5 is a timing diagram showing an example of a multi-slice imaging sequence in which data acquisition is performed, for example, three times for a plurality of slice positions shown in FIG.

[Explanation of symbols]

1 subject 2 magnetic field generating magnetic circuit 3 gradient magnetic field generating system 4 transmitting system 5 receiving system 6 signal processing system 7 sequencer 8 CPU S ₁ slice position S ₂ slice position Sm slice position

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Claims (1)

[Claims] 1. A magnetic field generating means for applying a static magnetic field and a gradient magnetic field to a subject, and a transmission system for irradiating a high frequency signal in order to cause nuclear magnetic resonance in atomic nuclei of atoms constituting the biological tissue of the subject. A magnetic resonance imaging apparatus comprising a receiving system for detecting an echo signal emitted by the nuclear magnetic resonance, and a signal processing system for performing an image reconstruction operation using the echo signal detected by the receiving system, In response to a control command from the control means in the signal processing system, a high-frequency pulse applied to collect an echo signal of a tomographic image at each slice position and a tilt in three axial directions within a repetition time of one imaging with respect to the subject. The magnetic field pattern is added according to different imaging conditions corresponding to each slice position, and a plurality of imaging conditions are obtained depending on each slice position in one imaging. Multi-slice imaging method in a magnetic resonance imaging apparatus characterized by imaging the tomographic image.