JP2014008173A - Magnetic resonance imaging device and separation image imaging method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a separation image in a short period of time by setting a TE for imaging a separation image of a plurality of materials selected by an operator with usage of a magnetic resonance imaging device.SOLUTION: A plurality of echo time periods are set on the basis of a difference between a frequency selected on a frequency spectrum distribution obtained from an analyte and a frequency of water. On the basis of a plurality of echo signals measured in a plurality of echo time periods, a water image, and either an image of a material corresponding to the selected frequency or a suppressed image in which the component of the material is suppressed are obtained.

Description

  The present invention relates to a nuclear magnetic resonance imaging (hereinafter abbreviated as MRI) apparatus for detecting a nuclear magnetic resonance signal from a nuclide (mainly hydrogen atom) in an object and imaging a density distribution of magnetization. The present invention relates to a technique for separating and imaging a water signal and other clinically unnecessary substances.

  An MRI apparatus excites a nuclear spin of a subject placed in a static magnetic field by irradiating a high-frequency magnetic field (hereinafter abbreviated as RF) pulse having the Larmor frequency, and a nuclear magnetic resonance signal generated thereby (Hereinafter abbreviated as NMR signal or echo signal) is received by encoding spatial information using a gradient magnetic field. Then, the received echo signal is subjected to inverse Fourier transform, thereby imaging the form and function of the subject in two dimensions or three dimensions. At that time, based on a predetermined pulse sequence, the subject is imaged by controlling the application timing of the gradient magnetic field and the reception timing of the echo signal, thereby obtaining an image with a desired contrast.

In particular, an image in which adipose tissue contained in a large amount of a subject is suppressed is clinically useful, and its imaging method is a very important technique. As a method of obtaining a fat suppression image,
[1] A method of suppressing an echo signal from fat by selectively saturating only a fat signal by frequency selective excitation,
[2] A method of suppressing fat signal by inversion recovery method,
[3] A typical example is a method of obtaining a fat-suppressed image by obtaining a separated image obtained by separating a water signal and a fat signal.

  The Dixon method can be cited as a method [3] that can separate the water signal and the fat signal. In the Dixon method, the phase of the spin of the water tissue (hereinafter abbreviated as water spin) and the spin of the adipose tissue (hereinafter abbreviated as fat spin) are the same and opposite phases by spin echo method, gradient echo method, etc. Echo signals are measured with time. Then, the water / fat separated image is acquired by performing addition / subtraction between the in-phase image and the anti-phase image reconstructed using the respective echo signals, and acquiring the water image and the fat image.

  In this technology (Patent Document 1), a method of obtaining a water / fat separated image in a short time and with a high SNR by imaging without waiting for the time when the phase of the water spin and the fat spin are in phase is proposed. ing.

Japanese Patent Laid-Open No. 11-225995

  However, there are substances that interfere with diagnosis, such as silicon as well as fat, in clinical practice, and a method that can separate them is desired.

  In the method using the frequency selective excitation pulse such as [1], it is necessary to perform imaging by performing frequency selective excitation for each substance. However, if the chemical shift is small, the difference between the peaks of the frequency spectrum becomes small. It is difficult to perform frequency selective excitation. In addition, it is difficult to handle a plurality of substances with a single imaging, and thus time is consumed.

  Inversion recovery methods such as [2] cannot suppress multiple substances in principle.

  In the Dixon method related to [3], fat suppression images are acquired mainly by performing arithmetic processing on each image acquired based on chemical shifts of water spin and fat spin. It does not obtain a suppressed image in which a substance is suppressed.

  Accordingly, an object of the present invention has been made in view of the above problems, and an MRI apparatus and a separated image that can acquire separated images in a short time for a plurality of substances (nuclides) that are desired to be separated as well as fat. It is to provide an imaging method.

  In order to achieve the above object, the present invention sets a plurality of echo times based on the difference between the frequency selected on the frequency spectrum distribution obtained from the subject and the frequency of water, and measures with a plurality of echo times. Based on the plurality of echo signals thus obtained, either an image of the substance corresponding to the selected frequency and an image of water, or a suppression image in which the substance is suppressed are obtained.

  According to the MRI apparatus and the separated image imaging method of the present invention, it is possible to obtain separated images of not only fat but also a plurality of clinically unnecessary substances by one imaging, and it is possible to obtain separated images in a short time. Become.

Schematic diagram of nuclear magnetic resonance imaging equipment The functional block diagram of the arithmetic processing part of 1st Embodiment. The flowchart showing the processing flow of the first embodiment Diagram showing chemical shift of water, fat and silicon Schematic diagram showing the arrangement of spins where fat and silicon are out of phase and in phase Sequence diagram of GE sequence using Dixon method The figure which shows the separation image pick-up method from the antiphase of two or more substances, and the same phase image The figure which shows the imaging method which does not limit the phase to the opposite phase and the same phase

  Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. In all the drawings for explaining the embodiments of the invention, those having the same function are given the same reference numerals, and their repeated explanation is omitted.

  First, an MRI apparatus according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention.

  This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject 101. As shown in FIG. 1, a static magnetic field generating magnet 102, a gradient magnetic field coil 103, a gradient magnetic field power supply 109, and an RF transmission coil 104 and RF transmitter 110, RF receiver coil 105 and signal processor 107, measurement control unit 111, overall control unit 112, display / operation unit 118, and top plate on which the subject 101 is mounted generates a static magnetic field. And a bed 106 to be taken in and out of the magnet 102.

  The static magnetic field generating magnet 102 generates a uniform static magnetic field in the direction perpendicular to the body axis of the subject 101 in the vertical magnetic field method and in the body axis direction in the horizontal magnetic field method. A permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the.

The gradient magnetic field coil 103 is a coil wound in the three-axis directions of X, Y, and Z that are the real space coordinate system (stationary coordinate system) of the MRI apparatus, and each gradient magnetic field coil is a gradient magnetic field that drives it. A current is supplied to the power source 109. Specifically, the gradient magnetic field power supply 109 of each gradient coil is driven according to a command from the measurement control unit 111 described later, and supplies a current to each gradient coil. Thereby, gradient magnetic fields Gx, Gy, and Gz are generated in the three-axis directions of X, Y, and Z.
When imaging a two-dimensional slice plane, a slice gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 101, orthogonal to the slice plane and orthogonal to each other. Phase encoding gradient magnetic field pulse (Gp) and frequency encoding (leadout) gradient magnetic field pulse (Gf) are applied in the remaining two directions, and position information in each direction is encoded in the NMR signal (echo signal). .

  In addition, the gradient magnetic field coil 103 is also provided with a shim coil that generates a compensation magnetic field that is supplied with shimming current to reduce non-uniformity of the static magnetic field. Each shim coil has a component coil that generates a compensation magnetic field of each order. Specifically, a secondary component (x ^ 2, y ^ 2, xy, yz, zx, (x ^ 2-y ^ 2) component, etc.) or a further higher-order component may be included. The 0th-order (Bo component) component is compensated by the excitation frequency f0 of the RF pulse, and the 1st-order component is also used as the gradient magnetic field coil.

  The RF transmission coil 104 is a coil that irradiates the subject 101 with an irradiation RF magnetic field pulse (hereinafter abbreviated as an RF pulse), and is connected to the RF transmission unit 110 and supplied with a high-frequency pulse current. As a result, an NMR phenomenon is induced in the spins of atoms constituting the living tissue of the subject 101. Specifically, the RF transmission unit 110 is driven in accordance with a command from the measurement control unit 111 (to be described later), amplitude-modulates and amplifies the high-frequency pulse, and then the RF transmission unit 104 is placed near the subject 101 after being amplified. By supplying, the subject 101 is irradiated with the RF pulse.

  The RF receiving coil 105 is a coil that receives an echo signal emitted by the NMR phenomenon of spin that constitutes the living tissue of the subject 101, and is connected to the signal processing unit 107 so that the received echo signal is sent to the signal processing unit 107. Sent.

  The signal processing unit 107 performs detection processing of the echo signal received by the RF receiving coil 105. Specifically, in accordance with a command from the measurement control unit 111 described later, the signal processing unit 107 amplifies the received echo signal and divides it into two orthogonal signals by quadrature detection, For example, 128, 256, 512, etc.) are sampled, and each sampling signal is A / D converted into a digital quantity. Therefore, the echo signal is obtained as time-series digital data (hereinafter referred to as echo data) composed of a predetermined number of sampling data. Then, the signal processing unit 107 performs various processes on the echo data, and sends the processed echo data to the measurement control unit 111.

The measurement control unit 111 mainly transmits various commands for collecting echo data necessary for reconstruction of the tomographic image of the subject 101 to the gradient magnetic field power source 109, the RF transmission unit 110, and the signal processing unit 107. And a control unit for controlling them. Specifically, the measurement control unit 111 operates under the control of the overall control unit 112 described later, and controls the gradient magnetic field power source 109, the RF transmission unit 110, and the signal processing unit 107 based on a predetermined sequence of control data. Then, it is necessary to reconstruct the image of the imaging region of the subject 101 by repeatedly performing the irradiation of the RF pulse and the application of the gradient magnetic field pulse to the subject 101 and the detection of the echo signal from the subject 101. Control the collection of accurate echo data. In the repetition, the application amount of the phase encoding gradient magnetic field is changed in the case of two-dimensional imaging, and the application amount of the slice encoding gradient magnetic field is further changed in the case of three-dimensional imaging. Values such as 128, 256, and 512 are normally selected as the number of phase encodings, and values such as 16, 32, and 64 are normally selected as the number of slice encodings. With these controls, echo data from the signal processing unit 107 is output to the overall control unit 112.

  The overall control unit 112 controls the measurement control unit 111 and controls various data processing and processing result display and storage, and includes an arithmetic processing unit (CPU) 114, a memory 113, and a magnetic disk. And the like, and a network IF 116 that interfaces with an external network. Further, an external storage unit 117 such as an optical disk may be connected to the overall control unit 112. Specifically, the measurement control unit 111 is controlled to execute the collection of echo data, and when the echo data is input from the measurement control unit 111, the arithmetic processing unit 114 converts the encoded information applied to the echo data. Based on this, it is stored in an area corresponding to the k space in the memory 113. Hereinafter, the statement that the echo data is arranged in the k space means that the echo data is stored in an area corresponding to the k space in the memory 113. A group of echo data stored in an area corresponding to the k space in the memory 113 is also referred to as k space data. Then, the arithmetic processing unit 114 performs processing such as signal processing and image reconstruction by Fourier transform on the k-space data, and displays the resulting image of the subject 101 on the display / operation unit 118 described later. The data is recorded in the internal storage unit 115 or the external storage unit 117, or transferred to an external device via the network IF 116.

  The display / operation unit 118 includes a display unit for displaying the reconstructed image of the subject 101, a trackball or a mouse and a keyboard for inputting various control information of the MRI apparatus and control information for processing performed by the overall control unit 112. Etc., and an operation unit. The operation unit is disposed in the vicinity of the display unit, and an operator interactively controls various processes of the MRI apparatus through the operation unit while looking at the display unit.

  At present, the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as being widely used clinically. By imaging information on the spatial distribution of proton density and the spatial distribution of relaxation time in the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.

  The MRI apparatus and the separated image imaging method of the present invention display the frequency spectrum distribution obtained from the subject, accept the selection of a desired frequency on the displayed frequency spectrum distribution, and select the selected frequency and the frequency of water. Based on the difference, set multiple echo times for a given pulse sequence, based on multiple echo signals measured at multiple echo times, an image of water and an image of a substance corresponding to the selected frequency Or a suppression image in which the component of the substance is suppressed.

  Preferably, the plurality of echo times are times when the spins of the substance corresponding to the selected frequency and the spins of water are in phase with each other.

  Preferably, an in-phase image is reconstructed from an echo signal measured at an echo time having the same phase, and an anti-phase image is reconstructed from an echo signal measured at an echo time having an opposite phase. Based on the reverse phase image, either an image of water, an image of a substance corresponding to the selected frequency, or a suppressed image in which the components of the substance are suppressed are obtained.

  Preferably, water images obtained when images of the respective substances are obtained are added to generate a water image having a higher SNR than the original water image.

  Hereinafter, each embodiment of the present invention will be described in detail. In the following description, regarding the removal of the phase distortion based on the static magnetic field inhomogeneity, a description of the removal of the phase distortion based on the static magnetic field inhomogeneity is omitted assuming that a known technique is used.

(First embodiment)
A first embodiment of the MRI apparatus and separated image imaging method of the present invention will be described. In the first embodiment, when a plurality of substances to be separated (hereinafter abbreviated as separation substances) are selected on the frequency spectrum distribution, an echo time in which water and the separation substance are in phase for each separation substance. And the echo time that is in reverse phase are calculated, the in-phase image and the anti-phase image are reconstructed based on the echo signals measured at the calculated echo time, and the in-phase image and the anti-phase image are calculated. The water image and the separated image from which the separated substance is separated or the inhibited image from which the separated substance is suppressed are acquired.
Hereinafter, the first embodiment will be described in detail based on FIGS.

(Echo time (TE) setting)
First, a method for setting the echo time (TE) having the same phase and the opposite phase according to the first embodiment will be described. The time T _i [s] at which the phase is the same as the equation (1) using the chemical shift δ [ppm] of water and the separated substance, the static magnetic field strength B ₀ [T], and the gyromagnetic ratio γ [MHz / T] It is expressed in

同位相、逆位相になる周期の回数をNとして一般化すると次のように表される。ただし、NはN≧1の整数である。

When the number of periods having the same phase and the opposite phase is generalized as N, it is expressed as follows. However, N is an integer of N ≧ 1.

The time T _{i at} which the phase is the same is expressed as in Equation (2).

となる。分離物質が変わると水に対するケミカルシフトが変わるため、分離物質毎に式(2)、式(3)のδが変わることになる。静磁場強度が同じ装置では、水に対するケミカルシフトが大きいと同位相と逆位相を繰り返す時間が短くなり、逆にケミカルシフトが小さいと長くなる。 The time T _{0 at which the} phase is reversed is expressed as in Expression (3).

It becomes. Since the chemical shift with respect to water changes when the separation material changes, δ in Equations (2) and (3) changes for each separation material. In an apparatus having the same static magnetic field strength, when the chemical shift with respect to water is large, the time for repeating the same phase and the opposite phase is shortened, and conversely, when the chemical shift is small, the time is long.

(Generation of separated or suppressed images)
Next, a method for generating a separated image or a suppression image according to the first embodiment will be described.
Assuming that the image is I1 and the image of the separation substance 2 is I2, the in-phase image (Iip) and the anti-phase image (Iop) for water and the separation substance 1 can be expressed as follows.

Iip ＝ Iw + I1 ＋ a * I2 (4)
Iop = Iw-I1 + b * I2
Here, a is a constant representing the phase between the water spin and the spin of the separation material 2 at the timing when the water and the separation material 1 are in phase, and the water and the separation material 1 are in phase. It is a constant value determined based on the echo time and the chemical shift of the separated substance 2. Similarly, b is a constant representing the phase between the water spin and the spin of the separation material 2 at the timing when the water and the separation material 1 are in opposite phases, and the water and the separation material 1 are in opposite phases. It is a constant value determined based on the echo time and the chemical shift of the separated substance 2.

From the above formula (4), the water image Iw and the image I1 of the separation substance 1 are
Iw = (Iip + Iop) / 2-[(a + b) / 2] * I2 (5)
I1 = (Iip-Iop) / 2-[(ab) / 2] * I2
It is expressed. From Equation (5), the water image (Iw) is an image in which the separated substance 1 is removed (separated) or suppressed, and | (a + b) / 2 | <1, so that the separated substance 2 is suppressed. . Similarly, the image (I1) of the separated substance 1 is an image in which the separated substance 2 is suppressed because water is removed (separated) or suppressed and | (ab) / 2 | <1.

  From the above, the water image and the separation image of the separation material 2 can be similarly obtained from the in-phase image and the anti-phase image of the separation material 2, and the separation image 2 is removed from the water image ( Separation or suppression is an image in which the separation substance 1 is suppressed, and a separation image of the separation substance 2 is an image in which water is removed (separation) or suppression and the separation substance 1 is suppressed.

(Description of each function)
First, each function of the arithmetic processing unit 114 for realizing the separated image imaging method of the first embodiment will be described with reference to the functional block diagram of FIG. The arithmetic processing unit 114 of the first embodiment includes a frequency spectrum distribution measurement unit 201, a frequency spectrum distribution calculation unit 202, a chemical shift calculation unit 203, an echo time setting unit 204, a pulse sequence setting unit 205, and a separation An image generation unit 206 and a high SNR water image generation unit 207 are provided.

  The frequency spectrum distribution measurement unit 201 generates control data for a pre-scan sequence in order to detect the type of substance included in the imaging region of the subject, and notifies the measurement control unit 111 of the generated control data for measurement. Instructs the control unit 111 to execute a pre-scan sequence. The pre-scan sequence may be any pulse sequence that can excite the imaging region of the subject and acquire the FID signal or echo signal from the imaging region. Then, the measurement control unit 111 notifies the frequency spectrum distribution calculation unit 202 of the measured FID signal or echo signal data.

  The frequency spectrum distribution calculation unit 202 obtains the frequency spectrum distribution by performing inverse Fourier transform on the data of the FID signal or the echo signal, and displays the calculated frequency spectrum distribution on the display unit. The frequency spectrum distribution is displayed so that the spectrum of the other separated substance is relatively shown with respect to the spectrum of water.

The chemical shift calculation unit 203 specifies a frequency corresponding to a position selected by the operator via the operation unit on the frequency spectrum distribution displayed on the display unit. Since the frequency difference Δf between the identified frequency and the frequency of water is a frequency difference (= γB ₀ δ) based on the chemical shift δ [ppm], the chemical shift δ = Δf / γB ₀ can be obtained. In addition, when the separation substance corresponding to each chemical shift is stored in advance, the separation substance corresponding to the calculated frequency difference is specified, and the name and detailed chemical shift information of the specified separation substance are obtained and displayed. It is also possible to display on the part.

Based on the frequency difference Δf (= γB ₀ δ) calculated by the chemical shift calculation unit 203, the echo time setting unit 204 corresponds to the frequency selected using the above-described equations (1) to (3). The echo time when the separated substance is in phase, opposite phase, or arbitrary phase is calculated.

  The pulse sequence setting unit 205 sets the pulse sequence so that each echo signal is measured at each calculated echo time, generates control data for the set pulse sequence, and notifies the measurement control unit 111 of the generated control data Then, the pulse sequence set in the measurement control unit 111 is executed. The measurement control unit 111 notifies the separated image generation unit 206 of echo signal data measured at each set echo time.

  The separated image generation unit 206 reconstructs an image at each echo time by performing inverse Fourier transform on k-space data filled with echo signal data measured at each set echo time. Then, the above-described arithmetic processing is performed between the reconstructed images at the respective echo times, and a separated image or a suppressed image is acquired.

  The high SNR water image generation unit 207 obtains a water image having a higher SNR than the original water image by adding the water images obtained when obtaining the separated images of the respective separated substances.

(Description of processing flow)
Next, the processing flow of the first embodiment performed in cooperation with each of the functional units will be described based on the flowchart shown in FIG. This processing flow is stored in advance in the internal storage device 115 as a program, and is executed by the arithmetic processing unit 114 reading the program from the internal storage device 115 and executing it. Hereinafter, the processing contents of each processing step will be described in detail.

  In step 301, a frequency spectrum distribution is obtained by pre-scanning. Specifically, the frequency spectrum distribution measurement unit 201 generates prescan control data, notifies the measurement control unit 111 of the generated control data, and instructs the measurement control unit 111 to perform prescan. Then, the measurement control unit 111 notifies the frequency spectrum distribution calculation unit 202 of the measured FID signal or echo signal data.

  In step 302, the frequency spectrum distribution is displayed on the display unit. Preferably, this frequency spectral distribution is displayed to represent the chemical shift of the separated material relative to the frequency of water. Specifically, the frequency spectrum distribution calculation unit 202 acquires the frequency spectrum distribution by performing inverse Fourier transform on the data of the FID signal or the echo signal acquired in step 301, and displays the calculated frequency spectrum distribution on the display unit. .

In step 303, the operator selects and instructs a spectrum corresponding to the separated substance on the frequency spectrum distribution displayed on the display unit in step 302 via the operation unit. A plurality of separation substances may be selected. Then, the chemical shift calculation unit 203 identifies the frequency corresponding to the position instructed to be selected. The difference Δf between the identified frequency and the water frequency corresponds to the frequency difference (γB ₀ δ) based on the chemical shift Δ [ppm].

In step 304, the rotation period for each spectrum selected based on the frequency of water is calculated. Specifically, the echo time setting unit 204 calculates the relative rotation period (T) for each selected separation substance based on the frequency difference Δf (γB ₀ δ) calculated by the chemical shift calculation unit 203. To do. The relative rotation period (T) is
T = 1 / (frequency difference obtained in step 303 (Δf = γB ₀ δ))
Can be obtained as

In step 305, the echo time setting unit 204 uses the equations (1) and (2) based on the relative rotation period (T) for each separated substance calculated in step 304, with water as a reference. The time (T _i ) at which each separated substance has the same phase and the time (T _o ) at which it has the opposite phase are calculated. Specifically, assuming that N is an integer of N ≧ 1, the time of in-phase (T _i ) is T _i = N · T, and the time of anti-phase (T _o ) is T _o = (2N−1 ) ・ T / 2 can be calculated.

  In step 306, the following series of processing (306-1 to 306-3) is repeated for each separated substance.

  In step 306-1, the pulse sequence setting unit 205 sets and sets each echo time (TE) of the pulse sequence so that each echo signal is measured at the same phase as the opposite phase determined in step 305. The control data of the pulse sequence is generated, the generated control data is notified to the measurement control unit 111, and the pulse sequence set in the measurement control unit 111 is executed.

  In Step 306-2, the measurement control unit 111 measures in-phase and anti-phase echo signals at each echo time (TE), and notifies the separated image generation unit 206 of the data of the measured echo signals.

  In step 306-3, the separated image generation unit 206 generates k-space data by filling the k-space corresponding to each separated substance and each phase with echo data.

  In step 307, the separated image generation unit 206 repeats the following series of processing (307-1 and 307-2) for each separated substance.

  In step 307-1, an image at each echo time is reconstructed using the k-space data filled with the data of the echo signal measured in step 306.

  In step 307-2, a predetermined calculation process is performed between the reconstructed images at the respective echo times to obtain a separated image or a suppressed image. As a result of this calculation process, a water image is also obtained.

  In step 308, the high SNR water image generation unit 207 adds the water images obtained in step 307 to generate a water image having a higher SNR than the original water image.

In step 309, the separated image or the suppression image acquired in step 307 and / or the high SNR water image acquired in step 308 is displayed on the display unit.
The above is the description of the processing flow of the first embodiment.

  As described above, the MRI apparatus and the separated image imaging method of the first embodiment calculate the in-phase and anti-phase times in this way, and the in-phase and anti-phase echo signals (TE ) Is set. Thereby, each k-space data for obtaining separated images of a plurality of separated substances can be acquired by one imaging. Then, using the image for each echo time (TE) obtained by reconstructing each k-space data, the aforementioned addition / subtraction arithmetic processing is performed. As a result, it is possible to acquire separated images or suppression images for a plurality of separated substances by one imaging, and it is possible to acquire separated images in a short time. Moreover, the water image obtained when obtaining the separation image of each separation substance is added. Thereby, a high SNR water image can be generated.

(Specific examples)
Next, as a specific example of the first embodiment, a gradient echo sequence (GE method) is used, and silicon and fat are separated as separation materials other than water, and imaging is performed using a static magnetic field strength of 1.5T. Explain the case.

  The chemical shift from water of these two separated materials is 4.8 ppm for silicon and 3.5 ppm for fat. In addition, the number of echo signals acquired in one repetition time (TR) of the pulse sequence is a two-echo signal having the opposite phase and the same phase as water in the first period.

  First, the frequency spectrum distribution (step 301) acquired by the pre-scan is displayed on the display unit (step 302). FIG. 4 shows an example of the frequency spectrum distribution. The horizontal axis represents frequency, and the vertical axis represents spectral intensity. The operator selects the peaks of two separated substances, silicon and fat, on the displayed frequency spectrum distribution via the operation unit (step 303). Then, using equations (3) (time for anti-phase) and equations (2) (time for in-phase), the reverse phase of each separated substance with respect to water and the time for in-phase are obtained (step 304, step 305). ). In this embodiment, N = 1 because the same phase as the opposite phase of the first period is used. As a result, the time for the opposite phases of silicon and fat is 1.6 ms and 2.2 ms, respectively, and the times for the same phase are 3.3 ms and 4.5 ms, respectively. A schematic diagram of this state is shown in FIG.

  In FIG. 5, water spin (transverse magnetization) is represented by a dotted arrow, silicon spin (transverse magnetization) is represented by a solid arrow, and fat spin (transverse magnetization) is represented by a solid round arrow. Figure (a) shows that the spins of water and silicon are out of phase at TE = 1.6ms, and (b) shows that the spins of water and fat are out of phase at TE = 2.2ms. Show. Figure (c) shows that water and silicon spins have the same phase at TE = 3.3 ms, and figure (d) shows that water and fat spins have the same phase at TE = 4.5 ms. Show.

  Then, for each separated substance, a gradient echo sequence is set with these echo phases having the opposite phase and the same phase as TE1 and TE2, respectively. An example of the gradient echo sequence set in this way is shown in FIG. In FIG. 6, RF represents an RF pulse, GS represents a slice selective gradient magnetic field, GP represents a phase encode gradient magnetic field, GR represents a readout (frequency encode) gradient magnetic field, and Echo represents an echo signal. The echo signal is measured at each echo time by inverting the polarity of the readout gradient magnetic field at the echo times TE1 and TE2. In other words, all echo time (TE) data is acquired by one imaging. Specifically, the measurement of the TE1 echo signal is performed using a positive readout gradient magnetic field, and the measurement of the TE2 echo signal is performed using a negative readout gradient magnetic field.

  Note that the polarity of the read gradient magnetic field may be reversed in order of negative polarity first and then positive polarity. Then, the echo signal data measured at TE1 is filled into the k space corresponding to TE1, and the echo signal data measured at TE2 is filled into the k space corresponding to TE2 (step 306). Each k-space data is converted into an image by inverse Fourier transform, and a separation image or a suppression image for silicon and fat is obtained by the above-described addition / subtraction processing between the image data (step 307). Furthermore, a water image having a higher SNR than the original water image can be acquired by adding the water images obtained for each separated substance (step 308). FIG. 7 shows an imaging process flow of these steps 307 and 308.

(Second embodiment)
Next, a second embodiment of the MRI apparatus and separated image acquisition method of the present invention will be described. In the second embodiment, the phase at the time of measurement of the echo signal is not limited to the opposite phase and the same phase, but is measured with an arbitrary phase. However, each echo time (TE) is set so that the phase of the separated substance differs by π between adjacent echo times (TE) (that is, the phase advances by π). That is, an echo time in which water and the separated substance are in an arbitrary phase and an echo time in which the phase is advanced by π from the arbitrary phase are set. Specifically, the relative phase of the spin of the separation material with respect to the water spin at the first echo time (TE1) is different from the relative phase of the spin of the separation material with respect to the water spin at the second echo time (TE2) by π. Set each echo time as follows. Hereinafter, the second embodiment will be described in detail with reference to FIG.

(Relationship between arbitrary phase and echo time)
First, the relationship between the arbitrary phase and the echo time will be described.

となる。したがって、N周期目での位相θとなる時間T_nは、式(6)と式(2)を用いて、

と表すことができる。ここで、Nは、N≧0の整数である。さらに、πだけ位相が進む時間T_n＋πは

となる。 In order to acquire a separated image using an echo signal measured at an echo time of an arbitrary phase, not limited to in-phase and anti-phase, generalize the rotation period and acquire two sets of echo signals with different phases by π It shall be. The time T _m [s] at which the arbitrary phase θ is reached is the chemical shift δ [ppm], static magnetic field strength B ₀ [T], magnetic rotation ratio γ [MHz / T], and phase θ [rad] between water and the separated material. The echo signal of the arbitrary phase in the first cycle is

It becomes. Therefore, the time T _n that becomes the phase θ in the Nth period is _expressed by using the equations (6) and (2).

It can be expressed as. Here, N is an integer of N ≧ 0. Furthermore, the time T _{n + π} that the phase advances by _π is

It becomes.

  The arbitrary phase θ may be set to a predetermined value based on the chemical shift of water and the separated substance, the static magnetic field strength, and the imaging conditions, or may be set in response to an operator input.

(Generation of separated or suppressed images)
Next, a method for generating a separated image or a suppression image according to the second embodiment will be described. In the second embodiment, the phase at the first echo time (TE1) is θ, and the phase at the subsequent mth echo time (TEm) is θ + (m−1) π (m is a natural number of 2 or more). Therefore, the same phase exp (iθ) is applied to the image data measured at each echo time. Therefore, in the above equation (4), this phase exp (iθ) can be canceled and ignored. Therefore, also in the second embodiment, as shown in FIG. 8, a separated image or a suppressed image can be generated by the same calculation as in the first embodiment described above. Therefore, a detailed description of the method for generating a separated image or a suppression image according to the second embodiment is omitted.

(Function block)
Next, each function of the arithmetic processing unit 114 of the second embodiment will be described. The arithmetic processing unit of the second embodiment has the same functional units as those of the first embodiment described above, but the processing content of the echo time setting unit 204 is different. Hereinafter, only portions different from the first embodiment will be described, and description of the same portions will be omitted.

Based on the frequency difference Δf (= γB ₀ δ) calculated by the chemical shift calculation unit 203 and the arbitrary phase θ, the echo time setting unit 204 is selected using the above-described equations (6) to (8). Each echo time is calculated such that the spin phase of the separation material corresponding to the determined frequency becomes θ + (m−1) π (m is a natural number of 1 or more) at the m-th echo time (TEm).

(Processing flow)
Next, the processing flow of the second embodiment performed in cooperation with the above-described functions will be described. The processing flow of the second embodiment is the same as the processing flow of the first embodiment described above, but the processing in steps 305 and 306 is different. Hereinafter, only steps having different processing contents from those of the first embodiment will be described, and description of steps having the same processing contents will be omitted. However, in order to clarify that it is a processing step of the second embodiment, “ ^CASE2 ” is ^added to the step number.

In step 305 ^CASE2 , the echo time setting unit 204 uses the equations (7) and (8) as a reference for water based on the relative rotation period (T) for each separated substance calculated in step 304. As follows, the time (T _m ) when the phase of each separation substance becomes θ and the time (T _π ) when it becomes θ + π are calculated.

In step 306 ^CASE2, a series of processes are repeated (306-1 ^CASE2 ~306-3 ^CASE2) for each separation material. Incidentally, the step 306-2 ^CASE2 the step 306-2, the step 306-3 ^CASE2 since the same processing contents as the steps 306-3 not described, illustrating only the processing content of step 306-1 ^CASE2.

In Step 306-1 ^CASE2 , the pulse sequence setting unit 205 sets each echo time (TE) of the pulse sequence so as to measure the echo signal at the time when the phase of the separation substance determined in Step 305 ^CASE2 is θ and θ + π, respectively. Set, generate control data of the set pulse sequence, notify the generated control data to the measurement control unit 111, and cause the measurement control unit 111 to execute the set pulse sequence.
The above is the description of the processing flow of the second embodiment.

  As described above, in the MRI apparatus and the separated image imaging method of the second embodiment, the phase when measuring the echo signal is an arbitrary phase. However, each echo time (TE) is set so that the spin phase of the separated substance is different by π between adjacent echo times (TE). As a result, it is not necessary to wait for the first echo time until the spin phase of the separation substance is reversed, and the measurement of a plurality of echo signals can be shortened. Therefore, the imaging time can be shortened (shortened) as compared with the above-described large embodiment, and the SNR of the image can be improved by suppressing the attenuation of the echo signal.

(Specific examples)
Next, a specific example of the invention of the second embodiment will be described. Note that the processing from step 301 ^CASE2 to step 303 ^CASE2 is the same as that in the first embodiment described above, and a description thereof will be omitted.

In order to measure the echo signal in as short a time as possible after excitation, the echo signal in the first period is used. That is, N = 1. For example, the echo signal is measured at an echo time when the phase is π / 4 [rad], and then the echo signal is measured again at an echo time when the phase advances by π [rad]. (However, the second embodiment is not limited to these phases, and any phase is possible.) Therefore, the first time when the phase is π / 4 [rad] is T _{π / 4} = 1 The time when the phase has advanced by π [rad] after that becomes / 8γB ₀ δ is T _{sπ / 4} = 5 / 8γB ₀ δ.

The time _{T π / 4, T sπ /} 4 is set as the echo time (TE) (step 304 ^CASE2, 305 ^CASE2), performing imaging using thus set pulse sequence (step 306 ^CASE2). As the pulse sequence, a gradient echo sequence (GE method) can be used as in the first embodiment.

Then, after filling the measured echo signal into k-space (step 306 ^CASE2 ), imaging the k-space data by inverse Fourier transform, and performing the calculation process of equation (5), separation for each separated substance An image or a suppression image is acquired (step 307 ^CASE2 ). By performing this for each of the selected separation substances, it is possible to acquire separation images of the plurality of substances with one imaging, and it is possible to acquire in a short time without waiting for the phase to be in reverse phase and the same phase. Become. Furthermore, a water image having a higher SNR than the original water image can be acquired by adding the water images obtained for each separated substance (step 308 ^CASE2 ). FIG. 8 shows an imaging process flow of these 307 ^CASE2 and 308 ^CASE2 .

  101 subject, 102 static magnetic field generating magnet, 103 gradient magnetic field coil, 104 transmitting RF coil, 105 RF receiving coil, 106 bed, 107 signal processing unit, 108 overall control unit, 109 gradient magnetic field power source, 110 RF transmitting unit, 111 measurement Control unit 111, 118 Display / operation unit, 114 Arithmetic processing unit (CPU), 115 Internal storage unit, 116 Network IF, 117 External storage unit

Claims (6)

A display for displaying the frequency spectrum distribution obtained from the subject;
An input unit for receiving selection of a desired frequency on the displayed frequency spectrum distribution;
An echo time setting unit that sets a plurality of echo times of a predetermined pulse sequence based on the difference between the selected frequency and the frequency of water;
Separation for obtaining either an image of water and an image of a substance corresponding to the selected frequency or a suppressed image in which the substance is suppressed based on the plurality of echo signals measured at the plurality of echo times An image creation unit;
A magnetic resonance imaging apparatus comprising: The magnetic resonance imaging apparatus according to claim 1.
The echo time setting unit sets an echo time in which the water and the substance are in the same phase and an echo time in which the water and the substance are in opposite phases. The magnetic resonance imaging apparatus according to claim 1.
The echo time setting unit sets an echo time in which the water and the substance are in an arbitrary phase and an echo time in which the phase is advanced by π from the arbitrary phase. . The magnetic resonance imaging apparatus according to claim 1.
A magnetic resonance imaging comprising: a high SNR water image generation unit that obtains an image of water with improved SNR by adding the water images obtained for each frequency selected on the frequency spectrum distribution apparatus. A display step for displaying a frequency spectrum distribution obtained from the subject;
An input step for accepting selection of a desired frequency on the displayed frequency spectrum distribution;
An echo time setting step for setting a plurality of echo times of a predetermined pulse sequence based on a difference between the selected frequency and the frequency of water;
Separation for obtaining either an image of water and an image of a substance corresponding to the selected frequency or a suppressed image in which the substance is suppressed based on the plurality of echo signals measured at the plurality of echo times An image creation step;
A separated image imaging method in a magnetic resonance imaging apparatus. The separated image imaging method according to claim 5.
A separated image imaging method comprising: a step of obtaining an image of water with improved SNR by adding the images of water obtained for each frequency selected on the frequency spectrum distribution.

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