JP6169909B2 - Magnetic resonance imaging apparatus and real number component image acquisition method - Google Patents

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as “NMR”) signals from hydrogen, phosphorus, etc. in a subject, and images nuclear density distribution, relaxation time distribution, etc. In particular, the present invention relates to a technique for obtaining a real component image.

MRI equipment measures the NMR signals (echo signals) generated by the spins of the subject, especially the tissues of the human body, and the shape and function of the head, abdomen, limbs, etc. in two or three dimensions. It is a device that images. Shooting smell Te, the echo signals are different phase encoding by the gradient magnetic field is frequency encoded with is granted, is measured as time-series data. The measured echo signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  In diagnostic imaging using the MRI system described above, the fat component on the image is suppressed based on echo signals from fat protons (hereinafter abbreviated as fat signals) in order to increase the image contrast between the diseased tissue and surrounding tissues. An imaging method is desired.

  On the other hand, in medical diagnosis, an image with some anatomical information remaining may be more useful. For example, it is desirable to be able to select the degree of fat component suppression on the image.

  Selective suppression methods for acquiring images with suppressed fat components include CHESS (Chemical Shift Selective) method, STIR (Short TI Inversion Recovery) method, frequency selective IR pulse method (SPIR method or SPAIR method), and The Binominal IR method is known.

  The CHESS method uses the difference in resonance frequency between water protons and fat protons due to chemical shift, and saturates only the fat magnetization in advance using a frequency-selective high-frequency magnetic field pulse (hereinafter abbreviated as RF pulse). , Suppress fat signal.

  The STIR method uses the difference between the longitudinal relaxation times (T1) of water protons and fat protons, and as a prepulse, an inversion recovery RF pulse (hereinafter referred to as an IR pulse) that tilts the longitudinal magnetization 180 degrees with respect to the static magnetic field direction. (Abbreviated)), the longitudinal magnetization of water protons and fat protons is reversed 180 °, and then imaging starts at the timing when the longitudinal magnetization of fat protons reaches the null point by longitudinal relaxation (T1 recovery). Saturates the magnetization of fat protons and suppresses fat signals.

  The frequency selective IR pulse method combines the CHESS method and the STIR method, selectively applies an IR pulse at the resonance frequency of the fat proton, reverses only the longitudinal magnetization of the fat proton, and the longitudinal magnetization of the fat proton is the null point. The imaging is started when reaching.

  The Binominal IR method reverses only the longitudinal magnetization of fat protons using a Binominal IR pulse formed by combining a plurality of RF pulses (for example, Patent Document 1).

  The fat suppression method using frequency selective IR pulses and Binominal IR pulses has a feature that the fat suppression effect lasts longer than the CHESS method because it takes time to recover the longitudinal magnetization of fat protons by longitudinal relaxation. Therefore, using a high-speed imaging method with a short repetition time (TR), it is possible to acquire a plurality of echo signals in which fat signals are suppressed by one irradiation of these fat-selective RF pulses.

JP 2010-162096 A

Yoshihara Sadahara, Junko Tomiasu, Yasuhiko Horie, et al. Basic study on image contrast in Fluid Attenuated Inversion Recovery Balanced Turbo Field Echo (FLAIRB-TFE). Japanese Journal of Radiation Science 2005; 61 (5): 701-708

  However, in the above selective suppression method, the time until the longitudinal magnetization of the fat protons reaches the null point (TI) after applying the frequency selective IR pulse and measuring the echo signal for imaging. You just have to wait.

  Therefore, in a high-speed imaging method that repeats a pulse sequence, a combination of frequency-selective IR pulse application and image echo signal measurement multiple times, the waiting time (TI) depends on the frequency-selective IR pulse application interval. Therefore, in order to obtain an appropriate image contrast, it is necessary to adjust these two parameters (TI, frequency selective IR pulse application interval) (Non-patent Document 1).

  Therefore, the present invention has been made in view of the above problems, and selective excitation is performed when a signal from a specific nuclide is suppressed using a selective suppression method that reverses only the longitudinal magnetization of a specific nuclide such as a fat proton. The purpose is to substantially eliminate the waiting time (TI) from after the measurement until the measurement of the echo signal for the image.

In order to achieve the above object, the MRI apparatus and the real number component image acquisition method of the present invention are configured as follows. That is,
Measurement of echo signals for images within the period in which the longitudinal magnetization of the suppression nuclide excited prepulse part for suppressing the signal from the suppression nuclide and the suppression nuclide excited by the suppression nuclide excitation prepulse part remains negative An echo signal is measured from a subject using a pulse sequence that includes an image echo measurement unit that performs an image signal correction and a phase correction echo measurement unit that measures an echo signal for performing phase correction of the image. Then, using the phase correction map created using the echo signal measured by the phase correction echo measurement unit, the phase of the image formed using the echo signal measured by the image echo measurement unit is corrected. , Extract a positive pixel value from the real component of the phase-corrected image to obtain a positive image, extract a negative pixel value to obtain a negative image, and weight-add the positive image and the negative image. To obtain a superimposed image.

  According to the MRI apparatus and the real component image acquisition method of the present invention, when suppressing a signal from a specific nuclide using a selective suppression method that reverses only the longitudinal magnetization of a specific nuclide such as a fat proton, selective excitation is performed. It is possible to substantially eliminate the waiting time (TI) from the later measurement of the echo signal for the image. As a result, it is not necessary to adjust the waiting time (TI) and the frequency selective IR pulse application interval in high-speed imaging that repeats the combination of frequency selective IR pulse measurement and image echo signal measurement multiple times. be able to.

The block diagram which shows the whole structure of one Example of the MRI apparatus which concerns on this invention (a) is a block diagram showing the structure of one iteration of an example of a real component image acquisition sequence, and (b) shows the behavior of longitudinal magnetization of the suppression nuclide by the pulse sequence shown in (a). Functional block diagram showing an example of each function of the arithmetic processing unit 114 The flowchart which shows the processing flow for implement | achieving an example of the real component image acquisition method Sequence chart showing an example of a high-speed GE sequence as an example of the image echo measurement unit 202 Sequence chart showing an example of a GE sequence as an example of the phase correction echo measurement unit 203 Sequence chart showing an example of a frequency selective IR pulse as an example of the suppression nuclide excitation prepulse unit 201 of the first embodiment Sequence chart showing an example of a real component image acquisition sequence of the first embodiment Sequence chart showing an example of a Binominal IR pulse as an example of the suppression nuclide excitation prepulse unit 201 of Example 2 Sequence chart showing an example of a real component image acquisition sequence of the second embodiment Diagram showing an example of GUI for setting imaging parameters

  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 repeated explanation thereof 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 gradient magnetic field coil 103 is a coil wound in the three-axis directions of X, Y, and Z, which is 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 a measurement control unit 111 described later, and supplies a current to each gradient coil. As a result, gradient magnetic fields Gx, Gy, and Gz are generated in the three axis directions of X, Y, and Z. The gradient magnetic field coil 103 and the gradient magnetic field power supply 109 are included in the gradient magnetic field generator.

  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. The phase encoding gradient magnetic field pulse (Gp) and the frequency encoding (reading) gradient magnetic field pulse (Gf) are applied in the remaining two directions, and the position information of each direction is encoded in the nuclear magnetic resonance signal (echo signal). The

  The RF transmission coil 104 is a coil that irradiates the subject 101 with 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 coil 104 is disposed near the subject 101 after being amplified. By supplying, the subject 101 is irradiated with the RF pulse. The RF transmitter coil 104 and the RF transmitter 110 are included in the RF pulse generator.

  The RF receiving coil 105 is a coil that receives an echo signal emitted by the NMR phenomenon of the spin constituting 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, the signal processing unit 107 amplifies the received echo signal in accordance with a command from the measurement control unit 111 described later, divides the signal 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 the gradient magnetic field power source 109, the RF transmission unit 110, and the signal processing unit 107 are controlled based on control data of a predetermined pulse sequence. The control 101 repeatedly executes 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 to reconstruct the image of the imaging region of the subject 101. Control the collection of required 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 usually 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 display and storage of processing results, 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. The overall control unit 112 may be connected to an external storage unit 117 such as an optical disk.

  Specifically, when the measurement control unit 111 collects echo data by executing an imaging sequence and 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.

  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 arranged close to the display unit, and an operator interactively controls various processes of the MRI apparatus through the operation unit while looking at the display unit.

  Currently, the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton), which is the main constituent material of the subject, that is widely used in clinical practice. By imaging information about 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. can be imaged two-dimensionally or three-dimensionally.

<Overview of pulse sequence>
Next, an example of a real component image acquisition sequence using the specific nuclide selective suppression method according to the present invention will be described with reference to FIG. Fig. 2 (a) is a block diagram showing the structure of one repetition of an example of the real component image acquisition sequence, and Fig. 2 (b) shows the behavior of longitudinal magnetization of the suppression nuclide by the pulse sequence shown in (a). , Respectively. The real component image acquisition sequence shown in FIG. 2 (a) includes a suppression nuclide excitation prepulse unit 201, an image echo measurement unit 202, and a phase correction echo measurement unit 203. It shows that the combination is executed within one repetition time (TR), and the combination of three partial sequences is repeated a predetermined number of times for each repetition time (TR).

  The suppression nuclide excitation prepulse unit 201 is a partial sequence having an RF prepulse for exciting only the longitudinal magnetization of the suppression nuclide whose signal should be suppressed on the image to a predetermined angle. Here, the predetermined angle is an angle at which the longitudinal magnetization after excitation is opposite (negative) to the direction of the static magnetic field, and the polarity of the signal from the excited nuclide thus excited is from the desired nuclide that is not excited. The angle is opposite to the polarity of the signal. Specifically, it is greater than 90 degrees and less than 180 degrees. An example of using a frequency selective IR pulse as the suppression nuclide excitation prepulse will be described in Example 1 described later, and an example of using a Binominal IR pulse will be described in Example 2 described later.

  The image echo measurement unit 202 obtains an image in a state where the longitudinal magnetization of the suppression nuclide is excited at a predetermined angle by the suppression nuclide excitation prepulse unit 201 and the longitudinal magnetization of the suppression nuclide is maintained in a direction opposite to the static magnetic field direction. This is a partial sequence for measuring an echo signal. For that purpose, after execution of the suppression nuclide excitation prepulse unit 201, without waiting for time (without waiting for the longitudinal magnetization of the suppression nuclide to recover to T1 to the null point without waiting for the passage of time), preferably Immediately after the suppression nuclide excitation prepulse unit 201, an image echo measurement unit 202 is executed to measure an image echo signal.

  As a result, the state in which the longitudinal magnetization of the suppression nuclide is excited at a predetermined angle by the suppression nuclide excitation prepulse unit 201 is reflected in the echo signal for the image and is measured, and the image reconstructed using this echo signal is also displayed. The state of longitudinal magnetization of the suppression nuclide is reflected. The waiting time (TI) between the suppression nuclide excitation prepulse unit 201 and the image echo measurement unit 202 is arbitrary as long as it is the time from immediately after the excitation of the longitudinal magnetization of the suppression nuclide to T1 recovery to the null point. It can be set and does not depend on the change of waiting time (TI).

  As an example of the image echo measurement unit 202, a high-speed gradient echo (GE) sequence is possible, and a sequence chart of the example is shown in FIG. In the sequence chart of FIG. 5, in order from the top, the RF pulse (RF) irradiated from the RF transmitting coil 104, the slice selective gradient magnetic field (Gs) applied from the gradient magnetic field coil 103, the phase encoding gradient magnetic field (Gp), the frequency encoding ( (Leadout) Gradient magnetic field (Gr) and echo signal (Echo) from the subject 101 are shown, the vertical axis shows their intensity, and the horizontal axis shows time.

  The high-speed GE sequence in FIG. 5 shows an example in which eight echo signals are measured, and the unit sequence for measuring each echo signal is executed as follows. That is, the slice selective gradient magnetic field 514 is applied together with the RF pulse 512 set so as to include the resonance frequency of the suppression nuclide and the desired nuclide, thereby exciting the nuclear magnetization (spin) of the desired nuclide. The excitation angle by the RF pulse 512 is preferably 20 degrees to 90 degrees. After that, a phase encode gradient magnetic field 515 is applied in the phase encode direction, a frequency encode gradient magnetic field 517 is applied in the frequency encode direction, and position information in each direction is encoded into an echo signal 518, and an echo signal (gradient echo). ) 518 is measured. After measurement of the echo signal 518, a rewind gradient magnetic field 516 having the same area and different polarity as the phase encode gradient magnetic field 515 is applied, and the phase due to the phase encode gradient magnetic field 515 applied to the excited spin is cancelled.

In the case of the segment measurement in which one slice image or k space is divided into a plurality of segments by repeating the unit sequence for one repetition described above by changing the application amount of the phase encode gradient magnetic field 515 and the rewind gradient magnetic field 516. In this case, echo data for one segment is acquired.

  The image reconstructed from the echo signal measured by the image echo measurement unit 202 includes a phase change caused by incompleteness of the apparatus such as static magnetic field inhomogeneity or frequency encoding gradient magnetic field defect. In order to acquire a real component image, it is necessary to remove such a phase change. Therefore, phase correction data for removing this phase change is acquired by the next phase correction echo measurement unit 203.

  The phase correction echo measurement unit 203 is a phase correction map (phase correction map) for correcting the phase change in the image (complex image) reconstructed using the echo signal measured by the image echo measurement unit 202. This is a partial sequence for measuring an echo signal used to create (data). The phase correction echo signal is measured in the same slice as the slice measured by the image echo measurement unit 202, and the longitudinal magnetization of the suppression nuclide excited at a predetermined angle by the suppression nuclide excitation prepulse unit 201 indicates that the static magnetic field direction is It is executed in a period of longitudinal relaxation (T1 recovery) until it becomes the same (positive) direction.

  As a result, when the phase correction echo measurement unit 203 is executed, measurement is performed with the polarity of the echo signal from the suppression nuclide being positive, and phase correction data with good S / N can be obtained. In addition, the echo signal measured by the phase correction echo measurement unit 203 and the phase correction map (phase correction data) created from this echo signal are incomplete in the apparatus such as static magnetic field inhomogeneity and frequency encoding gradient magnetic field defect. In this case, the phase change caused by the nature of the property is included, but the polarity information of the longitudinal magnetization of the suppression nuclide or the desired nuclide is not included.

  The phase information included in the phase correction map (phase correction data) must substantially include phase change information inherent in the complex image reconstructed from the echo signal measured by the image echo measurement unit 202. Therefore, it is preferable that the pulse sequence of the phase correction echo measurement unit 203 is substantially the same as the pulse sequence for one repetition of the image echo measurement unit 202.

  In addition, since the spatial components of phase change information inherent in complex images are mostly low spatial frequency components, that is, most of the phase change components change slowly in space, the phase correction map (phase correction The spatial resolution of (data) may be lower than the spatial resolution of the complex image. Therefore, when the echo measurement unit for image 202 and the echo measurement unit for phase correction 203 are combined to form one repetition (TR) of the real component image acquisition sequence, measurement is performed by the echo measurement unit for phase correction 203. The number of phase encodes of the echo signal may be smaller than the number of phase encodes of the echo signal measured by the image echo measurement unit 202. In other words, every time a plurality of phase encode data is measured by the image echo measurement unit 202, the frequency at which one phase encode data is measured by the phase correction echo measurement unit 203 may be used.

  However, the echo measurement unit 203 for phase correction acquires continuous echo data including the center frequency 0 of the k space, and the application area of the phase encoding gradient magnetic field at that time is the same in the k space in the image echo measurement unit 202. It is desirable that the area is the same as the phase encoding gradient magnetic field when acquiring continuous echo data including the center frequency 0.

  Furthermore, the flip angle of the excitation RF pulse applied by the phase correction echo measurement unit 203 is reduced in order to reduce the loss of longitudinal magnetization due to the excitation, so that the flip angle of the excitation RF pulse in the image echo measurement unit 202 is reduced. A smaller angle is preferred.

  Therefore, as an example of the phase correction echo measurement unit 203, FIG. 6 shows an example of a gradient echo (GE) sequence that is substantially the same as the unit sequence that constitutes an example of the image echo measurement unit 202 shown in FIG. The meaning of each axis is the same as in FIG. A slice selective gradient magnetic field 614 is applied together with an RF pulse 613 that is set to include the resonance frequency of the suppression nuclide and the desired nuclide and is set to excite the same slice position as the image echo measurement unit 202. The nuclear magnetization (spin) of the desired nuclide is excited. The excitation angle of the RF pulse 613 is preferably about 5 °, which is smaller than that of the RF pulse 512.

  After that, a phase encode gradient magnetic field 615 is applied in the phase encode direction, a frequency encode gradient magnetic field 617 is applied in the frequency encode direction, and position information in each direction is encoded into an echo signal 618, and an echo signal (gradient echo). 618 is measured. After measurement of the echo signal 618, a rewind gradient magnetic field 616 having the same area and different polarity as the phase encode gradient magnetic field 615 is applied, and the phase due to the phase encode gradient magnetic field 615 applied to the excited spin is cancelled. However, since the spatial resolution of the phase correction map (phase correction data) may be lower than the image acquired by the image echo measurement unit 202, the application amounts of the phase encoding gradient magnetic field 615 and the frequency encoding gradient magnetic field 617 are for images. It may be smaller than the amount of gradient magnetic field applied by the echo measurement unit 202.

  When the real component image acquisition sequence configured as described above is executed for one iteration (TR), the echo data set acquired is one slice image or multiple slices of k space for one slice image. In the case of segment measurement obtained by dividing into segments, only one segment is obtained. For this reason, in the case of multi-slice measurement or segment measurement, it is necessary to repeatedly execute a real component image acquisition sequence corresponding to the number of times.

  Alternatively, in the example of FIG. 6, only one echo signal 618 is measured, but this number is arbitrary. After executing the suppression nuclide excitation prepulse unit 201 once, when the echo measurement unit 202 for the image measures all the echo signals necessary for reconstructing one slice image, the real component image acquisition sequence is one. It will be repeated only once. In this case, the phase correction echo measurement unit 203 continuously measures a plurality (for example, 8) of echo signals 618 including the center frequency 0 in the k space.

  Here, in the examples of FIGS. 5 and 6, eight echo signals 518 are measured and only one echo signal 618 is measured, but these numbers are arbitrary. Echo signals 518 are measured for a number of times necessary for imaging (for example, 256). When 256 pieces of data are acquired for imaging, the pulse sequence shown in FIG. 5 is repeated 32 times, and 32 echo signals for phase correction are also measured. When the echo signal for phase correction is measured by the phase correction echo measurement unit 203, the echo signal 618 is acquired while changing the intensity of the phase encode gradient magnetic field pulse 615 at each repetition, thereby obtaining the center frequency 0 of the k space. 32 consecutive data including can be collected.

  5 and 6 show an example of two-dimensional imaging, but it is possible to apply three-dimensional imaging by applying an encode gradient magnetic field also in the slice direction. 5 and 6 show an example of a high-speed GE sequence as the image echo measurement unit 202. As the image echo measurement unit 202, a TrueFISP sequence that applies a rewind gradient magnetic field also in the slice direction and the frequency encoding direction. It is also possible to use a fast spin echo (FSE) sequence.

<About the phase correction map>
As described above, in the complex image reconstructed from the echo signal measured by the image echo measurement unit 202, in addition to the signal phase inversion due to the polarity of the longitudinal magnetization, the static magnetic field inhomogeneity and the frequency encoding gradient magnetic field It also includes phase changes resulting from device imperfections such as defects. In order to remove this phase change and obtain a highly accurate real component image, the phase of the complex image is corrected using the phase correction map. Specifically, only the phase change caused by the device or the environment is corrected by subtracting the phase of the phase correction map for each pixel from the phase of the complex image. Thereby, only the polarity of the longitudinal magnetization of the suppression nuclide is reflected in the phase of the complex image after the phase correction, so that the signal of the suppression nuclide becomes a negative value and the signal of the desired nuclide becomes a positive value.

  In order to create the phase correction map, an echo signal is measured by the phase correction echo measurement unit 203. The echo signal measured by the phase correction echo measurement unit 203 is subjected to two-dimensional or three-dimensional Fourier transform to be converted into a phase correction map (phase correction data). This phase correction map represents a phase distribution in a two-dimensional space or a three-dimensional space. When only a part of the k-space is acquired as the echo data for phase correction, the remaining unacquired area of the k-space is filled with zeros, and the image data measured by the image echo measurement unit 202 is used. A two-dimensional or three-dimensional Fourier transform is performed in accordance with the size of the k-space region in which the echo data is filled. Since this two-dimensional or three-dimensional phase correction phase map does not maintain the polarity of the longitudinal magnetization of the suppression nuclide excited by the suppression nuclide excitation prepulse unit 201, only the phase change caused by the execution of the image echo measurement unit 202 is shown. Include.

Assuming that a complex image obtained by Fourier transforming the echo signal measured by the phase correction echo measurement unit 203 as described above is C (x, y), the phase correction map θ (x, y) is
θ (x, y) = tan ^-1 {Im (C (x, y)) / Re (C (x, y))} (1)
It becomes. Here, Re () and Im () represent the real component and the imaginary component of the argument, respectively. When the absolute value of the complex image reconstructed from the echo signal measured by the image echo measurement unit 202 is Iabs (x, y) and the phase is Φ (x, y), the complex image is Iabs (x, y) exp {iΦ (x, y)} and the above phase correction is
Iabs (x, y) exp {iΦ (x, y)} Xexp {-iθ (x, y)} (2)
It can be expressed as.

<About superimposed images>
As described above, for the complex image reconstructed using the echo signal measured by the image echo measurement unit 202 executed without any time interval after the execution of the suppression nuclide excitation prepulse unit 201, the phase correction is performed. After phase correction using the phase correction map created based on the echo signal measured by the echo measurement unit 203, in the real component of the complex image, the signal from the desired nuclide becomes a positive pixel value, and the suppression nuclide The signal becomes a negative pixel value. Therefore, if only the positive pixel value is extracted from the real component of the complex image after phase correction and the negative pixel value is replaced with zero, the image of the desired nuclide from which the signal from the suppression nuclide has been removed becomes a negative pixel. If only the value is extracted and the positive pixel value is replaced with zero, an image of the suppressed nuclide from which the signal from the desired nuclide is removed is obtained.

  Therefore, by obtaining the superimposed image by weighted addition of the image of the desired nuclide and the image of the suppression nuclide, in other words, an image obtained by extracting only positive pixel values from the real component of the complex image after phase correction, By obtaining a superimposed image by weighting and adding an image obtained by extracting only negative pixel values, an image capable of grasping an anatomical shape representing the distribution of the suppression nuclide while suppressing the signal from the suppression nuclide is obtained. Is possible.

In particular,
V (x, y) = Re (Iabs (x, y) exp {iΦ (x, y)} Xexp {-iθ (x, y)})
The image of the desired nuclide is
Idsr (x, y) = V (x, y) if V (x, y)> 0
= 0 otherwise (3)
The image of the suppression nuclide is
Ispr (x, y) = | V (x, y) | if V (x, y) <0
= 0 otherwise (4)
It can be expressed as. Therefore, the superimposed image is
I (x, y) = a · Idsr (x, y) + b · Ispr (x, y) {a + b = 1} (5)
It can be expressed as. Here, (x, y) is a two-dimensional coordinate of a pixel, and in the case of three dimensions, a z-coordinate is also added. By adjusting the weighting factors a and b, it is possible to adjust the degree of suppression of the signal from the suppression nuclide on the superimposed image, that is, how much the anatomical information of the suppression nuclide is visible on the superimposed image. become.

<About the functions of the embodiment>
Next, an example of each function of the arithmetic processing unit 114 for realizing an example of the MRI apparatus and the real component image acquisition method of the present invention will be described based on the functional block diagram shown in FIG. The arithmetic processing unit 114 according to this example includes an imaging parameter setting unit 301, a real component image acquisition sequence setting unit 302, an image reconstruction unit 303, a phase correction map creation unit 304, a phase correction unit 305, and a positive image. An acquisition unit 306, a negative image acquisition unit 307, and a superimposed image creation unit 308 are included.

  The imaging parameter setting unit 301 displays an imaging parameter setting GUI (Graphical User Interface) on the display unit, and accepts input settings of imaging parameter values by the operator. FIG. 11 shows an example of a GUI for inputting and setting these imaging parameters. In this imaging parameter, selection of whether to suppress the signal from the suppression nuclide as described above (check mark On / Off) 1101, and signal suppression from the suppression nuclide when suppressing the signal from the suppression nuclide Selection of specific method (radio button selection) 1102 and setting whether or not weighted addition of suppression nuclide image to desired nuclide image in reconstructed image (check mark On / Off) 1103 And a parameter for setting a weight (addition ratio) (input of a weighting factor) 1104 when performing weighted addition.

  Of the imaging parameter values that have been set and inputted, the value related to the setting of the suppression nuclide excitation prepulse unit 201 is set to the suppression nuclide excitation pulse setting unit 302-1 and the value related to the setting of the image echo measurement unit 202 is set to the image. In the echo measurement sequence setting unit 302-2, values related to the setting of the phase correction echo measurement unit 203 are input to the correction echo measurement sequence setting unit 302-3, and values related to reconstruction of the superimposed image are input to the superimposed image generation unit 308. , Notify each.

  The real number component image acquisition sequence setting unit 302 generates control data for executing the real number component image acquisition sequence based on the imaging parameter value notified from the imaging parameter setting unit 301. Specifically, the suppression nuclide excitation pulse setting unit 302-1 that generates specific control data of the suppression nuclide excitation prepulse unit 201 and the image echo measurement that generates specific control data of the image echo measurement unit 202 This includes a sequence setting unit 302-2 and a correction echo measurement sequence setting unit 302-3 that generates specific control data of the phase correction echo measurement unit 203.

  The image reconstruction unit 303 reads the echo data measured by the image echo measurement unit 202, notified from the measurement control unit 111, from the image k-space area of the memory 113, performs Fourier transform, and includes complex data. Reconstruct a complex image. Then, the reconstructed complex image data is stored in the memory 113.

  The phase correction map creation unit 304 reads the echo data measured by the phase correction echo measurement unit 203 notified from the measurement control unit 111 from the phase correction k-space region of the memory 113, performs Fourier transform, and performs image conversion. A phase correction map having the same dimensionality as the complex image reconstructed by the reconstruction unit 303 and having a low spatial resolution is created, and the created phase correction map data is stored in the memory 113. The method for generating the phase correction map is as described above.

  The phase correction unit 305 reads out the complex image data reconstructed by the image reconstruction unit 303 and the phase correction map data created by the phase correction map creation unit 304 from the memory 113 and outputs complex image data. Is subjected to phase correction using phase correction map data. The details of the phase correction process are as described above. Then, the complex image data after the phase correction is stored in the memory 113 again.

  The positive image acquisition unit 306 extracts a positive pixel value from the real number component in the complex image data phase-corrected by the phase correction unit 305 and replaces the negative pixel value with zero to obtain a positive image. The data is stored in the memory 113. This positive image is an image representing only the signal from the desired nuclide after the signal from the suppression nuclide to be suppressed is removed.

  The negative image acquisition unit 307 replaces the positive pixel value with zero from the real number component in the complex image data phase-corrected by the phase correction unit 305, extracts the negative pixel value, and converts it to an absolute value (absolute value). Are converted to a positive value), and a negative image is obtained and the data is stored in the memory 113. This negative image is an image representing only the signal from the suppression nuclide to be suppressed by removing the signal from the desired nuclide.

  The superimposed image creation unit 308 confirms the setting set by the imaging parameter setting unit 301 as to whether or not the suppression nuclide image is weighted and added to the desired nuclide image. When weighted addition is performed, the positive image data and the negative image data are read from the memory 113, and these are multiplied by the weighting factor set by the imaging parameter setting unit 301 to obtain a superimposed image, This is the final real component image. As a result, the signal from the suppression nuclide remains in the real number component image at a certain rate, and the real number component image is obtained by leaving the anatomical information that is the spatial distribution of the suppression nuclide to some extent.

<Processing Flow of Example>
Next, a processing flow performed by the above-described functional units in cooperation for realizing an example of the MRI apparatus and the real component image acquisition method of the present invention will be described based on the flowchart shown in FIG. This processing flow is stored in advance in the internal storage unit 115 as a program, and is executed by the arithmetic processing unit 114 reading the program from the internal storage unit 115 and executing it. Hereinafter, the processing contents of each processing step will be described in detail.

  In step 401, the imaging parameter setting unit 301 displays an imaging parameter setting GUI on the display unit, and accepts input settings of imaging parameter values by the operator. Specific imaging parameters are as described above.

  In step 402, the imaging parameter setting unit 301 determines whether or not signal suppression from suppression nuclides is selected from the imaging parameters input in step 401 (1101 check mark on / off). If it is not selected (check mark off; No), the process shifts to the imaging of the absolute value image by the normal imaging sequence and this processing flow ends. If it has been selected (check mark on; Yes), the imaging parameter setting unit 301 notifies each unit in the real component image acquisition sequence setting unit 302 of the value of the imaging parameter that has been set and input. Details are as described above. Then, the process proceeds to step 403.

  In step 403, the imaging parameter setting unit 301 confirms whether imaging is possible with the imaging parameters input and set in step 401. If imaging is impossible, the process returns to step 401 to accept imaging parameter change settings by the operator. If imaging is possible, the process proceeds to step 404.

  In step 404, based on the value of the imaging parameter input and set in step 401, the real number component image acquisition sequence setting unit 302 generates control data for executing the real number component image acquisition sequence shown in FIG. Details are as described above. Then, the real number component image acquisition sequence setting unit 302 notifies the measurement control unit 111 of the generated control data.

  In step 405, the measurement control unit 111 executes a real number component image acquisition sequence based on the control data of the real number component image acquisition sequence notified from the real number component image acquisition sequence setting unit 302 in step 404. The measurement control unit 111 stores the echo data measured by the image echo measurement unit 202 in the image k-space area of the memory 113, and stores the echo data measured by the phase correction echo measurement unit 203 in the memory 113. Save in the k-space area for phase correction.

  In step 406, the image reconstruction unit 303 reads the echo data measured by the image echo measurement unit 202 notified from the measurement control unit 111 in step 405 from the memory 113, performs Fourier transform, and performs complex transformation including complex data. The image is reconstructed and stored in the memory 113. Details are as described above. In addition, the phase correction map creating unit 305 reads out the echo data measured by the phase correction echo measuring unit 203 notified from the measurement control unit 111 in step 405 from the memory 113 and performs a Fourier transform to obtain a phase correction map. Create and save in memory 113. Details are as described above.

  In step 407, the phase correction unit 304 reads out the complex image data and the phase correction map data reconstructed in step 406 from the memory 113, and performs phase correction on the complex image data. Details are as described above.

  In step 408, the positive image acquisition unit 306 obtains a positive image from the real number component in the complex image phase-corrected in step 406 and stores the data in the memory 113. Details are as described above. The negative image acquisition unit 307 obtains a negative image from the real number component of the complex image phase-corrected in step 406 and stores the data in the memory 113. Details are as described above.

  In step 409, the superimposed image creating unit 308 confirms the setting (whether or not the check mark in 1103 is on / off) whether or not the suppression nuclide image is weighted and added to the desired nuclide image in step 401. If the setting is not weighted addition (check mark off; No), the process proceeds to step 410. If the setting is weighted addition (check mark on; Yes), the process proceeds to step 411.

  In step 410, the superimposed image creation unit 308 reads only the positive image data obtained in step 408 from the memory 113, and sets this as the final real component image. Thereby, the signal from the suppression nuclide to be suppressed is removed, and a real component image of only the signal from the desired nuclide is obtained. Then, the process proceeds to step 412.

  In step 411, the superimposed image creation unit 308 reads the positive image data and negative image data obtained in step 408 from the memory 113, and multiplies them by the weighting coefficient 1104 set in step 401 and adds them for each pixel. Thus, a real component image is obtained. Details are as described above.

In step 412, the real number component image obtained in step 410 or step 411 is displayed on the display unit.
The above is the outline of the processing flow of the present invention.

  Hereinafter, assuming the fat proton as the suppression nuclide and the water proton as the desired nuclide, the fast GE sequence shown in FIG. 5 as the image echo measurement unit 202 of the real component image acquisition sequence is shown as the phase correction echo measurement unit 203. Each embodiment of the present invention will be described assuming the GE sequence shown in FIG. 6. However, the present invention is not limited to these nuclides as suppression nuclides and desired nuclides.

  Next, Example 1 of the present invention will be described. The first embodiment is an example in which a frequency selective IR pulse is used as the suppression nuclide excitation prepulse unit 201.

  The frequency selective IR pulse is an inversion recovery (IR) pulse that has the resonance frequency of the suppression nuclide and inverts only the magnetization of the suppression nuclide by 180 degrees. In Example 1, since fat protons are assumed as the suppression nuclides, the frequency selective IR pulse of Example 1 has the resonance frequency of fat protons and reverses only the magnetization of fat protons by 180 degrees.

  An example of the suppression nuclide excitation prepulse unit 201 of the first embodiment using the frequency selective IR pulse is shown in FIG. The meaning of each axis is the same as in FIG. In order to disperse the phase of the transverse magnetization that remains after the frequency selective IR pulse 701 is applied in a spatial non-selection to reverse only the magnetization of the fat protons in the region including the imaging region by 180 degrees, the Gs, Gp, and Gr axes A spoiler gradient magnetic field pulse is applied to at least one of the axes. In the example of FIG. 7, a spoiler gradient magnetic field pulse 702 is applied to the Gs axis, a spoiler gradient magnetic field pulse 703 is applied to the Gp axis, and a spoiler gradient magnetic field pulse 704 is applied to the Gr axis.

  An example of a real component image acquisition sequence having the frequency selective IR pulse as the suppression nuclide excitation prepulse unit 201 is shown in FIG. The meaning of each axis is the same as in FIG. The real number component image acquisition sequence in FIG. 8 does not take time after the suppression nuclide excitation prepulse unit 201 shown in FIG. 7, that is, the longitudinal magnetization of the inverted fat protons is negative (in the direction opposite to the static magnetic field). In this period, preferably, immediately after the suppression nuclide excitation prepulse unit 201, an image echo signal is measured by the image echo measurement unit 202 including the high-speed GE sequence shown in FIG. An echo signal for creating a phase correction map is measured by the phase correction echo measuring unit 203 having the GE sequence shown in FIG.

  Since the phase correction and the acquisition of the real number component image after the execution of the real number component image acquisition sequence are as described above, detailed description thereof will be omitted.

  As described above, according to the first embodiment, it is possible to eliminate the need to wait for a waiting time (TI) before acquiring an echo signal for an image. As a result, it is possible to eliminate the need to adjust the application interval of TI and frequency selective IR pulses.

  Next, Example 2 of the present invention will be described. The second embodiment is an example in which a Binominal IR pulse is used as the suppression nuclide excitation prepulse unit 201.

  Binominal IR pulse is a frequency band in which each RF pulse has the resonance frequency of the desired nuclide as the center frequency and can also suppress the suppression nuclide, and two adjacent RF pulses have opposite phases alternately. An RF pulse sequence obtained by weighting the amplitude intensity ratio with a binomial coefficient, and each RF pulse is applied at a time interval until the transverse magnetization of the desired nuclide and the suppression nuclide change from the same phase to the opposite phase. It is. As a result, the longitudinal magnetization of the desired nuclide can be maintained in the direction of the static magnetic field, and the longitudinal magnetization of the suppression nuclide can be reversed 180 degrees (in the direction opposite to the static magnetic field). That is, as with the frequency selective IR pulse described above, only the longitudinal magnetization of fat protons can be reversed 180 degrees.

  Among the Binominal IR pulses, 1-1 pulses are two RF pulses weighted by an amplitude intensity ratio of π / 2 and -π / 2, and 1-2-1 pulses are three RF pulses. Is weighted by π / 4, -π / 2, π / 4, and the 1-3-3-1 pulse has an amplitude intensity ratio of π / 8, -3π / 8, 3π / 8, weighted by -π / 8. Here, the amplitude intensity π / 2 means the amplitude intensity of the RF pulse necessary for exciting the nuclide magnetization vector by π / 2 [rad] from the static magnetic field direction.

  FIG. 9 shows an example of the suppression nuclide excitation prepulse unit 201 of Example 2 using 1-1 pulses among the above Binominal IR pulses. The first excitation RF pulse 901 and the first slice selective gradient magnetic field pulse 903 constituting the 1-1 pulse are applied, and the magnetizations of water protons and fat protons in the slice selected region are both excited by 90 degrees. .

  Then, after the time τ when the transverse magnetization of the fat proton rotates by an angle of π with respect to the transverse magnetization of the water proton, the second excitation RF pulse 902 and the second slice selective gradient magnetic field pulse 904 constituting the Binominal IR pulse are When applied, the transverse proton magnetization of water protons is positive (static magnetic field direction) longitudinal magnetization, and that of fat protons is negative (anti-static magnetic field direction) longitudinal magnetization. Next, in order to disperse the phase of the remaining transverse magnetization, a spoiler gradient magnetic field pulse is applied to at least one of the Gs, Gp, and Gr axes. In the example of FIG. 9, a spoiler gradient magnetic field pulse 905 is applied to the Gs axis, a spoiler gradient magnetic field pulse 906 is applied to the Gp axis, and a spoiler gradient magnetic field pulse 907 is applied to the Gr axis. As described above, the Binominal IR pulse can use a slice selective gradient magnetic field, and is characterized by being capable of multi-slice imaging.

  FIG. 10 shows an example of a real component image acquisition sequence having 1-1 of the Binominal IR pulses as the suppression nuclide excitation prepulse unit 201. The meaning of each axis is the same as in FIG. The real number component image acquisition sequence in FIG. 10 is not delayed after the suppression nuclide excitation prepulse unit 201 shown in FIG. 9, that is, the longitudinal magnetization of the inverted fat proton is negative (in the opposite direction to the static magnetic field). In this period, preferably, immediately after the suppression nuclide excitation prepulse unit 201, an image echo signal is measured by the image echo measurement unit 202 including the high-speed GE sequence shown in FIG. An echo signal for creating a phase correction map is measured by the phase correction echo measuring unit 203 having the GE sequence shown in FIG.

  Since the phase correction and the acquisition of the real number component image after the execution of the real number component image acquisition sequence are as described above, detailed description thereof will be omitted.

  As described above, the second embodiment also has the same effect as the first embodiment. In particular, the use of Binominal IR pulses enables multi-slice imaging.

  101 subject, 102 static magnetic field generating magnet, 103 gradient magnetic field coil, 104 transmission RF coil, 105 RF reception coil, 106 bed, 107 signal processing unit, 108 overall control unit, 109 gradient magnetic field power source, 110 RF transmission unit, 111 measurement Control unit, 113 memory, 114 arithmetic processing unit (CPU), 115 internal storage unit, 116 network IF, 117 external storage unit, 118 display / operation unit

Claims (6)

A measurement control unit that controls measurement of echo signals from the subject based on a predetermined pulse sequence;
An arithmetic processing unit for reconstructing an image of the subject based on the echo signal;
With
The predetermined pulse sequence includes a suppression nuclide excitation prepulse unit for suppressing a signal from the suppression nuclide, an image echo measurement unit for measuring an echo signal for an image, and an echo signal for phase correction of the image And an echo measurement unit for phase correction for measuring
The arithmetic processing unit is
Using the phase correction map created using the echo signal measured by the phase correction echo measurement unit, the phase of the image formed using the echo signal measured by the image echo measurement unit is corrected. A phase correction unit;
A positive image acquisition unit that obtains a positive image by extracting a positive pixel value from the real number components of the phase-corrected image;
A negative image acquisition unit that extracts a negative pixel value from a real component of the phase-corrected image to obtain a negative image;
A superimposed image creating unit that obtains a superimposed image by weighted addition of the positive image and the negative image;
A magnetic resonance imaging apparatus comprising: The magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus according to claim 1, wherein the suppression nuclide excitation pre-pulse part includes a frequency selective IR pulse. The magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus according to claim 1, wherein the suppression nuclide excitation pre-pulse section includes a Binominal IR pulse. The magnetic resonance imaging apparatus according to any one of claims 1 to 3,
The magnetic resonance imaging apparatus, wherein the measurement control unit executes the phase correction echo measurement unit after the longitudinal magnetization of the suppression nuclide excited by the suppression nuclide excitation prepulse unit is restored to a positive state. In the magnetic resonance imaging apparatus according to any one of claims 1 to 4,
The measurement control unit executes the image echo measurement unit within a period in which longitudinal magnetization of the suppression nuclide excited by the suppression nuclide excitation prepulse unit maintains a negative state. Imaging device. A measurement control unit that controls measurement of echo signals from the subject based on a predetermined pulse sequence;
An arithmetic processing unit for reconstructing an image of the subject based on the echo signal;
A real component image acquisition method in a magnetic resonance imaging apparatus comprising:
Applying a suppression nuclide excitation prepulse to suppress signals from the suppression nuclide;
Measuring an echo signal for an image within a period in which the longitudinal magnetization of the suppression nuclide excited by the application of the suppression nuclide excitation prepulse maintains a negative state;
Measuring an echo signal for phase correction of the image;
Correcting a phase of an image configured using the echo signal for the image, using a phase correction map created using an echo signal for performing phase correction of the image;
Extracting a positive pixel value from the real number components of the phase-corrected image to obtain a positive image;
Extracting a negative pixel value from the real component of the phase-corrected image to obtain a negative image;
Obtaining a superimposed image by weighted addition of the positive image and the negative image;
A real number component image acquisition method comprising:

Images