JP2005152175A - Magnetic resonance imaging apparatus and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus which performs a radial scan or a propeller scan with a reduced band artifact. <P>SOLUTION: In the radial scan of SSFP (steady-state free-procession) with different readout gradient fields for each acquisition line, coincident conditions under which the centers of echoes for an FID (Free Induction Decay) echo and an SE (spin-echo) / STE (stimulated-echo) echo coincide with the origin on a K-space are acquired for each acquisition line by a step S507. Since a real scan requires that all reception echoes on the K-space should have the centers of the echoes on the origin by using these coincident conditions, deviations of the center of the echoes for the FID echo and SE/STE echo caused by the magnetic field inhomogeneity, or the like, are prevented. Moreover, the band artifact in tomogram information due to the deviation is reduced. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a magnetic resonance imaging apparatus that performs data collection using a pulse sequence including readout gradient magnetic fields in a plurality of directions perpendicular to each other.

  In recent years, image quality degradation caused by magnetic field inhomogeneity of a static magnetic field generated by a magnetic resonance imaging apparatus has been improved by improving the performance of a magnet and improving a pulse sequence used for imaging. Here, for example, in the case of a pulse sequence using SSFP (Steady State Free Precession), a plurality of received echoes (echo) are observed during application of the read gradient magnetic field due to magnetic field inhomogeneity. The The plurality of received echoes cause interference when an image is reconstructed using Fourier transform, and a striped shadow called a band artifact or a moire is generated on the reconstructed image. .

  Here, in order to reduce these artifacts, a plurality of received echoes observed with a time lag are superimposed on one to eliminate the interference (see, for example, Patent Document 1).

  On the other hand, a pulse sequence called a radial scan or a propeller scan is used to reduce artifacts due to the movement of the subject during imaging. In these pulse sequences, magnetic resonance signal information is acquired along a radial collection line centering on the origin where the spatial frequency is zero in the K space before image reconstruction. Thereafter, precise vertical and horizontal interpolation data arranged in a grid pattern is generated in a rectangular K space centered on the origin by interpolation or the like, and image reconstruction is performed.

As a result, it is possible to obtain a reconstructed image with little artifact due to motion by performing imaging using the magnetic resonance signal information on the collection line passing through the origin of the K space, which is less influenced by motion, as raw data.
Japanese Patent Application Laid-Open No. 2003-210431, (page 6, FIGS. 2 to 3)

  However, according to the background art described above, the radial scan or the propeller scan becomes weak in an uneven magnetic field. That is, in the radial scan or the propeller scan, one piece of tomographic image information is acquired by repeating a pulse sequence with a different readout gradient magnetic field each time. Here, the effect of the magnetic field inhomogeneity superimposed on the readout gradient magnetic field is different every time. As a result, a plurality of received echoes due to the magnetic field inhomogeneity are generated at different positions for each acquisition line, and many band artifacts are generated. Arise.

  In particular, the radial scan or propeller scan using SSFP is weak in the magnetic field inhomogeneous and easily generates a plurality of reception echoes. Therefore, in the pulse sequence using these, band artifacts are likely to occur.

  For these reasons, it is important how to realize a magnetic resonance imaging apparatus that performs radial scan or propeller scan with reduced band artifacts.

  An object of the present invention is to provide a magnetic resonance imaging apparatus capable of performing a radial scan or a propeller scan with reduced band artifacts.

  In order to solve the above-described problems and achieve the object, the magnetic resonance imaging apparatus according to the first aspect of the present invention uses the pulse sequence having readout gradient magnetic fields in a plurality of axial directions of three axes orthogonal to each other. A series of magnetic resonance signal information collected during the formation of the gradient magnetic field is a series of data along the collection line passing through the origin where the spatial frequency is zero on the K space, which is the basis of image reconstruction by Fourier transform. The pulse sequence includes an echo center and an origin of an echo included in the magnetic resonance signal information for each of the plurality of acquisition lines having the readout gradient magnetic field having different sizes. Correction means for matching is provided.

  In the invention according to the first aspect, the pulse sequence is generated by the correcting means for each of a plurality of acquisition lines having different magnitude readout gradient magnetic fields and the echo center of the echo included in the magnetic resonance signal information and the origin in the K space. To match.

  The magnetic resonance imaging apparatus according to the invention of the second aspect is characterized in that the echoes are FID echoes and SE / STE echoes collected in the pulse sequence using SSFP.

  In the invention of the second aspect, magnetic resonance signal information is collected by a pulse sequence using SSFP.

  In the magnetic resonance imaging apparatus according to the third aspect of the invention, the correction unit separates and extracts the FID echo and the SE / STE echo before collecting the magnetic resonance signal information using the SSFP. Magnetic resonance signal information is separately collected by a separation pulse magnetic field and a separation pulse sequence using the SSFP having the same readout gradient magnetic field as the pulse sequence.

  In the invention of the third aspect, before collecting magnetic resonance signal information using SSFP, the FID echo and the SE / STE echo are separated and extracted, and the position of the individual echo center corresponding to the readout gradient magnetic field is individually extracted. Get information.

  The magnetic resonance imaging apparatus according to the invention of the fourth aspect is characterized in that the individual collection is performed on a part of the collection lines of the plurality of collection lines constituting the K space.

  In the invention of the fourth aspect, the position information of the echo center is collected only on a part of the collection lines.

  Further, in the magnetic resonance imaging apparatus according to the invention of the fifth aspect, the correcting means includes an adjusting means for obtaining a coincidence condition between the FID echo and the SE / STE echo and the origin at the partial collection line. Interpolating means for obtaining matching conditions for all the collection lines from the matching conditions.

  In the fifth aspect of the invention, the correction means obtains the coincidence condition between the FID echo and the SE / STE echo and the origin in a part of the collection lines by the adjusting means, and from this coincidence condition, the interpolation means Find matching conditions on the collection line.

  In the magnetic resonance imaging apparatus according to the sixth aspect of the present invention, when the separation pulse sequence matches the FID echo, the separation gradient magnetic field is positioned after the readout gradient magnetic field of the separation pulse sequence. It is characterized by making it.

  In the invention of the sixth aspect, the phase of the SE / STE echo is dispersed by the separation gradient magnetic field, and only the FID echo is acquired.

  Further, in the magnetic resonance imaging apparatus according to the seventh aspect of the invention, the adjusting means obtains a spatial distribution of phase angles by Fourier-transforming the FID echo, and further obtains position information of the echo center from the gradient of the spatial distribution. It is characterized by acquiring.

  In the seventh aspect of the invention, the magnitude of deviation between the echo center of the FID echo and the origin of the collection line is detected.

  The magnetic resonance imaging apparatus according to the invention of the eighth aspect is characterized in that the adjusting means adjusts a phase angle of an RF pulse that excites the magnetic resonance signal.

  In the invention of the eighth aspect, the adjusting means adjusts the phase angle of the RF pulse to correct the position of the echo center of the FID echo.

  The magnetic resonance imaging apparatus according to the ninth aspect of the invention provides the separation gradient magnetic field when the separation pulse sequence matches the SE / STE echo, and the magnetic resonance signal of the separation pulse sequence. It is located between the RF pulse to be excited and the readout gradient magnetic field of the separation pulse sequence.

  In the ninth aspect of the invention, the phase of the FID echo is dispersed by the separation gradient magnetic field, and only the SE / STE echo is acquired.

  In the magnetic resonance imaging apparatus according to the invention of the tenth aspect, the adjustment means performs Fourier transform on the SE / STE echo to obtain a spatial distribution of phase angles, and further determines the position of the echo center from the inclination of the spatial distribution. It is characterized by acquiring information.

  In the tenth aspect of the invention, the magnitude of deviation between the echo center of the SE / STE echo and the origin of the acquisition line is detected.

  Further, the magnetic resonance imaging apparatus according to the invention of the eleventh aspect is characterized in that the adjusting means adjusts the magnitude of the dephase gradient positioned before the readout gradient magnetic field.

  In the eleventh aspect of the invention, the adjustment means adjusts the magnitude of the dephase gradient magnetic field to correct the deviation of the echo center position of the SE / STE echo.

  The magnetic resonance imaging apparatus according to a twelfth aspect of the invention is characterized in that the echoes are SE echoes and STE echoes collected in the pulse sequence using FSE.

  In the twelfth aspect of the invention, magnetic resonance signal information is collected by a pulse sequence using FSE.

  The magnetic resonance imaging apparatus according to the thirteenth aspect of the invention is the FSE having the same readout gradient magnetic field as the pulse sequence before the correction unit collects magnetic resonance signal information using the FSE. The position information of the echo centers of odd-numbered and even-numbered SE echoes or STE echoes that are repeatedly generated with a plurality of 180-degree pulses is collected by a pulse sequence using.

  In the thirteenth aspect of the invention, the position information of the echo centers of the odd-numbered and even-numbered SE echoes or STE echoes of the pulse sequence using FSE is collected.

  The magnetic resonance imaging apparatus according to the fourteenth aspect of the invention is characterized in that the collection is performed on a part of the plurality of collection lines constituting the K space.

  In the fourteenth aspect of the invention, the position information of the echo center is collected only on a part of the collection lines.

  Further, in the magnetic resonance imaging apparatus according to the invention of the fifteenth aspect, the correction means uses the partial collection line to match the odd-numbered and even-numbered SE echoes or STE echoes with the origin. And an interpolating means for obtaining matching conditions for all the collected lines from the matching conditions.

  In the fifteenth aspect of the invention, the correcting means obtains a matching condition between the odd-numbered and even-numbered SE echoes or STE echoes and the origin in a part of the collection lines by the adjusting means. The matching condition for all the collection lines is obtained by the interpolation means.

  Further, in the magnetic resonance imaging apparatus according to the sixteenth aspect of the invention, the correction means is configured so that the echo centers of the even-numbered and odd-numbered SE echoes or STE echoes coincide with each other based on the position information. The phase angle of the RF pulse for exciting the magnetic resonance signal or the magnitude of the dephase gradient positioned before the readout gradient magnetic field is adjusted.

  In the sixteenth aspect of the invention, the magnitude of the phase angle or the dephase gradient is adjusted so that the even-numbered and odd-numbered echo centers coincide.

  In addition, the magnetic resonance imaging method according to the invention of the seventeenth aspect provides a different read gradient before collecting magnetic resonance signal information by a pulse sequence having read gradient magnetic fields in a plurality of axial directions of three axes orthogonal to each other. Acquires the position information of the echo center regarding the echo contained in the magnetic resonance signal information of the magnetic field, stores the magnetic resonance signal information, and the spatial frequency on the K space, which is the basis of image reconstruction by Fourier transform, becomes zero. Along the plurality of acquisition lines passing through the origin, based on the position information, the echo center and the origin are determined for matching the different readout gradient magnetic fields, and based on the matching conditions, The echo center and the origin are made coincident to collect magnetic resonance signals of the pulse sequence.

  In the seventeenth aspect of the invention, before collecting magnetic resonance signal information by a pulse sequence having readout gradient magnetic fields in a plurality of directions of three axes orthogonal to each other, positional information of echo centers of the different readout gradient magnetic fields is collected. , And the coincidence condition for matching the echo center with the origin in the K space is obtained, and based on the coincidence condition, the magnetic resonance signal is collected by a pulse sequence in which the echo center and the origin coincide.

  In the magnetic resonance imaging method according to the eighteenth aspect of the invention, the coincidence condition is determined based on a phase angle of an RF pulse that excites the magnetic resonance signal and a magnitude of a dephase gradient positioned before the readout gradient magnetic field. It is characterized by becoming.

  In the magnetic resonance imaging method according to the eighteenth aspect of the invention, the phase is matched by the phase angle of the RF pulse and the magnitude of the dephase gradient positioned before the readout gradient magnetic field.

  As described above, according to the present invention, the pulse sequence is generated by the correction means for each acquisition line having different magnitude readout gradient magnetic fields, the echo center of the echo included in the magnetic resonance signal information, and the K space. Therefore, the mismatch of these echo centers can reduce band artifacts that occur during image reconstruction, thereby improving image quality degradation due to magnetic field inhomogeneity and the like.

The best mode for carrying out a magnetic resonance imaging apparatus according to the present invention will be described below with reference to the accompanying drawings. Note that the present invention is not limited thereby.
(Embodiment 1)
First, the overall configuration of the magnetic resonance imaging apparatus according to the first embodiment will be described. FIG. 1 shows the overall configuration of the magnetic resonance imaging apparatus. The apparatus has a magnet system 100. The magnet system 100 includes a main magnetic field coil unit 102, a gradient coil unit 106, and an RF (radio frequency) coil unit 108. And it has the gradient drive part 130, RF drive part 140, the data (data) collection part 150, the control part 160, the data processing part 170, the display part 180, and the operation part 190 which drive and control each of these coil parts.

  Here, each coil part has a substantially disk shape and is arranged in pairs one above the other. Further, an imaging space exists at an intermediate position between the upper and lower coil portions, and the cradle 101 and the subject 1 are arranged. The subject 1 is lying on the cradle 101 and is transported to the imaging space together with the cradle 101.

  The main magnetic field coil unit 102 forms a static magnetic field in the imaging space of the magnet system 100. The direction of the static magnetic field is parallel to the vertical direction of the main magnetic field coil section 102 that is disposed one by one in the vertical direction. That is, a so-called vertical magnetic field is formed. The main magnetic field coil unit 102 is configured using a superconducting coil or a permanent magnet.

  The gradient coil section 106 generates three gradient magnetic fields for giving a linear gradient to the static magnetic field strength in three axes orthogonal to each other, that is, in the x, y, and z axis directions shown in FIG. Then, these three gradient magnetic fields are combined, and a slice axis, a phase encode axis, and a frequency encode axis are set in the imaging space to perform imaging. Here, the slice axis, the phase encode axis, and the frequency encode axis may have an arbitrary inclination in the imaging space by combining the gradient magnetic fields of the x, y, and z axes while maintaining the orthogonality between them. it can.

  The gradient magnetic field in the slice axis direction is also called a slice gradient magnetic field, the gradient magnetic field in the phase encode axis direction is also called a phase encode gradient magnetic field, and the gradient magnetic field in the frequency encode axis direction is also called a frequency encode gradient magnetic field. The gradient magnetic field in the direction of the frequency encoding axis makes the phase of the magnetic resonance signal that is separated from the phase of the magnetic resonance signal excited by the subject 1 the same as the phase of the dephasing magnetic field. And a readout gradient magnetic field for refocusing. In order to make it possible to generate such a gradient magnetic field, the gradient coil unit 106 has three gradient coils in the x, y, and z axis directions (not shown). Hereinafter, the gradient magnetic field is also simply referred to as a gradient.

  The RF coil unit 108 forms a high-frequency magnetic field for exciting a magnetization vector (vector) in the body of the subject 1 located in the static magnetic field space. This formation of the high-frequency magnetic field excites magnetic resonance inside the subject 1 and is also referred to as transmission of an RF excitation signal. The RF excitation signal is also referred to as an RF pulse.

  The reception coil unit 109 receives an electromagnetic wave that generates an excited magnetic moment, that is, a magnetic resonance signal. Here, it is preferable that the receiving coil unit 109 is disposed in the vicinity of the subject 1 and receives a magnetic resonance signal emitted from the subject 1 with high sensitivity. The RF coil unit 108 can also serve as a receiving coil.

  A gradient driving unit 130 is connected to the gradient coil unit 106. The gradient driver 130 gives a drive signal to the gradient coil unit 106 to generate a gradient magnetic field. The gradient drive unit 130 has three systems of drive circuits (not shown) corresponding to the three systems of gradient coils in the gradient coil unit 106.

  The RF drive unit 140 is connected to the RF coil unit 108, gives a drive signal to the RF coil unit 108, transmits an RF pulse, and excites a magnetization vector in the body of the subject 1.

  A data collection unit 150 is connected to the reception coil unit 109. The data collection unit 150 performs sampling (sampling) and digitization of the magnetic resonance signal received by the RF coil unit 108 by an A / D (Analog To Digital) converter, and converts the digitized magnetic resonance signal to Digital data is collected in the K space of the data processing unit 170 described later.

  The control unit 160 is connected to the gradient driving unit 130, the RF driving unit 140, and the data collecting unit 150, and controls the gradient driving unit 130 to the data collecting unit 150 to perform imaging. The control unit 160 is configured using a computer or the like, and includes a memory (not shown). The memory stores a pulse sequence (pulse sequence) that is a control program for the control unit 160 and various data. The function of the control unit 160 is realized by the computer executing a program stored in the memory.

  The output side of the data collection unit 150 is connected to the data processing unit 170 and transmits the collected data to the data processing unit 170. The data processing unit 170 is configured using a computer or the like and has a memory (not shown). The memory stores a program for the data processing unit 170 and various data.

  The data processing unit 170 is connected to the control unit 160 and is superordinate to the control unit 160. The function of this apparatus is realized by the data processing unit 170 executing a program stored in the memory.

  The data processing unit 170 stores raw data, which is a magnetic resonance signal collected by the data collecting unit 150, in the K space. This K space is a data space that is the basis for image reconstruction by two-dimensional or three-dimensional Fourier transform, and is a grid in the spatial frequency Kx, Ky, and Kz axis directions corresponding to the x, y, and z axis directions. Data is arranged in a grid. Then, the tomographic image information is generated by Fourier transforming the data in the K space, and this tomographic image information is stored in the memory. The data processing unit 170 also includes interpolation means for generating grid-like data on which Fourier transform is performed from concentric data in the K space acquired by a radial scan described later.

  The data processing unit 170, the control unit 160, the gradient driving unit 130, and the RF driving unit 140 constitute correction means, and the control unit 160 and the RF driving unit 140 adjust the intensity of the RF pulse and also adjust the magnetic resonance frequency. Adjusting means for adjusting the phase of the RF pulse with respect to the basic clock having Thereby, at the time of excitation, the angle of the magnetization vector to be excited with the main magnetic field direction and the phase angle of the magnetization vector to be excited in a plane perpendicular to the main magnetic field direction are adjusted.

  In addition, the control unit 160 and the gradient driving unit 130 constitute an adjusting unit that adjusts the magnitude of the dephase gradient magnetic field. Thereby, the echo center position of the reception echo contained in the magnetic resonance signal is adjusted.

  The display unit 180 and the operation unit 190 are connected to the data processing unit 170 and configured by a graphic display or the like. The operation unit 190 includes a keyboard having a pointing device.

  The display unit 180 displays the reconstructed image and various information output from the data processing unit 170. The operation unit 190 is operated by an operator and inputs various commands and information to the data processing unit 170. An operator operates the apparatus interactively through the display unit 180 and the operation unit 190.

  Here, before describing the operations of the control unit 160 and the data processing unit 170 according to the present invention, a frequency encoding axis including a readout gradient magnetic field is obtained by a plurality of combinations of gradient magnetic fields in the x, y, and z axis directions. The radial scan will be described.

  FIG. 2A shows a pulse sequence called FIESTA or FISP as an example of a pulse sequence used when performing a radial scan. In this figure, each axis of the RF magnetic field, x, y, z gradient magnetic field, and received echo has a common time axis. This pulse sequence is an SSFP that continuously irradiates the subject 1 with RF pulses at a repetition time TR, and makes the magnetization vector excited by the subject 1 in a steady state, and in the x, y, and z-axis directions. Each time, the sum of all gradient magnetic fields is set to zero within the repetition time TR. The example of FIG. 2A is an example of acquiring a tomographic image in the xy plane perpendicular to the z axis.

  FIG. 2B illustrates an example of a magnetic resonance signal acquired in the K space by a scan using the pulse sequence of FIG. Here, since the radial scan is in the xy plane, the horizontal axis of the K space forming the Fourier space is Kx, the vertical axis is Ky, and the received echo acquired by one TR is the origin where the spatial frequency is zero. Present on the collection line through. In the radial scan, magnetic resonance signals are acquired on a plurality of collection lines that exist radially around the origin, and grid-like data with the Kx and Ky directions as indices is calculated from these data by interpolation. Note that the collection line shown in FIG. 2B is a typical one, and the actual number of collection lines has a resolution comparable to the matrix size of the K space (matrix size). To be set.

  Further, in the radial scan of this example, data collection on collection lines in a plurality of directions passing through the origin of the K space is performed by setting the phase encode amount to zero and rotating the frequency encode axis in the xy plane. In the pulse sequence of FIG. 2A, since the frequency encode axis is rotated in the xy plane, the readout gradient magnetic field exists in both the x and y axes, and the magnitude for each TR changes. Note that in FIG. 2A, the dotted line indicates that the magnitude of the readout gradient magnetic field of the x-axis gradient and the y-axis gradient changes for each TR.

  Here, in the radial scan, the magnitude of the readout gradient magnetic field changes in the x-axis and y-axis directions every time TR, so that the effect of magnetic field inhomogeneity superimposed on the gradient magnetic field is also different each time. Note that in grid-like data collection using phase encoding, the frequency encoding axis is always fixed in one axial direction, that is, the x, y, or z axis direction. It will be the same.

  FIG. 3 is observed when the vertical axis is signal intensity and the horizontal axis is a position on the acquisition line with the origin in FIG. 2B as the center, and the pulse sequence shown in FIG. 2A is used. A typical example of a magnetic resonance signal is shown. Here, the magnetic resonance signal includes two types of reception echoes of FID echo (free induction decay echo) and SE / STE echo (Spin Echo / Stimulated Echo). Contains.

  The FID echo is a reception echo observed immediately after excitation, which is excited by an RF pulse before the readout gradient magnetic field. In the example of FIG. 2A, a magnetization vector in the main magnetic field z direction is a magnetization vector excited in the xy plane by an RF pulse in TR. The SE / STE echo is a magnetization vector excited by RF pulses several times before, and the influence of the gradient magnetic field differs from the FID echo. The appearance position of the received echo changes depending on the magnitude of the gradient magnetic field before multiple times. To do. Note that the echo center of the FID echo and the SE / STE echo generally indicates a peak position where the signal intensity of the received echo is maximum.

  Here, the magnetic resonance signal including the FID echo and the SE / STE echo observed during the application of the read gradient magnetic field is −N / 2 as shown in FIG. 3 in the A / D converter of the data acquisition unit 150. ~ N / 2 points are sampled. The sampled magnetic resonance signal information becomes raw data along the collection line in the K space shown in FIG. Note that N is a positive integer, and a value such as N = 128, 256 is used. The center of sampling corresponds to the origin on the K space shown in FIG. 2B, and a component having a spatial frequency of zero appears.

  Next, the operations of the control unit 160 and the data processing unit 170 constituting the correction unit according to the present invention will be described with reference to FIG. FIG. 4 is a flowchart showing operations of the control unit 160 and the data processing unit 170 according to the present invention. First, the operator sets a pulse sequence to be scanned from the operation unit 190 to the control unit 160 via the data processing unit 170 (step S401). Here, a case of a radial scan using FIESTA as shown in FIG. Then, the control unit 160 performs prescan and adjusts the resonance frequency, the gain of the RF driving unit 140, and the like.

  Thereafter, the data processing unit 170 performs matching condition collection processing (step S403). In this matching condition collection process, the echo centers of the FID echo and the SE / STE echo illustrated in FIG. 3 are set as the origin at the time of sampling, that is, the origin in the K space, for each magnitude of the readout gradient magnetic field. Matching conditions consisting of phase angle information of RF pulses and magnitude information of dephase gradient magnetic field to be matched are collected. This matching condition collection process will be described in detail later.

  Thereafter, the control unit 160 performs a main scan using the matching condition for each magnitude of the readout gradient magnetic field acquired in step S403 (step S404). In this main scan, the echo centers of the FID echo and the SE / STE echo all coincide with the origin. In particular, in the case of a radial scan, all echo centers coincide with the origin on all acquisition lines centering on the origin of the K space shown in FIG.

  Thereafter, the data processing unit 170 performs image reconstruction (step S405). In this image reconstruction, tomographic image information is generated by performing Fourier transform on data in the K space. In particular, in the case of a radial scan, since raw data is acquired along a radial collection line centered on the origin shown in FIG. 2B, grid-like data along the Kx and Ky axes is obtained. , Generated by interpolation or the like. Then, a Fourier transform is performed on the interpolation data.

  Thereafter, the data processing unit 170 displays the generated tomographic image information on the display unit 180 (step S406), and interpretation by an operator or the like is performed.

  Next, the matching condition collection processing in step S403 will be described in detail with reference to FIG. FIG. 5 is a flowchart showing the operation of the matching condition collection process. First, the data processing unit 170 calculates the readout gradient magnetic field of the pulse sequence set in step S401 in FIG. 4, and sets the separation pulse sequence using this readout gradient magnetic field in the control unit 160 that is an adjustment unit ( Step S501). Here, an example of the separation pulse sequence is shown in FIG.

  FIG. 6 is an example of a separation pulse sequence when SSFP is used. Here, two types of separation pulse sequences are used, one for FID echo adjustment and one for SE / STE echo adjustment. FIG. 6A shows the FID echo adjustment and FIG. 6B shows the SE / STE echo adjustment.

  FIG. 6A shows a separation pulse sequence for FID echo adjustment, and a crusher 601 that is a separation gradient magnetic field exists on the y-axis. Since the crusher 601 exists after the readout gradient magnetic field, the FID echo excited by the RF pulse is not affected at all, and the phase of the subsequent SE / STE echoes is dispersed and the received echo is extinguished. . Therefore, in the separation pulse sequence of FIG. 6A, only the FID echo is observed at the position of the readout gradient magnetic field.

  FIG. 6B shows a separation pulse sequence for SE / STE echo adjustment, in which a crusher 602 that is a separation gradient magnetic field exists on the y-axis. Since the crusher 602 exists after the RF pulse and before the readout gradient magnetic field, the phase of the FID echo excited by the RF pulse is dispersed and the received echo is extinguished, while the previous RF pulse is used. In the excited SE / STE echo, the gradient magnetic field areas of the crusher 602 cancel each other, and the reception echo appears as usual. Therefore, in the separation pulse sequence of FIG. 6B, only the SE / STE echo is observed at the position of the readout gradient magnetic field. FIG. 6 illustrates the case where the read gradient magnetic field exists only in the x-axis direction for easy understanding.

  Thereafter, returning to FIG. 5, the control unit 160 as the adjusting means performs scanning using the separation pulse sequence as shown in FIGS. 6A and 6B (step S502). Then, the data processing unit 170 obtains the temporal distance between the echo center of the FID echo and the origin of the collection line and the echo center and collection of the SE / STE echo from the acquired FID echo and the received echo of the SE / STE echo. Echo center position information, which is a temporal distance from the line origin, is acquired. Here, the temporal distance between the echo center and the acquisition line origin is obtained from the phase angle between the real component and the complex component of the converted data obtained by Fourier transforming these received echoes.

FIG. 7 shows a method for obtaining the temporal distance between the echo center and the acquisition line origin. FIG. 7A shows an FID echo or an SE / STE echo, and the time difference between the collection line origin and the peak echo center is ΔΦ. FIG. 7B shows the distribution of the phase angle between the real component and the complex component generated when the received echo of FIG. 7A is Fourier-transformed by a solid line. Here, the transition rule of the time axis of the Fourier transform indicates that ΔΦ shown in FIG. 7A is equal to the slope of the phase angle distribution shown in FIG. 7B, that is, the first-order component. Therefore, for example, from the phase angle distribution of FIG. 7B, using the FOV (Field Of View) indicating the imaging range, and the phase angles Φ1 and Φ2 at both ends,
ΔΦ = (Φ2-Φ1) / FOV
Thus, a time difference ΔΦ which is echo center position information is obtained.

  Then, returning to FIG. 5, the data processing unit 170 determines whether the echo center position information exceeds a preset threshold value (step S504). When the echo center position information exceeds the threshold value (Yes at step S504), the echo center position is determined. From the position information, adjustment is performed on the control unit 160, which is an adjustment unit (step S505). Here, when adjusting the echo center of the FID echo, the phase angle of the RF pulse is adjusted by the adjusting means from ΔΦ, and when adjusting the echo center of the SE / STE echo, the adjusting means is used. Adjust the magnitude of the dephase gradient magnetic field. Then, the process proceeds to step S502, the scan by the separation pulse sequence is performed again, and the echo center position information is repeated until it falls within a preset threshold value.

  Further, when the threshold value is not exceeded (No in step S504), the data processing unit 170 assumes that the adjustment to the readout gradient magnetic field is sufficient, and stores the phase angle of the RF pulse and the magnitude of the dephase gradient magnetic field in the memory. Save (step S506). Then, it is determined whether or not the adjustment is performed with the main readout gradient magnetic field having a different size, which is used in the pulse sequence set in step S401 (step S507), and the adjustment is not performed with the main readout gradient magnetic field. In this case (No at Step S507), the process proceeds to Step S501, a main readout gradient magnetic field that is not adjusted is set, and Steps S502 to S505 are repeated. Here, the main readout gradient magnetic field is selected from all readout gradient magnetic fields used in the SSFP radial scan, for example, every several to several tens on the K space shown in FIG. Refers to the collection line specified in.

  When adjustment is performed using the main readout gradient magnetic field (Yes at step S507), the interpolation unit of the data processing unit 170 uses a process such as linear interpolation to perform RF pulse detection for all readout gradient magnetic fields. Adjustment values of the phase angle and the magnitude of the dephase gradient magnetic field are obtained (step S508), and this process is terminated. Then, the process proceeds to step S404 in FIG. 4 to perform the main scan. In step S503, a threshold value is set for the echo center position information, and the adjustment is repeated until the value falls within the threshold value. However, the number of adjustments is determined, and the adjustment for a specific readout gradient magnetic field is terminated when the number of adjustments ends. You can also

  As described above, in the first embodiment, the SSFP radial scan with different readout gradient magnetic field for each acquisition line, and for each acquisition line, the echo center of the FID echo and the SE / STE echo is on the K space. Collecting coincidence conditions that coincide with the origin, and using these coincidence conditions during the main scan, all received echoes in the K space have an echo center at the origin, so they are generated due to magnetic field inhomogeneity, etc. Shift of the echo center of the FID echo and SE / STE echo is prevented, and band artifacts of tomographic image information generated due to this shift are reduced.

  In the first embodiment, the echo center is adjusted with main readout gradient magnetic fields having different sizes. However, all readout gradient magnetic fields can be adjusted to eliminate the interpolation process.

In the first embodiment, an example in which a vertical magnetic field is used for the main magnetic field coil unit 102 has been described. However, a main magnetic field coil unit that similarly generates a horizontal magnetic field may be used.
(Embodiment 2)
By the way, in the first embodiment, the present invention exemplifies a case where a radial scan using SSFP is used. Similarly, in the case of a radial scan and propeller scan using FSE (FastSpinEcho) Can be used. Therefore, in the second embodiment, a case of radial scan using FSE will be exemplified.

  FIG. 8 shows a pulse sequence of radial scan using FSE. Note that the overall configuration of the magnetic resonance imaging apparatus that performs this scan is exactly the same as that of the magnet system 100, and thus description thereof is omitted.

  In FIG. 8, each axis of the RF magnetic field, the x, y, z gradient magnetic field, and the reception echo has a common time axis. In this pulse sequence, a magnetic resonance signal excited by a 90-degree RF pulse is repeatedly phase-inverted by a 180-degree RF pulse, and reception echoes are sequentially acquired. Note that the example of FIG. 8 is an example of acquiring a tomographic image in the xy plane perpendicular to the z axis.

  Further, a dephase gradient magnetic field and a read gradient magnetic field exist between 180-degree RF pulses, and reception echoes are generated. Here, the magnitudes of the dephase gradient magnetic field and the readout gradient magnetic field are different each time. In the example of FIG. 8, the frequency encode axis exists in the xy plane, and the origin in the K space shown in FIG. Magnetic resonance signal information is collected along different radial collection lines at the center. Then, collection using different readout gradient magnetic fields is repeated, and collection of magnetic resonance signal information that substantially covers the K space is performed.

  In a radial scan pulse sequence using FSE, the magnetic resonance signal information includes two received echoes, an SE echo and an STE echo. Note that the FID echo is not used in the FSE and is not observed. In FSE, in the example of FIG. 8, a magnetic resonance signal excited in the xy plane is inverted by a 180-degree RF pulse and observed as an SE echo, while a plurality of 180-degree RF pulses are observed. The STE echo by appears at a position on the time axis different from the SE echo. These echoes are illustrated on the time axis of the reception echo shown in FIG. After the first 180 degree RF pulse, the magnetic resonance signal excited by the 90 degree RF pulse is inverted and appears on the readout gradient magnetic field as the first SE echo. Thereafter, the first SE echo is inverted by the next 180-degree RF pulse, and appears on the next readout gradient magnetic field as the second SE echo.

  Here, the origin on the readout gradient magnetic field corresponding to the origin on the K space does not coincide with the echo centers of the first SE echo center and the second SE echo in most cases. The time difference t1 between the echo center of the first SE echo and the origin and the time difference t2 between the echo center of the second SE echo and the origin are equal in magnitude by t1 = t2 due to the gradient inversion by the 180-degree RF pulse. Appear at symmetrical positions. Thereafter, the echo centers of the odd-numbered SE echo and the even-numbered SE echo always appear at symmetrical positions on the time axis with respect to the origin of the readout gradient magnetic field.

  Further, an STE echo is generated on the readout gradient magnetic field after the second 180-degree RF pulse. Although this echo appears at a position different from the second SE echo, it is affected by the 180-degree RF pulse similarly to the SE echo, and the echo centers of the odd-numbered STE echo and the even-numbered STE echo are always read out by the gradient magnetic field. It appears at a symmetrical position on the time axis with respect to the origin of.

  Next, the operations of the control unit 160 and the data processing unit 170 constituting the correction unit according to the second embodiment will be described with reference to FIG. FIG. 9 is a flowchart illustrating the matching condition collection processing of the control unit 160 and the data processing unit 170 according to the second embodiment. This matching condition collection process corresponds to the matching condition collection process shown in step S403 of FIG. 4, and the other processes are exactly the same as those shown in FIG. .

  First, the data processing unit 170 calculates a readout gradient magnetic field in the FSE radial scan set in step S401 of FIG. 4, and sets a pulse sequence using the readout gradient magnetic field in the control unit 160 (step S901). ).

  Thereafter, the control unit 160 performs scanning (step S902). Then, the data processing unit 170 determines, for example, the temporal distance between the echo center of the odd-numbered SE echo and the collection line origin from the received odd-numbered and even-numbered SE echoes or STE echoes, and even-numbered Echo center position information, which is the time distance between the echo center of the SE echo and the acquisition line origin, is acquired. Here, the temporal distance between the echo center and the acquisition line origin is, for example, the phase between the real component and the complex component of the converted data obtained by Fourier transforming these received echoes, as in the method shown in FIG. Find from the angle. The STE echo is also included at the same time as the SE echo, but the STE echo has a small signal strength and is ignored.

  Then, the data processing unit 170 determines whether or not the echo center position information matches at the odd-numbered and even-numbered SE echo centers (step S904), and if they do not match (No at step S904), the adjusting means. The phase angle of the RF pulse or the magnitude of the dephase gradient magnetic field is adjusted by using the control unit 160, the RF drive unit 140, and the gradient drive unit 130 (step S905). Then, the process proceeds to step S902, the scan by the pulse sequence is performed again, and is repeated until the odd-numbered and even-numbered SE echo center position information matches.

  In addition, when the odd-numbered and even-numbered SE echo center position information matches (Yes in step S904), the data processing unit 170 assumes that the adjustment to the readout gradient magnetic field is sufficient, and the phase angle of the RF pulse Alternatively, the magnitude of the dephase gradient magnetic field is stored in the memory (step S906). The condition that the odd-numbered and even-numbered SE echo center position information matches is that the odd-numbered and even-numbered SE echo center positions do not change, or the odd-numbered and even-numbered SE echo center positions coincide with the origin. It is equivalent to the condition and can be replaced. At this time, the STE echo is also affected by the phase angle of the RF pulse or the dephase gradient magnetic field similar to the SE echo, and the odd-numbered and even-numbered STE echo center position information coincides.

  Then, it is determined whether or not adjustment has been performed with the main readout gradient magnetic field used in the pulse sequence set in step S401 (step S907). Step S907 negative), the process proceeds to Step S901, the main readout gradient magnetic field that is not adjusted is set, and Steps S902 to S906 are repeated. Further, when adjustment is performed with the main readout gradient magnetic field (Yes at Step S907), all of the RF pulse phase angle acquired at Step S906 or the magnitude of the dephase gradient magnetic field is subjected to processing such as interpolation. An adjustment value consisting of the phase angle of the RF pulse or the magnitude of the dephase gradient magnetic field is obtained for the different read gradient magnetic fields (step S908). This process is terminated, the process proceeds to step S404 in FIG. 4, and a main scan is performed.

  As described above, in the second embodiment, the echo centers of the odd-numbered and even-numbered SE echoes coincide with the origin in the K space in the FSE radial scan with different readout gradient magnetic fields for each acquisition line. The conditions are collected, and during the main scan, the received echoes in the K space are assumed to have an echo center at the origin by using these coincidence conditions. The shift of the echo center of the even-numbered SE echo is prevented, and hence the band artifact of the tomographic image information caused by this shift is reduced.

  In the second embodiment, the example of the radial scan using the FSE has been described. Similarly, in the propeller scan using the FSE, the echo center can be made coincident with the origin for each different readout gradient magnetic field.

It is a block diagram which shows the whole structure of a magnetic resonance imaging device. It is a figure which shows the SSFP pulse sequence and K space of Embodiment 1. FIG. 3 is a diagram illustrating a reception echo according to the first embodiment. FIG. 3 is a flowchart showing an overall operation of the first embodiment. 4 is a flowchart illustrating a matching condition collection process according to the first embodiment. FIG. 3 is a diagram showing a separation pulse sequence according to the first embodiment. 6 is a diagram illustrating a method for obtaining a deviation of an echo center according to Embodiment 1. FIG. FIG. 10 is a diagram illustrating an FSE pulse sequence according to the second embodiment. 10 is a flowchart showing a matching condition collection process according to the second embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Subject 101 Cradle 100 Magnet system 102 Main magnetic field coil part 106 Gradient coil part 108 RF coil part 109 Reception coil part 130 Gradient drive part 140 RF drive part 150 Data collection part 160 Control part 170 Data processing part 180 Display part 190 Operation part 601 and 602 crushers

Claims (18)

A series of magnetic resonance signal information collected during the formation of the readout gradient magnetic field based on a pulse sequence having readout gradient magnetic fields in a plurality of axial directions of three axes orthogonal to each other serves as a basis for image reconstruction by Fourier transform. A magnetic resonance imaging apparatus that is a series of data along a collection line passing through an origin on a space where the spatial frequency is zero,
The pulse sequence includes a correcting unit that matches the echo center of the echo included in the magnetic resonance signal information with the origin for each of the plurality of acquisition lines having the readout gradient magnetic field having different sizes. Magnetic resonance imaging apparatus.   The magnetic resonance imaging apparatus according to claim 1, wherein the echoes are FID echoes and SE / STE echoes collected by the pulse sequence using SSFP.   The correction means includes a separation gradient magnetic field that separates and extracts the FID echo and the SE / STE echo, and a readout gradient magnetic field that is the same as the pulse sequence before collecting magnetic resonance signal information using the SSFP. The magnetic resonance imaging apparatus according to claim 2, wherein the FID echo and the SE / STE echo are individually collected by a separation pulse sequence using the SSFP.   The magnetic resonance imaging apparatus according to claim 3, wherein the individual collection is performed on a part of the plurality of collection lines constituting the K space.   The correction means includes an adjustment means for obtaining a matching condition between the FID echo and the SE / STE echo and the origin for the partial collection line, and a matching condition for all the collecting lines based on the matching condition. The magnetic resonance imaging apparatus according to claim 4, further comprising an interpolation unit for obtaining the magnetic resonance imaging apparatus.   6. The separation pulse sequence according to claim 3, wherein, when the coincidence of the FID echoes is performed, the separation gradient magnetic field is positioned after the readout gradient magnetic field of the separation pulse sequence. The magnetic resonance imaging apparatus described.   The magnetic resonance according to claim 6, wherein the adjustment unit obtains a spatial distribution of phase angles by performing a Fourier transform on the FID echo, and further acquires position information of the echo center from an inclination of the spatial distribution. Imaging device.   The magnetic resonance imaging apparatus according to claim 7, wherein the adjustment unit adjusts a phase angle of an RF pulse that excites the magnetic resonance signal.   When the separation pulse sequence performs the coincidence of the SE / STE echoes, the separation gradient magnetic field is set between an RF pulse that excites a magnetic resonance signal of the separation pulse sequence and a readout gradient magnetic field of the separation pulse sequence. The magnetic resonance imaging apparatus according to claim 3, wherein the magnetic resonance imaging apparatus is positioned.   10. The adjustment unit according to claim 9, wherein the adjustment unit obtains a spatial distribution of phase angles by performing a Fourier transform on the SE / STE echo, and further acquires position information of the echo center from an inclination of the spatial distribution. Magnetic resonance imaging device.   The magnetic resonance imaging apparatus according to claim 10, wherein the adjustment unit adjusts a magnitude of a dephase gradient positioned before the readout gradient magnetic field.   The magnetic resonance imaging apparatus according to claim 1, wherein the echo is an SE echo and an STE echo collected by the pulse sequence using FSE.   The correction means is repeatedly generated with a plurality of 180 degree pulses by the pulse sequence using the FSE having the same readout gradient magnetic field as the pulse sequence before collecting the magnetic resonance signal information using the FSE. 13. The magnetic resonance imaging apparatus according to claim 12, wherein position information of the echo centers of the odd-numbered and even-numbered SE echoes or STE echoes is collected.   The magnetic resonance imaging apparatus according to claim 13, wherein the collection is performed on a part of the plurality of collection lines constituting the K space.   The correction means includes an adjustment means for obtaining a matching condition between the odd-numbered and even-numbered SE echoes or STE echoes and the origin in the partial collection lines, and all the collection lines based on the matching conditions. The magnetic resonance imaging apparatus according to claim 14, further comprising an interpolation unit that obtains a matching condition at.   Based on the position information, the adjusting means adjusts the phase angle of the RF pulse that excites the magnetic resonance signal or the readout so that the even-numbered and odd-numbered SE echoes or STE echoes coincide with each other. The magnetic resonance imaging apparatus according to claim 15, wherein the magnitude of a dephasing gradient positioned before the gradient magnetic field is adjusted. Before collecting magnetic resonance signal information by a pulse sequence having readout magnetic fields in a plurality of axial directions of three axes orthogonal to each other,
Obtaining the position information of the echo center related to the echo included in the magnetic resonance signal information of the different readout gradient magnetic field,
The magnetic resonance signal information is stored, and on the K space that is the basis of image reconstruction by Fourier transform, along a plurality of acquisition lines that pass through the origin where the spatial frequency is zero, based on the position information, Find the matching condition for matching the echo center and the origin for each different readout gradient magnetic field,
Collecting the magnetic resonance signals of the pulse sequence by matching the echo center and the origin based on the matching condition;
A magnetic resonance imaging method.   The magnetic resonance imaging method according to claim 17, wherein the matching condition includes a phase angle of an RF pulse that excites the magnetic resonance signal and a magnitude of a dephase gradient positioned before the readout gradient magnetic field. .

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