JP2016131847A - Magnetic resonance imaging apparatus and magnetic resonance imaging method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technology which can select an excitation range, freely set resolution, adjust contrast by TE, does not extend a measurement time, and can reduce noise in an MRI apparatus.SOLUTION: An echo signal is measured according to a sequence that is provided with; a slice excitation part which does not perform reconvergence (Refocus) after high-frequency magnetic field pulse is irradiated during measurement using an MRI apparatus; and an echo signal read-out part which does not perform phase dispersion (Dephase) and performs k space scanning radially from a displaced position of a k space by unperforming reconvergence.SELECTED DRAWING: Figure 3

Description

  The present invention measures a nuclear magnetic resonance signal (hereinafter referred to as an NMR signal) from hydrogen, phosphorus, etc. in an object, and images a nuclear density distribution, relaxation time distribution, etc. (hereinafter referred to as MRI). And a pulse sequence technique for suppressing noise.

  The MRI apparatus measures NMR signals generated by nuclear spins constituting a subject, particularly a human tissue, and images the form and function of the head, abdomen, limbs, etc. two-dimensionally or three-dimensionally. Device. In imaging, a high-frequency magnetic field pulse (RF pulse) is applied together with a gradient magnetic field pulse for slice selection in order to selectively excite a specific region after placing the subject in a static magnetic field (polarization magnetic field B0). The phase or frequency is encoded by applying a gradient magnetic field pulse for readout within the excitation range.

  For application of the gradient magnetic field pulse, a gradient coil designed to generate a gradient magnetic field along the X, Y, and Z axes of the MRI apparatus is used. When the applied intensity such as the rising and falling of the gradient magnetic field pulse changes, the gradient magnetic field coil vibrates, which causes noise.

  In order to reduce this noise, for example, there is a method of applying both the slice selective gradient magnetic field pulse and the readout gradient magnetic field pulse as one gradient magnetic field pulse instead of applying them separately. This is an imaging method called PETRA (Pointwise Encoding Time Reduction with Radial Acquisition) (see, for example, Non-Patent Document 1), which reduces the number of rising and falling times of gradient magnetic field pulses that cause noise. Noise is reduced.

  There is also an imaging method according to an ultra-short TE sequence (UTE) aimed at shortening TE (echo time) (see, for example, Patent Document 1). In the UTE, a slice selection gradient magnetic field pulse that does not require refocus (Refocus) called a Half RF pulse and a read gradient magnetic field pulse that starts scanning from the center of the k space are used. Since scanning is started from the center of the k-space, the readout gradient magnetic field pulse does not require a dephase pulse. Therefore, according to UTE, the number of inversions of the polarity of the gradient magnetic field is reduced, and noise is reduced.

Grodzki DM et al, “Ultrashort echo imaging using point encoding time reduction with radical acquisition (PETRA)” 12 Quantum Resonance.

European Patent Application No. 0495501

  However, in the method of Non-Patent Document 1, the slice selection gradient magnetic field pulse and the read gradient magnetic field pulse are combined into one pulse, and the application direction of these gradient magnetic field pulses is changed in a three-dimensional direction. For this reason, the excitation range becomes a spherical shape having the same size in each of the three-dimensional directions, and the image resolution is limited to those in which the directions of XYZ are the same. Therefore, the amount of data that must be measured is increased compared to a sequence that can limit the excitation range or increase the resolution only in the necessary direction, and the measurement time is extended and the required amount of the reconstruction memory is increased. .

  Further, in the method of Non-Patent Document 1, time is taken by switching of the RF receiving coil between the RF pulse irradiation and the start of echo signal readout. Therefore, since data cannot be collected during that time, data at the center of k-space is lost. The missing data at the center of k-space needs to be measured in a separate scan, leading to further extension of measurement time.

  Furthermore, in the method of Non-Patent Document 1, since the slice selection gradient magnetic field pulse and the read gradient magnetic field pulse are combined into one pulse, the read gradient magnetic field pulse is applied immediately after the irradiation with the RF pulse. For this reason, the TE of the first echo cannot be selected, and the contrast cannot be adjusted by selecting the TE.

  In UTE (Ultra Short TE sequence), the refocusing pulse of the slice selective gradient magnetic field is omitted by using the Half RF pulse. However, when the Half RF pulse is used, an NMR signal having one complete slice profile cannot be obtained unless two data excited by changing the polarity of the slice selective gradient magnetic field are added. Therefore, the measurement time is doubled as compared with the case of using a normal RF pulse.

  As described above, according to the conventional method, the number of gradient magnetic field pulses to be applied is reduced, so that the degree of freedom in setting the excitation range and resolution is limited, and the contrast cannot be adjusted by the selection of TE. The measurement time is extended.

  The present invention has been made in view of the above circumstances. In an MRI apparatus, selection of an excitation range and setting of resolution can be freely performed, contrast adjustment by TE can be performed, measurement time is not extended, and An object is to provide a technique for reducing noise.

  The present invention provides a slice excitation unit that does not apply a refocus pulse after irradiation with a high-frequency magnetic field pulse during measurement using an MRI apparatus, and a position in k space that is displaced by not applying a refocus pulse. The echo signal is measured according to a sequence that includes k-space scanning in a radial manner and an echo signal reading unit that does not apply a phase dispersion (dephase) pulse.

  According to the present invention, the excitation range can be selected, the resolution can be freely set, and the contrast can be adjusted by TE, as in the case of measurement according to the conventional pulse sequence, and the measurement time is extended. Therefore, it is possible to reduce noise during the execution of the pulse sequence.

It is a block diagram of the MRI apparatus of embodiment of this invention. (A) is a functional block diagram of the control system of the embodiment of the present invention, (b) is an explanatory diagram for explaining the processing of the signal processing unit of the embodiment of the present invention. (A) is explanatory drawing for demonstrating the pulse sequence of the conventional non-orthogonal system sequence, (b) is explanatory drawing for demonstrating the pulse sequence of embodiment of this invention. (A) is explanatory drawing for demonstrating the scanning locus | trajectory of k space by the conventional pulse sequence, (b) is description for demonstrating the scanning locus | trajectory of k space by the pulse sequence of embodiment of this invention. FIG. It is explanatory drawing for demonstrating the shape determination method of the slice selection gradient magnetic field pulse of embodiment of this invention.

  Hereinafter, an example of an embodiment of the present invention will be described with reference to the accompanying drawings. Note that in all the drawings for explaining the embodiments of the invention, the same reference numerals are given to components having the same function unless otherwise specified, and the repeated description thereof is omitted.

[MRI system configuration]
First, an overall outline of an example of the MRI apparatus of the present embodiment will be described. FIG. 1 is a block diagram showing the overall configuration of the MRI apparatus 100 of the present embodiment. The MRI apparatus 100 of the present embodiment obtains a tomographic image of a subject using an NMR phenomenon. As shown in FIG. 1, a static magnetic field generation system 120, a gradient magnetic field generation system 130, a transmission system 150, , Receiving system 160, control system 170, and sequencer 140.

  The static magnetic field generation system 120 generates a uniform static magnetic field in the direction perpendicular to the body axis in the space around the subject 101 if the vertical magnetic field method is used, and in the body axis direction if the horizontal magnetic field method is used. The apparatus includes a permanent magnet type, normal conduction type or superconducting type static magnetic field generation source disposed around the subject 101.

  The gradient magnetic field generating system 130 is a gradient magnetic field coil 131 wound in the three-axis directions of X, Y, and Z, which is a coordinate system (apparatus coordinate system) of the MRI apparatus 100, and a gradient magnetic field power source that drives each gradient magnetic field coil. 132, and in accordance with a command from the sequencer 140, the gradient magnetic field power supply 132 of each gradient magnetic field coil 131 is driven to apply gradient magnetic fields Gx, Gy, and Gz in the three axis directions of X, Y, and Z. . The gradient magnetic field strength is changed by controlling the value of the current flowing through the gradient magnetic field coil 131.

  At the time of imaging, for example, a 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, and the remaining two orthogonal to the slice plane and orthogonal to each other The gradient magnetic field pulse Gp and the gradient magnetic field pulse Gf are applied in the direction, and position information in each direction is encoded in the echo signal. Hereinafter, in this specification, the gradient magnetic field pulse applied to determine the slice plane is referred to as a slice selection gradient magnetic field pulse, and the gradient magnetic field applied at the time of echo signal readout is read out to encode the positional information in the echo signal. This is called a magnetic field pulse.

  The transmission system 150 irradiates the subject 101 with a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the living tissue of the subject 101. The transmission processing unit 152 includes a high-frequency oscillator (synthesizer), a modulator, and a high-frequency amplifier, and a high-frequency coil (transmission coil) 151 on the transmission side. The high frequency oscillator generates an RF pulse and outputs it at a timing according to a command from the sequencer 140. The modulator amplitude-modulates the output RF pulse, and the high-frequency amplifier amplifies the amplitude-modulated RF pulse and supplies the amplified RF pulse to the transmission coil 151 disposed in the vicinity of the subject 101. The transmission coil 151 irradiates the subject 101 with the supplied RF pulse.

  The receiving system 160 detects a nuclear magnetic resonance signal (echo signal, NMR signal) emitted by nuclear magnetic resonance of the nuclear spin constituting the living tissue of the subject 101, and receives a high-frequency coil (receiving coil) on the receiving side. 161 and a reception processing unit 162 including a combiner, an amplifier, a quadrature detector, and an A / D converter (A / D converter).

  The reception coil 161 is disposed in the vicinity of the subject 101 and detects the sampling by sampling the NMR signal (reception signal; echo signal) of the response of the subject 101 induced by the electromagnetic wave irradiated from the transmission coil 151. Each sampled echo signal is amplified in the reception processing unit 162, detected at a timing according to a command from the sequencer 140, converted into a digital quantity, and sent to the control system 170 as k-space data.

  The sequencer 140 repeatedly applies an RF pulse and a gradient magnetic field pulse according to a predetermined pulse sequence. The pulse sequence describes the high-frequency magnetic field pulse, the gradient magnetic field pulse, the timing and intensity of signal reception, and is stored in the control system 170 in advance. The sequencer 140 operates in accordance with an instruction from the control system 170 and transmits various commands necessary for collecting tomographic image data of the subject 101 to the transmission system 150, the gradient magnetic field generation system 130, and the reception system 160.

  The control system 170 controls the overall operation of the MRI apparatus 100, performs various operations such as signal processing and image reconstruction, displays and stores processing results, and the like, and includes a CPU 171, a storage device 172, a display device 173, and an input device 174. With. The storage device 172 includes an internal storage device such as a hard disk, a ROM, and a RAM, and an external storage device such as an external hard disk, an optical disk, and a magnetic disk. The display device 173 is a display device such as a CRT or a liquid crystal. The input device 174 is an interface for inputting various control information of the MRI apparatus 100 and control information of processing performed by the control system 170, and includes, for example, a trackball or a mouse and a keyboard. The input device 174 is disposed in the vicinity of the display device 173. The operator interactively inputs instructions and data necessary for various processes of the MRI apparatus 100 through the input device 174 while looking at the display device 173.

  The CPU 171 implements each process and each function of the control system 170 such as control of the operation of the MRI apparatus 100 and various data processing by executing a program stored in advance in the storage device 172 in accordance with an instruction input by the operator. To do. For example, when data from the receiving system 160 is input to the control system 170, the CPU 171 executes processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 101 as a result on the display device 173. At the same time, it is stored in the storage device 172.

  Note that all or a part of the functions of the control system 170 may be realized by hardware such as ASIC (Application Specific Integrated Circuit) or FPGA (Field-programmable gate array). Various data used for processing of each function and various data generated during the processing are stored in the storage device 172.

  In the static magnetic field space of the static magnetic field generation system 120 into which the subject 101 is inserted, the transmission coil 151 and the gradient magnetic field coil 131 are opposed to the subject 101 in the case of the vertical magnetic field method and are in the horizontal magnetic field method. It is installed so as to surround the subject 101. Further, the receiving coil 161 is installed so as to face or surround the subject 101.

  Currently, the nuclide to be imaged by the MRI apparatus, which is widely used clinically, is a hydrogen nucleus (proton) that is a main constituent material of the subject 101. In the MRI apparatus 100, information on the spatial distribution of the proton density and the spatial distribution of the relaxation time of the excited state is imaged so that the form or function of the human head, abdomen, limbs, etc. can be expressed two-dimensionally or three-dimensionally. Take an image.

[Control system]
In the present embodiment, the number of gradient magnetic field pulse intensity changes is suppressed in order to reduce noise during the execution of the pulse sequence. In order to realize this, measurement is performed according to a pulse sequence in which a refocusing (refocus) pulse of a slice selection gradient magnetic field pulse and a phase dispersion (dephase) pulse of a read gradient magnetic field pulse are not applied.

  At this time, since the application of the refocus pulse of the slice selective gradient magnetic field pulse is not performed, an offset (displacement; shift) occurs in the kz direction. In the present embodiment, the k-space is scanned starting from this displacement position.

  In order to perform such control and obtain an image, the control system 170 of the present embodiment, as shown in FIG. 2A, follows a predetermined pulse sequence, a gradient magnetic field generation system 130, a transmission system 150, a reception system. 160, applying a gradient magnetic field pulse and a high-frequency magnetic field pulse, sampling an echo signal, and a signal processing unit 420 for obtaining a reconstructed image from the echo signal sampled by the measurement control unit 410. Prepare.

  First, the pulse sequence that the measurement control unit 410 follows will be described.

[Conventional pulse sequence]
Prior to the description of the pulse sequence of this embodiment, a conventional pulse sequence for applying a refocus pulse of a slice selective gradient magnetic field pulse and a dephase pulse of a read gradient magnetic field pulse will be described.

  Here, as a measurement for obtaining an image, along a plurality of non-orthogonal coordinate system scanning trajectories obtained by rotating one non-orthogonal coordinate system scanning trajectory at different angles around a predetermined reference point in k-space. The radial sampling method in the non-orthogonal sampling method for measuring the echo signal is used. In the radial sampling method, echo signals are measured radially while changing the rotation angle around the origin of k-space.

  FIG. 3A illustrates a conventional pulse sequence 200a. This pulse sequence 200 a includes an excitation radio frequency (RF) pulse 201, a slice selection gradient magnetic field pulse 202 applied together with the excitation RF pulse 201, and a slice refocus pulse 202 r applied immediately after the slice selection gradient magnetic field pulse 202. , A dephase pulse 203d, and a readout gradient magnetic field pulse 203 applied immediately after the dephase pulse 203d.

  The excitation RF pulse 201 is a selective excitation RF pulse that excites nuclear spins in the imaging plane. Here, a case where a pulse whose waveform is symmetrical in the time direction is used as the excitation RF pulse 201 will be described as an example.

  The slice selection gradient magnetic field pulse 202 is a gradient magnetic field pulse for slice selection. The slice refocus pulse 202r is a gradient magnetic field pulse for returning (refocusing) the phase of the spin diffused by the slice selective gradient magnetic field pulse 202. The dephase pulse 203d is a gradient magnetic field pulse that disperses the spin phase in advance in order to adjust the convergence timing of the echo signal.

  The read gradient magnetic field pulse 203 is a pulse applied to give (encode) position information to the echo signal 205. Here, a case where a trapezoidal wave is used will be described as an example. The read gradient magnetic field pulse 203 is applied in a different direction for each TR using a gradient magnetic field coil 131 in three directions of X, Y, and Z. At the timing when the readout gradient magnetic field pulse 203 is applied, an NMR signal (echo signal) 205 is obtained. A scanning trajectory 300a in the k space by the readout gradient magnetic field pulse 203 applied in the pulse sequence 200a shown in FIG. 3A is indicated by an arrow in FIG.

  The gradient magnetic field pulse 204 is a rewinder gradient magnetic field pulse for compensating for the action of the read gradient magnetic field pulse 203, and is applied as necessary.

  According to the conventional pulse sequence 200a, substantially the entire k space is scanned. However, as the gradient magnetic field pulse, at least four gradient magnetic field pulses, the slice selective gradient magnetic field pulse 202, the slice refocus pulse 202r, the dephase pulse 203d, the readout gradient magnetic field pulse 203, are applied.

  When a current is passed through the gradient coil 131 in a static magnetic field, a force is applied to the gradient coil 131 by Lorentz force. When the strength and direction of the current flowing through the gradient coil 131 change, the strength and direction of the force applied to the gradient coil 131 also change. As a result, the gradient magnetic field coil 131 vibrates, and the vibration is transmitted to surrounding structures and air to generate noise. Since the Lorentz force generated in the gradient magnetic field coil 131 is proportional to the static magnetic field strength, the noise during execution of the pulse sequence increases as the MRI apparatus has a higher static magnetic field strength. This noise causes physical and mental discomfort to the subject (patient).

[Pulse sequence of this embodiment]
In the present embodiment, in order to reduce noise during the execution of the pulse sequence, the intensity of the current flowing through the gradient magnetic field coil 131 and the change in direction are reduced as much as possible. For this reason, in the pulse sequence of the present embodiment, the slice refocus pulse 202r and the dephase pulse 203d in the pulse sequence 200a are not applied.

  A pulse sequence 200 of the present embodiment is shown in FIG. The pulse sequence 200 of the present embodiment applies a slice refocus pulse 202r for refocusing the nuclear spin after irradiating the excitation RF pulse 201 and the slice selective gradient magnetic field pulse 202 to excite the nuclear spin in the imaging slice. And a readout period in which the echo signal 205 is sampled while applying the readout gradient magnetic field pulse 203 without applying the dephase pulse 203d for dispersing the phase of the nuclear spin.

  Note that the pulse sequence 200 of the present embodiment is the same as the conventional pulse sequence 200a except that the slice refocus pulse 202r and the dephase pulse 203d are not applied as described above.

  The measurement control unit 410 according to the present embodiment controls each unit according to the pulse sequence 200. A scanning trajectory 300 in the k space of the echo signal by this pulse sequence 200 is indicated by an arrow in FIG.

  In the pulse sequence 200 of the present embodiment, the slice refocus pulse 202r is not applied as described above. For this reason, the k-space scanning trajectory 300 is shifted in the kz direction, which is the slice direction (an offset 403 is generated). Further, since the dephase pulse 203d is not applied, scanning is performed radially in the three-dimensional direction, starting from a point offset in the kz direction.

  Hereinafter, the position where the scanning locus 300 of the present embodiment is the starting point, that is, the position where the scanning locus 300 in the k space is shifted from the k space origin 301 by not applying the slice refocus pulse 202r, is referred to as the k space reference position 303. Call it. The difference between the k-space origin 301 and the k-space reference position 303 is called a shift amount.

[Signal Processing Unit 420]
The signal processing unit 420 reconstructs an image from echo signals sampled on the k-space scanning locus 300 in accordance with the pulse sequence 200 described above. In order to realize this, the signal processing unit 420 uses the measurement condition 401 and the RF waveform information 402 as shown in FIG. 2B, and the k-space reference position 303 by not applying the slice refocus pulse 202r. A shift amount calculation unit 421 that calculates a shift amount (offset) 403 in the kz direction from the k-space origin 301, and each echo signal 205 sampled by the measurement control unit 410 according to the shift amount 403 is arranged in k space. In accordance with the k-space coordinate calculation unit 422 that calculates the k-space coordinates 404 of the space data and the calculated k-space coordinates 404, each k-space data is rearranged in the orthogonal coordinate system k-space, and the k-space data after the rearrangement An image reconstruction unit 423 for obtaining a reconstructed image.

  The measurement conditions 401 and the RF waveform information 402 are stored in advance in, for example, a RAM or the like in the storage device 172. Similarly, the k-space data group 405 obtained by sampling the echo signal 205, the calculated kz-direction shift amount 403, and the k-space coordinates 404 are also stored in the storage device 172, for example, in a RAM or the like. The

  The RF waveform information 402 includes the shape of the excitation RF pulse 201, the shape of the slice selection gradient magnetic field pulse 202, and the ratio of the application area of the slice refocus pulse 202r to the application area of the slice selection gradient magnetic field pulse 202. Since the application area of the slice refocus pulse 202r changes in proportion to the intensity (Amplitude) of the slice selection gradient magnetic field pulse 202, the RF waveform information 402 is held in advance and used to calculate the application area of the slice refocus pulse 202r. .

  For example, if the waveform of the excitation RF pulse 201 to be irradiated is symmetrical in the time direction, the application area of the gradient magnetic field pulse by the slice refocus pulse 202r is about half of the application area of the slice selection gradient magnetic field pulse 202. Therefore, in this case, 0.5 is stored. The application area of the slice refocus pulse 202r is finely adjusted and optimized by experiment or simulation.

  Details of the processing of each unit will be described below.

[Shift amount calculation unit 421]
First, shift amount calculation processing by the shift amount calculation unit 421 will be described. The shift amount calculation unit 421 calculates the shift amount (kz direction shift amount) 403 in the kz direction of the k space data group 405 on the k space using the measurement condition 401 and the RF waveform information 402.

  The shift in the kz direction occurs because the slice refocus pulse 202r is not applied. Therefore, the shift amount calculation unit 421 calculates the application amount (application area) of the slice refocus pulse 202r in the conventional pulse sequence 200a, and calculates the shift amount 403 in the kz direction based on the application amount. That is, the shift amount calculation unit 421 uses the application area of the refocus pulse 202r necessary for refocusing the nuclear spin excited by the application of the excitation RF pulse 201 and the slice selective gradient magnetic field pulse 202, and uses the shift amount. Is calculated.

  The shift amount calculation unit 421 first calculates the waveform (application area) of the slice selection gradient magnetic field pulse 202 in order to calculate the application area of the slice refocus pulse 202r. The application area of the slice selective gradient magnetic field pulse 202 is determined by the waveform of the excitation RF pulse 201 and a desired slice position. Therefore, first, the waveform of the excitation RF pulse 201 is obtained.

  In general, the shape of the waveform of the excitation RF pulse 201 is determined in advance according to the measurement condition 401. Therefore, the shift amount calculation unit 421 uses the predetermined shape as the waveform of the excitation RF pulse 201.

  The waveform of the slice selective gradient magnetic field pulse 202 is calculated by a known technique. Specifically, as illustrated in FIG. 5, the shift amount calculation unit 421 determines the duration 212 of the slice selection gradient magnetic field pulse 202 from the irradiation time of the excitation RF pulse 201, and the intensity 222. Is determined by the irradiation frequency of the excitation RF pulse 201 and the desired slice excitation position.

  As described above, the slice refocus pulse 202r is a gradient magnetic field pulse applied to make the phase of the nuclear spin in the slice direction (kz direction) constant after the slice excitation. The shift amount calculation unit 421 calculates the application area of the slice refocus pulse 202r according to the RF waveform information 402 using the calculated application area of the slice selection gradient magnetic field pulse 202.

  As described above, the shift amount calculation unit 421 obtains the waveform of the excitation RF pulse 201 from the measurement condition 401, and determines the slice selection gradient magnetic field pulse 202 from the irradiation time, irradiation frequency, and desired slice excitation position of the excitation RF pulse 201. Obtain the applied area. Then, the application area of the slice refocus pulse 202r is obtained from the application area of the slice selective gradient magnetic field pulse 202 and the RF waveform information 402.

ここで、γは磁気回転比［Ｈｚ／Ｔ］である。 Then, the shift amount calculation unit 421 calculates the shift amount 403 in the kz direction from the application area of the slice refocus pulse 202r. When the application area of the slice refocus pulse 202r is G_refocus [s · T / m], the shift amount 403 in the kz direction is calculated according to the following equation (1).

Here, γ is the magnetic rotation ratio [Hz / T].

[K-space coordinate calculation unit 422]
The k-space coordinate calculation process by the k-space coordinate calculation unit 422 will be described. The k-space coordinate calculation unit 422 uses the kz-direction shift amount 403 calculated by the shift amount calculation unit 421 and the measurement condition 401 to place each echo signal measured by the measurement control unit 410 as k-space data. Is calculated.

ここで、ｔは時間［ｓ］、ｘ、ｙ、ｚは傾斜磁場の印加軸、ｋｘ（ｔ）、ｋｙ（ｔ）、ｋｚ（ｔ）は各軸方向のｋ空間座標、ｉはデータ番号を示し、Ｇ＿ｒｅａｄ（ｘ，ｉ）、Ｇ＿ｒｅａｄ（ｙ，ｉ）、Ｇ＿ｒｅａｄ（ｚ，ｉ）は、ｉ番目のエコー信号サンプリング時の、各軸方向の読み出し傾斜磁場パルス２０３の印加面積を表す。 The k-space coordinates (kx (t), ky (t), kz (t)) where the echo signal measured (sampled) at time t is arranged are expressed by the following equation using the application area of the read gradient magnetic field pulse 203. Calculated in (2). The application area of the read gradient magnetic field pulse 203 is stored as the measurement condition 401. In general, the waveform (intensity Amplitude and duration Duration) of the read gradient magnetic field pulse 203 is held as the measurement condition 401.

Here, t is time [s], x, y, and z are gradient magnetic field application axes, kx (t), ky (t), and kz (t) are k-space coordinates in each axis direction, and i is a data number. G_read (x, i), G_read (y, i), and G_read (z, i) represent application areas of the readout gradient magnetic field pulses 203 in the respective axial directions at the time of sampling the i-th echo signal.

  As shown in Expression (2), kx (t) and ky (t) depend only on the shape (application area) of the read gradient magnetic field pulse 203. On the other hand, kz (t) is obtained by adding a kz-direction shift amount 403 to the shape (application area) of the read gradient magnetic field pulse 203.

[Image reconstruction unit 423]
An image reconstruction process performed by the image reconstruction unit 423 will be described. The image reconstruction unit 423 executes the k-space coordinates 404 calculated by the k-space coordinate calculation unit 422, the measurement condition 401, the pulse sequence 200, and the k-space data group 405 obtained by sampling the echo signal 205. The image is reconstructed using.

  The k-space data group 405 arranged in the k-space is rearranged (griding) at each lattice point of the orthogonal coordinates in the k-space according to the respective k-space coordinates. That is, k-space data for each lattice point is calculated. The k-space data at each grid point is calculated by a known technique such as interpolation. Then, the image reconstruction unit 423 reconstructs an image by, for example, Fourier transforming k-space data of each grid point after gridding.

  As described above, the MRI apparatus of the present embodiment applies the gradient magnetic field pulse and the high frequency magnetic field pulse 201 according to the predetermined pulse sequence 200 and samples the echo signal, and the measurement control unit 410. A signal processing unit 420 that obtains a reconstructed image from the sampled echo signal, and the pulse sequence 200 irradiates the high-frequency magnetic field pulse 201 and the gradient magnetic field pulse 202 for slice selection in the imaging slice. After excitation of a nuclear spin, a slice excitation period in which a slice refocus pulse 202r for refocusing the nuclear spin is not applied, and a dephasing pulse 203d for dispersing the phase of the nuclear spin is not applied, and reading is performed While applying the gradient magnetic field pulse 203 And a readout period for sampling the issue 205.

  Further, the measurement control unit 410 rotates a single non-orthogonal coordinate system scanning locus around the k-space reference position at different angles from a predetermined k-space reference position during the readout period. The echo signal 205 is sampled along the orthogonal coordinate system scanning locus 300, and the k-space reference position 303 is a position shifted from the k-space origin 301 by not applying the slice refocus pulse 202r.

  Thus, the pulse sequence 200 of the present embodiment is obtained by removing the slice refocus pulse 202r and the dephase pulse 203d from the conventional pulse sequence 200a for sampling the k-space radially. Accordingly, since the gradient magnetic field pulses are not used, noise associated with application of the gradient magnetic field pulses is reduced as compared with the conventional pulse sequence 200a.

  In the measurement of the present embodiment, the slice selection gradient magnetic field pulse 202 applied in a fixed direction and the excitation RF pulse 201 that is a selective excitation pulse are used, so that the excitation range is limited to a desired range. it can. Different image resolutions can be specified in the X, Y, and Z directions by changing the intensity or application time of the readout gradient magnetic field pulse 203.

  In the method of Non-Patent Document 1, the excitation range is a sphere having the same size in the three-dimensional direction, and the resolution must be equal in each direction. On the other hand, in the present embodiment, the excitation range can be limited to a desired range, and only the required range can be measured. Further, the resolution in a desired direction can be increased or decreased. For this reason, an image with a desired image quality can be obtained without extending the measurement time and with the minimum necessary capacity of the reconstruction memory.

  In the method of Non-Patent Document 1, since the slice selection gradient magnetic field pulse and the readout gradient magnetic field pulse are combined into one pulse, the application timing of the readout gradient magnetic field pulse cannot be arbitrarily set, and the TE of the first echo can be set. Cannot be selected. On the other hand, in this embodiment, since the slice selection gradient magnetic field pulse 202 and the read gradient magnetic field pulse 203 are separated, the application timing of the read gradient magnetic field pulse 203 can be arbitrarily set. That is, the TE can be set arbitrarily, and the contrast can be adjusted.

  In the UTE, since the half RF pulse is used, the measurement time is doubled. However, in the present embodiment, the normal selective excitation pulse is used as the excitation RF pulse 201, and therefore the measurement time caused by the RF pulse is used. There will be no extension.

  Thus, according to the present embodiment, noise can be reduced without various restrictions by the conventional method.

<Modification>
According to the pulse sequence 200 of the present embodiment, the k space is sampled along the scanning locus 300 shown in FIG. As can be seen from this figure, according to the pulse sequence 200, the defect region 310 in which the k space is not filled with the k space data due to the shift of the scanning trajectory in the kz direction that occurs because the slice refocus pulse 202r is not applied. Occurs.

  In the above-described embodiment, the case where the image reconstruction is performed with the defect region 310 as it is has been described as an example. However, in order to suppress a reduction in image resolution, the image reconstruction unit 423 may be configured to fill the k-space data of the missing region 310 before image reconstruction.

  Filling the deficient region 310 with k-space data can be performed using a known technique.

For example, there is a technique for filling the defect region 310 using the fact that k-space data in the k-space has a complex conjugate relationship. When the reconstructed image data is a real number, the k-space data on the k-space has a complex conjugate relationship as shown in the following equation (3).

  Because of this property, if k-space data is arranged only in a half area in k-space (a region in two quadrants not on the diagonal), Equation (3) is used from the k-space data F (kx, ky). Thus, the remaining F (−kx, −ky) can be calculated.

Details of the present technology are disclosed in Non-Patent Document 2 below.
[Non-Patent Document 2] David A. A. Feinberg et al. , Halving MR Imaging Time by Conjugation: Demonstration at 3.5kG, Radiology, 1986, Vol 161, No2, p527-531.

  As described above, in the method of Non-Patent Document 1, the center data of the k-space is missing, and it is necessary to measure the area by another scan, so that the measurement time is extended accordingly. On the other hand, in this embodiment, when the missing area 310 is other than the center of the k space and is equal to or less than half of the entire k space area, it can be estimated and filled by calculation. Therefore, the measurement time is not extended.

  In the above embodiment, the waveform of the excitation RF pulse 201 is symmetrical in the time direction. However, the excitation RF pulse 201 having an asymmetric waveform in the time direction may be used. In this case, the application area of the slice refocus pulse 202r changes according to the asymmetry rate of the excitation RF pulse 201.

  In the above embodiment, when calculating the k-space coordinates, the k-space coordinate calculation unit 422 uses the application area of the read gradient magnetic field pulse 203 as it is obtained from the measurement condition 401. However, it is not limited to this.

  For example, the k-space coordinate calculation unit 422 uses the waveform of the readout gradient magnetic field pulse 203 acquired from the measurement condition 401 as an input waveform, calculates an output waveform using a response function, and calculates k-space coordinates using the output waveform. It may be calculated.

  The response function is the sum of the element response functions of a plurality of elements that affect the output gradient magnetic field waveform, and the gradient magnetic field waveform input according to the designation in the measurement condition 401 under a predetermined condition, and the input An output gradient magnetic field waveform corresponding to the gradient magnetic field waveform is calculated and calculated using both.

  Examples of the plurality of elements that influence the output gradient magnetic field waveform include an eddy current, a gradient magnetic field power supply control circuit, and the like.

  The output waveform calculated using the response function is closer to the shape of the readout gradient magnetic field pulse 203 applied during actual imaging. Therefore, by using the calculated output waveform, k-space coordinates can be calculated more accurately, and more accurate rearrangement processing can be performed. As a result, the accuracy of the obtained reconstructed image is increased.

  Moreover, although the case where the readout gradient magnetic field pulse 203 is a trapezoidal wave has been described as an example, the present invention is not limited to this. For the readout gradient magnetic field pulse 203, for example, a waveform other than the trapezoidal wave used in spiral scan or the like may be used.

  Further, the echo signal 206 generated when the rewinder gradient magnetic field pulse 204 is applied may be used as the second contrast for image reconstruction. Further, when the TR is sufficiently long, the rewinder gradient magnetic field pulse 204 may not be applied.

  100: MRI apparatus, 101: subject, 120: static magnetic field generation system, 130: gradient magnetic field generation system, 131: gradient magnetic field coil, 132: gradient magnetic field power supply, 140: sequencer, 150: transmission system, 151: transmission coil, 152: Transmission processing unit, 160: Reception system, 161: Reception coil, 162: Reception processing unit, 170: Control system, 171: CPU, 172: Storage device, 173: Display device, 174: Input device, 200: Pulse sequence , 200a: pulse sequence, 201: excitation RF pulse, 202: slice selective gradient magnetic field pulse, 202r: slice refocus pulse, 203: readout gradient magnetic field pulse, 203d: dephase pulse, 204: rewinder gradient magnetic field pulse, 205: echo Signal: 206: Echo signal, 300: Scanning locus, 300a: Running Trajectory, 301: k-space origin, 303: k-space reference position, 310: missing area, 401: measurement conditions, 402: RF waveform information, 403: kz direction shift amount (offset), 404: k-space coordinates, 405: k Spatial data group 410: Measurement control unit 420: Signal processing unit 421: Shift amount calculation unit 422: k-space coordinate calculation unit 423: Image reconstruction unit

Claims (7)

A measurement control unit that applies a gradient magnetic field pulse and a high-frequency magnetic field pulse according to a predetermined pulse sequence and samples an echo signal;
A signal processing unit that obtains a reconstructed image from the echo signal sampled by the measurement control unit, and
The pulse sequence is
After irradiating the high-frequency magnetic field pulse and the gradient magnetic field pulse for slice selection, a slice excitation period in which no refocus pulse is applied,
And a readout period in which the echo signal is sampled while applying the gradient magnetic field pulse for readout without applying a dephase pulse. The magnetic resonance imaging apparatus according to claim 1,
The measurement control unit includes a plurality of non-orthogonal coordinate systems obtained by rotating one non-orthogonal coordinate system scanning locus around the k-space reference position at different angles from a predetermined k-space reference position in the readout period. Sampling the echo signal along the scanning trajectory,
The magnetic resonance imaging apparatus, wherein the k-space reference position is a position shifted from the origin of the k-space by not applying the refocus pulse. The magnetic resonance imaging apparatus according to claim 2,
The signal processing unit
A shift amount calculation unit that calculates a shift amount of the k space reference position from the k space origin;
A k-space coordinate calculating unit that calculates k-space coordinates of each echo signal sampled by the measurement control unit according to the shift amount;
An image reconstruction unit that rearranges each echo signal in an orthogonal coordinate system k-space according to the k-space coordinates, and obtains the reconstructed image from the rearranged echo signals. apparatus. The magnetic resonance imaging apparatus according to claim 3,
The magnetic resonance imaging apparatus, wherein the image reconstruction unit estimates an echo signal of a defect region due to the shift of the k-space reference position, and obtains the reconstructed image using the estimated echo signal. The magnetic resonance imaging apparatus according to claim 3 or 4,
The shift amount calculation unit calculates an application area of the refocus pulse necessary for refocusing the nuclear spins excited by applying the high-frequency magnetic field pulse and the gradient magnetic field pulse for slice selection. A magnetic resonance imaging apparatus characterized by being calculated using Using a magnetic resonance imaging apparatus, a plurality of non-orthogonal coordinates obtained by rotating one non-orthogonal coordinate system scanning locus around the k-space reference position in k-space at different angles from a predetermined k-space reference position A magnetic resonance imaging method for sampling an echo signal along a system scanning trajectory and obtaining a reconstructed image using the echo signal,
A measurement step of sampling the echo signal while applying the gradient magnetic field pulse for reading without applying the refocus pulse and the dephasing pulse after irradiating the high frequency magnetic field pulse and the gradient magnetic field pulse for slice selection,
A shift amount calculating step of calculating a shift amount of the k-space reference position from the k-space origin by not applying the refocus pulse;
A k-space coordinate calculating step of calculating k-space coordinates of each sampled echo signal according to the shift amount;
An image reconstruction step of rearranging each echo signal in an orthogonal coordinate system k-space according to the k-space coordinates and obtaining the reconstructed image from the rearranged echo signals. Method. The magnetic resonance imaging method according to claim 6, comprising:
The image reconstruction step estimates an echo signal of a defect region due to the shift of the k-space reference position from the rearranged echo signal, and obtains the reconstructed image using the estimated echo signal. A magnetic resonance imaging method.

Images