JP2014087442A5 - - Google Patents

Description

Flip angle control method of refocusing RF pulse and magnetic resonance imaging apparatus

The present invention measures nuclear magnetic resonance (Nuclear Magnetic Resonance: hereinafter referred to as NMR) signals from hydrogen, phosphorus, etc. in a subject and images nuclear density distribution and relaxation time distribution (Magnetic Magnetic Imaging). The present invention relates to a Resonance Imaging (hereinafter referred to as MRI) apparatus, and more particularly to control of a flip angle of a refocus RF pulse in a fast spin echo (hereinafter referred to as FSE) sequence.

MRI equipment measures NMR signals (echo signals) generated by nuclear spins that make up the body of a subject , especially the human body, and forms the shape and function of the head, abdomen, limbs, etc. two-dimensionally or three-dimensionally. It is a device that automatically images. In imaging, the echo signal is given different phase encoding depending on the gradient magnetic field and is frequency-encoded and measured as time-series data. The measured echo signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  The FSE sequence, which is one of the imaging sequences that measure the echo signal, is generally applied to an excitation RF pulse with a flip angle of 90 ° (excitation RF pulse) and then a plurality of flip angles with a 180 ° flip angle. By applying a refocusing (refocusing) RF pulse, many echo signals can be acquired within the repetition time TR.

  However, in a high magnetic field MRI system where the static magnetic field strength exceeds 1.5T, the frequency of the RF pulse increases, so the specific absorption rate (Specific Absorption Rate: the amount of energy absorbed per unit mass of the human body) (Hereinafter referred to as SAR) increases. In particular, in the FSE sequence, 180 ° RF pulses are frequently used in a short time, so it is easy to exceed the SAR limit value.

  For safety reasons, the upper limit of the average value of SAR within a given time (time average SAR) is defined in IEC standards (IEC 60601-2-33: Particular requirements for the safety of magnetic resonance equipment for medical diagnosis) Has been. For example, as the upper limit value of whole body SAR, which is the SAR for the whole body, the 10-second average value and the 6-minute average value of SAR are set to 12 [W / kg] and 4 [W / kg], respectively.

  Therefore, in order to reduce the SAR in the FSE sequence, a technique called variable refocusing FA (Variable Refocus Flip Angle) (hereinafter referred to as VRFA) is applied in which the refocusing RF pulse is applied with a flip angle different from 180 °. (For example, Patent Document 1). By applying VRFA, the echo signal obtained is a mixture of T1 relaxation and T2 attenuation, but by appropriately controlling the flip angle in consideration of the T1 and T2 values of the imaging target, the echo signal Attenuation can be delayed.

US Patent No. 7671590 Japanese Patent Application No. 2010-532950

  VRFA is an indispensable technique when performing FSE sequences with a high-field MRI system, but there remains a problem in the image where the contrast is a mixture of T1 relaxation and T2 attenuation.

  Therefore, an object of the present invention is to perform re-scanning in the FSE sequence, which can acquire an image having the same contrast as the conventional FSE sequence in which the flip angle of the refocusing RF pulse is constant 180 ° while reducing the SAR. It is to provide a flip angle control method and MRI apparatus for focused RF pulses.

  To achieve the above goal, the present invention provides a 180 ° -FSE sequence with a refocus RF pulse with a flip angle of 180 ° and a VRFA- with a refocus RF pulse with a variable flip angle with a flip angle of 180 ° or less. When imaging using an imaging sequence that is combined with an FSE sequence, the 180 ° -FSE sequence is used to measure the echo signal to fill the low-frequency region of k-space, and the high-frequency region of k-space is filled VRFA-FSE sequence is used to measure the echo signal.

  According to the flip angle control method and MRI apparatus of the refocus RF pulse of the present invention, an image having a contrast equivalent to that of a conventional FSE sequence in which the flip angle of the refocus RF pulse is constant 180 ° while reducing the SAR. It becomes possible to acquire.

The figure which shows the whole MRI apparatus basic composition which concerns on this invention Sequence chart showing the application timing of each RF pulse and the measurement timing of each echo signal in a 180 ° -FSE sequence using a refocus RF pulse with a flip angle of 180 ° Sequence chart showing application timing of each RF pulse and measurement timing of each echo signal in VRFA-FSE sequence 300 using VRFA refocus RF pulse Functional block diagram of the first embodiment Example of dividing a two-dimensional k-space into three regions Flow chart showing the processing flow of the first embodiment Example of dividing a 3D k-space into low and high frequency regions Sequence chart showing 180 ° -FSE sequence of Example 2 Sequence chart showing 180 ° -FSE sequence of Example 3 Sequence chart showing SE sequence of Example 4

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

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

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

Static magnetic field generating magnet 102, if vertical magnetic field type in the direction perpendicular to the body axis of the subject 101, in the body axis direction if the horizontal magnetic field type, intended to generate a uniform static magnetic field, respectively, the test A permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the person 101.

  The gradient magnetic field coil 103 is a coil wound in the three-axis directions of X, Y, and Z that are the real space coordinate system (stationary coordinate system) of the MRI apparatus, and each gradient magnetic field coil is a gradient magnetic field that drives it. A current is supplied to the power source 109. Specifically, the gradient magnetic field power supply 109 of each gradient coil is driven according to a command from the measurement control unit 111 described later, and supplies a current to each gradient coil. Thereby, gradient magnetic fields Gx, Gy, and Gz are generated in the three-axis directions of X, Y, and Z.

When imaging a two-dimensional slice plane, a slice gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 101, orthogonal to the slice plane and to each other. Phase encoding gradient magnetic field pulse (Gp) and frequency encoding (leadout) gradient magnetic field pulse (Gf) are applied in the remaining two orthogonal directions, and position information in each direction is encoded in the echo signal (echo signal). The

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

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

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

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

The measurement control unit 111 sends various commands for collecting echo data necessary for reconstruction of the tomographic image of the subject 101 mainly to the gradient magnetic field power source 109, the RF transmission unit 110, and the signal processing unit 107. It is a control part which transmits and controls these. Specifically, the measurement control unit 111 operates under the control of the overall control unit 112 described later, and controls the gradient magnetic field power source 109, the RF transmission unit 110, and the signal processing unit 107 based on a predetermined sequence of control data. and, the application of radiation and the gradient pulses of RF pulses to a subject 101, perform iterative detection and echo signals from the subject 101, a re-image of the imaging region of the subject 101 Control the collection of echo data required for configuration. In the repetition, the application amount of the phase encoding gradient magnetic field is changed in the case of two-dimensional imaging, and the application amount of the slice encoding gradient magnetic field is further changed in the case of three-dimensional imaging. Values such as 128, 256, and 512 are normally selected as the number of phase encodings, and values such as 16, 32, and 64 are normally selected as the number of slice encodings. With these controls, echo data from the signal processing unit 107 is output to the overall control unit 112.

  The overall control unit 112 controls the measurement control unit 111 and controls various data processing and processing result display and storage, and includes an arithmetic processing unit (CPU) 114, a memory 113, and a magnetic disk. And the like, and a network IF 116 that interfaces with an external network. Further, an external storage unit 117 such as an optical disk may be connected to the overall control unit 112.

  Specifically, when the measurement control unit 111 collects echo data by executing an imaging sequence and the echo data is input from the measurement control unit 111, the arithmetic processing unit 114 converts the encoded information applied to the echo data. Based on this, it is stored in an area corresponding to the k space in the memory 113. Hereinafter, the statement that the echo data is arranged in the k space means that the echo data is stored in an area corresponding to the k space in the memory 113. A group of echo data stored in an area corresponding to the k space in the memory 113 is also referred to as k space data.

The arithmetic processing unit 114 performs processing such as signal processing and image reconstruction by Fourier transform on the k-space data, and the resulting image of the subject 101 is displayed on the display / operation unit 118 described later. It is displayed and recorded in the internal storage unit 115 or the external storage unit 117, or transferred to an external device via the network IF 116.

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

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

  The basics of the application sequence and application sequence of RF pulses and gradient magnetic field pulses and the measurement order of echo signals are determined in advance for each type of imaging sequence, and the application intensity, application timing, etc. of each RF pulse and gradient magnetic field pulse are determined. It is specifically determined by the value of the specified control parameter. The values of the basic control parameters of the imaging sequence are set in advance and held in the internal storage unit 115, and the values of other control parameters are set as imaging conditions by the operator via the display / operation unit 118. The arithmetic processing unit 114 specifically determines the control data of the imaging sequence based on the values of these control parameters, and notifies the measurement control unit 111 of the determined control data.

(FSE sequence)
Next, the outline of the FSE sequence according to the present invention will be described based on the sequence charts shown in FIGS.

  The FSE sequence according to the present invention applies N (N is a natural number) refocus RF pulses after one excitation RF pulse, and each refocus RF pulse has a subscript n (n is 1 ≦ n ≦ N is a natural number that satisfies N). The subscript n is given in the order of application. In addition, the flip angle of the refocus RF pulse to be applied nth is represented as FAn. The echo number of the echo signal measured immediately after the nth refocus RF pulse to be applied is n.

  Note that the imaging sequence according to the present invention is not limited to the FSE sequence, and is an imaging sequence in which a plurality of refocus RF pulses are applied within the repetition time TR after the excitation RF pulse (for example, 90 ° RF pulse). I just need it.

FIG. 2 is a sequence chart showing the application timing of each RF pulse and the measurement timing of each echo signal in an FSE sequence (hereinafter referred to as 180 ° -FSE sequence) 200 using a refocusing RF pulse with a flip angle of 180 °. is there. The application timing of other gradient magnetic field pulses is omitted. As shown in FIG. 2, after the excitation RF pulse (90 ° RF pulse) 201 is applied, a plurality of repetitive times with a flip angle of 180 ° (here, five cases are illustrated as an example) are repeated during a repetition time TR204. A convergent RF pulse 202 (202 ₁ , 202 ₂ , 202 ₃ , 202 ₄ , 202 ₅ ) is applied. Then, after each refocus RF pulse 202 is applied, the echo signal 203 (203 ₁ , 203 ₂ , 203 ₃ , 203 ₄ , 203 ₅ ) is measured. Also, the time from the peak of the excitation RF pulse 201 to the peak (center) of the echo signal that is echo data placed at the origin of the k space is called the effective echo time (TE) 204, and in the example of FIG. The time up to ₃ is TE.

On the other hand, FIG. 3 is a sequence chart showing the application timing of each RF pulse and the measurement timing of each echo signal in the VRFA-FSE sequence 300 using the VRFA refocus RF pulse. As in FIG. 2, the application timing of other gradient magnetic field pulses is omitted. As shown in FIG. 3, after applying the excitation RF pulse (90 ° RF pulse) 301, during the repetition time TR 304, a plurality of (variable) flip angles of 180 ° or less are different (again, as an example, five A refocus RF pulse 302 (302 ₁ , 302 ₂ , 302 ₃ , 302 ₄ , 302 ₅ ) is applied. Then, after each refocus RF pulse 302 is applied, the echo signal 303 (303 ₁ , 303 ₂ , 303 ₃ , 303 ₄ , 303 ₅ ) is measured. For the flip angle control of each refocus RF pulse 302, for example, the method described in Patent Document 1 can be used.

  The imaging sequence of the present invention uses a 180 ° -FSE sequence and a VRFA-FSE sequence in combination as a refocus RF pulse. At that time, the 180 ° -FSE sequence is used for the measurement of the echo signal for filling the low frequency region of the k space, and the VRFA-FSE sequence is used for the measurement of the echo signal for filling the high frequency region of the k space. As a result, the contrast of the image is equivalent to that of the conventional FSE sequence in which the flip angle of the refocus RF pulse is fixed at 180 °, and the SAR is reduced.

(Example 1)
Embodiment 1 A flip angle control method for a refocusing RF pulse and an MRI apparatus according to a first embodiment of the present invention will be described. In Example 1, the number of echo signals measured using the 180 ° -FSE sequence and the number of echo signals measured using the VRFA-FSE sequence, or a ratio thereof, are set so that the SAR of the subject is equal to or lower than the upper limit. The k-space is divided according to the determined number of echo signals or its ratio, and the 180 ° -FSE sequence is used to acquire the echo data to be placed in the low-frequency region near the origin of the k-space. A VRFA-FSE sequence is used to acquire echo data to be arranged in a region (edge region).

  Hereinafter, Example 1 will be described in detail.

(Function according to Example 1)
First, each function of the arithmetic processing unit 114 for realizing the refocus RF pulse flip angle control method of the first embodiment will be described based on the functional block diagram shown in FIG. The arithmetic processing unit 114 according to the first embodiment includes an imaging condition setting unit 401, a sequence setting unit 402, a sequence combination unit 403, a k-space division unit 404, and an imaging execution unit 405.

  The imaging condition setting unit 401 displays a GUI (input screen) for receiving imaging information setting input related to the subject information and the FSE sequence on the display unit, and receives imaging condition setting input by the operator. As imaging conditions related to the FSE sequence, the 180 ° -FSE sequence 200 and the VRFA-FSE sequence 300 are set in common with the repetition time TR, the effective echo time TE, and the number of echo signals measured with one excitation ETL (echo train length), time interval between echo signals IET (inter echo time), etc., and the number of image matrices. The subject information includes sex, age, height, weight, and the like. These subject information is used for SAR estimation.

  The sequence setting unit 402 sets the value of each control parameter of the 180 ° -FSE sequence 200 and the VRFA-FSE sequence 300 based on the imaging conditions set by the imaging condition setting unit 401. For common parameters, the same value is set, and individual parameters are set independently. In particular, for the VRFA-FSE sequence 300, the clip angle (FA) of each refocus RF pulse is specifically set.

The sequence combination unit 403 calculates the SAR of the 180 ° -FSE sequence 200 and the VRFA-FSE sequence 300 set by the sequence setting unit 402 . Then, it is determined whether or not the calculated SAR is within a predetermined upper limit value (for example, the upper limit value of the 10-second average value and the 6-minute average value of the whole body SAR). When the SAR exceeds the upper limit value, the combination of the imaging sequences is adjusted so that the SAR is within the upper limit value.

  The first adjustment is to reduce the number of echo signals measured with the 180 ° -FSE sequence 200 and to distribute the number of echo signals measured with each imaging sequence so that the decrease is measured with the VRFA-FSE sequence 300. Adjust the percentage. As the number of echo signals measured by the VRFA-FSE sequence 300 increases, the SAR value decreases.

  In the second adjustment, the number of echo signals measured in each of the 180 ° -FSE sequence and the VRFA-FSE sequence is set to a predetermined number or a predetermined ratio (that is, constant without changing), and the VRFA-FSE sequence The flip angle of each refocus RF pulse is changed so that the SAR decreases. In this case, the sequence setting unit 402 is instructed to change the clip angle (FA) of each refocus RF pulse of the VRFA-FSE sequence 300.

  One or both of the first method and the second method may be combined.

  The SAR may be calculated using the method of Patent Document 2 based on the actual measurement obtained by the pre-scan, or the subject information (weight, gender) set in the imaging condition setting unit 401. Etc.) and the SAR may be predicted only from the imaging conditions.

The k-space division unit 404 divides the k-space and divides each divided region according to the distribution or ratio of the number of echoes measured by the 180 ° -FSE sequence 200 and the VRFA-FSE sequence 300 determined by the sequence combination unit 403 , respectively. Set the measurement order. Specifically, the phase encoding is determined so as to measure echo signals arranged in a divided region including the origin of k space or a divided region near the origin in the 180 ° -FSE sequence 200. On the other hand, the phase encoding is determined so that the echo signal arranged in the divided area on the high frequency side (edge) of the k space is measured by the VRFA-FSE sequence 200. As a result, the echo data arranged in the low frequency region of the k space is acquired by the 180 ° -FSE sequence 200, and the echo data arranged in the high frequency region of the k space is acquired by the VRFA-FSE sequence 200.

An example of k-space division is shown in FIG. FIG. 5 shows the phase encoding direction (ky) of the two-dimensional k-space 500, where the phase encoding direction is ky and the frequency encoding direction is kx, in the low frequency region 502 including the origin and outside the low frequency region 502. shows an example of dividing into three regions on the negative side of the high-frequency region 501-1 and the positive high frequency region 501-2. The echo data of the low frequency region 502 is acquired by the 180 ° -FSE sequence 200, and the echo data of the high frequency regions 501-1 and 501-2 is acquired by the VRFA-FSE sequence 300.

The imaging execution unit 405 performs the echo processing in which the 180 ° -FSE sequence 200 and the VRFA-FSE sequence 300 set by the sequence setting unit 402 are encoded in the encoding order that is the measurement order of each divided region set by the k-space division unit 404. The control data of each FSE sequence is generated so as to measure the signal, the measurement control unit 111 is notified, and the measurement control unit 111 executes imaging using these FSE sequences.

(Processing flow of Example 1)
Next, the processing flow of the first embodiment performed in cooperation with the above-described functional units will be described based on the flowchart shown in FIG. This processing flow is stored in advance in the internal storage unit 115 as a program, and is executed by the arithmetic processing unit 114 reading the program from the internal storage unit 115 and executing it. Hereinafter, the processing contents of each processing step will be described in detail.

  In step 601, the imaging condition setting unit 401 displays a GUI for accepting input of imaging condition settings related to subject information and FSE sequences on the display unit, and accepts setting input by the operator.

In step 602, based on the imaging conditions input and set in step 601, the sequence setting unit 402 sets the values of the control parameters of the 180 ° -FSE sequence 200 and the VRFA-FSE sequence 300. In particular, the clip angle (FA) of each refocus RF pulse of the VRFA-FSE sequence 300 is set.

In step 603, the sequence combination unit 403 determines whether or not it is within the SAR upper limit value for the 180 ° -FSE sequence 200 set in step 602. For example, the 10-second average value and the 6-minute average value of the whole body SAR are calculated, and it is determined whether or not the calculated value is within the upper limit value. When the SAR exceeds the upper limit value, adjustment is performed by combining one or both of the first adjustment and the second adjustment so that the SAR is within the upper limit value. Details are as described above.

  In step 604, the k-space division unit 404 divides the k-space and each division according to the distribution or ratio of the number of echoes measured in the 180 ° -FSE sequence 200 and the VRFA-FSE sequence 300 determined in step 603, respectively. Set the measurement order of the area. Details are as described above.

In step 605, the imaging execution unit 405 performs each echo in the encoding order that is the measurement order of each divided region set in step 604 by the 180 ° -FSE sequence 200 and the VRFA-FSE sequence 300 set in step 602. Specifically, the control data of each FSE sequence is generated so as to measure the signal, and the measurement control unit 111 is notified of the control data so that the measurement control unit 111 performs imaging. Based on the notified imaging sequence control data, the measurement control unit 111 combines the 180 ° -FSE sequence 200 and the VRFA-FSE sequence 300 to measure an echo signal for filling each divided region of k-space. To control.
The above is the outline of the processing flow of the first embodiment.

  In the above description, an example in which echo data is filled in a two-dimensional k space on the premise of two-dimensional measurement has been described, but the same can be applied to three-dimensional measurement.

  In the three-dimensional measurement, the k space is also three-dimensional including the axis (kz) corresponding to the slice encoding direction. Therefore, the echo data of the low frequency region including the origin of the three-dimensional k-space is acquired by the 180 ° -FSE sequence 200, and the echo data of the high frequency region (edge) is acquired by the VRFA-FSE sequence 300. Also in this case, the boundary between the low-frequency region and the high-frequency region in the three-dimensional k space may be determined by SAR determined by pre-scanning or SAR prediction based on subject information and imaging conditions.

  FIG. 7 shows a measurement example of the three-dimensional k-space. The echo data of the low frequency region 701 including the origin on the ky-kz plane 700 in the three-dimensional k space is acquired by the 180 ° -FSE sequence 200, and the echo data of the high frequency region 702 is acquired by the VRFA-FSE sequence 300. Alternatively, when measuring only in the inscribed circle (711, 712) of the ky-kz plane in the three-dimensional k-space, and the other outer region 713 is circularly measured with zero padding, Echo data of the low frequency region 711 including the origin is acquired by the 180 ° -FSE sequence 200, and echo data of the high frequency region 712 is acquired by the VRFA-FSE sequence 300.

As explained above, the refocus RF pulse flip angle control method and the MRI apparatus of the first embodiment use a 180 ° -FSE sequence to acquire echo data arranged in the low frequency region near the origin of the k space. The VRFA-FSE sequence is used to acquire echo data to be arranged in the high frequency region (edge region) of the k space. Therefore, based on the SAR of the subject, a sequence combination unit that adjusts the distribution or ratio of the number of echo signals measured by the 180 ° -FSE sequence and the number of echo signals measured by the VRFA-FSE sequence, and the echo signal A k-space dividing unit that divides the k-space into a plurality of parts and sets the measurement order of each divided region in accordance with the number distribution or the ratio adjustment. As a result, it is possible to obtain an image having a contrast equivalent to that of a conventional FSE sequence in which the flip angle of the refocus RF pulse is kept constant by 180 ° while reducing the SAR.

(Example 2)
Next, a second embodiment of the flip angle control method and MRI apparatus of the refocusing RF pulse of the present invention will be described. In this second embodiment, as a 180 ° -FSE sequence, an echo signal immediately after the application of the dummy refocus RF pulse is obtained by using at least one of the first 180 ° refocus RF pulses and at least one of the last as a dummy. Not measured or measured but not used for image reconstruction. That is, one or more refocus RF pulses immediately after the 90 ° RF pulse and the last one or more refocus RF pulses are used as dummy. The dummy refocus RF pulse is a dummy RF pulse that only applies the refocus RF pulse, and means that the echo signal refocused by the dummy refocus RF pulse is not filled in the k space.

  If the TE of each echo data arranged in the low frequency region of the k space is greatly different, a step difference in the signal strength of the echo data is increased, which causes an artifact on the image. Since the 180 ° -FSE sequence is an FSE sequence for acquiring echo data located in the low-frequency region of k-space, a series of 180 ° reconvergence is performed to avoid acquisition of echo data with significantly different TE. Among the RF pulses, at least one of the first and at least one of the last are dummy refocusing RF pulses. This eliminates the filling of echo data with significantly different TE in the low-frequency region of the k space, and makes it possible to suppress a step difference in the signal strength of the echo data. As a result, the image quality can be improved.

  The number of 180 ° refocus RF pulses to be used as dummy RF pulses can be determined in advance or based on imaging conditions. For example, if the number of echo signals to be acquired by 180 ° -FSE sequence is determined in advance in 1TR and an ETL exceeding that number is set, the remaining refocus RF pulses are automatically set as dummy RF pulses. At this time, the number of echo signals to be acquired can be set by the operator, and the number of effective echo signals is calculated from the imaging conditions such as ETL and IET, and the echo signals other than the effective echo signals are automatically calculated. It is also possible to set a refocusing RF pulse that induces a dummy RF pulse.

  Note that the VRFA-FSE sequence is applied to the sequence for acquiring the echo data to be filled in the high frequency region of the k space.

FIG. 8 shows a sequence chart of the 180 ° -FSE sequence 700 of the second embodiment. As in FIG. 2, the application timing of other gradient magnetic field pulses is omitted. The 180 ° refocus RF pulse 202 ₁ after the 90 ° excitation RF pulse 201 and the last 180 ° refocus RF pulse 202 ₅ are used as dummy refocus RF pulses, and the 180 ° refocus RF pulse 202 ₁ and the 180 ° refocus RF no echo signals 203 _1, 203 ₅ immediately after the pulse 202 ₅ filled in the k-space (i.e., is not measured, is not used for image reconstruction be measured).

  Each function and processing flow in the second embodiment are the same as those in the first embodiment except that the 180 ° -FSE sequence is the same as that in the second embodiment, and thus the description thereof is omitted.

  As described above, the flip angle control method and the MRI apparatus of the refocusing RF pulse according to the second embodiment, as a 180 ° -FSE sequence, at least one of the first 180 ° refocusing RF pulses and the last of the 180 ° refocusing RF pulses. At least one of these is a dummy refocus RF pulse. As a result, in the low frequency region of the k space, the echo data having a significantly different TE is not filled, and the step difference in the signal strength of the echo data can be suppressed. Therefore, the image quality can be improved.

(Example 3)
Next, a third embodiment of the refocus RF pulse flip angle control method and MRI apparatus of the present invention will be described. In the third embodiment, as a 180 ° -FSE sequence, at least one of the first 180 ° refocusing RF pulses is a dummy refocusing RF pulse, and no dummy refocusing RF pulse is applied after the effective TE. . This shortens the repetition time TR of the 180 ° -FSE sequence.

  By shortening the repetition time TR in this way, it is possible to shorten the imaging time, and it is possible to reduce SAR because no extra dummy refocus RF pulse is applied.

  Note that the VRFA-FSE sequence is applied to the sequence for acquiring the echo data to be filled in the high frequency region of the k space.

FIG. 9 shows a sequence chart of the 180 ° -FSE sequence 800 of the third embodiment. As in FIG. 2, the application timing of other gradient magnetic field pulses is omitted. Like the third embodiment, a 180 ° refocusing RF pulses 202 ₁ after the 90 ° RF pulse 201 as a dummy refocusing RF pulse, which according to the image reconstruction be measured or not measured echo signals 203 ₁ Do not use. On the other hand, it does not apply the dummy refocusing RF pulse 205 ₅ after TE205. Therefore the echo signal 203 ₅ is not generated.

  The functions and the processing flow in the third embodiment are the same as those in the first embodiment except that the 180 ° -FSE sequence is the same as that in the third embodiment, and a description thereof will be omitted.

As described above, the flip angle control method and MRI apparatus of the refocusing RF pulse according to the third embodiment performs at least one of the first 180 ° refocusing RF pulses as a 180 ° -FSE sequence. A dummy refocus RF pulse is used, and no dummy refocus RF pulse is applied after effective TE. As a result, the repetition time TR can be shortened. In addition, the imaging time can be shortened, and the SAR can be reduced because no extra dummy refocus RF pulse is applied.

(Example 4)
Next, a fourth embodiment of the flip angle control method and MRI apparatus of the refocus RF pulse according to the present invention will be described. In the fourth embodiment, as the 180 ° -FSE sequence of the first embodiment, an SE sequence in which one refocus RF pulse having a flip angle of 180 ° is applied after the 90 ° RF pulse is used. The same VRFA-FSE sequence 300 as in the first embodiment is used for the FSE sequence for acquiring echo data that fills the high-frequency region of the k space.

  By applying the SE sequence to the acquisition of echo data in the low frequency region of k-space, unlike the FSE sequence, the low-frequency region of k-space is filled with the same TE echo data. The contrast can be improved.

  FIG. 10 shows a sequence chart of the SE sequence 1000 in which one 180 ° refocus RF pulse is applied after the 90 ° RF pulse. As in FIGS. 2 and 3, the application timing of other gradient magnetic field pulses is omitted.

  Within the repetition time TR, after the 90 ° RF pulse 1001, one 180 ° refocus RF pulse 1002 is applied, and one echo signal 1012 is measured in TE.

  The functions and processing flow in the fourth embodiment are the same as those in the first embodiment except that the 180 ° -FSE sequence is the same as that in the fourth embodiment, and thus the description thereof is omitted.

  As described above, the refocus RF pulse flip angle control method and the MRI apparatus according to the fourth embodiment use the SE sequence as the 180 ° -FSE sequence 200. As a result, the low-frequency region of the k space is filled with the same TE echo data, so that the contrast of the image can be improved.

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

Claims (6)

A measurement control unit that controls measurement of echo signals from a subject based on an imaging sequence in which one or more refocus RF pulses are applied after the excitation RF pulse to measure one or more echo signals;
An arithmetic processing unit for reconstructing the image of the subject using the one or more echo signals;
A magnetic resonance imaging apparatus comprising:
The imaging sequence is a combination of a 180 ° -FSE sequence having a refocus RF pulse having a flip angle of 180 ° and a VRFA-FSE sequence having a refocus RF pulse having a variable flip angle having a flip angle of 180 ° or less. ,
The measurement control unit uses the 180 ° -FSE sequence to measure an echo signal for filling a low-frequency region in k space, and uses the VRFA-FSE to measure an echo signal for filling a high-frequency region in k space. A magnetic resonance imaging apparatus using a sequence. The magnetic resonance imaging apparatus according to claim 1.
The arithmetic processing unit adjusts the distribution or ratio of the number of echo signals measured by the 180 ° -FSE sequence and the number of echo signals measured by the VRFA-FSE sequence based on the SAR of the subject. A combination part;
A k-space dividing unit that divides the k-space into a plurality of parts according to the distribution of the number of echo signals or the adjustment of the ratio, and sets the measurement order of each divided region;
A magnetic resonance imaging apparatus comprising: The magnetic resonance imaging apparatus according to claim 1 or 2,
Wherein in a series of 180 ° refocusing RF pulse in the 180 ° -FSE sequence, the first at least a bract last at least one and dummy refocusing RF pulses, measuring the echo signal immediately after the dummy refocusing RF pulse application A magnetic resonance imaging apparatus that is not used for image reconstruction even if it is measured. The magnetic resonance imaging apparatus according to claim 1 or 2,
Of the series of 180 ° refocus RF pulses in the 180 ° -FSE sequence, at least one of the first 180 ° refocus RF pulses is a dummy refocus RF pulse, and no dummy refocus RF pulse is applied after effective TE. Resonance imaging device. The magnetic resonance imaging apparatus according to claim 1 or 2,
The magnetic resonance imaging apparatus according to claim 1, wherein the 180 ° -FSE sequence is an SE sequence in which one refocus RF pulse having a flip angle of 180 ° is applied. A measurement control step for controlling the measurement of echo signals from the subject based on an imaging sequence in which one or more refocus RF pulses are applied after the excitation RF pulse to measure one or more echo signals;
An arithmetic processing step for reconstructing the image of the subject using the one or more echo signals;
A flip angle control method of a refocusing RF pulse in a magnetic resonance imaging apparatus having:
The imaging sequence is a combination of a 180 ° -FSE sequence having a refocus RF pulse having a flip angle of 180 ° and a VRFA-FSE sequence having a refocus RF pulse having a variable flip angle having a flip angle of 180 ° or less. ,
In the measurement control step, the 180 ° -FSE sequence is used to measure an echo signal for filling a low-frequency region in k-space, and the VRFA-FSE is used to measure an echo signal for filling a high-frequency region in k-space. A flip angle control method for a refocus RF pulse, characterized by using a sequence.