JP2009034341A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus applicable to multiple types of pulse sequences without elongating an imaging time and reducing SAR (Specific Absorption Rate) in a simple constitution. <P>SOLUTION: This magnetic resonance imaging apparatus combines a pulse sequence with high duty cycle with a pulse sequence with low duty cycle and performs it by interposing the pulse sequence with low duty cycle so as not to continuously perform the pulse sequence with high duty cycle. For example, when performing FSE method at a first time TR, a GE method is performed at the next time TR. This constitution can improve the detection accuracy of a lesioned part by applying multiple types of pulse sequences and reduce the SAR in the simple constitution. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging apparatus, and more particularly to optimization of SAR (Specific Absorption Rate).

  In a magnetic resonance imaging apparatus (MRI apparatus), the thermal effect of a high-frequency magnetic field is defined by SAR (Specific Absorption Rate), and the size of the SAR is prevented in order to prevent an increase in the temperature of an object by examination using the MRI apparatus. Is obliged to irradiate a high-frequency magnetic field that does not exceed a predetermined upper limit.

  The irradiation method of the high frequency magnetic field is related to a measurement procedure called a pulse sequence that defines the application procedure of the high frequency magnetic field and the gradient magnetic field and the timing of signal measurement. As pulse sequences, SE (spin echo) method, GE (gradient echo) method, FSE (first spin echo) method, EPI (echo planar) method and the like are known as representative methods, among which FSE method is: A 180-degree excitation pulse having a large energy is frequently used.

  The SAR depends on the frequency of the high-frequency magnetic field, the static magnetic field strength, and the like.

  In recent years, a magnetic resonance imaging apparatus having a static magnetic field intensity exceeding 3T has been used. However, although the SNR of image quality can be improved as compared with a 1.5T or 1T apparatus, restrictions on the SAR become more severe. Therefore, as described in Non-Patent Document 1, there has been proposed a method for reducing SAR in the FSE method by combining excitation pulses with a low excitation angle that has the same effect as a 180-degree excitation pulse. .

  Further, Patent Document 1 predicts an SAR to be imaged when a pulse sequence is executed, and at least one of the number of RF pulses, the pulse waveform, and the pulse width in the pulse sequence so that the predicted value is within the limit. Techniques for adjusting the balance are described.

Japanese Patent Laid-Open No. 2001-70282 Multiecho Sequence With Variable Refocusing Flip Angles: Optimization of Signal Behavior Using Smooth Transitions Between Pseudo Steady States .; Magnetic Resonance in Medicine 49: 527-535 (2003), Reduced RF Power Without Blurring: Correcting for Modulation of Refocusing Flip Angle in FSE Sequences .; Magnetic Resonance in Medicine 51: 1031-1037 (2004)

  However, the technique described in Non-Patent Document 1 described above can be applied only to the FSE method and cannot be applied to the SE method, the GE method, and the EPI method.

  Further, the technique described in Patent Document 1 requires a pulse adjustment unit for adjusting the number of pulses, the pulse waveform, or the pulse width, and the configuration becomes complicated. Furthermore, since the number of pulses is adjusted in order to keep the SAR within the limit, the imaging time becomes long.

  An object of the present invention is to realize a magnetic resonance imaging apparatus that can be applied to various types of pulse sequences without extending the imaging time and can reduce SAR with a simple configuration.

  In order to achieve the above object, the present invention is configured as follows.

  In the magnetic resonance imaging apparatus of the present invention, the control means combines the first pulse sequence having a predetermined duty cycle and the second pulse sequence having a duty cycle smaller than the first pulse sequence, Each of the pulse sequence and the second pulse sequence is executed at least once.

  It is possible to realize a magnetic resonance imaging apparatus capable of improving the detection accuracy of a lesion by applying various types of pulse sequences without extending the imaging time and reducing the SAR with a simple configuration.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

  First, an MRI apparatus to which the present invention is applied will be described with reference to FIG. In FIG. 5, the MRI apparatus includes a superconducting coil 1 for generating a uniform magnetic field space and 3 for generating a gradient magnetic field in which the magnetic field strength changes linearly along the three axial directions of x, y, and z. A set of gradient magnetic field generating coils 2, a high frequency magnetic field generating coil 3 for inducing magnetic resonance of the subject 10, a receiving coil 4 for detecting a magnetic resonance signal of the subject, a gradient magnetic field power source 5, and a high frequency Magnetic field power source 6, bed 7 for transporting the subject into the static magnetic field space, control unit 8 for controlling the operation of the gradient magnetic field generating coil 2, the high frequency magnetic field generating coil 3 and the receiving coil 4, a control command, and image reconstruction And an operation console 9 having image display means for displaying an image.

  In an MRI apparatus, an image is taken by a measurement procedure called a pulse sequence that defines the application procedure of a high-frequency magnetic field and a gradient magnetic field and the timing of signal measurement.

  As this pulse sequence, the SE method, the GE method, the FSE method, the EPI method and the like are known depending on the photographing time and the difference in the obtained image contrast. In a normal examination, by applying a plurality of pulse sequences in the same imaging region, images with different contrasts are taken to improve diagnostic ability such as detection of a lesioned part.

  In the prior art, each pulse sequence is executed independently and separately.

  FIG. 2 is a diagram showing a pulse sequence of the FSE method, and FIG. 3 is a diagram showing a pulse sequence of the GE method.

  In FIG. 2, the application timing of the high-frequency magnetic field (RF) 21, the x-direction gradient magnetic field (Gx) 22, the y-direction gradient magnetic field (Gy) 23, the z-direction gradient magnetic field (Gz) 24, and the magnetic resonance signal (echo) 25 Occurrence is displayed along the time axis. RF and Gx have a function of exciting a region to be photographed, Gy has a function of encoding position information at an echo frequency, and Gz has a function of encoding position information at an echo phase.

  In the FSE method, first, a 90-degree excitation pulse is irradiated, then a first 180-degree excitation pulse is irradiated, and a first echo is obtained at time TE. Further, the operation of irradiating the 180-degree excitation pulse at the next time TE and obtaining an echo at the same time TE is repeated.

  The example shown in FIG. 2 shows a case where four echoes are obtained within the repetition time TR of the pulse sequence. For example, assuming that L echoes are required for image reconstruction, the pulse sequence is repeated L / 4 times, so the execution time of the pulse sequence is (L × TR) / 4. Although the case where four echoes are obtained is shown here, the number of echoes is not limited to four.

  In FIG. 3, the application timing of the high frequency magnetic field (RF) 31, the x direction gradient magnetic field (Gx) 32, the y direction gradient magnetic field (Gy) 33, the z direction gradient magnetic field (Gz) 34, and the magnetic resonance signal (echo) 35 Occurrence is displayed along the time axis.

  RF and Gx have a function of exciting a region to be photographed, Gy has a function of encoding position information in the echo phase, and Gz has a function of encoding position information in the frequency of echo.

  In the GE method, an α-degree excitation pulse is irradiated with α being a value of 90 or less, and an echo is obtained at time TE. In the GE method, one echo is obtained within the repetition time TR of the pulse sequence. Accordingly, assuming that L echoes are required for image reconstruction, the pulse sequence is repeated L times, and the execution time of the pulse sequence is L × TR.

  The duty cycle represents the rate of irradiation time per unit time of the high-frequency magnetic field. The FSE method shown in FIG. 2 irradiates a high frequency magnetic field of a total of 5 × t hours with respect to the repetition time TR, where t is the irradiation time of one high frequency magnetic field. The cycle is 5t / TR.

  On the other hand, in the GE method shown in FIG. 3, when the irradiation time of one high-frequency magnetic field is t, the high-frequency magnetic field is irradiated for t times with respect to the repetition time TR, so the duty cycle of the sequence of FIG. Becomes t / TR. That is, the duty cycle of FIG. 2 is five times larger than the duty cycle of FIG. The SAR depends on the duty cycle. For example, when TR is extended by a factor of 5 in the sequence of FIG. 2, the duty cycles of FIGS. 2 and 3 are equal, and the respective SARs are also equal. However, since TR is extended 5 times, the shooting time is also extended 5 times. Here, for simplicity, the effect of the amplitude of the high-frequency magnetic field is omitted.

  Next, according to an embodiment of the present invention, SAR can be reduced without extending the shooting time by alternately switching between the FSE method shown in FIG. 2 and the GE method shown in FIG. Explain the procedure.

  FIG. 1 is a diagram showing a pulse sequence in one embodiment of the present invention. In FIG. 1, the high-frequency magnetic field 11, Gx12, Gz13, and the echo 14 are equivalent to the high-frequency magnetic field, Gx, Gz, and echo shown in FIGS. In FIG. 1, Gy is omitted. This is a concatenation of one FSE sequence and four GE sequences. Assuming that 5TR shown in FIG. 1 is a unit of sequence repetition, a high frequency magnetic field of (9 × t) time is irradiated for (5 × TR) time, and therefore the duty cycle of the example shown in FIG. Become TR.

  As described above, the duty cycle in the example shown in FIG. 2 is 5 t / TR, and the duty cycle in the example shown in FIG. 3 is t / TR. Therefore, it can be seen that the duty cycle of the FSE method is reduced by 64% in the coupled type of the FSE method and the GE method, compared to the case where the FSE method is performed alone.

  The signal obtained by the FSE method and the signal obtained by the GE method may be reconstructed separately. If the image reconstruction requires L echoes, the imaging time of the sequence shown in FIG. × (L × TR) / 4, which is equal to the sum of the imaging time of the FSE method alone shown in FIG. 2 and the imaging time of the GE method alone shown in FIG. The combination method of the FSE method and the GE method is not limited to FIG.

  By the way, the ratio of the irradiation time per unit time of the high-frequency magnetic field in the first pulse sequence is D1, the ratio of the irradiation time per unit time of the high-frequency magnetic field in the second pulse sequence is D2, and the execution time of the first pulse sequence If the ratio of the execution time of the second pulse sequence to A is A, the ratio of the irradiation time per unit time of the high-frequency magnetic field when the first pulse sequence and the second pulse sequence are connected is A × It can be generalized as D1 + (1-A) × D2.

  When this is applied to the example of FIG. 1, since D1 = 5 t / TR, D2 = t / TR, and A = 0.2, the duty cycle of the example shown in FIG. 1 is A × D1 + (1− A) × D2 = 1.8 t / TR, which is the same as the above result.

  Next, the operation of the pulse sequence shown in FIG. 1 will be described with reference to the flow of programs and data on the memory in the MRI apparatus. FIG. 4A shows a memory 100 that stores a program of the FSE method, and its address is from 1 to L / 4. FIG. 4B shows a memory 200 for storing a GE program, whose addresses are 1 to L.

  FIG. 4C shows a memory 300 that stores signal data obtained by the FSE method, and the addresses are 1 to L. FIG. 4D shows a memory 4 for storing signal data obtained by the GE method, and its addresses are 1 to L. It is assumed that an instruction for executing a 1TR sequence is stored in one address of the program memory, and one signal is stored in one address of the data memory.

  In the first TR, the CPU (control unit 8) executes the program 411 at the address 1 in the memory 100 and stores four echoes from the address 1 431 to the address 4 434 in the memory 300 (step 1).

  Next, the CPU 8 switches tasks, executes the program 421 at the address 1 in the memory 200, and stores one echo in the address 1 441 in the memory 400 (step 2).

  Subsequently, the CPU 8 executes the program 422 at the address 2 in the memory 200 and stores one echo in the address 442 at the address 2 in the memory 400 (step 3).

  Next, the CPU 8 executes the program 423 at address 3 in the memory 200 and stores one echo at 443 at address 3 in the memory 400 (step 4).

  Subsequently, the CPU 8 executes the program 424 at the address 4 in the memory 200 and stores one echo in the address 441 at the memory 400 (step 5).

  Through the above steps 1 to 5, the sequence for 1TR shown in FIG. 1 is performed. If the first process is to execute the FSE method and store the obtained data in the memory, and the second process is to execute the GE method and store the obtained data in the memory, the step The first process is executed once at 1 and the second process is executed four times at steps 2 to 5.

  In the next TR, the CPU 8 switches tasks, executes the program 412 at address 2 in the memory 100, and stores four echoes from 435 at address 5 to 438 at address 8 in the memory 300 (step 6).

  Subsequently, the CPU 8 switches tasks, executes the program 425 at address 5 in the memory 200, and stores one echo at 445 at address 5 in the memory 400 (step 7).

  Next, the CPU 8 executes the program 426 at address 6 in the memory 200 and stores one echo at 446 at address 6 in the memory 400 (step 8).

  Subsequently, the CPU 8 executes the program 427 at the address 7 in the memory 200 and stores one echo at the address 447 in the memory 400.

  The CPU 8 executes the program 428 at address 8 in the memory 200 and stores one echo at 448 at address 8 in the memory 400 (step 10).

  By the above operation, after the first process is executed once, the second process is executed four times. Thereafter, after the first process is executed once, the second process is executed four times. .

  When the program is repeated in this way and L echoes are stored in the memory 300 and the memory 400, the measurement ends. If the addresses of the respective memories are stored in the task switching in steps 2, 6, and 7, the continuity of each sequence program and the continuity of data storage are maintained at the time of switching. That is, the interrupted process can be resumed.

  Furthermore, when switching pulse sequences, it is also effective to save adjustment values such as irradiation gain or reception gain in each sequence. In this case, the adjustment value can also include the frequency, irradiation gain, reception gain, and the like obtained by performing the prescan.

  In addition, since the excited magnetization is in a steady state by repeating the pulse sequence, it is also effective to execute a pulse sequence that does not measure data several times to change the magnetization to a steady state when switching tasks. . In the above description, two types of pulse sequences are combined. However, it is obvious that other types of pulse sequences can be combined in the same manner.

  In this case, if N is an integer equal to or larger than 2, the control unit 8 has a memory space for storing a program for executing N pulse sequences and N memories for storing signal data of N pulse sequences. A space is provided and N pulse sequences are switched.

  The operation of the pulse sequence and the flow of the program in the embodiment of the present invention have been described above. The user interface for the operator to execute the operation will be described below.

  FIG. 6 is an explanatory diagram of a user interface when the present invention is not applied. In FIG. 6, the operator inputs a pulse sequence 61 and a parameter 62 to be executed using the user interface. That is, the photographing condition is input.

  The SAR can be calculated from the input photographing conditions. When the SAR exceeds the upper limit value, the pulse sequence is not allowed to be executed together with the warning display 63 in the image display area 64.

  Therefore, the operator re-enters the parameters so that the SAR does not exceed the upper limit value.

  FIG. 7 is an explanatory diagram of a user interface (operation console 9) when the present invention is applied. In this example, as described above, the SAR is reduced by combining a plurality of pulse sequences shown in FIG.

  For this reason, the example shown in FIG. 7 shows a user interface for designating a plurality of pulse sequences.

  In FIG. 7, the user interface can input at least two types of pulse sequences to be linked. As shown in FIG. 1, since the FSE method, GE method, GE method, GE method, and GE method are executed in this order, 1 is displayed on the button 711, FSE is displayed on the button 712, and 4 is displayed on the button 713. And GE is displayed on the button 714.

  Here, 1 indicated by the button 711 and 4 indicated by the button 713 indicate that FSE is repeated once and GE is repeated four times. Further, the order of inputting the button 712 and the button 714 means the execution order of each pulse sequence. The parameters of each pulse sequence are input with buttons 715 and 716 in the same manner as the parameter 62 in FIG.

  Here, the button 715 means an FSE method parameter, and the button 716 means a GE method parameter. However, as described above, the duty cycle is 1.8 t / TR, and the SAR is calculated in consideration of the weight of the subject, the strength of the high-frequency magnetic lift, and the like. When the SAR exceeds the upper limit value, the pulse display is executed together with the warning display 72.

  Therefore, the operator re-enters the parameters so that the SAR does not exceed the upper limit value. For example, the number of repetitions displayed by the button 711 or the button 713 is changed, or the repetition time TR is changed by the button 715 or the button 716.

  Further, after specifying the pulse sequence to be photographed with the buttons 712 and 714, the number of repetitions of the buttons 711 and 713, the repetition time TR of the buttons 715 and 716, etc. are automatically calculated so that the SAR does not exceed the upper limit value. It is also possible to provide a function.

  As described above, according to the present invention, it is possible to perform imaging for reducing the SAR by combining a plurality of pulse sequences without optimizing the high-frequency magnetic field waveform to reduce the SAR and without extending the imaging time. It becomes possible.

  In other words, according to the present invention, the magnetic resonance imaging can improve the detection accuracy of a lesioned part by applying various kinds of pulse sequences without extending the imaging time and reduce the SAR with a simple configuration. An apparatus can be realized.

  In the above-described example, the FSE method and the GE method are combined. However, the present invention also includes an example in which the FSE method, the GE method, and the SE method are combined, and an example in which the FSE method and the SE method are combined. Is applicable.

It is a figure explaining the pulse sequence in one Embodiment of this invention. It is explanatory drawing of the pulse sequence implemented only by FSE method. It is explanatory drawing of the pulse sequence implemented only by GE method. It is a figure explaining the memory in the MRI apparatus for applying one Embodiment of this invention. 1 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied. It is explanatory drawing of a user interface in case this invention is not applied. It is explanatory drawing of a user interface at the time of this invention being applied.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Superconducting coil, 2 ... Gradient magnetic field generation coil, 3 ... High frequency magnetic field generation coil, 4 ... Reception coil, 5 ... Gradient magnetic field power supply, 6 ... High frequency magnetic field power supply, 7 ... Bed, 8 ... Control unit, 9 ... Console, 100, 200, 300, 400 ... Memory

Claims (7)

Static magnetic field generating means, gradient magnetic field generating means, high frequency magnetic field generating means for inducing magnetic resonance in the subject, receiving means for receiving a magnetic resonance signal from the subject, the static magnetic field generating means, and the generation of the gradient magnetic field A magnetic resonance imaging apparatus comprising: control means for controlling operations of the high-frequency magnetic field generation means and the reception means;
The control means combines the first pulse sequence having a predetermined duty cycle and the second pulse sequence having a duty cycle smaller than the first pulse sequence, and the first pulse sequence and the second pulse sequence. A magnetic resonance imaging apparatus, wherein each of the sequences is executed at least once.   2. The magnetic resonance imaging apparatus according to claim 1, wherein the control means includes a memory space for storing program data for executing the first pulse sequence, and program data for executing the second pulse sequence. A memory space for storing signal data obtained by performing the first pulse sequence, a memory space for storing signal data obtained by performing the second pulse sequence, The first pulse sequence is executed, the signal data obtained by executing the first pulse sequence is stored in the memory space, the second pulse sequence is executed, and the second pulse sequence is executed. And the second process of storing the signal data obtained in the memory space at different times. Magnetic resonance imaging apparatus.   2. The magnetic resonance imaging apparatus according to claim 1, wherein the first pulse sequence is a pulse sequence by a first spin echo method, and the second pulse sequence is a pulse sequence by a gradient echo method or a spin echo method. A magnetic resonance imaging apparatus.   3. The magnetic resonance imaging apparatus according to claim 2, wherein when one process being executed is switched to the other process being paused, an address of a memory storing a pulse sequence program and an address of a memory storing data are set. Including the internal state of the control means at the process switching time, and when switching to the same process again, the interrupted process is resumed by calling the internal state of the control means. Imaging device.   The magnetic resonance imaging apparatus according to claim 2, wherein the information to be stored when the process is switched includes a sequence adjustment value including a frequency, an irradiation gain, and a reception gain obtained by performing the pre-scan. Magnetic resonance imaging device.   3. The magnetic resonance imaging apparatus according to claim 2, wherein immediately after the process is switched, the control unit performs an operation not to store measurement data in a memory even if a pulse sequence is executed a plurality of times. Magnetic resonance imaging device.   3. The magnetic resonance imaging apparatus according to claim 2, wherein when N is an integer equal to or greater than 2, the control unit includes a memory space for storing a program for executing N pulse sequences, and N pulse sequences. A magnetic resonance imaging apparatus comprising N memory spaces for storing signal data and switching N pulse sequences.

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