JP5707066B2 - Magnetic resonance imaging system - Google Patents

Description

  Embodiments described herein relate generally to a magnetic resonance imaging (MRI) apparatus.

  MRI magnetically excites a nuclear spin of a subject placed in a static magnetic field with a radio frequency (RF) signal of Larmor frequency, and nuclear magnetic resonance (NMR) generated by this excitation. ) Is an imaging method for reconstructing an image.

  Conventionally, a multi-transmission MRI apparatus that forms an RF magnetic field (B1) using a plurality of RF transmission channels has been devised. In a multi-transmission MRI apparatus, it is important to improve B1 non-uniformity.

JP-T-2006-508759

  An object of the present invention is to form a more uniform B1 by performing multi-RF transmission using a plurality of RF transmission channels.

A magnetic resonance imaging apparatus according to an embodiment of the present invention includes an imaging unit and a transmission phase calibration unit. The imaging means performs imaging by transmitting a plurality of high-frequency signals for imaging corresponding to the plurality of high-frequency signal transmission channels to the subject using the plurality of high-frequency signal transmission channels and at least one high-frequency transmission coil. The transmission phase calibration means calibrates at least one phase of the plurality of high-frequency signals for imaging based on phase measurement values of the plurality of high-frequency signals corresponding to the plurality of high-frequency signal transmission channels measured in advance. The transmission phase calibration means includes a plurality of directional couplers, a phase acquisition unit, and a high frequency switch. The plurality of directional couplers detect a plurality of traveling waves corresponding to the plurality of high frequency signals from the plurality of high frequency signal transmission channels. The phase acquisition unit acquires the phase measurement value based on the plurality of traveling waves. The high-frequency switch switches the plurality of traveling waves and sequentially causes the phase acquisition unit to output the plurality of traveling waves via a common signal channel. The high-frequency switch causes the plurality of traveling waves to be output to the phase acquisition unit via a reception system circuit of a reception channel that is not used for receiving a magnetic resonance signal.

1 is a configuration diagram of a magnetic resonance imaging apparatus according to an embodiment of the present invention. The block diagram which shows the detailed function of the computer shown in FIG. 1, and the detailed structure of RF transmission / reception system. The figure which shows the flowchart showing the flow of imaging by the magnetic resonance imaging apparatus shown in FIG.

  A magnetic resonance imaging apparatus according to an embodiment of the present invention will be described with reference to the accompanying drawings.

  FIG. 1 is a configuration diagram of a magnetic resonance imaging apparatus according to an embodiment of the present invention.

  The magnetic resonance imaging apparatus 20 includes a cylindrical static magnetic field magnet 21 that forms a static magnetic field, a shim coil 22, a gradient magnetic field coil 23, and an RF coil 24 provided inside the static magnetic field magnet 21.

  In addition, the magnetic resonance imaging apparatus 20 includes a control system 25. The control system 25 includes a static magnetic field power supply 26, a gradient magnetic field power supply 27, a shim coil power supply 28, a transmitter 29, a receiver 30, a sequence controller 31, and a computer 32. The gradient magnetic field power source 27 of the control system 25 includes an X-axis gradient magnetic field power source 27x, a Y-axis gradient magnetic field power source 27y, and a Z-axis gradient magnetic field power source 27z. In addition, the computer 32 includes an input device 33, a display device 34, an arithmetic device 35, and a storage device 36.

  The static magnetic field magnet 21 is connected to a static magnetic field power supply 26 and has a function of forming a static magnetic field in the imaging region by a current supplied from the static magnetic field power supply 26. In many cases, the static magnetic field magnet 21 is composed of a superconducting coil, and is connected to the static magnetic field power supply 26 at the time of excitation and supplied with current. It is common. In some cases, the static magnetic field magnet 21 is composed of a permanent magnet and the static magnetic field power supply 26 is not provided.

  A cylindrical shim coil 22 is coaxially provided inside the static magnetic field magnet 21. The shim coil 22 is connected to the shim coil power supply 28, and is configured such that a current is supplied from the shim coil power supply 28 to the shim coil 22 to make the static magnetic field uniform.

  The gradient magnetic field coil 23 includes an X-axis gradient magnetic field coil 23 x, a Y-axis gradient magnetic field coil 23 y, and a Z-axis gradient magnetic field coil 23 z, and is formed in a cylindrical shape inside the static magnetic field magnet 21. A bed 37 is provided inside the gradient magnetic field coil 23 as an imaging region, and the subject P is set on the bed 37. The RF coil 24 includes a whole body (WB) coil for transmitting / receiving an RF signal built in the gantry, a local coil for receiving an RF signal provided in the vicinity of the bed 37 and the subject P, and the like.

  The gradient magnetic field coil 23 is connected to a gradient magnetic field power supply 27. The X-axis gradient magnetic field coil 23x, the Y-axis gradient magnetic field coil 23y, and the Z-axis gradient magnetic field coil 23z of the gradient magnetic field coil 23 are respectively an X-axis gradient magnetic field power supply 27x, a Y-axis gradient magnetic field power supply 27y, and a Z-axis gradient magnetic field coil 27z. It is connected to the magnetic field power supply 27z.

  The X-axis gradient magnetic field power source 27x, the Y-axis gradient magnetic field power source 27y, and the Z-axis gradient magnetic field power source 27z are supplied with currents supplied to the X-axis gradient magnetic field coil 23x, the Y-axis gradient magnetic field coil 23y, and the Z-axis gradient magnetic field coil 23z, respectively. In the imaging region, a gradient magnetic field Gx in the X-axis direction, a gradient magnetic field Gy in the Y-axis direction, and a gradient magnetic field Gz in the Z-axis direction can be formed, respectively.

  The RF coil 24 is connected to at least one of the transmitter 29 and the receiver 30. The transmission RF coil 24 has a function of receiving an RF signal from the transmitter 29 and transmitting it to the subject P, and the reception RF coil 24 is accompanied by excitation of the nuclear spin inside the subject P by the RF signal. It has a function of receiving the NMR signal generated in this way and giving it to the receiver 30.

  On the other hand, the sequence controller 31 of the control system 25 is connected to the gradient magnetic field power supply 27, the transmitter 29 and the receiver 30. The sequence controller 31 is control information necessary for driving the gradient magnetic field power supply 27, the transmitter 29, and the receiver 30, for example, operation control information such as the intensity, application time, and application timing of the pulse current to be applied to the gradient magnetic field power supply 27. And the gradient magnetic field power supply 27, the transmitter 29 and the receiver 30 are driven according to the stored predetermined sequence to drive the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy, and the Z-axis gradient magnetic field. It has the function of generating Gz and RF signals.

  The sequence controller 31 is configured to receive raw data, which is complex data obtained by detection of an NMR signal and A / D (analog to digital) conversion in the receiver 30, and supply the received raw data to the computer 32. The

  For this reason, the transmitter 29 is provided with a function of applying an RF signal to the RF coil 24 based on the control information received from the sequence controller 31, while the receiver 30 detects the NMR signal received from the RF coil 24. Then, by executing required signal processing and A / D conversion, a function of generating raw data that is digitized complex data and a function of supplying the generated raw data to the sequence controller 31 are provided.

  Further, the computer 32 is provided with various functions by executing the program stored in the storage device 36 of the computer 32 by the arithmetic unit 35. However, a specific circuit having various functions may be provided in the magnetic resonance imaging apparatus 20 instead of at least a part of the program.

  FIG. 2 is a block diagram showing the detailed functions of the computer 32 shown in FIG. 1 and the detailed configuration of the RF transmission / reception system.

  A transmitter coil 24 </ b> A and a receiver coil 24 </ b> B are provided as the RF coil 24 on the gantry side. As the transmission coil 24A, a WB coil is usually used, but the transmission coil 24A may be configured by a plurality of coil elements. The transmission coil 24A is provided with a plurality of RF input ports 24C. On the other hand, as the receiving coil 24B, a plurality of coil elements are usually used.

  The transmitter 29 includes a plurality of transmission channels, and an RF transmission phase control unit 29A is provided on each transmission channel. Note that components other than the RF transmission phase control unit 29A on the transmission channel are not shown.

  The RF transmission phase control unit 29A has a function of adjusting the phase of the RF transmission signal in each transmission channel based on the control information of the transmission RF signal acquired from the computer 32 through the sequence controller 31. That is, the relative phase difference between the RF transmission channels is controlled by the plurality of RF transmission phase control units 29A.

  Each transmission channel of the transmitter 29 is connected to the RF input port 24C of the transmission coil 24A. For this reason, each RF transmission phase control unit 29A is connected to the transmission coil 24A through the RF input port 24C. The transmitter 29 is configured to sequentially output a plurality of RF transmission signals to the transmission coil 24A through the RF input port 24C with a predetermined phase using a plurality of transmission channels.

  Thereby, the distortion of B1 formed by the transmission coil 24A can be corrected. In other words, the phases of a plurality of RF transmission signals are controlled by each RF transmission phase control unit 29A so that the distortion of B1 is corrected.

  Furthermore, directional couplers 24D are installed in the vicinity of the signal lines between the RF input ports 24C and the transmission coil 24A. Each directional coupler 24D has a function of detecting an RF transmission signal transmitted from the RF input port 24C of the corresponding transmission channel to the transmission coil 24A in a non-contact manner.

  On the other hand, the receiver 30 also includes a plurality of reception channels, and each reception channel is provided with a reception system circuit 30A. Each receiving coil 24B is connected to a corresponding receiving system circuit 30A. Usually, a larger number of reception channels are prepared than the number of reception coils 24B.

  Each directional coupler 24D is connected to the input side of a common RF switch 24E. The output side of the RF switch 24E is connected to a reception system circuit 30A of a reception channel that is not connected to the reception coil 24B. The RF detection signal detected by each directional coupler 24D is switched by the RF switch 24E and output to the reception system circuit 30A of the reception channel that is not used for receiving the RF reception signal. ing. In other words, by switching the RF switch 24E, an RF detection signal detected from a desired transmission channel can be output to the common reception system circuit 30A. The RF detection signal output to the reception system circuit 30A is output to the computer 32 through the sequence controller 31.

  Note that the RF switch 24E may be omitted and a plurality of RF detection signals may be guided to the computer 32 through individual paths, but the signal phase variation between the paths varies. For this reason, it is necessary to correct the phase shift between the RF detection signals. Therefore, it is desirable that the RF switch 24E is provided in the vicinity of the directional coupler 24D so that a plurality of RF detection signals are guided to the computer 32 through the common path as much as possible. In addition, there is an effect that the cable configuration from the RF switch 24E to the receiving system circuit 30A is unified to simplify the circuit configuration.

  On the other hand, the computing device 35 of the computer 32 functions as an imaging condition setting unit 40, a data processing unit 41, and a transmission phase calibration value calculation unit 42 by executing a program stored in the storage device 36. The storage device 36 functions as the phase information storage unit 43.

  The imaging condition setting unit 40 has a function of setting the imaging condition based on the instruction information from the input device 33 and controlling the drive by giving the set imaging condition to the sequence controller 31. Among the imaging conditions, phase control information for a plurality of RF transmission signals set to correct the distortion of B1 is given from the transmission phase calibration value calculation unit 42 to the imaging condition setting unit 40.

  The data processing unit 41 performs an image reconstruction process including a Fourier transform (FT) on the raw data for imaging acquired from the sequence controller 31, that is, the NMR reception signal from the subject P received by the receiving coil 23B. A function of reconstructing the image data by performing the processing, and a function of performing necessary image processing on the image data obtained by the reconstruction and causing the display device 34 to display the image data.

  The transmission phase calibration value calculation unit 42 acquires an RF detection signal detected by the directional coupler 24D from the sequence controller 31, and based on the acquired RF detection signal, a plurality of transmission phase calibration values for B1 distortion correction are obtained. A function of calculating, and a function of providing the imaging condition setting unit 40 with phase control information based on the calculated calibration value of the transmission phase.

The calibration value of the transmission phase can be obtained from the RF detection signal as follows, for example.
N detected in the directional coupler 24D (n = 1, 2, 3, ..., N) RF detection signal S _n of the channel can be represented by the formula (1).
S _n = A _n cos (2πf _n t + φ _n ) (1)
In Equation (1), _An is amplitude, f _n is carrier frequency, φ _n is phase, and t is time.

Then, the formula (2-1) and formula cos the RF detection signal S _n, as shown in (2-2) (2 [pi] f _n t) and sin real part signal and the imaginary part by multiplying (2 [pi] f _n t), respectively Find the signal.
Real: S _n cos (2πf _n t) = A _n / 2cos {2π (2f _n ) t} + A _n / 2cos (φ _n ) (2-1)
Imag: S _n sin (2πf _n t) = A _n / 2sin {2π (2f _n ) t} -A _n / 2sin (φ _n ) (2-2)

Next, when the obtained real part signal and imaginary part signal are respectively multiplied by a low pass filter (LPF), only the second terms of the equations (2-1) and (2-2) remain. Then, by dividing each second term by the amplitude _An / 2, the real part signal and the imaginary part signal shown in equations (3-1) and (3-2) are obtained.
Real: cos (φ _n ) (3-1)
Imag: sin (φ _n ) (3-2)

Can be determined the phase phi _n of the RF detection signal S _n of the n-channel as shown from equation (3-1) and (3-2) to Equation (4).
φ _n = tan ^-1 (φ _n ) (4)

Phase phi _n of the RF detection signals S _n as shown in equation (5) includes a control value phi _{set_n} of RF transmission phase in the RF transmission phase control unit 29A of the transmitter 29, the transmitter 29 to the directional coupler 24D _This is the sum of the phase change φ _{shift_n} of the RF transmission signal that occurs during propagation.
φ _n = φ _{set_n} + φ _{shift_n} (5)

Therefore, the phase change φ _{shift — n} of the RF transmission signal can be calculated by Expression (6).
φ _{shift_n} = φ _n -φ _{set_n} (6)

Therefore, when the target value of the phase at the time the RF transmission signal passes through the directional coupler 24D is phi _{Target_n} is a phase change phi _{Shift_n} of the RF transmission signal as a calibration value, RF as shown in Equation (7) The control value φ _{cor_n} after calibration of the RF transmission phase in the transmission phase control unit 29A can be determined.
φ _{cor_n} = φ _{target_n} -φ _{shift_n} (7)

The target value φ target — _n of the phase at the time of passing through the directional coupler 24D is often a different value for each imaging condition such as an imaging region, and is determined in advance for each imaging condition by any means such as a test scan. be able to.

In the phase information storage unit 43, in addition to the target value φ target — _n of the phase when passing through the directional coupler 24D for each imaging condition, the phase φ _n of the RF detection signal _Sn and the phase change of the RF transmission signal in each transmission channel At least one of φ _{shift_n} is stored.

The transmission phase calibration value calculation unit 42, transmitted along with the target value phi _{Target_n} saved phase in the phase information storage unit 43 using the phase change phi _{Shift_n} phase phi _n or RF transmission signal RF detection signal S _n-channel The control value φ _{cor_n} after calibration of each RF transmission phase is calculated, and the control value φ _{cor_n} after calibration of the calculated RF transmission phase is _provided to the imaging condition setting unit 40.
Next, the operation and action of the magnetic resonance imaging apparatus 20 will be described.

  FIG. 3 is a flowchart showing the flow of imaging by the magnetic resonance imaging apparatus 20 shown in FIG.

  First, in step S1, the constituent elements of the magnetic resonance imaging apparatus 20 for executing the scan of the sequence controller 31, the static magnetic field magnet 21 and the like measure the phase of the RF transmission signal of each transmission channel prior to the imaging scan. To perform a prescan.

  For this purpose, the subject P is set on the bed 37 in advance, and a static magnetic field is formed in the imaging region of the static magnetic field magnet 21 (superconducting magnet) excited by the static magnetic field power supply 26. Further, a current is supplied from the shim coil power supply 28 to the shim coil 22, and the static magnetic field formed in the imaging region is made uniform.

  Then, an imaging condition for pre-scan is given to the sequence controller 31 from the imaging condition setting unit 40 by operating the input device 33. In the prescan imaging conditions, the RF transmission phase is set to a value that corrects the distortion of B1 according to the conditions such as the imaging region.

  For this reason, the sequence controller 31 drives the gradient magnetic field power source 27, the transmitter 29, and the receiver 30 according to the imaging conditions to form a gradient magnetic field in the imaging region where the subject P is set, and the RF from the transmission coil 24A. Generate a transmission signal.

  In the transmitter 29, each RF transmission phase control unit 29A adjusts the phase of the RF transmission signal to be the control value of the RF transmission phase set as the imaging condition, and sequentially transmits the RF transmission signal through the RF input port 24C. Transmit to coil 24A. For this reason, the RF transmission signal from each transmission channel is sequentially transmitted toward the subject P from the transmission coil 24A.

  For this reason, the NMR signal generated by the nuclear magnetic resonance inside the subject P is received by the receiving coil 24 </ b> B and given to the receiving system circuit 30 </ b> A of the receiver 30. In the reception system circuit 30A, raw data is generated by signal processing including A / D conversion. The raw data is given to the data processing unit 41 of the computer 32 through the sequence controller 31. This raw data can be used for setting an imaging condition for an imaging scan.

  On the other hand, each directional coupler 24D detects the RF transmission signal of the corresponding transmission channel. The detected RF detection signal for each transmission channel is sequentially output from the directional coupler 24D to the common reception system circuit 30A by switching the RF switch 24E. Further, the RF detection signal is A / D converted in the reception system circuit 30A, and is provided as a digital signal to the transmission phase calibration value calculation unit 42 of the computer 32 through the sequence controller 31.

Next, in step S2, the transmission phase calibration value calculator 42 calculates the phase φ _n of the RF detection signal for each transmission channel. Then, the calculated phase φ _n of the RF detection signal can be considered as a measured value of the phase of the RF transmission signal when passing through the directional coupler 24D.

Next, in step S3, the transmission phase calibration value calculation unit 42 calculates the difference between the calculated phase φ _n of the RF detection signal and the control value φ _{set_n of the} RF transmission signal, that is, the phase change φ _{shift_n} of the RF transmission signal as B1 distortion. It is obtained for each transmission channel as a calibration value of the RF transmission phase for correction of.

In step S4, the transmission phase calibration value calculation unit 42, using the phase change phi _{Shift_n} of the RF transmission signal to a target value phi _{Target_n} of each transmission channel of the RF transmission phase for the imaging scan is a calibration value of the RF transmission phase And calibrate. Then, the control value φ _{cor_n} after calibration of the RF transmission phase is given to the imaging condition setting unit 40, which is used as the imaging condition for the imaging scan.

Next, in step S5, the constituent elements of the magnetic resonance imaging apparatus 20 for executing the scan of the sequence controller 31, the static magnetic field magnet 21 and the like perform an imaging scan using the control value φ _{cor_n} after calibration of the RF transmission phase as an imaging condition. Execute.

That is, the imaging condition for imaging scan is given to the sequence controller 31 from the imaging condition setting unit 40. Then, raw data is collected in the same flow as the pre-scan. However, since the RF transmission signal is transmitted using the control value φ _{cor_n} after the calibration of the RF transmission phase calibrated based on the measured value of the RF transmission phase, the distortion of B1 is corrected well. As a result, NMR data with little influence of B1 strain can be obtained.

  Then, the data processing unit 41 generates image data from the NMR data. Since this image data is generated from NMR data with little influence of B1 distortion, the image quality is good.

  In the above-described example, the case where the pre-scan for measuring the RF transmission phase is executed for each imaging scan has been described. However, the pre-scan for measuring the RF transmission phase may not be performed immediately before the imaging scan. For example, a pre-scan for measuring the RF transmission phase may be executed every desired period, such as performing a pre-scan for measuring the RF transmission phase once a month. That is, the RF phase information stored in the phase information storage unit 43 can be used in an arbitrary imaging scan.

  The magnetic resonance imaging apparatus 20 as described above can calibrate the control value of the RF transmission phase based on the measured value of the RF transmission phase when transmitting the RF transmission signal using a plurality of transmission channels. is there.

  For this reason, according to the magnetic resonance imaging apparatus 20, the distortion of B1 can be corrected more favorably. For example, even if an appropriate RF transmission phase control value changes due to a secular change in the transmission system or a change in output gain, the control value of the RF transmission phase can be calibrated following the change.

  Furthermore, it becomes possible to efficiently irradiate the subject P with the RF transmission signal by B1 shimming using a plurality of transmission channels. As a result, it is possible to reduce SAR (specific absorption rate), which is an index indicating the magnitude of the absorbed energy of the RF signal to the human body.

  In addition, since the magnetic resonance imaging apparatus 20 can measure the RF transmission phase, it is possible to detect abnormality of the components of the transmission system. For this reason, the quality of the magnetic resonance imaging apparatus 20 can be maintained.

  Further, in the magnetic resonance imaging apparatus 20, since it is not necessary to generate an image in order to obtain the calibration value of the RF transmission phase, the RF transmission phase can be calibrated in a short time with a simple process.

DESCRIPTION OF SYMBOLS 20 Magnetic resonance imaging apparatus 21 Magnet for static magnetic field 22 Shim coil 23 Gradient magnetic field coil 24 RF coil 25 Control system 26 Static magnetic field power supply 27 Gradient magnetic field power supply 28 Shim coil power supply 32 Computer 37 Bed P Subject

Claims (3)

Imaging means for performing imaging by transmitting a plurality of high-frequency signals for imaging corresponding to the plurality of high-frequency signal transmission channels to a subject using a plurality of high-frequency signal transmission channels and at least one high-frequency transmission coil;
Transmission phase calibration means for calibrating at least one phase of the plurality of high-frequency signals for imaging based on phase measurement values of a plurality of high-frequency signals corresponding to the plurality of high-frequency signal transmission channels measured in advance;
With
The transmission phase calibration means includes
A plurality of directional couplers for detecting a plurality of traveling waves corresponding to the plurality of high-frequency signals from the plurality of high-frequency signal transmission channels;
A phase acquisition unit for acquiring the phase measurement value based on the plurality of traveling waves;
A high-frequency switch that switches the plurality of traveling waves and sequentially outputs the plurality of traveling waves to the phase acquisition unit via a common signal channel;
I have a,
The high frequency switch is a magnetic resonance imaging apparatus configured to output the plurality of traveling waves to the phase acquisition unit via a reception system circuit of a reception channel that is not used for receiving a magnetic resonance signal . The transmission phase calibration unit is configured to acquire a phase measurement value corresponding to an imaging condition of the imaging by measuring phases of a plurality of high frequency signals corresponding to the plurality of high frequency signal transmission channels prior to the imaging. The magnetic resonance imaging apparatus according to claim 1 . The transmission phase calibration means includes
Storage means for storing the phase measurement value or a phase calibration value based on the phase measurement value;
The magnetic resonance imaging apparatus according to claim 2, wherein at least one phase of the plurality of high-frequency signals for imaging is calibrated using the phase measurement value or the phase calibration value stored in the storage unit.

Images