JP4762432B2 - Magnetic resonance imaging apparatus and method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging (MRI, MR imaging) apparatus. More specifically, the present invention reduces the image quality (IQ / image quality) of an image (hereinafter referred to as MR image) of a magnetic resonance imaging result, thereby reducing artifacts. The present invention relates to a magnetic resonance imaging apparatus (MRI apparatus) and a method for reducing the influence of noise as a factor.
[0002]
[Prior art]
An MRI apparatus introduces (introduces) a subject, for example, a subject into an internal space of a magnet system, that is, a space (gantry) in which a static magnetic field is formed, a gradient (gradient) magnetic field, and a radio frequency (RF) magnetic field. Is applied to the subject to generate a magnetic resonance signal in the subject, and the magnetic resonance signal received by the receiving coil of the magnet system is signal-processed to generate an MR image of the subject.
[0003]
In an MRI apparatus, a magnet assembly is installed in a gantry that shields electromagnetic waves and magnetism, and a magnetic resonance signal received by an RF coil is taken out of the gantry and transmitted to a signal processing apparatus installed at a location away from the gantry. Is transmitted via.
The magnetic resonance signal detected by the RF coil is several μV, amplified to about several mV by an amplifier circuit installed outside the gantry, and then transmitted to the signal processing device via the transmission line.
Since the magnetic resonance signal transmitted from the amplifier circuit to the signal processing device is a low-level analog signal having a high frequency of several mV, it is easily affected by noise in the transmission path.
[0004]
MR imaging has the feature that the contrast of the tissue of the subject can be easily changed by only slightly changing the parameters. On the other hand, since there are many parameters, there is a possibility that artifacts may occur in the MR image even if the parameters are set incorrectly. Since artifacts may cause misdiagnosis when diagnosing the health condition of a subject from MR images, various methods have been tried to prevent artifacts as much as possible and to reduce the generated artifacts.
[0005]
Artifacts can be classified into several types. For example, there are known artifacts caused by body movement of the subject, artifacts caused by the subject's breathing, artifacts caused by magnetic susceptibility imbalance, and the like.
Some artifacts can be prevented by operating the parameters by the operator, while other artifacts cannot be prevented by such an operation.
[0006]
In addition to the artifacts caused by the original cause of MR imaging, the deterioration of the image quality of MR images caused by the influence of irregular noise in the magnetic resonance signal detected in the transmission path cannot be ignored. Such a degradation of the image quality of the MR image can also be a kind of artifact.
Artifacts caused by irregular noise with no reproducibility are particularly problematic because they cannot be repaired by parameter manipulation by the operator.
[0007]
Various measures for reducing noise or reducing noise for a signal processed by a signal processing means such as a computer and / or an electronic circuit have been proposed and implemented.
As one method of such measures, a method of reducing the impedance of a noise band to ground (to ground) path is known.
[0008]
As another method for dealing with noise, the applicant of the present application disclosed in Japanese Patent Application No. 2000-40488 filed on Feb. 18, 2000, a high-frequency, low-level analog magnetic resonance signal detected by a receiving coil in an MRI apparatus. In the scan room near the gantry outside the gantry, it is converted into a digital signal before being transmitted to the image processing device, encoded, and operated via the scan room, making it less susceptible to noise in the transmission path A method has been proposed in which an MR image is generated after a signal transmitted to a room is decoded and a signal encoded by an image processing apparatus in the operation room that has received such a signal is decoded.
According to such a method, the digital signal is less susceptible to noise than the analog signal, and the encoded magnetic resonance signal is transiently processed by performing error correction or the like when encoding. There is an advantage that a normal magnetic resonance signal can be reproduced even when the above noise is superimposed.
[0009]
[Problems to be solved by the invention]
The first noise countermeasure method of reducing the impedance of the noise-to-ground (to-ground) path of the noise band has a problem that the noise resistance characteristics are affected by the strength of the GND system of a facility such as a hospital where an MRI apparatus is installed. is there.
[0010]
In recent years, facilities such as hospitals where MRI apparatuses are installed are equipped with a large number of computers and / or various apparatuses using computers, and various ground currents flow from these apparatuses to the ground. If such a ground current fluctuates the ground potential and the MRI apparatus is grounded for the purpose of reducing the impedance in the vicinity of such a grounding place, conversely, the MRI apparatus's electronic circuit, microcomputer, etc. are not connected to the ground line. Regular noise may enter and be superimposed on the magnetic resonance signal detected by the receiving coil. When an MR image is generated by performing image processing on such a magnetic resonance signal on which irregular noise is superimposed, a problem that the image quality of the MR image is not stable is encountered.
[0011]
The method disclosed in Japanese Patent Application No. 2000-40488 requires that an electronic device containing an analog / digital conversion circuit, an encoding circuit, etc. be disposed in the vicinity of the outside of the gantry in the MRI apparatus. In consideration of supplying power for driving the gantry, there is a disadvantage that the device configuration of the gantry becomes complicated.
[0012]
In the method disclosed in Japanese Patent Application No. 2000-40488, when an error correction code is added at the time of encoding, it is effective for transient noise, but the noise is usually superimposed as an irregular continuous signal. In many cases, the effect of error correction is limited for such noise.
[0013]
As described above, it is desired to generate an accurate MR image of a subject who is not affected by external factors and eliminates the influence of noise in the MRI apparatus.
[0014]
  Therefore,Problems of the InventionRegardless of whether the magnetic resonance signal is converted into a digital signal and encoded in the transmission path between the gantry disposed in the scan room and the image processing device disposed in the operation room. An object of the present invention is to provide a magnetic resonance imaging apparatus (MRI apparatus) capable of suppressing deterioration of the image quality of an MR image that occurs in an artifact due to the influence of noise or suppressing the generated artifact to a negligible level.
[0015]
[Means for Solving the Problems]
  Invention that solves the problemAccording to the first aspect of the present invention, the clock for performing other signal processing related to magnetic resonance imaging other than the processing in the first signal processing means for driving and controlling various coils for magnetic resonance imaging and image processing the results. As a second signal processing means that operates based on the above, a computer and / or an electronic circuit whose clock frequency and / or operating frequency is higher than the Larmor frequency is used. An imaging device is provided.
[0016]
Preferably, while the first signal processing unit drives and controls various coils for magnetic resonance imaging and reads the result into the first signal processing unit, the second signal processing unit performs exception interrupt processing. The processing operation that makes the operation speed of the computer and / or the electronic circuit or the like irregular is not performed.
More preferably, while the first signal processing means drives and controls various coils for magnetic resonance imaging, and the results are read into the first signal processing means, the second signal processing means is a table or the like. Stop the process of the control target that operates.
[0017]
  Invention that solves the problemAccording to the second aspect of the present invention, the first signal processing means for driving and controlling various coils for magnetic resonance imaging and image processing the results drives and controls the various coils for magnetic resonance imaging, While being read into the first signal processing means, the second signal processing means operating on the basis of a clock for performing other signal processing related to magnetic resonance imaging other than the processing in the first signal processing means is magnetic resonance. Provided is a magnetic resonance imaging apparatus that stops processing of a control target that operates at a low speed, such as a table in the imaging apparatus.
[0018]
  Invention that solves the problemAccording to the third aspect, while controlling various coils for magnetic resonance imaging and inputting the results to the image processing means, processing of a controlled object such as a table in the magnetic resonance imaging apparatus is stopped. A magnetic resonance imaging method is provided.
[0019]
  First embodiment
  With reference to FIGS.Invention that solves the problemA first embodiment of the magnetic resonance imaging apparatus (MRI apparatus) will be described. FIG. 1 is a diagram illustrating the arrangement of main parts of an MRI apparatus. FIG. 2 is a diagram illustrating the configuration of main elements of the MRI apparatus.
[0020]
As illustrated in FIG. 1, the MRI apparatus is arranged separately in a scan room and an operation room. For example, a gantry in which various coils are disposed is disposed in the scan room. In the operation room, doctors, technicians, etc. set various parameters for magnetic resonance imaging, various switches for operating the table, cradle, etc., operation unit equipped with a display device that displays image processing results, etc. (Or a console), various signal processing devices, and the like are arranged.
The MRI apparatus illustrated in FIG. 2 accommodates a magnet system 100 and provides a gantry having a lumen in which a table 500 carrying a cradle on which a subject rides can be moved, and a static magnetic field accommodated in the gantry. A magnet system 100 including a main magnetic field coil unit 102, a gradient coil unit 106, and an RF coil unit 108, and a table 500 on which the subject 300 is placed and moves within the lumen of the gantry. The table on which the subject is placed on the cradle is transported in the lumen of the gantry or discharged from the gantry in accordance with an instruction from the operator.
The MRI apparatus also has a gradient coil drive unit 130 that applies a drive signal to the gradient coil unit 106, an RF coil drive unit 140 that applies a drive signal to the RF coil unit 108, and a magnetic field received (detected) by the RF coil unit 108. A data collection unit 150 that receives a resonance signal, a control unit 160, a data processing unit 170, a display unit 180, and an operation unit (operation console) 190 are included.
The MRI apparatus further includes a table 500 on which the subject 300 is mounted, a table driving unit 210 that drives the table 500, and an operation control device 200.
The data collection unit 150, the control unit 160, and the data processing unit 170 correspond to the first signal processing means of the present invention.
The operation unit 190 and the operation control device 200 correspond to the first signal processing means of the present invention.
[0021]
The gantry in the scan room is a part that performs magnetic resonance imaging of the subject 300, and the main magnetic field coil unit 102, the gradient coil unit 106, and the RF coil unit 108 are accommodated therein.
In the gantry, various indicators that display and / or instruct the examination state to the subject 300, for example, a light emitting diode (LED), a voice output device for reporting the examination state to the subject 300 Etc. are provided.
[0022]
When the operator (operator) of the MRI apparatus, for example, a doctor, an engineer, or the like, picks up an MR image of a subject, the operation unit 190 sets various parameters, instructs a transfer operation of the table 500, and performs imaging. This is a part that performs interactive processing with the MRI apparatus such as viewing the MR image displayed on the display unit 180 that has been subjected to image processing, and indicates various switches for setting the operator's instructions, the operating state of the MRI apparatus, and the like. Various devices to display are installed. In many cases, the display unit 180 is also mounted on the operation unit 190.
[0023]
The data processing unit 170 controls the control unit 160 in accordance with an operator command set from the operation unit 190.
The control unit 160 outputs control signals to the gradient coil driving unit 130 and the RF coil driving unit 140 in response to a command from the data processing unit 170, and the gradient coil unit 106 from the gradient coil driving unit 130 and the RF coil driving unit 140, respectively. And the drive current which excites the RF coil part 108 is output.
[0024]
The data collection unit 150 receives the signal detected by the RF coil unit 108 and sends it to the data processing unit 170. Since the magnetic resonance signal detected (received) by the RF coil unit 108 is a signal of a very low level of about several μV, a high frequency amplifier circuit is usually provided outside the gantry in the scan room or near the outside of the gantry. (Not shown) is often installed and amplified to about several mV, for example, and then transmitted to the data collection unit 150 in many cases.
The data processing unit 170 also receives a signal transmitted from the data collection unit 150, performs predetermined signal processing and image signal processing, generates an MR image of the subject 300, and displays the MR image on the display unit 180.
[0025]
The operation control device 200 cooperates with the data processing unit 170 as necessary when the MR image of the subject 300 is captured, and further cooperates with the control unit 160 as necessary, thereby the table driving unit 210. To move the subject 300 on the table 500 into the lumen of the gantry and move the subject 300 inside the gantry according to the imaging situation. After imaging, the subject 300 is moved to the gantry. Move outside.
The table 500 is provided with a motor, a driving mechanism, and the like for moving in accordance with a driving signal from the table driving unit 210.
As described above, the operation control device 200 also drives and controls a display such as an LED and an audio output device installed in the gantry.
[0026]
As described with reference to FIG. 1, in the MRI apparatus according to the first embodiment, the gradient coil unit 106 and the RF coil unit 108 installed in the gantry are driven and controlled, and the magnetism detected by the RF coil unit 108 is detected. A data processing unit 170 that receives a resonance signal, processes the signal, and displays an MR image on the display unit 180, and an operation control device 200 that controls the conveyance of the table 500 on which a motor or the like is installed via the table driving unit 210. And are separated.
The data processing unit 170 performs various types of signal processing on the detected magnetic resonance signal, and performs image signal processing for displaying an MR image on the display unit 180, so that a high-speed signal processing device such as a digital signal processor (DSP) and Built-in microcomputer.
The operation control apparatus 200 also performs conveyance control of the table 500 via the table driving unit 210 and incorporates a microcomputer in order to cooperate with the data processing unit 170. However, since the drive control of the table drive unit 210 is not as fast as the signal processing in the data processing unit 170, the operation speed of the microcomputer built in the operation control device 200 is used from the viewpoint of reducing the price of the MRI apparatus. Is low speed. That is, the clock frequency and operation speed of the operation control device 200 are lower than the clock frequency and operation speed of the data processing unit 170. This microcomputer corresponds to the second signal processing means of the present invention.
[0027]
The signal transmission cable will be described.
Between the gradient coil drive unit 130 and the gradient coil unit 106, between the RF coil drive unit 140 and the RF coil unit 108, and between the RF coil unit 108 and the data collection unit 150, respectively, as transmission paths, It is wired with coaxial cables C1, C2, C3.
[0028]
A signal between the operation unit 190 and the data processing unit 170 is usually a TTL signal of about several volts, for example. Similarly, a signal between the operation unit 190 and the operation control apparatus 200 is also usually a TTL signal of about several volts. Therefore, the cables C4 and C13 are ordinary signal cables that connect the electronic devices.
[0029]
The cable C11 that transmits various drive control signals from the operation control device 200 to the table cradle drive unit 210 is also a normal signal for connecting electronic devices because the various drive control signals are, for example, TTL signals of about several volts. A transmission cable is used. The cable C12 that transmits the state detection signal of the table 500 from the table driving unit 210 to the operation control apparatus 200 is also a normal signal transmission for connecting the electronic devices because the state detection signal is a TTL signal of about several volts, for example. A cable is used.
Cables (not shown) that transmit drive signals from the operation control device 200 directly or via the table drive unit 210 to the LED display of the MRI apparatus, voice notification device, etc. also have several drive signals. Since it is a TTL signal of about V, for example, a normal signal cable for connecting electronic devices is used. However, when the power source of the voice notification device is, for example, AC 100V, a low power cable for supplying power to the electronic device is used.
[0030]
A motor, such as a stepping motor, is used to move the table 500. Therefore, power is supplied via the cable C15 for driving the motor to the table 500 via a power source built in the table driving unit 210 or directly via a separate power source. Since the voltage for driving the motor is about AC100V to AC200V, the power supply cable C15 to the motor driving the table 500 uses a low-voltage power cable.
[0031]
  As described above, in the MRI apparatus of the present embodiment,Invention that solves the problemA data processing unit 170 system corresponding to the first signal processing means;Invention that solves the problemThe operation control device 200 system corresponding to the second signal processing means is separated, but the data processing unit 170 and the operation control device 200 are generally disposed at close positions. The cables C1 to C3, the cables C11 and C12, and the power supply cable C15 are often routed close to the gantry in the scan room.
[0032]
Noise and countermeasures in the MRI apparatus illustrated in FIG. 2 will be described.
[0033]
First noise and countermeasures
A signal transmitted between the operation control device 200 and the table driving unit 210 is a low-frequency signal compared to a signal transmitted from the RF coil unit 108 to the data collecting unit 150, but has many switching signals. There is a high possibility that switching noise will occur for a low-level magnetic resonance signal transmitted from 108 to the data collection unit 150.
As a countermeasure against such noise, the cable C3 is a coaxial cable. More preferably, the coaxial cables C1 to C3 are accommodated in the electromagnetic / electrostatic shielding cable, the cables C11 and C12 are also accommodated in the shielding cable, and the two shielding cables are separated as much as possible.
[0034]
Similarly, a driving switching signal for starting rotation of a motor that moves the table and the cradle 500, stopping rotation of the motor, or changing the direction of movement is applied. Although such a driving switching signal has a low frequency, the voltage level is high, for example, about 100 VAC, and the motor is mounted on the table 500 and moves within the lumen of the gantry. Are likely to affect low level magnetic resonance detection signals.
As a countermeasure against such noise, basically, the cable C16 is a shielded cable so that a drive signal of a motor or the like riding on the table 500 is not leaked to the outside after the cable C3 is a coaxial cable as described above. To do. More preferably, the coaxial cables C1 to C3 are accommodated in the shielding cable, and the cables C15 and C16 are also accommodated in the shielding cable so that the shielding cables are separated as much as possible.
[0035]
Second noise and countermeasures
For the first noise countermeasure described above or after taking the first noise countermeasure, at least the effective second noise countermeasure is to drive the MR coil section 106 and the RF coil section 108 by driving the gradient coil section 106 and the RF coil section 108. The period in which the signal is received does not cause the switching operation described above.
In other words, the operation control device 200 adjusts the operation timing in cooperation with the data processing unit 170 and, if necessary, the control unit 160, and the gradient coil is adjusted by the operations of the data processing unit 170 and the control unit 160. The weakness detected by the RF coil unit 108 by driving the gradient coil unit 106 and the RF coil unit 108 via the driving unit 130 and the RF coil driving unit 140, detected by the RF coil unit 108, and detected by the RF coil unit 108 outside the MRI apparatus. After the magnetic resonance signal is amplified, the data acquisition unit 150 receives the data and imports it into the data processing unit 170 (in this specification, this is referred to as an MR imaging period). Stop the transfer operation. Furthermore, the operation control device 200 also stops the operation of auxiliary devices such as an LED display and a voice notification device installed in the gantry.
In this specification, the data processing unit 170 and the control unit 160 drive and control the gradient coil unit 106 and the RF coil unit 108 for magnetic resonance imaging, and the period until the data collection unit 150 reads the result is expressed as “ This is called “MR imaging period”.
According to such a method, at least the MR imaging period is not affected by the above-described high-frequency switching signal.
As a result, the MR image obtained by processing the magnetic resonance signal through the data acquisition unit 150 and imaged becomes a high quality MR image that is not affected by noise.
[0036]
If the second noise countermeasure is performed, it is not necessary to perform the first noise countermeasure described above or it is not necessary to perform the first noise countermeasure by a strict method, and the equipment price becomes very low. There is an advantage that the wiring work is also simplified.
[0037]
Third noise and countermeasures
The current supplied to the motor or the like mounted on the table 500 is low in level with respect to the low level magnetic resonance signal detected by the RF coil unit 108 and the magnetic resonance signal amplified by the high frequency amplifier circuit. There is a high possibility of electromagnetic induction noise in the resonance detection signal.
As a countermeasure against such noise, a coaxial cable is used as the cable C3.
More preferably, the coaxial cables C1 to C3 are accommodated in the shielding cable, the feeding cable C15 and the drive control cable C16 are also accommodated in the shielding cable, and the shielding cables are separated as much as possible.
[0038]
Fourth noise and countermeasures
As described above, the MRI apparatus performs various kinds of signal processing on the detected magnetic resonance signal and performs image signal processing for displaying the MR image on the display unit 180. Therefore, the high-speed signal processing apparatus such as a digital signal processor ( DSP), a data processing unit 170 incorporating a microcomputer, etc., and a table transport control via the table driving unit 210, and an operation control incorporating a microcomputer to cooperate with the data processing unit 170 The apparatus 200 is provided independently.
Further, as described above, the drive control of the table drive unit 210 is not as high-speed signal processing as the data processing unit 170, and therefore, from the viewpoint of reducing the price of the MRI apparatus, the micro built in the operation control apparatus 200 is used. Computers are slow to operate. A reference clock frequency of such a microcomputer is about 10 to 30 MHz.
[0039]
The inventor of the present application has found that a microcomputer and / or an electronic circuit built in the operation control apparatus 200 is a noise source.
Usually, the operation control device 200 and the data processing unit 170 are arranged in the vicinity. An operation signal of a microcomputer having a reference clock frequency of about 10 to 30 MHz incorporated in the operation control device 200 is propagated and radiated as high frequency noise to the data processing unit 170 and received from the data collection unit 150 to be received by the data processing unit 170. The problem is that the image quality (IQ, image quality) of the MR image processed by the data processing unit 170 is finally reduced by superimposing the signal on the signal processed in the image, thereby reducing the quality of the image processing signal. It was.
Therefore, this noise becomes a problem even when the encoding method proposed in Japanese Patent Application No. 2000-40488 is adopted. Details will be described below.
[0040]
The Larmor frequency ωφ in MRI is defined by the following equation.
[0041]
[Expression 1]
ωφ = γ · Bφ (1)
Where γ is the gyromagnetic ratio,
Bφ is the magnitude of the external magnetic field (unit: Tesla T).
[0042]
The proton precesses around the external magnetic field Bφ, and the frequency of this precession is proportional to the magnetic field strength.
This precession frequency is called the resonance frequency or Larmor frequency. Thus, the Larmor frequency is proportional to the magnitude of the external magnetic field Bφ.
Proton gyromagnetic ratio γ is 42.56 MHz / T.
[0043]
  The strength of the external magnetic field of the coil of the current mainstream MRI apparatus is 0.2T to 1.5T, and its Larmor frequency is 8.512 to 63.84 MHz. Such a Larmor frequency magnetic resonance signal is detected by the RF coil unit 108 and processed by the data processing unit 170. Therefore, it is desirable that a pulse signal having a frequency, a switching signal, or the like is output from the operation control device 200 outside the Larmor frequency band. However, since the reference clock frequency of the low-cost microcomputer currently used for the operation control apparatus 200 is about 10 MHz to 30 MHz, the frequency of the reference clock itself is a Larmor frequency whose band is 8.512 to 63.84 MHz. Enter the band,Invention that solves the problemThe microcomputer in the operation control apparatus 200 as the second signal processing means becomes a noise source. Of course, the degree of noise varies depending on the level of such a frequency signal.
[0044]
Further, it takes one to several clocks to execute one instruction in the microcomputer in the operation control apparatus 200. Therefore, the actual operation speed of the microcomputer in the operation control apparatus 200 is (reference clock frequency) / (number of clocks required to execute one instruction).
Reference clock frequency f_CLK If two clocks are required when one instruction is executed at 10 MHz, the actual operation speed is 5 MHz. Reference clock frequency f_CLK When two instructions are required when executing one instruction at 30 MHz, the frequency f_OPIs 15 MHz.
Further, when an irregular operation such as exception interrupt processing or counter processing waiting for input is added, the frequency f of the substantial operating speed of the microcomputer is obtained._OPFurther decreases from the above-mentioned value.
[0045]
For example, the clock frequency f_CLK When a microcomputer with a frequency of 30 MHz is used, the reference clock frequency f is within the Larmor frequency band described above._CLK Is the frequency f of the actual operating speed when one instruction is executed at 30 MHz for two clocks._OPFor example, a frequency f of 12 to 15 MHz_OPSignals of two frequencies are entered.
Frequency f of substantial operation speed when executing one instruction in three clocks_OPBecomes, for example, 8 to 9 MHz. In this case, three frequencies are included in the Larmor frequency band described above.
[0046]
Even if the operating frequency of an electronic circuit such as a microcomputer in the operation control apparatus 200 is not in the Larmor frequency band, a signal having a frequency close to the Larmor frequency is likely to be noise.
[0047]
As a countermeasure against such noise, if it is assumed that the method of reducing the impedance of the anti-ground (ground) path of the noise band is adopted as in the conventional method, in addition to the above-mentioned problems, all such noise components are considered. On the other hand, it is very difficult to perform noise processing.
In this way, when a low-speed microcomputer is operated in a Larmor frequency band of 8.5 to 63 MHz in an MR apparatus having an external magnetic field of 0.2 T to 1.5 T, which is currently mainstream, the radiation and conduction noise is removed. Measures require troublesome work and enormous costs.
In particular, when the frequency of a signal that becomes a noise source is close to the Larmor frequency, the problem becomes large.
[0048]
Further, since the signal waveform during the operation of the microcomputer is not a single sine wave, the clock frequency f_CLK A harmonic spectrum appears at a frequency that is an integer multiple of. For this reason, even if the operating frequency of the microcomputer is lowered, such a harmonic signal enters the Larmor frequency band, so that such a method is not an effective measure.
[0049]
3A and 3B are graphs illustrating the situation. 3A and 3B, the horizontal axis indicates the frequency, and the vertical axis indicates the signal (and noise) level.
A spectrum with a high signal level has a clock frequency f_CLK The frequency f of the actual operating speed in order of decreasing signal level._OPFurther, the harmonic spectrum is shown in a portion higher than the reference clock frequency.
[0050]
  Mentioned aboveTaskAs a solution forAn invention that solves the problemThe following measures were taken in the embodiment.
[0051]
(1) Clock frequency f_CLK The operation control device 200 uses a microcomputer and / or an electronic circuit (hereinafter referred to as a microcomputer and / or an electronic circuit) as the second signal processing means of the present invention, which is higher than the Larmor frequency band. To do. For example, the clock frequency f_CLK A 300 MHz computer or microcomputer is built in the operation control apparatus 200 to perform the above control operation. Instead of a computer or a microcomputer, an electronic circuit configured using such an electronic circuit, for example, an ASIC (Applied Special Integrated Circuit) may be used for the operation control apparatus 200.
Clock frequency f_CLK However, since a 300 MHz computer or microcomputer is currently widely used as a portable personal computer, for example, it is easy to apply such a computer to the operation control apparatus 200.
[0052]
(2) The frequency f of the substantial operating speed_OPUse a microcomputer and / or electronic circuit higher than the Larmor frequency for the operation control device 200.
[0053]
(3) Clock frequency f_CLK And frequency f_OPA microcomputer and / or an electronic circuit in which the harmonic spectrum of an integral multiple of is very low is used for the operation control device 200.
[0054]
4A and 4B are graphs illustrating the situation. 4A and 4B, the horizontal axis indicates the frequency, and the vertical axis indicates the signal (and noise) level.
A spectrum with a high signal level has a clock frequency f_CLK And the frequency f of the actual operating speed in the order of decreasing signal level._OPFurther, the harmonic spectrum is shown in a portion higher than the reference clock frequency.
In FIGS. 4A and 4B, three typical spectra are illustrated, but in reality there are many more spectra. Even in that case, the solution does not change, the part that is operating during the MR imaging period is operated at a frequency higher than the Larmor frequency, is operated at an operating speed higher than the Larmor frequency, and the Larmor frequency or its harmonic frequency is Larmor frequency. When it enters the band, the operation is stopped and dynamic control is not performed.
[0055]
For example, the clock frequency f of the microcomputer built in the operation control apparatus 200_CLK When the circuit design of the operation control apparatus 200 of the MR apparatus is set to 300 MHz with an external magnetic field of 1.5 T, 300 MHz / 63.84 MHz≈4.7 clocks, and therefore, within 4 clocks (frequency f_OP= 75 MHz), the microcomputer in the operation control apparatus 200 is designed to process one command.
[0056]
When an electronic circuit such as a microcomputer that operates at an operation speed of about 200 MHz is placed in the operation control apparatus 200, the spectrum illustrated in FIG. 4A is obtained.
200 MHz harmonics, for example, 400 MHz and 600 MHz signals are not generated during the MR imaging period. Therefore, the noise spectrum present during imaging is the frequency f of the substantial operating speed._OPThus, the image quality (IQ) of the MR image is not directly affected.
[0057]
As described above, in the case of an MRI apparatus for an external magnetic field of 0.2T to 1.5T, the Larmor frequency was 8.512 MHz to 63.84 MHz, but when the external magnetic field is 0.1T to 3T, As illustrated in FIG. 5, the Larmor frequency is 4.256 to 127.68 MHz. Therefore, an electronic circuit and / or microcomputer in which the frequency of the reference clock and the substantial operating speed of the electronic circuit and / or microcomputer used in the operation control device 200 depart from this Larmor frequency band is used.
[0058]
More preferably, the following measures are additionally performed.
[0059]
(A) During the MR imaging period, the operation control device 200 does not perform processing such as exception interruption processing that makes the microcomputer operating speed irregular. This is because the irregular operation causes switching noise.
[0060]
(B) During the MR imaging period, control target processing such as a table and cradle 500 that requires low-speed operation is not performed. The spectrum of the electronic circuit in the operation control device 200 and / or the microcomputer itself and its harmonics extend to the Larmor frequency band. Therefore, generation of high frequency noise can be prevented by not performing dynamic control during this MR imaging period.
[0061]
Second embodiment
In the first embodiment described above, as proposed in Japanese Patent Application No. 2000-40488, the magnetic resonance signal detected by the weak RF coil unit 108 in the high-frequency amplifier circuit provided outside the gantry in the scan room is used. Although the case where encoding is not performed after amplification has been described, such a signal is received by the data collection unit 150 even when encoding is performed and noise resistance is improved as proposed in Japanese Patent Application No. 2000-40488. After that, when the data collection unit 150 or the data processing unit 170 decodes the signal and then the data processing unit 170 performs signal processing, the fourth noise, that is, the noise of the operating frequency of the microcomputer incorporated in the operation control device 200 or the like. Is affected. Therefore, even when the noise resistance is improved by the method proposed in Japanese Patent Application No. 2000-40488, the fourth noise countermeasure is effective.
[0062]
  The configuration of the MRI apparatus illustrated in FIG. 2 is an example,Invention that solves the problemIs not limited to the illustrated configuration. For example, the data processing unit 170 and the data collection unit 150 may be integrated, or the data processing unit 170 and the control unit 160 may be integrated.
[0063]
  The above figures are examples,Invention that solves the problemIt is obvious to those skilled in the art that the present invention is not limited to the numerical examples described above.
[0064]
【The invention's effect】
  Invention that solves the problemAccording to the above, the operation is performed with reference to a clock for performing other signal processing related to magnetic resonance imaging other than the processing in the first signal processing means for driving and controlling various coils for magnetic resonance imaging and image processing the results. By making the clock frequency or operating frequency of the second signal processing means sufficiently higher than the Larmor frequency, it is possible to substantially eliminate noise that affects the magnetic resonance signal during the MR imaging period. As a result, the image quality of the MR image is improved.
[0065]
  Invention that solves the problemAccordingly, by stopping the operation of the control target by the second signal processing unit during the MR imaging period, it is possible to substantially eliminate noise that affects the magnetic resonance signal. As a result, the image quality of the MR image is improved.
[Brief description of the drawings]
[FIG. 1] FIG.Invention that solves the problemIt is a figure which shows arrangement | positioning of each part of the magnetic resonance imaging device of embodiment.
[FIG. 2] FIG.Invention that solves the problemIt is a block diagram of the magnetic resonance imaging device (MRI apparatus) of the embodiment.
FIGS. 3A and 3B show the Larmor frequency band when the external magnetic field is 0.2 T to 1.5 T, and the clock frequency of the microcomputer in the operation control device in the Larmor frequency band. It is a graph illustrating a spectrum in the case of being in the vicinity.
4 (A) and 4 (B) show the Larmor frequency band when the external magnetic field is 0.2T to 1.5T;Invention that solves the problemIt is the graph which showed the relationship of the drive frequency of the microcomputer in the operation control apparatus by embodiment of this.
FIG. 5 shows the Larmor frequency band when the external magnetic field is 0.1T to 3T, and the clock frequency f in the operation control device._CLK Is a graph illustrating a spectrum when a microcomputer operating at an operating speed of about 300 MHz is used.
[Explanation of symbols]
  100 ・ ・ Magnet system
  102 .. Main magnetic field coil section
  106 .. Gradient coil section
  108 .. RF coil section
  130 .. Gradient coil drive
  140 .. RF coil drive
  150..Data collection part
  160 .. Control part
  170 .. Data processing section
  180 .. display part
  190 .. Operation part
  200 .. Operation control device
  210 .. Table drive section
  300 .. Subject
  500 ... table

Claims (6)

  Second signal that operates on the basis of a clock that performs other signal processing related to magnetic resonance imaging other than the processing in the first signal processing means that drives and controls various coils for magnetic resonance imaging and performs image processing on the results. A magnetic resonance imaging apparatus using a computer and / or an electronic circuit whose clock frequency and / or operating speed is higher than the Larmor frequency as the processing means.   While the first signal processing means drives and controls various coils for magnetic resonance imaging and reads the result into the first signal processing means, the second signal processing means is the computer for exception interrupt processing and the like. 2. The magnetic resonance imaging apparatus according to claim 1, wherein a processing operation for making the operation speed of the electronic circuit or any one of the electronic circuits irregular is not performed.   While the first signal processing means drives and controls various coils for magnetic resonance imaging and reads the results into the first signal processing means, the second signal processing means is a control target that operates at a low speed such as a table. 3. The magnetic resonance imaging apparatus according to claim 1, wherein the process is stopped.   Conductive wire connecting the receiving coil unit and the data collecting unit, conducting wire connecting the transmitting coil unit and the RF coil driving unit, conducting wire connecting the gradient coil unit and the gradient coil driving unit, and connecting the table and the table driving unit. The magnetic resonance imaging apparatus according to claim 1, wherein the conducting wire or the conducting wire connecting the table driving unit and the operation control unit is a coaxial cable.   The magnetic resonance imaging apparatus according to claim 4, wherein the coaxial cable is accommodated in a shielding cable.   6. The magnetic resonance imaging apparatus according to claim 1, wherein a clock frequency of the computer and / or electronic circuit as the second signal processing means or any one of them is 300 MHz or more.

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