JP2019111211A - Magnetic resonance imaging apparatus - Google Patents

Abstract

To accurately detect noise in an MRI apparatus, thereby appropriately reducing noise and improving an image quality of an acquired image.SOLUTION: A magnetic resonance imaging apparatus comprises: a transmitting unit for transmitting a high-frequency magnetic field pulse to a subject placed in a static magnetic field; a gradient magnetic field generating unit for applying a gradient magnetic field to the static magnetic field; a receiving unit for receiving a nuclear magnetic resonance signal generated from the subject by application of the high-frequency magnetic field pulse and the gradient magnetic field; a noise measuring antenna for measuring noise simultaneously with the nuclear magnetic resonance signal; and an image reconstructing unit for reconstructing an image on the basis of the magnetic resonance signal.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an MRI apparatus applied to a magnetic resonance imaging apparatus (hereinafter referred to as "MRI apparatus") to remove noise affecting an image.

  Conventionally, a nuclear magnetic resonance (NMR) signal generated by applying a high frequency magnetic field (RF) pulse to a subject is measured in a state where the subject (particularly a human body) is disposed in a uniform static magnetic field, There is known an MRI apparatus which two-dimensionally or three-dimensionally images the form and function of the head, abdomen, limbs and the like. In the MRI apparatus, NMR signals are subjected to different phase encoding by gradient magnetic fields and frequency-encoded to be measured as time-series data, and the measured NMR signals are reprocessed as an image by performing two-dimensional or three-dimensional Fourier transform. Configured

  Incidentally, since the NMR signal has a smaller amplitude than the electromagnetic wave propagating in the air, the electromagnetic wave generated from a radio, a television, a mobile phone, a motor, a switch or the like becomes noise. Therefore, by installing the MRI apparatus in the electromagnetic wave shield room made of a highly conductive material such as copper foil, the external noise source and the MRI apparatus are isolated, and the MRI apparatus arrives from the outside of the electromagnetic wave shield room Shielding electromagnetic waves.

  On the other hand, in the MRI apparatus, in addition to such electromagnetic noise, noise called “shot noise” or “spike noise” may occur during imaging. FIG. 11A shows an example of shot noise. For example, by changing the phase encoding gradient magnetic field, the NMR signal measured at the time of imaging is filled in the k space having time on the horizontal axis and the phase encode direction on the vertical axis. There are cases where shot noise of is present in addition to the data making up the object image. When the data including shot noise shown in FIG. 11A is Fourier-transformed into an image as shown in FIG. 11A, a diagonal mesh pattern is generated as shown in FIG. The pattern differs depending on the position and number of the shot noise signal, and it is difficult to indicate the nature of the noise in general, but in many cases the geometric pattern can be seen on the background without a signal.

The causes of such noise generation can be divided into several types of patterns. Most likely it is due to metal-to-metal contact. In particular, the contact between dissimilar metals generates an electric potential at the moment of contact by the piezoelectric effect, causing a partial discharge and becoming a noise source. In addition, the contact of two metal members at different potentials causes noise as well. Besides, contact failure in the electric circuit also causes noise. If there are loose connectors, lines, devices, etc. in the circuit, the connection state changes with the vibration of the gradient coil, and noise synchronized with the drive of the gradient coil can be generated.
Also, static electricity can be a source of noise. For example, when different types of resin members are rubbed or separated by driving of the gradient magnetic field coil, one of them may be charged, and may be discharged during the repetition to generate static electricity.
As described above, in many cases, physical vibrations of a gradient magnetic field coil, a refrigerator, or the like propagate to a problem point, and finally noise is generated.

  If such noise affects the image, it is necessary to identify and correct the noise source to obtain an image with reduced noise. Therefore, Patent Document 1 discloses a transient noise detection method for detecting a broadband noise propagating along a signal cable and correcting an NMR signal based on the detected noise.

Patent No. 3992690

By the way, the receiving coil and preamplifier provided in the MRI apparatus are tuned to the frequency of the NMR signal. In the transient noise detection method of Patent Document 1 described above, the noise event detector that detects noise is disposed outside the imaging bore and in the vicinity of the signal cable, so the noise event detector outputs from the receiving coil or preamplifier The NMR signal may also be falsely detected as noise, and it is not always appropriate to use the detection signal from the noise event detector for the correction of the NMR signal.
On the other hand, a method of reducing noise by image processing is also conceivable. That is, since the shot noise described above indicates a significantly large signal amount, a portion showing a signal significantly larger than the signal amount of the surrounding data in the k-space data is extracted as shot noise to perform so-called 0 padding. It is conceivable to reduce noise by doing this. However, also in this case, the NMR signal may be erroneously extracted as noise. Therefore, it is desirable to improve the noise detection accuracy.

  The present invention has been made in view of the above-described circumstances, and its object is to detect noise accurately in an MRI apparatus, reduce noise as appropriate, and improve the quality of an image to be acquired.

In order to solve the above-mentioned subject, the present invention provides the following means.
One aspect of the present invention is a transmitter configured to transmit a high frequency magnetic field pulse to an object placed in a static magnetic field, a gradient magnetic field generation unit applying a gradient magnetic field to the static magnetic field, the high frequency magnetic field pulse and the gradient magnetic field A receiver for receiving a nuclear magnetic resonance signal generated from the subject when a voltage is applied, a noise measurement antenna for measuring noise simultaneously with the nuclear magnetic resonance signal, and reconstructing an image based on the magnetic resonance signal And an image reconstruction unit.

  According to the present invention, it is possible to accurately detect noise in an MRI apparatus, reduce noise appropriately, and improve the image quality of an acquired image.

It is a block diagram showing a schematic structure concerning an MRI apparatus concerning a 1st embodiment of the present invention. The MRI apparatus which concerns on the 1st Embodiment of this invention WHEREIN: It is a reference drawing which shows a mode that a noise measurement antenna measures the noise which propagated as electromagnetic waves in the air. The MRI apparatus which concerns on the 1st Embodiment of this invention WHEREIN: It is a reference drawing explaining the candidate position of the arrangement position of a noise measurement antenna. It is explanatory drawing which shows the relationship of the frequency of a NMR signal and a noise signal. (A) acquired in the MRI apparatus is an NMR signal, (B) is a noise, and (C) is an explanatory view schematically showing the NMR signal after noise removal. It is a flowchart which shows the flow of imaging | photography and noise measurement processing in the MRI apparatus which concerns on the 1st Embodiment of this invention. It is a flowchart which shows the flow of imaging | photography and noise measurement processing in the MRI apparatus which concerns on the 2nd Embodiment of this invention, (A) is a flow of a process in the case of performing re-imaging automatically, when shot noise exists. In the case where shot noise is present, (B) shows that the shot noise is present to the user and the flow of processing when recommending re-shooting is performed. In the noise source search device concerning a 2nd embodiment of the present invention, it is a reference drawing showing an example of a screen for notifying a user that shot noise exists and recommendation of re-photographing. (A) acquired in the MRI apparatus is an NMR signal, and (B) is an explanatory view schematically showing noise. It is a flowchart which shows the flow of imaging | photography and noise measurement processing by the MRI apparatus which concerns on the 3rd Embodiment of this invention. (A) is a reference diagram showing an example of a shot noise signal in K space data, (B) is a reference diagram showing an example of a shot noise signal when K space data of (A) is changed to a real space image. is there.

  A magnetic resonance imaging apparatus according to an embodiment of the present invention comprises: a transmission unit that transmits a high frequency magnetic field pulse to an object placed in a static magnetic field; a gradient magnetic field generation unit that applies a gradient magnetic field to the static magnetic field; A receiver for receiving a nuclear magnetic resonance signal generated from the subject when a high frequency magnetic field pulse and a gradient magnetic field are applied; a noise measurement antenna for measuring noise simultaneously with the nuclear magnetic resonance signal; And an image reconstruction unit that reconstructs an image based on the image.

(Configuration of MRI apparatus)
The MRI apparatus shown in FIG. 1 includes a static magnetic field generation unit 2, a gradient magnetic field generation unit 3, a sequencer 4, a transmission unit 5, a reception unit 6, a noise measurement unit 7, and a signal processing unit 8.

  The static magnetic field generation unit 2 is configured by arranging a static magnetic field generation source of a permanent magnet system, a normal conduction system, or a superconducting system around the subject 1. The static magnetic field generation unit 2 generates a static magnetic field uniform in the space around the subject 1 in the direction orthogonal to the body axis in the case of the vertical magnetic field method, and in the body axis direction in the case of the horizontal magnetic field method.

  The gradient magnetic field generation unit 3 includes gradient magnetic field coils 9 for applying gradient magnetic fields in the directions of three axes of X, Y and Z, which are coordinate systems (static coordinate systems) of the MRI apparatus, and gradient magnetic fields for driving the respective gradient magnetic field coils 9. A power supply 10 is provided. The gradient magnetic fields Gx, Gy, Gz are applied in the three axial directions of X, Y, Z by driving the gradient magnetic field power supply 10 of each coil in accordance with an instruction from the sequencer 4 described later.

  At the time of imaging, slice direction gradient magnetic field pulses (Gs) are applied in the direction orthogonal to the slice plane (imaging cross section) to set the slice plane with respect to the subject 1, and the remaining 2 orthogonal to the slice plane The phase encoding direction gradient magnetic field pulse (Gp) and the frequency encoding direction gradient magnetic field pulse (Gf) are applied in one direction to encode positional information of each direction in the NMR signal.

  The sequencer 4 controls the RF pulse and the gradient magnetic field pulse to be applied repeatedly in a predetermined pulse sequence. The sequencer 4 operates under the control of the CPU 1 described later, and transmits various commands necessary for data acquisition of tomographic images of the subject 1 to the transmission system 5, the gradient magnetic field generation system 3, and the reception system 6.

  The transmitting unit 5 irradiates the subject 1 with an RF pulse in order to cause nuclear magnetic resonance to atomic spins of atoms constituting the living tissue of the subject 1, and the high frequency oscillator 11, the modulator 12, and the high frequency An amplifier 13 and a high frequency coil (transmission coil) 14 a on the transmission side are provided. The RF pulse output from the high frequency oscillator 11 is amplitude-modulated by the modulator 12 at a timing according to a command from the sequencer 4, and the amplitude-modulated high frequency pulse is amplified by the high frequency amplifier 13 and then disposed close to the subject 1 The RF pulse is irradiated to the subject 1 by supplying to the high frequency coil 14a.

  The receiving unit 6 detects an NMR signal (nuclear magnetic resonance signal) emitted by nuclear magnetic resonance of nuclear spins constituting the living tissue of the subject 1, and receives a high-frequency coil (receiving coil) 14b on the receiving side. A signal amplifier 15, a quadrature phase detector 16, and an A / D converter 17 are provided.

The NMR signal generated in the subject 1 excited by the RF pulse emitted from the high frequency coil 14 a on the transmission side is detected by the high frequency coil 14 b disposed in the vicinity of the subject 1 and amplified by the signal amplifier 15. After that, the signal is divided into two orthogonal signals by the quadrature detector 16 at a timing according to a command from the sequencer 4, and each signal is converted into a digital signal by the A / D converter 17 to be measured data in the signal processing unit 8. Sent.
The A / D converter 17 simultaneously receives the noise measured by the noise measurement unit described later simultaneously with the NMR signal, and converts the noise into a digital signal.

  In FIG. 1, the high-frequency coil 14a on the transmission side and the gradient magnetic field coil 9 face the subject 1 in the static magnetic field space of the static magnetic field generation unit 2 into which the subject 1 is inserted. In the case of the horizontal magnetic field type, it is installed so as to surround the subject 1. Further, the high frequency coil 14b on the receiving side is installed to face or surround the subject 1.

  The noise measuring unit 7 measures a noise generated in the MRI apparatus simultaneously with the NMR signal to detect shot noise generated in the MRI apparatus, and a noise measuring antenna 210, and an amplifier 211 which amplifies noise measured in the noise measuring antenna 210. Is equipped. The noise measurement unit 7 is disposed near the subject space where the subject 100 is placed, for example, near the gantry, and simultaneously receives a signal generated in the subject space at the same time as the high frequency coil 14b for reception. The arrangement location of the noise measurement unit 7 will be described later. Further, FIG. 2 shows a reference diagram in the case where noise generated from the MRI apparatus is measured by the noise measurement unit 7.

  As the noise measurement antenna 210, it is preferable to apply an antenna that measures an electric field, or to apply an antenna in which a high pass filter is added to the antenna that measures a magnetic field. This is because the noise measurement antenna 210 is exposed to the gradient magnetic field in the vicinity of the object space. As a system of an antenna, for example, a dipole-type broadband antenna is preferable.

  In addition, the noise measurement antenna 210 needs to have a specific frequency characteristic so as to detect a shot noise reliably, and not erroneously detect an NMR signal or a magnetic field generated by a gradient magnetic field coil as a noise. Here, in general, since the shot noise contains a frequency component much higher than the MRI signal, the noise measurement antenna 210 is a resonant frequency of a hydrogen nucleus of the subject placed in the static magnetic field applied by the static magnetic field generation unit 2. It is configured to be able to receive a signal having a frequency of twice or more of (resonance frequency of 64 MHz with 1.5 T MRI apparatus and 128 MHz with 3 T MRI apparatus). As the amplifier 211, one having the same frequency characteristic as the noise measurement antenna 211 is applied.

  The noise measurement antenna 210 may be disposed in the vicinity of the object space. An example of the arrangement position of the noise measurement antenna 210 is shown in FIG. As shown in FIG. 3, as the vicinity of the bore, the inside of the high-frequency coil 14b for reception, the inside of the high-frequency coil 14a for transmission, the air gap in the cover, the inside or surface of the bed, the bed base, etc. are considered first. However, if it is arranged in a place where the RF pulse from the high frequency coil 14a is too strong, it becomes difficult to design a filter for removing this, so it is preferable to avoid inside the high frequency coil 14b for reception and inside the high frequency coil 14a for transmission. . The interior of the bed or the surface is not uniform in position with respect to the bore, which makes it difficult to design the filter. Therefore, it is preferable to install the antenna inside the cover of the gantry or at the bed base.

  The signal processing unit 8 performs various data processing and displays and saves processing results, and the CPU (central processing unit) 1, a storage unit 18 such as a RAM and a ROM, and an external storage such as a magnetic disk and an optical disk. A device 19 and a display 20 such as a CRT are provided.

  The CPU 1 controls the entire MRI apparatus. That is, the CPU 1 applies gradient magnetic field pulses to the subject 1 and irradiates high frequency magnetic field pulses according to an imaging sequence based on the input imaging conditions, acquires a plurality of NMR signals, and captures a tomographic image of the subject 1 Control the sequencer 4 to do so. When data from the receiving system 6 is input to the CPU 1, the CPU 1 executes processing such as signal processing and image reconstruction. The CPU 1 causes the display 20 to display the obtained tomographic image of the subject 1 and records the tomographic image on the storage device 18 or the external storage device 19.

The CPU 1 evaluates the signal from the noise measurement antenna 210 after acquiring an echo signal of one echo or after acquiring all echoes. In this evaluation, the measurement data from the noise measurement antenna 210 is comprehensively evaluated from the peak value of noise, the number of appearances of noise, and the like. Then, according to the evaluation result, for example, re-shooting of the entire shooting sequence, re-shooting of only echo with noise, notification of noise occurrence to the user and recommendation of re-shooting, correction of signal of noise portion, etc. Can. By these processes, high-quality images from which noises have been removed can be obtained as a result of correction or re-shooting.
In the present embodiment, shot noise is determined from measurement data by the noise measurement antenna 210, and this is removed to generate a reconstructed image.

  Further, as shown in FIG. 1, the CPU 1 includes a noise determination unit 31, a noise removal unit 32, and a notification unit 33, and each unit included in the CPU 1 detects shot noise generated from the MRI apparatus and as needed. Processing such as removing noise or notifying the user that shot noise has occurred is performed. At the time of processing, a memory (not shown) is used as a work area, and the storage device 18 stores in advance necessary programs and data used for processing, data generated during processing, data obtained as a result of processing, and the like.

  The functions of the units included in the CPU 1 can be realized as software by the CPU 1 reading and executing a program stored in advance in a predetermined storage device. In addition, part or all of the operations performed by each unit included in the CPU 1 can be realized by hardware such as an application specific integrated circuit (ASIC) or a field-programmable gate array (FPGA).

  The noise determination unit 31 receives an input of noise measured by the noise measurement antenna, amplified by the amplifier 211, and converted into a digital signal by the A / D converter 17, and the intensity of the noise exceeds a predetermined threshold. The noise is determined as shot noise. As shown in FIG. 2, when noise is generated in the noise source, it propagates through the air as an electromagnetic wave, and the noise measurement antenna 210 catches the noise and performs measurement. The noise captured by the noise measurement antenna 210 is amplified by the amplifier 211, input to the AD converter 17 via a cable, converted to a digital signal, and then input to the CPU 1. Then, the noise determination unit 31 of the CPU 1 determines noise exceeding a predetermined threshold as shot noise.

Here, as schematically shown in FIG. 4, the noise contains frequency components that are predominantly higher than the frequency of the NMR signal. For example, the noise contains frequency components from 1 GHz to about 20 GHz, and at least one digit for 64 MHz to 128 MHz, which is the resonance frequency of the hydrogen nucleus of the subject placed in the static magnetic field applied by static magnetic field generation unit 2 The frequency is composed mainly of high frequency components. Therefore, among the noise measured by the noise measurement antenna 210, shot noise can be extracted by determining a noise whose intensity exceeds a predetermined threshold as the shot noise.
FIG. 5 is a schematic view of an NMR signal (upper stage), noise (middle stage), and an NMR signal after noise removal (lower stage). As shown in FIG. 5, since the noise measurement antenna 210 measures noise at the same time as the imaging operation of the MRI apparatus, the generation time of the noise determined as the shot noise is the generation time of the noise in the NMR signal Can be identified.

  The noise removing unit 32 removes shot noise from the NMR signal when there is noise determined to be shot noise by the noise determining unit. That is, the noise removing unit 32 performs, for example, processing such as zero padding on a signal at the same time as the occurrence time of noise determined as the shot noise by the noise determining unit 31 among the NMR signals shown in the upper part of FIG. To correct the NMR signal by suppressing noise. As a result, the noise removing unit 32 can generate k space data after noise removal shown in the lower part of FIG. 5.

  The middle stage noise represents a pulse in a high band, so it is an impulse with a narrow width, but the NMR signal in the upper stage is in a state of being filtered at a low frequency range, and ringing occurs. Therefore, it is necessary to zero-fill the noise time extracted by the noise determination unit 31 with a width so that the ringing can also be removed. The appropriate width may be provided in consideration of the bandwidth determined by the imaging conditions and the frequency component of the noise.

The removal of noise by the noise removal unit 32 may be performed automatically, or may be performed using an instruction from the user as a trigger.
The notification unit 33 notifies the user that shot noise is present. The notification to the user by the notification unit 33 can be performed by displaying on the display unit, sounding an alarm, or the like.

The operation unit 25 is for inputting various control information of the MRI apparatus and control information of processing performed by the signal processing unit 8, and includes an input unit 23. As the input unit 23, one or more input devices such as a mouse, a keyboard, and a trackball can be applied in combination. Further, the input unit 23 receives an input of imaging conditions from the user, and transmits the input imaging conditions to the CPU 1.
By arranging the operation unit 25 close to the display 20, the user can control various processes of the MRI apparatus interactively through the operation unit 25 while looking at the display 20.

The imaging and noise measurement process in the MRI apparatus configured as described above will be described according to the flowchart of FIG.
As shown in FIG. 6, when imaging is started in the MRI apparatus, an NMR signal is received by the high frequency coil 14b in step S11, and at the same time, noise is measured by the noise measurement antenna 210. In step S12, the noise determination unit 31 determines whether noise (measurement data) measured by the noise measurement antenna 210 has noise with an intensity exceeding the threshold. As a result of the determination, if there is no noise having an intensity exceeding the threshold, the process proceeds to step S15, and if there is noise having an intensity exceeding the threshold, the process proceeds to step S13. At this time, the notification unit 33 may notify the user that noise having an intensity exceeding the threshold is present.

  In step S13, the noise determination unit 31 extracts noise determined to indicate an intensity exceeding the threshold in the determination in step S12 as shot noise. In step S14, noise can be suppressed or eliminated by performing zero padding or the like on the signal in the NMR signal at the same time as the shot noise extracted by the noise determination unit 31 in step S13. In step S15, when it is determined that there is shot noise in step S12, based on the NMR signal after noise removal, when it is determined that there is no shot noise in step S12, the NMR signal received in step S11 is Each image is reconstructed based on it.

  The image reconstructed and obtained by the CPU 1 is displayed on the display 20. At this time, when the image is reconstructed based on the NMR signal after noise removal, when the image is displayed on the display 20, a noise having an intensity exceeding the threshold is also present, and thus the image is corrected. By displaying on the display 20 via the notification unit 33, the user may be notified.

  As described above, according to the present embodiment, a noise measurement antenna, which is a dedicated antenna for measuring noise in the MRI apparatus, is provided, and the noise is received simultaneously with the reception of the NMR signal. Since the noise measurement antenna is configured to receive a signal having a frequency twice or more the resonance frequency of the MRI apparatus, the NMR signal is not erroneously measured as noise, and the image quality of the measured noise is improved. A threshold is determined to detect noise that may have an influence, and noise that indicates an intensity exceeding the threshold is detected as shot noise. By doing this, in the MRI apparatus according to this embodiment, the shot noise can be accurately detected, and the detected shot noise is removed from the NMR signal, or the user is notified of the presence of the shot noise. it can. Therefore, noise can be appropriately reduced, and the image quality of the acquired image can be improved.

Second Embodiment
As described above, the CPU 1 can comprehensively evaluate the measurement data from the noise measurement antenna 210 based on the peak value of noise, the number of appearances of noise, etc., and according to the result of the evaluation, for example, It is possible to perform re-shooting of the entire shooting sequence, re-shooting of only echo with noise, notifying the user of noise occurrence and recommending re-shooting, and correcting a signal of a noise portion. In the first embodiment described above, when shot noise is detected, the noise is removed, and the image is reconstructed based on the noise-removed NMR signal. In the present embodiment, a case where re-shooting is performed when shot noise is present will be described.

  Hereinafter, the flow of processing in the case of performing re-imaging when there is shot noise by the MRI apparatus according to the present embodiment will be described according to the flowchart of FIG. 7. FIG. 7A is a flowchart showing the flow of processing in the case of automatically performing re-shooting when there is shot noise, and FIG. 7B shows the shot noise for the user when there is shot noise. It is a flowchart which shows the flow of the process in the case of recommending that it exists and re-photographing.

  First, processing in the case of automatically performing re-shooting in FIG. 7A will be described. As shown in FIG. 7A, when imaging is started in the MRI apparatus, an NMR signal is received by the high frequency coil 14b in step S21, and noise is measured by the noise measurement antenna 210 at the same time. In step S22, the noise determination unit 31 determines whether noise (measurement data) measured by the noise measurement antenna 210 has noise with an intensity exceeding the threshold. As a result of the determination, if there is no noise having an intensity exceeding the threshold value, the process proceeds to step S23, and image reconstruction is performed based on the NMR signal received in step S21. On the other hand, as a result of the determination in step S22, if there is noise of an intensity exceeding the threshold value, the process returns to step S21 and re-shooting is performed. Re-imaging may be re-imaging of the entire imaging sequence or re-imaging of only echo with noise, etc., which can be set in advance in the MRI apparatus, and re-imaging based on the user's selection. You may decide to do it.

  In addition, the notification unit 33 may notify the user that re-shooting is to be automatically performed because noise of an intensity exceeding the threshold value is present at the time of re-shooting. In the case of performing re-shooting of the entire shooting sequence, it takes the same time as the original shooting, and the same noise may occur. On the other hand, when only the echo with noise is recaptured, although it may not be applicable depending on the imaging sequence, it is preferable in that it does not lead to a large extension of the imaging time.

  Next, the process shown in FIG. 7 (B) will be described in the case where shot noise is present in the user and recommendation for re-shooting is performed. As shown in FIG. 7B, when imaging is started in the MRI apparatus, an NMR signal is received by the high frequency coil 14b in step S31, and noise is measured by the noise measurement antenna 210 at the same time. In step S32, the noise determination unit 31 determines whether the noise (measurement data) measured by the noise measurement antenna 210 has noise with an intensity exceeding the threshold. As a result of the determination, if there is no noise having an intensity exceeding the threshold, the process proceeds to step S34, and if there is noise having an intensity exceeding the threshold, the process proceeds to step S33.

  In step S33, the notification unit 33 displays, for example, a screen as shown in FIG. 8 on the display 20 in order to notify the user that shot noise is present and the recommendation for re-shooting. Whether or not to perform re-shooting is left to the judgment of the user. In step S34, after the effect of shot noise and recommendation for re-shooting for a certain period of time, or the OK button of the screen shown in FIG. After pressing, the process proceeds to step S34.

  In step S34, image reconstruction is performed based on the NMR signal received in step S31. After confirming the reconstructed image on the display 20, the user can determine the necessity of re-imaging. Therefore, for example, if the noise in the obtained reconstructed image is a noise that can be diagnosed, it can be determined that the time extension by re-shooting is avoided.

The noise detection process in step S22 and step S32 may be performed after all echoes have been acquired, or may be performed for each echo while shooting. If one echo is performed, it is preferable in that it does not delay the reconstruction.
By doing this, it is possible to accurately detect shot noise, remove the detected shot noise from the NMR signal, or notify the user of the presence of the shot noise. Therefore, noise can be appropriately reduced, and the image quality of the acquired image can be improved.

Third Embodiment
In each of the embodiments described above, when shot noise is present, the case of removing the noise and correcting the NMR signal or performing the re-imaging is separately performed independently, but these may be combined. In the present embodiment, noise correction and re-shooting are combined.

  As shown in FIG. 9, when noise is generated near the center of the k space (here, referred to as Zone A), since there is a possibility that sufficient correction can not be performed, re-shooting is performed. That is, referring to FIG. 9, it can be seen that in Zone A, the portion where the signal strength is high in the NMR signal and the shot noise are received at almost the same time. Therefore, if the signal in this portion is removed, not only the noise but also the NMR signal will be removed, and the removal of the noise will lead to the reduction of the image quality. Therefore, in the case of Zone A, re-shooting is performed without removing noise. On the other hand, when noise is generated only in Zone B, correction can be performed without significantly affecting the image quality, so noise correction is performed without performing re-imaging.

  Zone A is defined by a circle centered on the origin of the k space, and is defined for each sequence and every BW. If it is difficult to determine theoretically, the boundaries may be defined empirically. By doing this, it is possible to minimize the two disadvantages of time extension of re-shooting and failure of noise correction.

A specific process flow will be described according to the flowchart of FIG.
As shown in FIG. 10, when imaging is started in the MRI apparatus, an NMR signal is received by the high frequency coil 14b in step S41, and noise is measured by the noise measurement antenna 210 at the same time. In step S42, the noise determination unit 31 determines whether the measurement data included in Zone A includes noise with an intensity exceeding the threshold. As a result of the determination, if there is noise of an intensity exceeding the threshold value, the process returns to step S41, and re-shooting is performed. As a result of the determination, if there is no noise having an intensity exceeding the threshold, the process proceeds to step S43, and the noise determination unit 31 determines whether the measurement data included in Zone B includes noise having an intensity exceeding the threshold. As a result of the determination in step S43, when there is noise of an intensity exceeding the threshold in Zone B, the process proceeds to step S44, and when there is no noise of an intensity exceeding the threshold, the process proceeds to step S45.

In step S44, noise determined to have an intensity exceeding the threshold in Zone B is extracted as shot noise, and noise correction is performed by performing zero padding etc. on the signal in the NMR signal at the same time as the shot noise, Go to S45.
In step S45, if there is no shot noise in zone B, if there is shot noise in zone B based on the NMR signal received in step S41 and this is corrected, then based on the NMR signal corrected in step S44. Perform image reconstruction. The obtained image is displayed on the display 20.
By doing this, it is possible to accurately detect shot noise, remove the detected shot noise from the NMR signal, or notify the user of the presence of the shot noise. Therefore, noise can be appropriately reduced, and the image quality of the acquired image can be improved.

1 central processing unit (CPU) 2 static magnetic field generation unit 3 gradient magnetic field generation unit 4 sequencer 5 transmission unit 6 reception unit 7 ... Noise measurement unit, 8 ... Signal processing unit, 9 ... Gradient magnetic field coil, 10 ... Gradient magnetic field power supply, 11 ... High frequency oscillator, 12 ... Modulator, 13 ... High frequency Amplifier, 14a, 14b: high frequency coil, 15: signal amplifier, 16: quadrature detector, 17: A / D converter, 18: storage device, 19: external storage Device 20 20 display 23 input unit 25 operation unit 210 noise measurement antenna 211 amplifier 31 noise determination unit 32 noise removal Part, 33 ... notification part

Claims (10)

A transmitter configured to transmit a high frequency magnetic field pulse to a subject placed in a static magnetic field;
A gradient magnetic field generating unit that applies a gradient magnetic field to the static magnetic field;
A receiver configured to receive a nuclear magnetic resonance signal generated from the subject by the application of the radio frequency magnetic field pulse and the gradient magnetic field;
A noise measurement antenna that measures noise simultaneously with the nuclear magnetic resonance signal;
An image reconstruction unit that reconstructs an image based on the magnetic resonance signal;
Magnetic resonance imaging device equipped with   The magnetic resonance imaging apparatus according to claim 1, further comprising a noise determination unit that determines the noise as a shot noise when the intensity of the noise measured by the noise measurement antenna exceeds a predetermined threshold. And a noise removing unit that removes the shot noise from the nuclear magnetic resonance signal when there is noise determined to be a shot noise by the noise determining unit.
The magnetic resonance imaging apparatus according to claim 2, wherein the image reconstruction unit reconstructs an image based on the nuclear magnetic resonance signal from which the shot noise has been removed.   4. The magnetism according to claim 3, wherein the noise removing unit removes noise by replacing the nuclear magnetic resonance signal corresponding to a time when noise determined to be shot noise by the noise determining unit is generated by a predetermined value. Resonance imaging device.   3. The magnetic resonance imaging apparatus according to claim 2, wherein the noise determination unit re-captures a nuclear magnetic resonance signal again when the shot noise is present.   3. The magnetic sensor according to claim 2, wherein the noise determination unit re-acquires a nuclear magnetic resonance signal again when the shot noise is present at a predetermined position in k space in which the nuclear magnetic resonance signal is arranged. Resonance imaging device.   The magnetic resonance imaging apparatus according to claim 2, further comprising: a notification unit that notifies the user that the shot noise is present when the shot noise is present.   The magnetic resonance imaging apparatus according to claim 7, wherein the notification unit causes the display unit that displays the image to display that the shot noise is present.   The magnetic resonance imaging apparatus according to claim 1, wherein the noise measurement antenna is an antenna that detects an electric field.   The magnetic resonance imaging apparatus according to claim 1, wherein the noise measurement antenna receives and measures noise including a frequency component twice or more the resonance frequency of a hydrogen nucleus in an imaging target of an object placed in the static magnetic field.

Images