JP5562010B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to an MRI apparatus for obtaining a tomographic image of an arbitrary part of a subject using a magnetic resonance phenomenon, and particularly to a technique for specifying a noise generation source.

  A magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) is a magnetic field generating means for applying a static magnetic field and a gradient magnetic field to a subject, and for causing nuclear magnetic resonance in atomic nuclei constituting the biological tissue of the subject. A transmission system that emits a high-frequency signal, a reception system that detects an echo signal emitted from the subject by the above-described nuclear magnetic resonance, and a signal processing system that performs an image reconstruction operation using the echo signal detected by the reception system And a signal processing system for performing image reconstruction calculation using echo signals emitted by nuclear magnetic resonance, and obtaining a tomographic image by repeatedly performing a sequence of measuring echo signals emitted by nuclear magnetic resonance It is like that.

  Here, in the surrounding environment surrounding the MRI apparatus, external wireless noise generated from the nearby electrical and electronic devices is scattered, and means for receiving an echo signal emitted from the subject through the external wireless noise For example, reception may be performed by a high-frequency coil on the reception side. In this case, the external radio noise is mixed in the echo signal measured from the subject, and the image quality of the obtained tomographic image may be deteriorated.

  On the other hand, there are noise sources inside the system, and the image quality of the tomographic image obtained in the same way may be deteriorated.

  In the prior art described in Patent Document 1, in the MRI apparatus, the receiving system is provided with means for receiving external radio noise at a position where the echo signal emitted from the subject is not received, and the external radio noise receiving means A magnetic resonance imaging apparatus comprising a circuit for removing external radio noise mixed in the echo signal receiving means from the noise signal and an output signal from the means for receiving the echo signal from the subject is disclosed. ing.

Japanese Unexamined Patent Publication No. 3-188831

However, the prior art described in Patent Document 1 cannot identify the type of noise, and the noise has to be identified and removed to reduce artifacts.
An object of this invention is to provide the MRI apparatus which can specify the kind of noise.

  In order to solve the above problems, according to the present invention, a magnetic field generating system for applying a static magnetic field and a gradient magnetic field to a subject, and a high frequency for causing nuclear magnetic resonance in an atomic nucleus constituting the biological tissue of the subject. A magnetic resonance imaging apparatus comprising: a transmission system that irradiates a magnetic field; a reception system that detects a nuclear magnetic resonance signal emitted by the nuclear magnetic resonance; and a central processing unit that controls the operation of the entire apparatus. Includes a specifying means for specifying a noise generation source, and the specifying means is configured to two-dimensionally analyze frequency spectra of a plurality of echo signals obtained without applying the gradient magnetic field and the high-frequency magnetic field by the receiving system. The magnetic resonance imaging apparatus is characterized in that the noise generation source can be specified by the appearance frequency of the noise component having a predetermined frequency. Provided.

  A magnetic field generation system that applies a static magnetic field and a gradient magnetic field to the subject, a transmission system that irradiates a high-frequency magnetic field for causing nuclear magnetic resonance to cause nuclei constituting the biological tissue of the subject, and the nuclear magnetic resonance. In a magnetic resonance imaging apparatus comprising a receiving system for detecting emitted nuclear magnetic resonance signals and a central processing unit for controlling the operation of the entire apparatus, the receiving system comprises a specifying means for specifying a noise source. The specifying means determines a noise source based on whether a change in a noise component of a predetermined frequency follows the change in the reception sensitivity when the reception sensitivity of the reception coil of the reception system is changed. A magnetic resonance imaging apparatus capable of being specified is provided.

  According to the present invention, it is possible to provide an MRI apparatus capable of specifying the type of noise.

The block diagram which shows the whole structure of this invention The figure which shows the concept of Example 1 and 2 of this invention The flowchart which shows operation | movement of Example 1 of this invention. Sequence diagram for signal acquisition Diagram showing the resulting spectrum obtained by Fourier transform The flowchart which shows operation | movement of Example 2 of this invention. Diagram showing the resulting spectrum obtained by Fourier transform

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a block diagram showing the overall configuration of an MRI apparatus to which a processing method according to the present invention is applied. This MRI apparatus uses a magnetic resonance phenomenon to obtain a tomographic image of a subject, and as shown in the figure, a static magnetic field generation circuit 1, a gradient magnetic field generation system 2, a transmission system 3, and a reception system 4 A signal processing system 5, a sequencer 6, a central processing unit (CPU) 7, and an operation unit 8.

  The static magnetic field generation circuit 1 generates a uniform static magnetic field around the subject 9 in the direction of the body axis or in a direction perpendicular to the body axis. Magnetic, normal conducting, or superconducting magnetic field generating means are arranged. The gradient magnetic field generation system 2 includes a gradient magnetic field coil 10 that generates a gradient magnetic field in three axis directions of X, Y, and Z, and a gradient magnetic field power source 11 that drives each coil. By driving the gradient magnetic field power supply 11 of the coil, gradient magnetic fields Gs, Gp, and Gf in three axis directions of X, Y, and Z are applied to the subject 9. By applying this gradient magnetic field, a slice plane for the subject 9 can be set, and an echo signal is encoded to provide position information. Applying a compensation current to the shim coil that forms part of the static magnetic field generation circuit 1 or the gradient magnetic field generation system 2 based on the spatial and temporal information of the eddy current resulting from the application of the gradient magnetic field I can do it.

  The transmission system 3 irradiates a high-frequency signal in order to cause nuclear magnetic resonance to occur in the atomic nucleus constituting the biological tissue of the subject 9 by the high-frequency magnetic field pulse transmitted from the sequencer 6. A transmitter 13, a high-frequency amplifier 14, and a transmission-side high-frequency irradiation coil 15, a high-frequency pulse output from the high-frequency oscillator 12 is amplified by the high-frequency amplifier 14, and is then placed close to the subject 9. By supplying the high-frequency irradiation coil 15, electromagnetic waves (high-frequency signals) are irradiated on the subject 9.

  The receiving system 4 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of the nucleus of the biological tissue of the subject 9, and receives a high-frequency receiving coil 16 on the receiving side, an amplifier 17, and a quadrature detector 18 and an A / D converter 19, and the electromagnetic wave (NMR signal) of the response of the subject 9 due to the electromagnetic wave irradiated from the high-frequency irradiation coil 15 on the transmission side is disposed close to the subject 9 Is detected by a high frequency coil 16 and input to an A / D converter 19 via an amplifier 17 and a quadrature phase detector 18 to be converted into a digital quantity, and further by a quadrature phase detector 18 at a timing according to a command from the sequencer 6. The two sets of sampled collected data are sent to the signal processing system 5.

  This signal processing system 5 performs image reconstruction calculation and image display using the echo signal detected by the reception system 4, and processes such as Fourier transform, correction coefficient calculation, image reconstruction and the sequencer for the echo signal CPU 7 for controlling 6; ROM (read-only memory) 20 for storing time-lapse image analysis processing and measurement program and invariant parameters used in its execution; measurement parameter and reception system obtained in previous measurement Echo signal detected in 4 and the image used for region of interest setting are temporarily stored and the RAM (Timely Write Read Memory) 21 that stores parameters for setting the region of interest and the image data reconstructed by the CPU 7 A magneto-optical disk 22 and a magnetic disk 24 serving as a data storage unit for recording the image data read from the magneto-optical disk 22 or the magnetic disk 24. And a display 23 serving as a display unit for visualizing the data and displaying it as a tomographic image.

  The sequencer 6 serves as a control means for repeatedly applying a high-frequency magnetic field pulse causing nuclear magnetic resonance to the atomic nucleus constituting the biological tissue of the subject 9 in a predetermined pulse sequence, and operates under the control of the CPU 7. Various commands necessary for collecting tomographic image data of the subject 9 are sent to the transmission system 3, the gradient magnetic field generation system 2, and the reception system 4. The operation unit 8 inputs control information for processing performed in the signal processing system 5 and includes a trackball or mouse 25 and a keyboard 26.

  Next, the concept of Embodiments 1 and 2 of the present invention will be described with reference to FIG. In FIG. 2, 26 is a receiving coil, 27 is a preamplifier, 28 is a variable gain, and 29 is an ADC. There are two types of noise to be measured in the present invention, one is the external noise 30 that directly enters the receiving coil 26, and the other is the system noise 31 that directly enters the ADC 29. .

Next, a flowchart showing the operation of the first embodiment of the present invention will be described with reference to FIG.
(Step 41)
A receiving coil and a phantom are arranged at predetermined positions (center of the gantry).

(Step 42)
Collect signals without applying gradient or high-frequency magnetic fields. The sequence diagram for signal acquisition is as shown in FIG. In FIG. 4, high frequency pulses, gradient magnetic field pulses, and the like are not applied, and only the ADC is moved at a predetermined time interval to collect only noise.

(Step 43)
The data obtained in step 43 is Fourier transformed.

(Step 44)
The resulting spectrum obtained by Fourier transform is shown in FIG. 5 (a). Steps 44 to 42 are repeated many times, and a spectrum is obtained many times in step 44. The spectra are arranged in the vertical direction as shown in FIG.

  According to this embodiment, it is possible to check whether noise of a specific frequency is frequently generated in the vertical direction or mottled. For example, if the frequency of noise is mottled and it is an interval between trains passing nearby (for example, every 5 minutes), the operator can estimate the noise caused by the train. Further, if the frequency of noise is frequent and it is an interval between vehicles passing nearby (for example, an interval of 30 seconds), it is possible to estimate the noise caused by the vehicle and the operator.

  According to the above embodiment, a magnetic field generation system that applies a static magnetic field and a gradient magnetic field to a subject, a transmission system that irradiates a high-frequency magnetic field for causing nuclear magnetic resonance in an atomic nucleus constituting the biological tissue of the subject, In a magnetic resonance imaging apparatus comprising a receiving system for detecting a nuclear magnetic resonance signal emitted by the nuclear magnetic resonance and a central processing unit for controlling the operation of the entire apparatus, the receiving system specifies a source of noise. Specifying means, and the specifying means includes means for two-dimensionally arranging frequency spectra of a plurality of echo signals obtained without applying the gradient magnetic field and the high-frequency magnetic field by the receiving system. The noise generation source can be specified by the appearance frequency of the noise component of the frequency.

  In other words, in order to distinguish between noise that is always generated or noise that occurs only at a specific timing, the frequency spectrum is expressed two-dimensionally in shades and the number of repetitions is displayed side by side. Thereby, it can be made easy to visually grasp whether or not the occurrence of noise changes with time.

  Next, a flowchart showing the operation of the second embodiment of the present invention will be described with reference to FIG.

(Step 61)
A receiving coil and a phantom are arranged at predetermined positions (center of the gantry).

(Step 62)
Collect signals without applying gradient or high-frequency magnetic fields. The sequence diagram for signal acquisition is as shown in FIG. In FIG. 4, only the noise is collected by moving only the ADC at a predetermined interval without applying a high frequency pulse, a gradient magnetic field pulse, or the like.

(Step 63)
The data obtained in step 62 is Fourier transformed.

(Step 64)
The resulting spectrum obtained by Fourier transform is shown in FIG. 7 (a).

(Step 65)
In this step, steps 62 to 64 are repeated while changing the sensitivity of the receiving coil. The resulting spectrum obtained by changing the sensitivity of the receiving coil is shown in FIG. 7 (b).

(Step 66)
A graph as shown in FIG. 7C is obtained by dividing the spectrum data obtained by changing the reception gain.

(Step 67)
As a result of the division, components having different magnitudes with different reception gains are discriminated as external noise, and components whose magnitudes are not changed are discriminated as intrinsic noise.

  According to the present embodiment, when the sensitivity of the receiving coil is changed by changing the gain, the signal is increased or decreased by the difference in sensitivity if it is external noise, and is not increased or decreased by the sensitivity if it is noise inside the system. Therefore, by dividing the noise data obtained by changing the sensitivity of the receiving coil to obtain the signal ratio and graphing it, it is possible to determine which frequency is the external noise or the noise inside the system. Specifically, when receiving coil sensitivity = a and receiving coil sensitivity = b, two spectra are obtained as shown in Figs. 7 (a) and (b), and the division is calculated as shown on the right side. A simple graph. If the signal rate in the graph on the right changes to the same extent as the change in receiver coil sensitivity (receiver coil sensitivity ratio), the noise at that frequency can be determined as external noise (external noise), and the graph on the right. When the signal rate is different from the change in the sensitivity of the receiving coil (sensitivity ratio of the receiving coil), it is possible to determine that the noise at that frequency is internal noise (internal noise).

  According to the above embodiment, a magnetic field generation system that applies a static magnetic field and a gradient magnetic field to a subject, a transmission system that irradiates a high-frequency magnetic field for causing nuclear magnetic resonance in an atomic nucleus constituting the biological tissue of the subject, In a magnetic resonance imaging apparatus comprising a receiving system for detecting a nuclear magnetic resonance signal emitted by the nuclear magnetic resonance and a central processing unit for controlling the operation of the entire apparatus, the receiving system specifies a source of noise. The specifying unit is configured to determine whether the change in the noise component of a predetermined frequency follows the change in the reception sensitivity when the reception sensitivity of the reception coil of the reception system is changed. The source of noise can be specified.

  The present invention can be used to specify noise in an MRI apparatus.

  27 Frequency spectrum, 28 Data received multiple times to obtain a frequency spectrum and arranged in the vertical direction

Claims (1)

A magnetic field generation system for applying a static magnetic field and a gradient magnetic field to a subject;
A transmission system for irradiating a high-frequency magnetic field for causing nuclear magnetic resonance in an atomic nucleus constituting the biological tissue of the subject;
A receiving system for detecting a nuclear magnetic resonance signal emitted by the nuclear magnetic resonance;
A central processing unit for controlling the operation of the entire apparatus;
Means for identifying a noise generation source based on whether a change in a noise spectrum follows a change in the reception gain by changing a reception gain of a reception coil of the reception system;
Have
The means for identifying the noise generation source determines that a noise component having a frequency at which a ratio of data of two noise spectra having different reception gains is approximately the same as a ratio of a change in the reception gain is external noise. The noise component of the frequency showing a change different from the ratio of the change in the reception gain is determined as intrinsic noise.
A magnetic resonance imaging apparatus.

Images