JP5037956B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as `` NMR '') signals from hydrogen, phosphorus, etc. in a subject and images nuclear density distribution, relaxation time distribution, etc. In particular, the present invention relates to filtering of a received signal.

  MRI equipment measures the NMR (echo) signal generated by the nuclear spins that make up the body of the subject, especially the human body, and the shape and function of the head, abdomen, limbs, etc. in two or three dimensions. A device for imaging. In imaging, the echo signal is given different phase encoding depending on the gradient magnetic field and is frequency-encoded and measured as time-series data. The measured echo signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  After receiving the echo signal from the receiving coil, it is reconfigured in the signal processing system via the receiving system. Conventionally, a receiving system contains a lot of noise mixed from the outside, and noise reduction is performed by an analog bandpass filter or a digital bandpass filter or both (Patent Document 1).

Japanese Unexamined Patent Publication No. 5-154128

  When noise reduction is performed by an analog bandpass filter and / or a digital bandpass filter, signals other than the desired frequency component also pass, and thus appear as noise in the reconstructed image and cause misdiagnosis. When only a desired frequency is passed, a band pass filter having a narrow signal pass band and a high Q value may be designed. However, this method is not useful when the characteristics of the permanent magnet change due to temperature changes and the center frequency shifts.

  Therefore, an object of the present invention is to provide an MRI apparatus capable of obtaining a received signal having a good SN by passing only a signal having a desired frequency.

  In order to achieve the above object, the MRI apparatus of the present invention is configured as follows. That is, 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 nuclear nuclei constituting the biological tissue of the subject, and the nuclear magnetic resonance. A receiving system for receiving the emitted nuclear magnetic resonance signal; a signal processing system for performing image reconstruction calculation using the received signal received by the receiving system; and a central processing unit for controlling the operation of the entire apparatus. The reception system includes an adaptive line spectrum enhancer that passes only a desired frequency component from the received signal.

  According to the MRI apparatus of the present invention, only a signal having a desired frequency component can be passed, and a high SN signal with little noise can be obtained.

  Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the invention, and the repetitive description thereof is omitted.

  First, an overall outline of an example of an MRI apparatus according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention. This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject.As shown in FIG. 2, the MRI apparatus includes a static magnetic field generation system 2, a gradient magnetic field generation system 3, a transmission system 5, A reception system 6, a signal processing system 7, a sequencer 4, and a central processing unit (CPU) 8 are provided.

  The static magnetic field generation system 2 generates a uniform static magnetic field in the direction perpendicular to the body axis in the space around the subject 1 if the vertical magnetic field method is used, and in the direction of the body axis if the horizontal magnetic field method is used. Thus, a permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the subject 1.

  The gradient magnetic field generating system 3 includes a gradient magnetic field coil 9 wound in the three-axis directions of X, Y, and Z, which is a coordinate system (stationary coordinate system) of the MRI apparatus, and a gradient magnetic field power source 10 that drives each gradient magnetic field coil. The gradient magnetic fields Gx, Gy, Gz are applied in the three axis directions of X, Y, and Z by driving the gradient magnetic field power supply 10 of each coil in accordance with a command from the sequencer 4 described later. At the time of imaging, a slice direction gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and the remaining two orthogonal to the slice plane and orthogonal to each other A phase encoding direction gradient magnetic field pulse (Gp) and a frequency encoding direction gradient magnetic field pulse (Gf) are applied in one direction, and position information in each direction is encoded into an echo signal.

  The sequencer 4 is a control means that repeatedly applies a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) and a gradient magnetic field pulse in a predetermined pulse sequence, and operates under the control of the CPU 8 to collect tomographic image data of the subject 1. Various commands necessary for the transmission are sent to the transmission system 5, the gradient magnetic field generation system 3, and the reception system 6.

  The transmission system 5 irradiates the subject 1 with RF pulses in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the living tissue of the subject 1, and includes a high frequency oscillator 11, a modulator 12, and a high frequency amplifier. 13 and a high frequency coil (transmission coil) 14a on the transmission side. The high-frequency 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 placed close to the subject 1. By supplying to the high frequency coil 14a, the subject 1 is irradiated with the RF pulse.

  The receiving system 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of nuclear spins constituting the biological tissue of the subject 1, and receives a high-frequency coil (receiving coil) 14b on the receiving side and a signal amplifier 15 And a quadrature detector 16 and an A / D converter (A / DC) 17. After the NMR signal of the response of the subject 1 induced by the electromagnetic wave irradiated from the high frequency coil 14a on the transmission side is detected by the high frequency coil 14b arranged close to the subject 1 and amplified by the signal amplifier 15, The signal is divided into two orthogonal signals by the quadrature phase detector 16 at the timing according to the command from the sequencer 4, and each signal is converted into a digital quantity by the A / D converter 17 and sent to the signal processing system 7.

The signal processing system 7 performs various data processing and display and storage of processing results, and has an external storage device such as an optical disk 19 and a magnetic disk 18 and a display 20 composed of a CRT, etc. Is input to the CPU 8, the CPU 8 executes processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 1 as a result on the display 20, and the magnetic disk 18 of the external storage device. Record in etc.
The operation unit 25 inputs various control information of the MRI apparatus and control information of processing performed in the signal processing system 7, and includes a trackball or mouse 23 and a keyboard 24. The operation unit 25 is disposed close to the display 20, and the operator controls various processes of the MRI apparatus interactively through the operation unit 25 while looking at the display 20.

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

  At present, the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as being widely used clinically. By imaging information on the spatial distribution of proton density and the spatial distribution of relaxation time in the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.

  Next, an embodiment of the present invention will be described with reference to FIG. FIG. 2 is a block diagram of the system of the present invention.

  In the receiving system 6, the echo signal is received by the receiving coil, then converted to an intermediate frequency by a reference signal from the synthesizer, subjected to noise processing by the analog filter 201, A / D converted by the A / D converter 17, and CPU 8 Is input. In the MRI apparatus of the present invention, the adaptive line spectrum enhancer 202 is arranged between the A / D converter 17 and the CPU 8 and passes only the center frequency.

  The adaptive line spectrum enhancer 202 is a filter that passes only the signal of the nuclear magnetic resonance frequency from the received signal and blocks other frequency components even when noise is mixed in the signal whose input frequency is unknown.

Specifically, as shown in FIG. 3, the adaptive line spectrum enhancer minimizes the mean square error of the error ε _k between the input signal d _k and the output y _k in which broadband noise is superimposed on the desired signal. Is provided with a coefficient variable filter 301 in which coefficients are set. That is, the coefficient variable filter 301 receives the delayed signal x _{k obtained} by delaying the input signal d _k, and the coefficient c to be applied to the number of delay units and the delayed signal x _k so as to minimize the mean square error of the error ε _{k k} is set. By setting the number of delay units and the coefficients in this way, a signal having a correlation between the input signal d _k and the delayed signal x _k , that is, a desired signal is emphasized.

  As described above, even when the input frequency is unknown, only the desired signal can be passed.

  As shown in FIG. 4, the adaptive line spectrum enhancer may be configured in an analog manner and disposed before the A / D converter 17 of the reception system 6. Alternatively, the adaptive line spectrum enhancer may be arranged in the signal processing system 7. Alternatively, as shown in FIG. 5, an adaptive line spectrum enhancer may be configured digitally in the transmission system 5 and arranged in the front stage of the D / A converter 501 to obtain a sinc waveform with good SN. Alternatively, as shown in FIG. 6, an adaptive line spectrum enhancer may be configured in analog in the transmission system 5 and arranged at the subsequent stage of the D / A converter 501 to obtain a sinc waveform with good SN.

  As described above, according to the MRI apparatus of the present invention, since only a signal having a desired frequency component can be passed, a high SN can be obtained. In addition, by digitally configuring the adaptive line spectrum enhancer using an FPGA or the like, the adaptive line spectrum enhancer can be realized at a low cost and the design specification can be easily changed.

The figure which shows the structural example of an MRI apparatus. The block diagram of an adaptive line spectrum enhancer. The figure showing the internal structure of an adaptive line spectrum enhancer. The figure showing the receiving system which has arrange | positioned the analog adaptive line spectrum enhancer. The figure showing the transmission system which has arrange | positioned the digital adaptive line spectrum enhancer. The figure showing the transmission system which has arrange | positioned the analog adaptive line spectrum enhancer.

Explanation of symbols

  1 subject, 2 static magnetic field generation system, 3 gradient magnetic field generation system, 4 sequencer, 5 transmission system, 6 reception system, 7 signal processing system, 8 central processing unit (CPU), 9 gradient magnetic field coil, 10 gradient magnetic field power supply, DESCRIPTION OF SYMBOLS 11 High frequency transmitter, 12 Modulator, 13 High frequency amplifier, 14a High frequency coil (transmission coil), 14b High frequency coil (reception coil), 15 Signal amplifier, 16 Quadrature phase detector, 17 A / D converter, 18 Magnetic disk, 19 optical disk, 20 display, 21 ROM, 22 RAM, 23 trackball or mouse, 24 keyboard, 51 gantry, 52 table, 53 housing, 54 processing device

Claims (1)

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 nuclear nuclei constituting the biological tissue of the subject, A magnetic system comprising: a receiving system for receiving a nuclear magnetic resonance signal; a signal processing system for performing an image reconstruction operation using the received signal received by the receiving system; and a central processing unit for controlling the operation of the entire apparatus. In a resonance imaging apparatus,
The receiving system may have a adaptive line enhancer for passing only a desired frequency component from the received signal,
The adaptive line spectrum enhancer includes a coefficient variable filter to which the received signal and a delayed signal obtained by delaying the received signal are input,
A magnetic resonance imaging apparatus characterized in that spectrum enhancement is performed using the coefficient variable filter whose coefficient is determined so that a mean square error between the received signal and the delay signal obtained is minimized.

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