JP2012223299A - Apparatus for measuring ununiformity in static magnetic field in magnetic resonance imaging system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus for measuring ununiformity in a static magnetic field in a magnetic resonance imaging system, which is improved in convenience when adjusting ununiformity in the static magnetic field in an MRI system.SOLUTION: The apparatus for measuring ununiformity in a static magnetic field in a magnetic resonance imaging system includes: a static magnetic field-generating means for generating a static magnetic field in an imaging space where a subject is disposed; a means for adjusting ununiformity in the static magnetic field for adjusting the static magnetic field in the imaging space to have uniformity; and a display means for displaying the result of measurement of the static magnetic field in each part obtained using the apparatus for measuring ununiformity in a static magnetic field with a frequency spectrum.

Description

  The present invention relates to a static magnetic field inhomogeneity measuring apparatus in a magnetic resonance imaging (hereinafter referred to as MRI) apparatus, and more particularly to a technique for improving the convenience when adjusting the static magnetic field inhomogeneity in an MRI apparatus.

  The MRI device measures NMR signals generated by the spins of the subject, especially the tissues of the human body, and visualizes the form and function of the head, abdomen, limbs, etc. in two or three dimensions Device. In imaging, the NMR signal is given different phase encoding depending on the gradient magnetic field, frequency-encoded, and measured as time series data. The measured NMR signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  In this MRI apparatus, it is necessary to measure a plurality of magnetic fields in a desired imaging space in shimming (for example, refer to Patent Document 1) for adjusting a static magnetic field necessary for imaging. There were an expensive and complicated method using a gauss meter and a method using the frequency measurement function of an inexpensive and manual MRI system.

JP 2004-73752 A

  However, in the method using the frequency measurement function of an inexpensive and manual MRI system, the range of frequency measurement with respect to the center frequency is limited, and only the number of the frequency of the measurement result is output at each measurement location. It was. For this reason, there is no material to judge whether the obtained results are appropriate values, and it is often necessary to judge whether re-measurement is necessary based on the experience of the user, or to spend extra work for checking the operation of the device. there were. In addition, with an MRI system that has just been adjusted, the value of the magnetic field to be measured may be measured with a significant deviation from the center frequency of the magnetic field measurement. There was a problem that adjustment work could be advanced with low accuracy.

  An object of the present invention is to enable accurate magnetic field measurement when adjusting static magnetic field inhomogeneity in an MRI apparatus.

  In order to solve the above problems, the present invention provides a static magnetic field generating means for generating a static magnetic field in an imaging space in which a subject is arranged, and a static magnetic field non-uniform adjustment for uniformly adjusting the static magnetic field in the imaging space. A static magnetic field inhomogeneous measuring apparatus in a magnetic resonance imaging apparatus, comprising: a display means for displaying a result of magnetic field measurement at each location obtained in the static magnetic field inhomogeneous measuring apparatus in a frequency spectrum An apparatus for measuring static magnetic field inhomogeneity in a magnetic resonance imaging apparatus is provided.

  The static magnetic field inhomogeneity measuring apparatus in the magnetic resonance imaging apparatus comprises means for displaying the magnetic field measurement result at an arbitrary position in the frequency spectrum after performing magnetic field measurement at a plurality of positions. An apparatus for measuring a static magnetic field inhomogeneity in a magnetic resonance imaging apparatus is provided.

  The static magnetic field inhomogeneity measuring apparatus in the magnetic resonance imaging apparatus comprises means for converting the frequency band of the measurement in the magnetic field measurement according to the obtained display result of the frequency spectrum. An apparatus for measuring static magnetic field inhomogeneity in an apparatus is provided.

  According to the present invention, when adjusting the static magnetic field inhomogeneity in the MRI apparatus, the convenience can be improved and the magnetic field measurement can be performed with high accuracy.

It is a figure for demonstrating the general outline | summary of an example of the MRI apparatus which concerns on this invention. 1 is an external view of an MRI apparatus according to the present invention. It is a figure explaining the measuring method of the static magnetic field uniformity in the space 33 which arrange | positions the test object of the open type MRI apparatus which concerns on this invention. It is a connection diagram of the simple magnetic field measuring device for both transmitting and receiving attached to the plate for simple magnetic field measurement according to the present invention. It is a figure explaining the display screen of the numerical result at the time of measuring each measurement location of imaging space.

  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. 1, 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 generation system 3 includes a gradient magnetic field coil 9 that applies a gradient magnetic field 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 that drives each gradient magnetic field coil. Gradient magnetic fields Gx, Gy, and Gz are applied in the X, Y, and Z axis directions by driving the gradient magnetic field power supply 10 of each coil according to a command from the sequencer 4 described later. . During imaging, a slice selection 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 encode gradient magnetic field pulse (Gp) and a frequency encode gradient magnetic field pulse (Gf) are applied in one direction, and position information in each direction is encoded in the 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 RF pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at the timing according to the command from the sequencer 4, and the amplitude-modulated RF 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 phase detector 16 and an A / D converter 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 quadrature phase detector 16 divides the signal into two orthogonal signals 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 includes an external storage device such as an optical disk 19 and a magnetic disk 18, and a display 20 including a CRT or the like. When data from the receiving system 6 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 an external storage device On the magnetic disk 18 or the like.

  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 external view of the MRI apparatus according to the embodiment of the present invention will be described with reference to FIG. According to FIG. 2, the MRI apparatus according to the present invention attaches the simple magnetic field measurement plate 32 to the gantry indicated by 31 when adjusting the static magnetic field.

  Next, a method for measuring the static magnetic field uniformity in the space 33 in which the subject of the open MRI apparatus according to the present invention is placed will be described with reference to FIG.

  However, FIG. 3 does not show the upper structure arranged to face the base plate 34 across the space 33 so that the explanation of the measurement method of the static magnetic field uniformity of this embodiment is easier to understand. ing. The upper structure in this case has substantially the same configuration as the lower structure. A static magnetic field is generated in the space 33 by a static magnetic field generator. The operator installs a static magnetic field uniformity measuring jig provided with a pedestal 35 and a plate 36 so that the pedestal 35 is parallel to the substantially central bottom of the space 33. The plate 36 is fixed to a part of the pedestal 35 so as to be perpendicular to the pedestal 35.

  Therefore, the plate 36 is installed by the operator in a direction substantially parallel to the static magnetic field in the space 33, that is, in a direction perpendicular to the base plate 34. In this case, the high frequency coil (transmission coil) 14a and the gradient magnetic field coil 9 of the MRI apparatus are removed by the operator, and the base 35 is placed on the base plate 34 that fixes the high frequency coil (transmission coil) 14a and the gradient magnetic field coil 9. Install. The RF probe 37 used in the static magnetic field uniformity measurement method of the present invention does not particularly require the high-frequency coil (transmission coil) 14a of the MRI apparatus when performing static magnetic field uniformity measurement because it irradiates itself with electromagnetic waves. The installed plate 36 includes a plurality of fixing jigs 38 capable of fixing the RF probes 37 substantially evenly on the outer periphery in addition to the central portion thereof. When measuring the static magnetic field uniformity in the space 33, the operator sequentially desires the single RF probe 37 of the simple magnetic field measuring instrument for transmission and reception to the fixing jigs 38 provided at a plurality of positions of the plate 36. The static magnetic field uniformity is measured by attaching to the fixing jig 38 at the location. Since the plate 36 having the pedestal 35 is stably installed on the base plate 34, the operator can stably measure the static magnetic field uniformity at each location in the space 33. .

  The pedestal 35 of the static magnetic field uniformity measuring jig is composed of an outer peripheral pedestal and an inner peripheral pedestal. Further, the plate 36 is fixed to the inner peripheral pedestal and is installed so as to be perpendicular to the inner peripheral pedestal. Has been. On the inner side of the outer pedestal pedestal, there are provided projections that are applied to the notches provided on the outer side of the inner pedestal pedestal, and the notches are provided, for example, every 30 degrees. The operator secures the outer pedestal to the base plate 34, rotates the inner pedestal with respect to the outer pedestal, and hangs the protrusions of the outer pedestal on each notch of the inner pedestal so that the space 33 of the MRI apparatus The plate 36 can be easily rotated by 30 degrees in the direction with the static magnetic field direction as the rotation axis. The operator can acquire the measurement data of the static magnetic field uniformity from more places in the space 33 of the MRI apparatus by performing the static magnetic field uniformity measurement at each rotation angle.

  Thereby, since the operator can collect the measurement data of the wide-area and high-dimensional static magnetic field uniformity in the space 33 of the MRI apparatus, more accurate static magnetic field uniformity measurement is possible.

  Here, in the static magnetic field uniformity measurement jig of the present embodiment, the plate 36 is fixed to the inner peripheral base, but the plate 36 may be fixed to the outer peripheral base instead of the inner peripheral base. In that case, the operator performs the static magnetic field uniformity measurement by fixing the inner peripheral pedestal, not the outer peripheral pedestal, to the base plate 34.

  The material of the static magnetic field uniformity measuring jig can be mainly composed of an inexpensive material such as vinyl chloride.

  Next, FIG. 4 is a connection diagram of a transmission / reception simple magnetic field measuring device attached to the simple magnetic field measuring plate 32 attached in FIG.

  In FIG. 4, 39 is an RF unit that generates an irradiation signal, 40 is an attenuator that is connected to the RF unit 39 and attenuates the irradiation signal generated by the RF unit 39, and 41 is connected to an Attenuator 40, and the Attenuator 40 RF Power Amp (RF PA) that amplifies the illumination signal attenuated at 42, 42 is a dummy load that terminates several unused lines of the RF Power Amp, 43 is connected to the RF Power Amp 42, and RF Filter Box 44 for reducing noise contained in the irradiation signal amplified by Power Amp, 44 is connected to Filter box 43, and Transmitting and Receiving box (T / R Box) 45 for switching between transmission and reception of signal is Transmitting and Receiving Box 44 is a Transmitting and Receiving Probe (T / R Probe) that sends an irradiation pulse to a liquid phantom placed at the tip of the box and receives an MR signal, and consists of a coil that sends and receives the signal. , 46 is connected to Transmitting and Receiving box 44 It is a bed (TB) that includes an amplifier for amplifying the MR signal that is continued and received by the Transmitting and Receiving box 44. The bed 46 also amplifies the MR signal and sends it to the RF Unit 39 via the Filter Box 43. By determining the frequency of each received signal at a location necessary for the magnetic field uniformity measurement using the connection diagram having such a configuration, it is possible to measure the static magnetic field inhomogeneity.

  FIG. 5 is a numerical result display screen (GUI) when each measurement point in the imaging space is measured with the simple transmission / reception magnetic field measuring device whose connection diagram is shown in FIG. In the GUI screen of the simple magnetic field measuring apparatus for transmission and reception according to the present invention in FIG. 5, 47 is a START button for instructing measurement, and 48 is a result of measuring the static magnetic field at the center of the imaging space at the start and end of the magnetic field measurement. A box for displaying, 49 is a box for inputting the band width (frequency band) of the magnetic field measurement, and 50 is a reference based on the result of measurement at each measurement location in the imaging space according to the position of each measurement location. Boxes for displaying in tabular form by increasing / decreasing with respect to the center frequency, 51 and 52 are boxes for displaying the frequency spectrum of the magnetic field measurement results obtained for each position of each measurement point in a graph. , 51 is for displaying the position of the measurement location, and 52 is a graph showing the result obtained by Fourier transforming the measured NMR signal.

  According to the above embodiment, when measuring at each measurement location, check each measurement result once in the frequency spectrum graph 52, and if appropriate, move to measurement at the next measurement location. Is possible. In addition, when the frequency spectrum 52 is seen, if the signal for magnetic field measurement is buried in noise, or if re-measurement is necessary due to the extremely small amount of signal, the mass of the measurement location is selected again, and the START button 47 Click to enable re-measurement. If the signal is not in the range such as there is no Peak in the frequency spectrum graph 52, the frequency range in the Bandwidth box 49 is changed, and the START button 47 is clicked to perform measurement again. Even after all the measurement points have been measured, the results of the measurement points that need to be confirmed by looking at the overall numerical results can be obtained by selecting the measurement points that are considered necessary in Box 50. It is possible to determine whether or not all the measurements are necessary again before starting the adjustment operation by referring to the graph 52 displayed on the spectrum.

  The present invention can be used for a static magnetic field inhomogeneity measuring apparatus in a magnetic resonance imaging apparatus.

  47 START button, 48 Static magnetic field measurement result, 49 Brandwidth, 50 measurement result table, 51 Position display, 52 Result graph

Claims (3)

  A static magnetic field in a magnetic resonance imaging apparatus, comprising: a static magnetic field generating means for generating a static magnetic field in an imaging space in which a subject is arranged; and a static magnetic field non-uniformity adjusting means for uniformly adjusting the static magnetic field in the imaging space. The inhomogeneous measuring apparatus comprises a display means for displaying the result of the magnetic field measurement at each location obtained in the static magnetic field inhomogeneous measuring apparatus as a frequency spectrum. Uniform measuring device.   The static magnetic field inhomogeneity measuring apparatus in the magnetic resonance imaging apparatus comprises means for displaying the result of the magnetic field measurement at an arbitrary position in the frequency spectrum after performing the magnetic field measurement at a plurality of positions. The static magnetic field inhomogeneity measuring apparatus in the magnetic resonance imaging apparatus according to claim 1.   The static magnetic field inhomogeneity measuring apparatus in the magnetic resonance imaging apparatus comprises means for converting a frequency band of measurement in the magnetic field measurement according to the obtained display result of the frequency spectrum. The static magnetic field inhomogeneity measuring apparatus in the magnetic resonance imaging apparatus of description.

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