JP4248588B1 - Compact magnetic resonance imaging system - Google Patents

Abstract

A small-sized magnetic resonance imaging apparatus that does not require a high-frequency electromagnetic shield room and can image the entire hand at once.
A permanent magnet 41 for forming a static magnetic field, a first gradient magnetic field coil 43 for forming a magnetic field gradient in the same direction as the static magnetic field, and first perpendicular to the first gradient magnetic field coil and orthogonal to each other. Imaging necessary and sufficient for imaging the entire hand including the RF probe 42 including the second and third gradient magnetic field coils 44 and 45 and an RF (radio frequency) coil for forming a radio frequency magnetic field in a radio frequency electromagnetic shield box. In a small magnetic resonance imaging apparatus that secures an area, a local high-frequency electromagnetic shield plate 49 having an electromagnetic shield portion 492 made of a conductor plate and an insulator sheet is applied to the subject's forearm, and the high-frequency electromagnetic shield box 425 By storing the LC balun circuit 424 inserted in series with the impedance matching circuit of the RF coil circuit 421, a high frequency electromagnetic shield loop is obtained. The to unnecessary.
[Selection] Figure 2

Description

  The present invention relates to a small-sized magnetic resonance imaging apparatus, and more particularly, to a magnetic resonance imaging apparatus (RA diagnostic MRI apparatus) for rheumatoid arthritis diagnosis that does not require a high-frequency electromagnetic shield room.

  Rheumatoid Arthritis (RA) is characterized by persistent polyarthritis of unknown cause, causing joint deformity while destroying cartilage and bone, and is said to have about 700,000 patients in Japan. Yes. It has been elucidated that this RA joint destruction progresses rapidly within 2 years after onset, and it is extremely effective to diagnose RA at an early stage and start treatment with a new drug such as a biologic. It has become clear.

  In addition to various serum markers, it has been found that MRI examinations targeting the hands are useful for early diagnosis of RA. Currently, the whole body MRI (WB-MRI: Whole Body MRI) apparatus is used. In Europe and the United States, early RA diagnosis using small MRI, which is advantageous for equipment costs, imaging charges, etc., is becoming widespread.

Further, in the MRI apparatus, as shown in FIG. 11, in order to prevent noise that appears in an MR image when external high frequency electromagnetic noise is received by a high frequency (RF) probe and mixed in an NMR signal, the following patent document is disclosed. 1 (FIG. 11 (A)), Patent Document 2 (FIG. 11 (B)) and a high-frequency electromagnetic shield room that accommodates the entire MRI apparatus, or a test as described in Patent Document 3 below. A high-frequency electromagnetic shield (not shown) that covers the entire body of the person and the entire work room is used.
United States Patent 6,255,823 United States Patent 6,882,547 United States Patent 4,613,820

  However, currently used whole body MRI apparatuses are optimized for imaging of the brain, cardiopulmonary, abdomen, etc., and it is wide to use the whole body MRI apparatus for hand imaging for early diagnosis of RA. There are problems such as an expensive imaging fee due to an expensive apparatus requiring installation space, difficulty in securing a long imaging time and an imaging time frame associated therewith, and requiring an unreasonable posture from the patient during imaging.

  In addition, the small limb MRI apparatus used in Europe and the US has a small imaging area, so in order to capture the entire hand, it must be divided into multiple imaging operations, which increases the examination time and interprets the interpretation. There was also a problem in terms of efficiency.

  Further, the high-frequency electromagnetic shield room and the like described in the above-mentioned patent documents require a large installation space for the MRI apparatus, are expensive, and often lack openness. A method that does not require a shield room or the like has been desired.

  The present invention has been made by intensive research in view of such problems, eliminates the need for a high-frequency electromagnetic shield room, is compact and inexpensive for imaging the entire hand, and is examined in an open space. It is an object of the present invention to provide an MRI apparatus that can take an image of the whole hand in a short time and interpret it in a short time with a comfortable posture with a large degree of freedom.

According to a first aspect of the present invention, there is provided a permanent magnet for forming a static magnetic field, a first gradient magnetic field coil for forming a magnetic field gradient in the same direction as the static magnetic field, and first perpendicular to the first gradient magnetic field coil and orthogonal to each other. An imaging region necessary and sufficient to image the entire hand at once with a radio frequency (RF) probe including a radio frequency (RF) coil circuit for forming a radio frequency magnetic field coil 2 and a third gradient magnetic field coil and a radio frequency magnetic field. In a small-sized magnetic resonance imaging apparatus for securing a high-frequency coupling with a forearm of a subject at the time of imaging, a grounded high-frequency electromagnetic shield is provided, and an LC balun circuit in series with an impedance matching circuit included in the RF coil circuit And the impedance matching circuit and the LC balun circuit are arranged in the high frequency electromagnetic shield box of the RF probe. It is a small magnetic resonance imaging apparatus characterized by. In this case, disposing the impedance matching circuit and the LC balun circuit in the high frequency electromagnetic shield box of the RF probe has good space efficiency and is effective for the high frequency electromagnetic shield.

A second invention is the compact magnetic resonance imaging apparatus according to the first invention, wherein the local high-frequency electromagnetic shield is formed by a local high-frequency electromagnetic shield plate made of a conductor plate and an insulator sheet.

According to a third invention, in the first or second invention, the LC balun circuit is mounted on the same substrate integrally with an impedance matching circuit included in the RF coil circuit. It is a resonance imaging apparatus.

According to the first invention, since it is a small-sized magnetic resonance imaging apparatus that does not require a high-frequency electromagnetic shield room by performing high-frequency coupling with the forearm of the subject at the time of imaging and applying a grounded local high-frequency electromagnetic shield, The MRI system can be installed in a narrower space than before, and the entire hand can be imaged in a short time in a small, inexpensive, open space with a high degree of freedom in the posture of the subject. Thus, the effect of being able to interpret is obtained.
In particular, it is effective for early diagnosis and early treatment of RA by making inexpensive and easy imaging related to RA occurring in the joints of unspecified hands.

  According to the second invention, in addition to the first invention, an LC balun circuit is further inserted in series with the impedance matching circuit included in the RF coil circuit, and the impedance matching circuit and the LC balun circuit are connected to the RF matching circuit. It is arranged in the high frequency electromagnetic shield box of the probe, and the abolition of the high frequency electromagnetic shield room can be made more reliable.

  According to a third invention, in the first or second invention, the local high frequency electromagnetic shield is formed by a local high frequency electromagnetic shield plate made of a conductor plate and an insulator sheet. According to the second or third aspect of the invention, since the LC balun circuit is mounted on the same substrate integrally with the impedance matching circuit included in the RF coil circuit, each of the LC balun circuits can be simply and more reliably. The effects of the invention can be obtained.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of an MRI apparatus according to an embodiment of the present invention. The outline of the configuration of the MRI apparatus is to control the entire system 1, collect NMR (Nuclear Magnetic Resonance) signals, and perform image reconstruction / display. A computer (PC) 10 for performing the above, a radio frequency (RF) signal for exciting a nuclear spin system, a radio frequency signal unit unit (transmission / reception unit) 30 for amplifying and detecting the received NMR signal, a gradient magnetic field coil (hereinafter referred to as “gradient”) A gradient magnetic field power supply unit 50 that drives the coils 43 to 45, a power amplifier (high-frequency transmitter) 60 that excites the nuclear spin system, and a permanent magnet (hereinafter referred to as “magnet”) that generates a uniform static magnetic field and generates nuclear magnetization. ”41, composed of gradient coils 43 to 45 for generating a gradient magnetic field, and an RF coil 421 for applying a high frequency to the nuclear spin system and receiving a signal.

  2A and 2B are explanatory views of the gantry of the MRI apparatus according to the embodiment of the present invention, where FIG. 2A is a front sectional view and FIG. 2B is a left side sectional view. In FIG. 2, the magnet 41 is usually made of an Fe—Nd—B magnet material. Reference numeral 411 denotes two upper and lower horizontal elements constituting the yoke, and 412 denotes a longitudinal element constituting the yoke. The yoke has a function of forming a column-type permanent magnet magnetic circuit and confining the magnetic flux in the magnetic circuit. Yes. Reference numeral 413 denotes a pole piece formed on the inner surface side of the two transverse elements 411 of the yoke. The pole piece 413 has a protruding rose shim around its periphery to increase the uniformity of the static magnetic field, and is composed of a large number of small pieces of magnetic material for locally correcting the non-uniformity of the static magnetic field on the surface. Has a passive shim. The gradient coils 43 to 45 are attached to the inner surface side of the pole piece 413, and the RF probe 42 is attached between the upper and lower pole pieces 413.

  The local high frequency electromagnetic shield plate 49 is formed of a conductor plate 491 such as a copper plate and an insulator sheet 492 covering the surface thereof. The local high-frequency electromagnetic shield plate 49 is laid on one end of the upper surface of the base material 493, and a copper foil or the like is attached to the other end side of the base material 493, and is grounded to provide a high-frequency electromagnetic shield. An electromagnetic shield box 425 is seated. The local high-frequency electromagnetic shield plate 49 is laid on the base material 493 so as to be joined to the high-frequency electromagnetic shield box 425.

  The insulator sheet 492 covering the surface of the conductor plate 491 that forms the local high-frequency electromagnetic shield plate 49 is for avoiding a conductor plate such as a copper plate from directly contacting the human body.

  Reference numeral 423 in FIG. 2B is a circuit board in which an impedance matching circuit and an LC balun circuit, which will be described later, are integrally mounted and attached to the vertical wall in the box of the RF probe 42, that is, the high-frequency electromagnetic shield box 425.

  The impedance matching circuit and the LC balun circuit are integrally mounted on the same substrate and arranged in the high-frequency electromagnetic shield box 425 is effective in improving space efficiency and high-frequency electromagnetic shield effect. In addition, it is essential to attach a conductive foil such as a copper foil to the outer surface of the storage box of the RF probe 42 to form the high-frequency electromagnetic shield box 425 in order to eliminate the need for a high-frequency electromagnetic shield room.

  In order to obtain an image that is easy to standardize and interpret the captured image, the subject stretches out the palm and fingers straight and puts it on the side plate 47, and covers the cover 48 from above to open the RF probe. From 422, the entire left or right hand is inserted and imaged.

FIG. 3 is a circuit diagram of the RF coil circuit and the LC balun circuit according to the embodiment of the present invention. The right side of FIG. 3 is an RF coil circuit 421, which includes a tuning capacitor (C _t1 , VC _t1 ) for tuning the coil and a matching capacitor (VC _m1 , VC _m2 ) for matching with the characteristic impedance. It is divided by a fixed capacitor (C _t ) having the same capacity as the tuning capacitor to suppress coupling with the subject.

  The left side of FIG. 3 is an LC balun circuit, which is a bridge circuit in which a capacitor (Cb) and a coil (Lb) are inserted. An LC balun circuit is inserted in series with an impedance matching circuit included in the RF coil circuit, the LC balun circuit is grounded, and the high-frequency electromagnetic shield box 425 is also grounded.

  The RF coil is housed in a high-frequency electromagnetic shield box 425 and shielded from external radio waves, the forearm of the subject is grounded at a high frequency by a local high-frequency electromagnetic shield plate 49, and the RF coil Since the center point of the circuit 421 is set to the same potential as the ground by the LC balun circuit 424, a high-frequency electromagnetic shield room can be made unnecessary.

In the embodiment of the present invention, a 3D-gradient echo method is employed as a T ₁ weighted image capturing sequence for obtaining anatomical information, and a fat suppression T ₂ weighted image capturing sequence for detecting a lesion is used. Although the STIR-3DFSE method is adopted, the imaging sequence is not limited to this.

  FIG. 8 is an explanatory diagram of the sequence of the 3D-gradient echo method according to the embodiment of the present invention, and FIG. 9 is an explanatory diagram of the sequence of the STIR-3DFSE method according to the embodiment of the present invention.

  In the operation procedure of the MRI apparatus according to the present embodiment, first, a hand R as a subject is put into the opening 422 of the RF probe 42 installed in the magnet 41, and the imaging program of the PC 10 is started. The imaging program starts the data acquisition program at the same time as starting the imaging pulse sequence according to the imaging parameters input from a keyboard (not shown).

  The pulse sequence is output as an accurate timing signal from a DSP (pulse generator) 14, sent to a DAC (DA converter) 12, converted into an analog signal by the DAC 12, and an RF pulse waveform is combined with an LPF (low-pass filter) 33 and an amplifier. 35 is supplied to the high frequency modulator 31, while the waveform of the gradient magnetic field current is supplied to the three current amplifiers 51 to 53 of the gradient magnetic field power supply unit 50. In the high frequency modulator 31, the Larmor frequency reference signal that is constantly output from the synthesizer 13 and the pulse waveform are mixed, and an RF pulse is output. The RF pulse is input to the power amplifier (high frequency transmitter) 60 via the VGA 36, and after power amplification is performed to generate a high frequency magnetic field in the RF coil 42, the RF pulse is supplied to the RF coil via the switch 70. Is done. The current amplifiers 51 to 53 of the gradient magnetic field power supply unit 50 supply constant current pulses proportional to the signal waveform to the gradient coils 43 to 45.

  The nuclear spin in the hand R, which is the subject excited by the RF pulse, induces an NMR signal in the RF coil, is sent to the preamplifier 80 via the switch 70, is amplified by the preamplifier 80, and further passes through the VGA 37 to be detected. A rotating system NMR signal is obtained in the device 32.

  The detected signal is sent to an ADC (AD converter) 11 through an amplifier 38 and an LPF 34, digitized by the ADC 11, and temporarily stored in a memory (not shown) of the PC 10. After the data collection necessary for the image reconstruction is completed, the reconstructed image is displayed on the image display 20 by the image reconstruction program.

  The transmission gain of the RF pulse at the time of measurement and the reception gain of the NMR signal are instructed from the PC 10 to the transmission / reception unit 30 through the USB I / F (interface) 35, and the VGA 36 is transmitted by the CPU built in the transmission / reception unit 30. , 37 and amplified to a predetermined gain by the VGA.

  Next, the present invention will be described in more detail with reference to examples. The MRI apparatus used in this example is based on the description of the embodiment of the present invention and the drawings, and the main specifications of each component are as follows.

(Permanent magnet 41)
・ Manufacturer: Hitachi Metals, Ltd.
-Permanent magnet material: Fe-Nd-B system-Dimensions:
Width x height x depth = 460mm x 780mm x 440mm
・ Static magnetic field strength: 0.3T
・ Magnet gap: 13cm
-Magnetic field uniformity:
50 ppm within the spheroid region (22 cm × 22 cm × 8 cm)
(Gradient coils 43-45)
・ Winding method:
Gradient coil in the x and y axis direction ... Target field method Gradient coil in the z axis direction ... Genetic algorithm-Number of windings:
x, y axis direction gradient coil ... 25 turns,
Gradient coil in the z-axis direction: 32 turns ・ Gradient magnetic field uniform region: spheroid (long axis 20 cm × short axis 6 cm)
-Coil winding frame:
Material: Bakelite plate (2.7mm thick)
Machining method: Milling winding pattern on seat plate with NC milling machine based on CAD data Coil spacing: 12cm
Current surface diameter: 40 cm or less • Winding:
Diameter x material = 1.0mm x copper wire (with polyethylene coating)
A three-dimensional magnetic field analysis is performed from the obtained winding distribution according to Bio-Savart's law, and the uniform field of gradient magnetic field according to the following equation 1 is set to within 10%.

(RF probe 42)
・ RF coil circuit 421
-Coil width x thickness x material x number of turns = 10 mm x 0.1 mm x copper foil x oval type (short axis 6.5 mm x long axis 12.5 cm) 14 turns-Capacitor C _t = 100 pF x 3, C _t1 = 100 pF, VC _t1 = 2-40 pF,
VC _m1 = _{2 to} 40 pF, V _cm2 = _{2 to} 40 pF
・ LC balun circuit 424
Coil L _b = 0.623μH × 2 pieces capacitors C _b = 249pF × 2 pieces, high-frequency electromagnetic shielding box 425 ... resin plate made, copper foil adhered (local high-frequency electromagnetic shield plate 49)
-Conductor plate 491 ... Copper plate, thickness 1mm
・ Insulator sheet 492 FRP sheet, thickness 0.5mm
・ Base material of local high-frequency electromagnetic shield plate 493 ・ ・ ・ Resin plate, thickness 10mm

[Evaluation of local high frequency electromagnetic shielding performance]
As artificial noise, a loop coil with a diameter of 2 cm is installed at a position 59 cm high 2 m away from the opening of the magnet, and a high frequency of 12.5787990 MHz, 16 dBm is supplied from the signal generator (ANRITSU, MG3641A) to the loop coil. Measure the frequency characteristics of disturbance noise power by changing the combination of the local high-frequency electromagnetic shield plate and the LC balun circuit while the healthy subject puts his hand in the RF probe, and set the reception gain of the high-frequency transmitter / receiver to the maximum In the state of the high-frequency power amplifier OFF, only the residual noise component was measured without generating an NMR signal, and the collected data was subjected to frequency analysis by fast Fourier transform.

  FIG. 6 shows the measurement results of the frequency characteristics of the residual noise power when the combination of the LC balun circuit and the local high-frequency electromagnetic shield plate is changed. From FIG. 6, an average shielding effect of −17.7 dB was obtained by using a local high-frequency electromagnetic shielding plate, and a shielding effect of −22.9 dB was obtained by using an LC balun circuit together. In this way, by taking a local shield measure, an image can be acquired with a good signal-to-noise ratio (SNR) without a shield room.

[Imaging healthy subjects]
FIG. 8 shows a captured image of a T1-weighted image by the following 3D-GRE method of a healthy subject (53-year-old male), and shows an image in which the slice position has changed on the dorsal side according to (A) to (D). With this T1-weighted image, the position of each joint from the distal interphalangeal joint to the carpal bone, the ligament attachment, and cartilage morphological information such as the cartilage can be confirmed as in the conventional whole body MRI apparatus. It was. * 3D- gradient echo method, _{T 1-weighted} sequences
Pulse sequence repetition time TR = 35 ms
Echo time TE = 5.5ms
Flip angle FA = 60 °
Voxel Size ・ ・ ・ 0.4mm × 0.8mm × 1.6mm
Number of pixels: 512 × 192 × 32 (512 × 512 × 16 after Fourier interpolation)
Number of excitations NEX = 2
Imaging time: 7 minutes 10 seconds

FIG. 9 shows a captured image of a fat-suppressed T2 weighted image of the same subject by the following STIR-3DFSE method, and shows an image in which the slice position is changed on the dorsal side according to (A) to (D). This fat-suppressed T2-weighted image suppresses fat signals such as synovial fluid and venous blood in which bone marrow and subcutaneous fat signals are suppressed as in the conventional whole-body MRI apparatus, and a high signal in which a relatively long component of T2 is remarkable. Confirmed as an area.
* STIR-3DFSE method, fat suppression _{T 2-weighted} sequences
Pulse sequence repetition time TR = 1000ms
Inversion time TI = 100ms
Effective echo time TEeff = 60ms
Number of echoes ETL = 12
Voxel Size ・ ・ ・ 0.8mm × 0.8mm × 1.6mm
Number of pixels: 256 x 384 x 16
Imaging time: 8 minutes 30 seconds

  Note that FIG. 7 is a captured image of an MRI apparatus without a countermeasure against local high-frequency electromagnetic shielding, but it was not possible to confirm the position information of each joint, the ligament attachment portion, the morphological information of the imaging site such as cartilage.

  According to this embodiment in which local high frequency electromagnetic shielding measures are taken, a shield effect of −22.9 dB can be obtained, the high frequency electromagnetic shielding room is unnecessary, and the MRI apparatus becomes small. The entire hand can be imaged and interpreted in a short time in a simple and easy-to-follow posture with a small, inexpensive, open space with a high degree of freedom in the posture of the subject. .

1 is a block diagram of an MRI apparatus according to an embodiment of the present invention. It is explanatory drawing of the mount part of the MRI apparatus which concerns on embodiment of this invention, (A) is front view sectional drawing, (B) is left side sectional drawing. It is a circuit diagram of RF coil circuit and LC balun circuit concerning an embodiment of the invention. It is explanatory drawing of the sequence of the 3D-gradient echo method which concerns on embodiment of this invention. It is explanatory drawing of the sequence of STIR-3DFSE method which concerns on embodiment of this invention. This is a frequency characteristic of residual noise power when the combination of the LC balun circuit and the local high-frequency electromagnetic shield plate is changed. It is a picked-up image of the MRI apparatus without a local high frequency electromagnetic shielding countermeasure. It is the picked-up image of a RA patient's hand imaged with the 3D-gradient echo method based on the Example of this invention. It is the picked-up image of a RA patient's hand imaged by STIR-3DFSE method which concerns on the Example of this invention. It is the high frequency electromagnetic shielding room which concerns on a prior art example, (A) is what is described in patent document 1, (B) is what is described in patent document 2. FIG.

Explanation of symbols

1. MRI equipment 10. PC (computer)
11. ADC
12. ・ DAC
13. ・ Synthesizer 14. ・ DSP
15. ・ USB I / F
20. Image display 30 High frequency signal unit (transmission / reception unit)
31..Modulator 32..Detector 33..LPF (low pass filter)
34..LPF (low-pass filter)
35 ・ ・ Amplifier 36 ・ ・ VGA
37 ... VGA
38 ・ ・ Amplifier 39 ・ ・ Switch 40 ・ ・ Mounting frame 41 ・ ・ Magnet (Permanent magnet)
411 ・ ・ Yoke (transverse element)
412 ・ ・ Yoke (vertical element)
413 ・ ・ Polepiece 42 ・ ・ RF probe 421 ・ ・ RF coil circuit
422 ... Open 423 ... Circuit board
424 ··· LC balun circuit 425 · · High frequency electromagnetic shield box 43 · · Gradient coil (x-axis direction)
44 .. Gradient coil (y-axis direction)
45 .. Gradient coil (z-axis direction)
46 ·· Base 47 · · Sub-board 48 · · Cover (gloves)
49..Local high frequency electromagnetic shielding plate
491 .. Conductor plate 492 .. Insulator sheet 493 .. Base material of local high frequency electromagnetic shield plate 50 .. Gradient magnetic field power supply unit 51 .. Gradient amplifier (x-axis direction)
52 .. Gradient amplifier (y-axis direction)
53 .. Gradient amplifier (z-axis direction)
60 .. Power amplifier (high frequency transmitter)
70. ・ Switching device (switching circuit)
80 ・ ・ Preamp R ・ ・ Hand

Claims (3)

A permanent magnet that forms a static magnetic field, a first gradient coil that forms a magnetic field gradient in the same direction as the static magnetic field, and second and third gradients that are perpendicular to and perpendicular to the first gradient field coil Miniature magnetic resonance that secures an imaging area necessary and sufficient for imaging the entire hand at once with a radio frequency (RF) probe that includes a magnetic field coil and a radio frequency (RF) coil circuit that forms a radio frequency magnetic field in a radio frequency electromagnetic shield box In the imaging device,
A high frequency coupling is performed with the forearm of the subject at the time of imaging, and a grounded local high frequency electromagnetic shield is provided. Further, an LC balun circuit is inserted in series with the impedance matching circuit included in the RF coil circuit, the impedance matching circuit and the impedance matching circuit A compact magnetic resonance imaging apparatus, wherein an LC balun circuit is arranged in a high-frequency electromagnetic shield box of the RF probe. 2. The small magnetic resonance imaging apparatus according to claim 1, wherein the local high-frequency electromagnetic shield is formed by a local high-frequency electromagnetic shield plate made of a conductor plate and an insulator sheet. The LC balun circuit, small magnetic resonance imaging apparatus according to claim 1 or 2, characterized in that it is mounted on the RF coil circuit to the impedance matching circuit integrally with the same substrate contained.

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