JP4945207B2 - Magnetic resonance imaging apparatus and RF coil - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus and an RF coil.

  A birdcage coil is known as an RF coil that applies a high-frequency magnetic field to a subject and receives a magnetic resonance signal from the subject. One advantage of the birdcage coil is that a high-frequency magnetic field can be uniformly formed in the imaging space. In general, a birdcage coil is decoupled between a plurality of loop portions formed on the outer peripheral surface of a cylinder when a high frequency magnetic field is applied to the subject and a magnetic resonance signal is received from the subject. In other words, it has the same number of resonance frequencies as the number of loop portions.

  As the RF coil, a multi-coil such as a phased array coil is known. One advantage of the multi-coil is the use of a sense (SENSE) method or a smash (SMASH) method. That is, the multi-coil has one advantage of shortening the imaging time by generating an image based on the sensitivity distribution for each of the plurality of loop portions constituting the multi-coil. The Marcocoil is decoupled between a plurality of loop portions when a high frequency magnetic field is applied to the subject and a magnetic resonance signal is received from the subject, and has a single resonance frequency. Yes.

Patent Document 1 discloses a technology that uses a birdcage coil as a multi-coil. That is, in Patent Document 1, application of a high-frequency magnetic field and reception of a magnetic resonance signal are performed by a birdcage coil. During application and reception, decoupling is performed between a plurality of loop portions, and the birdcage coil is a single unit. Operates at a single resonant frequency.
JP-A-10-201738

  The distance between the subject and the plurality of loop portions constituting the birdcage coil differs for each of the plurality of loop portions due to the influence of the contour of the subject. Therefore, when the birdcage coil is used as a multi-coil, the load is different for each of the plurality of loop portions when the high-frequency magnetic field is applied. As a result, the high-frequency magnetic field cannot be uniformly applied, and the image quality is degraded.

  An object of the present invention is to provide a magnetic resonance imaging apparatus and an RF coil capable of uniformly applying a high-frequency magnetic field and receiving a magnetic resonance signal for each loop part.

  The magnetic resonance imaging apparatus of the present invention is a magnetic resonance imaging apparatus for generating an image of the subject based on a magnetic resonance signal received by applying a magnetic field to the subject, and applying a high-frequency magnetic field to the subject And an RF coil that receives the magnetic resonance signal from the subject, a data collection unit that collects data based on the magnetic resonance signal received by the RF coil, and a data collected by the data collection unit An image generation unit configured to generate a tomographic image of the subject; and a control unit configured to control the operation of the RF coil, wherein the RF coil is adjacent to each other while sharing a part of a conductor forming a loop. The plurality of loop portions and the mode of the RF coil are separated from each other by a single mode in which the plurality of loop portions do not interfere with each other and the resonance frequency is single, and the plurality of loop portions. And a switching unit capable of switching between a plurality of modes having a plurality of resonance frequencies, and the control unit, when the application of the high-frequency magnetic field is performed, the mode of the RF coil becomes the plurality of modes, When the magnetic resonance signal is received, the operation of the switching unit is controlled so that the mode of the RF coil is the single mode, and the data collection unit is one or more of the plurality of loop units The data based on the magnetic resonance signal received by the loop unit is collected for each loop unit.

  Preferably, the data collection unit collects data based on the magnetic resonance signals received by the plurality of loop units for each loop unit.

  Preferably, the image generation unit generates the tomographic image by a sense method or a smash method based on data for each of the plurality of loop units collected by the data collection unit.

  Preferably, the RF coil is a volume coil.

Preferably, the switching unit is connected in series to each of the adjacent loop units on a shared conductor shared by adjacent loop units in the single mode, and in the multiple mode, the switching unit A decoupling capacitor that is disconnected from adjacent loop portions, wherein the decoupling capacitor has a combined capacitance Cm on the shared conductor, and a mutual inductance between the adjacent loop portions M, When the resonance frequency of each of the adjacent loop portions is ω,
Cm = 1 / (ω ² M)
The capacitance is set so that

  Preferably, the resonance frequency in the single mode is set to be the same as any one of the plurality of resonance frequencies in the plurality of modes.

  Preferably, the switching unit is switched between a state connected to the loop unit and a state where the connection to the loop unit is disconnected in accordance with switching between the single mode and the plurality of modes. And an impedance portion including at least one of an inductor for each of the plurality of loop portions, and the impedance of the impedance portion is such that the resonance frequency in the single mode is the same as any one of the plurality of resonance frequencies in the plurality of modes. Is set to

  Preferably, the impedance unit includes at least one of a variable capacitor and a variable inductor.

  Preferably, each of the plurality of loop portions includes a capacitor connected in series to a conductor forming a loop, an inductor connected in parallel to the capacitor, an input portion connected in parallel to the capacitor, and the An output device connected in series to an inductor and outputting a signal in accordance with a signal input to the input unit to the data collection unit, the resonance of a parallel resonant circuit including the capacitor, the inductor, and the output device The frequency is set to be the same as the resonance frequency of the single mode.

  The RF coil of the present invention is an RF coil that applies a high-frequency magnetic field to a subject and receives a magnetic resonance signal from the subject, and is adjacent to each other while sharing a part of a conductor forming a loop. A plurality of loop portions and the mode of the RF coil, a single mode in which the plurality of loop portions do not interfere with each other and a single resonance frequency, and a plurality of loop portions interfere with each other, and a plurality of resonance frequencies A switching unit switchable between a plurality of modes, and the switching unit is connected to the loop unit in accordance with switching between the single mode and the plurality of modes, and the loop unit An impedance part including at least one of a capacitor and an inductor that can be switched between a state in which the connection of the impedance part is disconnected is provided for each of the plurality of loop parts. Nsu the resonant frequency in the single mode is set to be the same as any one of a plurality of resonant frequencies in the plurality of modes.

  According to the present invention, it is possible to uniformly form a high-frequency magnetic field and receive a magnetic resonance signal for each loop part.

  FIG. 1 is a block diagram showing a schematic configuration of a magnetic resonance imaging (MRI) apparatus 1 of the present embodiment. The MRI apparatus 1 includes a static magnetic field magnet unit 12, a gradient coil 13, an RF coil 15, an RF drive unit 22, a gradient drive unit 23, a data collection unit 24, a switching drive unit 25, and a control unit 26. , A cradle 27, a data processing unit 31, an operation unit 32, and a display unit 33.

  The static magnetic field magnet unit 12 forms a static magnetic field in the imaging space B in which the subject SU is carried in by the cradle 27. The direction of the static magnetic field is, for example, the body axis direction (z-axis direction) of the subject SU or the direction perpendicular to the body axis (x-axis direction or y-axis direction). The static magnetic field magnet unit 12 is constituted by a pair of permanent magnets, for example. The static magnetic field magnet unit 12 may be composed of a superconducting magnet.

  The gradient coil 13 forms a gradient magnetic field in the imaging space B in which a static magnetic field is formed, and adds position information to the magnetic resonance signal received by the RF coil 15. The gradient coil 13 has three systems: a first gradient coil that forms a frequency encode gradient magnetic field, a second gradient coil that forms a phase encode gradient magnetic field, and a third gradient coil that forms a slice selection gradient magnetic field. .

  In the imaging space B where the static magnetic field is formed, the RF coil 15 transmits an RF pulse that is an electromagnetic wave to the subject to form a high-frequency magnetic field, and excites proton spins in the imaging region of the subject SU. The RF coil 15 receives and outputs an electromagnetic wave generated from the excited proton in the subject SU as a magnetic resonance signal.

  The RF drive unit 22 outputs a drive signal for driving the RF coil 15 to form a high-frequency magnetic field in the imaging space B to the RF coil 15. Specifically, it includes a gate modulator (not shown), an RF power amplifier (not shown), and an RF oscillator (not shown). Based on the control signal from the control unit 26, the RF drive unit 22 modulates the RF signal from the RF oscillator into a signal having a predetermined timing and a predetermined envelope using a gate modulator. The RF signal modulated by the gate modulator is amplified by the RF power amplifier and then output to the RF coil 15.

  Based on the control signal from the control unit 26, the gradient driving unit 23 applies a gradient pulse to the gradient coil 13 to drive the gradient coil 13, thereby generating a gradient magnetic field in the imaging space B in which a static magnetic field is formed. The gradient drive unit 23 has three systems of drive circuits (not shown) corresponding to the three systems of gradient coils 13.

  The data collection unit 24 collects the magnetic resonance signals received by the RF coil 15 based on the control signal from the control unit 26 and outputs the collected magnetic resonance signals to the data processing unit 31. The data collection unit 24 collects the magnetic resonance signals subjected to phase encoding and frequency encoding so as to correspond to the k space. The data collection unit 24 detects the magnetic resonance signal received by the RF coil 15 using the output of the RF oscillator of the RF drive unit 22 as a reference signal, and the phase detector detects the analog resonance signal. The instrument converts it into a digital signal. Then, the collected magnetic resonance signals are stored in the memory and then output to the data processing unit 31.

  The switching drive unit 25 controls the operation of the switching unit for changing the mode of the RF coil 15 described later. Specifically, based on a control signal from the control unit 26, a forward or reverse bias voltage is applied to the switching unit.

  Based on the control signal output from the data processing unit 31, the control unit 26 outputs a control signal for causing the RF driving unit 22, the gradient driving unit 23, and the data collecting unit 24 to execute a predetermined pulse sequence. Take control. The control unit 26 is configured by a computer, for example.

  The data processing unit 31 outputs an operation signal based on the operation signal from the operation unit 32 to the control unit 26. The data processing unit 31 acquires data based on the magnetic resonance signal from the data collection unit 24, performs image processing on the acquired data, and generates image data of a tomographic image of the subject. The data processing unit 31 outputs a video signal based on the generated image data to the display unit 33.

  The operation unit 32 is configured by operation devices such as a keyboard and a mouse. The operation unit 32 is operated by an operator and outputs an operation signal corresponding to the operation to the data processing unit 31. The display unit 33 is configured by a display device such as a CRT. The display unit 33 displays an image based on the video signal output from the data processing unit 31.

  2A and 2B are diagrams showing an outline of the RF coil 15, FIG. 2A is a perspective view, and FIG. 2B is a side view.

  The RF coil 15 is configured as a so-called birdcage type coil. That is, the RF coil 15 is opposed to two loop conductors 41A and 41B (hereinafter simply referred to as “loop conductor 41”, which may not be distinguished from each other) and the two loop conductors 41 facing each other. A plurality of linear conductors 42 extending in the direction and connecting the two loop conductors 41 are formed, and are formed in a substantially cylindrical shape.

  A plurality of substantially rectangular loop portions 45 formed by two linear conductors 42 and a portion of the two loop-shaped conductors 41 separated by the two linear conductors 42 are provided on the outer peripheral surface of the cylinder. Is provided. Two loop portions 45 adjacent to each other share one linear conductor 42 disposed therebetween. Each loop unit 45 includes a capacitor 43 and an inductor (not shown) connected in series to form a series resonance circuit. In addition, the resonance frequency of each loop part 45 is mutually the same.

  Some of the plurality of loop portions 45 (for example, one loop portion 45 and two loop portions 45 having a phase difference of 90 ° from the loop portion) are connected to the RF drive unit 22 and the data via the TR switch 47A. A collection unit 24 is connected. The TR switch 47A allows the loop unit 45 and either the RF drive unit 22 or the data collection unit 24 to be selectively conducted. The TR switch 47A includes, for example, a switch that enables or disables the loop unit 45 and the RF drive unit 22, and a switch that enables or disables the loop unit 45 and the data collection unit 24. Each switch is configured to include, for example, a parallel resonant circuit, an FET, and the like. The operation of the TR switch 47A is controlled by the control unit 26.

  The rest of the plurality of loop units 45 are connected to the data collection unit 24 via the TR switch 47B (hereinafter simply referred to as “TR switch 47”, and the TR switch 47A and the TR switch 47B may not be distinguished). It is connected. The TR switch 47B allows the loop unit 45 and the data collection unit 24 to be conductive or non-conductive. The TR switch 47B includes, for example, a parallel resonance circuit, an FET, and the like, and each switch includes, for example, a parallel resonance circuit, an FET, and the like. The TR switch 47B may have the same configuration as the TR switch 47A. The operation of the TR switch 47B is controlled by the control unit 26.

  The signal output from the TR switch 47 is input to a plurality of preamplifiers 49 provided corresponding to the plurality of loop portions 45, respectively. The signal output from the preamplifier 49 is input to the data collection unit 24. Therefore, the data collection unit 24 can collect data based on the magnetic resonance signals received by the plurality of loop units 45 for each loop unit 45.

  The RF coil 15 has a mode in which the plurality of loop portions 45 do not interfere with each other and the resonance frequency becomes a single mode, and a plurality of loop portions 45 interfere with each other and the resonance frequency becomes plural. A switching unit 51 that can switch between a plurality of modes is provided. One switching unit 51 is provided for each of the two loop units 45 adjacent to each other. The RF coil 15 is a birdcage coil, and one loop portion 45 is adjacent to the other loop portions 45 on both sides in the circumferential direction, so that the number of switching portions 51 is the same as the number of loop portions 45. It has become. The plurality of switching units 51 can be regarded as one switching unit in the whole of the plurality of switching units 51.

  FIG. 2B illustrates a case where the TR switch 47 is provided on the loop conductor 41B and the switching unit 51 is provided on the loop conductor 41A. However, the TR switch 47 and the switching unit 51 may be provided only on one side of the two loop conductors 41, or the TR switch 47 may be provided on both of the two loop conductors 41. The switching unit 51 may be provided on both of the loop conductors 41.

  FIG. 3 is a diagram for explaining the switching unit 51. However, in the description with reference to FIG. 3 and FIG. 3, it is assumed that two loop portions 45 are extracted from the RF coil 15, and the entire RF coil 15 ′ is configured by the two loop portions 45.

  FIG. 3A shows a schematic configuration of the RF coil 15 ′ under such an assumption, FIG. 3B shows a state of the switching unit 51 in the single mode, and FIG. Indicates the state of the switching unit 51 in the multiple mode, FIG. 3D shows the sensitivity characteristic in one loop unit 45 in the single mode, and FIG. The sensitivity characteristic in the loop part 45 is shown. In the following description, they are referred to as “loop part 45-1” and “loop part 45-2”, and the two loop parts 45 may be distinguished.

  As shown in FIG. 3A, the switching unit 51 includes a decoupling capacitor 53 that can be connected in series to each of the two loop portions 45 on the linear conductor 42 shared by the two loop portions 45. ing.

  As shown in FIG. 3B, in the single mode, the decoupling capacitor 53 is connected in series to the two loop portions 45 on the linear conductor 42. Then, by appropriately setting the capacitance of the decoupling capacitor 53, the two loop portions 45 are isolated from each other, and as shown in FIG. There is one sensitivity peak. In other words, the resonance frequency of the RF coil 15 'is single.

  On the other hand, as shown in FIG. 3C, in the multiple mode, the decoupling capacitor 53 is disconnected from the two loop portions 45. Accordingly, the two loop portions 45 interfere with each other. In other words, there are two types of paths for the current flowing through the RF coil 15 ′: a path that flows through each loop section 45 and a path that flows through the entire RF coil 15 ′ composed of the two loop sections 45. As a result, as shown in FIG. 3E, the sensitivity peak of one loop portion 45 is two. In other words, the RF coil 15 'has a plurality of resonance frequencies. Note that the number of sensitivity peaks increases as the number of loop portions 45 increases.

  As shown in FIGS. 3D and 3E, the resonance frequency in the single mode is different from the plurality of resonance frequencies in the plurality of modes. Therefore, the switching unit 51 can be connected in series to the loop unit 45-1 in order to match the resonance frequency in the single mode with one of the plurality of resonance frequencies in the plurality of modes and efficiently receive the magnetic resonance signal. Variable capacitor 54-1 and variable capacitor 54-2 (hereinafter simply referred to as “variable capacitor 54”, which may not be distinguished from each other) that can be connected in series to loop unit 45-2. .

  As shown in FIG. 3B, in the single mode, the variable capacitor 54 is connected to the loop unit 45 in series. Specifically, the variable capacitor 54 is connected in series to a conductor other than the linear conductor 42 shared by the two loop portions 45, that is, the loop conductor 41.

  On the other hand, as shown in FIG. 3C, the variable capacitor 54 is disconnected from the loop unit 45 in the multiple mode. Therefore, the variable capacitor 54 does not affect the resonance frequency of the plurality of modes.

  Accordingly, when the capacitance of the variable capacitor 54 is appropriately set, the single-mode resonance frequency matches any one of the plurality of resonance frequencies in the plurality of modes. The variable capacitor 54 may be formed of various known variable capacitors such as an air variable condenser or a poly variable condenser.

  FIG. 4 shows an example of a configuration for switching between connection and disconnection of the decoupling capacitor 53 and the variable capacitor 54 to the loop portion 45 as shown in FIGS. 3B and 3C. .

  The loop-shaped conductor 41 </ b> A includes a main body 56, an inner conductor 57 connected in series to the main body 56, and an outer conductor 58 connected in series to the main body 56 and connected in parallel to the inner conductor 57.

  The linear conductor 42 includes a main body 60 and an end 61 connected in series to the main body 60. The main body 60 connects the inner conductor 57 of the loop conductor 41A and the loop conductor 41B facing the loop conductor 41A. The end portion 61 connects the inner conductor 57 and the outer conductor 58.

  The decoupling capacitor 53 is connected to the end 61 in series. The variable capacitors 54-1 and 54-2 are connected in series in each of the portions of the outer conductor 58 that are separated by the connection point with the end 61.

  Of the inner conductor 57, each part separated by a connection point with the linear conductor 42 is referred to as a PIN diode 63-1, 63-2 (hereinafter simply referred to as “PIN diode 63”), and the two may not be distinguished. )) In series, with the connection point side between the inner conductor 57 and the linear conductor 42 as the cathode and the connection point between the inner conductor 57 and the main body 56 (connection point between the inner conductor 57 and the outer conductor 58) side as the anode. It is connected. A PIN diode 64 is connected in series to the end portion 61 with the decoupling capacitor 53 side as a cathode and the connection point side between the end portion 61 and the inner conductor 57 as an anode. Each part of the outer conductor 58 that is separated by a connection point with the linear conductor 42 is referred to as a PIN diode 65-1, 65-2 (hereinafter simply referred to as “PIN diode 63”). Are connected in series, with the connection point between the outer conductor 58 and the main body 56 (the connection point between the outer conductor 58 and the inner conductor 57) as the cathode and the variable capacitor 54 side as the anode.

  When a positive bias is applied to the point P1 on the side where the cathode side of the PIN diode 63 and the anode side of the PIN diode 64 are connected, and the points P2 and P3 between the variable capacitor 54 and the PIN diode 65, That is, when a forward bias is applied to the PIN diodes 64 and 65 and a reverse bias is applied to the PIN diode 63, the end 61 and the outer conductor 58 are brought into conduction, and the inner conductor 57 is connected to the linear conductor 42. As shown in FIG. 3B, the RF coil 15 (15 ′) is in a single mode.

  On the other hand, when a negative bias is applied to the points P1, P2, and P3, that is, when a reverse bias is applied to the PIN diodes 64 and 65 and a forward bias is applied to the PIN diode 63, the end 61 and the outer conductor 58 are Among these, each part delimited by the connection point with the linear conductor 42 is in a non-conductive state, and the inner conductor 57 is in a conductive state, and as described with reference to FIG. 3C, the RF coil 15 (15 ′ ) Is a multiple mode. In this way, connection and disconnection of the decoupling capacitor 53 and the variable capacitor 54 to the loop portion 45 are switched.

  It should be noted that, in the inner conductor 57, each part separated by the connection point with the linear conductor 42 is referred to as an adjustment circuit 67-1, 67-2 (hereinafter simply referred to as “adjustment circuit 67”), and the two are not distinguished Is provided). The adjustment circuit 67 includes a capacitor 68 connected in series to the linear conductor 42, and an inductor 69 connected to the linear conductor 42 and a reference potential. The capacitor 68 is for cutting a direct current. The inductor 69 is for increasing the impedance to a high frequency (alternating current) and isolating the coil (forming the circuit) from the bias system and the ground. In other words, the bias system or the like is not seen as a loss from the coil. The adjustment circuit 67 looks like a high-pass filter at first glance, but does not need to resonate and does not operate as a high-pass filter.

  FIG. 5 is a diagram for explaining a method for setting the capacitance of the decoupling capacitor 53. However, in the description with reference to FIGS. 5 and 5, it is assumed that two loop portions 45 are extracted from the RF coil 15, and the entire RF coil 15 ′ is configured by the two loop portions 45. FIG. 5 is a circuit diagram showing an equivalent circuit of the RF coil 15 'in the single mode under such an assumption.

  The element X is an element equivalent to all elements having impedance on the linear conductor 42 shared by the loop portions 45-1 and 45-2. The capacitors C1 and C2 and the inductors L1 and L2 are all on the conductors (the linear conductor 42 and the loop conductor 41 on both sides) other than the shared linear conductor 42 in the loop portions 45-1 and 45-2, respectively. The capacitor and inductor equivalent to the capacitor and inductor are shown. The mutual inductance of the loop parts 45-1 and 45-2 is M.

Assuming that voltages V ₁ and V ₂ are applied to the loop portions 45-1 and 45-2 and currents i ₁ and i ₂ flow, the following equations are established.
V ₁ = X _C1 i ₁ + X _L1 i ₁ + Xi ₁ -Xi ₂ -X _M i _{2
V 2 = X C2 i 2 +} X L2 i 2 + Xi 2 -Xi 1 -X M i 1
Here, X _C1 , X _C2 , X _L1 , X _L2 , X, and X _M are impedances of capacitors C 1 and C 2, inductors L 1 and L 2, element X, and mutual inductance M, respectively.
Organizing the above formula,
V ₁ = (X _C1 + X _L1 + X) i ₁ − (X + X _M ) i ₂
V ₂ = − (X + X _M ) i ₁ + (X _C2 + X _L2 + X) i ₂

If the loop portions 45-1 and 45-2 are isolated, that is, if they are not affected by each other's current, the crosstalk term becomes 0.
− (X + X _M ) i ₂ = 0
− (X + X _M ) i ₁ = 0
And
X + X _M = 0
Therefore,
X = −X _M = −jωM = 1 / (j (1 / (ωM)))
Holds. This shows the capacitance, so if X = Cm,
ωCm = 1 / (ωM)
∴Cm = 1 / (ω ² M)
It becomes. Note that ω is the operating frequency (resonance frequency) of the RF coil 15.

Therefore, if the capacitance of the capacitor on the linear conductor 42 is set so that the electrostatic capacitance Cm obtained by synthesizing the electrostatic capacitance on the linear conductor 42 is 1 / (ω ² M), the loop portion 45 can be obtained. Interference between -1 and 45-2 is suppressed. Even in the case where there are three or more loop portions 45, as in the case where there are two loop portions 45, the electrostatic capacity of the decoupling capacitor 53 is set so that the above equation is established between two adjacent loop portions 45. If the capacitance (capacitance Cm obtained by synthesizing the capacitance on the linear conductor 42) is set, a single resonance frequency is realized.

  FIG. 6 is a diagram for explaining a decoupling method between the loop portions 45 that are not adjacent to each other, and shows one loop portion 45 with the TR switch 47 omitted.

  The preamplifier 49 has an input portion 49 c connected in parallel to the capacitor 43 via an inductor 75. Specifically, the input unit 49 c includes two input terminals 49 a and 49 b, and the input terminals 49 a and 49 b are connected to both sides of the capacitor 43. The inductor 75 is connected to one end of the capacitor 43 and the input terminal 49b, is connected in parallel to the capacitor 43, and is connected in series to the input unit 49c. The preamplifier 49 amplifies and outputs the signal input to the input unit 49c. The preamplifier 49 is composed of, for example, a low input impedance amplifier.

  A parallel resonance circuit 76 is configured by the input circuit of the capacitor 43, the inductor 75 and the preamplifier 49. However, when the preamplifier 49 is constituted by a low input impedance amplifier, the parallel resonance circuit 76 is substantially constituted by the capacitor 43 and the inductor 75. The parallel resonance circuit 76 is configured such that the resonance frequency matches the resonance frequency of the loop unit 45. In a single mode (when receiving a magnetic resonance signal), the loop unit 45 is substantially free due to high impedance due to resonance. Open loop state. Therefore, decoupling of the loop portions 45 that are not adjacent to each other is performed.

  FIG. 7A to FIG. 7C are time charts showing a control method of the switching unit 51 by the control unit 26. Specifically, FIG. 7A shows the transmission / reception timing of the RF signal by the RF coil 15, FIG. 7B shows the timing of the bias voltage applied to the points P1 to P3 in the switching unit 51, FIG. 7C shows the switching timing of the operation mode of the RF coil 15. In each figure, the horizontal axis is time. However, FIG. 7A to FIG. 7C conceptually show a rough relationship between the transmission / reception timing and the switching timing of the operation mode, and these timings are not accurately shown. And the interval between reception and reception are omitted.

  As shown in FIG. 7A, transmission and reception by the RF coil 15 are alternately performed at regular time intervals. At the time of transmission, the TR switch 47A enables the loop unit 45 and the RF drive unit 22 to be conductive, and disables the loop unit 45 and the data collection unit 24 from being conductive. The TR switch 47B disables the loop unit 45 and the data collection unit 24 from conducting. On the other hand, at the time of reception, the TR switch 47A disables conduction between the loop unit 45 and the RF drive unit 22, and enables conduction between the loop unit 45 and the data collection unit 24. The TR switch 47B allows the loop unit 45 and the data collection unit 24 to be electrically connected.

  At the time of transmission by the RF coil 15, a negative bias is applied to the points P1 to P3 of the switching unit 51 as shown in FIG. Accordingly, as shown in FIG. 7C, the operation mode of the RF coil 15 is a plurality of modes. On the other hand, at the time of reception by the RF coil 15, a positive bias is applied to the points P1 to P3 of the switching unit 51 as shown in FIG. Accordingly, as shown in FIG. 7C, the operation mode of the RF coil 15 is a single mode.

  The data processing unit 31 generates an image based on the signals received by the plurality of loop units 45 by, for example, the sum of squares method, the SENSE method, the SMASH method, or the like. In the SENSE method and the SMASH method, an image is generated using the difference in sensitivity distribution of the plurality of loop portions 45. In other words, the folded image portion is separated from the reconstructed images of the plurality of loop portions 45 to generate a new tomographic image having a large imaging area. In the SMASH method, data processing is performed before Fourier transform of k-space, and in the SENSE method, data processing is performed on image data after Fourier transform.

  According to the above embodiment, in the MRI apparatus 1, the control unit 26 sets the mode of the RF coil 15 to a plurality of modes when a high-frequency magnetic field is applied, and the RF coil 15 when receiving a magnetic resonance signal. The operation of the switching unit 51 is controlled so that the mode becomes a single mode, and the data collection unit 24 collects data for each loop unit 45. As a result, as compared with the conventional case where a high frequency magnetic field is applied at a single resonance frequency and data based on a magnetic resonance signal is collected, a loop portion is applied while uniformly applying a high frequency magnetic field to the imaging space B. Data can be collected every 45.

  Conventionally, volume coils such as birdcage coils collect data for each loop because it is difficult to uniformly apply a high-frequency magnetic field for each loop due to changes in the distance between the subject and the loop. However, it is not suitable for an RF coil for both transmission and reception. However, as described above, by switching between the single mode and the plurality of modes in synchronization with the transmission / reception timing, the volume coil can be suitably used as an RF coil for both transmission and reception that collects data for each of a plurality of loops. .

  In this embodiment, an image can be generated by a sense method or a smash method. That is, using the RF coil for both transmission and reception, both improvement in image quality by forming a uniform high-frequency magnetic field and high-speed image formation using sensitivity distribution in the plurality of loop portions 45 are realized. It is possible to use a volume coil as such an RF coil.

  Since the resonance frequency in the single mode is set to be the same as one of the resonance frequencies in the multiple modes, it is possible to efficiently collect data based on the magnetic resonance signal while suppressing power consumption. It becomes.

  Specifically, the switching unit 51 includes a variable capacitor 54 that is connected to the loop unit 45 in the single mode and disconnected from the loop unit 45 in the multiple mode, so that the resonance frequency in the single mode is set to the multiple mode. Since the frequency coincides with any one of the plurality of resonance frequencies, the configuration is simple.

  Furthermore, since the capacitance of the variable capacitor 54 is variable, for example, the capacitance of the variable capacitor 54 is changed each time the imaging region of the subject SU moves (the load changes), and a single mode resonance frequency and a plurality of resonance frequencies are obtained. It is also possible to appropriately adjust the resonance frequency of the mode.

  Since the parallel resonance circuit 76 also decouples the loop portions 45 that are not adjacent to each other, it is possible to collect data based on the magnetic resonance signal for each of the plurality of loop portions 45 more preferably.

  In the above embodiment, the data processing unit 31 is an example of the image generation unit of the present invention, the variable capacitor 54 is an example of the impedance unit of the present invention, and the preamplifier 49 is an example of the output device of the present invention. is there.

  The present invention is not limited to the above embodiment, and may be implemented in various aspects.

  The RF coil is not limited to a birdcage coil, as long as it includes two or more loop portions adjacent to each other while sharing a part of a conductor forming a loop. For example, the RF coil may be a surface coil. Further, the birdcage coil may be arranged with respect to any part of the subject, and may be a body coil or a head coil.

  The data collection unit only needs to be able to collect data based on the magnetic resonance signal for each loop of at least one of the plurality of loop units. For example, data based on magnetic resonance signals received by one loop unit may be collected to form an image. Even in this case, the high frequency magnetic field can be uniformly applied to the region to be imaged as compared to the case where the high frequency magnetic field is applied only by one loop portion.

  A method of making the resonance frequency in the single mode the same as any one of the plurality of resonance frequencies in the plurality of modes may be appropriately selected. For example, in the embodiment, an example is shown in which the variable capacitor is connected or disconnected in accordance with the switching between the single mode and the plurality of modes. However, at least one of the variable capacitor and the variable inductor is related to the switching of the switching unit. By changing the capacitance of the variable capacitor and the inductance of the variable inductor in accordance with the switching of the switching unit, either the single-mode resonance frequency or the multiple-mode resonance frequency is selected. Or one of them may be matched. However, as in the embodiment, the configuration in which the capacitor or the inductor is connected or disconnected in accordance with the switching of the switching unit is only the control of the switch, the control is simple, and transmission and reception are performed. There are various merits such as high-speed control corresponding to repetition.

  In addition, in the case of providing an impedance part that is switched between a state connected to the loop part and a state where the connection to the loop part is disconnected in accordance with switching between the single mode and the plurality of modes, the impedance part is It is not limited to what is connected in one mode and disconnected in a plurality of modes. Conversely, it may be disconnected in a single mode, connected in multiple modes, or a combination of what is connected in a single mode and what is disconnected. The impedance unit is not limited to one constituted by a variable capacitor, and may be an inductor having an inductance or a capacitance for changing the resonance frequency, and may be an inductor. Capacitors and inductors need not be variable. However, as described above, if the capacitor or the inductor is variable, it can be adjusted according to various circumstances, which is convenient. The impedance part may be connected in parallel or in series with the loop part.

  The means for connecting or disconnecting the decoupling capacitor or impedance unit is not limited to a switch for controlling conduction or non-conduction, for example, connecting a short-circuit line in parallel to the decoupling capacitor or impedance unit. May be substantially disconnected. Further, the switch for controlling conduction or non-conduction is not limited to the PIN diode, and may be, for example, an FET.

1 is a block diagram showing a configuration of a magnetic resonance imaging apparatus according to an embodiment of the present invention. The figure which shows the structure of RF coil of the magnetic resonance imaging device of FIG. The figure explaining the switching part of RF coil of FIG. The circuit diagram which shows the structural example of the switching part of FIG. The figure explaining the decoupling capacitor of the switching part of FIG. The figure explaining the preamplifier periphery of the RF coil of FIG. The timing chart explaining the control method of the switching part of FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Magnetic resonance imaging device, 12 ... Static magnetic field magnet part, 13 ... Gradient coil, 15 ... RF coil, 22 ... RF drive part, 24 ... Data collection part, 31 ... Data processing part (image formation part), 26 ... Control Part, SU ... subject, 45 ... loop part, 51 ... switching part.

Claims (10)

A magnetic resonance imaging apparatus for generating an image of the subject based on a magnetic resonance signal received by applying a magnetic field to the subject,
An RF coil for applying a high-frequency magnetic field to the subject and receiving the magnetic resonance signal from the subject;
A data collection unit for collecting data based on the magnetic resonance signal received by the RF coil;
An image generation unit that generates a tomographic image of the subject based on data collected by the data collection unit;
A control unit for controlling the operation of the RF coil;
With
The RF coil is
A plurality of loop portions adjacent to each other while sharing a part of a conductor forming a loop;
The mode of the RF coil includes a single mode in which the plurality of loop portions do not interfere with each other and the resonance frequency is single, and a plurality of modes in which the plurality of loop portions interfere with each other and the resonance frequency is plural. A switching section that can be switched between,
With
The control unit is configured such that when the high-frequency magnetic field is applied, the mode of the RF coil is the plurality of modes, and when the magnetic resonance signal is received, the mode of the RF coil is the single mode. To control the operation of the switching unit,
The data collection unit collects data based on the magnetic resonance signal received by one or more loop units among the plurality of loop units for each loop unit. The magnetic resonance imaging apparatus according to claim 1, wherein the data collection unit collects data based on the magnetic resonance signals received by the plurality of loop units for each loop unit. The magnetic resonance imaging apparatus according to claim 2, wherein the image generation unit generates the tomographic image by a sense method or a smash method based on data for each of the plurality of loop units collected by the data collection unit. The magnetic resonance imaging apparatus according to claim 1, wherein the RF coil is a volume coil. The switching unit is connected in series to each of the adjacent loop units on a shared conductor shared by adjacent loop units in the single mode, and the adjacent loops in the multiple mode. With a decoupling capacitor that is disconnected from the
The decoupling capacitor has a combined capacitance on the shared conductor of Cm, a mutual inductance between the adjacent loop portions as M, and a resonance frequency of each of the adjacent loop portions as ω,
Cm = 1 / (ω ² M)
The magnetic resonance imaging apparatus according to any one of claims 1 to 4, wherein a capacitance is set such that The magnetic resonance imaging apparatus according to claim 1, wherein a resonance frequency in the single mode is set to be the same as any one of a plurality of resonance frequencies in the plurality of modes. The switching unit includes at least a capacitor and an inductor that are switched between a state connected to the loop unit and a state where the connection to the loop unit is disconnected in accordance with switching between the single mode and the plurality of modes. An impedance part including one is provided for each of the plurality of loop parts,
The magnetic resonance imaging apparatus according to claim 6, wherein the impedance of the impedance unit is set such that a resonance frequency in the single mode is the same as any one of a plurality of resonance frequencies in the plurality of modes. The magnetic resonance imaging apparatus according to claim 7, wherein the impedance unit includes at least one of a variable capacitor and a variable inductor. Each of the plurality of loop portions is
A capacitor connected in series with a conductor forming a loop;
An inductor connected in parallel to the capacitor;
An output unit connected in parallel to the capacitor and connected in series to the inductor, and outputs a signal corresponding to the signal input to the input unit to the data collection unit;
With
The magnetic resonance imaging apparatus according to claim 1, wherein a resonance frequency of a parallel resonance circuit including the capacitor, the inductor, and the output device is set to be the same as a resonance frequency of the single mode. An RF coil that applies a high-frequency magnetic field to a subject and receives a magnetic resonance signal from the subject,
A plurality of loop portions adjacent to each other while sharing a part of a conductor forming a loop;
The mode of the RF coil includes a single mode in which the plurality of loop portions do not interfere with each other and the resonance frequency is single, and a plurality of modes in which the plurality of loop portions interfere with each other and the resonance frequency is plural. A switching section that can be switched between,
With
The switching unit is capable of switching between a state connected to the loop unit and a state in which the connection to the loop unit is disconnected in accordance with switching between the single mode and the plurality of modes. An impedance part including at least one is provided for each of the plurality of loop parts,
The impedance of the impedance unit is set so that a resonance frequency in the single mode is the same as any one of a plurality of resonance frequencies in the plurality of modes.

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