Magnetic Resonance Imaging Apparatus

Abstract

To suppress power consumption of a PIN diode contained in an RF coil.SOLUTION: A magnetic resonance imaging device of an embodiment includes: a transmission circuit for applying transmission pulses to an analyte; at least one high frequency coil provided with an active trap circuit having a PIN diode, and receiving a magnetic resonance signal from the analyte; a power source circuit capable of applying a reverse bias voltage and a forward bias voltage to the PIN diode, and configured to be capable of applying forward currents having a plurality of values to the PIN diode when applying the forward bias voltage; and a control unit for controlling the power source circuit so as to set the forward current to the plurality of values according to an imaging condition.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The embodiments disclosed in this specification and the drawings relate to a magnetic resonance imaging apparatus.

Background Art

[0002] A magnetic resonance imaging apparatus is an imaging apparatus that excites the nuclear spins of a subject placed in a static magnetic field with a transmission pulse of a high-frequency (RF: Radio Frequency) signal having a Larmor frequency, and reconstructs a magnetic resonance signal (MR (Magnetic Resonance) signal) generated from the subject accompanying the excitation to generate an image.

[0003] In a magnetic resonance imaging apparatus, for example, a transmission pulse is applied to a subject by a cylindrical coil called a WB (Whole Body) coil, and an MR signal emitted from the subject in response thereto is received by an RF coil (also called a local coil).

[0004] The RF coil has one or a plurality of loop-shaped element coils. Each element coil is provided with an active trap circuit including a PIN diode. The active trap circuit is provided to decouple between a plurality of element coils or to protect electronic components connected to the element coils from a powerful transmission pulse. In order to decouple the element coils or protect the element coils, it is necessary to apply a current in the forward direction of the PIN diode.

[0005] Recently, an RF coil called an array coil in which a large number of element coils are arranged in an array has become popular. Each of the large number of element coils included in the array coil includes a PIN diode. For this reason, the power consumed by the PIN diodes has also become non-negligible.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Laid-Open No. 1-164357 [Summary of the Invention] [Problems to be Solved by the Invention]

[0007] One of the problems to be solved by the embodiments disclosed in this specification and the drawings is to suppress the power consumption of the PIN diodes included in the RF coil. However, the problems to be solved by the embodiments disclosed in this specification and the drawings are not limited to the above problems. The problems corresponding to the respective effects of the respective configurations shown in the embodiments described later can also be regarded as other problems. [Means for Solving the Problems]

[0008] A magnetic resonance imaging apparatus according to an embodiment includes a transmission circuit that applies a transmission pulse to a subject, at least one high-frequency coil including an active trap circuit having a PIN diode, the high-frequency coil that receives a magnetic resonance signal from the subject, and a power supply circuit that can apply a reverse bias voltage and a forward bias voltage to the PIN diode, the power supply circuit being configured to apply forward currents of a plurality of values to the PIN diode when the forward bias voltage is applied, and a control unit that controls the power supply circuit to set the forward current to a plurality of values according to imaging conditions. [Brief Description of the Drawings]

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Modes for Carrying Out the Invention

[0010] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a block diagram showing the overall configuration of a magnetic resonance imaging apparatus 1 according to an embodiment. The magnetic resonance imaging apparatus 1 according to the embodiment includes a magnet gantry 100, a bed 500, a control cabinet 300, a console 400, and the like.

[0011] The magnet gantry 100 has a static magnetic field magnet 10, a gradient magnetic field coil 11, a WB (Whole Body) coil 12, etc., and these components are housed in a cylindrical housing. The bed 500 has a bed body 50 and a top plate 51. Further, the magnetic resonance imaging apparatus 1 has at least one high-frequency coil 20 (hereinafter referred to as an RF coil 20) disposed close to the subject.

[0012] The control cabinet 300 includes a static magnetic field power supply 30, gradient magnetic field power supplies 31 (31x for the X-axis, 31y for the Y-axis, 31z for the Z-axis), an RF receiver 32, an RF transmitter 33, and a sequence controller 34.

[0013] The static magnetic field magnet 10 of the magnet stand 100 has a generally cylindrical shape and generates a static magnetic field within the bore (the space inside the cylinder of the static magnetic field magnet 10), which is the imaging space for the subject (e.g., a patient). The static magnetic field magnet 10 incorporates a superconducting coil, and the superconducting coil is cooled to an extremely low temperature by liquid helium. The static magnetic field magnet 10 generates a static magnetic field by applying a current supplied from a static magnetic field power supply (not shown) to the superconducting coil in the excitation mode. After that, when it shifts to the persistent current mode, the static magnetic field power supply is disconnected. Once it shifts to the persistent current mode, the static magnetic field magnet 10 continues to generate a large static magnetic field for a long time, for example, over one year. Note that the static magnetic field magnet 10 may be configured as a permanent magnet.

[0014] The gradient magnetic field coil 11 also has a generally cylindrical shape and is fixed inside the static magnetic field magnet 10. This gradient magnetic field coil 11 applies a gradient magnetic field to the subject in the directions of the X-axis, Y-axis, and Z-axis by a current supplied from the gradient magnetic field power supplies (31x, 31y, 31z).

[0015] The bed body 50 of the bed 500 is movable in the vertical direction of the top plate 51, and moves the subject placed on the top plate 51 to a predetermined height before imaging. Then, at the time of imaging, the top plate 51 is moved horizontally to move the subject into the bore.

[0016] The WB coil 12 is fixed in a generally cylindrical shape so as to surround the subject inside the gradient magnetic field coil 11. The WB coil 12 transmits the RF pulse transmitted from the RF transmitter 33 toward the subject, and at the same time, receives the MR signal emitted from the subject due to the excitation of the hydrogen nuclei.

[0017] The RF coil 20 receives the magnetic resonance signal emitted from the subject at a position close to the body surface of the subject. The RF coil 20 is configured to include one or more element coils, as will be described later. There are various types of RF coils according to the imaging site of the subject. In FIG. 1, as the RF coil 20, a body coil 20a for imaging the chest and abdomen of the subject, a head coil 20b for imaging the head, and a spine coil 20c for imaging the spine are exemplified.

[0018] The RF transmitter 33 transmits RF pulses to the WB coil 12 based on an instruction from the sequence controller 34. On the other hand, the RF receiver 32 detects the MR signals received by the WB coil 12 and the RF coil 20, digitizes the detected MR signals, and sends them to the sequence controller 34.

[0019] The sequence controller 34 performs a scan of the subject by driving the gradient magnetic field power supply 31, the RF transmitter 33, and the RF receiver 32 under the control of the console 400. Then, the sequence controller 34 receives the MR signals collected by the scan from the RF receiver 32 and further sends them to the console 400.

[0020] The sequence controller 34 includes a processing circuit (not shown). This processing circuit is composed of, for example, a processor that executes a predetermined program, or hardware such as an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit). The console 400 is configured as a computer having a processing circuit 40, a memory circuit 41, an input interface 43, and a display 42.

[0021] The memory circuit 41 is a storage medium including external storage devices such as an HDD (Hard Disk Drive) and an optical disk device in addition to a ROM (Read Only Memory) and a RAM (Random Access Memory). The memory circuit 41 stores various kinds of information and data, and also stores various programs executed by the processor included in the processing circuit 40.

[0022] The input interface 43 includes, for example, various devices for the operator to input various information and data such as a mouse, a keyboard, a trackball, a touch panel, etc., and also includes an interface circuit between these devices and the processing circuit 40. The display 42 is a display device such as a liquid crystal display panel, a plasma display panel, an organic EL panel, etc.

[0023] The processing circuit 40 is, for example, a circuit including a CPU or a dedicated or general-purpose processor. The processor realizes various functions described later by executing various programs stored in the storage circuit 41. The processing circuit 40 may be composed of hardware such as an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). Various functions described later can also be realized by these hardware. Also, the processing circuit 40 can realize various functions by combining a processor and software processing by a program with hardware processing.

[0024] The console 400 controls the entire magnetic resonance imaging apparatus 1. Specifically, it receives imaging conditions and other various information and instructions by the operation of a mouse, a keyboard, etc. (input interface 43) by an operator such as a technician. Then, the processing circuit 40 causes the sequence controller 34 to execute a scan based on the input imaging conditions, and reconstructs an image based on the raw data transmitted from the sequence controller 34. The reconstructed image is displayed on the display 42 or stored in the storage circuit 41.

[0025] The left diagram of FIG. 2 is a diagram showing a configuration example of the RF coil 20 (for example, the body coil 20a). As shown in the left diagram of FIG. 2, the RF coil 20 includes a large number of element coils 200, and the large number of element coils 200 are arranged in an array. In the example shown in the left diagram of FIG. 2, 16 element coils 200 are arranged in 4 rows and 4 columns.

[0026] The right figure in FIG. 2 is a diagram showing one element coil 200 and several electronic circuits connected to the element coil 200. The element coil 200 is formed of a loop-shaped conductor. And a capacitor C, which is one of the components of the active trap circuit 201, is inserted in series in a part of the path of the loop-shaped conductor.

[0027] The active trap circuit 201 is composed of the above capacitor C, PIN diode D, and inductor L. A series connection circuit of the PIN diode D and the inductor L is connected in parallel to the capacitor C.

[0028] A power supply circuit 202 is connected to both ends of the PIN diode. The power supply circuit 202 can apply a forward bias voltage and a reverse bias voltage to the PIN diode D in a switchable manner. In particular, in the power supply circuit 202 of the magnetic resonance imaging apparatus 1 of the embodiment, it is configured to be able to apply forward currents of a plurality of values to the PIN diode D when applying a forward bias voltage.

[0029] A control circuit 310 is connected to the power supply circuit 202. The power supply circuit 202 switches between a forward bias voltage and a reverse bias voltage, and switches between a plurality of forward current values when applying a forward bias voltage, according to a control signal sent from the control circuit 310, or an instruction command or the like.

[0030] In addition to the active trap circuit 201, a preamplifier 203 is also connected to the element coil 200. The preamplifier 203 amplifies the MR signal received by the element coil 200 with low noise and sends it to the subsequent RF receiver 32.

[0031] FIG. 3 is a functional block diagram of the magnetic resonance imaging apparatus 1 according to the embodiment, and in particular, is a functional block diagram focusing on functions related to the control of the active trap circuit 201 in the RF coil 20. The left side of FIG. 3 is a block corresponding to the imaging unit 600, and the right side of FIG. 3 is a block corresponding to the console 400. Here, among the components of the magnetic resonance imaging apparatus 1 shown in FIG. 1, components other than the console 400 (the magnet stand 100 including the static magnetic field magnet, the control cabinet 300 including the sequence controller 34, and the bed 500) are called the imaging unit 600.

[0032] In the example shown in FIG. 3, the imaging unit 600 includes three RF coils 20 (RF coil 20a, RF coil 20b, and RF coil 20c). The number and type of RF coils 20 included in the imaging unit 600 are not particularly limited, but the RF coil 20a is, for example, a body coil 20a.

[0033] Each of the RF coils 20a, 20b, and 20c has a plurality of element coils 200. And corresponding to each of the plurality of element coils 200, a plurality of active trap circuits 201, a plurality of power supply circuits 202, and a plurality of preamplifiers 203 are provided.

[0034] The MR signals received by the plurality of element coils 200 of the RF coils 20a, 20b, and 20c are amplified by the corresponding plurality of preamplifiers 203 and input to the RF receiver 32. The RF receiver 32 digitizes the MR signal and sends it to the sequence controller 34. The sequence controller 34 further sends the received MR signal to the console 400.

[0035] The console 400 has the processing circuit 40 as described above, and various functions are realized by the processor included in the processing circuit 40 executing various programs. As shown in FIG. 3, the processing circuit 40 realizes, for example, an imaging condition setting function F01 and an image generation function F02.

[0036] The image generation function F02 performs reconstruction processing, such as Fourier transform, on the digitized MR signals (i.e., k-space data) sent from the sequence controller 34 to generate an MR image. The generated MR image is displayed on, for example, the display 42.

[0037] On the other hand, the imaging condition setting function F01 sets various imaging conditions input or selected by the user via the input interface 43 for the sequence controller 34. The imaging conditions include, for example, information regarding the imaging site and imaging region, information regarding the resolution such as the number of slices and matrix size, information regarding the imaging method (or type of pulse sequence) such as the FSE (fast spin echo) method, SSFP (steady-state free precision) method, DWI (diffusion weighted imaging) method, etc., and the values of various parameters within the pulse sequence (e.g., the flip angle and transmission peak power of RF pulses (i.e., transmission pulses) such as excitation pulses and refocusing pulses, the repetition period of the RF pulses, and the values of parameters regarding the magnitude and application timing of the gradient magnetic field).

[0038] The sequence controller 34 determines more detailed imaging conditions and their parameters based on the imaging conditions set by the imaging condition setting function F01 of the console 400.

[0039] For example, according to the set imaging site, such as imaging sites like the head, chest, abdomen, spine, etc., the RF coil 20 to be used for imaging is determined from among the RF coils 20 such as the head coil 20b, body coil 20a, spine coil 20c, etc. that are mounted. Additionally, the user may select the type of RF coil 20 to be used as one of the imaging conditions.

[0040] Also, for example, the sequence controller 34 determines the position and range of the FOV (field of view) from the information regarding the imaging region set as the imaging condition. The sequence controller 34 may distinguish, based on the determined position and range of the FOV, among the plurality of element coils 200 included in the selected RF coil 20, the element coils 200 corresponding to the region within the FOV and the element coils 200 corresponding to the region outside the FOV.

[0041] Also, the sequence controller 34 determines, from the information regarding the determined position and range of the FOV, the information regarding the resolution and matrix size, the information regarding the type of pulse sequence, the information regarding the flip angle of the RF pulse, etc., the parameters related to the RF pulse in the pulse sequence used for imaging (i.e., the waveform, generation timing, transmission peak power, etc. of the RF pulse), and the parameters related to the gradient magnetic field pulse (i.e., the waveform, generation timing, magnitude of the gradient magnetic field, etc. of the gradient magnetic field pulses in the X, Y, and Z axis directions).

[0042] When an imaging start instruction is issued, the sequence controller 34 controls the RF transmitter 33 to generate an RF pulse according to the determined parameters related to the RF pulse, and controls the gradient magnetic field power supplies 31x, 31y, and 31z to generate a gradient magnetic field pulse according to the determined parameters related to the gradient magnetic field pulse.

[0043] As described above, each active trap circuit 201 in the RF coil 20 of the embodiment is provided with a power supply circuit 202 for driving the PIN diode D. And, as shown in FIG. 3, a control circuit 310 is connected to each power supply circuit 202.

[0044] The control circuit 310 is provided, for example, inside the sequence controller 34. It controls the power supply circuit 202 according to the imaging conditions determined by the sequence controller 34, and performs a process of switching the forward bias voltage and the reverse bias voltage applied to the PIN diode D in the active trap circuit 201. Additionally, when applying the forward bias voltage, it applies forward currents of a plurality of different values to the PIN diode D according to the imaging conditions.

[0045] Note that the control circuit 310 does not necessarily have to be provided in the sequence controller 34. For example, it may be provided in the console 400, or may be provided in a distributed manner in each RF coil 20.

[0046] FIGS. 4(a) to 4(e) are diagrams schematically illustrating an example of the relationship between the control of the forward bias voltage and the reverse bias voltage for the above-described PIN diode D, the control of the forward current value, and the equivalent state of the element coil 200.

[0047] As shown in FIG. 4(a), in this example, the control circuit 310 switches between a state in which the power supply circuit 202 applies a reverse bias voltage to the PIN diode D and a state in which it applies a forward bias voltage. In the state of applying the forward bias voltage, the forward current is switched in multiple steps, for example, in three steps of [large], [medium], and [small].

[0048] FIG. 4(b) illustrates a state in which a forward bias voltage is applied to the PIN diode D and a large forward current equal to or greater than a predetermined value (a state in which the forward current value is [large]). In this state, as shown in the upper part of FIG. 4(b), the PIN diode D is in an on (closed) state, and the equivalent resistance value R of the PIN diode D with respect to the high-frequency signal f becomes almost zero. As a result, the active trap circuit 201 forms a parallel resonance circuit with the capacitor C and the inductor L.

[0049] Here, the capacitance of the capacitor C and the inductance of the inductor L are selected such that the resonance frequency of this parallel resonance circuit becomes the Larmor frequency. For this reason, when the forward current value of the PIN diode D is [large], with respect to the MR signal having the Larmor frequency, the impedance Z of the parallel resonance circuit becomes infinite, and as shown in the lower part of Fig. 4(b), the element coil 200 becomes open at the position of the parallel resonance circuit, and no MR signal flows through the element coil 200.

[0050] Therefore, when the forward current value of the PIN diode D is [large], it does not receive induction from other element coils 200, that is, the degree of decoupling is maximized. Also, since no MR signal flows through the element coil 200, the element coil 200 becomes an element coil 200 that is not used for generating the MR image, that is, a non - selected element coil 200. The state where the forward current value of the PIN diode D is [large] is the state where the power consumption of the PIN diode D is the largest.

[0051] Fig. 4(c) illustrates a state in which a forward bias voltage is applied to the PIN diode D, but the value of the forward current is a forward current smaller than a predetermined value in the state where the forward current value is [large] (that is, the state where the forward current value is [medium]). In this state, as shown in the upper part of Fig. 4(c), the resistance value R f of the PIN diode D is not zero, but shows a predetermined resistance value R f 1 with respect to the high - frequency signal. As a result, the capacitor C of the active trap circuit 201 is connected in parallel with the series circuit of the resistance value R f 1 and the inductor L. As shown in the lower part of Fig. 4(c), even at the Larmor frequency, the impedance Z1 of the active trap circuit 201 does not become infinite, but shows a finite value Z1.

[0052] In this state, that is, when the forward current value of the PIN diode D is [medium], although a certain amount of MR signal flows, the current value of the MR signal is severely restricted. Therefore, the element coil 200 becomes an element coil 200 that is not used for generating an MR image, that is, a non-selected element coil 200.

[0053] On the other hand, although it is affected by induction from other element coils 200 to a certain extent, that is, although the degree of decoupling is medium, decoupling is possible when other element coils 200 are not close to the element coil 200. When the forward current value of the PIN diode D is [medium], the power consumption of the PIN diode D is smaller than that in the state where the forward current value is [large].

[0054] FIG. 4(d) illustrates a state in which a forward bias voltage is applied to the PIN diode D, but the value of the forward current is set to be even smaller than the state where the forward current value is [medium] (that is, the state where the forward current value is [small]). Even in this state, as shown in the upper part of FIG. 4(d), the resistance value R of the PIN diode D f is not zero, but shows a predetermined resistance value R for the high-frequency signal f 2. However, the resistance value R f 2 is larger than the resistance value R in the state where the forward current value is [medium] f 1 (R f 2 > R f 1).

[0055] Also in this case, the capacitor C of the active trap circuit 201 is connected in parallel with the series circuit of the resistance value R f 2 and the inductor L. As shown in the lower part of FIG. 4(c), also at the Larmor frequency, the impedance Z2 of the active trap circuit 201 does not become infinite, but shows a finite value Z2. However, the impedance Z2 in the state where the forward current value is [small] shows a value smaller than the impedance Z1 in the state where the forward current value is [medium].

[0056] Even in this state, that is, even when the forward current value of the PIN diode D is [small], although a certain amount of MR signal flows, the current value of the MR signal is limited. Therefore, this element coil 200 becomes an element coil 200 that is not used for generating an MR image, that is, a non-selected element coil 200.

[0057] On the other hand, the degree of induction from other element coils 200 becomes larger than the state where the forward current value of the PIN diode D is [medium], and the degree of decoupling becomes smaller than the state where the forward current value of the PIN diode D is [medium]. That is, the induced current received from other element coils 200 becomes larger. However, even in this case, if other element coils 200 are separated from this element coil 200 by a certain distance or more, decoupling is possible. When the forward current value of the PIN diode D is [small], the power consumption of the PIN diode D becomes smaller than the state where the forward current value is [medium].

[0058] FIG. 4(e) illustrates a state in which a reverse bias voltage is applied to the PIN diode D. In this state, as shown in the upper part of FIG. 4(e), the PIN diode D is in an off (open) state, and the equivalent resistance value R of the PIN diode D with respect to the high-frequency signal f becomes almost infinite. As a result, the active trap circuit 201 connected in series to the element coil 200 substantially becomes only the capacitor C.

[0059] In this case, the impedance of the capacitor C at the Larmor frequency becomes almost zero, and as shown in the lower part of FIG. 4(e), the element coil 200 is in a state as if the switch at the position of the active trap circuit 201 is turned on.

[0060] That is, by applying a reverse bias voltage to the PIN diode D, this element coil 200 can receive an MR signal well, and this element coil 200 becomes an element coil 200 selected as an element coil 200 used for generating an MR image.

[0061] The state in which the reverse bias voltage is applied to the PIN diode D is the state where the power consumption of the PIN diode D is the smallest. However, in this state, when a large power such as an RF pulse is applied, electronic components such as the PIN diode D of the preamplifier 203 and the active trap circuit 201 are most likely to be damaged.

[0062] As described above, in the magnetic resonance imaging apparatus 1 of the embodiment, the bias control for the PIN diode D of the active trap circuit 201 is performed in multiple stages according to the imaging conditions. And, by this multi-stage bias control, the power consumption of the PIN diode D is reduced compared to the conventional case. Hereinafter, some examples of the multi-stage bias control for the PIN diode D will be shown.

[0063] FIG. 5 is a diagram showing a first embodiment. In the first embodiment, as shown in the upper part of FIG. 5, around the subject, a head coil 20b, a body coil 20a, and a spine coil 20c are arranged as examples of a plurality of RF coils 20.

[0064] Here, when the imaging site is set to be, for example, the head as an imaging condition, the sequence controller 34 or the control circuit 310 selects the head coil 20b as the RF coil 20 to be used for imaging, and non-selects the other two RF coils 20 (the body coil 20a and the spine coil 20c).

[0065] In this case, the head coil 20b is an RF coil corresponding to the imaging site, and since the head coil 20b becomes the selection coil (the first RF coil) that receives the MR signal, the control circuit 310 controls each power supply circuit 202 to apply a reverse bias voltage to the PIN diode D of each element coil 200 in the head coil 20b. On the other hand, the body coil 20a and the spine coil 20c do not correspond to the imaging site and are not used for imaging, so they become non-selection coils (the second RF coils).

[0066] Conventionally, for non-selected coils, in order to ensure reliable decoupling from the selected coil, a forward bias current equal to or greater than a predetermined value was applied to maximize the degree of decoupling. That is, both the body coil 20a and the spine coil 20c were set to the state of "forward current [large]" in FIG. 4. For this reason, the power consumption of the PIN diode D was in the maximum state.

[0067] However, since the body coil 20a and the spine coil 20c, which are non-selected coils, are not close enough to overlap with the head coil 20b, which is the selected coil, it is not always necessary to maximize the degree of decoupling, and a medium degree of decoupling is sufficient.

[0068] Therefore, in the first embodiment, the forward current of the PIN diode D is set to a value smaller than the predetermined value when the decoupling is maximum so that the degree of decoupling between the body coil 20a and the spine coil 20c, which are non-selected coils, is medium. That is, both the body coil 20a and the spine coil 20c are set to the state of "forward current [medium]" in FIG. 4. As a result, it is possible to reduce the power consumption of the PIN diode D compared to the conventional case.

[0069] FIG. 6 is a diagram showing a modification of the first embodiment. In the modification of the first embodiment, the region of the non-selected coil is divided into a plurality of regions according to the distance from the selected coil, and the value of the forward current of the PIN diode D of the element coil 200 in the region farther from the selected coil is made smaller.

[0070] For example, in the example shown in FIG. 6, the regions of the body coil 20a and the spine coil 20c, which are non-selected coils, are divided into a region A close to the head coil 20b, which is the selected coil, and a region B far from the head coil 20b. Since the region B is far from the head coil 20b, the degree of decoupling from the head coil 20b can be made smaller than the degree of decoupling in the region A.

[0071] Therefore, in a modification of the first embodiment, in region A of the body coil 20a and the spine coil 20c, while setting the PIN diode D in the "forward current [medium]" state as in the first embodiment, for region B of the body coil 20a and the spine coil 20c, the forward current applied to the PIN diode D is made even smaller than that in region A, and the PIN diode D is set in the "forward current [small]" state. By controlling the forward current in this way, in the modification of the first embodiment, the power consumption of the PIN diode D can be further reduced compared to the first embodiment.

[0072] FIG. 7 is a diagram showing a second embodiment. In the second embodiment, as shown in the upper part of FIG. 7, a body coil 20a is arranged on the subject as an example of at least one RF coil 20 used for imaging.

[0073] Here, as an imaging condition, the imaging region, that is, the FOV (field of view) is set by the user. For example, as illustrated in FIG. 7, a region of a predetermined range including the heart of the subject is set as the FOV.

[0074] When the FOV is set, the sequence controller 34 or the control circuit 310 selects one or more element coils 200 corresponding to the region of the FOV among the plurality of element coils 200 included in the body coil 20a, while non-selecting one or more element coils 200 corresponding to the region outside the FOV.

[0075] The control circuit 310 controls each power supply circuit 202 to apply a reverse bias voltage to the PIN diode D of the selected element coil 200 corresponding to the region of the FOV so that an MR signal can be received.

[0076] On the one hand, since the element coil 200 within the FOV and the element coil 200 outside the FOV are not close enough to overlap with each other, it is not always necessary to maximize the degree of decoupling, and a medium degree of decoupling is sufficient.

[0077] Therefore, in the second embodiment, the forward current of the PIN diode D is set to a value smaller than a predetermined value when the decoupling is maximum so that the degree of decoupling between the element coil 200 within the FOV and the element coil 200 outside the FOV becomes medium. That is, the element coil 200 outside the FOV is set to the state of "forward current [medium]" in FIG. 4.

[0078] Conventionally, a forward bias current equal to or greater than a predetermined value has been applied to the PIN diode D of the element coil 200 outside the FOV to maximize the degree of decoupling from the element coil 200 within the FOV. That is, the PIN diode D of the element coil 200 outside the FOV has been set to the state of "forward current [large]" in FIG. 4.

[0079] In contrast, in the second embodiment, by setting the element coil 200 outside the FOV to the state of "forward current [medium]" in FIG. 4, it is possible to reduce the power consumption of the PIN diode D compared to the conventional case.

[0080] FIG. 8 is a diagram showing a modified example of the second embodiment. In the modified example of the second embodiment, the region of the body coil 20a is divided into a plurality of regions according to the distance from the FOV, and the forward current value of the PIN diode D in the region far from the FOV (region B in FIG. 8) is made smaller than the forward current value of the PIN diode D in the region close to the FOV (region A in FIG. 8).

[0081] For example, in the example shown in FIG. 8, while the forward current value of the PIN diode D in region A is the same as that in the second embodiment described above, the forward current value of the PIN diode D in region B is set to a value smaller than the forward current value in region A. That is, the forward current value of the PIN diode D in region B is set to the state of "forward current [small]" in FIG. 4. By controlling the forward current in this way, in a modified example of the second embodiment, the power consumption of the PIN diode D can be further reduced compared to the second embodiment.

[0082] FIG. 9 is a diagram for explaining the bias control of the third embodiment. In the first and second embodiments described above, and their modified examples, the bias control of the PIN diode D during the period other than the on-period of the transmission pulse, that is, the reception period between the transmission pulses, has been described.

[0083] On the other hand, during the application period of the transmission pulse, from the viewpoint of maximizing the degree of decoupling with the coil for transmission (for example, the WB coil 12) to ensure the uniformity of the high-frequency magnetic field, and from the viewpoint of protecting the electronic components and preamplifiers connected to the element coil 200, a forward current equal to or greater than a predetermined value is applied to the PIN diode D.

[0084] Conventionally, during the application period of the transmission pulse (in FIG. 9, during the T ON period), regardless of the peak power of the transmission pulse (or the magnitude of the flip angle of the RF pulse), the forward current value of the PIN diode D was set to the state of "forward current [large]" in FIG. 4.

[0085] In contrast, in the third embodiment shown in FIG. 9, from the set imaging conditions, for example, the sequence controller 34 detects the peak power of the transmission pulse (or the flip angle of the RF pulse). Then, according to the peak power of the transmission pulse (or the magnitude of the flip angle of the RF pulse), the control circuit 310 changes the value of the forward current of the PIN diode D. As is well known, the type of pulse sequence varies depending on the type of imaging method, and the flip angle of the RF pulse is different, and accordingly, the transmission peak power of the RF pulse also varies.

[0086] For example, in the pulse sequence of FSE (fast spin echo) shown in the upper part of FIG. 9, following an excitation pulse with a flip angle of 90°, a refocusing pulse with a flip angle of 180° (i.e., having a high peak power) is repeatedly applied. On the other hand, in the pulse sequences of the GRE (gradient echo) system shown in the middle and lower parts of FIG. 9, the flip angle is set to a value smaller than 90°, for example, a small value such as a flip angle of 70° or 20°.

[0087] The transmission peak power of an RF pulse with a flip angle of 70° is lower than that of an RF pulse with a flip angle of 180°. Therefore, the required decoupling during the T ON period and the required withstand power of electronic components such as the PIN diode D can be smaller than those during the T ON period of an RF pulse with a flip angle of 180°. That is, during the T ON period of an RF pulse with a flip angle of 70°, the forward current of the PIN diode D can be set smaller than that during the T ON period of an RF pulse with a flip angle of 180°. For example, the forward current value can be set to the state of "forward current [medium]" in FIG. 4, and the power consumption of the PIN diode D can be reduced.

[0088] Furthermore, the transmission peak power of an RF pulse with a flip angle of 20° is lower than that of an RF pulse with a flip angle of 70°. Therefore, the T of an RF pulse with a flip angle of 20°ON During the period, the required decoupling and the required breakdown voltage of electronic components such as the PIN diode D may be smaller than those during the T of the RF pulse with a flip angle of 70°. ON During the period of the RF pulse with a flip angle of 20°, the T may be smaller than that during the period of the RF pulse with a flip angle of 70°. ON During the period of the RF pulse with a flip angle of 20°, the forward current of the PIN diode D can be set to be even smaller than that during the period of the RF pulse with a flip angle of 70°. ON For example, the forward current value can be set to the state of "forward current [small]" in FIG. 4, and the power consumption of the PIN diode D can be further reduced.

[0089] As described above, according to the magnetic resonance imaging apparatus according to at least one embodiment, the power consumption of the PIN diode included in the RF coil can be suppressed. Note that the RF transmitter and the control circuit in the description of each embodiment are examples of the transmitter circuit and the control unit in the description of the claims, respectively.

[0090] Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalent scope thereof.

Description of Reference Numerals

[0091] 1 Magnetic resonance imaging apparatus 20 (20a, 20b, 20c) RF coil 33 RF transmitter 40 Processing circuit 200 Element coil 201 Active trap circuit 202 Power supply circuit 203 Preamplifier D PIN diode L inductor C capacitor

Claims

1. A transmission circuit that applies a transmission pulse to a subject, At least one high-frequency coil including an active trap circuit having a PIN diode, the high-frequency coil receiving a magnetic resonance signal from the subject, A power supply circuit capable of applying a reverse bias voltage and a forward bias voltage to the PIN diode, the power supply circuit being configured to apply forward currents of a plurality of values to the PIN diode when the forward bias voltage is applied, A control unit that controls the power supply circuit to set the forward current to a plurality of values according to imaging conditions, A magnetic resonance imaging apparatus comprising the same.

2. The high-frequency coil includes one or a plurality of element coils, The magnetic resonance imaging apparatus according to Claim 1.

3. The active trap circuit is provided for each of the one or more element coils, The active trap circuit is configured to change the degree of decoupling between the high-frequency coil and other high-frequency coils, or the degree of decoupling between the element coil and other element coils, according to the value of the forward current applied to the PIN diode. The magnetic resonance imaging apparatus according to Claim 2.

4. The at least one high-frequency coil is a plurality of high-frequency coils arranged around the subject, The control unit, During the application period of the transmission pulse, the power supply circuit is controlled to set the forward current of the PIN diode to a predetermined first value or more so as to maximize the degree of decoupling and protect the electronic components connected to the element coil, During the non-application period of the transmission pulse, For each element coil in the first high-frequency coil corresponding to the imaging site among the plurality of high-frequency coils, a reverse bias voltage is applied to the PIN diode so as to be in a state capable of receiving the magnetic resonance signal, For each element coil in the second high-frequency coil not corresponding to the imaging site among the plurality of high-frequency coils, the power supply circuit is controlled to apply a forward current of a second value smaller than the first value to the PIN diode so that the degree of decoupling is lower than the degree of decoupling during the application period of the transmission pulse. The magnetic resonance imaging apparatus according to Claim 2 or 3.

5. The control unit controls the power supply circuit such that the second value of the forward current decreases as the distance between the first high-frequency coil and the second high-frequency coil increases. The magnetic resonance imaging apparatus according to claim 4.

6. The first high-frequency coil is a head coil, and the second high-frequency coil is at least one of a body coil or a spine coil. The magnetic resonance imaging apparatus according to claim 4 or 5.

7. The at least one high-frequency coil includes at least one first element coil corresponding to an imaging region of the subject and at least one second element coil corresponding to a region outside the imaging region. The control unit During the application period of the transmission pulse, the control unit controls the power supply circuit to set the forward current of the PIN diode to be equal to or greater than a predetermined first value so as to maximize the degree of decoupling and protect electronic components connected to the element coil. During the non-application period of the transmission pulse A reverse bias voltage is applied to the PIN diode so that the at least one first element coil is in a state capable of receiving the magnetic resonance signal. During the application period of the transmission pulse, the control unit controls the power supply circuit to apply a forward current having a second value smaller than the first value to the PIN diode so that the degree of decoupling of the at least one second element coil is lower than the degree of decoupling during the application period of the transmission pulse. The magnetic resonance imaging apparatus according to claim 2 or 3.

8. The control unit controls the power supply circuit such that the second value of the forward current decreases as the distance between the first element coil and the second element coil increases. The magnetic resonance imaging apparatus according to claim 7.

9. During the application of the transmission pulse, the control unit controls the power supply circuit such that the forward current applied to the PIN diode has different values according to the value of the peak power of the transmission pulse. The magnetic resonance imaging apparatus according to claim 1.

10. The control unit controls the power supply circuit such that the forward current applied to the PIN diode decreases as the value of the peak power of the transmission pulse decreases. The magnetic resonance imaging apparatus according to claim 9.

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