JP4393116B2 - Magnetic resonance imaging system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging apparatus that takes a tomographic image of a desired portion of a subject using a nuclear magnetic resonance phenomenon, and more particularly to cooling of an RF transmission coil.
[0002]
[Prior art]
When power is supplied to the RF transmission coil of the magnetic resonance imaging apparatus to generate a high-frequency magnetic field, the temperature of the RF transmission coil rises due to heat generated by passive elements such as diodes and capacitors that are constituent elements of the coil. In particular, the higher the magnetic field, the higher the frequency of the high-frequency magnetic field, so that the temperature rise of the RF transmitter coil becomes significant. If such an increase in the temperature of the RF transmitter coil is left unattended, the performance of the RF transmitter coil is reduced, and there is a possibility of causing a thermal hazard such as a burn to the subject approaching the irradiation coil. Therefore, in order to maintain the performance of the RF transmission coil and protect the safety of the subject, it is necessary to cool the RF transmission coil and control its temperature.
[0003]
As a method of cooling the RF transmission coil, it has been proposed to forcibly cool it using a refrigerant. For example, in [Publication 1], the RF transmission coil is cooled by using a liquid that does not contain protons as a refrigerant (for example, Freon FC-40). This suppresses the interference between the refrigerant and the high-frequency magnetic field, that is, the absorption of the high-frequency magnetic field by the refrigerant and the generation of a nuclear magnetic resonance signal from the refrigerant.
[0004]
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-224084
[Problems to be solved by the invention]
Generally, using water as a refrigerant is easier to handle and has higher cooling efficiency. However, when water is used as the refrigerant, the water contains protons, and as described above, the refrigerant absorbs a high-frequency magnetic field or an unnecessary nuclear magnetic resonance signal is generated from the refrigerant, causing unexpected artifacts in the image. It causes adverse effects such as generation. However, [Patent Document 1] does not propose a cooling means for suppressing interference with the above-described high-frequency magnetic field that occurs when the RF transmission coil is cooled using a refrigerant containing protons such as water. .
Accordingly, an object of the present invention is to cool an RF transmission coil while suppressing interference between such a refrigerant and a high-frequency magnetic field even if the refrigerant contains a proton such as water.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the present invention is configured as follows.
A magnetic field generating means for generating a static magnetic field and a gradient magnetic field in a space where the subject is placed, a high-frequency magnetic field transmitting means having an RF transmission coil for irradiating a high-frequency magnetic field to the imaging region of the subject, Receiving means for receiving a nuclear magnetic resonance signal; pulse sequence control means for controlling application of the gradient magnetic field, application of the high-frequency magnetic field and reception of the nuclear magnetic resonance signal; cooling means for cooling the RF transmission coil; In the magnetic resonance imaging apparatus including a refrigerant control unit that controls a refrigerant to be supplied to the cooling unit, the refrigerant control unit controls supply of the refrigerant to the cooling unit in response to the pulse sequence.
[0007]
As a result, when a refrigerant containing protons such as water is used, the RF transmission coil can be cooled while suppressing interference with a high-frequency magnetic field corresponding to an arbitrary pulse sequence.
According to a preferred embodiment, the refrigerant control means controls supply of the refrigerant so that the refrigerant is supplied to the cooling means when the high-frequency magnetic field is not applied to the subject.
Thereby, RF transmission coil can be cooled in real time, suppressing interference with a refrigerant and a high frequency magnetic field.
[0008]
Further, according to a preferred embodiment, the refrigerant control means controls the refrigerant so that the refrigerant is supplied to the cooling means between the end of the arbitrary pulse sequence and the start of the next pulse sequence. Control the supply.
Thereby, the RF transmission coil can be concentrated and cooled while suppressing interference between the refrigerant and the high-frequency magnetic field, and the temperature can be greatly reduced.
[0009]
According to a preferred embodiment, the refrigerant comprises a first refrigerant that is a liquid and a second refrigerant that is a gas, and the refrigerant control means is arranged in the cooling means during application of the high-frequency magnetic field. The supply of the first refrigerant and the second refrigerant is controlled so that is filled only with the second refrigerant.
As a result, the first refrigerant can be completely removed from the cooling means during application of the high-frequency magnetic field, so that interference between the refrigerant and the high-frequency magnetic field can be further suppressed.
[0010]
According to a preferred embodiment, the cooling means includes a gas-liquid separation means, and separates the first refrigerant and the second refrigerant that have passed through the cooling means, and then recirculates each refrigerant. Let
Accordingly, when the refrigerant is recirculated, each refrigerant can be easily controlled independently, and the refrigerant can be effectively used.
[0011]
According to a preferred embodiment, the cooling means is configured by combining tubes for flowing the refrigerant, and each tube is disposed in proximity to or in contact with each coil element constituting the RF transmission coil. The
As a result, the RF transmission coil can be efficiently cooled.
[0012]
Further, according to a preferred embodiment, the cooling means includes a tube substantially at the center of the RF transmission coil, and supplies the refrigerant that has been supplied from the periphery of the RF transmission coil to cool the RF transmission coil. The refrigerant is converged and discharged, or the refrigerant flow is reversed, the refrigerant is supplied from the substantially central pipe, and the refrigerant that has cooled the RF transmission coil is discharged from the periphery of the RF transmission coil.
[0013]
Thereby, in a vertical magnetic field type magnetic resonance imaging apparatus that generates a static magnetic field by a superconducting magnet in particular, it is possible to secure a space for easily making a hole in the center of the magnet, so that the tubes can be arranged spatially efficiently.
[0014]
According to a preferred embodiment, the magnetic field generating means for generating a static magnetic field and a gradient magnetic field in a measurement space where the subject is placed, and an RF transmitter coil for irradiating a radio frequency magnetic field to the imaging region of the subject are provided. A high-frequency magnetic field transmitting means; a receiving means for receiving a nuclear magnetic resonance signal from the subject; a high-frequency magnetic field shielding means arranged on the magnetic field generating means side of the RF transmission coil to shield the high-frequency magnetic field; a pulse sequence control means for controlling the reception of the the application of a magnetic field application of the high frequency magnetic field and the nuclear magnetic resonance signals, a cooling means for cooling the RF transmission coil, the magnetic resonance imaging apparatus comprising a city, the cooling means The high-frequency magnetic field shield means is disposed on the magnetic field generation means side and connected to the RF transmission coil via a heat conductive member.
[0015]
This eliminates interference between the refrigerant and the high-frequency magnetic field, so that the refrigerant can be constantly flowed during imaging of the subject.
According to a preferred embodiment, the refrigerant or the first refrigerant contains water.
Thereby, handling of a refrigerant becomes simple and cooling efficiency can also be made high.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiment of the invention, and the repetitive description thereof is omitted.
[0017]
FIG. 4 is a block diagram showing a configuration of a magnetic resonance imaging (hereinafter referred to as “MRI”) apparatus according to the present invention.
[0018]
This MRI apparatus generates a static magnetic field generating means 1 for applying a static magnetic field to the subject 116, an X-direction gradient magnetic field generating means 2 for generating a gradient magnetic field in the X direction, and a gradient magnetic field in the Y direction in an XYZ orthogonal coordinate system. Y direction gradient magnetic field generating means 3, Z direction gradient magnetic field generating means 4 for generating a gradient magnetic field in the Z direction, a gradient magnetic field power source 5 for supplying power to the gradient magnetic field generating means, and an imaging region of the subject An RF transmission coil 6 that irradiates a high-frequency magnetic field, an RF reception coil 7 that receives a high-frequency magnetic field emitted from the living tissue of the subject by nuclear magnetic resonance (hereinafter referred to as “NMR”), and the RF transmission coil 6 A cooling pipe 8 for cooling, a high-frequency magnetic field power supply 9 for supplying electric power for generating the high-frequency magnetic field, a high-frequency transmitting / receiving unit 110 for irradiating and receiving the high-frequency magnetic field, a pump 111 for conveying the refrigerant, Heat exchanger 112 to remove heat and tilt Control the execution of the pulse sequence by controlling the timing of the magnetic field application and the high-frequency transmission / reception unit, and the computer 113 that performs the image reconstruction calculation using the received signal, and the image signal generated by the computer is input to the tomogram The display 114 is displayed as an image.
[0019]
The gradient magnetic field generating means 2, 3, and 4 are each composed of a set of coils that generate gradient magnetic fields in three orthogonal directions of X, Y, and Z. A magnetic field coil generates a gradient magnetic field. An arbitrary cross section of the subject 116 can be selected depending on the application method of the gradient magnetic field, and position information can be given to the NMR signal. A gradient magnetic field that gives position information to an NMR signal is called a phase encode gradient magnetic field or a frequency encode gradient magnetic field, and thereby a measurement space (k space) in which measurement data is arranged is defined.
[0020]
The RF transmission coil 6 for transmission receives a power supply from the high frequency magnetic field power source 9 and generates a high frequency magnetic field. The frequency of the high-frequency magnetic field is tuned to the resonance frequency of the nuclear spin to be imaged. Usually, the imaging object of the MRI apparatus is a proton of a hydrogen nucleus that is a main constituent material of the subject.
[0021]
A signal from the RF receiving coil 7 is detected by the high frequency transmitter / receiver 110, processed by the computer 113, and converted into an image by calculation. The image is displayed on the display unit 114. The gradient magnetic field power supply 5 and the high frequency transmission / reception unit 110 are controlled by the computer 113 according to a pulse sequence.
[0022]
FIG. 1 shows the configuration and arrangement of a cooling pipe which is a cooling means in the first embodiment of the present invention. FIG. 2 shows the configuration of the RF transmission coil in the first embodiment. FIG. 2 is a schematic diagram when the entire RF transmission coil arranged on the XY plane is viewed from the positive direction of the Z axis. This RF transmission coil is composed of conductors 22 and 23, a capacitor 24, and a diode 25 on a substrate 21. The RF transmission coil in the first embodiment has an octagonal shape, and is composed of eight elements of the same type, each of which has a conductor 22 and a diagonal line of a conductor 23. Further, a capacitor 24 is disposed on the conductor constituting the side, and a diode 25 is disposed on the conductor constituting the diagonal line.
[0023]
FIG. 1 (a) shows a schematic diagram when the entire cooling pipe arranged on the XY plane is viewed from the positive direction of the Z axis. As shown in FIG. 2, the entire cooling pipe has an octagonal shape similar to that of the RF transmission coil, and is composed of eight elements of the same type. Then, each element of the cooling pipe is disposed in proximity to or in contact with the nearest coil element of the RF transmission coil (that is, in thermal contact so that heat exchange is sufficiently possible). FIG. 1 (b) shows a configuration relating to an arbitrary element of the cooling pipe. FIG. 1 (c) shows the arrangement of the tubes in the XZ section passing through the line connecting the tubes 11 and 15 of FIG. 1 (a) of the 8-element cooling tube. The positions of the pipes 11 and 15 are not limited to the positions shown in FIG. 1 (a), and may be any positions around the pipe 13.
[0024]
First, the refrigerant flow focusing on one coil element will be described. The refrigerant flows in from the pipe 11 and further flows into the pipe 13 disposed on the outer periphery of the conductor 22. The pipe 11 and the pipe 13 are separated by the valve 12, and the refrigerant in the pipe 11 flows into the pipe 13 when the valve 12 is open. The tube 14 is disposed along the top surfaces of the conductor 23 and the element 25 and is connected to the tube 18 at the center of the coil. Further, the pipe 16 branched from the pipe 14 is disposed along the upper surfaces of the conductor 22 and the element 24, and is joined to the pipe 18 via the pipe 17 which is a return path. The tube 18 penetrates the coil substrate 21 at the center of the RF transmission coil. The pipe 18 is connected to the pipe 15, and the refrigerant flows out of the pipe 15. The arrangement of the tubes on the other coil elements is the same. The pipe 11 and the pipe 15 are respectively connected to an outlet side of a pump not shown and an inlet side of a heat exchanger not shown.
[0025]
A method for controlling the temperature of the RF transmission coil using water as a refrigerant in the cooling pipe having the above structure will be described. RF pulse irradiation timing and cooling in synchronization with an arbitrary pulse sequence in order to avoid refrigerant excitation and generation of unnecessary NMR signals from the refrigerant by irradiation with a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) The supply timing of the refrigerant is controlled so that the refrigerant flows into the pipe at different timings so that the refrigerant is removed and does not exist in the cooling pipe when the RF pulse is applied.
[0026]
At the start time t0 of the pulse sequence (spin echo method) shown in FIG. 3, the refrigerant supplied from the pump of FIG. 1 (b) passes through the tube 11 and is filled in the tube 13 arranged on the outer periphery of the conductor 22. Suppose that At the start time t0, the valves 12 are all closed, and the refrigerant cannot flow freely before the pipe 13. In the spin echo method of FIG. 3, a 90 ° RF pulse is irradiated at time t1, a 180 ° RF pulse is irradiated after the TE / 2 hours, and a total of two RF pulses are irradiated during one repetition time TR. Is done. As shown in FIG. 5 (a), when the valve 12 is opened after the delay time τ1 from the irradiation time t1 of the 90 ° RF pulse, the refrigerant passes through the pipe 14 and the pipe 16 and merges in the pipe 15 via the pipe 18. Then, it enters a heat exchanger (not shown). Here, τ1> TE / 2. If the valve 12 is closed after a certain time, the refrigerant cannot pass through the pipe 13. As described above, FIG. 5 (a) shows a case where the valve is opened and closed at each repetition time TR. In FIG. 5 (a), the 180 ° RF pulse is omitted.
[0027]
In Fig. 5 (b), any protocol (pulse sequence) is used in a protocol for imaging at least multiple times for a subject (a combination of a series of pulse sequences to capture images with multiple purposes). It shows the case where the valve is opened and closed between the end of the first imaging and the start of the next imaging (pulse sequence). Here, the number of phase encoding is n, and τ1> n × TR. Thus, by making the timing of irradiation of the RF pulse different from the timing of inflow of the refrigerant into the tubes 14 and 16, it is possible to suppress the RF pulse from exciting the refrigerant and generating unnecessary signals. The opening / closing control of the valve 12 is performed via the computer 13 according to the photographing conditions or the photographing protocol.
[0028]
Next, a method for estimating the temperature rise gradient of the RF transmission coil from the imaging conditions will be described. Here, the spin echo method shown in FIG. 3 is applied as a reference pulse sequence. As shown in FIG. 3, RF pulses with flip angles of 90 ° and 180 ° are applied at half the echo time TE, respectively. Here, TE is the time from the irradiation time of the 90 ° RF pulse to the peak position of the echo signal. Gs, Gp, and Gf represent a slice gradient magnetic field, a phase encode gradient magnetic field, and a frequency encode gradient magnetic field, respectively, Gs is cut out of a cross section, and Gp and Gf are applied to give position information to the NMR signal. The above is repeated for the number of phase encodings at intervals of the repetition time TR.
[0029]
Here, if the energy intensity of an RF pulse with a flip angle of 90 ° is 1, the energy intensity of an RF pulse with a flip angle of 180 ° is 2. Also, when TR, TE, and the number of phase encodings change depending on the imaging conditions, it is possible to estimate the approximate temperature rise of the RF transmitter coil from the definitions of these parameters. For example, when TR is 1/2, the shooting time is also 1/2, so the temperature rise gradient is doubled. If the temperature rise of each element (capacitor 24 and diode 25) part and conductor part is measured once in advance by executing a pulse sequence under a certain condition, the temperature under the other imaging conditions for the same RF transmitter coil The ascending slope can also be estimated.
[0030]
Using this temperature rise gradient, the refrigerant flow rate can be optimized in the refrigerant circulation described below. That is, when a pulse sequence that increases the temperature rise is performed, the flow rate of the refrigerant is increased. For example, assuming that the temperature rise of the conductors 22 and 23 expected by the execution of this sequence is ΔTb, the temperature rise of the capacitor 24 is ΔTc, the temperature rise of the diode 25 is ΔTd, the length of the tube 14 is L14, and the tube 16 The length of L16, the cross-sectional area of the tube 14 is S14, the cross-sectional area of the tube 16 is S16,
S16 / S14 = L14 (ΔTb + ΔTd) / L16 (ΔTb + ΔTc) (1)
With the tube design, the flow rate of the refrigerant is controlled by the difference in the cross-sectional area of the tube, preventing overheating of specific parts of the RF transmitter coil. That is, by flowing a large amount of refrigerant through a portion where the temperature rise is large and flowing a small amount of refrigerant through a small portion, all the portions are cooled equally.
[0031]
Next, when the refrigerant is a liquid, the liquid refrigerant may remain in the tube 14 and the tube 16 during the RF pulse irradiation. Therefore, the liquid refrigerant is positively removed. Examples will be described. In the second embodiment, the refrigerant is used by controlling the supply of the first refrigerant that is liquid and the second refrigerant that is gas, and the first refrigerant is mainly responsible for cooling the RF transmission coil. The second refrigerant is mainly responsible for removing the first refrigerant.
[0032]
FIG. 6 shows a second embodiment. Gaseous refrigerant (second refrigerant) such as air is injected into the pipe, and the liquid refrigerant (first refrigerant) remaining in the pipe is positively pushed out to remove it. The supply of liquid refrigerant and gaseous refrigerant is controlled so that no liquid refrigerant is filled. That is, in the cooling section 66 including the pipe 14 and the pipe 16, air is injected from the nozzle 62 installed after the valve 12, as shown in FIG. 1 (a). The refrigerant that has passed through the RF transmission coil is separated into water and air by the gas-liquid separator 63, and the air is sent to the pump 60 and injected from the nozzle 62 again through the compressor 61. The water is sent to the heat exchanger 64 and circulated again by the pump 65.
[0033]
Further, as a third embodiment, an example in which the refrigerant is circulated constantly during the examination of the subject will be described. In this case, as shown in FIG. 1 (c), when the refrigerant circulates through the tubes 14 and 16 arranged on the upper surface of the RF transmission coil (that is, the surface on the measurement space side), unnecessary NMR signals are generated from the refrigerant by irradiation with RF pulses. Will occur. Therefore, in order to prevent the refrigerant from being affected by the RF pulse, as shown in FIG. 7, the tube is outside the substrate 21 and the high-frequency magnetic field shield 26 (that is, the static magnetic field generating means side, and the subject is arranged). Place it on the opposite side of the measurement space. However, compared to the case of FIG. 1 (c), the distance between the components (elements, conductors) of the tube and the RF transmission coil is increased, and the cooling effect of the tube is lost. As shown in FIG. The heat transfer member 41 is passed through the substrate 21, and the tube and the components of the RF transmission coil are brought into thermal contact with each other through the heat transfer member 41. Thereby, the heat energy emitted from the components of the RF transmission coil can be transferred to the refrigerant via the heat transfer member 41.
[0034]
As described above, the configuration and operation of the MRI apparatus using the cooling pipe for cooling the RF transmission coil have been described. However, the MRI apparatus of the present invention is not limited to the above-described embodiment, and various modifications can be made. For example, the configuration of the RF transmission coil and the cooling pipe can be a polygon other than an octagon or a circle. In addition, although the example in which the RF transmission coil and the cooling pipe are configured from the same type of element has been described, the RF transmission coil and the cooling pipe need not be the same type, and may be configured from elements having different shapes. In addition, although the example of water has been described as the (first) refrigerant, it is not particularly required to be water, and other liquids with high cooling efficiency (eg, alternative chlorofluorocarbon materials) or gaseous refrigerants (eg, nitrogen gas) Good. Moreover, although the case where the flow direction of the refrigerant is from the periphery to the center of the RF transmission coil has been described, the reverse may be possible. In the above description of the embodiment, the vertical magnetic field type MRI apparatus is taken as an example. However, the present invention can be similarly applied to a horizontal magnetic field type MRI apparatus.
[0035]
【The invention's effect】
As described above, according to the present invention, it is possible to cool the RF transmission coil using a refrigerant containing protons such as water while suppressing interference with the high-frequency magnetic field of the refrigerant. It is possible to use a high refrigerant such as water.
[Brief description of the drawings]
FIG. 1 is a diagram showing the configuration and arrangement of cooling pipes in a first embodiment of the present invention.
(a) The figure at the time of seeing the whole cooling pipe arrange | positioned on XY plane from the positive direction of a Z-axis.
(b) The figure which shows the structure of the cooling pipe regarding arbitrary 1 element of a cooling pipe.
(c) The figure which shows arrangement | positioning of the pipe | tube in the XZ cross section of a cooling pipe.
FIG. 2 is a diagram showing a configuration of an RF transmission coil in the first embodiment of the present invention.
FIG. 3 is a diagram of a typical pulse sequence (spin echo method).
FIG. 4 is a block diagram showing a configuration of an MRI apparatus according to the present invention.
FIG. 5 is a diagram showing refrigerant control synchronized with a pulse sequence.
(a) The figure in the case of flowing a refrigerant | coolant at the timing when RF pulse is not applied.
(b) The figure in the case of flowing a refrigerant | coolant between continuous pulse sequences.
FIG. 6 is a diagram showing a second embodiment.
FIG. 7 is a diagram showing a third embodiment.
[Explanation of symbols]
1 ... Static magnetic field generation means
2… X-direction gradient magnetic field generation means
3… Y direction gradient magnetic field generation means
4… Z-direction gradient magnetic field generation means
5 ... Gradient magnetic field power supply
6… RF transmitter coil
7… RF receiver coil
8 ... Cooling pipe
9… High frequency magnetic field power supply
60 ... Second refrigerant pump
61 ... Second compressor for refrigerant
62 ... Nozzle for second refrigerant
63… Gas-liquid separator
64 ... heat exchanger
65… First refrigerant pump
66 ... Cooling section
110 ... High-frequency magnetic field transceiver
111 ... Pump
112 ... Heat exchanger
113 ... Calculator
114 ... Display
115 ... Bed
116 ... Subject

Claims (5)

From the subject, magnetic field generating means for generating a static magnetic field and a gradient magnetic field in a measurement space where the subject is placed, a high-frequency magnetic field transmitting means having an RF transmission coil for irradiating a radiofrequency magnetic field to the imaging region of the subject, Receiving means for receiving the nuclear magnetic resonance signal, pulse sequence control means for controlling application of the gradient magnetic field, application of the high frequency magnetic field and reception of the nuclear magnetic resonance signal, cooling means for cooling the RF transmission coil, In a magnetic resonance imaging apparatus comprising: a refrigerant control means for controlling a refrigerant flowing through the cooling means,
The magnetic resonance imaging apparatus , wherein the refrigerant control unit controls supply of the refrigerant so that the refrigerant is not supplied to the cooling unit when the high-frequency magnetic field is applied . The refrigerant control means controls the supply of the refrigerant so that the refrigerant is supplied to the cooling means when the high-frequency magnetic field is not applied to the subject. The magnetic resonance imaging apparatus described. The refrigerant control means controls the supply of the refrigerant so that the refrigerant is supplied to the cooling means between the end of the arbitrary pulse sequence and the start of the next pulse sequence. The magnetic resonance imaging apparatus according to claim 1. The magnetic resonance imaging apparatus according to claim 1, wherein the refrigerant control unit controls the refrigerant to be removed from the cooling unit when the high-frequency magnetic field is applied to the subject. . The refrigerant comprises a first refrigerant that is a liquid and a second refrigerant that is a gas,
The refrigerant control means supplies the first refrigerant and the second refrigerant so that the cooling means is filled only with the second refrigerant when the high-frequency magnetic field is applied to the subject. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is controlled.

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