JP4330477B2 - Gradient magnetic field coil and magnetic resonance imaging apparatus using the same - Google Patents

Description

  The present invention relates to a gradient magnetic field coil used in a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), particularly an open type MRI apparatus, and more particularly to a cooling structure for efficiently cooling energization heat generation of the gradient magnetic field coil.

  In order to give position information to NMR signals, it is necessary to apply a gradient magnetic field superimposed on a static magnetic field in an MRI apparatus, and combine three sets of coils that generate gradient magnetic fields in the X, Y, and Z directions. Gradient field coils are widely used. A current is repeatedly applied to the gradient coil during imaging, thereby generating Joule heat. In recent years, with the spread of high-speed imaging methods, the magnetic field strength of the gradient magnetic field tends to increase, and the current value flowing through the coil and the number of conductor turns increase, resulting in an increase in Joule heat generated during energization. In particular, the active shield type gradient magnetic field coil combined with the shield coil so that the magnetic field generated by the gradient magnetic field does not leak around the coil other than the uniform magnetic field space has a low magnetic field generation efficiency. The calorific value of is very large.

In addition, as described above, a normal gradient magnetic field coil is composed of a combination of three types of coils that generate gradient magnetic fields in three axial directions, and the amount of generated heat and in-plane distribution are different. The exotherm and temperature have a large distribution.
Such an increase or distribution of the amount of heat generated by the gradient magnetic field coil causes thermal deformation in the gradient magnetic field coil itself or deteriorates the performance of the static magnetic field magnet, etc. Therefore, there is a technique for cooling the gradient magnetic field coil. Various proposals have been made.

For example, in Patent Document 1, in an active shield type gradient magnetic field coil provided with a main gradient magnetic field coil and a shield coil, a cooling pipe is arranged in the middle part of the main coil and the shield coil to cool the entire coil. A structure has been proposed. In Patent Document 2, layers having different thermal conductivities are laminated in the cross-sectional direction of the gradient magnetic field coil, and a cooling part is arranged in the outermost layer so that the thermal conductivity decreases from the heating part toward the cooling part. Gradient coils with improved heat flux have been proposed. Further, Patent Document 3 discloses a configuration in which a conductor constituting the gradient magnetic field coil is formed into a hollow cylinder in order to circulate a cooling medium.
JP 2001-46353 A Japanese Patent Laid-Open No. 2003-61930 JP 2001-353136 A

  However, since the cooling mechanism is not provided in the region having the largest heat generation density inside the coil, it is difficult to efficiently and evenly cool the entire surface described in Patent Document 1. In addition, since the structure described in Patent Document 2 has a structure in which layers having different thermal conductivities are laminated, the thickness of the gradient magnetic field coil itself is increased, and cooling provided in the outermost layer in order to secure an imaging space. The thickness of the part naturally has a limit, and the cooling efficiency also has a limit. Furthermore, in what is described in Patent Document 3, the shape and thickness are limited due to the characteristics of the coil conductor, and the cooling efficiency is also limited. Moreover, in the thing described in patent document 3, the gradient magnetic field coil which combined several coils is not considered, and the heat distribution of this kind of coil cannot be improved.

  Then, an object of this invention is to provide the gradient magnetic field coil provided with the cooling structure which can cool a gradient magnetic field coil efficiently and uniformly over the whole surface.

  In order to achieve the above object, a gradient coil for an MRI apparatus of the present invention has a substantially flat shape and includes a plurality of coils that generate gradient magnetic fields in different directions. With the features of

(1) A cooling pipe not having an electromagnetic function is disposed on at least one surface, and one of the plurality of coils constituting the gradient coil is a cooling coil having a refrigerant flow path therein.
(2) One of the plurality of coils constituting the gradient magnetic field coil is a cooling coil (cooling pipe having an electromagnetic function) having a refrigerant flow path therein, and is disposed between other coils. .
(3) When the gradient coil has a cooling pipe and a cooling coil that do not have an electromagnetic function, each piping system of the cooling pipe and the cooling coil has the same pressure loss, and the same refrigerant circulation system It is connected to the.
(4) When the gradient magnetic field coil has a cooling pipe and a cooling coil that do not have an electromagnetic function, the cooling pipe is made of a single layer wound non-inductively.
(5) When the cooling pipe is non-inductively wound, the distance between the forward path and the return path is maintained at a distance that prevents mutual heat exchange.
(6) In the case where the gradient magnetic field coil includes a main coil and a shield coil for preventing magnetic field leakage of the main coil, at least one of the main coil and the shield coil is any one of the features (1) to (6). It has.
(7) When both the main coil and the shield coil are provided with cooling pipes, both cooling pipes are connected in series.
(8) When both the main coil and the shield coil are provided with cooling pipes, both the cooling pipes are connected in series, and the cooling pipe on the shield coil side is connected to the refrigerant inflow side.
(9) The features (1) to (8) above are arbitrarily combined.
The MRI apparatus of the present invention includes any one of these features (1) to (9) as a gradient magnetic field coil, and a means for circulating the refrigerant having a heat exchanger and a circulation pump. .

  According to the gradient coil of the present invention, the heat generated by the gradient coil can be efficiently and uniformly cooled over the entire surface by adopting the above configuration.

  Embodiments of a gradient coil and an MRI apparatus having the same according to the present invention will be described below.

FIG. 1A is a block diagram showing an overall outline of an MRI apparatus to which the present invention is applied.
This MRI apparatus is a vertical magnetic field type open type MRI apparatus that generates a gradient magnetic field perpendicular to the body axis direction of the subject 101, and a pair of upper and lower sides that generate a static magnetic field in an imaging space into which the subject 101 is inserted. A magnet 102, a gradient magnetic field coil 103 that generates a gradient magnetic field in this space, an RF coil 104 that generates a high-frequency magnetic field in the imaging region of the subject, and a nuclear magnetic resonance (MR) signal generated by the subject 101 are detected. An RF probe 105 and a bed 112 for inserting the subject 101 into the static magnetic field space are provided.

In this open type MRI apparatus, a flat magnetic field coil 103 and a flat RF coil 104 are fixed to each of a pair of upper and lower magnets 102, and between support columns (not shown) that support the magnets 102. The imaging space can be accessed from.
The gradient magnetic field coil 103 is composed of gradient magnetic field coils in three directions (X, Y, Z) orthogonal to each other, and each generates a gradient magnetic field in response to a signal from the gradient magnetic field power supply 109. Further, the gradient magnetic field coil 103 is provided with a cooling means (described later), and the cooling means is connected to a heat exchanger 113 and a circulation pump 114 via piping for cooling, as shown in FIG. Yes.

The RF coil 104 generates a high-frequency magnetic field according to the signal from the RF transmission unit 110. The RF probe 105 can also be used as the RF coil 104. However, in the embodiment shown in the drawing, the RF probe 105 is arranged close to the measurement site of the subject 101 separately from the RF coil 104. A signal from the RF probe 105 is detected by the signal detection unit 106, processed by the signal processing unit 107, and converted into an image signal by calculation. The image is displayed on the display unit 108.
The gradient magnetic field power source 109, the RF transmission unit 110, and the signal detection unit 106 are controlled by the control unit 111 in accordance with a pulse sequence determined by the imaging method.

  FIG. 2 is a partial sectional view schematically showing a part of the gradient magnetic field coil 103 in detail, and shows only one of the gradient magnetic field coils arranged on the upper and lower sides (one arranged on the upper side). The gradient magnetic field coil 103 is an active shield type gradient magnetic field coil, and in addition to a main gradient magnetic field coil 201 (hereinafter also referred to as a main coil) that applies a gradient magnetic field to the imaging space 200, a magnetic field generated by the main coil 201 is generated. A shield coil 203 for generating a reverse magnetic field is provided so as not to leak to the static magnetic field magnet side. Similarly to the main coil, the shield coil 203 is a gradient magnetic field coil that generates gradient magnetic fields in three directions of X, Y, and Z. Like the main coil 201, the shield coil 203 has a flat plate shape in which the coils in these three directions are integrated. ing. The shield coil 203 is disposed with a predetermined interval in order to effectively cancel the magnetic field of the main coil 201.

  The gradient magnetic field coil 103 has cooling pipes 205 and 207 fixed to the imaging space side of the main coil 201 and the static magnetic field magnet side of the shield coil 203 in order to cool the heat generated by the main coil 201 and the shield coil 203, respectively. Has been. Each of the cooling pipes 205 and 207 has a forward path and a return path in the same plane, and a refrigerant supply and discharge outlet is provided on the outer surface of the coil. Generally, the cooling pipe structure has a bellows shape or a spiral shape, but the bellows shape causes a temperature gradient from one end of the gradient magnetic field coil to the other end, and the spiral shape has a total thickness for the return path. Becomes more than twice the thickness of the tube itself, which is not preferable as a cooling pipe for the gradient coil. For this reason, in the present invention, a cooling pipe having a spiral shape and having a forward path and a return path in the same plane is employed. FIG. 3 shows an example of the cooling pipe having such a structure.

  The cooling pipe shown in FIG. 3 has a structure in which a spiral outward path having a refrigerant inlet at the end of a flat plate is folded back in the vicinity of the center, and a spiral return path parallel to the outward path is connected to the refrigerant outlet at the end of the flat plate. This is called non-inductive winding. In such a non-inductive winding, the winding direction of the forward path and the winding direction of the return path are completely opposite directions, so even when it is made of a conductive material such as a copper tube or an aluminum tube, a magnetic field is generated by electromagnetic induction. There is no substantial electromagnetic effect. Further, since the refrigerant forward and return paths are formed in the same plane, the thickness required for the cooling pipe is only the thickness of the pipe, and an increase in the thickness of the gradient magnetic field coil due to the cooling pipe can be suppressed as much as possible. As a result, a sufficient area as an imaging space can be ensured.

  The shape of the non-inductive winding of the cooling pipe is compatible with the refrigerant supply device from the amount of heat to be removed by the cooling pipe, the flow rate of the refrigerant flowing inside the coiled pipe, the pressure loss determined from the cross-sectional dimensions and the total length of the flow path Designed to do. At this time, a predetermined interval is provided between the forward path and the return path of the cooling pipe that has been wound without induction in order to prevent heat exchange between the forward path and the return path. The distance between the forward path and the return path (turn interval) may be narrow if the flow rate is large, and wide if the flow rate is small. Therefore, it is designed appropriately according to the flow rate.

  As an example, FIG. 4 shows a case where a copper pipe has a cross-sectional shape having an outer diameter of 10 mm (inner diameter of 8 mm) and a length of 50 m, and a cooling pipe filled with epoxy resin between turns. In the figure, when the cooling water is flowed at about 0.5 L / min to the heat generation of 1.5 kW (solid line) and when the flow rate is doubled to about 1 L / min (dotted line), It is a graph showing the change of the cooling water temperature, and shows the values measured with different turn intervals (4 to 14 mm). As shown in the figure, it is necessary to provide a turn interval of 14 mm or more at a flow rate of about 0.5 L / min, and a turn interval of 10 mm or more at a flow rate of about 1 L / min.

  In the example of FIG. 4, the case where an epoxy resin is interposed between the forward path and the return path is shown, but only air existing between these may be used, or a material that blocks heat other than the epoxy resin (for example, a phenol resin). Or machinable ceramics). Further, it is possible to form the forward path with a material having good thermal conductivity and to form the return path with a material having low thermal conductivity. Thus, by blocking the heat exchange between the forward path and the return path, particularly in the vicinity of the refrigerant inlet and the refrigerant outlet, the temperature of the refrigerant flowing in the pipe and the temperature of the pipe rise uniformly from the pipe inlet to the outlet. Thus, the cooling efficiency of the gradient coil can be increased.

  The refrigerant is not particularly limited, but usually cooling water is used. The cooling water preferably has a conductivity of 50 μS / cm or less from the viewpoint of electrical insulation and corrosion. Such conductivity can be achieved by installing an ion exchanger for removing impurities in the circulation path of the cooling water.

  The cooling pipe 205 on the main coil side and the cooling pipe 207 on the shield coil side may be separate piping systems, but the piping system can be simplified by connecting them in series. In that case, it is preferable that the cooling pipe on the shield coil side where the temperature rise is relatively small be the refrigerant inflow side. These two pipes preferably have the same length and shape. Thereby, the winding process of a cooling pipe can be made common and work can be simplified. Alternatively, the length of the cooling pipe 205 on the main coil side having a large calorific value can be made longer than that on the shield coil side. In this case, the front surface on the main coil side can be efficiently cooled. The temperature gradient can be reduced.

  As described above, according to the gradient magnetic field coil of the present embodiment, the cooling pipe is fixed to at least one surface of the gradient magnetic field coil, and the forward path and the return path such as a non-inductive winding are typically coplanar as the cooling pipe. By adopting the piping having, it is possible to effectively suppress the temperature rise of the gradient magnetic field coil without increasing the thickness of the gradient magnetic field coil. In addition, in the case of an active shield gradient magnetic field coil equipped with a shield coil, heat transfer to the static magnetic field magnet can be effectively performed by arranging a cooling pipe not only on the main coil but also on the side surface of the shield coil on the static magnetic field magnet side. It is possible to suppress the change in the magnetic characteristics and the deterioration of the performance due to heat. In this case, the temperature gradient of the entire gradient magnetic field can be reduced by adjusting the length of the cooling pipes on the main coil side and the shield coil side and the refrigerant flow rate.

  Another embodiment of the present invention is shown in FIG. In the gradient magnetic field coil 103 of the embodiment shown in FIG. 5, the configuration of the cooling pipes 205 and 207 is the same as that of FIG. 2, but in addition to these, one of the main coils 201 of X, Y, and Z is a cooling coil. Also serves as. In other words, in the illustrated embodiment, the main Z coil 2011 is constituted by a hollow conductor, the refrigerant is passed through the inside, and the main X coil and the main Y coil are sandwiched between the cooling pipe 205 and the main Z coil 2011.

In general, in the gradient magnetic field coil 103 of the vertical magnetic field type MRI apparatus, when the vertical direction is the Z direction, the X coil and the Y coil have a complicated pattern for generating a magnetic field gradient in a direction parallel to the surface of the gradient magnetic field coil 103. Since it is wound, the electric resistance is higher than that of the Z coil, and therefore the heat generation amount is also large. Further, the heat generation amount of the main coil 201 is higher than that of the shield coil 203. Table 1 shows an example of the heat generation amount of a typical active shield gradient coil.

  The gradient magnetic field coil 103 of the present embodiment allows the main Z coil 2011 having a relatively simple winding pattern and having a small resistance as a refrigerant flow path to function as a cooling pipe, By adopting a structure in which the main Y coil is sandwiched between the two cooling pipes 2011 and 205 including the main Z coil, the temperature rise of the gradient magnetic field coil 103 and the thermal deformation of the gradient magnetic field coil can be more effectively suppressed. it can.

  FIG. 6 shows the cooling effect when the cross-sectional dimensions of the cooling pipe and the main Z coil are 6 mm × 6 mm, the wall thickness is 1 mm, and the cooling water flow rate is about 1 L (liter) / min. The graph of FIG. 6A shows the case where the cooling water flows through the cooling pipe and the main Z coil, and the graph of FIG. 6B shows the case where the cooling water does not flow through the cooling pipe. The measured values of the temperature of the main X coil are plotted against the energization time (200 A) for the main X coil. As is clear from the graphs (a) and (b), in the configuration in which the main Z coil is disposed on one surface of the gradient magnetic field coil, the cooling effect is not sufficient with only the main cooling coil, but on the other surface. By arranging the cooling pipe to sandwich another coil, a high cooling effect can be achieved.

  An example of the piping system of the cooling piping including the main Z coil is shown in FIG. As shown in the figure, this embodiment includes a piping system 701 dedicated to cooling and a piping system 702 to which a voltage is applied when energized, and the respective outlets C1 to C4 are provided on the outer peripheral portion of the gradient magnetic field coil. The cooling coil 205 on the main coil side and the cooling pipe 207 on the shield coil side belong to the piping system 701, and the main Z coil 2011 belongs to the piping system 702. A voltage is applied to the outlets C1 and C2 of the piping system 702 according to the imaging sequence. For this reason, C1 and C2 are connected to external piping using a hose made of an insulating material. Alternatively, the main Z coil pipe may be subjected to electrical insulation treatment, and in this case, a conductor may be used for connection to the outside.

  Rather than separate the cooling dedicated piping system 701 and the main Z coil piping system 702 from each other, the outlets C1 and C2 and C3 and C4 may be combined and branched in the gradient coil. Good. In this case, it is preferable to make the pressure loss of both the cooling-dedicated pipes 205 and 207 and the main Z coil 2011 equal. Thereby, the flow rate of the cooling water flowing through both the piping systems can be made equal without providing an external valve for adjusting the flow rate.

  According to the gradient magnetic field coil of the present embodiment, the main Z coil has a function as a cooling pipe, and the X coil and the Y coil that generate the most heat are sandwiched between the main Z coil and a cooling pipe dedicated to cooling. Thus, the temperature rise of the gradient coil can be more effectively suppressed. In this case, by equalizing the pressure loss of the cooling pipe that is the main Z coil and the cooling dedicated cooling pipe, a flow rate adjusting valve or the like is not required and the piping system can be simplified.

Next, still another embodiment of the present invention will be described.
FIG. 8 is a diagram showing another embodiment of the gradient coil according to the present invention. The gradient coil of this embodiment is also an active shield gradient coil including a shield coil 803 in addition to the main coil 801. In this embodiment, the main and shielded Z coils 8011 and 8031 are cooling coils provided with refrigerant flow paths therein, and the cooling coils 8011 and 8031 are X coils. It is characterized by being provided between the Y coil. These cooling coils 8011 and 8031 are provided with outlets for supplying and discharging refrigerant at the outer ends of the gradient magnetic field coils, respectively, and are electrically insulated and connected to the cooling circulation system. The main coil and the shield coil are electrically connected in series, and are connected to a gradient magnetic field power supply so that a predetermined voltage is applied when energized. The piping systems of the cooling coils 8011 and 8031 may be separate, or may be connected in parallel or in series to be the same system. However, in these connections, it is important not to disturb the electromagnetic function of each cooling coil.

In the gradient magnetic field coil of the present embodiment, the cooling coil is arranged between the X coil and the Y coil that generate a large amount of heat, so that the entire main coil or shield coil can be obtained without arranging a cooling-dedicated pipe on the surface. The temperature gradient can be reduced, and thermal deformation due to the temperature gradient can be effectively prevented. Further, by adopting the cooling coil for both the main coil and the shield coil, the temperature rise on the static magnetic field magnet side can be suppressed.
In this embodiment, piping dedicated for cooling is not essential, but cooling piping is provided on the static magnetic field magnet side of the shield coil 803 and the imaging space side of the main coil 801, as in the embodiment shown in FIG. 2 or FIG. Non-inductive winding can be adopted as a cooling pipe dedicated to cooling.

  As mentioned above, although embodiment of the gradient magnetic field coil of this invention was described, this invention is not limited to these embodiment, A various change is employable in the range described in a claim. For example, although the active shield type gradient magnetic field coil provided with the shield coil has been described in the above embodiment, the gradient magnetic field coil not provided with the shield coil or the gradient magnetic field coil provided with the correction coil for correcting the static magnetic field uniformity is also described. The same can be applied. Further, the present invention can be used in combination with other cooling means and heat shut-off means. For example, as shown in FIG. 3, not only cooling pipes in the coil surface but also cooling pipes may be provided on the outer surface of the coil, and heat conduction is performed on the mounting surface to the static magnetic field magnet as shown in FIG. It is also possible to arrange a thin plate 901 made of a material with a low rate (for example, a phenol resin plate).

  An example of incorporating the gradient magnetic field coil of the present invention into an MRI apparatus is shown in FIG. FIG. 10 shows an example in which the gradient magnetic field coil 103 having the same configuration as FIG. 5 is incorporated in the static magnetic field magnet 102. As shown in the figure, the shield coil side of the gradient magnetic field coil 103 is in contact with the static magnetic field magnet 102. It fixes to the recessed part formed in the static magnetic field magnet 102. FIG. At this time, a heat insulating plate 901 is disposed between the static magnetic field magnet 102 and the surface cooling pipe 207 of the shield coil. In this configuration, the heat generation of the gradient magnetic field coil is removed by the surface cooling pipe 207 of the shield coil, and the gradient magnetic field coil 103 and the static magnetic field magnet 102 are thermally cut off, thereby preventing fluctuations in the characteristics of the static magnetic field magnet 102. can do.

The figure which shows the whole outline | summary of the MRI apparatus with which this invention is applied. The figure which shows one Embodiment of the gradient magnetic field coil of this invention Diagram showing an example of cooling piping The figure which shows the design of the turn interval of the cooling piping of FIG. The figure which shows other embodiment of the gradient magnetic field coil of this invention. The graph which shows the cooling efficiency of the gradient coil of FIG. The figure which shows an example of the piping system of the gradient magnetic field coil of FIG. The figure which shows another embodiment of the gradient magnetic field coil of this invention. The figure which shows another embodiment of the gradient magnetic field coil of this invention. The figure which shows the state which incorporates the gradient magnetic field coil of this invention in an MRI apparatus.

Explanation of symbols

102 ... Static magnetic field magnet, 103 ... Gradient magnetic field coil, 201 ... Main coil, 2011 ... Main Z coil (cooling coil), 203 ... Shield coil, 205, 207 ... Cooling piping

Claims (6)

  A gradient coil for a magnetic resonance imaging apparatus having a substantially flat shape and including a plurality of coils for generating a gradient magnetic field in at least one direction, and a single layer wound non-inductively on at least one surface thereof A gradient coil having a cooling pipe made of   A gradient magnetic field coil for a magnetic resonance imaging apparatus having a substantially flat shape and including a plurality of coils that generate gradient magnetic fields in different directions, and at least one surface thereof does not have an electromagnetic function, A gradient magnetic field coil, wherein a cooling pipe composed of a single layer wound non-inductively is disposed, and one of the plurality of coils is a cooling coil having a coolant channel therein. The gradient coil according to claim 2,
Each of the cooling pipe and the cooling coil has the same pressure loss and is connected to the same refrigerant circulation system. A main coil comprising a plurality of coils having a substantially flat shape and generating gradient magnetic fields in different directions, and a gradient magnetic field having a substantially flat shape and canceling the gradient magnetic field generated by the main coil. A gradient coil for a magnetic resonance imaging apparatus comprising a generated shield coil,
The gradient magnetic field coil for a magnetic resonance imaging apparatus, wherein at least one of the main coil and the shield coil is the gradient magnetic field coil according to any one of claims 1 to 3. The gradient coil according to claim 4,
The main coil and the shield coil each include a cooling pipe, and the main coil and the cooling pipe for the shield coil are connected in series.   In the magnetic resonance imaging apparatus, comprising: a pair of static magnetic field magnets disposed opposite to each other with an imaging space interposed therebetween; and a flat plate-shaped gradient magnetic field coil disposed in proximity to each of the pair of static magnetic field magnets. 6. A magnetic resonance imaging apparatus comprising the gradient magnetic field coil according to claim 1 as a coil, and means for circulating the refrigerant having a heat exchanger and a circulation pump.

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