Cooling device for sub-MRI units and MRI apparatus

A cooling device for sub-MRI units of an embodiment includes a tank in which cooling water for cooling a heat generating unit that an MRI apparatus has is stored, a pump which circulates the cooling water stored in the tank through a circulation path starting from the tank and traveling around the heat generating unit, a heat exchanger which cools the cooling water, and a controller which decides that a water leakage has occurred on the circulation path when a decreasing rate of the cooling water in the tank is greater than a given reference value.

FIELD

Embodiments of the invention relate to a cooling device for sub-MRI units and an MRI apparatus.

BACKGROUND

An MRI apparatus is an apparatus which produces an image of a test object placed in a static magnetic field by applying a radio frequency magnetic field to the test object and detecting a magnetic resonance signal that the applied magnetic field causes the test object to generate. Not only the radio frequency magnetic field but a gradient magnetic field is applied to the test object, so that spatial position information of the test object is added to the magnetic resonance signal.

A large pulse current is repeatedly applied to a gradient magnetic field coil in order that a gradient magnetic field is generated. The gradient magnetic field coil is caused to generate heat by the applied pulse current, and thus rises in temperature. A technology for cooling the gradient magnetic field coil by circulating water as coolant through a cooling tube provided close to the gradient magnetic field coil in order to keep the gradient magnetic field coil in a certain temperature range is known, e.g., as disclosed in Japanese Unexamined Patent Publication No. 2011-87915.

The MRI apparatus includes units which generate heat such as a gradient magnetic field power source which generates the pulse current to be applied to the gradient magnetic field coil, an RF amplifier which amplifies radio frequencies to be applied to the test object, a helium compressor and so on, in addition to the gradient magnetic field coil. These units are each called a heat generating unit of the MRI apparatus, hereafter. These heat generating units are each cooled by the cooling water similarly as the gradient magnetic field coil is.

A cooling device for sub-MRI units is a device which cools the cooling water having been warmed by the heat generating units. The cooling water having been warmed by the heat generating units is cooled by a heat exchanger that the cooling device has. A circulation path for the cooling water is formed among the heat generating units and the cooling device. The cooling water having been cooled by the heat exchanger is sent to the heat generating units by a pump of the cooling device.

The circulation path for the cooling water described above is formed by a plurality of piping sections. Each of the piping sections is joined with one another by joints. If the piping structure is locally broken or one of the joints is put out of joint, the water leaks from the broken portion. Occurrence of a water leakage not only seriously affects cooling performance of the MRI apparatus, but could degrade another apparatus installed in the same building.

An ordinary cooling device for the MRI apparatus, however, does not have an effective means for sensing a water leakage. Thus, a cooling device for sub-MRI units and an MRI apparatus which can certainly sense a water leakage are demanded.

SUMMARY

A cooling device for sub-MRI units of the embodiment includes a tank in which cooling water for cooling a heat generating unit that an MRI apparatus has is stored, a pump which circulates the cooling water stored in the tank through a circulation path starting from the tank and traveling around the heat generating unit, a heat exchanger which cools the cooling water, and a controller which decides that a water leakage has occurred on the circulation path when a decreasing rate of the cooling water in the tank is greater than a given reference value.

DESCRIPTION OF EMBODIMENTS

An embodiment of the invention will be explained below on the basis of the drawings.

(1) Entire Constitution

FIG. 1is a block diagram which shows an entire constitution of an MRI (Magnetic Resonance Imaging) apparatus1of the embodiment. The MRI apparatus1is constituted by having a magnet for static magnetic field22, a gradient magnetic field coil26, an RF coil28, a bed32, a control unit30, a cooling device100and so on.

Further, the control unit30has a static magnetic field power source40, a gradient magnetic field power source44(for X-axis44x, for Y-axis44y, for Z-axis44z), an RF transmitter (RF amplifier)46, an RF receiver48, a sequence controller56and a computer58.

Still further, the computer58has an arithmetic operation device60, an input device62, a display device64and a storage device66.

The magnet for static magnetic field22is shaped substantially like a cylinder, and generates a static magnetic field in a bore (the hollow inside of the cylindrical magnet for static magnetic field22), i.e., the imaging area for the test object P. The magnet for static magnetic field22contains a superconductive coil, and generates the static magnetic field by applying a current supplied by the static magnetic field power source40to the superconductive coil. The superconductive coil contained in the magnet for static magnetic field22is cooled down to very low temperature by means of liquid helium, and the helium gas to keep a heat shield at low temperature in order to keep the inside of the static magnetic field at very low temperature is cooled by a helium compressor221.

The gradient magnetic field coil26is shaped substantially like a cylinder as well, and is fixed inside the magnet for static magnetic field22. The gradient magnetic field coil26applies a gradient magnetic field in each of the X-axis, Y-axis and Z-axis directions by means of a current supplied by the gradient magnetic field power source (44x,44yand44z), respectively. Pipes for coil cooling26athrough which the coolant (cooling water) to cool the gradient magnetic field coil26is let flow is provided close to the gradient magnetic field coil26.

The RF coil28is fixed around and opposite the test object P inside the gradient magnetic field coil26. The RF coil28irradiates the test object P with an RF pulse transmitted by the RF transmitter46, and receives a magnetic resonance signal emitted from the test object P. The bed32is constituted movably in the horizontal direction, and moves into the bore carrying the test object P in time of imaging.

The RF transmitter46transmits an RF pulse to the RF coil28as instructed by the sequence controller56. Meanwhile, the RF receiver48detects a magnetic resonance signal received by the RF coil28, and transmits raw data to the sequence controller56. The raw data is obtained by digitalizing the detected magnetic resonance signal.

The sequence controller56is controlled by the computer58so as to scan the test object P by driving the gradient magnetic field power source44, the RF transmitter46and the RF receiver48individually. Then, if the RF receiver transmits raw data as a result of scanning, the sequence controller56transmits the raw data to the computer58.

The computer58controls the MRI apparatus1as a whole. Specifically, the input device62accepts various inputs from an operator. Then, while making the sequence controller56scan on the basis of imaging conditions having been inputted, the arithmetic operation device60reconstructs an image on the basis of the raw data transmitted by the sequence controller56. The reconstructed image is displayed on the display device64, or stored in the storage device66.

In the above constitution, the gradient magnetic field coil26is a unit of a significant amount of heat generation, and so are the gradient magnetic field power source44, the helium compressor221, the RF transmitter (RF amplifier)46, etc. Those units are each called a heat generating unit, and the device which cools those heat generating units is the cooling device100. The constitution, operation and so on of the cooling device100will be explained below.

(2) Constitution of Cooling Device

FIG. 2shows an exemplary constitution of the cooling device100of the embodiment, and exemplary heat generating units of the MRI apparatus1to be cooled by the cooling device100as well. InFIG. 2, e.g., the gradient magnetic field coil26, the gradient magnetic field power source44and the helium compressor221are each a heat generating unit.

The cooling device100is constituted by having a tank110in which cooling water300is stored, a solenoid valve120, a pump130, a heat exchanger140, a controller150, a display portion160, a water gauge170and so on.

The tank110has a tank vessel111and a lid112. The tank vessel111is provided on its outer wall with a water intake113through which the cooling water in the tank110is supplied, a water outlet114through which the cooling water300is sent to the heat generating units, a back intake115through which the cooling water on its return after cooling the heat generating units is accepted, and so on.

The pump130is to circulate the cooling water300stored in the tank110through a circulation path starting from the tank110and traveling around the heat generating units. The heat exchanger140cools the cooling water sent out by the pump130by exchanging heat.

The solenoid valve120is to supply the tank110with the cooling water300from the outside. If the solenoid valve120is made open, the tank110is supplied with the cooling water300. If the solenoid valve120is closed, the water supply is stopped.

The water gauge170senses the level of the cooling water300stored in the tank110. The water gauge170shown inFIG. 2is a float type water gauge having a float171which moves up and down according to the buoyancy principle, and senses the water level (height of the surface301of the cooling water300stored in the tank110) by working a lead switch by means of a magnet put in the float so as to output a sensing signal. The water gauge170of the embodiment is not limited to one of the float type, and any type which can sense the water level in the tank110can be employed, such as an ultrasonic wave type, an electrostatic capacity type, a pressure type, etc.

The controller150not only entirely controls the cooling device100, but controls the operation of the pump130and make and brake of the solenoid valve120on the basis of the water level sensed by the water gauge170as described later. The controller150is constituted by having, e.g., a processor, a ROM, a RAM, etc. The processor runs a program stored in the ROM so that the control described above is performed. Otherwise, the controller150may be constituted by hardware such as an ASIC, or may be constituted in such a way as to perform the control described above by means of a combination of software and hardware processing operations.

The display portion160indicates information on a failure decided by the controller150, or information on a warning or an alarm as instructed by the controller150.

Piping structures200and210to circulate the cooling water are arranged between the cooling device100and the heat generating units. The cooling water having been cooled by the heat exchanger140of the cooling device100is split into branches by a branching unit201a, and flows into each of cooling means (not shown) that the respective heat generating units are provided with through each of input end joints202a. The cooling water that heat is exchanged with by the cooling means of each of the heat generating units (cooling water having risen in temperature) flows out of each of output end joints202b, gathers at a junction unit201band goes back to the cooling device100.

AlthoughFIG. 2shows an exemplary constitution in which three heat generating units (the gradient magnetic field coil26, the gradient magnetic field power source44and the helium compressor221) are cooled by the single cooling device100, the numbers of the cooling units100and the heat generating units are not limited to the example shown inFIG. 2and can be properly selected according to the amount of heat generation of the heat generating units to be cooled and the cooling performance of the cooling unit100. What is shown inFIG. 2is a typical constitution of, e.g., a 1.5 T (Tesla) type MRI apparatus.

Meanwhile, intensity of the gradient magnetic field needs to be higher as intensity of the static magnetic field grows, and so does the amount of heat generated by the gradient magnetic field coil and the gradient magnetic field power source. In order to be ready for that, the MRI device1may be provided with a plurality of the cooling devices100which share the load of cooling the respective heat generating units.

FIG. 3shows an exemplary constitution of, e.g., a 3 T type MRI apparatus, where two cooling devices100share the load of cooling four heat generating units. As shown inFIG. 3, e.g., the one of the cooling devices (1)100cools the gradient magnetic field coil26and the helium compressor221, and the other one of the cooling devices (2)100cools the gradient magnetic field power source44and the RF amplifier46.

If, by any chance, a portion of the piping structures200or210is damaged, or a portion of the input end joint202a, the output end joint202b, the branching unit201aor the junction unit201bis out of joint while the cooling device100is working, i.e., driving the pump130so as to circulate the cooling water, the water leaks from that portion. Occurrence of a water leakage seriously affects the performance in cooling the heat generating units of the MRI apparatus1. Further, the cooling water having leaked could degrade another apparatus installed in the same building.

Thus, the cooling device100of the embodiment is provided with means for sensing occurrence of a water leakage quickly and correctly, for immediately stopping the pump130from working upon the occurrence of the water leakage being sensed so as to prevent the damage from spreading, and for giving the operator an immediate notice of the occurrence of the water leakage.

Meanwhile, the tank110needs to be supplied with the cooling water300as necessary as the cooling water300in the tank110gradually decreases owing to natural evaporation, even without occurrence of water leakage. An exemplary operation of the cooling device100of the embodiment will be specifically explained below.

(3) Operation of Cooling Device

If the cooling water300stored in the tank110decreases at a decreasing rate V being greater than a given reference value Vs, the cooling device100of the embodiment decides that the decrease of the cooling water300is not caused by natural evaporation but is caused by water leakage having occurred somewhere on the circulation path, and then stops the pump130from working so as to stop the circulation of the cooling water. Meanwhile, if the cooling water300stored in the tank110decreases at a decreasing rate V being smaller than the given reference value Vs, the cooling device100decides that the decrease is caused by natural evaporation in the tank110. The controller150of the cooling device100makes the above decision, and performs the operation to stop the pump130from working.

More specifically, a first level L1and a second level L2which is lower than the first level L1are set as levels of the cooling water300stored in the tank110. Then, if the water gauge170senses the first level L1and then senses the second level L2within a lapse of time TL1−L2being shorter than a given reference lapse of time Ts, the controller150decides that water leakage has occurred somewhere on the circulation path and caused the drop of the water level, and stops the pump130from working so as to stop the circulation of the cooling water. In that case, the cooling water300decreases at a rate V corresponding to (L1−L2)TL1−L2, and the reference value Vs corresponds to (L1−L2)/Ts.

FIG. 4schematically shows a relationship between the water level301in the tank110and the first and second levels L1and L2, respectively, which are set for the decision. Further,FIG. 4shows a full storage level LF and a lowest storage level LL as well. While the water is being supplied, the solenoid valve120is open and the water level301in the tank110rises. While no water is being supplied, natural evaporation could cause a drop in the water level301, and so could water leakage. The water level301which is affected by those factors and changes can be sensed by the water gauge170. Incidentally, if neither water leakage nor evaporation is assumed to exist while no water is being supplied, the water level301does not change as a rule, as the cooling water circulates through the circulation path.

The first and second levels L1and L2are set higher than at least half of the full storage level LF (LF/2) in order that occurrence of a water leakage can be sensed early. Assume, e.g., an amount of water fully stored in the tank110to be 80 liters. Then, the first level L1is set to a water level corresponding to an amount of water of 70 liters (i.e., around 88 percent of the full storage level LF). Further, the second level L2is set to a water level corresponding to an amount of water of 65 liters (i.e., around 82 percent of the full storage level LF).

FIG. 5is a flowchart which shows an exemplary operation of the cooling device100of the embodiment, primarily performed by the controller150of the cooling device100.

Assume, at an initial phase (step ST1), that the water level is higher than the first level L1, and equals or stays lower than the full storage level LF. Further, assume that the solenoid valve120is closed and no water is being supplied at the initial phase.

The cooling device100starts a cooling operation at a step ST2. Specifically, while driving the pump130so as to start circulating the cooling water, the controller100starts a cooling operation by means of the heat exchanger140. The solenoid valve120remains closed and no water is being supplied at the phase of the step ST2. Thus, the water level drops from the initial level (higher than the first level L1) owing to natural evaporation or occurrence of a water leakage.

The controller150decides whether the water gauge170has sensed the first level L1at a step ST3. The controller150starts to count a lapse of time immediately after the water gauge170senses the first level L1(step ST4). The water level continues dropping owing to natural evaporation or occurrence of a water leakage.

The controller150decides whether the water gauge170has sensed the second level L2which is lower than the first level L1at a step ST5. The controller150obtains a lapse of time since the first level L1is sensed and until the second level L2is sensed, TL1−L2, immediately after the water gauge170senses the second level L2. Then, the controller150decides whether the lapse of time TL1−L2equals or stays shorter than a second reference time length TS2(step ST6).

If the lapse of time TL1−L2equals or stays shorter than the second reference time length TS2(YES of step ST6), the controller150then decides whether the lapse of time TL1−L2equals or stays shorter than a first reference time length TS1at a step ST7.

The second reference time length TS2is set longer than the first reference time length TS1, and is set, e.g., to 90 minutes in this occasion. On the other hand, the first reference time length TS1is set relatively short, and is set, e.g., to five minutes.

If the lapse of time TL1−L2is longer than the second reference time length TS2(e.g., 90 minutes) (NO of step ST6), the controller150decides that the water level has dropped owing to the natural evaporation, and the process goes to a step ST20. If the lapse of time TL1−L2equals or stays shorter than the second reference time length TS2, on the other hand, the controller150decides that there is a possibility of a water leakage having occurred (first decision). The controller150then decides whether the lapse of time TL1−L2equals or stays shorter than the first reference time length TS1(e.g., five minutes) at the step ST7(second decision). If the lapse of time TL1−L2equals or stays shorter than the first reference time length TS1(YES of step ST7), the controller150then decides that a water leakage has most probably occurred, immediately indicates a “water leakage alarm” (step ST8), and stops the pump130and the heat exchanger140from working as well (step ST9). The “water leakage alarm” is indicated locally on the display portion160of the cooling device100, and may be indicated remotely on the display device64of the MRI apparatus1or on a suitable display device installed in the test room where the MRI apparatus1main body is installed, as well. Further, the operator may be notified of the “water leakage alarm” not only by visual indication but by a sound, etc.

Meanwhile, if the lapse of time TL1−L2equals or stays shorter than the second reference time length TS2but is longer than the first reference time length TS1(NO of step ST7), the controller150decides that occurrence of a water leakage, although being possible, cannot be certainly concluded, or that a water leakage has occurred but to an insignificant extent. In this occasion, the controller150indicates a “water leakage warning” on the display portion160of the cooling device100or on the display device64of the MRI apparatus1or the suitable display device installed in the test room in order to draw attention of the operator (step ST10).

Upon being notified of the “water leakage warning” indication, the operator checks whether a water leakage has occurred. If the operator identifies no water leakage and resets the “water leakage warning” indication within a given period of time to check, e.g., ten minutes, the process goes to the step ST20.

If the “water leakage warning” indication is not reset within the given period of time to check, on the other hand, the controller150decides that the possibility of a water leakage still remains without being entirely excluded, and stops the pump130and the heat exchanger140from working to stop the cooling operation (step ST9).

As described above, the “NO” decision of the step ST6, or the “YES” decision of the step ST11leads to the step ST20. Although no water leakage has occurred in those two cases, the water level itself has dropped to the second level L2. Thus, water supply starts at the step ST20in this case. Specifically, the controller150opens the solenoid valve120and supplies the tank110with cooling water from the outside.

If the water gauge170senses the first level L1within a given period of time (e.g., five minutes) for reviewing the water supply after the water supply starts (YES of step ST21), the controller150closes the solenoid valve120and stops the water supply (step ST22). Then, the process returns to the step ST4, and the controller150restarts to count a lapse of time since the first level L1is sensed. If the apparatus is free from a trouble such as a water leakage and the water level drops owing to only natural evaporation in this way, the water level is kept between the first level L1and the second level L2in the tank110.

Meanwhile, if the water gauge170has not sensed the first level L1within the given period of time for reviewing the water supply (NO of step ST21), the controller150decides that some failure has occurred in the water supply system including the solenoid valve120resulting in that normal water supply is prevented. The controller150indicates a “water supply failure” on the display portion160of the cooling device100or on the display device64of the MRI apparatus1or the suitable display device installed in the test room in order to draw attention of the operator (step ST23). Upon being notified of the “water supply failure” indication, the operator checks a failed portion in the water supply system such as the solenoid valve120. If it is identified that the water supply system is free from failure or the failed portion is restored within a given period of time for checking, e.g., ten minutes, the operator resets the “water supply failure” indication. Upon the “water supply failure” indication being reset (YES of step ST24), then, the process returns to the step ST20so that the water supply is continued. Unless the “water supply failure” indication is reset within the given period of time for checking (NO of step ST24), the controller150decides that the possibility of the water supply failure still remains without being excluded, and stops the pump130and the heat exchanger140from working to stop the cooling operation (step ST9).

The cooling device100of the embodiment described above can decide whether a water leakage has occurred or not on the basis of how fast the water level drops in the tank110. Upon deciding that a water leakage has occurred, the cooling device100can immediately notify the operator of the fact, so that the MRI apparatus1can be previously protected from being damaged by the occurrence of the water leakage.

Further, the cooling device100can identify the cause of a drop of the water level as a water leakage or natural evaporation. If the cause of the drop is identified as the natural evaporation, the cooling device100can keep the water level within a specific range in the tank110by means of water supply through the solenoid valve120. Besides, the cooling device100can identify the water level and time when water is supplied so as to decide whether the water supply system including the solenoid valve120is in failure or not.

Although some embodiments of the invention have been explained, these are presented as exemplary only, and it is not intended to limit the scope of the invention. These embodiments can be implemented in other various forms, and various omissions, replacements or changes can be done within the scope of the invention. These embodiments and their modifications are included in the scope or gist of the invention, and included similarly in the invention written in the claims and equivalents of it as well.