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"Method for Adjusting Magnetic Resonance Imaging Apparatus and Superconductive Magnet Excitation Dock" in Patent Application Approval Process

August 28, 2014



By a News Reporter-Staff News Editor at Politics & Government Week -- A patent application by the inventor Tsuda, Munetaka (Tokyo, JP), filed on April 8, 2014, was made available online on August 14, 2014, according to news reporting originating from Washington, D.C., by VerticalNews correspondents.

This patent application is assigned to Hitachi Medical Corporation.

The following quote was obtained by the news editors from the background information supplied by the inventors: "MRI apparatuses are widely spread in medical facilities similarly to diagnostic apparatuses using X-rays. The reason is that the MRI apparatus has a different function from the diagnostic apparatus using X-rays. Not only the diagnostic apparatus using X-rays but also the MRI apparatus is an indispensable diagnostic apparatus for more reliable examination in diagnosis. The diagnostic function of the MRI apparatus is to obtain not only the morphological image information but also the functional diagnostic information. For example, the MRI apparatus can clearly draw a cerebral infarction lesion in the early stage of the disease. In particular, in an MRI apparatus using a magnet with high magnetic field performance in which the magnetic field strength exceeds 1 tesla and the magnetic field homogeneity is 3 ppm or less, many new diagnostic functions have been developed taking advantage of the magnetic field performance so that actual clinical applications are made.

"In order to achieve the high diagnostic function of the MRI apparatus, it is necessary to increase the magnetic field strength of the imaging space where a subject is disposed and to increase the magnetic field homogeneity. The increase in the magnetic field strength increases the strength of a nuclear magnetic resonance (NMR) signal detected from an examination area. Accordingly, since a signal-to-noise ratio or a contrast-to-noise ratio of a diagnostic image is increased, the quality of the diagnostic image is improved. In addition, the increase in the signal-to-noise ratio indicates that a high-speed imaging technique for shortening an examination time becomes possible. On the other hand, since even a slight difference in resonance frequencies of NMR signals acquired from the examination area can be identified by increasing the magnetic field homogeneity, the spatial resolution of the diagnostic image or the analytical capability of spectral diagnosis can be improved.

"In order to acquire a high-quality examination result in a diagnostic image or the like, not only the performance of the MRI apparatus but also realizing a comfortable examination environment, which relieves the tension of the subject, is important. In order to improve the examination environment, an open MRI apparatus with an open-structure imaging space where a subject is disposed has been realized (PTL 1). In the MRI apparatus of PTL 1, an open examination space is realized by using a superconducting magnet in which superconducting coils are disposed above and below the space, in which a subject is disposed, interposed therebetween.

"On the other hand, in a superconducting coil formed by pouring thermosetting resin (epoxy resin) between wound superconducting wires and curing it, distortion by cooling occurs due to a difference between the thermal contraction rates of the superconducting wires and the resin, as disclosed in PTL 2. If the superconducting wires move by several micrometers due to this distortion, the superconducting wires are locally heated. As a result, a quench by which a change to the normal conducting state is made occurs, as is known well. If a quench occurs in the superconducting magnet of the MRI apparatus, not only does the superconducting magnet not function as a superconducting magnet any more, but also a large amount of refrigerant is vaporized by the heating and is emitted through an emergency exhaust port. For this reason, it is necessary to avoid the occurrence of a quench in the MRI apparatus installed in the hospital and the like.

"In order to avoid the sudden occurrence of a quench in an MRI apparatus which holds a persistent current over a long period of time, PTL 2 proposes reducing distortion energy generated in superconducting wires and resin by repeating the electromagnetic force generated at the time of excitation and demagnetization several times (at least 3 times) or an adjustment method of a superconducting magnet which releases distortion energy of a superconducting coil by causing a quench forcibly with the heat from the outside."

In addition to the background information obtained for this patent application, VerticalNews journalists also obtained the inventor's summary information for this patent application: "Technical Problem

"The effect of an adjustment method of a superconducting magnet disclosed in PTL 2 is accepted in a superconducting magnet with a relatively low magnetic field strength, which has a static magnetic field strength of 1 tesla or less and in which a current flowing as a persistent current when operating the superconducting magnet is set as a much lower value than the maximum current (this is called a critical current) flowing through superconducting wires.

"However, if the value of the persistent current flowing through the superconducting wires is increased in order to increase the static magnetic field strength, distortion cannot be released in the adjustment method disclosed in PTL 2. This is because energy of the electromagnetic force generated by a compulsory quench or excitation and demagnetization disclosed in PTL 2 is calculated as about 1/10 of distortion energy generated at the time of cooling of superconducting wires and resin and this is too small to release total distortion energy. For this reason, if a high magnetic field superconducting magnet is adjusted by the method disclosed in PTL 2, distortion remains. Accordingly, a problem occurs in that a quench occurs in the high magnetic field superconducting magnet even with slight disturbance (heating due to movement of a superconducting coil or the influence of a magnetic field) which does not cause a quench in a low magnetic field superconducting magnet.

"In particular, since a superconducting magnet used in the open MRI apparatus has a complicated structure in which superconducting coils are disposed up and down with the imaging space interposed therebetween, the superconducting magnet is easily influenced by cooling distortion or electromagnetic force distortion. Accordingly, since the number of times of quench required until the distortion is completely released at the time of adjustment tends to increase, it takes time to release the cooling distortion.

"Hereinafter, causing a quench in order to release distortion is called training. In addition, the number of times of quench required until distortion is released is called the number of times of training.

"Another cause of making difficult the operation of completely releasing the cooling distortion energy of the superconducting magnet is that the required number of times of training can not be predicted in the current technology. For this reason, it is difficult to decide a construction period until the superconducting magnet can be used after being transported to medical facilities, as in the related art. On the other hand, it is preferable to shorten the period of a process of installing a superconducting magnet in medical facilities in order to shorten the duration of the examination service suspended.

"In addition, at the time of training, a superconducting coil generates heat by a quench, and a large amount of liquid helium (for example, tens of thousands of liters due to training performed multiple times) evaporates. Then, helium gas which is 700 times the liquid helium generated to cool the air, and the water vapor in the air rises like white smoke. For this reason, if training is performed in a hospital or near the hospital, hospital employees or neighboring residents may mistake it for fire.

"However, if training is performed at the location distant from the hospital, it is necessary to transport the superconducting magnet to the hospital. Accordingly, even in a superconducting magnet determined that the distortion was completely released, training may be necessary due to distortion at the time of re-cooling when the temperature of the superconducting coil is increased and cooled again at the time of transportation.

"In addition, there is also a problem of cost because a large amount of liquid helium evaporates in training. Until the distortion is completely released, it is necessary to cause a quench multiple times. As an example, this requires tens of thousands of liters of liquid helium, which costs.

"The invention has been made in view of the above, and it is an object of the invention to provide a cooling and excitation method of a superconducting magnet capable of deciding and shortening the installation period after transportation to medical facilities.

"Solution to Problem

"In order to solve the above-described problem, in a first aspect of the invention, the following adjustment method of an MRI apparatus including a superconducting magnet is provided.

"That is, this is an adjustment method of an MRI apparatus including: a cooling and excitation step in which work of transporting a superconducting magnet manufactured in advance to a facility for excitation different from a facility where the superconducting magnet is to be installed and temporarily installing the superconducting magnet, cooling a superconducting coil of the superconducting magnet with a refrigerant, and supplying a current from an external power supply for excitation is repeated until a predetermined rated current flows; a demagnetization and transportation step of demagnetizing the superconducting coil excited by the rated current as a preliminary step and transporting the superconducting magnet to the facility where the superconducting magnet is to be installed in a state where the superconducting coil is cooled by the refrigerant; and an installation step of installing the superconducting magnet in the facility where the superconducting magnet is to be installed and supplying a predetermined rated current from an external power supply to the superconducting coil in order to excite the superconducting coil. Thus, since the cooling and excitation step is performed in the facility for excitation, distortion of the superconducting coil can be eliminated (training) by causing a quench, the number of times of which is not decided and the number of times of which until the rated current flows is not known, in the facility for excitation. After the demagnetization, the superconducting magnet is transported in a cooled state. Accordingly, in the installation step performed in the facility where the superconducting magnet is to be installed, it is not necessary to perform the training again. As a result, it is possible to decide and shorten a time required for the installation step.

"When a plant for condensing helium gas to liquid helium, a frame on which the superconducting magnet is mounted, a supply pipe used to send liquid helium from the plant to the superconducting magnet, and a recovery pipe used to send helium gas from the superconducting magnet to the plant are provided in the facility for excitation, the liquid helium can be directly supplied from the plant to the superconducting magnet. Therefore, it is possible to reduce the cost in transporting helium. In addition, the helium gas discharged from the superconducting magnet can be recovered and reused.

"In the cooling and excitation step, for example, a current value is increased with a predetermined increase rate when supplying a current from the external power supply to the superconducting coil. When a quench occurs in the superconducting coil and the refrigerant evaporates, training is performed using a method of injecting a refrigerant and increasing a value of a current from the external power supply with a predetermined current value increase rate.

"In the cooling and excitation step, preferably, the external power supply is separated to make a closed loop in which a persistent current flows after the rated current flows through the superconducting coil, and the state is maintained for a predetermined time. By confirming that a quench does not occur during the state maintained, it is possible to confirm that distortion is released (end of training).

"In the demagnetization and transportation step, it is preferable to transport the superconducting coil while maintaining the superconducting coil at a liquid helium temperature or less after the demagnetization.

"In the demagnetization and transportation step, it is possible to store the superconducting coil for a predetermined number of days while maintaining the superconducting coil at a liquid helium temperature or less after the demagnetization. In the storage, it is preferable to execute at least one of an operation of a cryocooler of the superconducting magnet and refrigerant supplementation. Moreover, in the storage, it is preferable to operate the cryoccoler and a cryoheater of the superconducting magnet in order to maintain the pressure in a refrigerant vessel of the superconducting magnet in a fixed range.

"When the temperature of the superconducting coil rises by a predetermined value or more in the demagnetization and transportation step, distortion may occur again in the superconducting coil at the time of re-cooling. Accordingly, it is preferable to increase a current, which is supplied from the external power supply at the time of excitation in the installation step, with a predetermined current value increase rate and perform the training.

"In addition, according to a second aspect of the invention, the following adjustment method of an MRI apparatus including a superconducting magnet is provided.

"That is, this is an adjustment method of an MRI apparatus including: a training step of performing training for releasing distortion occurring in a superconducting coil by transporting a superconducting magnet manufactured in advance to a facility for excitation different from a facility where the superconducting magnet is to be installed and temporarily installing the superconducting magnet, cooling the superconducting coil of the superconducting magnet with a refrigerant, and supplying a current from an external power supply; a demagnetization and transportation step of demagnetizing the superconducting coil after the training as a preliminary step and transporting the superconducting magnet to the facility where the superconducting magnet is to be installed in a state where the superconducting coil is cooled by the refrigerant; and an installation step of installing the superconducting magnet in the facility where the superconducting magnet is to be installed and supplying a predetermined rated current from an external power supply to the superconducting coil in order to excite the superconducting coil.

"In addition, according to a third aspect of the invention, a dock for superconducting magnet excitation which is used for adjustment of an MRI apparatus is provided.

"That is, this is a dock for superconducting magnet excitation including: a plant for condensing helium gas to liquid helium; a frame on which a superconducting magnet is mounted; a supply pipe used to send liquid helium from the plant to the superconducting magnet; a recovery pipe used to send helium gas from the superconducting magnet to the plant; a power supply for supplying an excitation current to the superconducting magnet; and a power lead which connects the power supply to the superconducting magnet. By providing such a dock, the superconducting magnet can be cooled and excited at low cost at the place directly connected with the helium plant. In addition, helium gas evaporating in the event of a quench can be recovered and reused.

"Preferably, at least two recovery pipes with different diameters used to send the helium gas from the superconducting magnet to the plant are disposed, and the recovery pipe with a large diameter is connected to an emergency exhaust hole for exhausting helium gas, which evaporates in the superconducting magnet in the event of a quench, to the outside. Then, even if a large amount of liquid helium evaporates at once in the event of a quench at the time of training, it can be recovered through the recovery pipe with a large diameter.

"Preferably, a constant-pressure valve which is opened when the pressure in the recovery pipe exceeds predetermined pressure is provided in the recovery pipe with a large diameter. In addition, it is preferable to include a fan for diffusing helium gas emitted when the constant-pressure valve is opened. Being mistaken for a fire or the like can be avoided by diffusing white smoke caused by the helium gas.

"As a fourth aspect of the invention, the following adjustment method of an MRI apparatus including a superconducting magnet is provided.

"That, this is an adjustment method of an MRI apparatus including: a first transportation step of transporting a superconducting magnet manufactured in advance to a facility for excitation different from a facility where the superconducting magnet is to be installed; a second transportation step of repeating work of cooling the superconducting magnet with a refrigerant and exciting the superconducting magnet with a current supplied from an external power supply in the facility for excitation until a predetermined rated current flows and then transporting the superconducting magnet to the facility where the superconducting magnet is to be installed in a state where the superconducting coil demagnetized as a preliminary step is cooled by the refrigerant; and an installation step of installing the superconducting magnet in the facility where the superconducting magnet is to be installed and supplying a predetermined rated current from an external power supply to a superconducting coil in order to excite the superconducting coil.

"In addition, according to a fifth aspect of the invention, the following adjustment method of an MRI apparatus including a superconducting magnet is provided.

"That is, this is an adjustment method of an MRI apparatus including: a cooling and excitation step in which work of transporting a superconducting magnet manufactured in advance to a facility for excitation and temporarily installing the superconducting magnet, cooling a superconducting coil of the superconducting magnet with a refrigerant, and supplying a current from an external power supply for excitation is repeated until a predetermined rated current flows; and a demagnetization and transportation step of demagnetizing the superconducting coil excited by the rated current as a preliminary step and transporting the superconducting magnet to the facility where the superconducting magnet is to be installed in a state where the superconducting coil is cooled by the refrigerant.

"Advantageous Effects of Invention

"The following effects are obtained by the adjustment method of an MRI apparatus of the invention.

"(1) A superconducting magnet after completion of training of a superconducting magnet can be transported to the medical facilities.

"(2) A training process a required time for which is not decided can be eliminated from the installation period, and it is possible to shorten the installation period. In particular, when the current MRI apparatus in the medical facilities is replaced with an open MDI apparatus using a superconducting magnet which has a high magnetic field strength and a release structure, the duration of the current examination service suspended can be minimized.

"(3) The supply of helium needed at the time of cooling, excitation, and training becomes easy. Thus, since the facility of the helium plant can be utilized for recovery of helium, there is an effect of cost reduction.

BRIEF DESCRIPTION OF DRAWINGS

"FIG. 1 is a block diagram showing the entire configuration of an MRI apparatus of the invention.

"FIG. 2 a sectional view showing the structure of a superconducting magnet of the MRI apparatus shown in FIG. 1.

"FIG. 3 is a flow showing a method of cooling and excitation of the superconducting magnet of a first embodiment.

"FIG. 4 is a block diagram showing the configuration used to maintain the temperature and the pressure of a helium vessel 203 when stored in the first embodiment.

"FIG. 5 is a block diagram showing the configuration of a superconducting dock 50 used in a second embodiment."

URL and more information on this patent application, see: Tsuda, Munetaka. Method for Adjusting Magnetic Resonance Imaging Apparatus and Superconductive Magnet Excitation Dock. Filed April 8, 2014 and posted August 14, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=7816&p=157&f=G&l=50&d=PG01&S1=20140807.PD.&OS=PD/20140807&RS=PD/20140807

Keywords for this news article include: Hospital, Hitachi Medical Corporation.

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