Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
2000-02-09
2001-05-29
Barrera, Ray (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S217000, C335S300000, C324S318000, C324S319000
Reexamination Certificate
active
06239680
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic field stabilization method, magnetic field generating apparatus and magnetic resonance imaging apparatus, and more particularly to a method of stabilizing a magnetic field of a magnetic field generating apparatus employing permanent magnets, a magnetic field generating apparatus that generates a stable magnetic field, and a magnetic resonance imaging apparatus that employs such a magnetic field generating apparatus.
A magnetic field generating apparatus that employs permanent magnets for magnetic resonance imaging has a pair of permanent magnets facing each other in the vertical direction with a space interposed, and yokes constituting a magnetic circuit for the permanent magnets. In order to generate a stable magnetic field while preventing a change in magnetic field strength due to a variation in the ambient temperature, the magnetic field generating apparatus is kept at a constant temperature. The constant temperature is selected so that it is slightly higher than the ordinary temperature, and yet the permanent magnet does not lose its magnetism. For example, a temperature of 30° C. is selected.
The temperature is maintained by using an electric heater as a heat supply source, and the heat release amount of the electric heater is controlled by a controller. The controller is supplied with a detected signal indicative of the temperature measured at a predetermined position in the magnetic field generating apparatus. The temperature measurement point to obtain the detected signal is positioned within a yoke immediately above an upper permanent magnet, for example. The controller controls the heat release amount of the electric heater so that a value of the detected signal is equalized to a predetermined value.
The electric heater employed is one having a large heat release amount corresponding to a large heat capacity of the magnetic field generating apparatus, and the controller on-off controls the electric heater with PWM (pulse-width-modulation) signals. The duty ratio between ON and OFF is PID-controlled (proportional-plus-integral-plus-derivative-controlled) according to the deviation of the detected signals from the predetermined value.
In attempting to maintain the constant temperature of the magnetic field generating apparatus by the temperature control as described above, a difference arises between the temperatures of the upper and lower permanent magnets when a temperature difference is increased between upper and lower positions in a space in which the magnetic field generating apparatus is installed, as is often the case with a mobile magnetic resonance imaging apparatus in the hard winter, etc., leading to a problem that the stability of the magnetic field is deteriorated.
Moreover, since an electric heater having a large heat release amount is employed, frequency of switching between ON and OFF is increased when the temperature control is conducted with a small temperature deviation, i.e., in the low-ripple state, causing the control to be prone to instability due to a lag of heat conduction within the magnetic field generating apparatus. Furthermore, the increase in frequency of switching between ON and OFF gives rise to noises, which deteriorates the quality of captured images.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a magnetic field stabilization method, magnetic field generating apparatus and magnetic resonance imaging apparatus that is not affected by the temperature distribution in an installation space.
It is another object of the present invention to provide a magnetic field stabilization method, magnetic field generating apparatus and magnetic resonance imaging apparatus whose control system has good stability.
In accordance with a first aspect of the present invention, there is provided a magnetic field stabilization method of stabilizing a magnetic field of a magnetic field generating apparatus having a pair of permanent magnets facing each other with a space interposed and yokes constituting a magnetic circuit for the permanent magnets, comprising the steps of: detecting temperatures of a plurality of positions in the yokes; and controlling respective heat quantities to be applied to the plurality of positions based on the detected temperatures of the plurality of positions to stabilize the respective temperatures of the plurality of positions.
In accordance with a second aspect of the present invention, there is provided a magnetic field stabilization method for a magnetic field generating apparatus having a pair of permanent magnets facing each other with a space interposed and yokes constituting a magnetic circuit for the permanent magnets, comprising the steps of: detecting a temperature of the yoke; and controlling a heat quantity to be applied to the yoke based on the detected temperature to stabilize a magnetic field, wherein the heat quantity to be applied to the yoke is obtained by a combination of heat quantities generated by a plurality of heat generating sources.
In accordance with a third aspect of the present invention, there is provided a magnetic field generating apparatus comprising: a pair of permanent magnets facing each other with a space interposed; yokes constituting a magnetic circuit for the permanent magnets; a plurality of temperature detecting means for detecting temperatures of a plurality of positions in the yokes; a plurality of heat generating means for generating respective heat quantities to be applied to the plurality of positions; and a plurality of temperature regulating means for controlling the plurality of heat generating means, respectively, based on the detected temperatures of the plurality of positions to stabilize the respective temperatures of the plurality of positions.
In accordance with a fourth aspect of the present invention, there is provided a magnetic field generating apparatus having: a pair of permanent magnets facing each other with a space interposed; yokes constituting a magnetic circuit for the permanent magnets; temperature detecting means for detecting a temperature of the yoke; heat generating means for generating a heat quantity to be applied to the yoke; and temperature regulating means for controlling the heat generating means based on the detected temperature to stabilize the temperature of the yoke, wherein the heat generating means comprises a plurality of heat generating sources, and the temperature regulating means also controls a combination of heat quantities of the plurality of heat generating sources.
(Effect)
According to the present invention, the entire magnetic field generating apparatus is kept at a constant temperature by detecting temperatures of a plurality of positions in the yokes, and controlling respective heat quantities to be applied to the plurality of positions based on the respective detected temperatures to stabilize the respective temperatures of the plurality of positions. Moreover, the control is stabilized by also controlling a combination of heat quantities of a plurality of heat generating sources for generating the heat quantity to be applied to the yoke.
Therefore, the present invention can provide a magnetic field stabilization method, magnetic field generating apparatus and magnetic resonance imaging apparatus that is not affected by the temperature distribution in an installation space. Moreover, the present invention can provide a magnetic field stabilization method, magnetic field generating apparatus and magnetic resonance imaging apparatus whose control system has good stability.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.
REFERENCES:
patent: 3810254 (1974-05-01), Utsumi et al.
patent: 5652517 (1997-07-01), Maki et al.
Higa Yoshifumi
Inoue Yuji
Nagano Shigeo
Suzuki Tsunemoto
Tsujii Yoshiaki
Barrera Ray
GE Yokogawa Medical Systems Limited
Kojima Moonray
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