The present invention relates to a magnetic resonance imaging (MRI) apparatus which provides a magnetic resonance image of a subject under examination, and more particularly to, a method for controlling a magnetic field generating unit by using means for controlling a static magnetic field power source and a cooling unit, and also to a system for executing this method.
Since an MRI apparatus needs a time-stable magnetic field, i.e., a static magnetic field, a constant current power source is used in a normal conductive MRI apparatus to apply a time-stable current to a normal conductive coil.
The intensity of a static magnetic field generally depends on the stability of the output voltage of a static magnetic field power source and also on the stability of the shape of the normal conductive coil. These stabilities may be deteriorated by a change in temperature. Various methods for controlling the temperature are employed to stabilize the intensity of a static magnetic field. With regarded to the static magnetic field power source, (1) parts receiving less influence of a temperature change are used, (2) essential parts are disposed under temperature-controlled conditions, or (3) the power source is set under temperature-controlled conditions. With regard to the normal conductive coil, (1) the temperature of a medium such as cooling water for cooling the normal conductive coil is controlled or (2) the normal conductive coil is set under temperature-controlled conditions.
Once the static magnetic field power source is turned off, the power consumed by the parts such as a shunt resistor and a normal conductive coil, which can influence the stability of a static magnetic field, becomes zero, thus reducing the temperature. When the static magnetic field power source is energized again, it would take a long time to put the temperature of these parts in a stable state again. This time is called "pre-heating time." As shown in FIG. 1, the intensity of a static magnetic field varies within that time, so that no scanning by an MRI apparatus is possible during the time.
According to prior art, therefore, even when the MRI does not carry out a scanning operation while, for example, the patient is being moved, the static magnetic field power source is energized to provide a stable static magnetic field. However, this method requires more power, thereby the running cost of the MRI apparatus is increased and the life of the apparatus is shorten.
Under the circumstances, it is required a control method which can stabilize the static magnetic field generated in an MRI apparatus and can reduce the running cost of the apparatus as well.