The present invention relates generally to superconducting magnet systems and, more particularly, to minimizing heat load to a superconducting magnet system during magnet ramping. The present invention also relates to reducing the thermal load on a magnet during persistent operation of the magnet.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MR systems typically use superconducting magnets, often with multiple coils to generate the uniform magnetic field. These superconducting magnets are part of a cold mass cooled by liquid helium. The magnets are made typically of niobium-titanium material that is cooled to a temperature of 4.2 K with liquid helium. Exemplary superconducting magnet systems operating in MRI systems require occasional ramping of the superconducting magnet to charge the magnet for use of the MRI system. After the superconducting magnet is ramped, the current supply used for the magnet ramping is disconnected and is not needed until further magnet ramping is necessary, such as for demagnetization of the superconducting magnet or for remagnetization of the superconducting magnet after, for instance, scheduled service, a magnet quench, and the like.
Frequently, retractable current leads are used that are only connected to the superconducting magnet during magnet ramps. After the superconducting magnet is ramped, and during normal operation, the current leads are retracted to remove the conduction heat load from the leads to the superconducting magnet. Such retractable leads are normally maintained at room temperature and, together with the equipment used to power the leads, when connected to the superconducting magnet, serve as a heat load on the superconducting magnet.
It would therefore be desirable to have a system and method capable of ramping a superconducting magnet while minimizing the heat load on the superconducting magnet.