Patent ID: 6242915
Filing Date: 2001-06-05
Classification: G01R

Abstract:
In an MRI system having a magnet for producing a polarizing magnetic field B.sub.0 in a region, and a gradient coil assembly for producing magnetic field gradients in a subject to be imaged located in the region, a field-frequency lock system which comprises:a microcoil located outside the gradient coil assembly and in the region in which the polarizing magnetic field B.sub.0 is produced;a resonant sample disposed adjacent the microcoil and containing a material that produces a signal when the microcoil is energized with an rf pulse;transmit means for producing an rf pulse that is applied to the microcoil;receive means for acquiring a signal from the microcoil that is produced by the resonant sample material;a frequency comparator for measuring the difference frequency (.DELTA..omega.) between the frequency of the acquired signal and a reference frequency which corresponds to a frequency produced by the resonant sample material when the polarizing magnetic field B.sub.0 is at a nominal value; andmeans responsive to the difference frequency .DELTA..omega. for compensating the operation of the MRI system as it performs a scan on the subject for variations in the magnitude of the polarizing magnetic field B.sub.0.