Nmr and esr probes for closed loop control of gradient fields

Abstract

Magnetic resonance imaging apparatus uses magnetic field gradients X, Y, Z to spatially encode the magnetic signals arising from a patient on a couch,in bore,of a main magnet. Thermal stresses arising from aggressive gradients during multiple acquisitions result in imperfectly repeated gradients and resulting image artefacts. The invention uses a probe,provided with a gradient coil set similar to that for imaging, and fed by currents derived from the imaging gradient coils, connected so as to produce an opposing gradient surrounding an MR active substance in the probe. This enables a probe with a sufficiently large amount of MR active substance to produce a useful signal to be used to monitor the gradient, while overcoming the de-phasing problem. which would otherwise occur. Closed loop control of gradients thereby becomes possible.