The use of adaptive algorithms for obtaining optimal electrical shimming in magnetic resonance imaging (MRI)

A method of determining the dc coil current values to electrically shim the static magnetic fields used in magnetic resonance imaging (MRI) using modified steepest descent adaptive algorithm is described. Using a 32 cm diameter by a 40 cm long water phantom as the test volume, the algorithm achieved field homogeneities of 0.2 parts per million (ppm) peak-to-peak within a 20 cm diameter spherical imaging volume, and 1.3 ppm peak-to-peak within the entire phantom. The algorithm achieved an inhomogeneity variance of 0.18 ppm2. The shim system was successfully modeled as a sum of adaptive linear combiners. The model contains 13 parameters that can be varied, 12 shim coil currents, and the receiver mixer frequency. The model was then used to predict key adaptive algorithm parameters. Experimental verification of these parameters lends support to the accuracy of the model.