Picker International, Inc.

Picker Corporation

Frederick T Goldie

Magnetic coils (12) create a generally uniform magnetic field through an examination region (10). A radio frequency transmitter applied RF pulses to an RF antenna (26). A gradient field controller (30) generates gradient excitation current pulse profiles which pass through an eddy current calibration circuit (50) to gradient coils (38). A radio frequency receiver (92) receives magnetic resonance signals from the examination region, which signals are Fourier transformed (94) into an image representation (96). The eddy current calibration circuit (50) modifies the current pulse profiles in order to compensate for eddy currents, i.e. to remove the effect of eddy currents such that the resultant gradient magnetic field has a selected profile. An automatic calibration circuit (60) adjusts the decay constant or center frequency of a band pass filter (56) and the gain of an amplifier (58). A sine wave generator (62) applies a sine wave voltage to the gradient coils as a search coil monitors (64) the resultant magnetic field and a current probe (70) monitors the current flowing through the gradient coil. The magnetic field and current are phase sensitively detected (66, 72) to determine the relative amplitude and phase of each. The relative amplitude and phase are compared as the frequency of the sine wave signal is swept. The time constant or center frequency of the band pass circuit is set to the frequency (78) at which the field phase lags the current phase by a maximum and the gain is set in accordance with a fractional fall-off (f) in the relative amplitude.

Long time constant eddy current compensation

A method of operating an electromagnet arrangement comprising a main electromagnet and a number of correction coils for applying correction fields to the field produced by the main electromagnet to produce a resultant desired homogeneous magnetic field in a given volume. The method comprises obtaining the least squares fit values for the correction coil currents and, if the required current in any coil lies outside a range of permissible values for that coil current, following a special procedure to find the best selection of coil currents consistent with the permissible currents in the coils.

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London, United Kingdom

Frederick T Goldie