Emi, LLC

Picker International, Inc.

Colin G Harrison

Ian R Young

Michael Burl

Hugh Clow

The invention concerns the production of gradient fields for nuclear magnetic resonance apparatus particularly imaging apparatus. It has been earlier proposed that samples of the resonance signal be taken at intervals such that the field gradient integrals in each such interval are the same. This sampling is at unequal time intervals. It is now proposed to provide the gradient fields in discrete pulses with spaces in between at which samples can be taken. It is then straightforward to make the pulses of equal field integral or to provide an extra small pulse in one or both of the orthogonal gradients if the field integral should need adjusting before the next sample.

Nuclear magnetic resonance systems

In known NMR imaging apparatuses it has been the practice to arrange the G.sub.R field gradient, which rotates in the plane of the examined slice, with a zero crossing substantially in the center of the slice and to demodulate at the Larmor frequency at the zero crossing. It is now proposed to demodulate the detected resonance signals at a frequency which, in the presence of the field gradient, is for a part of the plane lying outside the body slice. This may be by arranging the zero-crossing to be outside the slice. In that case it is preferable that the zero-crossing of G.sub.R lies on a straight line (27) maintained tangential to a circle (28) lying outside the body (23) about an axis (24) through the slice. Control may then be by precalculated coil currents or by precalculating fields at probe positions and adjusting the coil currents to give the expected fields. Alternatively the zero-crossing may be maintained at the center and demodulation may be at a different frequency. It is then desirable also to measure the resonance frequency in the absence of the field gradient across the slice. The resonance signals are then effectively further demodulated at that measured frequency. The effective demodulation is preferably achieved by complex multiplication of the two signals.

Imaging systems

Nuclear magnetic resonance (NMR) apparatuses have been proposed to examine slices or volumes of bodies, perhaps patients for medical purposes, by exciting resonance and detecting the resonance signals. In an NMR apparatus an G.sub.R field gradient, for selecting lines within a slice or volume, is formed of Gx and Gy components. These should match exactly in shape if not in magnitude. In practice this is difficult to achieve and it is proposed to monitor these fields and derive error signals for their deviation from each other or from a standard. The error signals are used to reduce the deviations so that continuous adjustment of the fields is provided.

In a medical NMR apparatus it is desirable that the axial magnetic field, Ho, is uniform. The result can in theory be obtained by accuracy of construction but in practice this cannot readily be achieved. It is now proposed to use a field sensing probe system which indicates the value of that field at a plurality of positions in the examined region and provides error signals indicating deviations from uniformity. The field is automatically corrected in response to the error signals by applying currents to field correcting coils. The errors may be considered as distinct gradient and dishing errors which are corrected individually.

A pulsed power supply for driving an X-ray tube is described. Four p.f.n. devices are coupled in pairs to respective transformers and the individual devices supply pulses of relatively short duration to their respective transformers. Each pulse supplied by one device is followed by a pulse supplied by a device of the other pair, and pulses supplied by all of the devices in sequence are combined to produce a pulse of relatively long duration for application to the tube.

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High Wycombe, United Kingdom

Colin G Harrison