Abstract: 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.

Abstract: 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.