Gradient coils for therapy tomographs

Gradient coil system for the production of a magnetic transverse gradient field G.sub.x =dB.sub.z /dx in an nuclear magnetic resonance (NMR) tomograph with a main field magnet for the production of a homogeneous static main magnetic field B.sub.z in a measuring volume whose center coincides with an origin of a Cartesian x-, y-, z-coordinate system, wherein the main magnetic field B.sub.z is directed along the z-axis, and the magnetic transverse gradient field G.sub.x varies along the x-axis of this coordinate system, wherein the gradient coil system comprises four partial coils (S.sub.1x, S.sub.2x, S.sub.3x, S.sub.4x) each having two current connections (A.sub.1, A.sub.2) which are arranged mirror symmetrically with respect to the xy-plane (z=0) and mirror symmetrically with respect to the zy-plane (x=0), wherein each partial coil contains winding sections on an inner and on an outer cylinder (Z.sub.ix, Z.sub.ax) extending about the z-axis and, in each case, in a radial connecting plane (V.sub.+x, V.sub.-x) essentially parallel to the xy-plane, wherein the winding sections of each partial coil have current flowing through them in series during operation and wherein the radial connecting planes (V.sub.-x, V.sub.-x) of those partial coils (S.sub.1x, S.sub.4x ;

• S.sub.2x, S.sub.3x) which lie across from each other relative to the xy-plane have a distance, and wherein the partial coils (S.sub.1x, S.sub.2x, S.sub.3x S.sub.4x) are configured such that the winding sections on the inner and on the outer cylinder (Z.sub.ix, Z.sub.ax) are axially more distant from the xy-plane than the corresponding radial connecting surface (V.sub.+x, V.sub.-x);
• the axial distance (d.sub.Vx) of the radial connecting surfaces (V.sub.+x, V.sub.-x) are in a range between 0.8 and 1.45 times the radius (R.sub.ax) of the outer cylinder (Z.sub.ax);
• the radius of the inner cylinder (Z.sub.ix) is between 0.35 and 0.8 times the radius R.sub.ax of the outer cylinder (Z.sub.ax);
• the axial distance (d.sub.Vx) of the connecting surfaces (V.sub.+x, V.sub.-x) is larger than 30 cm, the inner and the outer cylinder (Z.sub.ix, Z.sub.ax) each are circular cylinders about the z-axis;
• the axially outermost winding sections on the outer cylinder (Z.sub.ax) are separated further from the xy-plane than the axially outermost winding sections on the inner cylinder (Z.sub.ix);
• the outer cylinder (Z.sub.ax) comprises less winding sections than the inner cylinder (Z.sub.ix);
• the partial coils (S.sub.1x, S.sub.2x, S.sub.3x S.sub.4x) are so-called streamline coils with current conducting sheets and quasi-continuous current distribution;
• the distribution of the winding sections on the outer cylinder (Z.sub.ax) is calculated such that axially outside the connecting surfaces (V.sub.+x, V.sub.-x) there are no radial gradient field components on the cylinder surface;
• and the winding sections of the partial coils on the inner (Z.sub.ix), on the outer cylinder (Z.sub.ax) and on the connecting surface (V.sub.+x, V.sub.-x) each extend about approximately 180.degree..