The invention relates to a mixture of biodegradable polyesters comprising: a polyhydroxy acid of the poly-\u03b5-caprolactone type and its copolymers, aliphatic polyester, and a polymer of polylactic acid, in which the concentration of varies with respect to in the range between 40 and 70% by weight, and the concentration of with respects to lies between 2 and 30%. The casting body is filled with small spherical granules having a smooth and slippery surface. The radio frequency shield may be configured to extend behind the radio frequency coil, and the axial length of the radio frequency shield may be at least two times the axial length of the radio frequency coil.