Design and Construction of a Low-Cryogen, Lightweight, Head-Only 7T MRI Magnet

A newly developed compact 7.0 T MRI system (C7T) is a dedicated brain imaging system with the weight and footprint of a clinical whole-body 3.0 T MRI scanner. This new 7T MRI scanner will be easier for deployment and installation than a whole-body 7T thanks to lighter weight and more compact size. Its superconducting coils wound with NbTi wires are mounted inside of aluminum support cylinders. The magnet has a fully sealed cooling system that comprises of 3 cryocoolers, liquid and gas helium tanks and stainless-steel cooling tubes thermally attached to aluminum support cylinders. It needs only about 1% of liquid helium as that of conventional superconducting magnet. Helium will be permanently sealed inside the helium tanks and cooling tubes even after a quench. As such, no vent stack is needed for this magnet. The C7T magnet is designed for a B0-field homogeneity of <1.0 ppm over a 26-cm field-of-view. This magnet incorporates a superconducting shim assembly comprising 6 axial and 12 transverse shim coil sets, which reduces the amount of magnetic material required in the passive shimming system. The design and construction of this novel compact 7T MRI magnet is reported in this paper.