Magnetic resonance imaging (MRI) is a medical imaging modality that can create images of the inside of a human body without using X-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z-axis,” by convention). A MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis (i.e., the “longitudinal axis”), and that varies linearly in amplitude with position along one of the x, y, or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonance frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. These coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
One type of magnet assembly for an MRI system is cylindrical and annular in shape and includes, among other elements, a superconducting magnet, a gradient coil assembly and an RF body coil assembly. The RF body coil assembly can be mounted on the outside of a patient bore tube and mounted inside the gradient coil assembly. The gradient coil assembly is disposed around the RF body coil assembly in a spaced-apart coaxial relationship and the gradient coil assembly circumferentially surrounds the RF body coil assembly. The gradient coil assembly is mounted inside the superconducting magnet and circumferentially surrounded by the superconducting magnet. Interconnections for supply and return of electricity, control signals, coolant and the like are typically routed from a “service end” of the MRI scanner around the cylindrical magnet assembly, while a patient table and other patient-directed aspects are placed at another end, the “patient end,” of the MRI scanner.
The gradient coil assembly used in an MRI system may be a shielded gradient coil assembly that consists of inner and outer gradient coil assemblies bonded together with a material such as epoxy resin. The inner gradient coil assembly or winding and the outer gradient coil assembly or winding are disposed in concentric arrangement with respect to a common axis. Typically, the inner gradient coil assembly includes inner (or main) X-, Y- and Z-gradient coils and the outer gradient coil assembly includes the respective outer (or shielding) X-, Y- and Z-gradient coils. In order to improve gradient coil performance as well as to reduce the radial space used in the magnet assembly, combined (or integrated) gradient coil/RF coil designs have been developed (for example, as described in U.S. Pat. No. 6,930,482, entitled “Time-Variable Magnetic Fields Generator For A Magnetic Resonance Apparatus,” issued on Aug. 16, 2005, naming Oliver Heid and Markus Vester as inventors). Such designs allow the main gradient coils to be brought closer radially to the imaging region, which can improve gradient performance. In an integrated gradient coil/RF coil configuration, a main (or inner) gradient coil assembly and RF coil are combined into a single unit by splitting the main gradient coil (i.e., the X, Y- and Z-main gradient coils of the main (or inner) gradient coil assembly) into two portions thereby creating a gap along the length of the cylindrical main gradient coil. An RF resonator (e.g., an RF coil) is positioned in the gap between the two portions of the main gradient coil. A birdcage type RF coil can be created by bridging the gap with rungs spanned by capacitors.
For a combined (or integrated) gradient coil/RF coil configuration, each portion of the split main gradient coil (X, Y and Z main coils) requires electrical connections (or leads). In addition, cooling lines have to be connected to and routed through the gradient coil/RF coil assembly. The coolant and lead connections are complicated by the need to deliver coolant and current to each of the separate portions of the main X, Y and Z gradient coils. Accordingly, there is a need for robust electrical and hydraulic connections for the each portion of the split main gradient coil that maintain or improve performance limits.