Active vibration isolation applied to a Magnetic Resonance Imaging (MRI) system

A magnetic resonance imaging system (MRI) produces high acoustic noise levels during scanning. In these systems, a strong static magnetic field is used in the order of a few Tesla. Additionally, a magnetic gradient field is generated which varies in time. This gradient field is created by means of a so-called gradient coil, in which electrical currents are used in the order of a few hundred amperes. The combination of these alternating electrical currents running through the gradient coil and the strong magnetic field that is created by the superconducting main magnet causes Lorentz-force induced vibrations and thus acoustic noise. Especially at the structural resonance frequencies of the gradient coil, high vibration levels occur. Some typical resonance modes of the gradient coil will be discussed.The gradient coil radiates acoustic noise directly. Due to structural transmission paths vibrations are caused in other parts of the system, which causes acoustic noise as well. At Philips, active vibration isolation techniques were investigated to reduce the structural transmission of the induced vibrations to other parts of the MRI-system. Provided that the direct radiation of the gradient coil is treated sufficiently well, active vibration isolation opens the road to a further reduction of the radiated acoustic noise. It is shown that the force transmissibility from the gradient coil to the other parts of the MRI-system can be reduced by approximately 10 dB over a frequency range between 800 and 2000 Hz, where the highest acoustic noise levels occur, by means of active vibration isolation.