The invention relates to a method and an apparatus for measuring the
position and the orientation of an interventional device within a
magnetic resonance imaging apparatus, the magnetic resonance image
apparatus providing a static magnetic field B.sub.0 and magnetic field
gradients with known magnetic flux densities and directions. The
invention includes attaching a Faraday sensor to the interventional
device, the Faraday sensor being connected to an optical measuring
device. The method also includes using the optical measuring device for
measuring the rotation of the plane of polarization of linearly polarized
light in the Faraday sensor, while exposing the interventional device:
(i) to the static magnetic field B.sub.0, and (ii) to the field gradients
in all three directions of space in addition to the static magnetic field
B.sub.0. The method also includes determining the position and the
orientation of the interventional device by evaluating the measured
rotation of the plane of polarization of the linearly polarized light.