This invention provides a MRI guided ultrasound therapy apparatus. It comprises a static field magnet adapted to apply a static magnetic field in an magnetic resonance volume at a predetermined disposition at least one ultrasound energy applicator adapted to apply energy within an energy application zone at a predetermined disposition and The mechanical positioning means for moving said ultrasound energy applicator to position the applicator so that the energy application zone intersects said magnetic resonance volume within said region of the subject. In that apparatus, the static field magnet is open at both ends or open at side. The sided open is upward or downward and the mechanical positioning means of this ultrasound energy applicator is close to and is located outside of this sided open. This invention reduces the space limitation for the mechanical positioning means of the ultrasound transducer. Meanwhile, the non-magnetic requirement on the mechanical positioning means become less greatly and particularly the problem of interference from magnetic field produced by working current of the transducer power cord to MRI system can be solved.
