A method for performing computed tomography, in which method X- radiation is produced with an X-ray source, which is collimated into a cone beam by means of a collimation system comprising at least one collimator to an object being imaged, and the X-radiation that has been transmitted through the object is received with an X-ray detector to produce image information of the object being imaged, said detector having a width and imaging area, and X-ray imaging is performed from different imaging angles by moving the X-ray source and the X-ray detector with respect to the object being imaged by following a scanning imaging movement where the X-ray source and the X-ray detector are moved synchronically with respect to one another, a control system moves the collimator for the collimation system synchronically with the movement of the X-ray detector with respect to the X-ray source and projections of the object are taken at desired imaging angle intervals, in which method: a first imaging phase is performed by scanning at least a part of the object to be imaged by following a first arc of a first rotating movement to produce first image information, said first rotating movement being a rotation about a mechanical rotation centre, the centre being in a horizontal x-y plane perpendicular to an axis of rotation, said first rotating movement having a first offset where the offset is defined with respect to a centreline of the detector, the offset being a shortest distance, of the said centreline between the X-ray source and the X-ray detector, from the said mechanical rotation centre, the offset of the imaging is changed between imaging phases, at least one other imaging phase is performed with at least one changed offset to produce second image information of at least part of the object, wherein the mechanical rotation centre in the other imaging phase is moved in the plane relative to the centre of the first rotating movement of the first imaging phase, and projection information, which was produced in the first phase and the other phase, is combined by image processing carried out by means of an image processing unit for producing three-dimensional image information of the object. a first imaging phase is performed by scanning at least a part of the object to be imaged by following a first arc of a first rotating movement to produce first image information, said first rotating movement being a rotation about a mechanical rotation centre, the centre being in a horizontal x-y plane perpendicular to an axis of rotation, said first rotating movement having a first offset where the offset is defined with respect to a centreline of the detector, the offset being a shortest distance, of the said centreline between the X-ray source and the X-ray detector, from the said mechanical rotation centre, the offset of the imaging is changed between imaging phases, at least one other imaging phase is performed with at least one changed offset to produce second image information of at least part of the object, wherein the mechanical rotation centre in the other imaging phase is moved in the plane relative to the centre of the first rotating movement of the first imaging phase, and projection information, which was produced in the first phase and the other phase, is combined by image processing carried out by means of an image processing unit for producing three-dimensional image information of the object.