The invention relates to a waveguide (24) for focusing electromagnetic energy on an area of interest, said waveguide (24) comprising outer walls forming a channel with a width (w) and a height (h) for the propagation of the electromagnetic waves. In order to be able to focus the EM waves on an area of interest, i.e. the chest of a patient (20), so that influences which other moving objects in the environment have on the waves, are shielded away, the channel is provided with a first opening and a second opening wherein said walls are non-metal hollow walls filled with an electrically conductive liquid. The waveguide (24) can be used for example with a computed tomography system (12) CT with a rotating gantry (16) and a patient table (18). The waveguide (24) will not produce artefacts in the CT image by scattering the X-ray, like a metal wall would do it. Nevertheless the conductive liquid makes the construction work as a waveguide for EM waves.