Abstract: An apparatus for use in determining the orientation and location of an image plane, particularly when imaging the head of a human being, has an elongated flexible channel provided for containing an imaging-opaque fluid which is visible in the image. First and second carriers are each provided for supporting three respective portions of the elongated flexible channel at respective orientations transverse to the image plane and in predetermined space relation with respect to one another. Preferably, the portions of the said elongated flexible channel are arranged substantially as legs of a triangle. A support arrangement maintains the first and second carriers in fixed spatial relation to one another and to the head of the human being. The present invention can be used in any of several known imaging modalities by using appropriate contrast agents.