1. An apparatus for defining the location of an image plane of an image through a selected portion of the body of a being, the apparatus comprising: a plurality of elongated channel sections for containing a material which is visible in the image; first carrier means for supporting a first predetermined number of said elongated channel sections in respective orientations transverse to the image plane and in predetermined space relation with respect to one another, said first carrier means being provided with respective first and second carrier portions for supporting respective ones of said plurality of elongated channel sections therealong, said first and second carrier portions being coupled to one another at respective first ends thereof, and to a base member at respective second ends, said first and second carrier portions and said base member forming a triangle, and a third carrier portion arranged substantially to bisect said triangle; and support means for maintaining said first carrier means in fixed spatial relation to the body of the being.