. Radiography and radio-therapeutics . e distortion, which is unavoidableat all distances within the 2-metre limit, may be uniform in all platesexamined. When this is so the distortion need not seriously enter intothe matter, since it is the same in all cases. A convenient distance between the plate and the anti-cathode of thetube would be 70 centimetres (28 inches). This might be designated thestandard distance for all thoracic and abdominal work. At this distancethe distortion of any structure, or portion of a structure, or organ in the centreline is comparatively slight. The distortion of t
. Radiography and radio-therapeutics . e distortion, which is unavoidableat all distances within the 2-metre limit, may be uniform in all platesexamined. When this is so the distortion need not seriously enter intothe matter, since it is the same in all cases. A convenient distance between the plate and the anti-cathode of thetube would be 70 centimetres (28 inches). This might be designated thestandard distance for all thoracic and abdominal work. At this distancethe distortion of any structure, or portion of a structure, or organ in the centreline is comparatively slight. The distortion of the contour of the organtowards the periphery increases as the rays leave the central axis. Kohlerhas shown that the displacement of the image of the left border of the heartis greater by 1 centimetre with a focus distance of 70 centimetres, than witha focus distance of 2 metres. At the latter distance the distortion is practi-cally negligible for all parts of the organ. Seventy centimetres is therefore THE EXAMINATION OF THE HEART 303. a good distance at which to place the patient. A fixed point on the plate-holder can be arranged so that the distance is readily obtained. At the distance of 70 centimetres the diaphragm of the tube-box isarranged so that the rays will completely cover a 15 x 12 plate. The tubeshould be centred over the mid-point of the plate, and for thoracic workthe point on the patient over which the central ray is fixed would be thexiphoid cartilage in front and a line between the angles of scapulae it is possible to obtain full exposures of the thorax at a distanceof 2 metres, then tele-rontgeno-graphy of the thorax is of decidedadvantage in cardiac work, becausethe heart shadow is shown with-out any distortion. In makingexposures at intervals of time thisis of great service, enabling us todispense with the orthodiagraph. The repeated examination ofcardiac cases will enable the radi-ologist to record the varying con-dition of the heart at interva
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