. Radiography, X-ray therapeutics and radium therapy . gnbody. In the plate taken in position (1), the distance from a given point,probably a bony landmark, is taken. The plate taken in position (2) showsthe depth from the surface. In most instances this should be sufficient, asthe operator has only to measure the distances and make a mental note ofthe position. In order to get a graphic record of the measurements, a simple plate-holder can be constructed with an inch or centimetre rule, which slides overthe surface of the plate so that it may be placed in relation to a bone or foreignbody. A
. Radiography, X-ray therapeutics and radium therapy . gnbody. In the plate taken in position (1), the distance from a given point,probably a bony landmark, is taken. The plate taken in position (2) showsthe depth from the surface. In most instances this should be sufficient, asthe operator has only to measure the distances and make a mental note ofthe position. In order to get a graphic record of the measurements, a simple plate-holder can be constructed with an inch or centimetre rule, which slides overthe surface of the plate so that it may be placed in relation to a bone or foreignbody. A second inch or centimetre scale runs at right angles to the longi-tudinal one. The marks on the scale are rendered opaque by insertingpieces of wire into the wood at the correct distances. The second plate-holderis fixed at right angles to the first one and also has sliding scales. The 110 RADIOGRAPHY foreign body may be located by screening prior to the taking of the the two negatives are examined it is easy to locate accurately the a Qh. 1 Fig. 91.—Diagram to show method of taking a lateral view. A, Source of X-rays. E, Shadow of foreign body on , Foreign body. /. Shadow of bones. C, Bones. 6, Graduated scale. B, Limb. foreign body. The plates have on the surface an exact rule for exact localisation the Mackenzie Davidson method should be employed in Mackenzie Davidson Localisation.—Thecentral ray emitted from an X-ray tube has to be definitely fomid. To dothis it is necessary to have an arrangement for determining the central tube is accurately fixed on the box beneath the table ; the latter movesin two directions on trolley wheels. The central ray is located by means ofcross wires, or a plumb-line running on pulleys and moving with the tube,so that whatever the position of the tube the plumb-line always indicates theposition of the focus of rays upon the anti-cathode. The distance of theanode from the top of the c
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