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Stereoscopy (23/10) Click here to see our objects about stereoscopy A well-known mechanism of depth perception is the “stereopsis”, the ability to see two slightly different images, one by each eye, which are converted in the brain to one single picture with depth ( example: the Viewmaster®). Stereoscopic radiography was described first by Sir James Davidson McKenzie (1856 – 1919) in the British Medical Journal (1898): taking two slightly discrepant radiographs and view them in a manner that fuses them in this single image with depth information. Two films are taken with the tube in two shifted positions, the patient remaining absolutely still between the two exposures. Since the objective is to produce discrepant images, the tube shift must cross the long axis of the part to be examined; but the shifting should also be parallel to the lead strips of the grid, to avoid grid cut-off. The viewing procedure must reproduce the direction of the shift horizontally, and if no viewing equipment is used, the viewer has to cross his eyes: look at the film on the left by the right eye, and the film on the right with the left eye. Since not everyone can perform this eye crossing, and it is causing eyestrain, a variety of viewing devices, “stereoscopes”, were developed with mirrors or prism to combine the two images in a single image. (O-23) (O-279) (O-288) (O-236) (O-131). The primary disadvantage of stereoscopy is the increased exposure to radiation of the patient: indeed at least two images are needed, sometimes more if the cooperation of the patient to remain still is not easy. This technique was principally used in USA. |