STEREOSCOPY in the field of radiography.pptx

Steroescopy

1. STEREOSCOPY DR REJO JOHN

2. Learning objectives: Introduction to Stereoscopy Physiology of depth perception Steroescopic filming Tube shift Stereoscopy viewing systems Advantages Disadvantages

3. Introduction J. MacKenzie Davidson introduced stereoscopy to radiology in 1898, and it won immediate and widespread acceptance. Stereoscopy grew in popularity, and by 1930 most radiographs were taken stereoscopically. Stereoscopy had reached the height of its popularity. Then, gradually, the pendulum began to swing. R outine stereoscopy is expensive and time-consuming. The death blow came with the discovery that radiation causes mutations. The popularity of stereoscopy dropped precipitously, and the pendulum made its full swing.

4. PHYSIOLOGY OF DEPTH PERCEPTION It depends on two completely independent mechanisms: the first, monocular or photographic depth perception, requires only one eye; the second, stereopsis, requires binocular vision. Binocular depth perception, called "stereopsis,“ is a unique characteristic of man and other primates. It is dependent on the brain's ability to receive slightly different images from each eye, discrepant images, and then to fuse them into a single image that has depth. Discrepant images are the heart of stereopsis, and fusion occurs in the brain. The degree of discrepancy is of vital importance. If the images are too different, the brain cannot fuse them, and the stereoscopic effect is lost. If the difference is too small, the fused images appear flat.

5. STEREOSCOPIC FILMING Two films are exposed, one for each eye. Figure 17-2 shows the relationship between the x-ray tube, patient, and film. The film is changed between exposures. The second film should be placed in exactly the same position as the first film. The tube is shifted from the left to the right eye position between exposures, and it is the only part that moves. The patient must remain absolutely still. Usually the film is placed in a Bucky tray under the grid so that it can be changed without disturbing the patient.

6. Magnitude of Tube Shift In the past, radiologists thought that the stereoscopic tube shift had to be equal to the interpupillary distance (2.6 in.), but now we know that this is neither necessary nor desirable. The optimal quantity of tube shift is empiric. We have learned by trial and error that a tube shift equal to 10% of the target-film distance produces satisfactory results. A 10% shift is simple to apply and produces discrepant images, which is the only prerequisite to stereoscopic image formation. For example, if the filming distance is 40 in., the tube shift will be 4 in.

7. Direction of Tube Shift The direction of tube shift is important for two reasons. First, many stereoscopic examinations are done with grid techniques, and it is undesirable to shift across the long axis of a grid because of grid cutoff from lateral decentering . In general, all grid-type techniques should be done with a longitudinal tube shift (i.e., along the long axis of the grid). If there is a compelling reason for using a cross shift, then you should use a low-ratio grid. Usually you can expect satisfactory films with a 10% cross shift on an 8:1 or lower ratio grid, but at the expense of increased patient exposure. Both exposures should be made with exactly the same tube shift to either side of grid center .

8. The direction of tube shift is important for a second reason. The objective in stereoscopic filming is to produce discrepant images. The tube shift should be in the direction that produces maximum discrepancy. STEREOSCOPIC VIEWING The stereoscope is an optical instrument used to superimpose two pictures taken from slightly different vantage points. It was invented by Wheatstone in 1838, long before Roentgen discovered x rays.

9. Preliminary Steps Before we can view films stereoscopically, there are three preliminary steps we must follow to determine how the films should be oriented for stereoscopic viewmg . The first step is to identify the tube side of the film, that is, the side that is closer to the patient and thus facing the x-ray tube during the exposure. Usually we read x-ray films as if the patient were facing us. The next step is to determine the direction of the x-ray tube shift. In Figure 17-5, the films in the upper illustration were obtained with a cross shift, and those in the lower illustration with a longitudinal shift. The films must be viewed as shown to present the eyes with discrepant images. If the films are viewed in any other way, there will be no stereoscopic effect. The last preliminary step is to determine which film is to be viewed by the left eye, and which by the right eye. Each eye must view its appropriate film. The whole concept of film placement is simplified if we merely think of ourselves as the x-ray tube. Our right eye represents the tube when it is on the right side, and our left eye represents the tube from the left side.

10. In summary, to keep the image of the patient properly oriented with regard to left and right, we must view stereoscopic films from the tube side, with our eyes converging along the direction of tube shift, and with each eye viewing its appropriate film.

11. Cross-Eyed Stereoscopy. Cross-Eyed Stereoscopy. In the routine interpretation of roentgenograms, both eyes are focused at a single area of interest, accommodation and convergence are coordinated, and both eyes see identical structures in sharp focus. With cross-eyed stereoscopy, two films are interpreted at the same time (Fig. 17-8).

12. Wheatstone Stereoscope. Wheatstone Stereoscope. The principal disadvantage of cross-eyed stereoscopy is that it requires a dissociation of accommodation and convergence. All other stereoscopic systems are designed to overcome this disadvantage. The Wheatstone stereoscope will be used to illustrate how they function. With a Wheatstone unit, convergence is assisted by a pair of mirrors that are located halfway between the films (Fig. 17 -9). We accommodate to the plane of the films and view reflected mirror images. The mirrors are adjusted to superimpose the right and left eye films without changing our convergent effort from what it would normally be for the viewing distance. We see a stereoscopic image hanging in space behind the mirrors and, because accommodation and convergence are coordinated to the same distance, our eyes feel no strain.

13. Figure 17-10 shows a stereoscope designed by Caldwell in 1906.1 It consists of a pair of prisms mounted into binoculars. The unit is compact and convenient to use. Gass and Hatchett have described a similar system The simplest stereoscope, described by Kerekes , requires only a small pocket mirror and a steady hand.5 The mirror is held against the side of the nose and both eyes focus on the left eye film (Fig. 17-ll). The mirror is adjusted to interrupt the view of the right eye, which then sees a mirror image of the right eye film. The mirror is carefully adjusted until the left and right eye images superimpose and are fused into a combined image having depth. Because the right eye sees a mirror image, the film must be reversed so that both images are seen in the same orientation.

14. Viewing Systems Stereopsis, or binocular depth perception, is only qualitative. With stereoscopic radiographs we are incapable of judging the absolute distance between objects. All we can appreciate is which of two objects is closer to us (i.e., a rank order type of depth perception). All we need for stereopsis are discrepant images, and the degree of discrepancy must be sufficiently great to permit an appreciation of depth, and yet not so great that the brain cannot fuse the images. With this fact in mind, we can understand why the film reading distance is not important. Whatever distance is convenient and comfortable is perfectly adequate. The same stereoscope can be used to interpret all stereoscopic films, no matter what distance was used for the exposure.

15. advantages The advantages (or perhaps uses is a better term) of stereoscopy depend on the skill of the stereoscopist . Education . The teaching of normal anatomy is simplified with stereoscopic images. Lines that are superimposed into a confusing array for the novice migrate to their true position in depth, and they create a perspective that is not possible on a single film. Foreign Body Localization . Stereoscopy cannot compete with a pair of right-angle radiographs for localization of a single foreign body. Stereoscopy is superior for the localization of many foreign bodies, such as multiple pellets from a shotgun wound. It allows us to localize each object from a single vantage point. With two right- anglefilms , it is not always possible to identify one particular foreign body on both films,because they all look alike.

16. Localization of Intracranial Calcifications. Some intracranial calcifications are so small that they can only be seen on the lateral view, and the only way they can be localized is with stereoscopic films. In fact, some calcifications can only be recognized stereoscopically. Unimposing Confusing Shadows. The more confusing a radiograph is, the more likely stereoscopy is to be helpful. For example, a collection of gas in the colon may be impossible to differentiate from a destructivelesion in the pelvis on a single film. A good stereoscopist can recognize the true situation with ease.

17. disadvantages Expense . Stereoscopy requires two films, which adds to the cost of an examination. It requires more technician time, is wearing on the equipment, and doubles the cost of films and processing. Patient Exposure. The principal disadvantage of stereoscopy, the reason it should not be used routinely, is the increased patient exposure required to produce two films.

18. Need for Patient Cooperation. Because the patient must remain perfectly still between exposures, stereoscopy cannot be used on patients who are unable to cooperate. Need for Practice. To become and remain a good stereoscopist , you must practice. The more stereoscopy you do, the better you will become.. Stereoscopy should be practiced daily; routine lateral skull films are probably the most satisfactory study to use, because they frequently give information that cannot be obtained in any other way.

19. SUMMARY Stereoscopic filming techniques are simple and require no special equipment. Two films are exposed, one for each eye. Between exposures, the x-ray tube is shifted 10% of the target-film distance (6°); the films are changed, taking care to position the second film in exactly the same position as the first film. The films of the stereoscopic pair present the eye with slightly different, or discrepant, images.

20. Many viewing systems are available, but all accomplish the same purpose. They assist the eyes in coordinating accommodation and convergence. Stereoscopic depth perception is only relative, a rank order type of depth perception, which ranks objects in their order of closeness but does not disclose the distance between them. A failure to appreciate this fact is one reason why some radiologists have been disappointed in stereoscopy. Stereoscopy has several serious disadvantages. The examination requires two films, so that it is costly and doubles the patient's radiation exposure. Because the patient must remain perfectly still between exposures, it cannot be used on patients who are unable to cooperate. In spite of these disadvantages, stereoscopy is still a valuable tool, and frequently offers the simplest solution to a difficult radiologic problem.

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