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When X-rays cross the human body, they are more absorbed by bone than by weak tissue, due to the physics of mainly the photoelectric interactions. This results in a negative image that can be represented on a radioscopic screen and photographic glass plates.
The first registration of such a medical X - ray investigation was a positive picture transferred to photographic paper. This technique was used till circa 1920.
The quality of the photography is influenced by the used photographic material, the developing technique and the quality of applied X-rays. The latter is dependent upon the kilovoltage applied as well as the intensity and penetration power of the X-rays. These had also a direct effect on the exposure time: the major bottleneck in the early period. Beside this technical problem the adaptation of photographic material and its processing was of decisive importance, because in spite of progress in radiologic equipment the exposure time remains very long. This drawback had two main disadvantages: firstly he harmful biological effects of radiation and secondly, a long exposure time, inducing artefacts produced by movement.
Intensifying screens were rapidly developed, already in 1896, and used by pioneers in X-ray applications to shorten the exposure time; an important improvement. Intensifying screens increase the efficiency of the X-rays by producing fluorescence light also captured on the film, allowing a reduction of beam intensity.
A new phase in the history of radiography began with attempts to solve the problem of the sensitivity of emulsions. At the beginning of the 19th century the use of a suspension of small particles of silver bromide crystals in jelly started. Even with this method the quality of the pictures depended on the wavelength of the X-rays. Moreover, during World War I many of these glass plates got broken in the frontlines (O-19).
At that time an innovation was introduced in the X-ray technology. Eastman, later Kodak company, launched already in 1914 a typical X-ray film: a cellulose nitrate base with a photographic emulsion on one side. The adoption of such a material (already used from the beginning in dental radiological) solved the problems caused by weight, volume and the inconvenience of handling glass plates. At that time, the use of photographic paper also was gently abandoned.
About 1920, a thick film with emulsion on both sides was introduced under the name Dupli-Tized. A double emulsion on glass plate was never used because the depth of the glass did not allow the coincidence of the two images and could interfere with the sharpness of the picture. The effect of a dose of X-ray on the image was in doubled. This film offered substantial advantages in sensitivity: the combination of a thick emulsion layer and the two emulsions allowed greater absorption of X-rays, both for pictures taken without or with intensifying screens.
Later on cellulose mono-acetate as base of X-ray film came available, it was far more expensive and it bend more easily. But a tragic event was to change procedures completely. A large blaze involving X-ray film occurred in the Cleveland Clinical Hospital in 1929 claiming one hundred and twenty-five lives. This disaster proved that there was a need to substitute this material. Cellulose tri-acetate became the solution. The last significant change that radiographic film underwent was the substitution of this base by polyester. Cellulose acetate presented serious problems when automation was introduced into the developing process at the end of the 1950s.