Oral Radiology Ram Kumar Srivastava
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IntroductionChapter 1

2
The discovery of X-ray in 1895 by Wilhelm Conrad Roentgen started with the history of radiography. Roentgen used a vacuum tube, an electric current and special screens covered with a material that glowed (fluorescent) when exposed to radiation. He discovered a glow or fluorescence which was coming from screens when cathode rays (streams of electrons) passing from one end of the tube to the other. He realized that something from the tube was striking the screens and causing glow and named this unknown rays as X-rays.
Johana Wilhelm Hittorf, a German physicist, used the vacuum tube to study fluorescence. In 1870, William Crookers, an English chemist, designed the tube which was later known as the Hittorf-Crookes tube. In 1913, William D Coolidge, an electrical engineer, developed the first hot cathode X-ray tube, a high vacuum tube that contained a tungsten filament. Weston Price introduced the bisecting technique in 1904, and the paralleling technique was first introduced by C Edmund Kells in 1896.
X-ray forms part of the electromagnetic spectrum at the high energy end, the visible light in the middle and microwaves and radio waves at the low energy end. The X-rays are energetic enough to ionize atoms and break molecular bonds as they penetrate tissues and are therefore, called ionizing radiation. X-rays are produced when high-energy electrons strikes a high atomic number material. This interaction is produced within an X-ray tube. A high voltage is passed across two tungsten terminals. One terminal (cathode) is heated until it liberates free electrons. When a high voltage is applied across the terminals, the electrons accelerate toward the anode at high speed. On hitting the anode target, X-rays are produced.3
X-ray picture is produced due to interaction of the ionizing radiation with tissues as it passes through the body. Tissues of different densities are displayed as distinct areas depending on the amount of radiation absorbed. The four different densities are: gas (air), fat, soft tissue and fluid and calcified structure (bone). Air absorbs the least amount of X-ray and, therefore, appears black (radiolucent) on radiograph, whereas calcified structures (bone) absorb the most, resulting in white radiopacities, soft tissue and fluid appear grey on a radiograph.