E-Book, Englisch, Band 28, 169 Seiten
Reihe: QuintEssentials of Dental Practice
ISBN: 978-1-85097-317-1
Verlag: Quintessence Publishing
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)
Autoren/Hrsg.
Weitere Infos & Material
Chapter 01: The Historical Perspective
Chapter 02: Intraoral X-ray Equipment and Imaging
Chapter 03: Panoramic Equipment and Imaging
Chapter 04: Conventional Image Receptors
Chapter 05: Digital Imaging
Chapter 06: Direct Digital Imaging
Chapter 07: Indirect Digital Imaging
Chapter 08: Image Storage and Handling
Chapter 09: Implant Imaging
Chapter 1
The Historical Perspective
Aim
The aim of this chapter is to provide an historical perspective for the current developments in dental x-ray imaging. Outcome
The reader will have knowledge of the earliest use of x-rays in dentistry and of the subsequent developments, setting the scene for the important developments in dental imaging in the early years of the 21st century. The First Steps
Dentists first made radiographs of their patients in the early months of 1896, within weeks of the 1895 publication of Röntgen’s discovery of x-rays. It is difficult to say exactly who produced the first dental radiograph, but it is certain that at least four individuals were independently experimenting with x-rays in the oral cavity: Walkhoff (Fig 1-1) and Koenig in Germany, Kells in the USA and Harrison in England. Frank Harrison is of particular interest for two reasons. First, because he carefully recorded his work and published this in the Journal of the British Dental Association in June 1896; second, because his radiographs were of acceptable clinical quality (Fig 1-2). Fig 1-1 One of the first radiographs of teeth, taken by Otto Walkhoff in Germany in the early months of 1896. Fig 1-2 Dental radiograph of a 7-year-old girl taken by Frank Harrison in Sheffield in 1896. The exposure time was several minutes. How was it done? Harrison’s records show that, in the absence of a mains electrical supply, radiography needed substantial battery power. To this was added a transformer (homemade) to produce high voltages; finally, he used a hand-crafted glass x-ray tube. An example of such an installation is shown in Fig 1-3. In these respects he was simply following the methods of Röntgen. Such equipment was unreliable in its output and was constantly subject to breakdown. A further challenge was the means of recording an image. Following Röntgen’s own method, Harrison initially used photographic glass plates, cut down to size and wrapped in rubber dam, but he soon switched to “Eastman’s Kodac” [sic] film. With this apparatus and material, he was able to produce radiographs with 6-minute exposures. Fig 1-3 An x-ray installation from 1896/97. On the left is the x-ray tube and on the right is the electrical apparatus required to produce the electrical supply. (Courtesy of the Science Museum, London). Harrison was among the earliest to report radiation injuries. He reported practising his techniques on his male assistant, who was acting as a patient, over a four-week period in 1896. The consequences were dramatic: On June 4 the patient complained of an itching and burning sensation, with slight redness over the area subjected to the x-rays; shaving had to be discontinued on account of pain.... On the 6th the hair follicles of the beard and right side of the moustache appeared to postulate. On June 8... the skin commenced to desquamate; on the next day the hair over the affected region began to fall... The hair has continued to fall up to the date of writing (June 24), and the skin of the face is quite bald, and the glossy skin in slight wrinkles. Journal of the British Dental Association (1896) In most respects, Harrison’s report was a microcosm of the collective experiences of early pioneers of dental radiography: ramshackle collections of x-ray generating equipment, homemade adaptations of photographers’ materials and sporadic radiation-induced injuries. It was a faltering start to x-ray imaging in dentistry and it is unsurprising that Harrison ended his paper with the statement: “the work is altogether too complicated and too expensive to be added to the dental outfit”. Progress in X-ray Equipment
The early years of the 20th century saw rapid improvements in medical x-ray generating equipment. Within months of Röntgen’s discovery, several scientists had independently developed a “focus tube”. In simple terms, this moved the anode of the x-ray tube from a position perpendicular to the electron beam to one at an angle, leading to a smaller source of the x-rays (and hence sharper images) and a longer tube life. Early x-ray tubes, far from being evacuated, contained a lot of air. Indeed, in the beginning this air was essential to provide an electron source for x-ray production. These tubes were highly unpredictable in operation: during use the vacuum improved, but it then deteriorated when the tube was idle. To deal with this challenge, tubes had small projections in the glass bulb that contained a screw cap (Fig 1-4); this cap could be briefly opened to allow a little air to enter, thus restoring tubes that had been used heavily and had too high a vacuum. This “hit and miss” x-ray production meant that the operator was forced to test the x-ray output frequently, usually on himself or an unfortunate assistant. The process of “setting the tube” stored up many future problems in terms of delayed radiation injuries and cancer induction. Fig 1-4 This early x-ray tube shows the additional side protuberance, which allowed “regulation” of the tube to keep it working optimally. Mains electrical supply was far from universal at the start of the 20th century. Instead, power supplies were limited to pulsating direct voltage from induction coils, with the reverse currents (anode to cathode) giving poor efficiency and shortening tube life. The introduction of transformers and AC mains supplies in the first and second decades of the 20th century improved the situation substantially. The key development, however, was the patenting of the “hot cathode” x-ray tube by Coolidge in 1913 (Fig 1-5). Coolidge used a tungsten filament as the cathode and a good vacuum was achievable, allowing a vastly superior efficiency of x-ray production. Although numerous modifications were made over the subsequent years, this invention is the real ancestor of the x-ray tube found in modern x-ray machines, including dental sets. Fig 1-5 William Coolidge in his laboratory, with an example of his “hot cathode” x-ray tube. In the early years, dentists simply copied medical equipment, putting together components bought at the local chemists or hardware store or by mail order. It was not until 1905 that the first “dental x-ray set” was manufactured in Germany, followed in 1912/13 by two US manufacturers. Early x-ray equipment had exposed high-voltage wires, which were a serious danger to the operator; indeed, the risk of electrocution was as serious as that of x-ray injury. In 1918, the first shockproof dental x-ray set was introduced in the USA. The Victor CDX used an oil-filled container to house the x-ray tube and electrical wires, a method of insulation that is still used today. Nonetheless, equipment with exposed wiring continued to be used for some years. Thus, the basics of a safe and efficient x-ray machine for dentists were in place by the 1920s. By the next decade, designs incorporating the new streamlined art deco style were being produced (Fig 1-6), with a position-indicator cone to aid in radiographic positioning. Such equipment had a long life, and older dentists may even remember sets like this persisting beyond the middle of the century. Fig 1-6 An advertisement for a state-of-the-art dental x-ray set from the 1930s. Note the pointed cone to aid in positioning during radiography. (Courtesy of the Science Museum, London). While the revolution in dental x-ray set design was complete, certain developments were still to come though a slower and more evolutionary process. One important change was the introduction of electronic rather than clockwork timers, a change that was essential to cope with the shorter exposure times achievable with faster films. The “pointed cone” position indicating device seen in Fig 1-6 was gradually displaced by open-ended cylinders during the 1970s, reducing scatter and clearly demarcating the irradiated area. In the last decade of the 20th century, constant-potential x-ray equipment gradually became available to dentists, shortening exposure times and avoiding the “pulsating” x-ray production associated with mains voltage. One key development in dental radiography in the 20th century was “panoramic imaging”. The full story has been told in Chapter 1 of Panoramic Radiology (Quintessentials of Dental Practice – 20; Imaging – 2, 2006) and the reader is referred there for information. Briefly, however, early attempts to produce panoramic images of the jaws can be traced back to 1922. Panoramic imaging using intraoral x-ray sources was tried in the 1940s and early 1950s, but it gave poor image quality and very high radiation doses to the mouth. What we now recognise as panoramic radiography was developed in the 1940s by the Finnish scientist Yrjö Veli Paatero, who built on previous ideas of other researchers (Fig 1-7). Paatero employed a slit-collimated x-ray beam and a film cassette, with relative movement of these and the patient producing a scan image of the jaws. From early machines with single rotation centres, developments included increasing the number of centres to three and, more recently, to a continuously moving centre of rotation. Today, panoramic equipment offers several sophisticated options, including imaging of selected areas and cross-sectional imaging for implantology. Fig 1-7 Dr Paatero making fine adjustments to patient positioning. The device was subsequently installed in Helsinki University in 1951. (Kindly supplied by Instrumentarium Imaging, Tuusala, Finland). Progress in...
