Disadvantages of Indirect Digital Radiography compared with Direct Digital Radiography

Quintessentials of Dental Practice – 28
Imaging – 3

21st Century Imaging

Authors:

Keith Horner

Nicholas Drage

David Brettle

Editors:

Nairn H F Wilson

Keith Horner

Quintessence Publishing Co. Ltd.

London, Berlin, Chicago, Paris, Milan, Barcelona, Istanbul, São Paulo, Tokyo, New Delhi, Moscow, Prague, Warsaw

British Library Cataloguing in Publication Data

Horner, Keith
21st century imaging. - (Quintessentials of dental practice; v. 28)
1. Teeth - Radiography
I. Title II. Drage, Nicholas III. Brettle, David
617.6′07572

ISBN: 1850973172

All rights reserved. This book or any part thereof may not be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, or otherwise, without the written permission of the publisher.

ISBN: 1-85097-317-2

Foreword

Diagnostic imaging is the commonest form of clinical investigation used in oral diagnosis. In contrast to what existed in the last quarter of the 20th century, practitioners now have a bewildering choice of imaging systems for use in everyday clinical practice. Imaging in the 21st century – Volume 28 in the unique Quintessentials of Dental Practice series, provides excellent insight and authoritative guidance on the use of contemporary imaging systems. As expected of titles in the Quintessentials series, this book has been carefully prepared to appeal to, in particular, busy practitioners and students at all levels. The text is generously illustrated, with each chapter concluding with carefully selected references or helpful suggestions for further reading.

With the prospect of different forms of digital imaging being widely used, if not largely replacing traditional diagnostic imaging in clinical practice in a matter of five to ten years, existing and future practitioners need to get to grips with the relevant technologies and clinical techniques as soon as is practically possible. This book provides the means to meet this challenge, with lots to interest and information for all members of the dental team. As has come to be expected of all volumes in the Quintessentials series, this is a read-in-a-few-hours, and then keep-to-hand book of immediate practical relevance. This book is a great addition to the Quintessentials series, which continues to go from strength to strength. It is a privilege and honour to be Editor-in-Chief of the series, helping to make books such as this volume available to busy colleagues across the world.

Preface

The 20th century saw a slow, steady evolution in dental x-ray imaging. Despite obvious improvements in the quality and sophistication of equipment and materials for dental radiography over the first hundred years after Roentgen’s discovery of x-rays, the basic photographic based technologies are the direct descendents of those used in the 1890s. In the 21st century, the foot has been put firmly down on the accelerator as far as dental imaging is concerned. Digital imaging is rapidly becoming the method of choice for dentists in many countries and now we are faced with exciting new developments that promise to revolutionise the way we use images to help in managing our patients.

In writing this book, we had in mind the dentist who was looking for information about “state of the art” dental imaging. While commencing with some historical information to set the scene, we decided to concentrate particularly on digital imaging. Thus, three chapters are devoted specifically to this subject. Nonetheless, despite a “high tech” emphasis in much of the book, we have still included information on conventional image receptors; film remains a cheap and adequate way of imaging dental patients that should not be consigned to the history books just yet. We have also included information on intra oral and panoramic equipment, to help the reader understand what is available and the ideal features to seek when buying. In the final chapter, “Implant Imaging” we have focused on the more complex imaging techniques and equipment that a dentist is unlikely to consider buying, but for which he or she may refer patients.

Acknowledgements

We thank our families, friends and colleagues for their encouragement and support while writing this book.

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:

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 Image Receptors

Roll photographic film was the choice of early dental radiography practitioners, and this method persisted for two decades. Although one manufacturer produced ready-wrapped film in 1913, most dentists persisted with preparing films on an individual patient basis. It would not be until 1919 that the very first machine-wrapped dental film was manufactured: Kodak “Regular” film. This, as much as developments in equipment, made radiography a more attractive proposition for consideration by the average dentist.

While film remained the main focus of radiography, the use of intraoral fluoroscopy should not be forgotten. This involved the use of an instrument that, ostensibly, looked like a dental mouth mirror (Fig 1-8) but which had a fluorescent material in place of the mirror. If this was placed in the mouth behind a tooth and exposed then a “real time” image would appear. Despite its “instant imaging” attractions, this technique led to excessive x-ray exposures, not least to the operator, as the temptation to keep the x-rays turned on was strong.

Fig 1-8 Intraoral fluoroscopy; a) a small fluorescent screen is fitted to a mouth mirror type handle; b) early drawing of this instrument in use.

In 1924, Kodak produced a double-sided film emulsion (Kodak “Radiatized”), which halved the exposure times needed for imaging. Thereafter, developments were principally related to improving film speed. In 1940, “Ultraspeed” film allowed exposure times to be reduced by half. This was followed by further speed improvements in the 1950s. In the following decade, the polyester base was introduced, which was more robust than the cellulose acetate that had been in use since the 1930s. In 1980, “E-speed” film was introduced, once again halving the required exposure times, while in recent years we have seen a move to film speeds that fall into the “F-speed” category (see Chapter 4).

In 1989, there was a major development in dental radiography, with the first digital system for recording images: RadioVisioGraphy (Fig 1-9). This was developed by a French dentist, Francis Mouyen, and was based on a combination of the old fluoroscopy methods (Fig 1-8) with new video charged-couple device (CCD) camera technology. As with most clever inventions, the idea was simple: link a fluorescent screen to a video camera using some optical coupling. This combination was contained in a rather bulky sensor. Despite its primitive form and limitations, this development started the process of bringing dental radiography towards the 21st century. Today, there are many manufacturers of digital radiography equipment and a number of underlying technologies. These will be the focus of much of this book, notably in Chapters 5, 6 and 7.

Fig 1-9 RadioVisioGraphy, the first digital dental radiography system, marketed in 1989.

Progress in Techniques

In the 1890s, dental radiography was a case of placing the glass plate or film where it would fit reasonably comfortably, placing the patient close to the x-ray tube, and hoping for the best while turning on the electrical supply. This method certainly produced images of teeth, but by no means could these images by described by our modern terminology of bitewing or periapical radiographs.

Table of Contents

21st Century Imaging

Authors:

Keith Horner

Nicholas Drage

David Brettle

Editors:

Nairn H F Wilson

Keith Horner

Quintessence Publishing Co. Ltd.

London, Berlin, Chicago, Paris, Milan, Barcelona, Istanbul, São Paulo, Tokyo, New Delhi, Moscow, Prague, Warsaw

Foreword

Preface

Acknowledgements

Chapter 1

The Historical Perspective

Aim

Outcome

The First Steps

Progress in X-ray Equipment

Progress in Image Receptors

Progress in Techniques