Efficacy of digital intra-oral radiography in clinical dentistry

PII:

Journal of Dentistry, Vol. 25, Nos 34, pp. 215-224, 1997 Copyright 0 1997 Elsevier Science Ltd. All rights reserved

SO300-5712(96)00026-7

Printed in Great Britain 0300-5712/97$17.00+0.00

ELSEVIER

Review

Efficacy of digital intra-oral clinical dentistry

radiography

in

C. H. Versteeg, G. C. H. Sanderink and P. F. van der Stelt Department

of Oral Radiology

Academic

Cenfre for Dentistry, Amsterdam,

The Netherlands

ABSTRACT Objectives: This article emphasizes the comparison of intra-oral digital imaging to film-based imaging. Additional possibilities of digital imaging that may contribute to system efficacy are discussed as well. study selection: The main subjects for research in digital imaging are image quality, image acquisition, diagnostic quality, image manipulation, automated analysis, and application software. Data sources: Representative articles on these subjects from the international literature are used for this review. Indirect digital imaging still requires film processing, sophisticated film digitizers, and time to digitize film. Although it is not an efficient method for the dental practice, digitization can be very useful for quantitative analysis of radiographs. Direct digital imaging is more efficient than indirect digital imaging. The main advantages are (semi) real time imaging, low X-ray dose requirements, and no need for chemical processing. In spite of a more limited resolution of the images, direct imaging may perform as accurately as film-based imaging. Direct image plate systems can well be used, for instance, for full-mouth series. The main application of direct sensor systems appears to be endodontology and implantology. In summary, direct digital imaging may be as efficient as film-based imaging in clinical dentistry. The computer provides for many additional options in digital imaging, such as the digital storage, compression, and exchange of radiographic information. Image manipulation (e.g. image enhancement, subtraction radiography and image reconstruction) and automated analysis may benefit radiodiagnosis. Conclusion; It can be concluded that digital imaging certainly has great potential, especially with respect to improvement of diagnostic quality and automated image analysis. 0 Elsevier Science Ltd. All rights reserved KEY WORDS:

Review, Efficacy, Digital radiography,

J. Dent. 1997; 25: 215-224

(Received

Dentistry

13 April 1995; accepted 26 February

INTRODUCTION Film has been an inexpensive and reliable image receptor in dental radiography for a long time. This may be the reason why attempts to introduce alternatives to (non-screen) dental film never were as successfuli4. However, the challenge to find alternatives is compreCorrespondence should be addressed to: Dr C. H. Versteeg, ACTA, Louvesweg 1, 1066 EA Amsterdam, The Netherlands.

1996)

hensible because of some disadvantages of film-based techniques. These disadvantages include the time required to process the film interrupting the dental treatment, and variability of image quality associated with chemical film processing. In addition, film processing and storage require much space, and manipulation of film-based images is impossible after processing. For some years now it has been possible to digitally acquire, manipulate, store, retrieve and exchange radiographic information, and in many aspects this is a

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Table 1. Variables for analysis of differences and film-based imaging

between digital imaging

Economy

Diagnosis

Imaging technique

Image acquisition Image receptor Feasibility Safety/dose System management Time Room/archive Environment costs

Image quality Size Resolution Diagnostic quality

Digital image management

Application software Algorithms Integration Image storagekompressron Telecommunication Confidentiality

Image manipulation Automated analysis

Variables regarding imaging technique can also be used for comparison. Digital image management, however, creates additional, uncomparable possibilities.

major technical development. Since the breakthrough of dental digital imaging, many systemsand application software packages have been developed. The computer, which is an indispensable device in digital imaging, provides many additional possibilities in dental radiography. This implies that comparison of digital imaging with film-based imaging in fact is complicated; the surplus value of computer and software may overshadow the radiographic value. Table I shows variables for analysis of differences between digital imaging and film-based imaging. The two imaging techniques can well be compared, whereas digital image management creates additional, incomparable options. Many studies and review articles discuss digital imaging by simply describing a new system or applications, without stressing the comparison to conventional film-based imaging5-9. The purpose of the present article is to emphasize the comparison of the two imaging techniques, and to discuss additional possibilities of digital imaging as well. The following order is used: image quality, image acquisition and diagnostic quality, image manipulation and automated analysis, and application software.

IMAGE QUALITY Any image used for diagnostic purposes, requires optimal quality. In radiology, the image quality of film as well as of digital images is described in terms of physical properties, such as contrast, brightness and resolution. Contrast and brightness differ for different conventional films, processed under optimal conditions. Improper processing procedures, as regularly occur in general practice, can change the contrast and brightness

considerably. In particular, exhausted, diluted, or cold developer liquids deteriorate image quality”. The advantage of digital images over conventional film is that contrast and brightness can (automatically) be adjusted. In case of underexposure, the contrast of a digital image can be raised to proper blackening, but the image thus treated will be noisier due to the lower exposure dose used. Conversely, an overexposed image can be made lighter. In this case, however, the areas of the image that correspond to maximum irradiation, for instance, interproximal spaces,may display the typical blooming effect due to saturation. However, under- or overexposed images may be improved or, to some extent, corrected after exposure’ ‘. In clinical use, the smallest detectable object is a function of resolution, density and contrast, angulation, shape of the object, and whether the object is superimposed on high density structures such as tooth roots or restorations. Resolution is the ability to distinguish between small objects that are close to another. A digital image consists of pixels (picture elements). In digital radiology, the number of grey levels (contrast resolution), and size of the pixel (spatial resolution) determine the resolution of the digital image. Each pixel in a digital image is represented by a number corresponding to its grey level (shade of grey). In other words, each number corresponds to one small area of the visual image, and the number gives the level of darkness or brightness of the area. The number of shades of grey (dynamic range) normally used is 256. This implies that each pixel is coded in the computer as a byte, which is 8 bit (2’=256). The darkest grey (black) is assigned zero, and the lightest grey (white) is assigned 255. Other formats are possible, and vary somewhat depending on the computer architecture. Twelve, 16 or 32 bit per pixel allow a greater range of pixel values, but also require more computer memory to store the image. Most commercially available systems for digital radiography in dentistry use 8 bit contrast resolution. The spatial resolution of the system must equal twice the spatial frequency of the smallest details to be detected (Nyquist frequency). For instance, if the smallest detectable object has a diameter of 100 urn, then a spatial resolution of at least 50 urn is required to resolve the object unambiguously. The spatial resolution of digital images depends on the way the digital image is acquired, but is not as high as film radiographs, in any case. Figure 2 shows similar digital images with substantial variation of resolution. The smallest detectable object is mainly a function of the spatial resolution, with some interaction from the grey level resolution, whereas the minimum differential grey level depends on the grey level resolution, with second-order effects from the spatial resolution. Resolution can also be expressed as line pairs per millimetre visible (lp/mm). All dental films currently

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et a/.: Efficacy of digital intra-oral radiography

217

Fig, 7. (A) Digital image. Pixels (picture elements) are arranged in a matrix of rows and colums. In this image, the matrix size is 385x576 pixels (spatial resolution). The image contains 210 shades of grey (contrast resolution).(B) Similar image; lower spatial resolution. Rows of pixels are arranged in a matrix of 30x44 pixels. Number of grey shades is 210.(C) Similar image; lower contrast resolution (three shades of grey). Matrix size is 385x576 pixels.

available have a resolution of approximately 12 lp/mm, even the fastest speedgroup E films”. Most direct digital systems are in the range of 7-10 lp/mn~‘~~‘~. It

probably depends on the diagnostic task under consideration if this is adequate for diagnosis and analysis of dental disease.

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IMAGE ACQUISITION QUALITY

AND DIAGNOSTIC

Indirect systems Indirect acquisition of a digital image is the conversion of a film radiograph to a digital image (digitization). The input signal for the conversion is obtained from a video camera or an image scanner. Digitization does not increase the information available over that contained in the original radiograph. It only turns the analog image into a form that can be read and analysed by the computer (A/D conversion). An important obstacle in this respect is the considerably smaller optical density range in digitized images when compared with conventional film. Hence, some possibly valuable diagnostic information is lost during digitization, and disturbing information or noise will be introduced. In addition, digitization may take a rather long time, and conventional film processing is still required. However, many possibilities for digital contrast enhancement and other image processing features exist after digitization. Accordingly, digitization is useful for quantitative analysis of radiographs. Digital processing features will be discussed in the digital image manipulation section. The most commonly used digitizers are video cameras, and image scanners. With video cameras, the radiograph is placed on a light box which is focused directly onto a photosensitive tube, or a charge-coupled device (CCD) sensor. For intra-oral radiographs, digitization cards (frame grabbers) are usually defined at a 512 x 5 12 x 8 bit or 512 x 480 x 8 bit matrix resolution (512x 512 or 512~480pixels; 256 shades of grey). Video cameras cannot provide sufficient improvement in spatial resolution to have a significant impact on the diagnostic accuracy of conventional dental radiographs. In addition, distorted digital images emanate from the lens system, non-uniform photosensor responses, and light-source variations, as well as electronic noise derived from the video camera and A/D converter systemsl5. Whereas video-based digitizing systemsare cheap, high-quality scientific-grade cameras and video framegrabbers (10 bit or better) are required to eliminate potential artifacts. A video system was tested by Hildebolt l6 for its suitability in quantitative studies of periodontal disease. He reported that the video system (DAGE-MT1 Series 68, 512 X 512 pixels, 8 bit) appears to be satisfactory for measurements of disease extent in alveolar bone, but only marginally acceptable for the detection of subtle changes in radiodensities as signs of diseaseactivity. A film scan system, including a CCD linear photo diode array slide scanner, is perfectly suitable for digitizing intra-oral film because of the close correspondence in size between slide film and intra-oral dental film. 111film scanners, light transmitted through a radiograph is detected by photosensors (photodiodes or CCDs) and converted to digital signals. Film scanners

offer some improvements over video cameras, although there are concerns about the flat exposure latitude curves at densities of 1 0.D.17. In a study by Shrout et al.‘*, a slide scanner (Nikon LS-3510AF, 8 bit) is compared with a video system (DAGE-MT1 Series 68, 512 x 512 pixels, 8 bits), and film scan system (Barneyscan, 1520 x 1024pixels, 8 bit). Image quality was evaluated in terms of resolution, sharpness, contrast, distortion and noise. It was found that the scanner offers substantial advantage over the other two digitizing systems for grey scale information from clinically important optical densities. Laser scanners are the most likely systemsto digitize radiographs in the dental office, although there are difficulties when applying laser scanners to small dental films”. These systems basically comprise collimated low-power (5 mW) laser beams to scan radiographs in a light-tight environment. Precise laser scanners for dental film are in the developmental stage, and not yet available for the dental office.

DIRECT IMAGE PLATE SYSTEMS Direct image plate systems include a phosphor photo stimulation screen. The (reusable) screen stores the photon energy when hit by an X-ray beam, and emits light when it is scanned by UV light. After the light is measured by a scanner, the measurementsare displayed on the monitor, and stored in the computer as a digital image. Direct image plate systemsinclude many advantages over film-based systems. The primary advantages of storage phosphors are a wide dynamic range because of automatic exposure control, and low dose requirements. The amount of energy stored in the image plate is linearly proportional to the X-ray exposure. The linearity is maintained throughout the entire dose range, which means that the image plate can hardly be over- or underexposed2’. In this sense,the image plate differs from both conventional film and direct sensors, which are much more vulnerable to changes in the exposure dose. The primary disadvantage is the fact that time (30 s) is required to read out the images. Nevertheless, the reading does not take as much time as film processing, and neither darkroom nor chemicals are needed. So far, one intra-oral image plate system has been introduced: the Digora (Soredex, Finland)**. Two sizes image plates (comparable with film size #O and #2) are available. Familiar X-ray equipment is used, the only adjustments required are the settings of patient dose, and calibration of the Digora system. The laser beam is focused into a spot with a diameter of 70 urn, and directed on the surface of the plate. In a study of dose requirements, it was found that images exposed with 10% of the dose required for E speedfilm still give a satisfactory image quality, when determining endodontic file length using file No. 202’. Intra-oral phosphor plate imaging seemsvery promising, although

Versteeg

Table II. Dimensions of sensitive area, image size and effective pixel size of intra-oral image receptors Dimensions Receptor

tmm)

Dental film size #O Dental film size #I Dental film size #2 Digora CDR Flash Dent RadioVisioGraphy Sens-A-Ray Sidexis Visualix

22x35 24x40 31x41 31x41 26x37 20x24 19x28 18x26 18x30 18x24

Image size (pixek)

Pixel size (effective; pm)

-

-

-

-

416x560 525x760 400x480 380x480 385x576 410x658 288x384

71x71 48x48 50x50 50x58 45x45 45x45 63x63

further research regarding diagnostic quality of images and system economy is necessary on behalf of system efI?cacyg,23.Comparative data with other imaging devices are presented in Table II.

DIRECT SENSOR

SYSTEMS

Direct sensor systems include a CCD sensor, a processor unit, a digital interface card, computer and software, Current systems require an IBM 386 PC (or higher) with a minimum of 640 kb internal memory, and should be equipped with a Super VGA graphics card and a high resolution monitor. Some systems are supplied with a dedicated electronic timer to synchronize the X-ray production and the image acquisition. Other systems are not hard-wired to the X-ray equipment: the sensor automatically starts the image acquisition when it detects an increase of the (background) radiation level. The digital image resolution is restricted by the limited number of pixels that can be grouped together in the CCD sensor. As a result, the spatial and grey-scale resolution cannot exceed the accuracy of conventional film-based images. Direct sensor systems are capable of ‘real time’ imaging; an image will be displayed on the monitor in a few seconds. The systems are built around a CCD sensor. CCDs are arrays of X-ray- or light-sensitive pixels. Intra-oral CCD sensors currently fall into two categories. Fibre and lens optically coupled sensors (e.g. RadioVisioGraphy (RVG), Trophy, France; Flash Dent, Villa, Italy) use a scintillation (intensifying) screen, coupled to a conventional CCD by optical fibres or a series of lenses. The fluorescence of the intensifying screen due to incident X-radiation is conducted by fibres or lenses to the CCD, which converts this energy into a voltage potential. The CCD is thus protected against radiation, and indirectly exposed. An RVG image, for instance, taken from the intensifying screen, is a relatively high contrast, and low noise image. As a result of using optical fibres, the image can be slightly

et

al.: Efficacy of digital intra-oral radiography

219

distorted at the borders. However, this is hardly visible under clinical circumstances. The other type of sensor is directly exposed and captures the image directly (Sens-A-Ray, Regam, Sweden; Visualix, Gendex, Italy; Sidexis, Siemens, Germany; CDR, Schick Technologies Inc., USA). This indicates that a thinner sensor box can be used. The Sens-A-Ray sensor has been examined by Nelvig12. He found that the CCD sensor survives at least 30000 exposures. Some of these sensors use an intensifying screen, which reduces the noise level. For instance, the surface of the Visualix sensor contains a layer of scintillator phosphor24. Compared with fibre-optically coupled sensors, the Visualix sensor still is relatively thin. Dimensions of sensitive receptor areas, digital image sizes, and effective pixel sizes (dimensions of sensitive area divided by number of pixels) of intra-oral image receptors are shown in Table II. For accurate diagnostics in dental radiography, the signal to be detected should well exceed the noise inherent in the radiographic system. This system noise is partly the result of the discrete nature of photons in the X-ray beam. A paramount difference between X-ray imaging with conventional film, and with electronical sensors is related to the higher sensitivity to X-rays of the latter. The imparted dose required by direct digital systemsis much less than that of conventional film. At these low dose conditions, one of the most important parameters to assessis the signal-to-noise ratio (SNR). The SNR can be defined as the ratio between the fraction of output variable (voltage, or current, or charge) which is unequivocally related to the diagnostic information (the signal) and the fraction of output variable which is not bearing diagnostic information (the noise). Noise increases with increasing exposure time; once a sensor is saturated, any additional radiation dose will reduce the SNR. In principle, more sensitive sensorswill result in lower SNRs. In addition, the maximum useful dynamic range (number of grey shades) is limited by the SNR of an image. Moreover, the SNR is particularly significant becausenoise is more evident than in films, due to the enlargement with which images are displayed on screen25.In a study concerning image quality, Wenze126compared the level of system noise in current direct digital systems (RVG, Sens-aRay and Visualix). In the study, the fastest system (Visualix) provided the most noisy images. The dose response of a digital sensor is linear, that is, the electronic charge created in each pixel is directly proportional to the integrated light or X-ray intensity (Fig. 2). Grey levels of digital systems and film density are not equivalent, and can therefore not be compared. To facilitate comparison between the dose response function of the sensor and the corresponding function for a film or the characteristic curve of film, Fig. 3 shows a dose response function for Kodak Ektaspeed dental film. The exposure range or latitude of the sensors is smaller than that of film. A direct sensor is

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;rev values

I- RVG-PC 2- Sens-A-Ray B+Visualix 4-Flash Dent HR

100

200

300

4;0

Exposure (pGy) Fig. 2. Dose response curves of four digital sensors.

of 0

100

200

300

400

Exposure &Gy) Fig. 3. Dose response curve of Kodak Ektaspeed

film.

more sensitive to X-ray radiation and, consequently, reaches full saturation at a lower exposure than film. Since sensors are smaller than dental film, the area of interest may not be completely visible, requiring more exposures in the samearea. Kantorowitz et al.‘7 showed that the minimal exposure time (MET) of a full-mouth periapical series with direct sensor systems (RVG, Sens-a-Ray) is lower than for D-speed film. When the X-ray beam is adapted to the size of the sensor, there will be an even larger reduction of the patient dose. When maximum image resolution is essential, a higher exposure is required, and the dose advantages over conventional radiography become less significant5. It is not clear, therefore, how large the dose reduction will be when films are replaced by digital sensors. Diagnostic quality of direct sensor systems has been investigated to a certain extent. Shearer et aL2*, for instance, found that RVG was of equal value to conventional film for imaging root canal systems. The Digora, RVG and Sens-a-Ray systemshave been compared with Ektaspeed films in a study by Sanderink et al. 29. Accuracy of endodontic file length determination was chosen as a means of testing image quality. His group reported that the Digora, RVG and Sens-aRay systems render a comparable result with conventional radiography using size 15 files. For occlusal and

approximal caries, studies have demonstrated that the RVG provides comparable diagnostic accuracy to bitewing E-speed and D-speed films30-33. Diagnostic accuracy in the detection of simulated periodontal bone lesions using film and electronic sensor was investigated in another study34. It was demonstrated that the Sensa-Ray system rendered comparable results with Dspeed and E-speed film in detection of the periodontal lesions. From these findings it can be concluded that in spite of the more limited resolution, direct digital imaging seems to perform as accurately as conventional film radiography. Primary advantages are real-time imaging, low X-ray dose, and no need for chemical film processing. Disadvantages are the small sensor sizes, and the high initial costs.

IMAGE MANIPULATION AND AUTOMATED ANALYSIS A distinct difference between film-based radiography and digital radiography is that the latter facilitates image processing of existing image data. The purpose of digital image manipulation or processing is to selectively present the information that is diagnostically useful, and to suppress or discard the rest. As mentioned earlier, the inherent information content of a digital image cannot be increased by mere image processing. Digital image processing includes digital image enhancement, subtraction radiography, automated image analysis, and image reconstruction. When an image is in digital mode, contrast may be enhanced numerically by pixel value mapping methods, so-called histogram-based modifications, which are techniques to scale and transform the input pixel values. The contrast and density level can easily be adjusted to each diagnostic task by digital image manipulation. Accordingly, an important feature of digital radiography is that the same image may be used for various diagnostic tasks, for instance marginal bone loss, for which radiographs should be lighter,

Versteeg et a/.: Efficacy of digital intra-oral radiography

and caries, requiring darker images. Low-pass spatial filtering (smoothing) reduces the image noise, at the expense of a decrease in resolution. High-pass spatial filtering (hardening) enhances edges thus returning a crisper image, but with more noise. It facilitates the detection of boundaries of low contrast regions. Several studies have evaluated the effects of image enhancement algorithms35-37. It seems that digital systems should contain task-dependent image processing routines, but this proves to be very difficult to achieve38939. Subtraction radiography is an example of a utility used for quantitative and qualitative analysis of standardized images4’. It requires a pair of images taken over some time interval with identical projection geometry4*. Subtraction is based on the assumption that so-called ‘anatomic noise’ can be eliminated through subtraction of identical structures in two radiographs to be compared. Anatomical structures that have not changed between examinations are cancelled out and shown as a neutral grey in the radiographic image. Small tissue changes not readily detectable by direct comparison of the radiographs may thus appear in the subtraction image, or processed subtraction image. The usefuhress of subtraction techniques in clinical trials depends on the reproducibility of the radiographic recordings, and on the efficacy of the subtraction program to restore such variations. Computerized systems used for geometric image standardization are under development. These systems may provide computer controlled positioning of the patient to obtain the required projection geometry5. Studies on digital subtraction for diagnosis and analysis of the periodontium reported good results4246. Conventional radiographs are diagnostically insensitive for alveolar bone lesions, since a 30-50% mineral loss is required for their delineation44. Investigations have clearly shown that the removal of structural noise produces a marked increase in the detectability of a small substance loss; only a 5% loss is required for delineation of alveolar bone lesions42. Studies on detectability of changes in dental hard tissues have been reported, with contradictory observations4749. Accordingly, the major benefit of subtraction for bone may not apply to ename149. Another option for image manipulation, which is not directly available in film-based radiography, is image reconstruction. It includes the construction of one or more new images, based on the information contained in a set of basis images. Three-dimensional reconstruction of radiographic images can be of primary importance for the diagnosis and treatment planning in malformations, trauma, tumour investigation and surgery planning5G52. CT reconstruction software, for instance, is used for acquisition of 3-D images. The 3-D-CT image is made up of a series of slices, each of which represents the anatomy of a layer of tissue, with a slice thickness chosen by the operator (given the technical limitations of the system). Jeffcoat described a low-cost imaging workstation with a dental implant

221

treatment tool. Structures on any slice can be viewed from the frontal, lateral or occlusal aspect. Root from dental implants can be placed on screen and the mandibular curve can be defined using commercial CT reconstruction software (3-D/Dental Software). Bianchi54 notes that the method he studied for acquisition of animated 3-D models by means of a low-cost personal computer still takes rather long computing time, and requires much user interaction. Interpretation of digital images may be improved by automated image analysis55p57. Rather than recognizing the manifestations of a disease entity, the computer assistedanalysis system recognizes patterns that may be indicative for the manifestation of a diseaseentity. Prior knowledge of the size, shape and location of anatomical structures and pathognomonic features is very useful for improving the process of computer-aided image analysis58.A high degree of automation is required for routine use in general practice; the measuring process should be simple and fast.

APPLICATION

SOFTWARE

Algorithms The currently available direct digital systems and software packages are not yet able to perform all image processing routines mentioned above. Instead, less sophisticated algorithms often included are: automatic or interactive contrast and density control, measurement of density and histogram of density values, spatial image filtering (noise reduction and edge enhancement), pseudocolour and negative display, zoom on an area of interest, and electronic millimetre grids and rulers. Increasing brightness is a straightforward image improvement. Contrast enhancement and contrast stretching make low contrast areas more visible. Measurement of densities, and display of the histogram of an image may be very useful. A digital image, stored as a number of pixels and grey values, can be read as such by the user of the system. Thus, the distribution and mean of grey values can be used to determine correct exposure times for acceptable images. Spatial image filtration can be used to enhance small details. Whether and how much this kind of processing can improve the actual diagnostic potential is still a matter of debate, but it is probably task-dependent and observer-dependent. The pseudocolour, negative and zoom mode still need to be studied carefully. Pseudocolour display of density changes might improve the inter- and intraobserver agreement in estimating alveolar bone density changes43.Kiinzel had positive results with a certain colour scale modification59. Electronic millimetre grids and rulers that may be included in application software do not correct for differences in magnification (geometrical enlargement). However, electronic measurement of directly acquired images is free from errors caused by bending of film.

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Integration An important development is integration of software. Only if systems are developed with graphical user interfaces which are easy to use and simple in design, will the use of direct digital systems grow. Application software, developed to work in the Microsoft Windows environment (e.g. Windows 93, provides for a common graphical user interface and operating system infrastructure, to develop integration and communication. An integrated radiography system has the capability to act as one of the many components of an on-screen electronic patient record. Examples of specially designed software are the Image Management, Archive and Communication System (IMACS)60 and Emago6’. In the final stage, these software packages will be capable of integrating different direct digital image acquisition modalities such as intraoral, panoramic and extra-oral radiography. Emago provides the dentist with a convenient tool for the handling and manipulation of digital images obtained from direct intra-oral sensors. This software package includes advanced image processing algorithms and communication facilities. In the near future, all application software will probably be integrated into a complete computer-based patient record and database.

DISCUSSION For analysis of efficacy of digital intra-oral radiography, system economy is an important aspect. Digitizing film requires high-quality equipment, and cost-effective indirect systems are not yet available for the dental office. For that matter, a central digitizing service can be helpful. Direct digital systemsseemmore efficient for the dental practice. Before the dentist even considers buying a direct system, questions will come to mind, such as: ‘Do I have enough room and where should I put this system?‘, ‘Is it convenient and economically viable?‘, and ‘How should the system be serviced?’Even with the costs of computers declining, the dentist must make a substantial investment in time, energy and money(j2. The two direct digital devices, the sensor system and image plate system, are different in several ways. The advantages of direct image plate systems over direct sensor systemsinclude a thin, wireless and bigger screen (two sizes), accurate bite-wing and posterior region exposures, and outstanding dynamics. Thus, these systems can well be used, for instance, for full-mouth series.When using a direct sensor system, the size of the sensor compared with the small effective detector area is important to consider. Because of the small detector area, the chances of obtaining too little information increase. More exposures will be necessary to provide the same diagnostic information as in conventional size #2 dental film. Because of the real time character, and the dose advantages, the practitioner might be tempted

to take more images. It is clear that correct positioning of the sensor is more critical as compared with film. Fortunately, most direct systemsinclude a sensor positioner. Aiming devices help to reduce errors, although in endodontics a biteblock may interfere too much with the endodontic file. An indicator ring helps position the X-ray tube accurately and enables stronger collimation beams. The main fields of application of direct sensor systems appear to be endodontology and implantology. The interpretation of digital images differs from interpretation of conventional film. Digital images, displayed on screen, are often scaled much larger than conventional film images, due to monitor screen resolution, and digital image file sizes. Dental digital images may best be displayed on a S-VGA or high resolution monitor (e.g. 1024x 768 pixel, 256 colours). Lower resolution monitors are not capable of showing all pixels and grey values of the digital image. The best way to perform diagnosis of a digital image is on screen in a darkened glare-free environment, with a downward gaze, an appropriate image blackening and contrast, and a viewing distance between 50 and 70 cm63. Storage of digital intra-oral images requires a great deal of disk memory capacity. A low-cost personal computer may not always be capable to store online all information that the dentist needs to have accessto in seconds. In addition, reliable image information can be acquired if unerasable storage devices are used. Accordingly, it should be noted that erasable storage devices include the risk of undesirable data corruption. Compression of files is a way to reduce the amount of storage space required. A high factor of compression automatically indicates low speed of the compression process. This is an important issue, especially in clinical settings. Compression of digital images definitely introduces legal questions. It needs to be studied more thoroughly in the near future, before compression techniques can be applied on a routine base. It is advisable to wait for clarity in the legal field. Telecommunication enables the general practitioner to acquire a second opinion or advise about radiographic information. It also facilitates communication with insurance companies. Thus, it can be very helpful and time saving: the procedure only takes minutes or seconds,without loss of quality. It is clear that sending images does not automatically imply less confidentiality concerning the patient. Very important, on the other hand, is the instant trust and credibility gained by displaying a large image that the patient can see and understand, using a ‘TV’ picture. In conclusion, digital intra-oral radiography seems to have good potential. Improvement of image quality by image manipulation, and automated analysis of digital images may contribute to better radiodiagnosis. Direct digital imaging is an efficient technique, in both economic and diagnostic sense.However, this imaging

Versteeg et a/.: Efficacy of digital intra-oral radiography

technique, like any other, needs to be studied continuously, with the emphasis on safety of patients and diagnostic quality of the images.

References 1.

2.

3.

4. 5. 6 I.

Gratt BM, Sickles EA, Gould RM, Jeromin LS, White SC. Xeroradiography of dental structures. IV. Image properties of a dedicated intra-oral system. Oval Sung 1980; 50: 572-579. Stephens RG, Kogon SL, Reid JA. An investigation of potential applications of intensifying screensin intra-oral radiography. Oral Surg 1982; 54: 591-596. Kogon SL, Stephens RG, Reid JA. A clinical trial of rare earth screen/film system in a periapical cassette.Oval Surg 1984; 57: 455461. Wilson NFH, Grant AA. A clinical trial of dental xeroradiography. Br Dent J 1986; 161: 327-335. Webber RL. Computers in dental radiography: a scenario for the future. JADA 1985; 111: 419424. Stelt van der PF. Improved diagnosis with digital radiography. Curr Opin Dent 1992; 2IV: l-6. Zubery Y. Computerized image analysis in dentistry: present status and future applications. Compend Contin

19.

maxillofac

21.

Digora brochure. Soredex. Finland: Orion Corporation, 1994. Velders XL, Sanderink GCH, van der Stelt PF. The effect of different exposure times on the detectability of endodontic files in the new Digora indirect digital intra-oral X-ray system. Proceedings 10th Congress of the IADMFR. Seoul, 25 June-l July 1994; p 73. Matsuda U, Okano T, Igeta A, Seki K. Effects of exposure reduction on the accuracy of an intraoral photostimulable-phosphor imaging system in detecting incipient proximal caries. Oral Radio1 1995; 11: 11-16. Molteni R. Direct digital dental X-ray imaging with VisualixNIXA. Oral Surg Oral Med Oral Path01 1993;76: 235-243. Welander U, Nelvig P, Tronje G. Basic technical properties of a system for direct acquisition of digital intraoral radiographs. Oral Surg Oral Med Oral Path01 1993; 75: 506-516. Wenzel A. Sensor noise in direct digital imaging (the RadioVisioGraphy, Sens-a-Ray, and VisualixNixa systems) evaluated by subtraction radiography. Oral Surg Oral Med Oral Path01 1994; 77: 70-74. Kantorowitz Z, Mol A, Black PM, Malmstrom HS, Handelman SL. Preliminary evaluation of two direct digital radiographic systems. J Dent Res 1993; 73

1995; 74 (Abstract): 463.

22.

23.

24.

25.

26.

ogy. Oral Surg Oral A4ed Oral Path01 Oral Radio1 Endod

Dent Res 1988; 96: 149-160. 12.

Nelvig P, Wing K, Welander U. Sens-A-Ray. A new system for direct intraoral radiography. Oral Surg Oral

13.

Welander U, McDavid WD, Sanderink GCH, Tronje G, Morner AC, Dove SB. Resolution as defined by line spread and modulation transfer functions for four digital intraoral radiographic systems. Oral Surg Oral Med Oral Path01 1994; 78: 109-l 15. Kiinzel A: Benz C. Flash-Dent, Radio-Visio-Graphie und Visualix: Drei digitale dentale rontgen-systeme im vergleich. Dtsch Zahniirztl Z 1995; 50: 303-307. Craine ER, Forbes S, Scott JS. Very high resolution fast image digitization. In: Schneider RH, Dwyer SJ, eds. Medicine XIVIPACS IV Proceedings SPIE. Bellingham, Washington: SPIE 1986; Vol 626, pp 386-391. Hildebolt CF, Vannier MW, Pilgram TK, Shrout MK. Quantitative evaluation of digital dental radiograph imaging systems. Oral Surg Oral Med Oral Path01 1990; 70: 661-668. Shrout MK, Hall JM, Hildebolt CF. Differentiation of periapical granulomas and radicular cysts by digital radiometric analysis. Oral Surg Oral Med Oral Path01 1993; 76:

Med Oral Path01 1992; 74: 818-823.

14.

15.

16.

11.

356-361. 18.

Shrout MK, Potter BJ, Yurgalavage MH, Hildebolt CF, Vannier MW. 35-mm Film scanner as an intraoral dental radiograph digitizer. I: a quantitative evaluation. Ora/ Surg Oral Med Oral Path01 1993; 76: 502-509.

Radio1 1994; 23: 183-191.

Lim KF, Loh EE-M, Hong HY. Quantitative assessment of a new intra-oral digital imaging system. J Dent Res

Vandre RH, Webber RL. Future trends in dental radiol-

1995; 80: 471478. 9. Wenzel A, Griindahl H-G. Direct digital radiography in the dental office. I Dent J 1995; 45: 27-34. 10. Sanderink GCH. Imaging: new versus traditional technological aids. I Dent J 1993; 43: 3355342. 11. Wenzel A. Effect of image enhancement for detectability of bone lesions in digitized intraoral radiographs. Stand J

Lavelle CLB, Wu CJ. When will excellent radiographic images be available to the general dental office? Dento-

20.

Educ Dent 1992; 11: 964-973. 8

223

27.

(Abstract): 268.

Shearer AC, Horner K, Wilson NHF. Radiovisiography for imaging root canals; an in vitro comparison with conventional radiography. Quintessence Int 1990;21: 789% 794. 29 Sanderink GCH, van der Stelt PF, Velders XL. Image quality of a new indirect digital intraoral X-ray sensor system. The Digora system compared to direct digital systems and film in assessingroot canal length. Proceedings 10th Congress of the IADMFR. Seoul, 25 June-l July 1994; p 117. 30. Wenzel A, Hintze H, Mikkelsen L, Mouyen F. Radiographic detection of occlusal caries in noncavitated teeth. A comparison of conventional film radiographs, digitized film radiographs, and RadioVisioGraphy. Oral Surg Oral Med Oral Path01 1991; 72: 621-626. 31. Russell M, Pitts NB. Occlusal caries diagnosis: radiovisiography versus bitewing radiography. Caries Res 1991;

28

25 (Abstract): 217. 32.

Pitts NB, Russell M. Radiographic diagnosis of approxima1caries in vitro: different modalities of third-generation ‘Radiovisiography’ versus E-speed bite-wings. Caries Res 1993; 27 (Abstract): 206.

Hintze H, Wenzel A, Jones C. In vitro comparison of Dand E-speed film radiography, RVG, and Visualix digital radiography for the detection of enamel approximal and dentinal occlusal caries lesions. Caries Res 1994; 28: 363-367. 34. Furkart AJ, Dove SB, McDavid WD, Nummikoski P, Matteson S. Direct digital radiography for the detection of periodontal bone lesions. Oral Surg Oral Med Oral 33.

Path01 1992; 74: 652-660.

224

J.

Dent. 1997;25:

Nos 3-4

35. Wenzel A, Fejerskov 0, Kidd E, Joyston-Bechal S, Groeneveld A. Depth of occlusal caries assessedclinically, by conventional film radiographs, and by digitized, processedradiographs. Caries Res 1990; 24: 327-333. 36. Wenzel A, Larsen MJ, Fejerskov 0. Detection of occlusal caries without cavitation by visual inspection, film radiographs, xeroradiographs, and digitized radiographs. Caries Res 1991; 25: 365-371. 37. Verdonschot EH, Kuijpers JMC, Polder BJ, De LengWorm MH, Bronkhorst EM. Effects of digital grey-scale modification on the diagnosis of small approximal carious lesions. J Dent 1992; 20: 4449. 38. Wenzel A, Pitts N, Verdonschot EH, Kalsbeek H. Developments in radiographic caries diagnosis. J Dent 1993;21: 131-140. 39. Mol A, Black PM. Direct digita image characteristics by region of interest, J Dent Res 1995;74 (Abstract): 463. 40. Griindahl H-G, Griindahl K, Webber RL. A digital subtraction technique for dental radiography. Oral Surg Oral Med Oral Path01 1983; 55: 96-102. 41. Jeffcoat MK, Reddy MS, Webber RL, Williams RC, Ruttiman UE. Extraoral control of geometry for digital subtraction radiography. J Periodont Res 1987; 22: 3966 402.

Hausmann E, Christersson L, Dunford R, Wiskesjo U, Phylo J, Genco RJ. Usefulness of subtraction radiography in the evaluation of periodontal therapy. J Periodont 1985; 56 (Suppl. 11): 4-7. 43. Bragger U, Pasquali L. Color conversion of alveolar bone density changes in digital subtraction images. J Clin Periodont 1989; 16: 209-214. 44. Okano T, Mera T, Ohki M, Ishikawa I, Yamada N. Digital subtraction of radiographs in evaluating alveolar bone changes after initial periodontal therapy. Oral Surg Oral Med Oral Path01 1990; 69: 258-262. 45. Jeffcoat MK, Page R, Reddy MS, et al. Use of digital radiography to demonstrate the potential of naproxen as an adjunct in the treatment of rapidly progressive periodontitis. J Periodont Res 1991; 26: 415421. 46. Reddy MS, Bruch JM, Jeffcoat MK, Williams RC. Contrast enhancement as an aid to interpretation in digital subtraction radiography. Oral Surg Oral Med Oral Path01 1991; 71: 763-769. 47. Grondahl H-G, Grondahl K, Okano T, Webber RL. Statistical contrast enhancement of subtraction images for radiographic caries diagnosis. Oral Surg Oral Med Oral Path01 1982; 53: 219-223. 48. Wenzel A, Halse A. Digital subtraction radiography after stannous fluoride treatment for occlusal caries diagnosis. Oral Surg Oral Med Oral Path01 1992; 74: 824-828. 49. Halse A, Espelid I, Tveit AB, White SC. Detection of mineral loss in approximal enamel by subtraction 42.

radiography. Oral Surg Oral Med Oral Path01 1994; 77; 177-182. 50. Vannier MW, Marsh JL, Warren JO. Three-dimensional CT reconstruction images for craniofacial surgical planning and evaluation. Radiology 1984; 150: 1799184. 51. Ono I, Ohura T, Narumi E, ef al. Three-dimensional analysis of craniofacial bones using three-dimensional computer tomography. J Craniomaxillofac Surg 1992; 20: 49-60. 52. Mooney MP, Siegel MI, Kimes KR, Tohdunter JS, Smith TD. Anterior paraseptal cartilage development in normal and cleft lip and palate human fetal specimens. Cleft Palate Craniofac J 1994; 31: 239-245. 53. Jeffcoat MK, Jeffcoat RL, Reddy MS. A dental implant treatment planning tool for low-cost imaging workstations. Annual International Conference of the IEEE Engineering in Medicine and Biology Society. 1991; Vol. 13, pp 3488349. 54. Bianchi SD, Tornincasa S, Morello S, Ramieri G. 3D reconstruction of 2D radiographic data on a low-cost personal computer. Dentomaxillofac Radio1 1992; 21 (Abstract): 22.5.

Pitts NB, Renson CE. Reproducibility of computer-aided image-analysis-derived estimates of the depth and area of radiolucencies in approximal enamel. JDent Res 1985;64: 1221-1224. 56. Mol A, van der Stelt PF. Digital image analysis for the description of periapical bone lesions. Int Endodont J 1989; 22: 299-302. 57. Heaven TJ, Firestone AR, Feagin FF. Computer-based image analysis of natural approximal caries on radiographic films. J Dent Res 1992; 71: 846-849. 58. Stelt van der PF. Inference systemsfor automated image analysis. Dentomaxillofac Radio1 1992; 21: 180-183. 59. Kiinzel A, Lehmann T, Benz C, Schmitt W, Kaser A. Colouring digital dental radiographs. Dentomaxillofac Radio1 1995; 24: 100-101. 60. Dove SB, McDavid WD, Welander U, Tronje G, Wilcox CD. Design and implementation of an image management and communications system (IMACS) for dentomaxillofacial radiology. Dentomaxillofac Radio1 1992; 21: 216221. 61. Stelt van der PF. The E-mago image processing environment for direct digital radiography in dentistry. Proceedings of the 15th Annual International Conference qf the IEEEIEMBS. San Diego, 28-31 Ott 1993; pp 1623-1624. 62. Maness WL. The future of diagnostic workstations. In: Preston JD, ed. Computers in Clinical Dentistry. Carol Stream, Illinois: Quintessence, 1993; pp 204-215. 63. Taptagaporn S, Saito S. Visual comfort in VDT operation: physiological resting states of the eye. Ind Health 1993; 31: 13-28. 55.