Table 4

Digital radiography 

Digital radiography is now possible with either charge-coupled devices (CCD) or phosphor imaging plates.  The equipment is connected to a personal computer (PC).  A charge-coupled device is an intra-oral silicon sensor, sensitive to X-rays, that is directly connected to a computer.  The image is displayed on the screen immediately after exposure and this is time saving because there is no "processing".  In the phosphor imaging system, the latent image on the plates has to be digitised.  This takes place in a processing unit connected to the computer and takes, depending on the system, 20 s to 2 minutes.  Both systems require lower doses of radiation compared with E-film.  By manufacturers reduction in exposure time of 20-60% (CCD systems) and approximately 50% (phosphor imaging plates) are claimed.  In daily practice it seems that the dose reduction is less then these just mentioned percentages.   Something that will be enhanced by the fact the dentists using CCD are taking more X-rays.  The bulky CCD sensors are usually smaller than conventional films and more exposures are needed to cover the same area compared to conventional radiography or phosphor imaging plates and more retakes are reported in connection with CCD sensors compared to conventional film (Versteeg et al., 1998).

The image quality could be as good as with conventional films but depends of the digital system used.  At the moment there are large variations in quality between the different systems.  In a recent study dentists consider the user friendliness of the handling of the two different digital systems before taking a radiograph as less than for the conventional film (Berkhout et al., in press).  The patient's comfort was also mentioned as unfriendly especially when the systems were used for children.  In the case of digital radiography, the elimination of the chemistry of film processing after taking the radiograph was considered an advantage.  Digital images are best viewed on a good computer screen and often loose quality when printed.  Such images are like any computer file and may be stored on disks and easily transferred to other computers.  In the future, "expert" systems may provide decision support based on automated image analysis (Firestone et al., 1998, White, 1999).  In conclusion, digital radiography has advantages over conventional radiography, but the bulky sensor systems with attached cable and the need for a computer are clinical inconveniences.  No studies concerning the use of digital radiography in children are available, but it seems likely that at present the advantages of these systems are cancelled out by the disadvantages such as acceptance of the sensor or phosphor plate by the child.  In future improvements of the devices can be expected, but for the moment, on balance, conventional films may be more suitable for young children. 

Extra-oral radiography 

Extra-oral radiography comprises the lateral oblique projection, dental panoramic radiography and cephalometry.  In all extra-oral techniques intensifying screens are used.  In specialized clinics for maxillo-facial radiography, advanced techniques like computer-tomography are commonly used.  In pediatric practice panoramic radiography is useful when a more complete evaluation of the patient's jaws and teeth are needed, but the image does not have such fine resolution as intra-oral radiography so quality of the image of the radiographed teeth is lower.  The dose is relatively low and the method is convenient to use.  It requires an exposure time of several seconds and uneasy patients may move during exposure.  Panoramic radiography is not indicated for general screening purposes.

If an intra-oral radiograph shows uncommon structures or findings that cannot be explained by normal anatomy or covered by a single exposure, the examination has to be supplemented by extra-oral radiography.  There is no reason to screen for jawbone lesions in healthy, asymptomatic children and adolescents (Matteson et al., 1991).

Principles for interpretation of radiographs 

Radiographs should be reviewed systematically and under proper viewing conditions.  An X-ray viewer with a magnifying lens and radiographs in a mount ensure that extraneous light is not transmitted to the eye.  Areas of special interest should be compared with previous radiographs if available.  The basis of judgment of pathology is knowledge and experience of normal anatomy and its variants. 

The principle of radiographic interpretation can be compared with any laboratory test in medicine or dentistry.  A perfect test should always be positive in presence of disease and negative in its absence.  Unfortunately in reality, tests are biased and two types of errors occur: Over-registration (false positives) and under-registration (false negatives).  The other two possible outcomes of a test are true positive and true negative findings.  Usually low disease prevalence increases the probability of false positive diagnoses and high prevalence increases the likelihood of false negative diagnoses.  There are many aspects that should be taken into consideration in the diagnostic process such as within and between observer variation, quality of radiographs, the validity of the 2 dimensional representation of a 3 dimensional object.  Many aspects related to the radiographic diagnosis of caries, is discussed by Gröndahl (Gröndahl, 1996) and this textbook chapter is recommended as supplementary reading.

Evidence-based guidelines?

The guidelines presented are based on the current dental literature combined with the clinical experience and judgement of the authors.  We have tried to find the best research evidence in the clinically relevant studies and clinical expertise.  There are some systematic attempts to search the literature.  The NIH Consensus Conference on caries (including diagnosis) held in Bethesda 2001 reviewed 1407 diagnostic studies (Agency for Healthcare Research and Quality, 2001), but it was concluded that there was limitations in terms of the number of studies, the number of methods tested, the numbers of teeth and surfaces studied, and weakness in the research designs of the studies (Coulter, 2001).  One of the panel's conclusions was that radiology has acceptable diagnostic efficacy in detecting larger cavitated lesions.  Regarding the future needs in the field of caries diagnosis the panel says (National Institutes of Health, 2001): "There is currently no diagnostic modality which can differentiate between microbiologically active caries and demineralized dentin without caries activity beneath a restoration.  This is a critical weakness in view of the significant percentage of restorations inserted to replace existing restorations.  The need for the identification and clinical staging of the presence, activity, and severity of dental caries is of paramount importance in the deployment of treatment strategies that employ increasingly important nonsurgical modalities, such as fluoride, antimicrobials, sealants, and no treatment.  Some diagnostic modalities are currently in various stages of development and testing; these modalities will need to be evaluated, using rigorously controlled clinical trials.  Such studies will promote true staging of carious lesions, based on highly sensitive and specific diagnoses, followed by appropriate, linked, treatment-planning decision algorithms."  Whether these needs may be fulfilled by other means than dental radiography remains to be seen, but until then the most convenient and cost-effective method for caries detection of approximal caries and dentin lesions in occlusal surfaces, is dental radiography.

In the "Selection criteria for dental radiography" developed in the UK (Faculty, 1998) the authors have linked recommendations with levels of evidence. This work demonstrates how difficult it is to find good evidence from dental literature for many of the procedures used in dental radiography. This opens many perspectives for dental research in the future.

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