[Radiography in parodontology using the long-cone parallel technic: justification, technological principles and practical application].

Radiographs are an important aid in the periodontal examination and diagnosis. Since they are only the two-dimensional situation, it is important to use a technique that gives the best images. The long-cone paralleling technique meets the highest standards of image-quality. Geometric factors have an important influence on the dimensions and on the proportions of the different structures, especially the crown and the root length, the bone-height. Due to the greater distance between the radiation source and the film, the amount of radiation energy has to be increased. This is partly realised by an increased kilovoltage and milli-amperage. This increase also improves the contrast and the sharpness of the image. The focus-diameter, the film-thickness and the movements during the exposition-time are potential sources of blurring. However, the increased distance between the radiation source and the film reduces considerably the influence of these factors, so that the blurring is nearly invisible. Extra-oral radiographs, as orthopantomographs, are valuable as a first general image. Intra-oral long-cone radiographs are however essential if detailed images are required. The long-cone paralleling technique is based on the following principles: an increased distance between the radiation source and the film to obtain a bundle of parallel rays; an increased distance (except for the lower molar region) between the tooth and the film to obtain parallelism between the film and the long-axis of the tooth; the central beam must be directed perpendicularly to the long-axis of the tooth. If these simple principles are respected, the long-cone paralleling technique becomes a highly reliable method, although easy and fast to realise.