Adriaens P A, De Boever J
Rev Belge Med Dent. 1982 Mar;37(2):44-9.
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.
X光片在牙周检查和诊断中是一项重要的辅助手段。由于它们只是二维图像，因此采用能提供最佳图像的技术很重要。长锥平行投照技术符合图像质量的最高标准。几何因素对不同结构的尺寸和比例有重要影响，尤其是牙冠和牙根长度、骨高度。由于放射源与胶片之间的距离更大，必须增加辐射能量。这部分是通过提高千伏和毫安来实现的。这种增加也改善了图像的对比度和清晰度。焦点直径、胶片厚度以及曝光期间的移动是造成图像模糊的潜在因素。然而，放射源与胶片之间距离的增加大大降低了这些因素的影响，以至于模糊几乎不可见。口外X光片，如曲面断层片，作为初步的整体图像很有价值。然而，如果需要详细图像，口内长锥X光片则必不可少。长锥平行投照技术基于以下原则：增加放射源与胶片之间的距离以获得一束平行射线；增加牙齿与胶片之间的距离（下颌磨牙区除外）以获得胶片与牙齿长轴之间的平行度；中心射线必须垂直于牙齿的长轴。如果遵循这些简单原则，长锥平行投照技术尽管易于实现且快速，但会成为一种高度可靠的方法。
