Plain (or conventional) tomography is a radiographic technique that has been available for many years, generally in institutions such as dental schools or hospitals, due to the size and expense of the equipment. However, tomographic capability has been added to some sophisticated computer-controlled panoramic x-ray machines, making tomography potentially more readily available in dental offices and clinics.
In conventional tomography, an image is made of a thin layer of tissue; tissues that are superficial and deep to the desired region blur out due to movement of the x-ray tube and film. Some machines use a relatively simple linear movement of the tube, which can unfortunately produce streaking artifacts. More complex tube movements (such as spiral and hypocycloidal movements) can blur out undesired tissues more completely, thus making the area of interest more prominent.
In the past, the primary use of tomography in dentistry has been for detailed evaluation of the osseous structures of the TMJ (Figure 3-7, A). However, some of the newer machines will also produce cross-sectional images of the jaws (see Figure 3-7, B). While these are excellent for the assessment of the bone prior to the placement of dental implants, they also can be used whenever a cross-sectional view of the jaws would be
helpful, such as when determining the relationship between a lesion and the apex of the tooth or when evaluating the integrity of the buccal or lingual cortical bone.
For the evaluation of large or complex lesions such as facial fractures or tumors, conventional tomography has generally been superceded by CT or magnetic resonance imaging because of the clarity of the images, the lack of blurring from other structures, and the ability to produce images in multiple planes. However, plain tomography may still be of value in imaging lesions confined to the jaw bones.