Chest tomography: a practical approach
Tomography is a specialized radiographic technique that allows one to better visualize an area of interest by selectively blurring out superimposed obscuring structures. It does not improve the sharpness of any portion of the radiographic image; in fact, it produces images which, in terms of sharpness and contrast, are degraded as compared to those produced by standard chest radiographs. The apparent improvement in visualization of the area of interest results solely from the blurring out of superimposed structures. Although tomography is of unquestioned value in evaluation of various chest lesions, the optimal technique for performing it in this context remains controversial. Clearly, the goal of tomography is to provide effective blurring of superimposed structures, while at the same time producing the least amount of image degradation and the smallest radiation exposure possible. Thus, the tomographic mode selected in an individual case should be a function of the amount and proximity of information superimposed upon the area of principal interest. In the chest, where ribs and vessels are frequently the major obscuring structures, effective blurring can usually be achieved using 15° linear tomography. The tomograms produced by this mode, as compared to those produced by 40° linear tomograms, have clearer images and larger 'effective' section thickness. In addition, they require less radiation exposure per tomographic cut and fewer total tomographic cuts per examination, thus markedly reducing the total radiation exposure to the patient. These differences in image clarity and radiation exposure are further accentuated when 15° linear tomograms are compared to hypocycloidal tomograms. Indeed, hypocycloidal tomography is rarely required in chest work. Evaluation of the mediastinum, however, is more effectively performed using a circular tomographic mode because of the anatomical orientation of obscuring structures in this area.
Ravin, CE; Wallin, K; Sorenson, J
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