Since the first brain scanner was constructed several years ago, computed tomography (X 线断层摄影术)or computed medical imagery, has become fairly widely used. Its rapid acceptance is due to the fact that it has overcome several of the drawbacks of conventional X-ray technology.
To begin with, conventional two-dimensional X-ray pictures cannot show all of the information contained in a three-dimensional object. Things at different depths are superimposed, causing confusion to the viewer. Computed tomography can give three-dimensional information. The computer is able to re- construct pictures of the body’’s interior by measuring the varying intensities of X-ray beams passing through sections of the body from hundreds of different angles. Such pictures are based on series of thin ’’slices’’.
In addition, conventional X-rays generally differentiate only between bone and air, as in the chest and lungs. They cannot distinguish .soft tissues or variations in tissues. The liver and pancreas are hot discernible at all, and certain other organs may only be rendered visible through the use of radiopaque (辐射透不过的)dye. Since computed tomography is much more sensitive, the soft tissues of the kidneys or the liver can be seen and clearly differentiated. This technique can also accurately measure different degrees of X-ray absorption, facilitating the study of the nature of tissue.
A third problem with conventional X-ray methods is their inability to measure quantitatively the separate densities of the individual substances through which the X-ray has passed. Only the mean absorption of all the tissues is recorded. This is not a problem with computed tomography. It can accurately locate a tumor and subsequently monitor the progress of radiation treatment, so that in addition to its diagnostic capabilities, it can play a significant role in therapy.
According to the passage, computed tomography can be used for all of the following EXCEPT _________.