Thus CT is essentially a 3D version of X-ray radiography, and therefore
offers high contrast between bone and soft tissue and low contrast between among
different soft tissues. See Figure 3.2(b). A contrast agent (some chemical solution
opaque to the X-rays) can be injected into the patient in order to artificially increase
the contrast among the tissues of interest and so enhance image quality. Because
CT is based on multiple radiographs, the deleterious effects of ionizing radiation
should be taken into account (even though it is claimed that the dose is sufficiently
low in modern devices so that this is probably not a major health risk issue). A
CT image can be obtained within one breath hold which makes CT the modality
of choice for imaging the thoracic cage.