What is the use of contrast for treatment planning for CT?
Radiographic contrast absorbs X-radiation wavelengths resulting in the contrast material appearing light on a radiograph since fewer X-rays reached the film (they were absorbed before getting to the film). Bone, a radio-dense material also appears light on a radiograph for the same reason. Since computer-assisted tomography is a technology utilizing X-rays, contrast functions to absorb the radiation just as it does in classical radiography. The contrast is injected into vessels and blood flow delivers the contrast to tissue vessel beds. If the CT scan is performed immediately after contrast injection, the major veins and chambers of the heart will be seen ...
Contrast enhanced CT scanning is very helpful for defining the boundaries of tissue abnormalities, some of which may require surgical or radiological treatment. In critical organs, e.g., brain, precise knowledge of abnormality boundaries is critical to avoid damage to normal tissue during treatment.