X-ray Computed Tomography

• A pixel within the CT image represents the ability of the atoms within the voxel to attenuate (decrease) an X-ray beam. The brighter the pixel the greater the ability of the tissue to attenuate X-rays.
• To produce a CT image an X-ray tube rotates around the patient projecting X-rays through the patient from many angles. A ring of X-ray detectors collects an x-ray image from each projection angle around the patient. A computer algorithm then calculates the attenuation that each vowel in a rectangular grid would have to have to produce the pattern of Xrays in the multitude of projections.
• Contrast within the image varies from white (high attenuation) to black (low attenuation) with the type of tissue within the voxel:
  • Bone
  • Soft tissue
    • Gray matter
    • White matter
  • Water
  • Fat
  • Air
• Pathological processes are identified by alterations in anatomy and attenuation.
  • Obviously abnormal anatomy is identified by adequate knowledge of normal anatomy (which is why you are studying normal anatomy).
  • Pathological processes typically increase the water content in tissues. The attenuation of a voxel is an average of both the remaining normal tissue and the abnormal added water. Consequently, pathological processes decrease the attenuation/brightness of soft tissues. Similarly, when the pathological processes invade the surrounding fat they increase its attenuation/brightness due to the added water.
  • The blood within acute hemorrhage has a higher attenuation/brightness than the surrounding soft tissue. Its attenuation first increases as a clot forms and then gradually declines over the following days.
  • Intravenous X-ray contrast dye has higher attenuation than soft tissue. Due to the blood brain barrier, injecting X-ray contrast normally only brightens blood vessels and tissues without a blood brain barrier like the choroid plexus. Pathological processes typically disturb the blood brain barrier allowing contrast to enter. Consequently, pathological processes within the brain brighten following contrast administration.