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Basic Principles of CT

• X-rays are absorbed to diffreent degrees by different tissues.

• Dense tissue such as bone absorb the most x-rays and hence only few reachs the detector opposite. Conversely, tissues with low density absorb (eg: air & fat)almost none of the xrays, allowing most to pass through to the film or detector opposite.

• As opposed to conventional radiographs , with CT scannin an X-ray source and detector, situated 180 degrees across from each other, move 360 degrees around the patient, continously detecting and seeding information about the attenuation of X-ray as they pass through the body.

• Computer then manipulates ant integrates the acquired data and assigns numerical values based on the subtle differences in x-ray attenuation. Based on these values, a grey scale axial image is generated that can distinguish between objects with even small differences in density.

Attenuation Coefficient

• The tissue contained within each image unit absorbs a certain proportion of the X-rays that pass through it. This ability to block x-rays as they pass through a substance is known as attenuation.

• For a given body tissue, the amount of attenuation is relatively constant and is known as that tissue's attenuation coefficient.

• In CT, these attenuation coeficients are mapped to an arbitary scale between -1000 HU and + 1000 HU. This scale is the Hounsfield Scale.

Artifacts

• CT of the bain is subjected to a few predictable artifacts that can potentially inhibits the ability to accurately interpret the images .

• Besides motion and metal artifact, the most important common effects are called

  • Beam hardening

  • Volume averaging

• Beam hardening is a phenomenon that causes an abnormal signal when a relatively small amount of hypodense brain tisue is immediately adjacent to dense bone.The posterior fossa, where there is extremely dense bone surronding the brain, is particularly subject to this phenomenon.

• It appears as either linear hyper or hypodensities that can partially obscure the brainstem and cerebellum.

• Volume averaging (also called partial volume artifact) arises when the imaged area contains different types of tissue (eg. Bones and brain). For that particular image unit, the CT pixel produced will represent an average density for all the contained structures.