CT Filtration

Compensating filtersare used to shape the x-ray beam.

They reduce the radiation dose to the patient and help to

minimize image artifact. Radiation

emitted by CT x-ray tubes ispoly-chromatic. Filtering

the x-ray beam helps to reduce the range of x-ray energies

that reach the patient by removing the long-wavelength

(or “soft”) x-rays. These long-wavelength x-rays are readily

absorbed by the patient, therefore they do not contribute

to the CT image but do contribute to the radiation dose

to the patient. In addition, creating a more uniform beam

intensity improves the CT image by reducing artifacts that

result from beam hardening.

Different filters are used when scanning the body than

when scanning the head. Human body anatomy typically

has a round cross section that is thicker in the middle than

in the periphery. Hence, body-scanning filters are used to

reduce the beam intensity at the periphery of the beam,

corresponding to the thinner areas of a patient’s anatomy.

Because of their shape they are often referred to as bowtie filters.

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