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A CT image consists of an array of picture elements called pixels. Each pixel represents the mean attenuation of an axial column of tissue about 1 cm in length. The attenuation is expressed as a Hounsfield number. Typical Hounsfield numbers for various tissues are given below:
A CT scan image is displayed as if the observer is looking at the patient from the feet. Cranial and cervical scans may be displayed as if looking from above.
Because only a limited number of grey-scale steps can be perceived, the observer may select the range of Hounsfield numbers over which the grey-scale should be applied. This range is called the window. Different windows are appropriate for different tissues: