Hounsfield unit

(Sir Godfrey Newbold Hounsfield, 20th century, British research scientist, Nobel prize laureate in medicine/physiology in 1979), a system to measure the attenuation coefficient of tissues in computed tomography CT . Hounsfield units are also termed CT numbers (Table 1). Also see Hounsfield unit.

Hounsfield unit, Table 1. Sample CT numbers for various tissues.

In accordance with this system, lesions whose attenuation values are close to that of water are consistent with, but not specific for, cysts. Lesions composed solely or predominantly of fat produce negative CT numbers; however, some types of liposarcoma contain great amounts of fat, and some forms of lipoma reveal abundant nonfatty tissue. haematomas characteristically demonstrate inhomogeneous areas with regions of both high attenuation (approximately 50 HU) and low attenuation (approximately 10 HU) in the subacute stage and homogeneous areas of low attenuation (120 HU) in the chronic stage. The measurement of attenuation values of bone lesions may be more difficult, especially in narrow bones in which the contribution of the cortex may prohibit accurate assessment.

The identification of gas in soft tissue or bone by CT is possible owing to its very low attenuation value. Gas within a vertebral body documented by CT, for example, is an important sign of ischaemic necrosis of bone. Intraosseous gas is also identified in some cases of osteomyelitis and in subchondral cysts (pneumatocysts), particularly in the ilium and vertebral bod

Revised by WM 23.03.09

DR