HU-based differentiation of material applies to medical-grade dual-energy CT scans but not to cone beam computed tomography (CBCT) scans, as CBCT scans provide unreliable HU readings. Values for different body tissues and material CT scan of the thorax with window level set to -700 HU (lung) CT scan of the thorax with window level set to -1,000 HU (air) CT scan of the thorax with window level set to 0 HU (water) CT scan of the thorax with window level set to 60 HU (liver) The above standards were chosen as they are universally available references and suited to the key application for which computed axial tomography was developed: imaging the internal anatomy of living creatures based on organized water structures and mostly living in air, e.g. Variation in the measured values of reference materials with known composition, and variation between and within slices may be used as part of test procedures. This is particularly important for CT scans used in radiotherapy treatment planning, where HU is converted to electron density. : 259Ĭalibration tests of HU with reference to water and other materials may be done to ensure standardised response. Thus, a change of one Hounsfield unit (HU) represents a change of 0.1% of the attenuation coefficient of water since the attenuation coefficient of air is nearly zero. H U = 1000 × μ − μ water μ water − μ air are respectively the linear attenuation coefficients of water and air.
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