From the case:
Hounsfield unit formula
{"current_user":null,"step_through_annotations":true,"access":{"can_edit":false,"can_download":true,"can_toggle_annotations":true,"can_feature":false,"can_examine_pipeline_reports":false,"can_pin":false},"extraPropsURL":"/studies/123146/annotated_viewer_json?_c=1696822527\u0026lang=us"}
The equation for the calculation of Hounsfield unit used in CT scans is sometimes written with a second term in the denominator for the attenuation of air. As this value is essentially zero it can be ignored.