Explaining the Hounsfield Unit

A Computed Tomography (CT) scan comprises of many tens, if not hundreds, of seperate X-ray exposures which are composited together to form a cohesive image of the subject of the scan.

With all these composited images being taken and meshed together, there needed to be a way to seperate out the body into meaningful data that can be interpreted by trained radiologists and doctors.

Different systems and anatomical structures of the body have varying levels of density (think of low-density fat compared to high-density bone) and thus the ability to identify varying density from the X-rays in a standardised manner heralded the invention of the CT scanner.

To determine the varying density, Sir Godfrey Hounsfield, amongst others, were able to measure the attentuation (reduction in strength of X-rays) caused by absorption from different anatomical structures1. The value of the attentuation became the Hounsfield Unit (HU). With a baseline of 0, being the density of pure water, Hounsfield values can be both negative and positive. The higher the value, the more dense the matter.

Once calculated, images in the axial, coronal and sagittal planes have been coloured in gray-scale based upon their HU. Higher density areas such as bone are whiter than low-density areas such as skin, fat and even the air in the lungs.

Windowing Settings

In a medical/radiological setting, Windowing relates to the grey-level mapping/contrast of a scan, changing the Hounsfield Units/CT numbers to highlight particular areas within the scan. 

By changing both the upper & lower threshold values, it is possible to occlude (hide) certain features from the scan, such as 'removing the skin' or hiding the skeletal system. View our short video below to see how easy it is to change the Upper & Lower threshold values in our Free 3D DICOM Viewer Software.

Using Our Windowing Slider To Change Hounsfield Unit Range

As seen in the video above, the process for changing the CT number is very simple. Dragging the lower end of the slider to the right increases the lowest HU visible, resulting in the occlusion of low-density tissue from the 3D render. 

Should you choose to reduce the top value, this will have the opposite effect of reducing the highest HU visible which will result in the skeletal system being hidden from view and more focus being drawn to the soft-tissue areas and organs. 

We recommend experimenting with the Windowing Settings, including the HU values threshold bar and the Clipping slider to understand how you can actually visualise more from your scans by hiding non-essential data.