Hounsfield unit

Hounsfield units (HU) are a dimensionless unit universally used in computed tomography (CT) scanning to express CT numbers in a standardized and convenient form. Hounsfield units are obtained from a linear transformation of the measured attenuation coefficients ¹. This transformation (figure 1) is based on the arbitrarily-assigned radiodensities of air and pure water:

• radiodensity of distilled water at standard temperature and pressure (STP) equals 0 HU

• radiodensity of air at STP equals –1000 HU

Note: STP is defined as temperature of 0 °C and pressure of 10⁵ pascals (i.e. sea-level).

This results in a scale running from –1000 HU for air, between –100 HU and 100 HU for most tissues, around 2000 HU for very dense bone (cochlea) and over 3000 HU for metals ⁷. How these values are displayed is determined by the application of specific window and level values. See figure 2 for typical values encountered in a CT scan.

CT images traditionally stored data at 12-bit depth, allowing for 2¹² = 4096 different values. These were usually mapped to represent radiodensities between –1024 HU and 3071 HU ⁷. This range adequately covers all tissues in a human body.

CT images from newer scanners store data at 16-bit depth, allowing for 2¹⁶ = 65 536 different values ⁸. How these are mapped and what HU range is covered differs between manufacturers. The extended HU range has several applications:

• high noise (low dose, sharp kernel) scans of lungs can produce values far below –1000 HU, clipping these values at –1024 HU (as in 12-bit images) could disrupt image analysis

• metals in implants have HU values in the order of tens of thousands, exact representation of these values (as opposed to clipping at 3071 HU in 12-bit images) allows for more accurate radiotherapy planning in patients with such implants ^8,9

The software of all CT scanners and PACSs has the ability to measure the density of a region of interest (ROI) electronically overlaid the image.

Hounsfield units are measured and reported in a variety of clinical applications. One well-known use is the evaluation of the fat content of the liver, with fatty liver diagnosed by the presence of a liver-to-spleen ratio <1.0 or 0.8 ². Other less common uses include assessing bone mineral density (BMD) ³, predicting the presence of anemia ⁴, and guiding the management of kidney stones ⁵.

No equivalent to Hounsfield units exists in any other form of structural imaging.

Typical values

Although values will vary somewhat between healthy individuals and between manufacturers, different tissues have predicable normal values on non-contrast CT ^10-12.

• air: -1000 HU

• bone (cortical): >1000 HU

• bone (trabecular): 300 to 800 HU

• brain (grey matter): 40 HU ¹¹

• brain (white matter): 30 HU ¹¹

• subcutaneous fat: -100 to -115 HU ¹⁰

• liver: 45-50 HU ¹⁰

• lungs: -950 to -650 HU ¹²

• metal: >3000 HU

• muscle: 45 to 50 HU ¹⁰

• renal cortex: 25 to 30 HU ¹⁰

• spleen: 40 to 45 HU ¹⁰

• water: 0 HU (by definition)

History and etymology

Hounsfield units are named after Sir Godfrey Hounsfield, the Nobel prize winner and inventor of CT.