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 densities of air and pure water:
- radiodensity of distilled water at standard temperature and pressure (STP) = 0 HU
- radiodensity of air at STP = -1000 HU
STP: standard temperature is 0 °C and pressure is 105 pascals (i.e. sea-level).
This results in a scale running from -1000 HU for air to +~2000 HU for very dense bone (cochlea).
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.
History and etymology
- 1.Hounsfield GN. Computed medical imaging. Nobel lecture, December 8, 1979. J Comput Assist Tomogr. 1980;4 (5): 665-74. Pubmed citation
- 2. Zeb I, Li D, Nasir K et-al. Computed tomography scans in the evaluation of fatty liver disease in a population based study: the multi-ethnic study of atherosclerosis. Acad Radiol. 2012;19 (7): 811-8. doi:10.1016/j.acra.2012.02.022 - Free text at pubmed - Pubmed citation
- 3. Pickhardt PJ, Pooler BD, Lauder T et-al. Opportunistic screening for osteoporosis using abdominal computed tomography scans obtained for other indications. Ann. Intern. Med. 2013;158 (8): 588-95. doi:10.7326/0003-4819-158-8-201304160-00003 - Free text at pubmed - Pubmed citation
- 4. Bruni SG, Patafio FM, Dufton JA et-al. The assessment of anemia from attenuation values of cranial venous drainage on unenhanced computed tomography of the head. Can Assoc Radiol J. 2013;64 (1): 46-50. doi:10.1016/j.carj.2011.08.005 - Pubmed citation
- 5. Ouzaid I, Al-qahtani S, Dominique S et-al. A 970 Hounsfield units (HU) threshold of kidney stone density on non-contrast computed tomography (NCCT) improves patients' selection for extracorporeal shockwave lithotripsy (ESWL): evidence from a prospective study. BJU Int. 2012;110 (11b): E438-42. doi:10.1111/j.1464-410X.2012.10964.x - Pubmed citation
- 6. Thomas AMK, Banerjee AK, Busch U. Classic Papers in Modern Diagnostic Radiology. (2004) ISBN: 9783540219279
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