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