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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 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.
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
- 7. C. Glide‐Hurst, D. Chen, H. Zhong, I. J. Chetty. Changes realized from extended bit‐depth and metal artifact reduction in CT. (2013) Medical Physics. 40 (6Part1): 061711. doi:10.1118/1.4805102 - Pubmed
- 8. Jayamani J, Osman N, Tajuddin A, Salehi Z, Ali M, Abdul Aziz M. Determination of Computed Tomography Number of High-Density Materials in 12-Bit, 12-Bit Extended and 16-Bit Depth for Dosimetric Calculation in Treatment Planning System. J Radiother Pract. 2019;18(03):285-94. doi:10.1017/s1460396919000013
- 9. Ese Z & Zylka W. Influence of 12-Bit and 16-Bit CT Values of Metals on Dose Calculation in Radiotherapy Using PRIMO, a Monte Carlo Code for Clinical Linear Accelerators. Current Directions in Biomedical Engineering. 2019;5(1):597-600. doi:10.1515/cdbme-2019-0150