The Hounsfield unit (HU) is a relative quantitative measurement of radio density used by radiologists in the interpretation of computed tomography (CT) images. The absorption/attenuation coefficient of radiation within a tissue is used during CT reconstruction to produce a grayscale image. The physical density of tissue is proportional to the absorption/attenuation of the X-ray beam. The Hounsfield unit, also referred to as the CT unit, is then calculated based on a linear transformation of the baseline linear attenuation coefficient of the X-ray beam, where water is arbitrarily defined to be zero Hounsfield Units and air defined as -1000 HU. The linear transformation produces a Hounsfield scale that displays as gray tones. More dense tissue, with greater X-ray beam absorption, has positive values and appears bright; less dense tissue, with less X-ray beam absorption, has negative values and appears dark. The Hounsfield unit was named after Sir Godfrey Hounsfield, recipient of the Nobel Prize in Physiology or Medicine in 1979, for his part in the invention of CT, as it had immediate recognition as a revolutionary diagnostic instrument.
This linear transformation of the original linear attenuation makes the Hounsfield scale a relative scale, rather than absolute. Different X-ray beam energies will result in different tissue absorption and hence, different HUs. Early studies showed the HU to be dependent on the various CT parameters. The type of reconstructing algorithm, design of the CT, and X-ray kilovoltage, were the most important factors identified. These factors need to be standardized to help make the HU a reliable diagnostic measurement tool.
CT artifacts can affect Hounsfield unit measurements. One of the most encountered CT artifacts, beam-hardening artifact, affects the measurement of radiodensity. Polychromatic energies comprise the conventional CT X-ray. High-density tissue selectively absorbs X-rays of lower energy, thus altering the X-ray beam. This absorption, in turn, can alter the X-ray beam absorption in the center of high-density tissue and results in a change in the HU, leading to falsely lowered HU measurements and appears less dense, or darker, on CT images. Modern CT machines can correct for this artifact in the reconstruction process.
Continued advancements in CT as a diagnostic tool have led to different CT designs. Different CT designs, in turn, can alter the HU. For example, cone-beam computed tomography (CBCT), used mainly in dentistry, cannot show the actual HU similar to conventional CT but does show a strong correlation. Dual-energy CT (DECT) uses X-ray beams of two different energies for deriving additional information to produce both anatomic and functional information. Given the dependence of the HU on energy, the use of HU as a quantitative diagnostic parameter is limited in DECT. The same is true for reduced energy protocols used in CT imaging today.
Lastly, one should remember that the visualization of images on a CT has as its basis differences in tissue density and radiodensity. In the case of foreign body evaluation on CT imaging, if the foreign body has a similar physical density to the tissue that it is embedded, it will have similar HU and will be hard to detect by visually CT. The radiologic evaluation of a wooden foreign body is complicated, given the varied appearance of wood and changes within the wood. There is documentation showing that a wooden foreign body demonstrates increasing HU over time.
The use of the HU to measure tissue density has aided radiologists in the interpretation of images and diagnosis of disease. One of the earliest uses of the HU as a quantitative measurement came in the evaluation of solitary pulmonary nodules. More recently, HU measurements of bone on quantitative CTs and conventional CTs have been obtained to determine bone mineral density. Hounsfield unit measurements of bone have also recently been studied to estimate bone quality before spinal instrumentation. Measurements of paraspinal muscles have been obtained to help identify patients at risk for sarcopenia.
With continued further advancements in technology, semi-automated measurements of tissue are being studied to aid the radiologist in the evaluation and diagnosis of disease. Semi-automated HU measurements of solitary pulmonary nodules have shown to be an accurate approach to determining malignant from benign solitary pulmonary nodules. Other semi-automated measurements of HU are likely to aid in the evaluation of CT images and become part of the landscape of clinical practice in the near future.
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