Hounsfield-unit density

Berber E, Foroutani A, Garland AM, et al (2000) Use of CT Hounsfield unit density to identify ablated tumour after laparoscopic radiofrequency ablation of hepatic tumors. Surg Endosc 14 799-804... 

The attenuation values are given in Hounsfield units (HU), the density scale ranging from -1,000 (= air) through 0 (= water) to +1,000 (= bone). These 2,000 shades of brightness are recorded by the computer, but... 

In correlation with fatty infiltration, the fatty liver shows a diffuse reduction in density with decreased Hounsfield units. The fatty liver appears much darker than the spleen (so-called grey liver). Hepatomegaly with smooth contours is visible. The parenchyma is iso-dense or hypodense compared with the vessels and bile ducts, which is why (depending on the degree of fatty infiltration) there is a reversal of density (so-called negative scan ). A linear correlation exists between the extent of the fatty infiltration and the decrease in the density recorded. An increase in the relative fat content by 10% leads to a reduction in density of 17(-20) HU. 

The first liposarcoma observed in the liver was reported by J.I. Kim et al. in 1987. (289) Up to now, about 10 cases have been published. Bright yellow tissue containing mature fat cells admixed with scattered immature lipoblasts were found in various stages of development. Moreover, hyperchromatic nuclei in pleomorphic tumour cells, eosinophilic cytoplasm and fusiform cells were evident. (290) On US, liposarcoma is generally inhomogeneous with hypoechoic or hyperechoic features. Also on CT, this myxoid tumour is inhomogeneous and has Hounsfield units ranging from near-water density to that of muscle and may exhibit contrast enhancement. In addition, cystic areas and septated areas may be seen in MRI. 

Another hepatocyte-specific contrast formulation, dysprosium ethoxybenzyltris(carboxylatomethyl)triazaun-decanedioic acid diethylenetriaminepenta-acetic acid (Dy-EOB-DTPA), has been used for Uver CT imaging (266). It contains calcium-EOB-DTPA (0.5 mg/ml), tro-metamol (1.2 mg/ml), and hydrochloric acid (final pH 7.4). It has been described as a stable metal chelate with high tolerability in vitro and in animal studies, in which a long-lasting increase in CT density of about 30 Hounsfield units has been reported. A total of 40 healthy male volunteers (mean age 33 years) received intravenous infusions... 

The first CT system was shown to be approximately 100 times more sensitive than the conventional X-ray system (837). Contrast enhancement in CT is measured in terms of a density unit. In the original article, water was arbitrarily set at a value of 0 unit air, 500 unit fat, 50 unit, and so forth. On this scale, a number of tissues in the body fell within 20 units above water or "4% window" in scanning. This unit was later named the Hounsfield Unit (HU) in honor of the author (840). Using HUs, air has a value of -1000, water a value of 0, and dense bone a value of +1000. The white tone on the picture represents dense material with enhanced contrast. Materials such as tumor will be less enhanced by the contrast medium because of limited blood supply and will appear hypodense, gray, or less white. Organs or tissues not contrast enhanced will appear black on the picture. A similar unit, known as the CT number, for measuring the image contrast or conspicuity, discussed by Dawson (836), is based on a com-... 

CT image pixel values are often negative because they are in Hounsfield (1980) units or Hounsfield scale, which is a quantitative scale for describing radio density. The radio density of distilled water at standard pressure and temperature (STP) is defined as zero Hounsfield units (HU). The radio density of air at STP is defined as -1,000 HU. Other important HUs are fat -50, soft tissue such as musde + 40, calculus + 100 to + 400 and bone + 1,000. The range of possible values is -1,000 to +1,000. 

As Hounsfield units should be related hnearly with attenuation, the calculation can be performed based on CT density values measured for the respective substance. However, as the definition of Hounsfield units implies that an attenuation of 0 is reflected by a value of -1,000 for air, the formula for the Dual-Energy Index resolves to (Eq. 5.2) ... 

Fig. 16.3a-d. Hypothetical density of lumen versus plaque on representative cross-sections, a The coronary lumen (filled with an iodine contrast agent at 400 HU) is surrounded by soft plaque and epicardial fat. The problem zone in this situation is the delineation of soft plaque versus epicardial fat b Part of the soft plaque structures are densely calcified. The calcified plaque with a density of 700 HU is oversized due to partial volrnne averaging, but delineation against the lumen is not compromised by contrast issues, c For a hypothetical plaque structure at an initial stage of calcification (Hounsfield units... 

The subjects underwent computed tomography (CT) scanning within 3 days of each blood sample being taken. Transverse tomograms were taken at the umbilical level and at fixed intervals in which the spleen and liver were observed in the same plane. Total fat area, visceral fat area, and subcutaneous fat area were determined from the CT images at the umbilical level by the method of Tokunaga et al. (1983). Hepatic fat content was estimated by the method of Kato et al. (1984). Hounsfield units (HU a measure of density) of the liver and spleen were also determined from the CT images, and the liver-to-spleen ratio of the values (L-HU/S-HU) was calculated as an index of the hepatic fat content. 

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