Hounsfield unit

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. 

Fig. 8.3 Pronounced focal (regional) fatty degeneration of the hver of varying intensity both visual and confirmed by Hounsfield units (-14, -9, +4, +30). Native CT, normal value +65 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-... 

HU Hounsfield unit W window-width CT review setting L center-level CT review setting... 

The presence of water in biological substances, such as soft tissue, has lead to the creation of a unit of attenuation coefficient for tomographic imaging. This is the Hounsfield unit (HU), defined as... 

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. 

The low-contrast and high-contrast resolution of the 320-detector-row CT does not differ from the conventional 64-detector-row CT scan from the same manufacturer. Phantom measurements demonstrated a low-contrast resolution of 2 mm at 0.3% Hounsfield unit (HU) difference and a high-contrast resolution of 0.35 mm. 

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 challenge for spinal artery CTA is to provide sufficient arterial enhancement but to scan before arrival of contrast medium in the venous system. An ROI of the bolus tracking system placed in the ascending aorta might be affected by inflow artifacts of the SVC and may result in a mistimed early scan. Therefore, placement of the ROI in the aortic arch or descending aorta is recommended. In the presence of aortic dissection, caution should be taken that the ROI is not too big or positioned in the false lumen or across the dissection membrane, respectively. In these cases, manual start of the scan should be considered. The Hounsfield unit threshold should be around 100 HU above baseline. Scan start is usually delayed by time for table movement (<3 s), which is usually right above the origin of the vertebral arteries. An additional scan delay of 3 s is recommended for scanners with equal to or more than 16 rows and rotation time equal or less than 0.4 s. Hounsfield unit values of attenuated blood in the thoracic aorta should never be lower than within the pulmonary trunk. 

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