Hounsfield values

Within the software architecture the volumetric dataset is organized in terms of voxels. These are near cubic elements that arise from each image pixel. This latter is the basic building block for the voxel to which a Hounsfield value (in case of CT) or an intensity value (in case of MR) is assigned each voxel has also a defined spatial position and dimension in the volume. 

Water and water-methylcellulose solution have been replaced by polyethylene glycol (PEG) electrolyte solution or low Hounsfield value barium suspension, 0.1% ultra-low-dose barium with sorbitol, a nonadsorbable sugar alcohol that promotes luminal distention and limits resorption of water across the length of the small bowel (Megibow et al. 2006). 

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... 

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 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-... 

The elastic (noperties of all the nuterials were detomined by following the British Standard BS750S on a Hounsfield instrument model HSKS. From these tests it was possible to obtain die values for tensimi ratio (IrX working extension (we) and dif raitial extension ctor (deQ, all teqnessed as a percenti. Ihese inoperties are used to determine the compresaon charactaistics and limits of support pressure garments. 

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. 

VR offers an alternative approach to segmentation although it is still dependent on thresholds, the modulation of opacity values (see Chaps. 4 and 6) allows a wider range of tissue combination and display (Rubin et al. 1996 Johnson et al. 1996). With VR it is possible to visualize in the same view different tissue (having different Hounsfield density) or combine the visualization of tissue and metallic components, i.e., cochlear implants (Fig. 10.1) (Neri 2001a). 

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) ... 

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. 

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. 

Data taken from Nagao et al (2000). Values are means SEM (n=19). See Section D.3 of text for further details. =16 week value minus initial (0 week) value. - Significantly different from the initial value by Student s t-test / <0.05, p<0.01. Significantly different from TAG group by Student s t-test for paired values /j<0.05, p<0.01. Significantly different from TAG group by analysis of covariance, p<0.05. W/H, waist to hip circumference ratio. V/S, visceral to subcutaneous fat area ratio. L-HU/S-HU, liver to spleen ratio of Hounsfield units. 

Design of Research T en successive patients were included in the study. The W/O/W emulsion, consisting of IPSO microdroplets 70 pm in diameter, was injected through a catheter inserted into the hepatic artery or a segmental branch thereof. The mean amount of the W/O/W emulsion per one injection was 10 ml. The mean CT values for the tumor (Hounsfield unit, FlU/pixel) were calculated from the CT values of all pixels in the whole area corresponding to the largest cross section of the main tumor. The mean CT values for the... 

Figure 10.7 Mean CT attenuation values (Hounsfield unit, HU/pixel) immediately and 7 days after injection with W/OAV emulsion. The values were calculated by using numeric data of randomly selected 250 pixels in a main tumor, juxta-tumor nontumorous tissue within an area up to 10mm from the edge of the tumor, and nontumorous tissue 10 mm distant from the tumor.

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