CT attenuation values

An X-ray fluorescence method has been developed for in vivo determination of iodine in thyroid (Aubert et al., 1981 Jonckheer and Deconinck, 1982). This method is based on the irradiation of iodine in the thyroid by o(-rays provided by a -source, such as Am. The excited iodine atoms emit a characteristic X-ray fluorescence radiation, which is proportional to the amount of iodine present in the gland. The reported detection fimit reaches 0.01 mg/ml thyroid, this value is much lower than the iodine concentration in thyroid. The reported radiation dose equivalent is only 60mSv per measurement. This method has been successfully used for the clinical determination of in thyroid (Milakovic et al., 2006 Reiners et al., 1996, 2006 Briancon et al., 1992). An indirect method was also reported to determine of thyroid iodine in vivo (Imanishi et al., 1991), which is based on the relationship of CT attenuation values with iodine concentration in the thyroid. It was reported that the CT value correlated finearly with iodine concentration in thyroid nodules when iodine concentration was higher than 0.02 mg/g. 

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.

In the case of conventional sources, the ability to discriminate among materials with closely similar linear attenuation values (or bulk density) strongly depends on the accuracy of the /(voxc value determination (Denison et al., 1997). For each individual object voxel within a digital image, it is possible to compute a normalized attenuation coefficient known as CT number from the linear attenuation ... 

With consecutive tomograms, the thickness of each section of the body is 5-8-10 (-12) mm. In individual cases, additional thin-section tomograms of 1 mm can be obtained. Resolution is 1 x 2 mm in the hepatic area, with an accuracy in attenuation values of up to 0.5%. In this way, the values for a particular cross-section and their spatial distribution are visualized in a scan. This results in blur-free, anatomically precise imaging of a layer of the body in an axial plane. CT scans provide satisfactory information if an object diameter of 1.5 to 3 mm is resolved with a density gradient of 0.5% to the surrounding area at an integral dose of 10 mGy. 

Fig. 8.2a-c. Ovarian location in a woman of childbearing age. CT scans at the level of the uterine corpus (a-c) The right ovary (arrow) is located in the ovarian fossa (a). Atypical location of the left ovary (arrow) anterior to the uterine corpus near the anterior abdominal wall (b). A corpus luteum cyst (asterisk) displays attenuation values higher than water and a distinct enhancing wall (c)... 

Ovaries can be identified on CT and MRI due to their location and soft tissue characteristics. The landmark of the ovaries are follicular structures which can be best identified on T2-weighted MRI [8]. On CT, normal ovaries can be best identified after bowel contrast opacification. They are ovoid soft tissue structures with low attenuation areas which represent normal follicles (Fig. 8.2). Presence of a dominant folhde ranging more than 1 cm in size assists in ovarian identification. Hemorrhagic corpus luteum cysts may he identified by high attenuation values or a fluid-fluid level [9]. 

Fig.9J5. Mucinous q adenoma in CT. At the level of L5, a cystic ovarian lesion extending to the upper abdomen and measuring 25 cm in diameter is demonstrated. It bulges the abdominal wall and displaces bowel loops posteriorly. It displa) multiple thin septations (arrow). LocuU in the left periphery display attenuation values which are higher than water. The large lesion size and different densities of the loculi are findings suggesting the diagnosis of a mucinous cystadenoma...

Fig. 9.29. Dermoid torsion in CT. Noncontrast transaxial CT in a 37-year-old female who presented with acute pelvic pain. A well-demarcated left adnexal lesion with fatty attenuation values ( s-terisk) is located adjacent to the uterus. Linear calcifications are found at its medial wall. An area of higher density within the fatty tissue correlated with floating hair in the macroscopic specimen. The homogenous wall thickening (arrow) is caused by edema due to torsion of the dermoid...

The accuracy of radiotherapy dose calculations, based on the attenuation values of the CT scan of the patient, needs to be very high to prevent underexposure of the tumor and overexposure of the healthy tissue. Typically the isodose surfaces are calculated and viewed in 3D in the context of the local anatomy. Increasingly the images made in the linear accellerator with the electronic portal imaging device" (EPID) are used for precise localization of the beam and repeat positioning of the patient. 

The CT appearance of a cavernoma depends on the amount of internal thrombosis, hemorrhage, and calcification. Examples are shown in Figures 2.2, 2.4a, 2.7a, 2.8a, 2.10a, 2.12a, and 2.13a,b. The lesions appear hyperdense compared to adjacent brain parenchyma, but can have variable attenuation values. Because the density of blood on CT depends on clot formation, the attenuation of a thrombosed cavernoma changes with time. Calcifications do not change that much however, cavernomas tend to calcify only partially (see Figs. 2.12 and 2.13). In patients with a recent hemorrhage, the cavernoma may be suspected on CT mainly by taking into account... 

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) ... 

Performing a standard coronary CTA, CT density values within the myocardium can give insight into pathologic ischemia of the myocardium, i.e., hypoperfusion or myocardial infarction, both reflected by a reduced CT density or hypoattenuation. Ischemic changes in the myocardium after coronary arterial occlusion consist of disruption of cell membrane function and integrity and increased permeability of small vessel walls. In contrast-enhanced CT, the initial area of low attenuation primarily reflects myocardial edema, i.e., a pronounced water content of the myocardium, which is followed by infiltration of inflammatory cells. Subsequently, necrotic myocardium is replaced by fibrous and/or fatty... 

On non-enhanced CT scans a hepatic cyst appears as a round or ovoid well-defined lesion, with no evident wall. It has a homogeneous and hypoattenuating content with attenuation values similar to water (<20 HU) (Fig. 7.1c). After contrast media injection, both the wall or its content do not show any enhancement (Fig. 7.1b) (Mathieu et al. 1997). Higher attenuation values (>20 HU) are present in cyst with hemorrhage or inflammation inside in these cases complicated cysts are difficult to differentiate from... 

In 45 patients with known colorectal cancer. Cohade etal. [28] compared retrospectively the accuracies for staging and restaging of [ F]-FDG-PET/CT versus [1 F]-FDG-PET alone. All p F]-FDG-PET and [ F]-FDG-PET/CT studies were separately evaluated in randomized order. [ Ge] attenuation-corrected images were assessed to reassure that only the added value of CT information was... 

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