Tissue density

Tissue volumes may be expressed in mass or volume units. When volume units are used, a tissue density of 1 is normally assumed. Predicted concentrations from a blood flow-limited compartment may be compared directly to observed values. 

Why are barium- and iodine-based materials selected for contrast media The production of X-ray images depends on the differences between the X-ray absorbing power of various tissues. This difference in absorbing power is called contrast and is directly dependent on tissue density. To artificially enhance the ability of a soft tissue to absorb X-rays, the density of that tissue must be increased. The absorption by targeted soft tissue of aqueous solutions of barium sulfate and iodized organic compounds provides this added density through the heavy metal barium and the heavy nonmetal iodine. 

A clear colorless liquid with a strong odor of vinegar. Flash point 129°F. Corrosive to metals and tissue. Density 9.0 lb /gal. Used to make fibers, plastics, pharmaceuticals, dyes, and explosives ( AAR,... 

Xerosis with itch is present in more than 20% of patients with HIV infection, and most commonly is localized on the lower extremities.19 One study revealed the cutaneous neural tissue density within the papillary dermis and epidermis to be significantly decreased in HIV infected patients versus healthy controls.20 The study also reported significant differences in neuropeptide concentrations within HIV infected individuals. For example, calcitonin gene-related peptide (CGRP) was reduced in the epidermis in both the upper arms and legs, whereas substance P was found to be reduced only in the upper arms of HIV patients with itch. Dry skin and itch can also be induced iatrogenically via commonly prescribed medications for HIV such as indinavir etc. The standard in HIV treatment involves protease inhibitors, such as indinavir, which have been shown to induce dry skin and itch in over 40% of patients.21... 

For soft human tissue that would be otherwise have too low a contrast to be resolved in conventional high-intensity planar X-ray radiography, a thorough study of X-ray absorbance as a function of tissue density and thickness allowed the development of hardware and software for computed axial tomography (CAT scanning) which performs a three-dimensional analysis of X-ray absorption at low intensities and convenient two-dimensional displays of resolved human tissue features. 

There is CT, for computerized tomography, a wedding of x-ray and the computer that scans a portion of, say, the brain by revolving an x-ray tube around the head, converting the pictures into a digital computer code to make high-resolution, three-dimensional video images accurate to. 9 millimeter. Differences in tissue density, between abnormal and normal tissue, are revealed fine details of bone structure appear clearly the location of tumors is pinpointed. 

The maximum moisture content for a wood can be estimated easily. In a timber having a basic density of 300 kg m the oven-dry cell tissue (density 1500 kg m )... 

ID=c.IDNum % A temporary variable for compartment IDs c. N Compar tmen ts = length (c. CompName) / c.Density =1.0 % Body and tissue density... 

In the evaluation of cardiac masses, CT scanning shows the mass as a distinct space-occupying entity. Tissue density differentiation as seen on a CT scan allows characterization of density, aiding in determination of the nature of masses. Masses as small as 0.5 to 1 cm can be identified on CT scans. 

ECHO is based on the principle of differential acoustic impedance (or tissue density) and the laws of reflection and refraction. Sound waves directed across tissues from a transducer will reflect back sound waves of different frequencies. The ability of the ultrasonic beam to penetrate chest wall structures is inversely proportional to the frequency of the signal. With transthoracic ECHO, frequencies of 2.0 to 5.0 MHz are used commonly in adults, and frequencies of 3.5 to 10.0 MHz are used in children. Serial determinations in a given patient using the same conditions and ECHO images (windows) provide the best form of internal control to allow comparisons of test results. In clinical trials, echocardiograms are read and interpreted independently by two or three clinicians to provide a means of control. 

CT represents Computer-aided Tomography the computer reconstructs a series of measured images of tissue density (tomography). Small differences in tissue density may indicate the presence of a tumor. 

Here, T is the tissue temperature, C is the tissue heat capacity, p is the tissue density, k is the tissue thermal conductivity, Qm is the rate of metabolic heat generation, and Qb is the rate of heat exchange with blood. Qm is generally small and is calculated from the oxygen consumption of the tissue (Jain, 1984). The estimation of Qb is discussed next. 

CBV can be calculated from (5.4) if one takes into account the brain tissue density, p, and a correction factor, CH, to adjust the difference between arterial and capillaiy hematocrit [70,116]. In vivo experiments in the 1970s demonstrated the markedly lower hematocrit in capillaries compared to arterial hematocrit [117-119] hence the introduction of the CH is required for the quantification of CBV as follows ... 

The output from LDF is measured not in easily interpretable units of flow but rather in hertz. It would be ideal to define a single calibration factor that could be used in all tissues to convert laser output to flow in absolute units. Unfortunately, the calibration to determine an absolute flow is limited by the lack of a comparable standard and the lack of preset controlled conditions. This may be due to varying tissue optical properties affected by tissue density (Obeid et al., 1990). Further, LDF signals can be affect by movement of the probe relative to the tissue. 

Two other common measures of bone density in biomechanical studies are termed tissue and apparent densities. Tissue density p i is defined as the ratio of mass to volume of the actual bone tissue. It is similar for cortical and trabecular bone, varies little in adult humans, and is about 2.0 g/cm. Apparent density p pp is defined as the ratio of the mass of bone tissue to the bulk volume of... 

FIGURE 27.15 Attenuation correction. To obtain accurate SPECT results, corrections must be made for tissue attenuation. In regions where there are large variations in tissue density such as the thorax, this requires an independent transmission study. This shows one possible configuration where line sources of Gd-lS3 are translated across the field of view to collect the transmission data. This information is reconstructed to obtain a crude CT image of the thorax to correct myocardial perfusion studies. (Courtesy of GE Medical Systems.)... 

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