Tissue properties

Drugs that affect tissue properties, particularly blood flow at the absorption site, may be used to control the rate of absorption. Reduced drug absorption may be achieved physiologically with an IM preparation by incorporating epinephrine, which causes a local constriction of blood vessels at the site of injection. Increased muscular activity may enhance drug absorption because of increased drug flow. 

Sugihara, T., and C. J. Martin. Simulation of lung tissue properties in age and irreversible obstructive syndromes using an aldehyde. J. Gin. Invest. 56 23-29. 1975. 

Experimentally, / and a have been poorly quantified (Levick, 1994 McGuire and Yuan, 2001 Parameswaran et al., 1999). Retardation and reflection coefficients may depend on flow rate, solute and tissue properties. 

Suitability of Various Regions of the Oral Mucosa for Transmucosal Drug Delivery based on Various Tissue Properties... 

Fischer U, Rebrin K, von Woedtke T, Abel R Clinical usefulness of the glucose concentration in subcutaneous tissue properties and pitfalls of electrochemical biosensors. Hormone Metabolism Research 1994, 26, 515-522. 

With regard to this hypothesis, only the first term in Equation (1) is a function of tissue properties, since... 

The specific carrier-wave amplitudes (field intensities) which have been found to be effective in producing Ca ion efflux are discussed in terms of tissue properties and relevant mechanisms. The brain tissue is hypothesized to be electrically nonlinear at specific field intensities this nonlinearity demodulates the carrier and releases a 16 Hz signal within ljie tissue. The 16 Hz signal is selectively coupled to the Ca ions by some mechanism, perhaps a dipolar-typ +(Maxwell-Wagner) relaxation, which enhances the efflux of Ca ions. The hypothesis that brain tissue exhibits a slight nonlinearity for certain values of applied RF electric field intensity is not testable by conventional measurements of e because changes... 

A number of variables, such as the perfusion flow rate, the membrane surface area and geometry, the MWCO of the membrane, the diffusion characteristics of the collected analyte, the composition of the perfusion medium, and the temperature, influence recovery parameters. When the microdialysis is carried out in vivo, the recovery can also be affected by some tissue properties, including tissue tortuosity, the extracellular space volume, the tissue blood fluid and the tissue metabolism of the substance. 

U. Fischer, K. Rebrin, T. Woedtke, and P. Able, Clinical Usefulness of the Glucose Concentration in the Subcutaneous Tissue—Properties and Pitfalls of Electrochemical Biosensors, Horm. Metab. Res., 26, 515 (1994). 

Chemical composition is an important contributor to bone quality, a term that encompasses the effects of architecture, composition and remodeling dynamics. An important advantage of infrared (IR) and Raman imaging in bone studies is that they allow the imaging of parameters that measure tissue quality and competence. These are usually measured as band height or band area ratios, and in some cases as band widths. Although measures of tissue properties are similar in both IR and Raman spectroscopy, the IR metrics have been validated with other techniques. 

Heller A 1990 Electrical wiring of redox enzymes Accounts Chem. Res. 23 1280034 Janata J 1989 Principles of Chemical Sensors (New York Plenum) p 317 Fischer U, Rebin K, v Woedtke T and Abel P 1994 Clinical usefulness of the glucose concentration in the subcutaneous tissue—properties and pitfalls of electrochemical sensors Horm. Metab. Res. 26 515-22... 

In multicellular organisms, thin lipid membranes serve as semipermeable barriers between aqueous compartments (Figure 5.1). The plasma membrane of the cell separates the cytoplasm from the extracellular space endothelial cell membranes separate the blood within the vascular space from the rest of the tissue. Properties of the lipid membrane are critically important in regulating the movement of molecules between these aqueous spaces. While certain barrier properties of membranes can be attributed to the lipid components, accessory molecules within the cell membrane—particularly transport proteins and ion channels—control the rate of permeation of many solutes. Transport proteins permit the cell to regulate the composition of its intracellular environment in response to extracellular conditions. 

The observed tissue time-density curve represents a combination of the effects of the AIF and the inherent tissue properties. Thus, to fit the model, the effects of the AIF on the tissue concentration curve must be removed using a mathematical process known as deconvolution to derive R(t), which is dependent only on the hemodynamic properties of the voxel under consideration. R(t) demonstrates an abrupt (indeed, instantaneous) rise, a plateau of duration equal to the minimum transit time through the tissue of interest, and then decay towards baseline. It was shown by Maier and Zierler [110] that the tissue concentration curve can be represented as the CBF multiplied by AIF convolved with R(t) ... 

A comprehensive overview of frequency-domain DOT techniques is given in [88]. Particular instraments are described in [166, 347, 410]. It is commonly believed that modulation techniques are less expensive and achieve shorter acquisition times, whereas TCSPC delivers a better absolute accuracy of optical tissue properties. It must be doubted that this general statement is correct for any particular instrument. Certainly, relatively inexpensive frequency-domain instruments can be built by using sine-wave-modulated LEDs, standard avalanche photodiodes, and radio or cellphone receiver chips. Instruments of this type usually have a considerable amplitude-phase crosstalk". Amplitude-phase crosstalk is a dependence of the measured phase on the amplitude of the signal. It results from nonlinearity in the detectors, amplifiers, and mixers, and from synchronous signal pickup [6]. This makes it difficult to obtain absolute optical tissue properties. A carefully designed system [382] reached a systematic phase error of 0.5° at 100 MHz. A system that compensates the amplitude-phase crosstalk via a reference channel reached an RMS phase error of 0.2° at 100 MHz [370]. These phase errors correspond to a time shift of 14 ps and 5.5 ps RMS, respectively. 

Early detection of the response of hepatocellular carcinoma (HCC) to microsphere brachytherapy may be important to permit repeat radioemboliza-tion or to alter treatment strategies. Water-mobility measurements with use of diffusion-weighted MRl appear useful for noninvasive interrogation of microstructural tissue properties. Findings of diffusion-weighted MRl may serve as an early biomarker of HCC response and represents a promising technique for non-invasive assessment of tumor response after radioembolization [11]. 

There is a duality in the electrical properties of tissue. Tissue may be regarded as a conductor or a dielectric. In frequencies of 100 kHz or less, most tissues are predominantly electrolytic conductors. Therefore, we start Chapter 2 with a look at electrolytes. Bulk electrolyte continuity is broken in two important ways by electrode metal plates and by cell membranes. This break in continuity introduces capacitive current flow segments. At the electrodes, electric double layers are formed in the electrolyte the cell interiors are guarded by membranes. With high-resolution techniques, it is possible to extract important capacitive (i.e., dielectric) properties even at low frequencies, such as 10 Hz. At higher frequencies, such as 50 kHz, the dielectric properties of tissue (discussed in Chapter 3) may dominate. At the highest frequencies, tissue properties become more and more equal to that of water. Pure water has a characteristic relaxation frequency of approximately 18 GHz. 

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