Ultrasound absorption coefficients

The absorption of ultrasound increases the temperature of the medium. Materials that possess higher ultrasound absorption coefficients, such as bone, experience severe thermal effects as compared to muscle tissue, which has a lower absorption coefficient [5]. The increase in the temperature of the medium upon ultrasound exposure at a given frequency varies directly with the ultrasound intensity and exposure time. The absorption coefficient of a medium increases directly with ultrasound frequency resulting in temperature increase. 

Kamlet-Taft) hydrogen bond donation ability ultrasound absorption coefficient isobaric expansibility... 

Absorption of ultrasound results in a temperature increase of the medium. Materials which posses higher ultrasound absorption coefficients, such as bones, experience severe thermal effects as compared to muscle tissues which have a lower absorption coefficient (a). 

The PRF method has been used - to measure the ultrasound absorption coefficient of tissue, the value of which is difficult to obtain using invasive temperature measurements. The tissue sample was placed in a block of agarose gel and heated by ultrasound. The energy absorbed by the tissue diffuses slowly into the surrounding gel. Knowing the thermal capacity of the gel, and the temperature distribution measured by MRI, an estimate of the total absorbed energy can be made. The ultrasound pressure absorption coefficient (a) can be calculated as ... 

Since 1970 the main anomalies in the propagation of ultrasonic waves in non-oriented samples of liquid crystals have been discovered these are the phase velocity dispersion and the nonclassical (a/f = constant) behavior of the ultrasound absorption coefficient due to the relaxation phenomenon. The anomalies are manifested in the megahertz Irequency range and are most pronounced in the vicinity of phase transitions. Kapustin [14] was the first to begin extensive investigations on the anomalies, and look at all types of liquid crystals. Most of the work done since 1980 has involved the use of a dc magnetic or electric field to... 

Perturbation of a spin-state equilibrium by ultrasound results in the absorption of sound, the absorption coefficient a (neper cm ) being determined as a function of frequency /(Hz) according to ... 

The first area involves low amplitude (higher frequency) sound and is concerned with the physical effect of the medium on the wave and is commonly referred to as low power or high frequency ultrasound . Typically, low amplitude waves are used for analytical purposes to measure the velocity and absorption coefficient of the wave in a medium in the 2 to 10 MHz range. Information from such measurements can used in medical imaging, chemical analysis and the study of relaxation phenomena and this will be dealt with later. 

Perturbation of a chemical equilibrium by ultrasound results in absorption of the sound. Ultrasonic methods determine the absorption coefficient, a (neper cm-1), as a function of frequency. In the absence of chemical relaxation the background absorption, B, increases with the square of the frequency f (hertz) that is, a/f2 is constant. For a single relaxation process the absorption increases with decreasing frequency, passing through an inflection point at the frequency at (radians sec-1 = 2nf) which is the inverse of the relaxation time, t (seconds), of the chemical equilibrium [Eq. (6) and Fig. 3]. 

As ultrasound energy penetrates the body tissues, biological effects can be expected to occur if the tissues absorb the energy. The absorption coefficient (a) is used as a measure of the absorption in various tissues. For an ultrasound consisting of longitudinal waves with perpendicular incidence on homogeneous tissues, the following equation applies ... 

Similar to ultrasound, the strength of an electric field resulting from the absorption of plane parallel microwaves in a homogeneous tissue decreases exponentially (Bladel, 1964). The absorption coefficient, a, is related to the dielectric constant, e, the electrical conductivity, cr (mho/cm) and wavelength in air, A (cm). Both e and a are functions of the wave frequency and temperature. A number of investigators have measured e and normal tissues. We have measured cr and e of various mammary carcinomas and 9L-glioma at 37 and 43°C, and have compared the results with normal tissues (Peloso et al., 1984). These data have been fitted by the following equations ... 

The method of nuclear acoustic resonance proves to be suitable for studying the nuclear spin-phonon interaction using a moderate magnetic field. Resonance absorption of tiie ultrasound can be regarded as the phenomenon inverse to magnetic relaxation through one-phonon processes (Al tshuler and Kozyrev 1972). Therefore the considerations discussed earlier are also relevant to estimates of the magnitude of sound absorption by nuclei of the VV ions. The absorption coefficient of sound due to transitions between nuclear sublevels i, f takes the form of... 

Interaction between ultrasound and tissue can be characterized by a number of parameters like speed of propagation, attenuation, absorption coefficients etc. As ultrasound velocity is characteristic for a particular tissue the possibility is opened for ultrasound velocity tomography. The principles hereof and implications for medicine have been described by Greenleaf et al. (1975). In ultrasound velocity tomography transmitters and receivers are positioned around the object under study. Times of flight of ultrasonic pulses from transmitter to receiver are measured accurately. By rotating the transmitters and receivers in a plane around... 

Ultrasonic absorption coefficient Scattering methods Density fluctuations Ultrasound... 

The absorption of ultrasound in smectic phases is significantly more anisotropic than that in nematics, and even the velocity has a measurable anisotropy of about 5%. Details of the behaviour of SmA, SmB, SmC and SmE phases can be found in the literature [14—16, 18, 86, 94-97]. The usual approach to the analysis of smectic phases, based on the linear theory of elasticity and hydrodynamics, results in the relationship a—f, which does not agree with the experimental data. In the low-frequency range the coefficients a, o, 4 and c% demonstrate singularity, induced by nonlinear effects, in the form of oT. This results in a linear frequency dependence of the ultrasound absorption. The corrections for the coefficients of elasticity B and K, taking into account the nonlinear fluctuation effects in smectic phases, depend on the wavevector of the smectic phase layer structure B=(ln9,)- [96, 97]. In... 

---