Citations

Theories on acoustic radiation pressure associated with plane acoustic waves and lossless media have been examined extensively. The radiation pressure produced by a non-plane wave in a lossy unbounded medium was analyzed. The distinction between Lagrangian and Eulerian pressure was used to clarify the understanding of radiation pressure. The radiation pressure exerted on a perfectly absorbing pointwise target by an arbitrary acoustic wave in an unbounded lossy medium was then considered. By using an indirect approach developed in Chu and Apfel [B.-T. Chu and R. E. Apfel, 1673–1687 (1982)], the radiation pressure was analyzed without appealing to the detailed solutions of the governing fluid dynamic equations. It was shown that even in lossy media with arbitrary waves, the radiation pressure experienced by a pointwise target is equal to the Langevin pressure which is the time average energy density at that point. The radiation pressure exerted on a specific target by an arbitrary acoustic wave in an infinite lossy medium was studied and a concise theoretical result was obtained. [The work was supported by grants CA 43920 and HL 41046 from the National Institutes of Health.]

Background and Purpose— With the changes in management of atrial fibrillation (AF) over time, it is possible that the time trends of post-AF stroke incidence and mortality have changed. We sought to determine whether the incidence and survival of ischemic stroke after AF diagnosis have improved. Methods— We identified the Olmsted County, Minn, residents who developed first AF from 1980 to 2000 and followed them in medical records to 2004. The outcomes were first ischemic stroke and death. Results— Of the 4117 subjects diagnosed with first AF and without previous stroke, 446 (11%) sustained a first ischemic stroke during a mean follow-up time of 5.5±5.0 years. The age- and sex-adjusted incidence of stroke decreased, on average, by 3.4% per year ( P =0.0001), concurrent with an increase in warfarin and aspirin use (both P <0.0001) and reduction of systolic blood pressure ( P <0.001). The age-adjusted ischemic stroke incidence was higher in women ( P =0.039), but not after adjusting for systolic blood pressure ( P =0.41). Compared with the general Minnesota white population, the relative mortality hazard ratio was 1.88 for men and 1.84 for women without stroke and 3.03 for men and 3.80 for women ( P <0.05) with stroke. The relative mortality hazard did not vary by age or calendar year of AF diagnosis. Conclusions— Post-AF ischemic stroke incidence decreased significantly from 1980 to 2000, during which time a substantial increase in the use of antithrombotic therapy and reduction of systolic blood pressure was evident. The relative mortality risk of stroke, however, had not improved over time.

A general equation is derived that describes the behavior of a piezoelectric transducer with a nonuniform distribution of piezoelectric coefficient within its bulk, when submitted to an arbitrary distribution of acoustic pressure. Based on this equation, an expression for the receiving transfer function of the transducer is calculated. The results demonstrate the dependence of the transfer function on the distribution of piezoelectric coefficient, and that it is possible to benefit from a nonuniform distribution to optimize the transfer function. The general equation also describes the influence of the external electric circuit loading the transducer, which leads to another independent means of optimizing the transfer function. The proposed model combines effects of piezoelectric material characteristics, acoustic backing, and electric loading, without resorting to Mason or other equivalent circuits for the transducer.

The heart is a three-dimensional structure that performs its function by undergoing complex deformation. Current clinical evaluation of the heart is limited to silhouettes, outlines, projections, or tomograms, obligating the clinician or investigator to sequentially evaluate a series of two-dimensional images using mental assimulation of data. To provide more easily understandable three-dimensional images, techniques have been developed for acquiring data in sequential planes through the heart to display the beating heart in three dimensions. Analysis of such data on a user-friendly workstation can be accomplished using an extensive set of software tools developed at our institution. Three-dimensional imaging can be used as an effective means of teaching interrelationships of volume, contents, and function, in addition to the physiologic and anatomic interrelation-ships. Three-dimensional images of the beating human heart obtained in vivo using new data acquisition techniques will be shown. Evaluation of the method using balloons and excised hearts has shown that software and geometry of scanning are appropriate for high-accuracy measurements of surfaces.

The orotic acid dietary “liver lipid” model has successfully been used to study ultrasonic properties in excessively fatty rat livers [O'Brien et al., J. Acoust. Soc. Am. 83, 1159–1166 (1988)]. The model was used again in order to evaluate more fully ultrasound properties within a more condensed (2%–10%) lipid concentration range. The fatty livers were first evaluated in the intact animal (in situ) with an ultrasonic imaging system (≅ 5 MHz) and then the excised (in vitro) livers were evaluated over the 1- to 100-MHz frequency range. Water, lipid, total protein, and collagen were evaluated for each liver. Significant (p &lt; 0.001) correlations of ultrasonic properties (absorption, all attenuation, speed, and Markovian entropy) against tissue properties (water and lipid) were obtained, thus suggesting that the independent measure of texture may relate to basic ultrasonic propagation properties. [Work supported by NIH Grant CS36O29.]

On the basis of many measurements on attenuation, absorption, and speed of ultrasound in biological materials, one can make the following generalizations: (a) Biological tissues have high attenuation coefficients, about two to three orders greater than their major component, water; (b) the amplitude attenuation coefficient increases with frequency near-linearly in soft tissues and near-quadratically in fluids with frequency in the range 1 to 10 MHz, and possibly even up to 100 MHz; (c) speed dispersion is extremely small (∼0.1 to 1 m/s/ MHz); (d) absorption and attenuation coefficients either increase or decrease with an increase in temperature, depending on the nature of the tissue, and the frequency of irradiatiOn; (e) fluids containing proteins show a sharp increase in excess absorption in acidic and basic pH ranges, and finally; (f) at sufficiently high intensity of irradiation the absorption coefficient increases with intensity. Comprehensive models for the propagation of ultrasonic waves through lossy media should explain these experimental findings. We will review some of the current models and critically examine them in light of the definition of absorption in relation to attenuation and scattering. [Work supported by NIH CA 24085, CA 41324, and NSF 8310626.]

The validity of wave equations employed as system models in acoustical diffraction tomography is investigated using simulations and measurements of the scattering of plane ultrasound waves by cylinders. It is demonstrated by simulation and experiment that it can be appropriate to neglect density fluctuations and shear waves, implying that the commonly used form of the wave equation suitably describes scattering by fluctuations of acoustic speed and absorption. Diffraction tomographic reconstructions of simulated data reveal the importance of absorption, the behavior of the real and imaginary parts of the reconstructed refractive index, and the relative advantages and limitations of the Born and Rytov approximate transformations.

Changes in the nonlinear parameter B/A caused by the addition of monohydric alcohols to water have been investigated. The measurements, made by using an aqueous solution of n-propanol, isopropanol, and t-butanol, show that as the alcohol concentration is increased, B/A decreases to a minimum at approximately 10% concentration (by volume). Further increase in alcohol concentration raises B/A rapidly to a maximum at about 80% by volume. This complex variation in B/A is attributed to solute–solvent interaction which is believed to be due to an enhancement of water structure in the water-rich region. These results suggest that the magnitude of B/A may be closely related to the quasilattice structure of water, which to a first approximation can be expressed by relative amounts of bound and unbound water.

A study of the statistics of ultrasound speckle indicates that the noise-to-signal ratio (NSR) of the echo envelope peaks (EEP) in a B scan is monotonically related to the variance of the mean power of the backscattered echoes. If it is assumed that the backscattered echoes all have the same mean power, the NSR of the EEP’s in a B scan will be expected to reach a theoretical minimum value NSR0 (0.40∼0.42). In practical situations, the variance of the mean power of the echoes is increased due to the presence of both attenuation (including beam spreading) and the spatial variation in the backscattering cross section (BCS) of the tissue. Accordingly, the measured value of the NSR of the EEP’s is expected to be greater than the NSR0. In principle, the effects of beam pattern and the attenuation can be compensated for by system calibration and a depth-related gain function, respectively. The attenuation coefficient of the tissue may then be obtained by adjusting the gain function to minimize the NSR of the EEP’s. Due to the random nature of the heterogeneity of diseased tissue, the effect of the variation in the BCS, however, cannot be compensated for by the depth-related gain function. Therefore, the minimum value of the NSR indicates the spatial variation in the BCS and may finally correlate to the disease state of the tissue.