Ultrasonic interferometric measurements of the shear elastic properties of MgSiO3 perovskite were conducted on three polycrystalline specimens at conditions up to pressures of 8 gigapascals and temperatures of 800 kelvin. The acoustic measurements produced the pressure (P) and temperature (T) derivatives of the shear modulus (G), namely ( partial differentialG/ partial differentialP)T = 1.8 +/- 0.4 and ( partial differentialG/ partial differentialT)P = -2.9 +/- 0.3 x 10(-2) gigapascals per kelvin. Combining these derivatives with the derivatives that were measured for the bulk modulus and thermal expansion of MgSiO3 perovskite provided data that suggest lower mantle compositions between pyrolite and C1 carbonaceous chondrite and a lower mantle potential temperature of 1500 +/- 200 kelvin.
The application of therapeutic ultrasound for the treatment of atrial fibrillation (AF) is investigated. The results of theoretical and experimental investigation of ultrasound ablation catheter are presented. The major components of the catheter are the high power cylindrical piezoelectric element and parabolic balloon reflector. Thermal elevation in the ostia of pulmonary veins is achieved by focusing the ultrasound beam in shape of a torus that transverses the myocardial tissue. High intensity ultrasound heating in the focal zone results in a lesion surrounding the pulmonary veins that creates an electrical conduction blocks and relief from AF symptoms. The success of the ablation procedure largely depends on the correct choice of reflector geometry and ultrasonic power. We present a theoretical model of the catheter's acoustic field and bioheat transfer modeling of cardiac lesions. The application of an empirically derived relation between lesion formation and acoustic power is shown to correlate with the experimental data. Developed control methods combine the knowledge of theoretical acoustics and the thermal lesion formation simulations with experiment and thereby establish rigorous dosimetry that contributes to a safe and effective ultrasound ablation procedure.Keywords ultrasound therapy; high intensity focused ultrasound; atrial fibrillation 1.INTRODUCTIONAtrial fibrillation (AF) is identified as a leading health risk cardiac arrhythmia. A patient with AF is more likely to have a stroke than the general population. With the aging of the population in industrialized countries the rise in the prevalence of AF becomes a major health concern. The current treatments for AF are either highly invasive or intricate and time consuming, and all are accompanied by a significant risk of morbidity and mortality. This creates a backlog of individuals who would be treated if an effective and simple procedure were developed.AF has been shown to initiate from triggers within pulmonary veins (PV) [1]. Initially it was treated by the highly invasive surgical maze procedure [2], which was primarily performed only in conjunction with other open-heart surgeries, such as valve replacement. This stimulated the development of less invasive procedures focusing on PV isolation, with catheter based NIH Public Access Author ManuscriptAcoust Phys. Author manuscript; available in PMC 2010 July 21. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript radio frequency (RF) ablation leading the way. Since then catheter ablation to cure AF has developed to be the primary focus of many electrophysiologists around the world, spawning numerous variations ranging from "fire" [3] to "ice" [4]. Pappone et al. [5] developed a technique for the treatment of AF that combines both aspects of the maze and PV isolation approaches, and applied this technique to both paroxysmal and chronic AF.While catheter-based RF ablation has been proven to be successful, the procedure is very lengthy (4-8 hours), requires a large inf...
Garnets with compositions between majorite and pyrope, Mj38, Mj48, Mj75 and Mj79 were synthesized at high pressures and temperatures in a 2000‐ton uniaxial split‐sphere apparatus (USSA‐2000) and investigated using high resolution synchrotron X‐ray powder diffraction and transmission electron microscopy. The results from both techniques are consistent with the tetragonal field for these garnets extending to a majorite composition just below Mj75. The cubic‐tetragonal structural phase transition in garnet along the majorite‐pyrope join is sensitive to both composition and temperature and is expected to result in anomalous behavior in elastic shear moduli. This phase transition may occur in the transition zone of the earth's mantle and will have important effects on the elastic and rheological properties of this region where these garnets are stable phases.
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