Faez, T.

Subharmonic Venture (Doctoral Thesis)

2012-10-18T00:00:00Z

As a person, always fascinated with the presence of physics in the daily life challenges, let me share this joy with you! But before leaving you with complicated medical terms, allow me to tell you a story. My grandmother passed away when I was nine years old. She was the sweetest person I have ever known. She died on a hospital bed due to heart attack. Few minutes before her death, she had asked my uncle for a slice of watermelon. Her son, happy for the signs of fast recovery of his mom from the earlier heart attack she had just survived few days before, leaves her bedside to fulfill her wish. Within those couple of minutes he was away, my grandmother had a second heart attack and this time she never recovered from it. Unfortunate stories of this type are not unique. Many may have similar experiences or have heard similar ones from friends and acquaintances. What strikes me most is that these stories mostly have one thing in common: abruptness and unexpectation. It should be noted that a heart attack is not only reserved for aged fellows or those who do not follow a healthy diet or lifestyle. Many cases have been observed among young people and athletes who have never had any sign of heart trouble in their lives.

Second harmonic inversion for ultrasound contrast harmonic imaging (Article)

2011-06-07T00:00:00Z

Ultrasound contrast agents (UCAs) are small micro-bubbles that behave nonlinearly when exposed to an ultrasound wave. This nonlinear behavior can be observed through the generated higher harmonics in a back-scattered echo. In past years several techniques have been proposed to detect or image harmonics produced by UCAs. In these proposed works, the harmonics generated in the medium during the propagation of the ultrasound wave played an important role, since these harmonics compete with the harmonics generated by the micro-bubbles. We present a method for the reduction of the second harmonic generated during nonlinear-propagation-dubbed second harmonic inversion (SHI). A general expression for the suppression signals is also derived. The SHI technique uses two pulses, p′ and p″, of the same frequency f0and the same amplitude P0to cancel out the second harmonic generated by nonlinearities of the medium. Simulations show that the second harmonic is reduced by 40 dB on a large axial range. Experimental SHI B-mode images, from a tissue-mimicking phantom and UCAs, show an improvement in the agent-to-tissue ratio (ATR) of 20 dB compared to standard second harmonic imaging and 13 dB of improvement in harmonic power Doppler.

Characterizing the Subharmonic Response of Phospholipid-Coated Microbubbles for Carotid Imaging (Article)

2011-06-01T00:00:00Z

The subharmonic vibration of BR14 (Bracco Research S.A., Geneva, Switzerland) contrast agent microbubbles is investigated within the preferable frequency range for carotid ultrasound imaging (8-12 MHz). The response of the bubbles was recorded optically with an ultra-fast recording camera (Brandaris 128) at three acoustic pressures (50, 100 and 120 kPa). The vibration of the microbubbles was measured as a function of the excitation frequency and its frequency content was determined. Among 390 recordings, 40% showed subharmonic oscillations. It was observed that for smaller microbubbles (diameter < 3 μm) the frequency of the maximum subharmonic response increases for increasing pressures (shell hardening) opposite to what has been reported for larger microbubbles (3 μm < diameter < 15 μm). These findings are well predicted by the model proposed by Marmottant et al. (2005) after including the dilatational shell viscosity of the microbubbles measured by Van der Meer et al. (2007), which indicates a marked shear-thinning behavior of the phospholipid shell.

Characterization of Definity™ Ultrasound Contrast Agent at Frequency Range of 5-15 MHz (Article)

2011-02-01T00:00:00Z

The status of vasa vasorum, which can be imaged using ultrasound contrast agents, is an indication for the progression of atherosclerosis. The preferred ultrasound frequency for this purpose is between 5 and 15 MHz. Therefore, it is essential to have knowledge about the acoustic properties of microbubbles such as elasticity and viscosity to be able to implement the current models for lipid encapsulated microbubbles developed for frequencies used in precordial imaging. In this study, the shell parameters, stiffness Spand friction Sf, of Definity™ microbubbles have been calculated at frequency range of 5-15 MHz by comparing the theoretical modeling of acoustic bubble response and experimental measurements. Derived parameters are in good agreement with previous estimations on SonoVue™ and Sonazoid™ contrast agent. However, the value of Sfis higher than previously estimated for Definity™ between 12-28 MHz.