Date of Award

1-1-2013

Document Type

Open Access Thesis

Department

Mechanical Engineering

First Advisor

Sourav Banerjee

Abstract

Acoustic microscopy provides extraordinary advantages over state-of-the-art invasive imaging techniques to determine the mechanical properties of living colonies of pathogens and micro-organisms. It is possible to obtain the morphomechanical parameters of the pathogenic colonies e.g. variation of thickness, stiffness and the coefficients of attenuation, using scanning acoustic microscope (SAM). However, the process requires an expert with extensive understanding of SAM and ultrasonic signals which is very time consuming and expensive for complex form of analysis. Due to lack of a suitable computational tool, presently the ultrasonic wave scattering, reflection and transmission through the biological specimens cannot be properly visualized. Without any reliable simulated environment, it is extremely difficult to extract the morphomechanical parameters from the invading pathogens. To understand the ultrasonic signals that are reflected or scattered back from the biological specimens, one would need to compute the Pupil Function (PF), i.e. generated by a particular SAM lens. PF is the total pressure field in front of the lens at focal plane generated by the lens and cannot be experimentally measured without placing a reflecting surface in front of the lens. Hence to determine the PF one could change the interpretation of PF. Thus a detailed computer simulation platform for the SAM experiments is necessary. Particularly it is mandatory to obtain the accurate PF that is generated by a particular SAM lens used in the experiments before decoding the morphomechanical properties of the biological specimens.

To obtain the accurate PF in front of an acoustic lens, this dissertation presents a detailed development of Distributed Point Source Method (DPSM) for modeling SAM experiments. The ultrasonic field in front of the focused 100 MHz lens, obtained from the simulation can be further used to determine the material properties of the biological specimens. An accurate modelling of SAM lens using the distributed point source method (DPSM) is proposed for its proven capability to simulate ultrasonic fields at higher frequencies. DPSM is computationally cheap and efficient than the Finite Element Method (FEM).

The acoustic lenses used in the SAM are commonly made of sapphire but enclosed with a brass casing. The sapphire head consists of four different geometrical shapes and each segment has individual influences on the visualization of the ultrasonic field produced by the transducer. Thus, the accurate geometry of the acoustic lens is an important factor for modeling. Using the DPSM accurate geometry of a 100 MHz lens is modeled and the PF is computed in front of the lens. It is shown that as per the design specification of the lens, the pressure field is accurately focused at the focal point. The peak pressure at the focal point and the rippled wave effect away from the focal point are verified in the DPSM based simulation environment.

Rights

© 2013, Rowshan Ara Rima

Download

Share

COinS