Date of Completion

5-6-2016

Embargo Period

5-6-2017

Major Advisor

Leila Ladani

Associate Advisor

Jiong Tang

Associate Advisor

Nejat Olgac

Associate Advisor

Bi Zhang

Associate Advisor

Quing Zhu

Field of Study

Mechanical Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Ultrasonic measurement allows for nondestructive testing and inspection of the internal structure of tissues and materials. Ultrasound has a number of advantages over other commonly used nondestructive methods. It is safe, relatively inexpensive, and is the most commonly used health diagnostic imaging method. Nevertheless, traditional ultrasound does have disadvantages. Standard methods detect the presence of flaws and inclusions from amplitude and time-of-flight information. However, these techniques provide little insight into the actual material characteristics of the sample. Available techniques use a limited, low range of frequencies. The measurements also need to be interpreted by trained technicians and can vary among ultrasound systems. The field of Quantitative Ultrasound was developed to address these shortcomings by looking deeper into the characteristics of the received signals.

The objective of this dissertation is to understand the scattering behavior of high-frequency signals for relatively small specimens and relate that to the microstructure of materials. In particular, a recently developed quantitative parameter which was found to be responsive to tissue microstructure, peak density, is investigated. Due to the infancy of using peak density, the physical characteristics that affect it are not fully understood.

The procedures and algorithms developed for characterizing materials based upon peak density are discussed. The results of experimental studies using tissue-like phantom materials containing internal inclusions are presented. Two-dimensional images created using peak density measurements were found to locate the inclusions better than standard amplitude based images.

It was found that peak density was highly reliant on the specific signal processing methods used. The process is then optimized and standardized to generate high quality measurements. The response to microstructure is investigated by studying tissue-mimicking phantoms containing glass scatters of various sizes and number densities. Finite element simulations of ultrasound waves traveling through tissue phantoms containing glass microspheres are developed, and the results are compared against experiment. From the simulations and experiments it was established that peak density is most responsive to larger numbers of scatterers present in the material and is more reliable for scatterers with diameters near the wavelength. The peak density was also found to be more sensitive than amplitude-based techniques.

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