|M.Sc Student||Gargir Oren|
|Subject||Application of Coded Excitations in Ultrasonic|
Thermal Therapy Monitoring
|Department||Department of Biomedical Engineering||Supervisor||Professor Haim Azhari|
|Full Thesis text|
The ability to track thermal changes in non-invasive therapeutic procedures, namely High Intensity Focused Ultrasound surgery, is currently very limited. Magnetic Resonance Imaging (MRI) is commonly used, but this method of thermal monitoring is very expensive and access to the patient is restricted. Ultrasonic thermal monitoring may potentially provide a cost effective real time alternative to MRI.
The objectives of this research thesis were: (i) to investigate the acoustic properties of tissues during externally induced ultrasonic thermal changes using both pulse echo and through transmission ultrasonic scans, and (ii) to investigate the potential advantages of transmitting coded excitations in low noise environments.
The research comprised of computer simulations and experimental studies of through transmission and pulse echo ultrasonic waves. In each experiment, three different transmission signals were used: (i) a binary phase modulated Barker sequence, (ii) a linear frequency modulated chirp and (iii) a bandwidth limited Gaussian pulse.
A data processing algorithm, designed for advantages inherent to the coded excitation signals, was developed to track echo time shifts resulting from thermal speed of sound changes. In the through transmission scans, echo-shifts were used to measure the speed of sound directly. In the pulse echo imaging, echo-shift pairs were evaluated to create normalized speed of sound curves.
The computer simulation served as a proof of concept to validate the feasibility of our suggested method for pulse echo imaging.
Phantom results showed that speed of sound measured by ultrasonic through transmission method is highly correlated with the temperature changes. Normalized pulse echo measurements were then evaluated directly against the through transmission speed of sound calculations. This evaluation showed that the pulse echo speed of sound changes trend and mimic that of the through transmission measurements, but with less consistency. It was found that the use of coded excitation signals in pulse echo imaging performed statistically on par with the Gaussian pulse in low noise environments, and statistically better in low signal to noise ratio (SNR).A method for tracking thermal changes based on echo time shifts in tissues was presented.
In conclusion, it was found that ultrasonic through transmission can be used as a thermal monitoring gold standard. It was shown that the pulse echo, echo-shift imaging with the developed algorithm is feasible to trend thermal changes in tissue. The use of coded excitations was proven to be a useful tool for improving the detectability of temperature induced echo-shifts in low SNR environments.