M.Sc Student | Yeshurun Lilach |
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Subject | Ultrasonic Imaging and Measurement of Thermal Diffusivity of a Tissue |
Department | Department of Biomedical Engineering | Supervisor | Professor Haim Azhari |
Full Thesis text | ![]() |
The major mechanism in noninvasive surgery using HIFU is thermal
ablation. Extensive research was conducted in order to obtain temperature
mapping of the procedure, however, this is not the only important feature in
monitoring thermal ablation. The thermal diffusivity at the ablation site also
plays an important role in the final therapeutic outcome, as it influences the
temperature's distribution achieved in the tissue in space and time. Moreover,
tissue thermal diffusivity is different in tumors as compared to normal tissue.
This study presents a method for estimating, non-invasively, the thermal
diffusivity using Through Transmission (TT) ultrasound imaging and HIFU
heating. A computed ultrasound system for scanning the tissue in a TT mode was
used during its cooling process. The experiments were conducted on a tissue
phantom made of Agar and on porcine fat. A HIFU system was used for heating
the tissue. The temperature was kept lower than to
avoid irreversible damage. TT scanning provided the time of flight (TOF) values
of a chosen area in the tissue. The TOF values were consequently converted into
average values for the Speed of Sound (SOS). The average SOS profiles, during
cooling, were registered. A model for evaluating the spatial distribution of
the SOS and its relation to temperature, in one slice of the tissue, was developed.
Using the average SOS profiles along with the model, the changes in temperature
profiles over time were estimated. These changes in the temperature profiles
were then used for calculating the corresponding thermal diffusivity of the
studied specimen. The results have demonstrated the ability to assess the
thermal diffusivity non-invasively using TT ultrasound. The empirically obtained
values matched the reference values found in the literature and values obtained
by a simple heat transfer experiment. Thermal diffusivity for porcine fat was found
to be one order of magnitude lower than the one for agar. This stems from the
fact that fat is a natural insulating material. The fact that the results for
agar and porcine fat were significantly different indicates that thermal
diffusivity may also be used for characterizing tissues and for differentiating
between different tissue types. This method may potentially be used as a
diagnostic tool for breast tumors characterization and for success assessment
of thermal ablation treatments.