DESIGN AND EVALUATION OF MICROWAVE METHODS FOR MEASURING BLOOD PERFUSION IN TISSUE USING A FLOW MIMICKING PHANTOM SETUP

Open Access
Author:
Pardeshi, Jayendrasingh R
Graduate Program:
Electrical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 24, 2016
Committee Members:
  • Mohammad Reza Tofighi, Thesis Advisor
Keywords:
  • Microwave heating
  • microwave radiometry
  • blood perfusion measurement
  • bioheat transfer
  • temperature decay
  • microwave heating
  • temperature decay curve
  • microwave flowmetry
  • perfusion measurement
Abstract:
Blood perfusion is an essential constituent in the physiology of a biological tissue, having an essential role in the transportation of oxygen, nutrients, and pharmaceuticals to the tissue. Evaluation of skin perfusion is an important issue in the clinical medicine area such as the treatment of peripheral vascular diseases. Despite its significance, there is no widely acceptable method of noninvasively measuring local absolute perfusion without expensive radiological imaging devices. Therefore, in this thesis, a method is designed and proposed to use microwave heat for noninvasively measuring blood perfusion. Here, a microwave antenna is used to heat a tissue. Then the temperature signature of the tissue while cooling provides a measure of perfusion. A complete system would be made up of two parts, i.e., a microwave source for heating and a radiometric system, for temperature measurement of the target tissue. The focus of this project is on the microwave heating system. Although, a preliminary radiometry system was also tested. In order to achieve the main objective of the measurement of blood perfusion in tissue, a phantom setup has been developed. This setup includes dual-mode microwave antennas (two annular slot antennas on a single substrate) of two frequencies, precisely 0.9 GHz and 3.5 GHz. The 3.5 GHz antenna is used to feed a custom made radiometer fabricated utilizing a commercially available C band satellite low noise block. The 0.9 GHz antenna has been designed to heat a porous medium, (a soaked sponge) placed under it. This porous medium mimics a real tissue to provide more realistic results. To mimic perfusion during the testing, water flows through the sponge similar to the blood flow in a real body tissue. A thermistor is placed inside the sponge to measure the temperature variation for the experiment. Also, a flowmeter is used to precisely set flow levels between zero to a few mL/min (e.g., 5 mL/min) which corresponds to the expected rate of blood perfusion within tissues. The temperature in the setup is raised by different amounts, typically 0.2 °C to 0.5 °C in different experiments to observe results. The sponge is irradiated by 0.9 GHz microwave power at a range of 1 - 5 W. Measurements were performed with different sponge thicknesses, and with or without the radiometer in place. Circulation within the sponge was improved by boring a hole through it. Consistent thermistor readings were obtained without the radiometer. However, when the radiometer was used, readings for both the thermistor and radiometer were not consistent. This is speculated to be due to thermal interference caused by the DC electrical power consumed by the radiometer. The annular slot antennas used in the setup were designed by using ANSYS HFSS (ANSYS, Inc., Canonsburg, Pennsylvania). For radiometry, different frequencies enable RF reception from different depths in the tissues, as the penetration depth of the microwaves decreases as the frequency increases. To understand flow distribution in the developed blood perfusion measurement system, the flow in the setup was also simulated by a computational fluid dynamics (CFD) modelling software, i.e. ANSYS CFX (ANSYS, Inc., Canonsburg, Pennsylvania). To further study the experimentally observed microwave heating, the power absorption pattern of the antenna is exported from HFSS as the local specific absorption rate (SAR) in the tissue. This power absorption pattern of the antenna is in turn imported in CFX to be utilized in the simulation. This process would help in verifying the procedure, which has been developed for the measurement of blood perfusion, by comparing the results of simulation with those obtained from the tests performed on the phantom setup.