| Hyperthermia becomes an important tumor treatment modality, after surgical treatment, chemotherapy, radiotherapy and biological immunotherapy. For hyperthermia there are two modes for heating, one mode is warm-heating method, killing tumor cells by heating the tumor to reach the highest temperature of its endurable limitation. The other mode is high temperature coagulation method, which heating tumor to reach above 65℃in a short time and to coagulate the proteins inside the tumor. It's important to monitor the temperature during hyperthermia so that the tumor cells were most killed and the surrounding normal tissues were less injured.At present, invasive methods was used for measuring tissue temperature in clinical. Temperature achieved by inserting thermocouples into the tumour. Invasive methods can only provide temperature measurements at a limited number of points in the tissue, and the implantation of probes is traumatic for the patient. To overcome these problems, much effort is now being put into the development of non-invasive methods of temperature monitoring. Non-invasive techniques are preferable for clinical thermometry and a number of different physical principles have been explored for this purpose includes ultrasonic, nuclear magnetic resonance, x-ray compute tomography, microwave, electrical impedance tomography.Electrical impedance tomography method monitors the changes of the tissue's bio-impedance to estimate temperature based on the relationship between resistivity and temperature of tissue. EIT has several advantages over other tomographic methods: the instrumentation is relatively inexpensive, transportable and can easily be accommodated in a hyperthermia treatment environment.There are a few studies on the relationship between resistivity and temperature of liver tissue and the results differ much. The previous studies cannot obtain statistically significant conclusion because of many reasons such as disaccord of materials, methods and condition control etc. There is not yet an international widly-acknowledged standard. The relationship is generally simplified into linear relationship of 2%/℃. Which notably becomes the bottleneck problem for futher development of related studies.In order to estimate the temperature during hyperthermia, the more accurate relationship between resistivity and temperature should be acquired. In this study, the impedance changes of materials(NaCl solution, agar, fresh rabbit liver) were measured during heating and cooling processes. Also the heating and cooling processes of pig liver sample were monitored by EIT based on phantom. The main contents are as follows:⑴set-up of the standardized liver impedance spectroscopy measurement platform.Based on a variety electrical impedance spectrum measurement of tissues such as animals, human skull,and breast tissue were completed by our team. We established the standardized measurement system of liver impedance spectroscop, designed and made the measurement assembly of liver impedance spectroscopy, and verified the measurement precision of the measurement system.①The measurement system of liver impedance spectroscopy was established based on impedance analyzer (Agilent 4294A). Agilent impedance analyzer 4294A was connected to a laptop via GBI to USB converter, its driving current was set to 0.5mA. Impedance data was recorded at each integer temperature points. At each temperature a total of 201 logarithmically spaced frequencies ranging from 100Hz10MHz were swept and the measurement was performed for at least 6 s.②Designing and making of the impedance spectroscopy measurement assembly for liver. According to the construction features of liver and practical requisition, the measurement assembly of liver impedance spectroscopy using the standard four-electrode method was designed. The main material of the assembly was nonconducting plexiglass and the electrode material was pure silver.③Designing and making of the heating cabinet. Cabinet was quickly heated by two crystal heating tube (300W). The temperature in the cabinet was controlled by a temperature auto-control instrument. A temperature sensor (Thermocoax, fluke17B) was inserted into the tissue to monitor tissue temperature in real time.⑵Study on the relationship between resistivity and the temperature of saline. Based on the measurement system mentioned above, the impedance spectrum of saline were measured during heating process and cooling process in the temperature range of 25℃80℃.The findings suggest that the Temperature-Resistivity Variation Factor (TRVF) mainly fluctuates between -1.0% -2%.⑶Study on the relationship between resistivity and the temperature of agar.Based on the measurement system mentioned above, the impedance spectrum of agar were measured during heating process and cooling process in the temperature range of 25℃80℃. At the frequency of 1kHz, with temperature below 55℃the TRVF mainly fluctuates between -2.0% -3%. From 55℃to 67℃, TRVF rapidly changes from -2.0% to -5%. From 68℃to 82℃, TRVF changes rapidly from -5% to + 1.0%.⑷Study on the relationship between resistivity and the temperature of fresh rabbit liver tissue in high temperature coagulation method.The impedance changes of 12 fresh rabbit livers were measured in the frequency range of 100Hz 10MHz during heating and cooling processes in the temperature range of 30℃80℃. At the frequency of 1kHz, with temperature below 58℃the TRVF mainly fluctuates between -1.0% -2.2%. From 58℃to 69℃, TRVF rapidly changes from -2.0% to -14.7%. At 69℃, 70℃and 71℃, TRVF maintains around -14.7%. From 71℃to 76℃, TRVF changes sharply from -14.7% to - 1.0%. TRVF is mainly -1% at temperature above 76℃.⑸Applying EIT to monitor hyperthermia treatment based on phantomThe cylinder phantom is 284mm in diameter and 240mm in height. Sixteen electrodes were equally placed around phantom. NaCl solution with a resistivity of 480Ω·cm was injected to simulate abdominal resistivity (500Ω·cm) distribution. A cartridge heater was inserted into the tissue of pig liver about 30mm in depth. The temperature of the cartridge heater was controlled by the temperature auto-control instrument. Another 3 temperature sensors (T2, T3, T4) also placed in different positions to monitor the temperature distributions inside the tissue during heating and cooling. EIT system (1250μAp-p, 50kHz) applies polar driving and adjacent measurement mode and its imaging speed is 1frame/s.From the series of images, it can be seen that the color of region of interest (ROI), corresponding to the heating tissue, becomes redder and the size of the ROI grows larger as temperatures elevation. This indicates that the changes in images are correlated with the temperature of the tissue. Analyses were conducted on the relationships between average boundary voltage and temperature and reconstructed value of ROI and temperature.The results of curve fitting shows that better outcome would be obtained when the temperature is characterized by boundary voltage and reconstruction of ROI. The maximum absolute fitting error of boundary voltage and reconstruction of ROI are around 3.6 and 1 degree celsius respectively. This indicates that EIT may be applied to precisely monitor the temperature change and its distribution. |
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