Study on Method and Key Technique of Tissue Temperature Estimation in Hyperthermia
|School||Beijing University of Technology|
|Keywords||Hyperthermia Microwave coagulation Noninvasive temperature estimation Specific Absorption Rate (SAR) Ultrasonic Tissue Character (UTC)|
Hyperthermia means to heat tumor tissue until exceeding its durable temperature by some kinds of heating methods, including traditional hyperthermia and thermal coagulation. Traditional hyperthermia kills tumor cells by making tissue temperature at 42.5-45℃for a moment (commonly keeping several minutes or tens minutes).Thermal coagulation therapy means to heat the local tissue by microwave or others up to the coagulating temperature (commonly heating to 60℃instantaneously or keeping 54℃for one minute). Those two mechanisms are both adopted to kill tumor tissue. Especially, with the development of percutaneous microwave, radio frequency and ultrasound, thermal coagulation has been widely used in clinical treatment of cancer in recent years, and has been one of the therapeutic methods to cure malignant tumor.During the treatment, the temperature of tissues must be kept within a suitable range so that the therapy can be carried out effectively and safely. If the temperature inside the area of treatment drops below given value, there will be no intended effect. On the other hand, if the temperature is too high, the“healthy”cells will be also affected, not only the tumor’s ones, which is not intended. Current clinical methods used for measuring tissue temperature are invasive in nature. However, such invasive temperature measurements are potentially dangerous and may cause higher health risks to patients. There are many research reports about noninvasive temperature estimation in hyperthermia; ultrasound temperature estimation is one of them. This technique is using some parameters of sound characteristic and its temperature correlation to obtain the information of tissue temperature, compared with other methods, it has more advantages. Therefore, the use of noninvasive temperature measurement methods with relatively high precision is more preferred.After analyzing the advantages and limitations of each kind of method for temperature estimation, and according to the practical demands for noninvasive temperature estimation of tissues in liver cancer hyperthermia, the method of noninvasive estimation and its key technique in hyperthermia are studied. The technique of ultrasound temperature estimation is discussed by signal and image processing aspects, in order to find a practical method for hyperthermia monitoring in clinic.It can be divided as following.(1) The noninvasive ultrasound temperature estimation system was established. Based on the degassed water and fresh in-vitro pig liver, the correlation of echo shifts of backscattered ultrasound and temperature in tissue was studied. The results show that velocity of ultrasound varied in accordance with the temperature, and it caused the change of the echo shifts with the tissues heat expanding in heat field; the echo shifts varied in accordance with the temperature, and there is a stable linear function between them. According to the temperature correlation of echo shifts, an ultrasound temperature estimation system has been established, which mainly consists of ultrasound emitter and receiver, high speed AD card and PC. The time delay of backscattered ultrasound can be detected and temperature of tissue can be measured and displayed in the thermometry system.(2) The variation of characteristic parameters of B-mode ultrasound in hyperthermia was studied, in order to using the temperature correlation of characteristic parameters of B-mode ultrasound to realize noninvasive temperature estimation. The experiments were carried out on in-vitro animal tissues that were heated by hot water and microwave coagulation, and a series of B-mode ultrasonic images of liver were obtained at different temperatures in the process of traditional hyperthermia and thermal coagulation. Then the images were processed to extract some characteristic parameters of B-mode ultrasound (gray and its gradient variation, power spectral density, texture structure, etc.), and the temperature correlations of these ultrasonic tissue characteristic parameters were calculated. It has been found that the gray of B-mode image varied in accordance with the temperature changes in the heated tissue, and the average correlation coefficient is about 0.9. The value of image gray is increased with temperature rising, and there is a stable linear function between them. The correlation coefficient between gray gradient of the images and temperature is about 0.7. Besides, the power spectral density of the images, the standard deviation of the density, and the unevenness are also changed with temperature rising, it is consistent with the experimental results of tissue protein solidification in microwave ablation. The results show that the method is feasible to temperature estimation.(3) The heat field distribution of the novel water-cooled microwave antenna in clinical liver cancer therapy was studied, in order to predict temperature field accurately before microwave coagulation. Compared with the normal microwave radiator, water-cooled antenna can reduce the carbonization of tissue, and increase coagulation area of the tissue under the same heating condition. Using the thermocouple made by ourself, which is small diameter and high impedance, the Specific Absorption Rate (SAR) distribution in equivalent phantom of standard static liver tissue was also determined, so it is the basis of the computer simulation in temperature field and surgical planning of the microwave coagulation. Considering the character of tissue thermophysical properties varied in accordance with the temperature changes, in order to improve the fidelity of the temperature field near the antenna, the shape and the size of the thermo coagulation area and temperature rising law were obtained after correction on in-vitro experiments.(4) Ultrasonic Tissue Character (UTC) means to reflect tissue physiological and pathological information by tissue ultrasonic characteristic, its development and improvement will bring more developing prospect in hyperthermia. As an important parameter, the change trend of the Integrated Backscatter (IBS) and the relativity between IBS and temperature are presented and the results show that the value of the IBS is firstly increased and then reduced according to temperature rising. The process is non-reversible and can affect the ablation of tissue. The preliminary work shows a possibility prospect of using the ultrasonic tissue characterization as a base to detect and monitor tissue state of microwave ablation.