Study On Multiple-mode Thermal Fields In Tissues By Ultrasound Phased Arrays

Cancer is one of the life-threatening diseases. Surgical resection is presently the treatment of choice for both well-localized primary and metastatic malignancies. However, the majority of the patients are not candidates for surgical resection due to certain criteria, such as multifocal disease or location of tumor to key vessel. Thus, there is a demand for thermotherapy techniques for cure of malignancies. The thermotherapy technique has become increasingly interesting in cancer therapy. The phased focused ultrasound technique, as a noninvasive treatment technique, has become a hot topic in the research of cancer treatment. At present, the phased focused ultrasound technique involves using driving signal phase shift in conjunction with amplitude regulation to produce high temperature fields of single focus and multiple foci patterns in tissues where the temperatures can reach 70°C and above within a few seconds inducing irreversible local coagulation necrosis. Furthermore the phased focused ultrasound technique can involve using phased patterns combination in conjunction of amplitude regulation to create mild uniform temperatures over various tissue volumes where the temperatures are elevated and maintained at 40°C-45°C during the therapy. The tissue with mild hyperthermic temperature deep in the body gives condition for combined cancer therapy by chemotherapy and hyperthermia, thermo-sensitive liposome and hyperthermia and heat-sensitive trans-gene and hyperthermia etc.We proposed an ideal for cure of malignancies by using the phased focused ultrasound technique based method of combination of multiple thermal field modes after investigation of progress of phased focused ultrasound technique. For this purpose, we studied the modeling of multiple-mode thermal-acoustic fields from ultrasound phased arrays, the fabrication of phased arrays system and optimal design of array applicator, key technique for generating controllable local thermal field and multiple-mode thermal distributions from ultrasound phased-array under different physiological and treatment conditions. Accurate models are important in understanding thermal distribution in the tissue better and evaluating the performances of transducers of new design and new treatment application. Optimal design of phased array system offers hardware foundation for generating mild temperature distribution over various volumes. Key technique for generating controllable local temperature field is used to creating even mild temperature distribution tailored to various tumor shapes. The analysis of multiple-mode thermal fields from ultrasound phased-array under different physiological and treatment conditions facilitates improving treatment planning.In this paper, the theory and method for calculating multiple-mode thermal-acoustic fields were studied. An ultrasound thermal model, with finite element representation for modeling thermal diffusion was developed. The model incorporates dynamic behavior of acoustic parameters and blood perfusion as a function of temperature and thermal dose and the ultrasonic power deposition as function of time. A prototype system of spherical-section ultrasound phased arrays with phased precision of 1.40625°, which can generate desired multiple-focus field, was designed and developed. Furthermore, the effects of the design parameters of arrays on acoustic field, including the shape and curvature radius of applicator, the arrangement pattern, number and size of element were investigated. Based on the results, the array design of prototype system was optimized. It has been found that the holed annular spherical-array comprising 108 elements with unequal space between elements is able to heat large deep seated tumor with mild hyperthermic temperature as well as ablate small tumor.A new method for targeted heating of deep tissues was developed by using phased low intensity ultrasound which can generate various multiple foci patterns by electronically changing its amplitude or phase pattern. This method involves using a technique of combining switching and rotating of multiple foci patterns to create a uniform temperature over various tissue volumes. Using this method, the target tissue deep in the body can be heated to a specified temperature, which gives conditions for thermo-sensitive liposomes release. A simulation study for a 108-element, spherically sectioned array was performed to determinate an optimal heating scheme from a set of multiple focus fields which were produced by inputting different combinations of phases and amplitudes. The simulation results show that the therapeutic heating zones in various sizes (Ф5 mm-Ф40 mm) with uniform temperature distributions can be obtained employing the combined method. Furthermore, the influence of perfusion and circle time on the uniform temperature distributions by the technique of combining switching and rotating of multiple patterns was investigated. With the knowledge of perfusion effect, an approach for controlling of the focal intensities based on rise velocity of temperatures of key points was presented. Using this approach, the focal intensities of multi-modes can be regulated adaptively and the expected uniform temperature over various tissue volumes can be achieved.The role of dynamic changes in blood perfusion and ultrasound attenuation in lesion formation of multiple-focus pattern during ultrasound surgery was analyzed. The results show that thermal-dose-dependent ultrasound attenuation is the dominating factor in the full dynamic model. If the dynamic ultrasound attenuation is ignored, the diameter of the coagulation will be significantly underestimated. Then we studied the effects of blood perfusion and thermal conductivity on the size and shape of necrosed volume of multiple-focus pattern during ultrasound surgery with different inputs of intensity, treatment duration and focal spacing. The results show that for the pattern with overlapping foci with the power deposited between foci larger than 75% of peak power, the short-duration and high-intensity heating scheme can completely reduce the cooling effect of the blood perfusion. This overlapping foci pattern is an appropriate option for ultrasound ablation in high perfused tissue. In addition, a method for constructing a finite-element model with realistic vessel geometry was presented. Using this method, we constructed three-dimensional FEM models consisting of portal vein of real geometry, hepatic tissue and dynamical blood perfusion. We employed this FEM model to obtain the temperature distribution for a transfer equation based on large blood vessels convection effect and investigate influences of blood vessel location and blood velocity on lesion of multiple-focus pattern during focused ultrasound surgery. The simulation results show that if the distance of the large blood vessel from the tumor is small, the blood flow may play an important role in the thermal dose distribution on the two tips of focal zone next to the large blood vessel. Underdosed areas might be present from which the tumor can regenerate. When the distance of the large blood vessel from the tumor is larger than 3 mm, the cooling effect of large blood vessels is not significant in focused ultrasound surgery.