| In China, brain tumor is one of the top ten malignant tumors which endanger national health. At present, the main treatment is surgery and radiation therapy.However, surgical treatment is not only risky but also easily leads to complication, in addition, toxic side effects limit the radiation therapy. The high-intensity focused ultrasound(High intensity focused ultrasound, HIFU) treatment technology which rose in recent years is followed by many researchers for its non-invasive(or minimally invasive), the treatment which can be repeated, non-toxic side effects, and other advantages. Currently, HIFU therapy has been applied to the clinical treatment of entities soft tissue tumors such as uterine fibroids, breast cancer and liver tumors.In terms of the treatment of brain tumors which is wrapped by the skull, because the factors of the non-uniformity of the structure of the skull and the large differences of the skull and the around soft tissue in acoustic properties could cause phase distortion and amplitude distortion, reflection and scattering, the formation of standing waves and other phenomena, and then lead to some troubles such as out of focus, focal position offset, focal region shape distortion, thermal damage of skull and the surrounding brain tissue or insufficient focal region temperature to extirpate the diseased tissue. To address these problems, the effect of brain tumor tissue, irradiated frequency, radiation dose, irradiation ways and other factors on the acoustic pressure field and temperature field resulted by transcranial HIFU must be analyzed in the processing of brain tumor therapy to promote HIFU tumor therapy to being applied safely and effectively in clinical as soon as possible.ObjectiveCurrently, it exist these problems that skull is burned because of the high temperature of surrounding soft tissue and skull, and normal brain tissue may be burned because of inappropriate regulation of focus position and the size of focal region during HIFU transcranial brain tumor treatment. Taking glioma patients as an example, the numerical simulation model of HIFU transcranial treatment is established with the CT image data of head of patient to study the effect of brain tumor tissue to the focal region and analyze the effect of different operating frequency,irradiation sound intensity, irradiation methods of phased transducer to focal region and skull temperature, and finally to provide theoretical data and treatment method of promoting HIFU transcranial tumor therapy to clinical therapy safely and effectively as soon as possible.MethodsTaking glioma patients as an example, the numerical simulation model is established with the CT image data of patient's head, spherical phase transducer of 82 elements distributed randomly and water. The excitation signal of elements of phased transducer is obtained based on the time inversion method, and Westervelt nonlinear propagation equation and Pennes bio-heat transfer equation are central difference with finite difference time domain method to simulate numerically and analyze acoustic pressure field and temperature field of transcranial focus. The differences of acoustic parameters between brain tumor tissue and brain tissue and the impact on the HIFU focal region are researched, and the impact of different operating frequencies,sound intensity, time of phased transducer on the size of HIFU focal region and skull temperature are also studied when the focus is single. Under repeated irradiation ways,the variation of skull temperature, shape of focal region and volume of focal region are studied on the basis.Results1. There is no significant difference in acoustic parameters of brain tumor tissue and surrounding brain tissue; porosity of necrotic brain tumor is larger, sound velocity,density and absorption coefficient are smaller. Brain tumor necrosis tissue has smaller influence on pressure field of HIFU, but it has light impact on the temperature distribution.2. When transcranial HIFU focuses, the standing wave effect gradually weakens,, the volume of focal region gradually becomes smaller, the sound pressure of the focus and maximum sound pressure of side lobe gradually increases, maximum sound pressure of skull fluctuates as the input frequency increases.3. When irradiation time is same, focus temperature,maximum temperature at the skull, long and short axial of focal region gradually increase as sound intensity increases. When focus temperature is same, irradiation time, maximum temperature atskull and long and short axial of focal region gradually decrease as sound intensity increases.4. When irradiation sound intensity is same, the ratio of maximum temperature rise at skull and temperature rise of focus(Ratio) first decreases and then increases as irradiating time extends; when irradiating time is same, Ratio gradually reduces as irradiating sound intensity increases.5. When the focus distance Lf of secondary irradiation is less than3 mm, time of focus temperature of second irradiation to 90? gradually increases and increased margin gradually decreases as the time interval tc between secondary irradiation extends. When the focus distance Lf of secondary irradiation is greater or equal to3 mm, time of focus temperature of second irradiation to 90? is same; When the focus distance of secondary irradiation is constant, maximum temperature at skull gradually decreases, but decreases margin is tiny as the time interval tc between secondary irradiation extends.6. When the time interval tc between secondary irradiation is constant,cumulative focal region volume of secondary irradiation first increases and then decreases as the focus distance of secondary irradiation increases, and the cumulative focal region volume of secondary irradiation reached Max when Lf is 3mm; when the focus distance Lf of secondary irradiation is 1mm and 2mm, the cumulative focal region volume of secondary irradiation gradually increases as the time interval tc between secondary irradiation extends; when the focus distance Lf of secondary irradiation is 3mm-5mm, the cumulative focal region volume of secondary irradiation gradually decreases as the time interval tc between secondary irradiation extends;when the focus distance Lf of secondary irradiation is 5mm, the area below 55 ?that cannot ablate brain tumor between the two foci occurs, and the area gradually increases as the time interval tc between secondary irradiation extends.7. Under the conditions that repeated irradiation is at the same focus position,when focus peak temperature of each irradiation is at the same temperature, the volume of focal region gradually increases with increasing irradiation times, but increased margin is tiny; when irradiation time of each irradiation is same, the volume of focal region gradually increases with increasing irradiation times, and increasedmargin gradually decreases; under the conditions repeated irradiation is at multi-focal position, when focus peak temperature of each irradiation is at the same temperature,the focal region shape is irregular, and maximum temperature at the skull and irradiation time fluctuates; when irradiation time of each irradiation is same, the area below 55? that cannot ablate brain tumor may occur.Conclusions1. There is no significant difference in acoustic parameters of brain tumor tissue and surrounding brain tissue, but some differences with necrotic brain tumor tissue;brain tumor tissue has smaller influence on pressure field distribution of HIFU, but it has light impact on the temperature field distribution.2. Larger irradiation sound intensity, shorter irradiation time of the focus temperature to same temperature, shorter long and short axial of focal region, lower the temperature at the skull.3. Temperature of focal region, temperature at skull and the size of volume of focal region are regulated by different irradiation ways when transcranial HIFU focuses. |
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