| The high-intensity focused ultrasound has been successfully applied in the treatment of soft tissue tumors,such as uterine fibroids,breast cancer,prostate cancer and liver cancer which can be repeated,non-toxic side effects,and other advantages for its non-invasive,non-toxic side effects,repeatable treatment and other advantages.However,for the treatment of brain tumors wrapped by the skull,due to the heterogeneity structure of the skull and its acoustic characteristics are quite different with the surrounding brain tissue,phase distortion and the amplitude attenuation could occur during the transcranial transmission of the sound waves,followed by the damage of skull and surrounding tissue due to the energy deposition in skull or insufficient focal region energy to extirpate the pathological tissue.In recent years the development of the hemisphere phased array with large-aperture could maximize covering skull surface,thus reducing energy deposition at skull,and become one of the focuses of many researchers.ObjectiveIn the HIFU transcranial therapeutic of brain tumors,Phased transducer is the core part,the structure and operating parameters of the transducer directly affect the distribution of sound pressure and temperature field in the focal region,The study of its structure design and regulation method is beneficial to the realization of non-invasive,accurate and effective HIFU treatment of brain tumors.In this paper,the influence of the structure and operating parameters on the temperature field and the change law of the small opening transducer are studied by numerical simulation and then designed a large opening hemispherical phased transducer based on these studies,the sound pressure and temperature field of the large opening transducer are studied by numerical simulation,by selecting the incentive method of the array element(full or partial array excitation),studied the phased transducer used for HIFU transcranial therapy with small and large opening hemisphere phased transducer,enhance the adjustability of focal region and provide technical methods and effective theory for the clinical treatment of brain tumors.MethodsBased on the design method of 82 array random distribution phased array transducer,a small opening and a large opening hemispherical phased transducer whose aperture diameter are respectively 100 mm and 300 mm are designed on the Matlab computing platform,combined with human head CT scan data,a 3D HIFU numerical simulation model which is spread through the brain tissue and a 3D HIFU transcranial numerical simulation model are established in which the transducer are respectively small opening and large opening hemispherical phased transducer;Control the excitation signal of the transducer array by numerical fitting phase modulation method which is based on the time inversion method,and Westervelt nonlinear propagation equation and Pennes bio-heat transfer equation are numerical analyzed with finite difference time domain method.Numerical simulation of the Sound pressure field and temperature field are achieved on GPU parallel computing platform using CUDA C language,take 0.25 as safety threshold to judging its sound field characteristics,here,0.25 refers to the maximum sound intensity ratio between the sidelobe and main lobe,take the area in which the temperature is above 54? as therapeutic focal region to study the characteristics of the focal region.Results 1.small opening transducer:(1)When the excitation frequency of the transducer is 0.5MHz-0.8MHz,the central temperature of the focal region increases with the increase of the excitation frequency,the long axis of the therapeutic focal region is decreased gradually while the short axis is almost the same;When the excitation frequency is 0.8MHz-1.0MHz,the sidelobe increases gradually in the temperature field,the central temperature of the focal region decreases with the increase of the excitation frequency,the long axis and short axis of the therapeutic focal region are both decreased gradually.(2)When the excitation area ratio of the transducer are respectively 32% and 35%,the central temperature of the focal region first increased and then decreased gradually with the increase of the radius and the decrease of the number of elements,the variation of long axis and short axis of focal region is consistent with the change of temperature;when the excitation area ratio of the transducer are 41%,the central temperature of the focal region is increased gradually with the increase of the radius and the decrease of the number of elements,the long axis and short axis of the therapeutic focal region are almost the same.2.large opening transducer:(1)When the excitation area ratio of the large opening transducer is 32%,the focal area is small and the temperature of the side lobe is high;when the excitation area ratio of the large opening transducer is 41%,the central temperature of the focal region is high,and there is almost no sidelobe in the temperature field.(2)When the large opening transducer is focused along the acoustic axis(X axis)in each set focus,with the increase of the focal length,the sound pressure amplitude of the focal point first increases and then decreases,the maximum sound intensity ratio of the sidelobe and the mainlobe first decreases and then increases;the central temperature of the focal region increases first and then decreases,the temperature of the sidelobe decreases first and then increases,the long axis and short axis of the therapeutic focal region are both increase first and then decrease.When the focal length is 144 mm,the central sound pressure of the focal region is the lowest,the maximum sound intensity ratio of the sidelobe and main lobe is far greater than 0.25,and can not form the therapeutic focal region in which the temperature is exceeding 54?;When the focal length is 144 mm,the central temperature of the focal region is less than 54?.(3)When the large opening transducer is focused along the Y axis whose geometric focus is(150,150,150)(unit:mm),with the increase of the distance from the acoustic axis,the central sound pressure of the focal region decreases gradually,the number of the sidelobe increases gradually and the sound pressure increases gradually,the maximum sound intensity ratio of the sidelobe and the mainlobe increases gradually;the central temperature of the focal region decreases gradually,the temperature of the side lobe increases gradually,the long axis and short axis of the therapeutic focal region are both decrease gradually.when the focus is 6mm away from the acoustic axis,a lot of side lobes would appear in the focal plane,and the maximum sound intensity ratio of the sidelobe and main lobe is greater than 0.25,the central temperature of the focal region is less than 54?.(4)When the large opening transducer is focused along the Z axis whose geometric focus is(150,150,150),with the increase of the distance from the acoustic axis,the central sound pressure of the focal region decreases gradually,the number of the sidelobe increases gradually and the sound pressure increases gradually,the maximum sound intensity ratio of the sidelobe and the mainlobe increases gradually,but still less than 0.25;the central temperature of the focal region decreases gradually,the temperature of the side lobe increases gradually,the long axis and short axis of the therapeutic focal region are both decrease gradually.when the focus is 6mm away from the acoustic axis,no therapeutic focal region is formed in which the temperature is exceeding 54?.(5)When the focusing is achieved by driving all arrays,the regulation range of large opening hemisphere phased array transducer along the X axis is 150-168mm;the safe regulation range along the Y axis and Z axis is 3mm off-axis.(6)By driving the 45 transducer arrays which are close to the basal center,therapeutic focal region could be formed along the X axis when the focal length are respectively 138 mm and 144 mm,therapeutic focal region could be formed at 6mm off-axis.along the Y axis and Z axis,therefore,the regulation range of large opening hemisphere phased array transducer along the acoustic axis is expanded to 138~168mm,the regulation range of the focus region along the Y axis and Z axis is expanded to 6mm off-axis.Conclusions1.The excitation area ratio of the small opening transducer could be applied in the design of the large opening transducer.2.Large aperture transducer can be used in the treatment of deep brain tumors,but its adjustable range is small.3.By the excitation of partial array,focusing of the small opening transducer could be achieved on the basal center of the large opening transducer.Expand the focal region regulation range of the hemispherical phased transducer.This study designed a 256 array random distribution hemispherical phased transducer according to the change law of the temperature field of the small opening transducer.By selecting the partial excitation array of the large aperture transducer,which realized small aperture transducer focusing in large aperture transducer and made their respective advantages complementary to small aperture and large aperture transducer.The regulation range of the focal region was expanded. |
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