| High intensity focused ultrasound transcranial therapy for brain disease has received much attention since it is non-invasive and repeatable.It has been used to treat brain tumor and other neurological diseases in clinical trials.Due to the focal region temperature control during treatment and the individual differences in skull,it would cause serious complications or lead to the treatment failure.In order to solve the problems,it is necessary to study the method to modulate the temperature distribution,shape and the volume of focal region.What's more,it is also necessary to study the effect of the structure parameters and acoustic parameters of skull on HIFU transcranial focusing.ObjectiveThe poor temperature control and individual differences of skull could induce some clinical problems such as failing to treat because of the low temperature at target or some serious complications because of over high temperature.In order to deal with these problems,this research aimed to modulate the thermal field to realize the temperature uniform distribution in focal region as well as adjust the shape and volume of focal region by superimposing two signals which can focus at the preset targets to emit the transducer elements and modulating the trigger delay time and the amplitude ratio factor of the two signals.At the same time,the head CT data of 32volunteers were used to establish numerical simulation models to statistically analyze the effect of skull curvature radius,thickness and density on HIFU transcranial focusing.This study will provide theoretical method and theoretical data for the development of safe and effective HIFU clinical treatment of brain tumors and planning the treatment path.MethodsThis study established the simulation models based on the CT scan data and 82elements randomly distributed phased array and numerically computed the pressure field as well as the temperature field of HIFU using the finite difference in time domain method.The temperature distributions of focusing at the two preset targets were compared which the stimulating signals of the elements were obtained by the time reversal combined with correlation method and focus pressure combined with correlation method.The method used to obtain the stimulation signals of elements to modulate the focal region of HIFU brain tumor treatment was screened out from that.The parameters and method to modulate the temperature distribution,shape and volume of HIFU focal region were study by analyzing the influence factor of the temperature field formed by superimposing two signals that focus at the two preset targets to stimulate the transducer elements.Based on head CT data of 32 volunteers,the numerical simulation models were established.The correlation between the skull shape parameters,acoustic parameters and HIFU transcranial focusing was studied through numerical simulation combined with statistical analysis.Results(1)In the craniotomy state,the uniform temperature distribution(temperature difference was no more than 1°C)along the direction of acoustic axis in the focal region was realized when the distance between the two preset targets was 7.5~12.5mm through adjusting the triggering time delay as well as the amplitude ratio coefficient of the two signals that focus at the preset targets.The volume of treatable focal region can be modulated in 30.1~71.6mm~3.The uniform temperature distribution on the direction perpendicular to the acoustic axis in the focal region was realized with the distance between the two preset targets was 2.0~3.0 mm and the volume of treatable focal region can be modulated in 58.7~201.8 mm~3.(2)In the state of HIFU transcranial focusing,the temperature at focus can be rose using the focus pressure combined with correlation method to obtain the signals and stimulate the transducer elements.On the base of considering the mean thickness and mean attenuation coefficient of the skull in acoustic window,the uniform temperature distribution along the acoustic axis in focal region was realized when the distance between the two preset targets was 7.5~12.5 mm through adjusting the triggering time delay as well as the amplitude ratio coefficient of the two signals that focus at the preset targets.The volume of treatable focal region can be modulated in26.8~95.3mm~3.The uniform temperature distribution on the direction perpendicular to the acoustic axis in focal region was realized with the distance between the two preset targets was 2.0~3.0 mm and the volume of the treatable focal region can be modulated in 88.0~226.7mm~3.(3)The focus pressure combined with correlation method is better than the time reversal combined with correlation method in reducing the ratio of the acoustic pressure at skull and at focus.The ratio of acoustic pressure at skull and focus increased with the curvature radius and the thickness of skull.(4)Based on the head CT data of the 32 volunteers,the acoustic pressure at focus formed by the focus pressure combined with correlation method was significantly higher than that formed by the time reversal combined with correlation method and the ratio of the sound pressure at skull and at focus formed by the focus pressure combined with correlation method was significantly lower than that formed by the time reversal combined with correlation method.The ratio of sound pressure at skull and at focus was positively correlated with the curvature radius of skull and the average thickness respectively.The average density of skull is related to the average thickness.With the increase of irradiation power,the ratio of sound pressure at skull and at focus decreased but the change was small.With the increase of the set focal length,the ratio of sound pressure at skull and at focus decreased.Conclusion(1)Compared with the time reversal combined with correlation method,focusing with the elements stimulation signals obtained by the focus pressure combined with correlation method can effectively increase the accumulation of transcranial acoustic energy at the focal region.(2)Using the double excitation signals superposition method,the temperature in the focal region can be uniformly distributed in the direction of the acoustic axis and in the direction perpendicular to the acoustic axis by adjusting the trigger delay time and the amplitude ratio coefficient of the two signals.At the same time,the shape and volume of the focal region can be modulated.(3)The factors that influence the HIFU transcranial focusing effect are the curvature radius of skull and the average thickness of skull in the acoustic window.In this study,the method to modulate the temperature distribution in focal region as well as the shape and volume of focal region was developed by numerical simulation and the uniform distribution of temperature in the focal region was achieved.The effects of shape parameters and acoustic parameters of skull along with the ultrasound irradiation parameters on the HIFU transcranial focusing effect were statistically analyzed.It would provide theoretical method and theoretical data for clinical HIFU brain tumor treatment plan development. |
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