Numerical Simulation Of The Temperature Distribution For HIFU Transcanial Brain Therapy

High intensity focused ultrasound is a non-invasive cancer therapeutic technique with little side effect and toxic,which made the temperature of the target rise to more than 60 ? in several seconds through the ultrasound effects,such as mechanical effect,heating effect and cavitation effect.HIFU has been applied to the clinical treatment for various solid tumors currently.As for the treatment of the transcranial brain tumor,it is difficult to produce an intracranial focus using this technique because there is strong phase and amplitude aberration of the acoustic wave when propagating though the skull.Moreover,the difference of the acoustic impedance between the skull and the surrounding soft tissue is obvious which could lead to the heat deposited in the skull in the process of the treatment of the tumor located at the superficial tissue.Therefore,HIFU is limited in the treatment of brain tumors in the clinical application for the issues that may damage of the skull and the normal tissue.ObjectiveIn this study,the adopted numerical simulation was based on a transcranial ultrasound therapy model taking the CT images of a human skull as a reference.The acoustic propagation formula and the Pennes bio-heat conduction equation were applied to the simulation of the transcranial temperature distribution.According to the temperature distribution and the Time Reversal theory,the position of the treatable focal region was corrected and the hot spot existing in the skull was eliminated.Furthermore,the influence of the exposure time,input power and the distance between transducer and skull on the temperature distribution were analyzed.The results can provide the theory reference data and theoretical basis for clinical HIFU therapeutic dose formulation.MethodsThe HIFU transcranial therapeutic models were established and the temperaturedistribution was calculated in the study.Under the cylindrical coordinate system,the homogenizing head parameter model was set taking the human skull as a reference combined with the eight elements transducer.The ultrasound propagation of the nonlinear theory was applied to the simulation with the Finite Difference Time Domain method to calculate the pressure in space at every time.And the temperature field were obtained using the Pennes biological heat conduction equation under different treatment conditions.And the focus position was corrected and the hot pot in the skull was eliminated so as to optimize the treatment.Therefore,the effect of the exposure factors to the focal region and the temperature field were analyzed.As to the simulation model based on the CT scan of a human head,the simulation was performed under the rectangular coordinate system combined with the 64-array transducer.The temperature distribution was simulated under different conditions of the HIFU treatment for brain tumors.The phase compensation method that was discussed in the homogenizing model was applied to focus in the brain tissue.Furthermore,the influence of the exposure factors on the temperature distribution was analyzed.ResultsThe simulation in the homogenizing head parameter model1.The focus position deviated 0.4mm from the set focal length 80mm after using the time reversal method.2.The highest temperature rise in the skull was only 2.4 ? after using the hot-spot elimination method and the purpose of attenuating the temperature was achieved.3.Under the same condition,when obtaining the same volume of the focal region,the skull more need more the input power and the exposure time than the non-skull model.4.The start time to form the focal region differs from the input intensity.The more input intensity,the shorter the exposure time.Moreover,the volume of the focal region increases in a nonlinear form with the exposure time and it increases in a higher growth rate when given more input intensity.5.The greater the input intensity,the shorter time was needed to reach the same size.The smaller the input intensity,the longer time was needed to reach the same size.6.When the therapeutic depth was 20mm,the distance between the transducer and the skull is set as different value,the increasing rate of the volume was varied in the process of exposure and the volume of the focal region was diverse from each other after exposure 20s.The simulation in the in-homogenizing head parameter model1?The focus position deviated 0.75mm from the set focal length 80mm after using the time reversal method.2?The highest temperature rise in the skull was only 9.71? after using the hot-spot elimination method and the purpose of attenuating the temperature was achieved.3?The temperature rise in the skull and the focus increases in a nonlinear form with the exposure time when given the same input intensity.The temperature rise of the focus increases slightly and after using the hot-spot elimination method and there is little effect on the tendency of the temperature rise of the focus during the exposure time.4?The volume of the decreases when the therapeutic depth from deep location to superficial location using the time-reversal method and the hot-pot elimination method,respectively.The focal region was not produced when the therapeutic depth was set as 20mm,so what cases should the hot-spot elimination be applied to need to be analysis based on the tumor location.5?When the therapeutic depth was constant as 2.5mm and the distance between the transducer and the skull was diverse from each other,the optimal location of the case to produce the greatest volume after using the hot-spot elimination method.In the clinical treatment process,the numerical simulation should be performed to calculate the volume of the focal region in order to select the most optimal distance between the skull and the transducer.Conclusions1?The results showed that the position of the focal region could be corrected and the location could reach to the target based on the Time Reversal theory.2?When target located at the superficial tissue,the high temperature in the skull was eliminated using the hot-spot elimination method without affecting the focus.The focal region above 60?? could be formed at the superficial tissue located at the superficial tissue using the hot spot elimination method3?HIFU transcranial brain tumor therapy needs more energy than the treatment without the skull.4?When obtaining the same volume of the focal region,the more the input power the less the exposure time.5?The volume of the focal region increases with the exposure time and the input power in a nonlinear form in the HIFU transcranial brain tumor therapy.6?The volume of the focal region is influenced by the distance between the transducer and the skull.