The Numerical Study Of The Phase Control Method And The Acoustic Pressure Distribution For The HIFU Transcranial Focusing

The High Intensity Focused Ultrasound(HIFU)treatment has been successfully applied in the clinical therapy of the solid tumor in the soft tissue.However,for the HIFU transcranial focusing and treatment of brain tumors,since the acoustic feature of skull is completely different with the one of soft tissue,the problem of peak temperature in the skull is rising.ObjectiveIn order to solve the problems in the HIFU clinical treatment.it is necessary to master the process of HIFU transcranial transmission and the acoustic pressure and energy distribution.In this paper,a brain tumor clinical case was regarded as the object of study,and.by building 3D numerical simulation model which includes skull.combining the 64 elements phased array transducer,to investigate the peak pressure reduction method by adjusting the phase configuration of each element on transducer,which provides an effective method for HIFU brain tumor treatment in clinical application.MethodThe 3D numerical simulation is based on the clinical case that the HIFU transcranial focusing would lead the peak pressure in the skull.Congruent order reduction of Westervelt acoustic wave propagation equation by using Finite Difference Time Domain(FDTD)method to calculate pressure field in three dimensional space.And by using the method of obtaining the acoustic characteristic parameters of skull from CT data image put forward by Aubry,the skull and transmission medium could be imported into the simulation model.In order to achieve the HIFU transcranial focusing and reducing the peak pressure in the skull,water model was first chosen for numerical simulation and experimental verification,second in the water model engaged with a homogeneous bone plate,compare the self focusing method,time reversal phase method and peak reduction method.then the last two methods were applied in HIFU transcranial focusing simulation.where the effect of the focusing depth was preliminary discussed in the acoustic pressure distribution.ResultsI.Water model1.Under the same condition of the simulation parameters.the Westervelt acoustic wave propagation equation was operated respectively by descending order difference and direct difference for acoustic field numerical simulation.where the distribution of the acoustic pressure were almost same,but the calculating time of former method was less than the one of latter by about 30.9%.2.The average sound pressure transducer surface was given,the pressure profile on the acoustic axis and lateral axis obtained by the numerical simulation and experiment in a water tank were agreed well.II.Bone plate model1.In the bone plate model.it was showed that the position of the focal region could be corrected and the location could reach to the target based on the time reversal method.2.Compared with the time reversal,time reversal.joint peak reduction method could decrease the peak pressure in skull and reduce the rate between bone pressure and focus pressure,but the effect on the focus location and amplitude was neglected.III.Skull model1.Compared with the time reversal method,time reversal joint peak reduction method could decrease the peak pressure in skull and reduce the rate between bone pressure and focus pressure,but the effect on the focus location and amplitude was neglected.2.When the focal length was given,by using the time reversal method,with the decreasing of focusing depth L2 the peak acoustic pressure in skull increasing,the real focus amplitude reducing,the rate between bone pressure and focus pressure increasing.3.The focal length was given.by the use of the time reversal method,when the focusing depth L,was greater than 30 mm,the outer skull cortex acoustic pressure was greater than the inner one;When focusing depth L2 was less than 30 mm.the outer skull cortex acoustic pressure was less than the inner one.4.When focusing depth is less than 30 mm.it was necessary to use time reversal joint peak reduction method to reduce the peak pressure in the skull,which would damage to the skull and surrounding tissue.Conclusions1.It was showed that the position of the transcranial focal region could be corrected and the location could reach to the target based on the time reversal method.2.When there was a peak pressure inside the skull,using the time reversal joint peak reduction method could reduce the peak acoustic pressure inside the skull.3.The focal length was given,by using the time reversal method,the focusing depth L2 was negatively correlated with the rate between bone pressure and focus pressure.4.The focal length was given.when the focusing depth was less than the critical focusing depthL L2 it was necessary to use time reversal joint peak reduction method to reduce the peak pressure in the skull.which would damage the skull and surrounding tissue.