The Effective Of Skull Acoustic Window And Standing-wave For HIFU Transcranial Focusing

High Intensity Focused Ultrasound（HIFU）transcranial focusing treatment have problems that sound wave reflection product by heterogeneous structure and large different acoustic impedance between skull and perienchyma which inducted undesired standing waves that caused space energy retention and inducted energy loss.At the same time,it suffers severe feasibility of significant heat in the skull or skin as the high absorption with acoustic wave of skull.The degree of degradation is dependent on the structure of the bone.The goal of this study is to analyze the effective of array element excitation frequency of the different bone windows（temporal scales area,occipital area and parietal area as example）optimize the acoustic pressure and temperature field with different acoustic window of the skull and evaluate the effect of the acoustic window in focusing result and on-axis or off-axis regulative scope with the most appropriate array element excitation frequency of each acoustic window;use 0 or?random phase transformation method to break the standing wave and to analyze the influence of standing wave on the acoustic pressure and temperature fields.The simulation showed that the suitable frequency and adjustable range of focus were different to the parts of the skull that had large structural differences;the standing wave can be suppressed after 0 or?random phase transformation method.The sound pressure and rate of temperature increase at the focal region improved with the decrease of the standing waves.It is confirmed that the excitation frequency and phase transformation frequency are relevant.ObjectiveIn High Intensity Focused Ultrasound（HIFU）transcranial focusing treatment,we can use the advantage of the 64 array element concave spherical surface transducer that it has smaller apertures with which can choose different bone windows with different position of the cancer.The most appropriate array element excitation frequency was chose for each acoustic window to research the adjustable range of focus of different bone windows.Reflections induced by heterogeneous structures and large differences in acoustic impedance between the skull and perienchyma which encounter continuous wave and in the condition of the same amplitude,frequency,wavelength and phase difference.It results in undesired standing waves.Phase transformation method was used to reduce the standing wave by randomly changing the phase in time segments and decrease the intensity of the standing wave.And research the effective of the method to HIFU focal region.MethodsThe simulations performed in this paper are based on a full three-dimensional（3D）finite-difference time-domain（FDTD）simulation of acoustic propagation and thermal behavior and a 64-element phased transducer（an aperture of 100 mm and curvature of 80 mm）and temporal scales area,occipital area and parietal area were chose to bone windows,as example.Combine the time reversal method and the method that reduced peak acoustic pressure in skull together to study the most appropriate array element excitation frequency for each acoustic window and the adjustable range of focus for different bone windows.Then 0 or?random phase transformation method was used to each bone windows with their own appropriate array element excitation frequency to research the effect of the time segment of random phase transformation for the result of standing wave suppression.At the same time,the acoustic pressure and temperature field before and after the phase transformation method were compared and the relationship excitation frequency and phase transformation frequency was researched.Finally,the sound pressure field and focal regions of HIFU were measured in sink when sound wave spread through thinner scapula of sheep and thicker human skull to verify the effect of bone windows is different in ultrasound focusing by experiment.Results1.Temporal scales area,occipital area and parietal area were chose as bone windows,respectively.The optimum excitation frequency for the temporal bone window was chose as 0.6 MHz after the excitation frequency selection.The optimum excitation frequency for the occipital bone and the parietal bone windows were both0.7 MHz.It was studied on different patients that the optimum excitation frequency for the bone windows in the same part of skull was different.2.The maximum L_1max of temporal bone window is 55 mm when chosen the optimum excitation frequency（0.6MHz）.It is 50 mm for both occipital bone and the parietal bone window.3.The ability of off-axis focusing adjustable range of temporal bone window is largest above all which can focus in brain tissue only 20 mm from outer surface of skull and without thermal damage at skull.It can focus in brain tissue 22.5 mm from outer surface of skull with occipital bone window.With the parietal bone window,it can focus in brain tissue 27.5 mm from the outer surface of skull.The on-axis focus adjustable range（near the skull direction）for the parietal bone window is 7.5 mm.It is nearer compare to the occipital bone and the parietal bone window（2.5 mm）.4.The focus position has a maximum acoustic pressure at T_r=6μs when the excitation frequency was 0.6MHz for temporal bone window,an increase of 14.1%compared to T_r=0.The focus position of occipital bone and parietal bone window at0.7 MHz both have the maximum acoustic pressure at T_r=5μs.This coincides with a minimum R_αvalue.The temperature rise at the focus increase with phase transformation method is about 14.1℃when irradiation time t=10 s（temporal bone window as example）.The acoustic pressure and the peak temperature in the skull increase slightly.The necrosis generated through the bone windows increased greatly with the application of the technique with the same irradiation time.5.The minimum standing wave intensity occurs when the ratio of the phase transformation frequency and excitation frequency fr./f=0.3 for each bone window with the phase transformation standing wave suppress method.The standing-wave ratio（R_α）value and volume of necrosis field for different sound intensities only increases slightly as the volume of necrosis enlarges with intensity.6.The sound pressure decrease greatly when through bone with experimental verification.7.The hydrophone got the sound pressure curve of the human skull is smoother but the sound pressure is smaller than the scapula of sheep when driving power and the irradiation time is same.8.The focal regions are small when through the thinner scapula of sheep and absent when through the thicker human skull in the condition of the single-element.Increasing the irradiation time is easy to produce thermal damage in bones.Conclusions1.The effect to HIFU transcranial focusing and the suitable frequency of different bone windows were different.There were best focusing result and on-axis or off-axis regulative scope with the most appropriate array element excitation frequency of each acoustic window2.The ability of off-axis focusing adjustable range bone windows with small thickness（such as temporal bone window）is better than others which can focus in brain tissue more superficial and can treatment the cancer which at superficial position of the brain.3.The standing wave can be suppressed after 0 or?random phase transformation method.The sound pressure and rate of temperature increase at the focal region improved and the treatment time become short with the decrease of the standing waves.4.The optimum phase transformation frequency was chose for better standing wave suppression result.And it is confirmed that the excitation frequency and phase transformation frequency are relevant5.The experiment proves that the effect of bone on ultrasound focusing is great in the HIFU treatment and the effect is different when sound wave spread through different bone windows.Scapula of sheep and human skull both resulted in severe attenuation of sound pressure.For the scapula of sheep,a focal region was generated in egg-white tissue-mimicking phantom,the region is irregular.No focal region was produced when sound wave spread through human skull that with complex structure.In conclusion,the different effect of bone windows to HIFU focusing and the ability of focusing adjustable range were calculated after optimum excitation frequencies of array elements have been chosen for each bone window with noninvasive numerical model.It can offer a reference to clinical treatment.The 0 or time segment phase transformation method that can be used to suppress the standing wave was proposed for HIFU treatment.The method was based on the time reversal method and the method that reduced peak acoustic pressure in skull.The excitation signal of arrays was modulated with the time segment of random phase transformation to suppress the standing wave.It can not only suppress the standing wave but also improve the energy of the focal region with the appropriate time segment at optimum excitation frequencies of each bone windows to speed up the temperature rise at the same time.