| Brain tumors now rank among the top ten malignant tumors that seriously endanger human health,the main treatment methods were surgical resection and postoperative radiotherapy and chemotherapy.High-intensity focused ultrasound(HIFU)for the treatment of tumors was the non-invasive or minimally invasive therapeutic new technologies which to accurately focus the tissue-non-destructive low energy ultrasound on the body to be treated,so that the temperature of the target area rises above 55°C in a short period of time to the tumor tissues of the fatal target area.It has been applied in the clinical treatment of uterine fibroids,prostate cancer,and breast cancer.For the HIFU treatment of brain diseases encased by the skull,defocusing due to the acoustic characteristics of the skull and the influence of the structure,the insufficient energy in the focal domain makes the HIFU treatment focus problems such as small focal area and irregular shape;Because of the large number of key functional areas in the brain,HIFU treatment must strictly and strictly control the thermal injury area of HIFU treatment.In order to solve these problems,it was necessary to study the total input power of the transducer and the irradiation frequency during the treatment of HIFU transcranial brain tumors.When two focal points were combined,the influence of factors such as the distance between two set focus target points?the two signal source trigger delays and the setting of the focus target position on the formation of the sound pressure field and the temperature field in the transcranial HIFU facilitates the safe and effective application of the HIFU transcranial brain tumor treatment to the clinic as soon as possible.ObjectiveHIFU has been introduced into brain tumor treatment because of its non-invasive,repeatable treatment,etc.However,how to control the thermal injury area,cavitation-induced injury,and the treatment of focal area in HIFU transcranial brain tumor therapy was still limited the application of HIFU transcranial tumor therapy to clinical.This article uses the volunteers' head CT image data to establish a numerical simulation model for the treatment of HIFU transcranial brain tumors,and studies the input power and operating frequency of the HIFU phased-control transducer,and the factors such as the distance between the two set focus target points when the two focal points are merged?the trigger delay of the signal source and the setting of the focus target position influence the focal region of the HIFU;screened HIFU therapy parameters that avoid the risk of thermal damage and cavitation damage in normal tissues;selection of clinical HIFU safety treatment plans and choice of treatment parameters to provide theoretical basis and data reference.MethodsA numerical simulation model for the treatment of HIFU transcranial brain tumors was performed using volunteer head CT image data,a random array of 82 array concave spherical phased transducers,and a water body to obtain phased transducer transducer excitation signals based on the time reversal method.The acoustic pressure field and temperature field formed by HIFU transcranial focusing were numerically simulated and analyzed.Based on the equivalent thermal dose assessment of thermal damage and mechanical index to evaluate the cavitation damage analysis,the areas of thermal damage and cavitation damage were discussed,and the effects of the input power and operating frequency of the phase-controlled transducer on the focal region formation of HIFU were studied.The distance between the focused target point,the triggering delay of the two signal sources and the setting of the focus target point position to influence the size,shape,and thermal damage and cavitation damage in the focal region of the HIFU were studied.Results1.When the total input power of HIFU irradiation was low,only low-frequency ultrasound focusing could be used and the HIFU transcranial focal area could reach the treatment temperature for a long time,which might be caused heat injury between the skull and normal brain tissue that opposite to the set focus position and the ultrasonic phased transducer.2.When the total input power of HIFU irradiation ws too high,the time required for low-frequency ultrasound focusing and the HIFU transcranial focal region to reach the treatment temperature could only be shortened,possibly resulting in cavitation damage of the skull and normal brain tissue outside the focus field.3.The sound pressure of the HIFU forming focal region increases with the increase of the total input power,but the input total power has little effect on the sound pressure distribution,while the sound pressure of the focal region decreases first and then increases with the increase of the frequency.Then,the trend of increasing at the end was then reduced;the temperature rise at the skull decreases with the increase of the total input power,and the temperature rise at the skull first increases and then decreases and then increases at a later time.4.The total input power of single foci to avoid normal tissue damage at different operating frequencies ranges from:?the total input power was 20.6-61.8 W at 0.5 MHz;?the total input power was 41.2-82.4 W at 0.6 MHz;?the total input power was 41.2-61.8 W at 0.7 MHz;?the total input power was 41.2-61.8 W at 0.8 MHz;?there was no treatable total input power at 0.9 MHz;?the total input power was 61.8 W at 1.0 MHz.5.In the case of bifocal integration in the direction of the acoustic axis,setting the distance between the two set focus target points to 12 mm effectively expands the overlap between the equivalent thermal dose thermal damage area and the MI > 1.9 threshold area;when the trigger delays of 600 ns and 800 ns for the two signal sources as the distance between the target points was 14 mm could also effectively expand the overlap between the equivalent thermal dose thermal damage region and the MI > 1.9 threshold region.6.The total input power of double foci to avoid normal tissue damage at different operating frequencies ranges from:?the total input power was 41.2-123.7 W at 0.5 MHz;?the total input power was 41.2-103.1 W at 0.6 MHz;?the total input power was 61.8-144.3 W at 0.7 MHz;?the total input power was 41.2-103.1 W at 0.8 MHz;?the total input power was 61.8-144.3 W at 0.9 MHz;?no treatable total input power at 1.0 MHz.7.When the two focal points in the direction perpendicular to the acoustic axis were fused symmetrically,the size of the focal region formed when the two set focal points were symmetrically set was smaller than the size of the focal region formed when HIFU was set asymmetrically.8.In the case of bifocal fusion in the direction perpendicular to the acoustic axis,when the distance between two set focus targets was greater than 3 mm,the HIFU transcranial focal field begins to separate into two focal regions;when the two signal sources have different trigger delays,HIFU transcranial foci form fusion or separation.9.In the case of bifocal integration perpendicular to the acoustic axis,when the distance between two set focus target points was 2 mm,the triggering delay of the two signal sources could be controlled to 200 ns to control the shape of the focal region to resemble a parallelogram.10.When perpendicular to the bifocal focus in the direction of the acoustic axis,setting the distance between the two set focus targets to 3 mm effectively expands the overlap between the equivalent thermal dose thermal damage region and the MI > 1.9 threshold region;Setting the triggering delay of the two sources to be 100 ns,1300 ns,and 1500 ns when setting the distance between the focused target points to 3.5 mm could also effectively expand the overlap area between the equivalent thermal dose thermal damage region and the MI > 1.9 threshold region.Conclusions1.In order to avoid thermal damage and cavitation damage in the skull and normal brain tissue,when HIFU transcranially treats brain diseases,it was necessary to screen the total input power range of the transducer under the operating frequency.2.By controlling the total input power and operating frequency of the transducer,adjusting the distance between two set focus target points during the two-focus fusion,the trigger delays of the two signal sources,and setting the focus target point could control the relative size of the equivalent thermal dose thermal damage zone where HIFU forms the focal domain and the region where MI is greater than 1.9.3.Compared with single-focus domain focusing,bifocal fusion focusing could effectively expand the total input power range that could be safely used for treatment,thereby reducing treatment time and improving the safety of HIFU transcranial brain tumor treatment. |
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