The hemispherical phased transducer maximizes the coverage of the skull and the ultrasonic energy per unit area of the skull is minimized,thereby reducing the risk of skull burns,but the transducer has a small focal area adjustment range,increasing the focal length of treatment is an urgent question for this type of transducer.In this paper,a three-dimensional high-intensity focused ultrasound(HIFU)transcranial propagation model is established based on the human head structure.The finite difference time domain(FDTD)is combined with the Westervelt acoustic wave nonlinear propagation equation and Penne's biological heat conduction equation for numerical simulation of the sound pressure field and temperature field.Forming a treatable focal area in a small-opening hemispherical transducer with a small amount of numerical simulation calculation focusing at a set position to determine the minimum partial excitation area ratio of focusing.And then,applying these preliminary results to a large-opening diameter hemispherical transducer and the temperature field formed by it or full excitation is studied.The results show that the focus area with the excitation area ratio of less than 22%moves forward to the transducer side when the excitation sound is formed.When the excitation area ratio is greater than or equal to 23%,it focuses at the set position.In the case of partial incentives,using 23%of the partial array,the adjustable range of the treatable focal area formed in the three-dimensional space is larger than that of the full excitation.
Recently, the phase compensation technique has allowed the ultrasound to propagate through the skull and focus into the brain. However, the temperature evolution during treatment is hard to control to achieve effective treatment and avoid over-high temperature. Proposed in this paper is a method to modulate the temperature distribution in the focal region. It superimposes two signals which focus on two preset different targets with a certain distance. Then the temperature distribution is modulated by changing triggering time delay and amplitudes of the two signals. The simulation model is established based on an 82-element transducer and computed tomography （CT） data of a volunteer＇s head. A finite- difference time-domain （FDTD） method is used to calculate the temperature distributions. The results show that when the distances between the two targets respectively are 7.5-12.5 mm on the acoustic axis and 2.0-3.0 mm in the direction perpendicular to the acoustic axis, a focal region with a uniform temperature distribution （64-65 ℃） can be created. Moreover, the volume of the focal region formed by one irradiation can be adjusted （26.8-266.7 mm3） along with the uniform temperature distribution. This method may ensure the safety and efficacy of HIFU brain tumor therapy.
