Study Of Phased Array Transducer And Driving Circuit In Multi-frequency HIFU Transcranial Focusing

High-intensity Focused Ultrasound(HIFU)has been applied to the clinical treatment of solid soft tissue tumors such as breast cancer,prostate cancer,and uterine fibroids because of its non-invasive/minimally invasive,reproducible,and other advantages.In the transcranial treatment process,due to the complex structure of the skull and the large differences in speed of sound,acoustic impedance with surrounding soft tissue,severe distortion of the sound field after transcranial focusing may occur,which may result in clinical problems such as offset of focus position,insufficient energy in the focal domain and heat deposition at the skull.In recent years,improving the focusing performance of ultrasonic phased array transducer under the limited output power of HIFU treatment device,and the method of reducing the system volume and delay error by increasing the system integration has become one of the hot spots for many researchers.ObjectiveDuring the HIFU transcranial treatment,due to the strong attenuation of the skull and the high non-uniformity of the skull structure and density,the attenuation of the ultrasonic beam emitted by the transducer array element laid in different position is different in the transcranial transmission process,following a big difference in energy distribution in the focal region,which ultimately leads to a lower temperature in the focal region and fails to achieve therapeutic goals.The multi-frequency ultrasonic excitation proposed in recent years can increase the temperature of focal domain under certain input power and irradiation time.In this paper,a 82-element randomly distributed phased array transducer is applied to study the influences of the number of frequency partitions and the frequency difference between different partitions on the HIFU focal region in multi-frequency partitioned excitation,and to select the array elements partition method and corresponding optimal operating frequency combination which can improve the focusing efficiency,realizing the design of phased array transducer with different working frequencies in different array elements partitions.Based on the numerical simulation,phase control and drive circuit of the multi-element phased array transducer is designed and fabricated.By increasing the system integration,the system volume and the delay error are both reduced,which provided technical method and theoretical reference for realizing the miniaturization of phase control and drive system.MethodsUsing the volunteer's head CT scan data established HIFU craniotomy/transcranial numerical simulation model;Acquiring the excitation signal of the transducer array elements based on time-reversal numerical fitting or autocorrelation cross-correlation method.Using CUDA C language FDTD numerical analysis Westervelt acoustic nonlinear propagation equation and Pennes biological heat conduction equation to simulate HIFU craniotomy/transcranial focusing temperature field on the GPU parallel computing platform(Yongtian SCS4450),study the influences of the number of frequency partitions and the frequency difference between different partitions on the HIFU focal region in multi-frequency partitioned excitation of 82-element randomly distributed phased array transducer,and to select the array elements partition method and corresponding optimal operating frequency combination which can improve the focusing efficiency,realizing the design of phased array transducer with different working frequencies in different array elements partitions.Based on the numerical simulation,using FPGA master chip,high-speed DA,integrated drive amplifier,combined with Labview graphical programming software,quartus II programmable logic device design software,ModelSim simulation software,Altium Designer PCB design software for the design and Printed Circuit Board implementation of Labview user interface,phase controll circuit and array element drive circuit in the phased array transducer phase control and drive system;the phase control and drive system are completed and the phase resolution,delay error and harmonic characteristic of the designed system are evaluated through experiments.Results 1.Numerical simulation of multi-frequency HIFU phased array transducer temperature field in craniotomy mode(1)When the frequency difference between adjacent zones is greater than or equal to 0.025 MHz,the dual-frequency,triple-frequency,and quad-frequency excitations satisfy the focal region temperature under multi-frequency excitation is higher than that of single-frequency's,and the maximum temperature in the focal region decreases first and then increases as the frequency difference between adjacent zones increases;in the case of five-frequency excitation,the focal region temperature is not higher than that of single-frequency's any more.(2)When the number of frequency partitions is greater than 2,with the increase of the number of partitions,HIFU focusing performance(focal temperature,treatable focal volume,etc)has not been improved,and 0.7 MHz+0.725 MHz dual-frequency excitation could achieve the best focusing effect.2.Numerical simulation of multi-frequency HIFU phased array transducer temperature field in transcranial mode(1)As the frequency difference between dual frequencies increases,the maximum temperature in the focal region increases first and then decreases,the maximum temperature at the skull rises slowly,considering the temperature in the focal region and the skull,the best focusing effect is obtained when the excitation frequencies are respectively 0.7 MHz and 0.75 MHz unter dual-frequency excitation mode.(2)With the dual-frequency excitation mode whose excitation frequencies are respectively 0.7 MHz and 0.75 MHz,under the condition of certain maximum focal region temperature and treatment time,the required input power for dual-frequency excitation is lower than that of single-frequency excitation,while the volume of treatable focal region is larger than that of single-frequency excitation;under the condition of certain maximum focal region temperature and input power,the treatment time required for dual-frequency excitation is shorter than that of single-frequency excitation,while the volume of treatable focal region is larger than that of single-frequency excitation.3.Design and implementation of the phase control and drive circuit of phased array transducer(1)The phase control circuit based on the FPGA master chip EP4CE30F23C8 N,the high-speed digital-to-analog converter AD9744 and the differential amplifier AD8047 can stably output multiple sine waves with frequencies of 1 MHz and phase resolution of 2 ns.(2)The array element drive circuit based on the ADA4870 integrated drive amplifier can output sinusoidal signal with peak-to-peak value of 36.2 V and no high-order harmonics,the circuit delay error is less than 1 ns.In addition,in order to avoid damaging to the transducer array element due to excessive power,an enable switch and automatic power off and alarm function when overheating occurs are considering in the design of the drive circuit.(3)The HIFU phase control and drive system user program based on Labview graphical programming software can realize the import of delay data text file of each array element,and can adjust the delay data directly on the PC user interface according to the experimental requirements.Conclusions1.The focusing performance of multi-frequency excitation is better than single-frequency excitation,and the focusing effect of dual-frequency excitation is the best.2.The optimal frequency combinations corresponding to the craniotomy and the transcranial model is different under dual-frequency excitation.With this combination of frequencies,the focal region has the highest temperature(when input acoustic power and irradiation time are certain),the required input power for tissue ablation is the smallest while the volume of treatable focal region is the largest(when the highest focal region temperature and irradiation time are certain),the required treatment time is the shortest while the volume of treatable focal region is the largest(when the highest focal region temperature and input power are certain).3.The use of integrated drive amplifier for power amplification can reduce system volume and delay error,and there are no high-order harmonic components in the output sine wave.