The Numerical Simulation Study Of Acoustic Pressure Field And Temperature Field About HIFU Phased Array Transducer

Ultrasound transducer is one of the key components of high intensity focused ultrasound(HIFU) tumor therapy system. Single element transducer with fixed focal length has been used in clinical treatment such as breast cancer, uterine fibroids,prostate cancer which needs to be adjusted mechanically the position between patient and transducer. While phased array transducer has the advantages of dynamic focusing and forming multiple foci by phase control, which could shorten treatment time, take control of the location of the focal region effectively. It’s research receives extensive attention from many scholars.ObjectivePhased array transducer has the advantage of adjustable focal region, but there are some problems which include numerous element number, complex structure, high cost of phase and excitation circuit, high acoustic pressure sidelobe prone to occur in acoustic pressure field. These problems limit clinical application of transducer. In order to solve it, the key is to ensure good focus performance under the condition of less number of array elements. The reducing of high acoustic pressure sidelobe and improvement of acoustic field distribution is accomplished form the point of element distribution and the optimization of each array element excitation signal based on the numerical simulation model of 82-element phased array transducer. And the treatment of a larger volume of tumors can be realized by adjusting the focal number and distance between foci which provide theoretical data and referenced basis for clinical application of phased array transducer.MethodsThe three-dimensional numerical simulation model is established based on the82-element phased array transducer whose elements are distributed in period,concentric and quasi-random manner. The phased control signal of each element is abstracted based on time reversal method. The acoustic pressure field and temperature field is formed by finite difference time domain method based Westervelt acoustic nonlinear transmission formula and Pennes bio-heat transfer equation. The acoustic pressure field formed by different element distribution array is compared, thevariation with different focus location and adjustable acoustic field range are also discussed under the single focus mode. And the adjustable acoustic field range can be extended by the high acoustic pressure sidelobe elimination method. The merge of multiple foci is accomplished by adjusting the distance between foci, and the variation of acoustic pressure field and temperature field with different number of foci are discussed under the multi-foci mode.Results1. The variation of the main lobe with three different distribution’s transducer is consistent when focusing at the geometric focus. While the ratio between the maximum sidelobe( r value) and the main lobe of period and concentric array is significantly bigger than quasi-random array.2. As the distance off the acoustic axis increasing, the acoustic pressure amplitude of the main lobe decreases, r value increases, and the sidelobe which distributes around the focal region moves to the negative direction off the acoustic axis gradually with increasing quantity and amplitude.3. The results when phased array transducer is focusing along the acoustic axis are as follows:(1) The acoustic pressure amplitude of the main lobe increases firstly and then decreases, while r value decreases firstly and then increases with the increasing of setting focal length along the acoustic axis.(2) The position of the maximum acoustic pressure amplitude of the main lobe and the minimum r value are different when focusing different distance off the acoustic axis. And the adjustable acoustic focal range along the acoustic axis decreases gradually with the increasing distance off the acoustic axis.(3) The acoustic pressure amplitude of high acoustic pressure sidelobe around the focal region can be reduced to some extent by the high acoustic pressure sidelobe elimination method, while the one of the main lobe is lower slightly.4. As the setting distance between foci reduces from 4 mm to 2 mm, the isolated two foci and three foci merge into a focal region gradually. And the acoustic pressure amplitude and the number of the sidelobe which occurs between the setting foci decreases gradually, the acoustic pressure amplitude of the main lobe decreases firstlyand then increases with the decreasing of setting distance between foci.5. As the increasing number of setting foci, the amplitude and quantity of the sidelobe which occurs between foci increase under the multi- foci mode gradually.6. There is not obvious temperature rise outside the focal region whose shape is regular ellipsoid when focusing at the geometric focus in vitro liver tissue.Simultaneously axial temperature curve rises steeply, and the temperature rise outside the main lobe is close to zero. The bifocal focusing can enlarge the volume of treatable focal region compared with single focus.Conclusions1. Element distribution of phased array transducer can influence the information of the sidelobe in the acoustic pressure field, but does not affect the main lobe.2. The phased array transducer with quasi-random distribution can suppress the acoustic pressure amplitude of the sidelobe and improve the acoustic pressure distribution significantly compared with period and concentric transducer.3. The acoustic pressure amplitude of the main lobe increases firstly and then decreases, while r value decreases firstly and then increases with the increasing distance between the focusing location along acoustic axis and the position of the transducer self-focusing. As the distance off the acoustic axis increasing, the acoustic pressure amplitude of the main lobe decreases, r value increases, and the adjustable acoustic focal range along the acoustic axis decreases gradually.4. The adjustable acoustic focal range of the phased array transducer can be extended to some extent by the high acoustic pressure sidelobe elimination method based on time reversal method.5. The isolated two foci and three foci can be merged into a focal region when the distance between the foci reduces to 2 mm, but the shape may not be regular.6. The volume of treatable focal region can be enlarged by multi-foci focusing.