Numerical Simulation Of Temperature Field Focused By Transcranial HIFU And Modulation Of Driving Signals Of Transducer Array

High Intensity Focused Ultrasound(HIFU) has been applied to clinical treatment of substantive liver tumor and uterine fibroids and so on. Because of its non-invasive and repeatable therapeutic, it is introduced into transcranial tumor treatment and drug delivery. As the target surrounded by the skull and the large difference of acoustic parameters between the skull and brain, the aberration of phase and amplitude of HIFU is appeared after HIFU propagating after the skull that leads to defocusing and thermal damage to the skull and tissue around it. It is very necessary to study the methods of focusing precisely and effectively and reducing thermal damage to the skull and tissue around it.Objective In order to correct the aberration of phase and amplitude and defocusing and focusing at the target accurately and avoiding thermal damage to the skull, the driving signals of transducer elements is modulated by correcting their phase and amplitude and superposed signals of reducing high pressure in skull. The pressure and temperature fields are modulated and focusing accurately in brain is realized to make good reference for clinic treatment.Methods The 3D simulation model used in our research was made of a 64-element cap-shaped phased transducer with a small aperture and reconstructed images of the skull based on CT scans. The transcranial pressure fields of the model were calculated by Finite Difference Time Domain(FDTD) with the Westervelt nonlinear propagation equation. The transcranial temperature fields of the model were calculated by FDTD with the Pennes bioheat equation. The phase and amplitude of driving signals of the transducer was modulated based on time reversal. The pressure and thermal fields focused by the modulated driving signals were used to study and modulated the signals further. The effects of different modulated driving signals on the transcranial pressure and temperature fields were analysed and discussed through the numerical simulation. The temperature threshold and the thermal dose were applied to evaluate the possible thermal damage to the skull and tissue around it and calculate the treatable volume of focal region. Under the condition of safe treatment, the transcranial modulating and focusing region was studied and the method of compensating the treatable volume was proposed.Results1. The focus was appeared at the setting target when focusing with the phase of driving signals of transducer modulated. But there was a high pressure existing in the skull and the radio between the pressure in skull and at focus was still high.Combined with amplitude correction of driving signals, the pressure at focus was higher than only phase correction and the one in the skull and the radio were a little lower. The radio of pressure in skull and at focus and the high pressure in skull were decreased but the pressure at focus was decreased after modulated with the phase correction and reducing high pressure in skull. By modulating the phase and amplitude of driving signals and superposing signals of reducing high pressure in skull, the radio was least and the pressure in skull was decreased. The pressure at focus was almost same with the one after only phase correction.2. When focusing on acoustic axis, the pressure at focus after modulating the amplitude of driving signals of transducer was higher than the one without amplitude modulation. At the same time, the volume of treatable focal region after amplitude modulation was larger.3. The phase used to modulating the driving signals obtained by numerical fitting and cross-correlation based on time reversal were both used to realized the transcranial focusing.4. When modulating the phase and amplitude of the driving signals of elements located at different rings of transducer, there could form good focal region and the temperature at the critical surface between skull and water was lowest by modulating the amplitude of signals of elements located at the outer rings.5. The pressure and temperature and thermal deposition at focus focusing nonlinearly were larger than those focusing linearly.6. When HIFU propagating through the occipital bone window, at the depth of 25 mm, the safe distance of focusing vertically off the acoustic axis along Y axis was 5mm. But safe focusing vertically off the acoustic axis along Z axis could not realize.7. The volume of treatable focal region after modulating was decreased as the distance vertically off acoustic axis getting larger. When focusing off axis at the constant distance, the volume of focal region was larger with the intensity increasing approximately linearly. The volume of focal region focusing off axis could be modulated by changing the intensity of transducer elements to make the same volume as focusing on acoustic axis.Conclusions1. The driving signals of transducer affect the transcranial focusing. The focus could appear at the setting target accurately and the pressure and temperature at focus were increased to deposited more energy at focus by modulating the phase and amplitude of driving signals of elements of transducer.2. By superposing signals used to reducing high pressure in skull to the driving signals, the thermal deposition in skull was decreased avoiding thermal damage to skull. When modulating the amplitude of driving signals of elements located at outer rings of transducer, the temperature at critical surface between skull and water was also decreased.3. The nonlinearity has an influence on transcranial focusing. It should be taken into consideration during transcranial focusing.4. When HIFU propagating through the fixing acoustic window, there exists a safe focusing distance vertically off acoustic axis. As the distance getting larger, the volume of treatable was reduced. The volume focusing off axis could be the same as the one focusing on axis by modulating the intensity of driving signals of transducer based on the linear relationship between the distance off axis and the intensity.In conclusion, the method of modulating the temperature fields was studied by numerical simulations to figure out the issue that trascranial waves didn’t focus at target and did thermal damage to the skull. Modulating the amplitude of driving signals of transducer can increase the thermal deposition concentrating at focus under the condition of focusing accurately in brain. The relationship between the intensity and the distance vertically off acoustic axis could make a reference for clinical treatment dose plan.