Design And Fabrication Of Ultrasound Annular Phased Array For Transcranial Neurostimulation

Direct modulation of neural activity has substantial relevance for treatment of neurologic and psychiatric disease as well as for scientific investigation of the neural mechanisms underlying the sensory,motor,and cognitive functions of the human brain.Brain stimulation has become an important method in research and therapeutic applications,all currently implemented approaches to the stimulation of brain suffer from a limitation or weakness.For example,transcranial magnetic stimulation(TMS)and transcranial direct current stimulation(tDCS)do not require invasive procedures but suffer from poor spatial resolution(~1 cm).Deep brain stimulation(DBS)offer a higher targeting specificity but requires surgically implanted electrodes.In recent years,transcranial focused ultrasound stimulation(tFUS)has emerged as a novel noninvasive brain stimulation showing a greater spatial resolution and does not require surgery in the whole stimulation process,it can stimulate the deeper brain tissue and attracting more and more interest from researchers.The existence of the skull brings difficulty for quantitative ultrasonic brain stimulation when carry out transcranial neurostimulation experiment.To deliver the necessary acoustic power to the target region in the brain effectively and safety,one must know is the transmittance of the skull.Estimation of transmittance through human skulls is the most fundamental and important factor for equipment-system designs.For clinical applications,uncertainty of the transmittance is undesirable.In addition,the traditional single-element transducer can not achieve higher spatial resolution and stimulate brain regions more specifically and deeply,in some cases,need a acoustic collimator to restrain wave beam.Comprehensive consideration,we designed and fabricated a ultrasound transducer for transcranial neurostimulation: A 5-element,0.5 MHz annular phased array based on a kind of highly heat-resistance piezoelectric ceramic PZT-4 whose diameter was 30 mm,and the transmittance of skulls was tested by this transducer.The annular array can achieve real-time dynamic focusing by applying different electronic delay for each element individually.In this way,the focal point of transducer is controllable and flexible,quite suitable for brain tissue stimulation of different depth,and enhance spatial resolution.Then a systematic measurements such as electrical properties,acoustic properties and sound field characteristics showed that the five elements of transducer have good consistency,electromechanical factor kt is above 0.42 and capacitance is 500 pF approximately according to the electrical test results,the transducer possessing a central frequency of 500 kHz and a-6 dB broad bandwidth from 0.38 MHz to 0.60 MHz,respectively.The acoustic field was measured by an Acoustic Intensity Measurement System(AIMS)indicate that the size of the acoustic focal point was 2 mm in diameter.The transmittance of skulls are also measured by the AIMS in a different condition.By changing two factors(ultrasound frequency,thickness of skull)we achieved the experimental transmittance of actual situation,one can use curve fitting methods to extract these values such that the experimental results conform with calculations,obtained the law of transmittance rate change with ultrasonic frequency while experimental values are a little bit lower than the theoretical values and we verified that the designed transducer can meet the demand of the transcranial neurostimulation in this paper.