Creating uniform ultrasound fields in the brain without an array

There has been a recent interest in using patient specific ultrasound lenses to correct for the phase aberration caused by the skull in focused transcranial ultrasound. Here, we apply acoustic lenses to the problem of creating a highly uniform field inside the skull, motivated by the requirements of targeted and ultrasound responsive nanodroplets being developed for the treatment of brain metastases. The phase and amplitude required for uniform sonication was determined from simulations of ultrasound propagation utilising CT data as a basis for the geometry of the patient’s skull. From these simulations, silicone lenses were produced, capable of introducing phase inhomogeneity in the incident ultrasound beam to compensate for the effects of transmission through the skull. Experimental measurements with an ex vivo skull showed that phase uniformity could be at least partially restored within the skull cavity. Methods for modulating the amplitude of incident ultrasound have also been investigated. Stereolithography was used to directly print in highly attenuating (15 dB/cm at 1 MHz) materials, allowing rapid production of customised and selective acoustic absorbers. A 3D printed transducer backing was also studied, able to suppress transducer output in certain regions, and provide up to 15 dB dynamic range across the surface of a planar ultrasound transducer.