In vitro and in vivo characterization of a cranial window prosthesis for diagnostic and therapeutic cerebral ultrasound

OBJECTIVE The authors evaluated the acoustic properties of an implantable, biocompatible, polyolefin-based cranial prosthesis as a medium to transmit ultrasound energy into the intracranial space with minimal distortion for imaging and therapeutic purposes. METHODS The authors performed in vitro and in vivo studies of ultrasound transmission through a cranial prosthesis. In the in vitro phase, they analyzed the transmission of ultrasound energy through the prosthesis in a water tank using various transducers with resonance frequencies corresponding to those of devices used for neurosurgical imaging and therapeutic purposes. Four distinct, single-element, focused transducers were tested at fundamental frequencies of 500 kHz, 1 MHz, 2.5 MHz, and 5 MHz. In addition, the authors tested ultrasound transmission through the prosthesis using a linear diagnostic probe (center frequency 5.3 MHz) with a calibrated needle hydrophone in free water. Each transducer was assessed across a range of input voltages that encompassed their full minimum to maximum range without waveform distortion. They also tested the effect of the prosthesis on beam pressure and geometry. In the in vivo phase, the authors performed ultrasound imaging through the prosthesis implanted in a swine model. RESULTS Acoustic power attenuation through the prosthesis was considerably lower than that reported to occur through the native cranial bone. Increasing the frequency of the transducer augmented the degree of acoustic power loss. The degradation/distortion of the ultrasound beams passing through the prosthesis was minimal in all 3 spatial planes (XY, XZ, and YZ) that were examined. The images acquired in vivo demonstrated no spatial distortion from the prosthesis, with spatial relationships that were superimposable to those acquired through the dura. CONCLUSIONS The results of the tests performed on the polyolefin-based cranial prosthesis indicated that this is a valid medium for delivering both focused and unfocused ultrasound and obtaining ultrasound images of the intracranial space. The prosthesis may serve for several diagnostic and therapeutic ultrasound-based applications, including bedside imaging of the brain and ultrasound-guided focused ultrasound cerebral procedures.