Transcranial ultrasound focus reconstruction with phase and amplitude correction

Abstract

Therapeutic and diagnostic ultrasound procedures performed non-invasively through the skull require a reliable method for maintaining acoustic focus integrity after transmission through layered bone structures. This study investigates the use of amplitude correction in combination with phase correction using multiple-element phased transducer arrays to reconstruct ultrasound focuses through the human skull. It has been previously demonstrated that adaptive phase correction using a multiple-element focused transducer array yields a significant correction to an acoustic field that has been distorted by the heterogeneities of the skull bone. The introduction of amplitude correction, in a regime where acoustic pressures from individual transducer array elements are adjusted to be normalized at the focus. We demonstrate a 6% average decrease in acoustic sidelobe acoustic intensity relative to the focal intensity and a 2% average decrease in the full-width-at-half-maximum (FWHM) of the acoustic intensity profile at the focus. These improvements come at the expense of significant ultrasound intensity loss - as much as 30% lower - at the focus since the amplitude correction method requires that, at constant power, a larger proportion of energy is absorbed by regions of the skull with higher attenuation. In contrast, a second correction method that distributes pressure amplitudes such that the less attenuating sections of the skull are exposed with more energy has demonstrated an average sidelobe intensity decrease of 3% with no change in the FWHM at the focus. On average, there was a 2% increase in the acoustic intensity at the focus for this inverse amplitude correction method. These results indicate that amplitude correction according to the attenuating properties of various segments of the skull can provide nominal improvements or hindrances depending on the application.