Transcranial shear-mode ultrasound: assessment of imaging performance and excitation techniques.
Transcranial ultrasound imaging is limited by poor acoustic windows and skull induced distortions to the beam. Shear waves in the skull have a better impedance match with longitudinal waves in water and thereby produce a more coherent focus inside the skull. This study presents work on an imaging technique that utilizes shear-wave propagation through the skull. The pulse-echo lateral distortion introduced by the skull was analyzed by imaging a point scatterer behind ex vivo human craniums at 1 MHz. Brightness images of the target obtained with either shear-mode or conventional longitudinal-mode transmission in the bone were assessed to quantify lateral resolution. As compared to longitudinal-mode transmission, it was found that the use of shear-mode resulted in improved localization along the propagation (depth) axis at the expense of degraded lateral resolution. The signal-to-noise ratio (SNR) limitations introduced by severe attenuation of shear-waves in the skull were overcome with frequency modulated (FM) coded excitations. This gain in SNR was exchanged with resolution and used for compensation of frequency-dependent attenuation in the skull, resulting in a greater than 20% improvement in lateral resolution for both modes of transcranial transmission. The results are an important step towards enhancing the quality of transcranial sonography.
Yousefi
Ali
A
Electrical Engineering Department, University of Toronto, Toronto, ON M4N 3M5 Canada. ali.yousefi@utoronto.ca
Goertz
David E
DE
Hynynen
Kullervo
K
D000465:Algorithms; D016477:Artifacts; D004867:Equipment Design; D005583:Fourier Analysis; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D012815:Signal Processing, Computer-Assisted; D012886:Skull / Q000033:anatomy & histology; D017585:Ultrasonography, Doppler, Transcranial / Q000379:methods*