Transcranial Shear-Mode Ultrasound: Assessment of Imaging Performance and Excitation Techniques

• Published in: IEEE transactions on medical imaging Vol. 28; no. 5; pp. 763 - 774
• Main Authors: Yousefi, A., Goertz, D.E., Hynynen, K.
• Format: Journal Article
• Language: English
• Published: United States IEEE 01.05.2009; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Acoustic beams; Acoustic distortion; Acoustic imaging; Algorithms; Attenuation; Brain; diagnosis; Equipment Design; Focusing; Fourier Analysis; Frequency modulation; Humans; Image Enhancement - methods; imaging; Impedance; shear waves; Signal Processing, Computer-Assisted; Signal resolution; Skull; Skull - anatomy & histology; Ultrasonic imaging; Ultrasonography, Doppler, Transcranial - methods; ultrasound
• ISSN: 0278-0062; 1558-254X
• DOI: 10.1109/TMI.2008.2012017

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.