Contrast-enhanced transcranial two-dimensional ultrasound imaging using shear-mode conversion at low frequency

Area of Science:

• Medical Imaging
• Biomedical Engineering
• Acoustics

Background:

• Transcranial ultrasound (US) signal distortion and attenuation pose challenges for brain imaging.
• Shear-mode transmission is a novel approach to mitigate US beam distortion through the skull.

Purpose of the Study:

• To evaluate the efficacy of transcranial shear-mode transmission for improving ultrasound brain imaging.
• To assess the impact of shear-mode transmission on image quality and resolution at different US frequencies.

Main Methods:

• An ex vivo human skull section was used to image a contrast-agent-filled tube with a 32-element 750 kHz linear phased array transducer.
• Shear-mode transmission was tested alongside fundamental, second harmonic, and subharmonic imaging frequencies.
• Image resolution was measured laterally and axially.

Main Results:

• Subharmonic imaging with shear-mode transmission successfully visualized the tube in 6 of 9 cases, outperforming fundamental and second harmonic frequencies (1 of 9 cases each).
• Minor axial image location improvement was observed at the fundamental frequency with shear mode.
• Skull presence and incident angle did not affect signal resolution, which was 5mm lateral/3mm axial for fundamental/harmonic and 10mm lateral/5mm axial for subharmonic imaging.

Conclusions:

• Subharmonic shear-mode ultrasound shows the greatest potential for skull penetration in transcranial imaging.
• The enhanced penetration comes at the expense of reduced image resolution compared to other frequencies.