Transcranial ultrasound hydrophone scanning and X-ray CT data

Additional laser beams along Y and Z axes confirmed the measurement point was at the skull’s outermost surface, compensating for curvature. Measurements used a focused transducer (Imasonic A101, 0.55 MHz, 80 mm diameter, 64 mm radius of curvature) and a 1 mm PVdF needle hydrophone (Precision Acoustics, UK; 0.1–10 MHz calibrated range). The transducer, driven by a signal generator and ENI 2100 L RF amplifier, emitted 500 kHz pulses at 10 kHz repetition rate.

The –6 dB focal spot diameter was 3 mm. The hydrophone, mounted on a 3-axis computer-controlled system, scanned a 35×35 mm² area in XY and XZ planes. Scanning was continuous along X, with 0.3 mm steps in Y and Z. Spatial averaging was performed over 0.3 mm along X. Scan limits were verified to avoid hydrophone-skull collisions.

Reference scans were performed post-measurement with the skull removed, maintaining identical transducer and hydrophone positions. Measurements were taken at parietal and frontal bone regions, with bone thicknesses of ~5.2 mm and ~6.1 mm, respectively, at normal and 40° incidence. All measurements were conducted in a tank filled with degassed, de-ionized water.