Acoustic holography for characterisation of sub-megahertz frequency medical ultrasound sources

Ultrasonic sources operating in the 0.2-1 MHz range are employed in transcranial ultrasound applications to minimise skull-induced aberrations and absorption. Characterizing these acoustic sources is crucial to modelling in situ fields in the brain. However, measurement best practice for sub-MHz sources is not well-established, and guidelines developed for MHz medical ultrasound sources may not apply when similar source apertures are operated at lower frequencies. Here, errors in acoustic fields projected from measured holograms are investigated experimentally using three transducers operating frequencies between 270 and 750 kHz. Errors in reconstruction of the focal regions compared to measurement were less than 2% for measurement planes approximately 1.3 times the -20 dB beam diameter, and root mean square errors were less than 2% for measurement planes more than two times the -20 dB diameter, acquired between the source and focus. Larger measurement planes are required to reduce errors for these lower frequencies compared to fields generated by sources operating at 1 MHz and above.