Real-time ultrafast holography of focused ultrasound wave fields using an ultrasound scanner
Acoustic pressure field measurements are required for marketing clearance of medical ultrasound instruments, but the gold standard method, hydrophone scanning, is time consuming. The objective of this work was to develop an ultrafast holographic method for real-time imaging of focused ultrasound fields. The technique is based on parallel acquisition with a sensor array, and FUS field reconstruction based on time reversal assuming a homogeneous lossless medium. The geometric characteristics of the sensor to enable accurate reconstruction were evaluated through numerical modelling. A practical example involving a commercially-available 128-element linear-array ultrasound imaging probe is presented. Finally, preliminary experiments were conducted on in-vitro soft tissues to validate the concept of simultaneous dual-mode imaging, combining conventional B-mode tissue imaging with FUS field holography. In modelling of ideal sensor characteristics (2D matrix; intercepted energy > 99%), parallel holography allowed reconstructing wave fields with excellent agreement with the Rayleigh Integral reference (errors << 0.01%). Successfully experimental demonstration of real-time ultrafast wave field holography was achieved with the commercial imaging probe (7-10% error in the -6dB focus vs hydrophone measurements, acquisition time: 100 microseconds, display rate: 15 frames/s), despite a suboptimal intercepted energy (44%). Real-time dual-mode imaging of continuous wave fields allowed targeting localized tissues regions, while holographic movies highlighted the propagation of transient waves. Overall, real-time ultrafast holography offers a practical way to perform metrology and guidance of ultrasonic devices, by visualising waves fields typically used for ultrasound imaging, or those used during thermal or non-thermal ultrasound therapy.