Passive twin-layer spatial-temporal phase-interference compensator for improved ultrasound propagation: A computer-simulation and experimental study in acrylic step-wedge samples

Transcranial ultrasound wave degradation created by variations in both thickness and tissue composition is a significant impediment to diagnostic and therapeutic interventions of the brain. The current ‘active’ solution is to vary the transmission delay of ultrasound pulses, inherently necessitating electronic control of each individual transducer element. This paper advances the sonic-ray concept of ultrasound wave propagation by hypothesising that wave degradation can be minimised if both the transit-time and propagation path-length for all sonic-rays are made constant. A computer simulation was performed to investigate the dependence of an ultrasound signal upon spatial and temporal matching of all sonic-rays propagating through a sample exhibiting significant variation in transit time. Ultrasound propagation through a cylindrical acrylic 20-step wedge sample was considered, showing that phase-interference is removed only if there is complete spatial and temporal matching. A ‘passive’ concept for achieving this is introduced within this paper, consisting of a twin-layer ultrasound phase-interference compensator (TL-UPIC). An experimental study was performed in transmission-mode on six cylindrical acrylic step-wedge samples, ranging from 2 to 20 steps, each creating a corresponding number of sonic-rays. A TL-UPIC model corresponding to each step-wedge sample was designed, such that the propagation path-length (spatial) and transit-time (temporal) was constant for all sonic-rays, and replicated using 3D-printing. Time- and frequency-domain analysis demonstrated that incorporation of the TL-UPIC successfully removed phase-interference in all cases. It is hypothesised that the TL-UPIC concept may be applied to both pulse-echo mode diagnostic imaging and transmission mode therapeutic applications; and that it is also applicable to either single-element or multi-element array transducers, of any practicable frequency and dimension.