Congruence of frequency-dependent spatial coherence between linear frequency-modulated pulses and conventional pulses
Adaptive transmit frequency selection, filtering, and frequency compounding all offer the ability to improve target conspicuity by balancing the effects of imaging resolution, signal-to-clutter ratio, and speckle texture, but the use of pilot pulses at a series of individual frequencies to calculate image quality at each interrogable frequency restricts real-time implementations. We propose the use of linear frequency-modulated transmissions, also known as chirps, to interrogate frequency-dependent trends in spatial coherence, a well validated measure of signal quality, for the purpose of monitoring image quality to more efficiently perform these adaptive strategies. These measurements of spatial coherence are compared to those acquired by individually transmitted conventional pulses over a range of frequencies. Experimental results in porcine and human liver tissue models indicate that chirps replicate the mean coherence in a region-of-interest. Results also show that prediction accuracy improves with chirp length. Furthermore, chirps are able to predict frequency-dependent decreases in coherence in both porcine abdominal and human liver models. This work indicates that the use of chirps is a viable strategy to improve the efficiency of variable frequency coherence mapping, thus presenting an avenue for real-time implementations for frequency-based adaptive imaging strategies.
