Evaluation of Harmonic SLSC Beamforming Theory

Short-lag spatial coherence (SLSC) beamforming has significantly improved target boundary visibility and reduced acoustic clutter by measuring and displaying the similarity of closely separated backscattered ultrasound signals. Also, harmonic imaging can offer additional improvements as it is known to reduce acoustic clutter. SLSC imaging has been combined with harmonic imaging to combine the benefits of SLSC beamforming and harmonic imaging. The objective of this work is to evaluate spatial coherence theory associated with harmonic SLSC beamforming of ultrasound images, with an emphasis on the received center frequency and the location of the evaluated region. The harmonic spatial coherence function generally has lower values and decreases faster with increasing lag compared to the fundamental spatial coherence function, particularly at target edges. For example, at the edge of an 11 mm-wide hyperechoic target, harmonic spatial coherence functions were up to 28.6% lower than corresponding fundamental spatial coherence functions. In addition, simulation results from tissue regions were compared with tissue-mimicking phantom and in vivo breast tissue results, each demonstrating that harmonic spatial coherence values can also surpass fundamental spatial coherence values. This work lays the theoretical foundation necessary to interpret nuanced differences among harmonic spatial coherence images in various clinical applications.