Mechanism of acoustic pressure spectrum shifting toward lower frequencies in applied current thermoacoustic imaging

Abstract

Objective: Thermoacoustic tomography (TAT) is a promising imaging technique used for early cancer diagnosis, tumor therapy, animal study and brain imaging. Although it's widely known that the TAT frequency response depends on the pulse width of the source and the size of the object, a thorough comprehension of the quantitative frequency modulation in TAT and the mechanism governing the shift in the thermoacoustic pressure spectrum towards lower frequencies with respect to the excitation source is still lacking. This study aims to understand why the acoustic pressure spectrum and the final voltage signals shift towards lower frequencies in TAT. Approach: We employed a linear time-invariant model. In the proposed model, the applied current thermoacoustic imaging (ACTAI) processes is divided into the thermoacoustic stage and the acoustoelectric stage. These two stages are characterized by the thermoacoustic transfer function and the transducer transfer function respectively. We confirmed the effectiveness of our model through a rigorous examination involving both simulations and experiments. Main results: Simulation results indicate that the thermoacoustic transfer function behaves as a low-pass filter. The inherent low-pass nature induces a shift towards low frequencies in the acoustic pressure spectrum. Experiments further confirm this behavior, demonstrating that the final electrical voltage also shift towards low frequencies. Notably, employing the proposed model, there is a remarkable consistency between the main frequency bands of the synthesized and measured final voltage spectrum. Significance: The propoed model thoroughly explains how the thermoacoustic transfer function causes shifts to low frequencies in both the acoustic pressure spectrum and the final voltage spectrum in TAT. These insights deepen our understanding of optimizing TAT systems in the frequency domain, including aspects like filter design and transducer selection. Furthermore, we underscore the potential significance of this discovery on medical applications, particularly in the context of cancer diagnosis.