Evaluation of Adjoint Methods in Photoacoustic Tomography with Under-Sampled Sensors

TL;DR

This paper analyzes the performance of adjoint methods in photoacoustic tomography with under-sampled sensors, demonstrating that Truncated Back-Projection improves contrast and resolution compared to traditional methods.

Contribution

The study provides a theoretical and numerical comparison of Time-Reversal and Back-Projection methods under under-sampling, introducing Truncated Back-Projection to enhance image quality.

Findings

TR has better contrast but lower axial resolution due to side lobes.

BP has higher resolution but lower contrast with under-sampled sensors.

TBP improves both omnidirectional resolution and contrast in under-sampled scenarios.

Abstract

Photo-Acoustic Tomography (PAT) can reconstruct a distribution of optical absorbers acting as instantaneous sound sources in subcutaneous microvasculature of a human breast. Adjoint methods for PAT, typically Time-Reversal (TR) and Back-Projection (BP), are ways to refocus time-reversed acoustic signals on sources by wave propagation from the position of sensors. TR and BP have different treatments for received signals, but they are equivalent under continuously sampling on a closed circular sensor array in two dimensions. Here, we analyze image quality with discrete under-sampled sensors in the sense of the Shannon sampling theorem. We investigate resolution and contrast of TR and BP, respectively in one source-sensor pair configuration and the frequency domain. With Hankel's asymptotic expansion to the integrands of imaging functions, our main contribution is to demonstrate that TR and BP have better performance on contrast and resolution, respectively. We also show that the integrand of TR includes additional side lobes which degrade axial resolution whereas that of BP conversely has relatively small amplitudes. Moreover, omnidirectional resolution is improved if more sensors are employed to collect the received signals. Nevertheless, for the under-sampled sensors, we propose the Truncated Back-Projection (TBP) method to enhance the contrast of BP using removing higher frequency components in the received signals. We conduct numerical experiments on the two-dimensional projected phantom model extracted from OA-Breast Database. The experiments verify our theories and show that the proposed TBP possesses better omnidirectional resolution as well as contrast compared with TR and BP with under-sampled sensors.