Evaluation of Reconstruction Methodology for Helical Scan Guided Photoacoustic Endoscopy

Photoacoustic endoscopy (PAE), combining both advantages of optical contrast and acoustic resolution, can visualize the chemical-specific optical information of tissues inside human-body. Recently, its corresponding reconstruction methods have been extensively researched. However, most of them are limited on cylindrical scan trajectories, rather than a helical scan which is more clinically practical. On this note, this article proposes a methodology of imaging reconstruction and evaluation for helical scan guided PAE. Different from traditional reconstruction method, synthetic aperture focusing technique (SAFT), our method reconstructs image using wavefield extrapolation which significantly improves computational efficiency and even takes only 0.25 seconds for 3-D reconstructions. In addition, the proposed evaluation methodology can estimate the resolutions and deviations of reconstructed images in advance, and then can be used to optimize the PAE scan parameters. Groups of simulations as well as ex-vivo experiments with different scan parameters are provided to fully demonstrate the performance of the proposed techniques. The quantitatively measured angular resolutions and deviations agree well with our theoretical derivation results <inline-formula> <tex-math notation="LaTeX">${D}{\sqrt {r_{s}^{2} + \overline {h}^{{2}}} } / {[{1.25}({r}_{s} {r}_{d} + \overline {h}^{{2}})] }$ </tex-math></inline-formula> (<italic>rad</italic>) and <inline-formula> <tex-math notation="LaTeX">$- \overline {h} {l} / ({r}_{s} {r}_{d} + \overline {h}^{{2}})$ </tex-math></inline-formula> (<italic>rad</italic>), respectively <inline-formula> <tex-math notation="LaTeX">${D},{r}_{d},\;{r}_{s},\overline {h} $ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${l}$ </tex-math></inline-formula> represent transducer diameter, radius of scan trajectory, radius of source position, unit helical pitch and the distance from targets to helical scan plane, respectively). This theoretical result also suits for circular and cylindrical scan in case of <inline-formula> <tex-math notation="LaTeX">$\overline {h} = {0}$ </tex-math></inline-formula>.