Improving Axial Resolution of Optical Resolution Photoacoustic
Microscopy with Advanced Frequency Domain Eigenspace Based Minimum Variance
Beamforming Method
Optical resolution photoacoustic microscopy (OR-PAM) leverages optical
focusing and acoustic detection for microscopic optical absorption imaging.
Intrinsically it owns high optical lateral resolution and poor acoustic axial
resolution. Such anisometric resolution hinders good 3-D visualization; thus
2-D maximum amplitude projection images are commonly presented in the
literature. Since its axial resolution is limited by the bandwidth of acoustic
detectors, ultrahigh frequency, and wideband detectors with Wiener
deconvolution have been proposed to address this issue. Nonetheless, they also
introduce other issues such as severe high-frequency attenuation and limited
imaging depth. In this work, we view axial resolution improvement as an axial
signal reconstruction problem, and the axial resolution degradation is caused
by axial sidelobe interference. We propose an advanced frequency-domain
eigenspace-based minimum variance (F-EIBMV) beamforming technique to suppress
axial sidelobe interference and noises. This method can simultaneously enhance
the axial resolution and contrast of OR-PAM. For a 25-MHz OR-PAM system, the
full-width at half-maximum of an axial point spread function decreased
significantly from 69.3 μm to 16.89 μm, indicating a significant
improvement in axial resolution