Super-Resolution Photoacoustic Microscopy Using Structured-Illumination

A novel super-resolution volumetric photoacoustic microscopy, based on the theory of structured-illumination, is proposed in this paper. The structured-illumination will be introduced in order to surpass the diffraction limit in a photoacoustic microscopy (PAM) structure. Through optical excitation of the targeted object with a sinusoidal spatial fringe pattern, the object’s frequency spectrum is forced to shift in the spatial frequency domain. The shifting in the desired direction leads to the passage of the high-frequency contents of the object through the passband of the acoustic diffraction frequency response. Finally, combining the low-frequency image with the high-frequency parts in four regular orientations in the spatial frequency domain is equivalent to imaging the targeted object with an imaging system of two-fold bandwidth and thus half lateral resolution. In order to obtain the image of out-of-focus regions and improve the lateral resolution outside the focal region of a PAM imaging system, Fourier-domain reconstruction algorithm based on the synthetic aperture focusing technique (SAFT) using the virtual detector concept is utilized for reduction in the required computational load and time. The performance of the proposed imaging system is validated with
in vivo
and
ex vivo
targets. The experimental results obtained from several tungsten filaments in the depth range of 1.2 mm, show an improvement of −6 dB lateral resolution from 55–
$287~\boldsymbol \mu \text{m}$
to 25–
$29~\boldsymbol \mu \text{m}$
and also an improvement of signal-to-noise ratio (SNR) from 16–22 dB to 27–33 dB in the proposed system.