  Photoacoustic microscopy with large depth of focus is significant to the
biomedical research. The conventional optical-resolution photoacoustic
microscope (OR-PAM) suffers from limited depth of field (DoF) since the
employed focused Gaussian beam only has a narrow depth range in focus, little
details in depth direction can be revealed. Here, we developed a computed
extended depth of field method for photoacoustic microscope by using wavelet
transform image fusion rules. Wavelet transform is performed on the max
amplitude projection (MAP) images acquired at different axial positions by
OR-PAM to separate the low and high frequencies, respectively. The fused low
frequency coefficients is taking the average of the low-frequency coefficients
of the low-frequency part of the images. And maximum selection rule is used in
high frequency coefficients. Wavelet coefficient of the MAP images are compared
and select the maximum value coefficient is taken as fused high-frequency
coefficients. And finally the wavelet inverse transform is performed to achieve
large DoF. Simulation was performed to demonstrate that this method can extend
the depth of field of PAM two times without the sacrifice of lateral
resolution. And the in vivo imaging of the mouse cerebral vasculature with
intact skull further demonstrates the feasibility of our method.
