  Photoacoustic imaging (PAI) is a promising medical imaging modality providing
the spatial resolution of ultrasound (US) imaging and the contrast of pure
optical imaging. For linear-array PAI, a beamformer has to be used as the
reconstruction algorithm. Delay-and-sum (DAS) is the most prevalent beamforming
algorithm in PAI. However, using DAS beamformer leads to low resolution images
along with significant effects of the off-axis signals. Coherence factor (CF)
is a weighting method in which each pixel of the reconstructed image is
weighted, based on the spatial spectrum of the aperture, to improve the
contrast. In this paper, it has been shown that the numerator of the formula of
CF contains a DAS algebra, and it was proposed to use the
delay-multiply-and-sum (DMAS) beamformer instead of the available DAS on the
numerator. The proposed weighting technique, modified CF (MCF), has been
evaluated numerically and experimentally compared to CF, and it was shown that
MCF leads to lower sidelobes and better detectable targets. The quantitative
results of the experiment (using wire targets) show that MCF leads to for about
45% and 40% improvement, in comparison with CF, in the terms of signal-to-noise
ratio and full-width-half-maximum, respectively.
