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In photoacoustic imaging, an adaptive beamforming method with coherence factor (ABF-CF) was previously
introduced for improving spatial resolution and signal-to-noise ratio (SNR) over a conventional delay-and-sum
beamforming method (DAS). However, the ABF-CF method is not suitable for being used in practical diagnosis since it
is overly-sensitive for off-axis interferences and noises. In this paper, a new adaptive beamforming method with
spatially-smoothed coherence factor (ABF-SSCF) is presented for ultrasound and photoacoustic combined imaging to
enhance the contrast and spatial resolution while preserving the target information by applying a spatial-smoothing
technique into CF coefficients from multiple sub-arrays within an array probe. To evaluate the ABF-SSCF method,
computer simulation and ex vivo experiments were conducted. For the computer simulation, 64-channel radio-frequency
(RF) data with one channel amplitude-varying off-axis interference was generated. Also, The ex vivo experiments were
conducted where 128-channel pre-beamformed RF data were captured from a microcalcification-contained breast core
specimen with a commercial ultrasound system equipped with a research package by using a 7-MHz linear array probe
(SonixTouch, Ultrasonix Corp., BC, Canada) and an Nd:Yag laser excitation system (Surelite III-10 and Surelite OPO
Plus, Continuum Inc., Santa Clara, CA, USA). From the simulation and ex vivo experiments, the proposed ABF-SSCF
method provides better contrast and spatial resolutions comparable than the DAS method. Also, compared to the ABFCF
method, image information is clearly presented without being degraded by off-axis interferences. These results
indicate that the proposed ABF-SSCF method can simultaneously enhance the image quality and efficacy of the ABF
method for ultrasound and photoacoustic combined imaging.
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