Photoacoustic (PA) imaging requires channel data acquisition synchronized with a laser firing system. Unfortunately,the access to these channel data is only available on specialized research systems,and most clinical ultrasound scanners do not offer an interface to obtain this data. To broaden the impact of clinical PA imaging,we propose a vendor-independent PA imaging system utilizing ultrasound post-beamformed radio frequency (RF) data,which is readily accessible in some clinical scanners. In this paper,two PA beamforming algorithms that use the post-beamformed RF data as the input are introduced: inverse beamforming,and synthetic aperture (SA) based re-beamforming. Inverse beamforming recovers the channel data by taking into account the ultrasound beamforming delay function. The recovered channel data can then be used to reconstruct a PA image. SA based re-beamforming algorithm regards the defocused RF data as a set of pre-beamformed RF data received by virtual elements; an adaptive synthetic aperture beamforming algorithm is applied to refocus it. We demonstrated the concepts in simulation,and experimentally validated their applicability on a clinical ultrasound scanner using a pseudo-PA point source and in vivo data. Results indicate the full width at the half maximum (FWHM) of the point target using the proposed inverse beamforming and SA re-beamforming were 1.33 mm,and 1.08 mm,respectively. This is comparable to conventional delay-and-sum PA beamforming,for which the measured FWHM was 1.49 mm.