Acoustic-resolution photoacoustic microscopy (AR-PAM) has great advantage over deep imaging depth when compared to optical-resolution PAM (OR-PAM). This is because that the point spread function (PSF) of AR-PAM is determined by the acoustic focus, which is relatively less scattered in biological tissues. In general, to maintain a high signal-to-noise ratio (SNR) and lateral resolution at a deep depth, AR-PAM uses an acoustic lens with a high numerical aperture (NA). The high NA lens provides high resolution and SNR in focal region, but significantly degrades the SNR and resolution in out-of-focus region. To overcome this problem, many researchers have introduced the synthetic aperture focusing technique (SAFT), which sums up the corresponding signals in the solid angle of the acoustic NA. However, the image enhancement of the conventional SAFTs has been limited because those techniques accumulate the signals without considering the actual photoacoustic (PA) wavefronts. In this study, we propose a novel SAFT that can overcome the existing limitation by exploiting each enhanced frequency components of the 1D SAFT images performed in multiple directions. As a result, we confirmed that the output AR-PAM image of our novel SAFT is superior to the existing SAFT image quality.
Photoacoustic microscopy (PAM) is a hybrid imaging technology using optical illumination and acoustic detection. PAM is divided into two types: optical-resolution PAM (OR-PAM) and acoustic-resolution photoacoustic microscopy (AR-PAM). Among them, AR-PAM has a great advantage in the penetration depth compared to OR-PAM because ARPAM relies on the acoustic focus, which is much less scattered in biological tissue than optical focus. However, because the acoustic focus is not as tight as the optical focus with a same numerical aperture (NA), the AR-PAM requires acoustic NA higher than optical NA. The high NA of the acoustic focus produces good image quality in the focal zone, but significantly degrades spatial resolution and signal-to-noise ratio (SNR) in the out-of-focal zone. To overcome the problem, synthetic aperture focusing technique (SAFT) has been introduced. SAFT improves the degraded image quality in terms of both SNR and spatial resolution in the out-of-focus zone by calculating the time delay of the corresponding signals and combining them. To extend the dimension of correction effect, several 2D SAFTs have been introduced, but there was a problem that the conventional 2D SAFTs cannot improve the degraded SNR and resolution as 1D SAFT can do. In this study, we proposed a new 2D SAFT that can compensate the distorted signals in x and y directions while maintaining the correction performance as the 1D SAFT.