Table 1. Summary of methods used for SR-PAI.
| Depth regime | Imaging modality | Mechanism | Resolution | Imaging speed | Pros | Cons | Ref | |
|---|---|---|---|---|---|---|---|---|
| Lateral | Axial | |||||||
| Ballistic | OR-PAM | Nonlinear photobleaching effect | 90 nm | 400 nm | Medium | Can be applied to both fluorescent and nonfluorescent samples. Capable of optical sectioning | The high frame number or excitation intensity may result in severe distortion between ‘bleached’ and ‘unbleached’ regions | (10) |
| Nonlinear absorption coefficient | 88 nm | – | Medium | High-order PA image leads to higher resolution. Capable of optical sectioning | Long imaging time, slow pixel-by-pixel high-order fitting, and potential heating and photobleaching damage to the sample with repeated excitation | (11) | ||
| Diffusive | AR-PAM | Deconvolution with blind speckle illumination | 80 ìm | – | Low | Uses blind structure illumination to achieve F imaging. Efficiently exploits dynamic temporal fluctuations | Requires sparsity of the structures | (13,26) |
| The small fluctuations over a large background may be challenging under low SNR conditions | ||||||||
| Improved optical focus through wavefront shaping | 30 ìm | – | Low | Can create an optical focal spot five times smaller than the acoustic focal spot size in scattering medium | Feedback optimization of wavefront for each pixel is very slow | (15) | ||
| PACT (OAT) | Photoacoustic signal fluctuation induced by flowing particles | 72 ìm | – | Low | Free from common photoacoustic imaging artifacts caused by band-limited acoustic detection. Potential for label-free microvessel imaging | Lacks high temporal resolution intrinsically. Requires high frame rate for fluctuation analysis | (12) | |
| Photoacoustic signal fluctuation with multiple speckle illumination | 160 ìm | – | Low | Simple experimental setup. Reveals high spatial frequencies by considering fluctuations in PA images | Requires a very large number of frames for high SNR | (25) | ||
| Localization of flowing absorbing particles | 78 ìm | 112 ìm | Very low | Not affected by flow rate. Enhances the visibility of structures under limited-view conditions | Needs high frame rate and high particle density | (14,27) | ||
SR-PAI, super-resolution photoacoustic imaging; OR-PAM, optical-resolution photoacoustic microscopy; AR-PAM, acoustic-resolution photoacoustic microscopy; PACT, photoacoustic computed tomography; SNR, signal-to-noise ratio.