TABLE 3.
Comparison of salient features of the label-free optical imaging modalities for AD.
| Imaging Techniques | Spatial Resolution | Sample Preparation | Biomarkers in AD | Advantages |
| FT-IR spectroscopic imaging | 0.54–12 μm |
Ex vivo
Flash-freezing; Fixation and freezing |
Aβ plaques; Lipids; Creatine | Rapid; High specificity; High resolution. |
| Raman spectroscopic imaging | 0.3–0.5 μm, lateral; 1–2 μm, axial |
Ex vivo
Fresh tissues; Fixation and flash-freezing |
Aβ plaques; Lipids | Real-time; High specificity; High resolution; Three-dimensional imaging; Minimal sample preparation. |
| OCT | >5 μm | In vivo | Ocular structures | Rapid; Non-invasive; Minimal sample preparation; Cost-effective. |
| OCM | 0.4–2 μm, lateral; 0.88–5.6 μm, axial |
Ex vivo
Flash-freezing; Fixation; Fixation and optical clearance |
Aβ plaques | High resolution; Three-dimensional imaging; Minimal sample preparation. |
| OCT-A | >5 μm | In vivo | Retinal vascular density | Rapid; Non-invasive; Minimal sample preparation. |
| Autofluorescence imaging | − |
Ex vivo
Fresh tissues; Fixation |
Aβ plaques; NFTs; Macular pigment | Rapid; Simple; Low-cost; High specificity. |
| Optical harmonic generation imaging | 0.31–0.57 μm3, 3D |
Ex vivo
Fresh tissues; Fixation; In vitro |
Aβ plaques; NFTs; Collagen | Rapid; High specificity; High resolution; Three-dimensional imaging; Low-cost. |
| Photoacoustic imaging | 5–150 μm | In vivo | Blood oxygenation | High sensitivity; Deep imaging depth. |
| Quantitative phase imaging | >0.8 μm |
Ex vivo
Fixation and dehydration |
Refractive index | Rapid; High resolution; Quantifiable; High sensitivity. |