Table 1. Strengths and weaknesses of representative imaging modalities (Baker 2010) .

Modality	Labels	Readout	Depth	Resolution	Pros	Cons
TPI	Fluorophores with two-photon absorption (700–1100 nm) cross-section in GM units	Vis–NIR light	~1 mm	1–2 μm	Less photo-damage and photo-bleaching than the corresponding FLI	Relatively poor depth penetration
BLI	Bioluminescent enzymes (typically luciferases)	Light (500– 630 nm emission)	~3 cm	3–5 μm	No external light, zero background emission	Inherently weak emission, expensive (~3 × 105 USD) charge-coupled device (CCD) camera
PAI	Probes (ε > 10 4/(M·cm)) that absorb light (680–950 nm) and create sound signals	Sound	5–7 cm	~50 μm	Better depth than light	Information processing and machines still being optimized