Table 2.

Imaging modalities of injectable hydrogel and their tissue engineering applications

Imaging modalities	Source	Typical penetration depth	Contrast agents and molecular probes	Spectral resolution	Gelation method	Tissue engineering applications	References
Computer tomography	x-ray	>10 cm	Iodine, barium	12–50 μm	Thermal	Prevention of fragile fracture, bone regeneration	[49, 51–53]
PET/Micro-PET	γ-rays	>10 cm	Fluorodeoxyglucose (FDG) 18F. 11C, 15O	1–2 mm	Cross-linking	Tissue regeneration	[43, 47, 50, 56]
Ultrasound	Acoustic reflection	0.1–10 cm	Microbubbles, nanoparticles	50 μm	Self-assembling, thermal	Bone regeneration	[44, 46, 57, 58]
Magnetic resonance imaging	Radio wave	>10 cm	Gadolinium, dysprosium, iron oxide particles	4–100 μm	Charge-based cross-linking	Regenerative medicine, drug delivery	[45, 48, 60–68]
Optical imaging	light	Several mm		>0.3 μm	Charge-based cross-linking	Regenerative medicine, drug delivery	[66, 69, 73]