Stem cell homing, tracking and therapeutic efficiency evaluation for stroke treatment using nanoparticles: A systematic review

Table 1 Molecular imaging modalities

Image type	Technique	Physical principle	Tracrer	In vitro imaging	Preclincal imaging	Clinical Imaging	Spatial Resolution	Temporal resolution	Penetration depth	Sensitivity	Strengths	Limitations
Optical imaging	BLI	Visible light	Luminescent proteins	Yes	Yes	No	3-5 mm	Seconds to minutes	1-2 cm	High (+++)	High sensitivity, non-radioactive, cell expansion	Low penetration depth, non-translational
	FLI	Visible or NIRF light	Proteins or fluorescent dyes	Yes	Yes	No	2-3 mm	Seconds to minutes	< 1 cm	High (++)	High sensitivity, non-radioactive	Low penetration depth, autofluorescence
Nuclear imaging	PET	High-energy γ-rays	Radioisotopes [89 Zr (78.4 h), 18 F (1.83 h), 11 C (0.34 h), 64 Cu (12.7 h), 68 GA (1.13 h)]	No	Yes	Yes	1-2 mm	Seconds to minutes	Limitless	High (++)	High penetration depth, high sensitivity	Radiation exposure, high cost
	SPECT	Low-energy γ-rays	Radioisotopes [99 mTc (6.03 h), 123 I (13.2 h), 111 In (67.4 h)]	No	Yes	Yes	1-2 mm	Minutes	Limitless	High (++)	High penetration depth, high sensitivity	Radiation exposure, high cost
Magnetic imaging	MRI	Radio waves	Contrast agents	No	Yes	Yes	0.02-0.1 mm	Minutes to hours	Limitless	Low	High penetration depth, non-radioactive, high spatial resolution	High cost, low sensitivity and contrast

PET: Positron emission tomography; SPECT: Single-photon emission computed tomography; BLI: Bioluminescence; FLI: Fluorescence; MRI: Magnetic resonance imaging; NIRF: Near-infrared fluorescence.