. 2020 Jan 12;10(4):1923–1947. doi: 10.7150/thno.39915

Table 2.

Trackers, advantages and disadvantages of various imaging techniques.

Techniques	Trackers / Advantages / Disadvantages
MRI, MPI	Trackers: Gd³⁺-, Mn²⁺-, ¹⁹F-based NPs, IONs and SPIONs. Advantages: (1) Arbitrary orientation tomography and non-osseous artifacts for accurate diagnosis; (2) Near-ideal depth penetration, spatial resolution (10~100 μm), high contrast and quantitative analysis; (3) Several weeks to months tracking for in vivo fate and aging of stem cells; (4) Non-ionizing and low frequency magnetic fields in MPI. Disadvantages: (1) Long scanning time and slow imaging speed; (2) Very insensitive to tumor calcification and in vivo calculus.
Fluorescence imaging	Trackers: inorganic QDs-, FNDs- and organic material-based NPs. Advantages: (1) Easy operation, low cost, non-invasive and fast imaging from seconds to minutes; (2) Relatively good biocompatibility for FNDs and organic material-based NPs; (3) High photostability for SP-based NPs and AIE dots; (4) Several weeks tracking for in vivo studies; (5) Theranostic capability based on some luminogens owning ROS generation. Disadvantages: (1) Limited tissue penetration depth in several micrometers for traditional optical probes, except for NIR-II probes (several millimeters); (2) Interference from biological auto-fluorescence; (3) Difficult to quantitative analysis.
PET/SPECT imaging	Trackers: radioisotopes-based materials such as ¹⁸F, ⁶⁴Cu, ⁸⁹Zr, ¹¹¹In and ⁸⁶Y/¹⁷⁷Lu-based reagents. Advantages: (1) Non-invasive inspection for early stage cancers and further precise medication with high quality; (2) Unlimited detection depth, spatial resolution (1~2 mm) and quantitative analysis; (3) Short tracking period depending on the half-life of radioactive reagents; (4) Whole-body imaging with high sensitivity. Disadvantages: (1) High cost; (2) Use of radioisotopes leading to hard-to-assess in vivo security; (3) Low sensitive to tumors in lung, liver and gastrointestinal tract in comparison with MRI or CT.
PA imaging	Trackers: Au nanorods, graphene, carbon nanotubes, organic smell-molecules and SPNs etc. Advantages: (1) Non-invasive and non-ionizing detection and fast imaging from seconds to minutes; (2) Deep tissue penetration depth in centimeters with good SNR and spatial resolution in tens to hundred micrometers, rich contrast and high sensitivity; (3) Days to weeks tracking for in vivo fate and aging of stem cells; (4) Low cost and portable system. Disadvantages: (1) Limited imaging resolution in comparison with optical imaging; (2) Lack of stability in detecting methods, and shallow detection depth in comparison with mature clinic detection technology such as CT and MRI.