Table 1 Summary of the features of the four most commonly used imaging modalities in preclinical research

Optical Imaging: (Bio)luminescence and fluorescence Imaging (BLI; FLI)

Spatial resolution: 2–5 mmTemporal resolution: seconds to minutesPenetration depth: < 1 cm for fluorescence, 1–2 cm for bioluminescenceSafety: completely safe

Cells transduced with reporter gene can be tracked; the signal disappears with cell death (no false-positives). Good for tracking cell fate.

Tracking of biological processes and molecular pathways such as cell signalling.Gene transfer efficiency in gene therapy preclinical research.¹²⁸Tumour imaging.^92,129Cell differentiation.⁷⁴

Semi-quantitative method. Output measured in relative light units (RLUs), which vary between different luminometers.

Good cell tracking with fluorescent quantum dots, however signal weakens with cell division and quantum dots from dead cells can be phagocytosed by macrophages and yield false positives—not suitable for tracking cell fate.¹²⁷

Alternatively, persistent luminescent particles have excellent signal to noise ratio, and reduced tendency to be released from cells and so can be used to track cells for longer periods of time than most other optical probes.¹⁰⁸

Magnetic resonance imaging (MRI)

Spatial resolution: 40–100 umTemporal resolution: minutes to hoursPenetration depth: no limitSafety: completely safe

Cells can be labelled with superparamagnetic iron oxide nanoparticles (SPIONs) or paramagnetic metal chelates.

Oncology (tumour growth, perfusion, ablation and oxygenation).¹³¹Cardiology (heart perfusion).¹³²Musculoskeletal tissue structures.¹³³

Magnetic reporters can also be used to track cells, but lack sensitivity.^55,130

Nuclear Imaging: PET and SPECT

Spatial resolution: 1–2 mmTemporal resolution: seconds to minutesPenetration depth: unlimitedSafety: there are some safety concerns over the use of radioactive tracers, however doses are very low and the risks are carefully monitored

Cells can be labelled with tracers for short-term tracking, for example ¹¹¹In (SPECT) or ¹⁸F-Fluoro-Deoxyglucose (PET).

PET or SPECT provide high sensitivity, which is an advantage for tracking anatomical localization of stem cells and nuclear imaging using reporter genes permits long-term engraftment studies.

SPECT and PET reporter gene imaging use the principle of interactions between an exogenous probe and the protein produced by the reporter gene. There are predominantly three genes: herpes simplex virus type 1 thymidine kinase (HSV1-tk), dopamine type 2 receptor (D2R), and, sodium/iodide symporter (NIS).⁹⁷

Photoacoustic Imaging

Spatial resolution: 20–300 μmTemporal resolution: seconds to minutesPenetration depth: 4–5 cmSafety: completely safe

Can image cells labelled with gold nanorods³⁰ or carbon nanotubes¹³⁴, or cells expressing NIR reporter genes.¹¹⁷ Can image down to 10,000 cells³⁰ and can quantify cell numbers.¹³⁵

Excellent tumour imaging.²²Functional imaging of some organs/tissues (using either endogenous or exogenous contrast).¹³⁶Imaging of 3D scaffolds.⁸¹