Table 2.

Minimal- or non-invasive imaging modalities for theranostic NPs.^{5, 58, 59}

Modality	Probe(s)	Pros	Cons
Nuclear imaging: PET, SPECT	Radionuclides (e.g. F-18, In-111, Cu-64)	Quantitative analysis High sensitivity	Radioisotope exposure Expensive procedures
MRI	Paramagnetic atoms (e.g. Gd, Mn) Superparamagnetic NPs (e.g. SPION)	Able to image physiological and anatomical details Soft tissue contrast	Limited acquisition time High cost
CT	Heavy elements (e.g. iodine)	Quantitative anatomical information High spatial resolution	Radiation exposure Limited to morphological information
Ultrasound imaging	Gas filled microbubbles	Ease of procedure Low cost	Low resolution Low sensitivity
NIR imaging	Fluorophores (e.g. fluorescence dye, QD)	High sensitivity and spatial resolution Low autofluorescence Cost efficiency and simplicity	Limited penetration (<5 mm)
Photoacoustic imaging	Light absorbates (e.g. fluorophore, quencher)	High spatial resolution Functional information	Limited penetration (<5 cm)

Abbreviations used are: PET, positron emission tomography; SPECT, single photon emission computed tomography; MRI, magnetic resonance Imaging; NP, nanoparticle; SPION, superparamagnetic iron oxide nanoparticle; QD, quantum dot