Table 1.

Overview of contrast agents for UMF, USF, and PAI.

		UMF	USF	PAI
Endogenous		NR	NR	Jiang and Pu, 2017; Zeng et al., 2018
Exogenous	biomolecules	NR	NR	Upputuri and Pramanik, 2016; Wang and Yao, 2016; Jiang and Pu, 2017; Zeng et al., 2018
	organic dyes	(Lin et al., 2012b; Liu et al., 2014)	NR
	inorganic	(McNeil, 2009; Miller et al., 2017)	NR
	dyes@polymer nanocomposites	NR	Yuan et al., 2013; Cheng et al., 2014, 2016, 2017; Pei et al., 2014; Yu et al., 2016
	dyes-labeled polymer	NR	Kobayashi et al., 2006; Cheng et al., 2014	NR
	dyes/microbubble	(Loving et al., 2010; Lin et al., 2015; Liu et al., 2015)	NR	NR
Functioning mechanism		Under debate	Ultrasound → Heat → phase change → Microenvironment → Quantum yield (dye) increases → Fluorescence “ON”	Absorb photons → heat
stability		NR	>6 months (Yu et al., 2016)	NR
			>10 months (Cheng et al., 2016)
Sensitivity (switchable)		No	Yes	Yes
Selectivity (targeting)		Yes*	Yes	Yes
Versatility of detection		Yes	Yes	No
Commercial availability		Yes	No	Yes

NR, not yet reported; FRET, Förster (or Fluorescence) Resonance Energy Transfer.

^*limited due to the size of microbubble.