TABLE 1

Comparison of Existing MR Reporter Genes

Gene	Contrast	Substrate	Advantages	Disadvantages
CK	³¹P-MRS	Endogenous adenosine triphosphate	No background signal in most tissues	Spectroscopic image (low resolution)
Tyrosinase	T2/T2*	Endogenous or exogenous iron	Rapid straightforward gene product	Signal dependent on availability of iron
β-gal	T1	Exogenous EgadMe	Positive contrast	Exogenous agent also gives partial signal without reporter gene
Transferrin receptor	T2/T2*	Exogenous transferrin-MION	High sensitivity	Accessibility and nonspecific uptake of substrate nanoparticles
Ferritin	T2/T2*	Endogenous or exogenous iron	High sensitivity	Delay of change in signal that is dependent on iron availability and ferritin loading factor
Mag A	T2/T2*	Endogenous or exogenous iron	High sensitivity	Delay of change in signal that is dependent on iron availability
LRP	CEST	None	Multicolor detection, on–off switchable	Current low sensitivity