a Approach 1 to synthesize HDL-MNS A particles where oleic acid coated hydrophobic MNS were first coated with a neutral lipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and later coated with apolipoprotein A1 (apoA-1). In approach 2, hydrophilic MNS were first coated with apoA1 and then coated with DPPC, resulting in HDL-MNS B particles. b r2 relaxivity plot of HDL-MNS B particles measured at 1.4 T. c Comparison of r2 values of HDL-MNS A and B with commercially available contrast agents (ferumoxytol, and ferumoxide). d T2-weighted MR images shows darker signal (decrease in T2 relaxation time) with the increase of Fe concentration. e Amount of Fe ion taken up per cell was measured via ICP-MS of cell pellets fed with difference concentrations of Ferumoxytol and HDL-MNS f cholesterol efflux from J774 macrophage cell lines by HDL-MNS.