A novel T-1 agent, antiferromagnetic alpha-iron oxide-hydroxide (alpha-FeOOH) nanocolloids with a diameter of 2-3 nm, has been successfully prepared. These nanocolloids, together with a post synthetic strategy performed in mesoporous silica, are a great improvement over the low T,weighted contrast common in traditional magnetic silica nanocomposites. The intrinsic antiferromagnetic goethite (alpha-FeOOH) shows very low magnetization (M-2) of 0.05 emu g(-1) at H = 2 T at 300 K (0.0006 emu g(-1) for FeOOH/WMSN-PEG), which is 2 orders of magnitude smaller than any current ultrasmall iron oxide NPs (>5 emu g-(-1)) reported to date, hence ensuring the low r(2) (proportional to M-2) (7.64 mM(-1) s(-1)) and r(2)/r(1) ratio (2.03) at 4.7 T. These biodegradable alpha-FeOOH nanocolloids also demonstrate excellent in vitro cellular imaging and in vivo MR vascular and urinary trace imaging capability with outstanding biocompatibility, which is exceptionally well secreted by the kidney and not the liver as with most nanoparticles, opening up a new avenue for designing powerful antiferromagnetic iron T-1 contrast agents.