Magnetic Resonance Imaging (MRI) biosensors with enough sensitivity to analyze specific biomolecules in living tissue and in vitro have been developed by David E. Wemmer, Alexander Pines, and coworkers at the University of California, Berkeley, and Lawrence Berkeley National Laboratory (Science 2006, 314, 446). They use hyperpolarized (nuclear spin-enriched) 129Xe to improve target molecule detection limits by several orders of magnitude over conventional MRI. The technique uses a cryptophane molecular cage that encapsulates xenon atoms and binds via a linker group to specific biological targets. The cages would localize at target sites when administered to patients. They would then inhale hyperpolarized xenon, which would enter the bloodstream and be distributed. Entry of xenon into molecular cages at sites of interest changes the element's resonance signal and would subtract intensity from the background level of hyperpolarized xenon in the body. This quenching could be sensed and used to map and quantify the molecular target. A radiologist commenting in Science notes that the technique is far from human use but is "promising."