Weissleder is currently testing the nanoparticle sensors using a research tool called a micro-NMR. Ultimately, he and the other T2 Biosystems researchers hope to develop a portable, commercial, handheld system that could be used at a patient's bedside or in an ambulance. The system works the same way as MRI but with much smaller components, says David Lee, another of the company's cofounders and the director of engineering at Analog Devices. "Instead of having coils for [radio-frequency] detection and magnets on the centimeter scale, the idea is to build them on the micrometer scale," he says. "The electronics in a cell phone are not too different from what you'd want in a detector." Lee says that making such a small detector is a matter of design, not basic research.

Developing a smaller detecting system is key to the company's long-term goal of enlisting the nanoparticles to monitor cancer and chronic diseases like diabetes using implantable devices, says Jacks. Michael Cima, also a T2 Biosystems cofounder, is developing small silicone implants that contain the magnetic nanoparticles and are permeable to biomarkers circulating in the body. (See "Real-Time Tumor-Monitoring Rod.") Such an implant could be inserted into a tumor during a needle biopsy.

A cancer patient's progress is currently monitored with infrequent MRI scans to determine if tumors are shrinking. With an implant, clinicians could take readings frequently and easily using a handheld device that could provide information including how metabolically active a tumor is and whether the drugs are reaching it. After a tumor seems to have gone away, the implant could be monitored for molecular signs that it might be recurring. Or a diabetes patient with an implant could use a handheld device to monitor her glucose levels without having to prick a finger.