Injectable Sensor Shows Oxygen Level Under MRI

Tumors, unlike healthy tissues, generally don’t do well in high-oxygen environment, but thrive when deprived of it. Being able to measure oxygen saturation within blood around a tumor can provide a look at how well therapy is affecting it, whether its growing or dying, and which approach is more effective in killing it.

Researchers at MIT have reported in Proceedings of the National Academy of Sciences on new injectable oxygen bead sensors that can stay embedded within the body for weeks and can be “read” using magnetic resonance imaging. The beads are made of two types of silicone, including one that provides the structural integrity and the other that changes the its proton spins when oxygen is absorbed. This change in spins can be detected under MRI, offering an indicator of the concentration of oxygen within the beads.

The new MRI sensor combines two forms of silicone — a solid called PDMS and a substance known as DDMPS, which has an oily consistency. The oily DDMPS can be dissolved in PDMS, creating what’s called a “swollen polymer.” The researchers shaped this polymer into a 1.5-millimeter sensor that could be implanted in tissue during a biopsy; they also created smaller particles (tens of microns long) that can be injected through a needle. After injection, these particles clump together to form a solid sensor.

DDMPS absorbs molecular oxygen, which alters the proton spins inside the silicone — a phenomenon that can be detected with MRI. Measuring this shift in the MRI signal reveals how much oxygen is present.
To test the sensors, the researchers implanted them in the hind legs of rats and then measured how the signal changed as the rats breathed pure oxygen, regular air, and pure oxygen again. The sensors detected changes in oxygen pressure as small as 15 millimeters of mercury, and it took less than 10 minutes to see the effects of a change in inhaled gas.
When the experiment was repeated four weeks later, the sensors yielded the same results.

Study in Proceedings of the National Academy of Sciences: Solid MRI contrast agents for long-term, quantitative in vivo oxygen sensing…

Press release: Tracking oxygen in the body…