An “in-body GPS” system was developed that can pinpoint the location of ingestible implants inside the body using low-power wireless signals. These implants could be used as tiny tracking devices on shifting tumors to help monitor their slight movements. The system, called ReMix, can track the implants with centimeter-level accuracy.
To test ReMix, a small marker was implanted in animal tissues. To track its movement, a wireless device was used that reflects radio signals off the patient. A special algorithm then uses that signal to pinpoint the exact location of the marker. The marker inside the body does not need to transmit any wireless signal; it simply reflects the signal transmitted by the wireless device outside the body. Therefore, it doesn't need a battery or any other external source of energy.
A key challenge in using wireless signals in this way is the many competing reflections that bounce off a person's body. In fact, the signals that reflect off a person's skin are actually 100 million times more powerful than the signals of the metal marker itself. To overcome this, an approach was designed that essentially separates the interfering skin signals from the ones being measured. This was accomplished using a small semiconductor device (diode) that mixes signals together so the skin-related signals can then be filtered out. If the skin reflects at frequencies of F1 and F2, the diode creates new combinations of those frequencies, such as F1-F2 and F1+F2. When all of the signals reflect back to the system, the system only picks up the combined frequencies, filtering out the original frequencies that came from the patient's skin.
One potential application for ReMix is in proton therapy, a type of cancer treatment that involves bombarding tumors with beams of magnet-controlled protons. The approach allows doctors to prescribe higher doses of radiation, but requires a very high degree of precision, which means that it's usually limited to only certain cancers. Its success hinges on a tumor staying exactly where it is during the radiation process. If a tumor moves, then healthy areas could be exposed to the radiation. But with a small marker like ReMix's, doctors could better determine the location of a tumor in real time and either pause the treatment or steer the beam into the right position.
Researchers next hope to combine the wireless data with medical data, such as that from magnetic resonance imaging (MRI) scans, to further improve the system's accuracy. In addition, the team will continue to reassess the algorithm and the various tradeoffs needed to account for the complexity of different bodies.