MIT’s CSAIL Creates “In-Body GPS" System to Track Tumors

To investigate inside the human body, long tubes with integrated cameras had to be swallowed or surgeries had to be performed. Wouldn’t it be better if physicians could get a preview in a less invasive, expensive, and time-consuming manner?

A team from MIT’s Computer Science and Artificial Intelligence Laboratory (CSAIL) led by Professor Dina Katabi is working on achieving precisely that with an “in-body GPS" system christened ReMix. The new technique can identify the location of ingestible implants within the body using low-power wireless signals. These implants could be used as miniature tracking devices on shifting tumors to help track their smallest movements.

In animal tests, the team showed that they can monitor the implants with centimeter-level accuracy. The researchers say that, in the future, similar implants could be used to transport drugs to particular areas in the body.

ReMix was created in partnership with scientists from Massachusetts General Hospital (MGH). The team defines the system in a paper that will be presented at this week's Association for Computing Machinery's Special Interest Group on Data Communications (SIGCOMM) conference in Budapest, Turkey.

Tracking inside the body

To put the ReMix to test, Katabi’s team first implanted a small marker in animal tissues. To monitor its movement, the scientists employed a wireless device that reflects radio signals off the patient. This was based on a wireless technology that the scientists earlier showed to detect breathing, heart rate, and movement. A special algorithm then uses that signal to identify the precise location of the marker.

Fascinatingly, the marker within the body does not need to convey any wireless signal. It just reflects the signal communicated by the wireless device outside the body. Thus, it does not necessitate a battery or any other external source of energy.

A significant challenge in using wireless signals in this manner is the several competing reflections that reflect off a person's body. In fact, the signals that bounce off a person’s skin are really 100 million times stronger than the signals of the metal marker itself.

To handle this, the team put forth an approach that basically separates the interfering skin signals from the ones they are attempting to measure. They performed this using a small semiconductor device, known as a “diode,” that combines signals together so the team can then strain out the skin-related signals. For instance, if the skin reflects at frequencies of F1 and F2, the diode develops new combinations of those frequencies, such as F1-F2 and F1+F2. When all of the signals bounce back to the system, the system only chooses the combined frequencies, straining out the original frequencies that radiate from the patient’s skin.

One possible application for ReMix is in proton therapy, a type of cancer treatment that involves blitzing tumors with beams of magnet-controlled protons. The method allows clinicians to prescribe higher doses of radiation, but requires a very high degree of accuracy, which means that it’s normally limited to only specific cancers.

Its success centers on something that is really quite undependable: a tumor staying precisely where it is during the radiation process. If a tumor moves, then healthy areas could be open to the radiation. But with a small marker like ReMixs, clinicians could better establish the location of a tumor in real-time and either pause the treatment or direct the beam into the correct position. (To be clear, ReMix is not yet sufficiently accurate to be used in clinical settings. Katabi says a margin of error closer to a few millimeters would be essential for actual implementation.)

"The ability to continuously sense inside the human body has largely been a distant dream," says Romit Roy Choudhury, a professor of electrical engineering and computer science at the University of Illinois, who was not involved in the study. "One of the roadblocks has been wireless communication to a device and its continuous localization. ReMix makes a leap in this direction by showing that the wireless component of implantable devices may no longer be the bottleneck."

Looking ahead

There are still many continuing challenges for enhancing ReMix. The team subsequently hopes to integrate the wireless data with medical data, such as that from magnetic resonance imaging (MRI) scans, to additionally enhance the system’s accuracy. Furthermore, the team will continue to reevaluate the algorithm and the several tradeoffs required to account for the complexity of various bodies.

"We want a model that's technically feasible, while still complex enough to accurately represent the human body," says MIT PhD student Deepak Vasisht, lead author on the new paper. "If we want to use this technology on actual cancer patients one day, it will have to come from better modeling a person's physical structure."

The scientists say that such systems could help enable more extensive acceptance of proton therapy centers. At present, there are just about 100 centers worldwide.

"One reason that [proton therapy] is so expensive is because of the cost of installing the hardware," Vasisht says. "If these systems can encourage more applications of the technology, there will be more demand, which will mean more therapy centers, and lower prices for patients."

Katabi and Vasisht co-wrote the paper with MIT PhD student Guo Zhang, University of Waterloo professor Omid Abari, MGH physicist Hsaio-Ming Lu, and MGH technical director Jacob Flanz.