A Better Way to Burn Tumors

Surgeons could torch tumors faster and more accurately with help from a new thermal imaging system.

by John Tibbetts
August 20, 2018

Surgeons could torch tumors faster and more accurately with help from a new thermal imaging system.

For some minimally invasive procedures, surgeons use a needle-thin probe to target radio frequency waves at tumor tissue. Radiofrequency ablation applies this electrical energy to destroy cancer cells with heat up to 60°C. But surgeons can’t judge the effectiveness of each heat treatment in the operating room. Instead, they track the procedure by taking follow-up images after each burst of heat.

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University of Southern California Viterbi School of Engineering researchers have developed a real-time imaging system to help surgeons provide thermal doses to the correct locations the first time to avoid unnecessary damage to tissues.

“Surgeons often don’t know how much they are heating the tumor and whether they are heating the right location for the treatment,” says Mahta Moghaddam, director of USC’s Microwave Systems, Sensors, and Imaging Lab. “They have to stop therapy, remove the patient, take an MRI or other image, look at it, and bring the patient back and resume treatment.” Moghaddam, along with USC’s John Stang, co-authored the study recently published in IEEE Transactions on Biomedical Engineering.

The USC researchers’ system would provide a 3D thermal monitor image of a tumor region that is co-registered with a prior baseline MRI or other image. A surgeon would view a 3D image of a head or lung or some other feature to be operated on. During surgery, microwave signals would be continually transmitted into the treatment zone and received by the thermal monitor in the operating room. The microwave signals would be computationally analyzed to provide doctors with a 3D temperature map based on the electrical permittivity of tissues. Each volume pixel of the 3D image would have a value of permittivity roughly between 5 and 60. At 55 to 60 °C, tissues are destroyed.

“Permittivity value shows how rapidly electromagnetic waves travel through that object,” Moghaddam says. “Permittivity is a strong function of temperature, determining the degree to which waves are attenuated in an object as they travel through. When you heat up something, the temperature goes up and the permittivity changes. When we see a real-time change in permittivity, we computationally relate that to a change in temperature.” 

The researchers plan to work closely with surgeons to devise a visualization tool for them to act on in real time. “We could give them a 3D picture in terms of a color map, red as hot, blue as cold, and then set a threshold of color transition,” Moghaddam says. When a surgeon see an area with a temperature above 55 or 60 °C, that region could show red, indicating that appropriate heat had been applied.

“The most difficult part of this project was developing interpretation algorithms to translate the microwave measurements into the 3D temperature map,” she says. “Once we measure the reflected or scattered fields in tissue, we have huge amounts of data that we have to combine in complicated ways for the temperature map. These algorithms are computationally intensive. We started with minutes between updates of the image and temperature. Now we can update the frame once every second.”

The system’s hardware is made up of low-cost components that are much simpler than those of MRI or X-ray. The next challenge is developing a system suitable for clinical applications. “We have proved the concept in the lab, but our next goal is to develop a version that is compact and versatile with the right interfaces for surgeons to work with,” she says.  

For the next phase, their system will undergo animal testing later this year in liver cancer studies with support from the USC Alfred E. Mann Institute for Biomedical Engineering and in collaboration with the USC Keck School of Medicine. If animal testing shows effective results, the technique may be three to five years away from clinical trials, Moghaddam says. The system could help surgeons reduce collateral damage to the surrounding healthy tissue, particularly sensitive structures such as blood vessels. It could also reduce the number of heat treatments needed until all of the tumor is destroyed.

John Tibbetts is an independent technology writer.

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