Efficiency of MRI as Internal ‘Thermometer' Improved

"We can get five to 10 times better accuracy in temperature maps than is possible with the best possible conventional methods,” said Dr. Warren Warren, a chemistry professor at Duke University (Durham, NC, USA), who is corresponding author of a new report appearing in the October 17, 2008, issue of Science.

The new technique "is suitable for imaging temperature in a wide range of environments,” added the report. MRI is a radiation-free technology for imaging patients' interior anatomies. MRI scans can also be used to estimate interior temperature changes in procedures such as hyperthermia cancer therapy, where focused heat is utilized to kill internal tumors. This is because hydrogen atoms in water shift their MRI broadcasting frequencies in a predictable way as the temperature of water changes. Furthermore, water is a major component of bodily tissues.

Although precise in evaluating water temperature changes in isolation, traditional MRI works as accurately as an internal thermometer within actual patients. This is because the magnetic field's interactions with hydrogen atoms vary widely within patients' bodies, and those interactions also shift from minute to minute, according to Dr. Warren. "Current methods break down in the very systems that are of greatest interest, those that are inhomogeneous and that change with time,” the report stated. "As a result, they only provide relative temperature maps,” Dr. Warren added. "So we're developing methods to do MRI differently.”

The Duke researchers' approach involves selective detection of what are called intermolecular multiple quantum coherences (iMQCs) in hydrogen atoms. Dr. Warren reported that the use of iMQCs is an application of his lab's 1998 correction of an early "subtle mistake” in the way MRI's inventors exploited quantum mechanical hypotheses.

Whereas MRI theory sees nuclei of hydrogen as miniscule bar magnets spinning in characteristic ways within magnetic fields, it originally ignored specific interactions between those spins, according to Dr. Warren. "We had to completely rewrite the theory of magnetic resonance to figure out where the mistake was made,” he added.

By incorporating these missing interactions, the Duke chemists modified both the electronics and interpretation of data from MRI scans to improve heat measurements. The Duke method exploits three sets of facts: First, water and fat never mix. Secondly, hydrogen atoms in water respond to heat changes but those in fat do not. Thirdly, water and fat molecules in the body are likely to be positioned within tens of millionths of a meter (or microns) of each other.

Fat and water molecules occurring so close together are subjected to the same magnetic conditions, the Duke chemists hypothesized. Therefore, the differences between the two types of MRI signals they emit should represent the effect of temperature changes on the hydrogen in water. Calculating the effects of iMQCs--the subtle interactions between atomic spins--additionally improves the accuracy of the comparison.

"So the difference between water and fat is an absolute magnetic resonance thermometer,” Dr. Warren said. The Duke team's report noted that the technique has been demonstrated in live rodents, including obese animals whose cells mimic those in fatty breast tissue. Because of fat cells' effects on magnetic fields, breast tissue cannot be temperature-checked using conventional MRI, the researchers also observed.

The technique could improve clinical applications of hyperthermia against cancer, and be applied in other types of therapy, Dr. Warren suggested. "Temperature regulation is an extremely important part of how biological processes in us work,” he said.

Related Links:
Duke University