A new thermometer may help cure cancer

A new magnetic resonance technique makes it possible to measure the temperature in internal body tissues in a non-invasive way and could therefore help to target heat therapy for cancer.

When one needs to measure the temperature in certain spots inside the body, one uses probes or needles, in other words, invasive techniques. These techniques are not without their flaws and dangers (infections come to mind, or even the accidental destruction of vital tissues). However, Paula Gould reports in Nature that getting precise temperature readings from inside the body without using needles or probes may soon become reality. Heat therapy for cancer, still largely experimental, looks very promising for the battle against certain cancers. Heating them can destroy them, other cancers become more sensitive to radiation treatment or drugs, thanks to the heat. Non-invasive techniques for measuring the temperature of internal tissues are extremely important for heat therapy. One wants to deliver the exact amount of heat needed. Overcooking the tissues is not an option. Ideally, the techniques used for measuring temperature must be non-invasive and cover a rather large area. They must also, obviously, be very precise. Current MRI techniques can only measure temperature changes in vivo, they do not give an absolute temperature. The measurements are also less accurate when the magnetic field varies in the measuring area. Unfortunately, this is hard to combat since humans just happen to be constructed out of mixed tissues. MRI uses radio-frequency pulses to push the nuclei of hydrogen atoms (that can be found mainly in water and fat) out of alignment with the magnetic field of the scanner. When they jump back to their normal position, they emit a radio signal of their own. This signal is then used to construct an image of the tissue. A team of scientists at Duke University, Durham, North Carolina has modified these radio pulses in order to produce an MRI thermometer that is between five and ten times more accurate than the best alternative solutions. The method is called HOT thermometry. The traditional methods of measuring and monitoring temperature with MRI exploit the property of water molecules to vibrate at different frequencies, depending on the temperature. This affects the radio signal they put out. Unfortunately, this does not give an absolute temperature measurement, only a relative one. This new technique however, uses specific sequences of radio-pulses. It looks at water molecules and fat molecules and compares their resonance frequencies. The frequency differences are directly related to the absolute temperature of the tissues of which they are a part. Warren Warren, a chemist at Duke University explains: "For example, if we find that this difference in a certain scanner magnet is 950 Hz, that would tell us that the temperature in that region is 40.2 °C." This technique of comparing the signals from different molecules also makes it possible to eliminate uncertainties that are caused by fields that are not uniform. The researchers have tested their technique on live, obese mice. They chose these because the levels of water and fat in their tissues are comparable to those in normal human breasts. The mice were heated by contact with a warm water tube. They were repeatedly scanned while their temperature rose from 28.6C to 39C. According to Chrit Moonen, director of the Laboratory for Molecular and Functional Imaging at the University Victor Segalen in Bordeaux, France, this new method could be a solution to one of the main difficulties encountered in temperature mapping. "The main benefit is that it allows measurements of absolute temperature independent of magnetic field inhomogeneity," he says. "But the low spatial and temporal resolution may make it difficult to use this technique to guide therapy." Mr. Moonen also thinks that further analysis is needed to determine how sensitive the technique is to motion and how precise it is when measuring the temperature in tissues that have very low fat concentrations. The people at Duke University are now reprogramming their MRI scanners to use the radio-frequency sequences that they need for HOT thermometry. The technique will then be used in an ongoing clinical trial that is studying targeted hyperthermia as a breast cancer treatment. "My expectation is that we are six months, at most, away from human trials," Warren says.