Conventional MRI works by polarizing the protons in the water of a patient's body by placing the patient in a strong magnetic field. However, this technique yields very poor images of the lungs because they are filled with air and contain little water. Instead of polarizing the protons in the patient's body, the new technique polarizes noble or inert gases such as xenon or helium. The patient inhales the polarized gas, which can then be detected in the airways of the lungs and dissolved in blood flowing to the nearby heart and brain.
The two keys to producing hyperpolarized gases are optical pumping and spin exchange. Happer and his colleagues use lasers to produce very high polarizations - more than 100,000 times more than the polarizations of the body's hydrogen protons in an MRI machine - in the nuclei of alkali-metal atoms such as cesium, potassium, and sodium, which are then transferred through spin exchange collisions to noble gases. Alkali-metal noble gas spin exchange, first accomplished at Princeton in the 1960s, allows one to take the polarization from the alkali-metal atoms and transfer it to noble gas atoms.
"Slowly but surely, through these spin exchange collisions, the angular momentum of the alkali-metal atoms is transferred to the nuclei of the noble gas atoms," said Gordon Cates, who developed the technique with Happer and Arnold Wishnia, a chemist at SUNY-Stony Brook. "That may sound simple, but you have to understand many details of the physics of what is going on if you expect this to work."
According to Happer, many exciting possibilities exist for the use of this new technique in medicine. For instance, it should enable physicians to better detect pulmonary embolisms - a blood clot in the lungs that must be treated before it dislodges and kills the patient. An additional benefit is that the technique does not require costly new MRI equipment, since existing machines - of which there are some 3,000 in the U.S. and 6,000 worldwide - can be retrofitted to incorporate the capability of imaging hyperpolarized gas.
Other promising applications include imaging brain function by tracing how various stimuli affect the flow of blood to the brain, and imaging the blood vessels in the heart as an alternative to the current method of angiography, which uses x rays to image iodine injected into a patient's blood to diagnose various heart conditions. The Princeton physicists are working with researchers at Duke University to extend the new technology to the use of laser-polarized helium gas
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