When we think of Magnetic resonance we think of the massive multimillion dollar magnetic resonance imaging machines into whose gaping mouth we are slowly propelled on a motorized table, ready to have our smallest flaws exposed. But the phenomenon of magnetic resonance has other medical uses. A team of physicians and scientists led by Prof. Ralph Weissleder of Massachusetts General Hospital (MGH) has developed a handheld diagnostic magnetic resonance (DMR) device that can diagnose cancer in an hour with greatly improved accuracy compared to the current gold standard. The DMR technique is sensitive enough that only material from a fine needle aspiration biopsy is needed for the test – a far less painful experience compared to the usual surgical or core needle biopsies.
The principle underlying magnetic resonance can be illustrated by imagining a magnetic compass, which is a small magnet free to pivot around a balance point. The magnetic force between the magnet and the Earth’s magnetic field causes the compass to align along the Earth’s magnetic field – the needle points north. But as the needle settles down, it oscillates clockwise and counterclockwise from the ideal alignment with the Earth’s field. This oscillation has a characteristic rate called the magnetic resonance frequency.
Now comes the tricky part. Place the compass between the poles of an electromagnet so that the field of the electromagnet is perpendicular to that of the Earth, and replace the Earth’s magnetic field by a static magnetic field from a magnet. If you send a DC current through the electromagnet, the compass needle just shifts into a new alignment. However, if you power the electromagnet with AC current, the compass needle will oscillate back and forth.
If the frequency of the AC current is equal to the magnetic resonance frequency, there will be a net transfer of energy to the compass needle – it will vibrate more and more strongly as the needle absorbs energy from the AC magnetic field. When the needle gains energy from the AC field, it is necessary to provide more energy to the AC field to compensate for those losses. DMR detects magnetic spins in a sample by measuring the power lost from the AC magnet.