The human body pulses with electric fields that are caused by the movement of charge through nerves and across muscle tissue. Physicists have long measured these currents directly with techniques such as electrocardiographs, which reveal heart function, and electroencephalographs, which reveal brain function.
But the same processes also produce magnetic fields, and these have the potential to be just as useful in diagnosing disease, perhaps even more so. In particular, magnetic sensors do not need to touch the skin to do their work. That’s useful for measuring signals from fetal hearts or from burn victims, for example.
But there is a problem. The body’s magnetic field is tiny, and detecting it requires hugely sensitive sensors. The only commercially available gadgets that can do this job are superconducting quantum interference devices, or SQUIDs, which can measure changes in magnetic fields measured in femtoTesla (10-15). These need to be cooled to the temperature of liquid helium, and the measurements made in screened rooms that are shielded from external magnetic fields.
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| How nitrogen atoms (blue) become embedded in a diamond lattice. |
And that makes them expensive. A typical magnetocardiograph system costs in the region of $1 million, compared to a few thousand dollars for an electrocardiograph.
So a way to make magnetocardiographs or (magnetoencephalographs) cheaper would be hugely useful.
As it happens, there is a technology waiting in the wings that promises to do just that: diamond sensors capable of measuring tiny magnetic fields at room temperature. The hope is that these sensors could make magnetocardiographs significantly cheaper while removing the need for hospitals to build expensive shielded rooms to operate them in.
Today, Matthew Dale and Gavin Morley at the University of Warwick in the U.K. say that diamond sensors are poised to revolutionize the way physicians use magnetic field measurements in diagnostic medicine. They map out the state of the art in this area and say that the business opportunity is significant.
First some background. At the heart of these diamond sensors is an atomic-scale device called a nitrogen-vacancy, or NV center. This is a type of defect in a diamond lattice consisting of a nitrogen atom sitting next to a vacancy.
NV centers have interesting properties when they accept an electron and become negatively charged. The electron can be made to emit red light, which is easily detected. The amount of light it emits depends on the electron’s spin polarization, and this is highly sensitive to any external magnetic field.
So any change in an external magnetic field can be measured by watching the amount of light emitted by an NV center. This process works at room temperature, and physicists have used it to measure field changes measured in picoTesla (10-12). They expect to be able to make the technique significantly more sensitive in the future, perhaps even able to match the sensitivity of SQUIDs.
Although currently less sensitive, NV centers have other advantages over SQUIDs. For a start, sensors using NV centers can always get closer to the signal than SQUIDs, which have to be insulated because of their freezing temperature.
And diamond-based sensors can operate without being carefully shielded from external magnetic fields. That’s because they can be used in groups, with the signals from more distant sensors used to cancel out the effect of any unwanted external field.
Of course, there are some challenges ahead to make diamond sensors useful. One is that the NV center emits light in all directions, and this makes it difficult to collect. However, carefully shaped mirrors should be able to capture most of this light.
Another is a competing technology called alkali metal magnetometers. These depend on the ability of certain kinds of spin-polarized atoms to absorb light depending on the local magnetic field. So these devices work by measuring the amount of light that passes through a small container of heated atomic gas.
However, diamond sensors are solid-state devices that are likely to be more robust than any of the competitors. “Even if NV magnetometers do not ultimately exceed others in sensitivity they might offer significant advantages in ruggedness, cost, and proximity to the subject,” say Dale and Morley.
The market for diamond-based magnetocardiographs is likely to be significant. Dale and Morley estimate that there are about 100 SQUID-based magnetocardiograph systems around the world. But there are some 100,000 hospitals around the world that could benefit from cheaper devices. “We estimate that 100,000 magnetocardiograph systems could be sold if the functionality were the same as existing SQUID systems and the price was below $150k,” they conclude.
That’s an interesting study. The body’s magnetic field is largely unused as far as medical diagnostics are concerned. If Dale and Morely are correct, that’s likely to change in the next few years.