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Switchable Metamaterial Makes Itself Invisible

A metamaterial that turns itself off could dramatically improve magnetic resonance imaging.

Magnetic resonance imaging works by blasting biological tissue with a strong, uniform radiofrequency magnetic field and then listening for the weak field generated by hydrogen nuclei in response.

Various physicists have pointed out that one way of improving these devices is with lenses designed to channel and focus these fields. These lenses have become possible thanks to the development of metamaterials, periodic arrays of electronic components that together bend these fields in the desired way.

But there’s a problem. Metamaterials designed to focus the weak fields generated by hydrogen nuclei can distort the much stronger field that stimulates them. Similarly, metamaterials designed for the stronger field can produce unwanted distortions in the weaker fields.

What’s needed is a metamaterial that works well for both fields.

Today, Marcos Lopez at the University of Seville and a few pals reveal the solution: an adaptable metamaterial that adjusts its properties according to the fields around it.

Here’s how it works. The metamaterial in question consists of a periodic array of split ring resonators, c-shaped pieces of copper, the size of pennies. Any field of the right frequency induces currents in these split rings that resonate back and forth. This resonance, in turn, interacts with the local field focusing or bending it.

Lopez and co have modified these c-shaped pieces of copper in a clever way: they’ve added a pair of diodes that close the ‘c’ making o-shaped conductors. In a strong field, the induced currents in the rings pass through the diodes, effectively shorting the split rings. When that happens, there is no resonance and so little or no interaction.

In that case, the properties of the metamaterial are similar to air and the metamaterial is essentially invisible to the field.

But in a weak field, the metamaterial behaves entirely differently. In this case, the field induces much weaker currents in the split rings that are not powerful enough pass through the diodes. So the shorting does not occur. In this case, the metamaterial behaves as usual, bending the field in the required way.

That’s a clever idea: a switchable metamaterial that becomes invisible when not needed.

Lopex and co have built and tested this idea and say it improves the signal to noise ratio of the resulting images. They describe the benefits so far as moderate (a 15% increase in the signal) but say this can be improved on by optimising the design.

That should help to improve MRI machines. Better signals translate into clearer images that can be taken more quickly and peer deeper into tissue. And for relatively little extra cost.

Ref: arxiv.org/abs/1102.2646: Active Split-Ring Metamaterial Slabs For Magnetic Resonance Imaging

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