Arxiv – Active split-ring metamaterial slabs for magnetic resonance imaging. (6 pages) Researchers have created a switchable metamaterial that becomes invisible when not needed. It has been built and tested and it improves the signal to noise ratio of the resulting images by 15%. The MRI improvement will be improved further by optimising the design.
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. 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. Marcos Lopez at the University of Seville et al have an adaptable metamaterial that adjusts its properties according to the fields around it.
In this work, it is analyzed the ability of split-ring metamaterial slabs with zero/high permeability to reject/confine the radiofrequency magnetic field in magnetic resonance imaging systems. Using an homogenization procedure, split-ring slabs have been designed and fabricated to work in a 1.5 Tesla system. Active elements consisting of pairs of crossed diodes are inserted in the split-rings. With these elements, the permeability of the slabs can be automatically switched between a unity value when interacting with the strong excitation field of the transmitting body coil, and zero or high values when interacting with the weak field produced by protons in tissue. Experiments are shown for different configurations where these slabs can help to locally increase the signal-to-noise-ratio.