Microstructured magnetic matieral enhances images

Using ideas directly derived from photonic bandgap materials, researchers at Marconi Caswell (Caswell, Northants, England) have developed a novel microstructured electromagnetic material that has significantly improved the performance of magnetic-resonance-imaging (MRI) machines. According to Mike Wiltshire, a chief engineer at the New Technology Centre of Marconi Caswell, these materials have strong magnetic properties in the radio-frequency (RF) band, but exhibit no dc magnetism. Because of this, he says, they can be used to guide RF flux from a remote object to the receiver coil in an MRI machine, thus permitting a clear image to be obtained where none is normally detectable.

A microstructured material consists of a fine structure that is small or comparable to the wavelength of operation. At Caswell, a "swiss roll" structure has been developed that consists of a central cylindrical spindle upon which is wound a spiral of conductor with an insulator backing. Consequently, there is no electrical contact between the layers. The rolls are then stacked into an array.

When an alternating magnetic field is applied along the axis of the cylinders, it induces a current in the conducting sheet. But this current cannot flow freely, because the sheet is not continuous. It can only flow because of the self-capacitance of the structure, which completes an ac circuit. According to Wiltshire, the swiss-roll material suits operation in the RF range, because the self-capacitance of the structure is large, and hence the resonant frequency of interaction is within the RF range.

Wiltshire choose a microstructured material for use at the 21.3-MHz operating frequency of a Marconi Medical Systems (Cleveland, OH) Apollo 0.5T MRI machine. An 8-mm-diameter glass-reinforced plastic spindle was wound with 35 turns to give a resonant frequency at 22.1 MHz. A set of 19 swiss rolls, 200 mm long, were made to provide a hexagonal block of material. These rolls were characterized as individual rods and as bundles of first seven and then 19 rods. To demonstrate the characteristics of the material, the rolls of material were used as flux guides in the bore of an MRI system. Placed below a 1-cm-thick water phantom that provided a reference plane, the object under test was held 200 mm above the water phantom, supported initially on an inert plastic block. According to Wiltshire, the material changes the conventional approach to optimizing the coil filling factor in MRI systems. "Exploiting this class of materials could fundamentally change existing approaches to MRI," he says.