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Ceramic dielectric components enable next generation medical MRI systems

02 July 2012

Morgan Technical Ceramics is working with the University Medical Centre (UMC) Utrect in The Netherlands to design an antenna for the next generation of ultra high field medical magnetic resonance imaging (MRI) systems. Scientists have designed a radio frequency (RF) coil array element for use in MRI systems with increased magnetic strength, which will enhance the quality of images from MRI body scans.

MRI at a magnetic field strength of seven Tesla has already demonstrated an impressive improvement in imaging quality for brain imaging, in comparison to the conventional 1.5 and three Tesla scanners.  However, achieving these same improvements for body imaging is more difficult because of the increased frequency of the RF signals.

RF signals are used in MRI to obtain resonance conditions for the hydrogen nuclei in the human body.  The resonance frequency increases proportionally with the increased magnetic field strength. This poses problems for imaging deeply located structures, since the wavelength of the signals decreases with increasing frequency. A possible solution may come from an alternative design of the RF coils that transmit and receive the signals.

Most RF coils in MRI equipment are designed as near field antennas, i.e. the coils are tuned to resonance to generate a large magnetic field in the near field region.  However, for high field MR imaging in the body, they are not ideal, as many organs are located at a depth of more than one wavelength.  To penetrate beyond the near field region, far-field antennas are needed, which are designed to emit an electromagnetic wave into the nearby medium.

A study by UMC, Utrect to investigate the design and effectiveness of RF coils for ultra high field MR body imaging has proven that radiative antennas are now a promising alternative to use as a transmit and/or receive coil array element.

“The study compared the radiative antenna against conventional elements of a loop coil and a microstrip,” says Alexander Raaijmakers, researcher at UMC Utrecht. “The radiative antenna consists of a block of dielectric substrate with a dipole antenna mounted on one side. This structure efficiently emits a propagating wave of RF signals towards the imaging target.”

Morgan Technical Ceramics recommended its D36 dielectric material for its exceptional electrical and mechanical properties. The material is non magnetic and has a dielectric permittivity of 37 and a low conductivity (3.12 x 10-5 S/m).  It has as a low electrical loss QO (21000), almost linear thermal response, and high RF power handling capacity.

“The permittivity of the substrate has to be close to the effective permittivity of the body to minimise reflections at the antenna-body interface,” says Paul Turnbull, Business Manager – HV and RF components at Morgan Technical Ceramics. “The conductivity of the substrate also has to be as low as possible to minimise antenna losses. Thanks to the high permittivity of our ceramic, UMC can easily increase the directivity of the antennas to obtain the best results.”

The results from the study show that the high excitation field level frequencies at the deeper regions (more than four centimetres depth) per unit of delivered power is highest for the radiative element compared with loop coil and microstrip elements.  

The high excitation field distribution of the radiative antenna is symmetrical and more uniform than other investigated elements, promising better image homogeneity. The radiative antenna efficiently emits a propagating wave into the tissue and prostate and imaging with an array of radiative antennas is now feasible.  

“Eight radiative antennas have been combined into a belt-like surface array for prostate imaging,” continues Alexander. “This opens up many new opportunities and we are currently investigating extending the application range to include heart imaging and working on understanding how the antenna efficiency depends on the design parameters.”

“We are delighted to be working with the UMC Utrecht, enabling them to push boundaries in the medical sector and develop the next generation of medical MRI equipment,” says Paul. “At Morgan Technical Ceramics we have over 50 years experience manufacturing high quality electro ceramic components and have built up extensive expertise in materials control, component manufacture and applications data in this field. Our extensive material and engineering expertise allows us to produce a wide range of experimental and prototype components in addition to the standard range of products.”

The images show (top) the results of radiative antennas on a volunteer (pelvis and prostate), and (bottom) a radiative antenna (courtesy Alexander Raaijmakers, UMC Utrecht).

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