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19.09.2011 Category: Issue 44, Fraunhofer IAF, From the institutes

New key components for magnetic resonance imaging

Often, doctors have to “look inside” the human body in order to be able to make a comprehensive diagnosis. But examinations involving endoscopes or catheters can be quite traumatic for patients. Magnetic resonance imaging (MRI) is gentler in many cases. As part of a research initiative, Fraunhofer IAF is now developing devices with higher spatial resolution – and shorter measuring times.

 

Picture of Fraunhofer IAF’s GaAs amplifier chip, as used in system tests. The dimensions are a mere 1.25 mm x 1.25 mm.
Photo: Fraunhofer IAF
Picture of Fraunhofer IAF’s GaAs amplifier chip, as used in system tests. The dimensions are a mere 1.25 mm x 1.25 mm.
Photo: Fraunhofer IAF
To test the new combination of receiving coil and pre-amplifier with a GaAs amplifier chip, a cherry tomato with a diameter of approximately 3 cm was examined with an MRI. The strength of the magnetic field is high, at 9.4 T. By positioning the IAF pre-amplifier near the receiving coil, particularly good picture quality with good local resolution is possible.
Photo: Fraunhofer IAF / fotolia
To test the new combination of receiving coil and pre-amplifier with a GaAs amplifier chip, a cherry tomato with a diameter of approximately 3 cm was examined with an MRI. The strength of the magnetic field is high, at 9.4 T. By positioning the IAF pre-amplifier near the receiving coil, particularly good picture quality with good local resolution is possible.
Photo: Fraunhofer IAF / fotolia

In magnetic resonance imaging, the patient is moved into a narrow tube where a magnetic field and radio waves generate an image of the inside of the body. Devices common today usually operate with between 8 and a maximum of 32 receiving coils. To achieve good image quality, these are positioned as close to the patient as possible. The amplifiers that are also needed must be placed further away due to their size and the heat they generate. The problem, however, is that the transmission distance from the coil to the amplifier impairs signal quality.

Compact and closer to the patient

Scientists at the University Clinic of Freiburg, the Fraunhofer Institute for Applied Solid State Physics IAF, and the Department of Microsystems Engineering IMTEK at the University of Freiburg are now working jointly on an improved generation of devices. The main aim is to be able to place a larger number of receiving coils, including pre-amplifiers, nearer to the patient. This would not only improve image quality, but also reduce measuring times. The Freiburg-based researchers have developed flexible receiving coils that can be adapted optimally to the surface of the region to be examined. To allow the pre-amplifiers to be mounted directly beside the receiving coil, amplifiers had to be designed that develop as little heat as possible and provide good amplification without additional interference (noise).

Gallium arsenide for VHF frequencies?

To achieve this goal, the project partners took a new approach to the semiconductor material: Previously, the low VHF frequencies at which MRIs usually operate were the domain of inexpensive silicon. Thanks to the metamorphous high-electron mobility transistor technology from IAF, the scientists have now succeeded in producing low-noise amplifier chips made of gallium arsenide (GaAs) with extremely low energy consumption. While conventional amplifiers have power consumption of about 2 W, the GaAs chips manage with only 7.5 mW. This means that a larger number of coils with integrated pre-amplifier can be positioned right on top of patients without causing cooling problems. Data transmission to the tomograph’s computer is also simplified, as thick coaxial cables are no longer needed to send the signal without interference to the remote MRI spectrometer. Examinations with a helmet on

The new, much more compact design offers other options: for example, a special “MRI helmet” with many individual receiving coils that could be adapted to the patient’s anatomy. It would further reduce the measurement time, and the doctor would also receive images with better information content. Moreover, the examination would be less traumatic for patients with claustrophobia.

 

 

INUMAC research project

The research work described in the article is part of a very comprehensive joint project funded by the German Ministry of Education and Research BMBF: “Imaging neurodisease using high field MR and contrastophores.” The plan arose from a Franco- German initiative for implementing new scientific discoveries in industrial innovations.

 

Contact:

 

Dr. Harald D. Müller
Phone +49 761 5159-458
harald.mueller(at)iaf.fraunhofer.de
Fraunhofer Institute for Applied Solid State Physics IAF
Tullastrasse 72
79108 Freiburg
Germany
www.iaf.fraunhofer.de