uni'wissen 01(3)-2011_ENG

When doctors need pictures of a broken bone or a dislocated cervical vertebra, x-ray ap- paratuses deliver precise images. If they need to examine muscles or other soft internal structures, they turn to a magnetic resonance imaging scan- ner. The radiation exposure from magnetic reso- nance imaging is much less dangerous for the body than that from x-rays. A large coil inside its tube creates a constant magnetic field that acts on hydrogen nuclei and their so-called nuclear spin in soft tissue and moves them in a certain direction. The various gray scales of magnetic resonance images reveal the density of water in the various types of tissue. Abnormalities are vis- ible immediately when the pattern reveals con- spicuous differences. Tumors, for example, have a different water content than healthy tissue. The Magnetic Answer The constant magnetic field in the tube is overlaid with a high-frequency field that makes the nuclear spins resonate, thus producing clear images of the region of the body being scanned. Another coil measures the resonance of the in- teraction between the magnetic fields, the so- called magnetic answer. “This measuring coil, which takes the images, should be as close as possible to the object,” says Prof. Dr. Ulrike Wall- rabe, who conducts research on small-scale magnetic systems in the Laboratory for Microac- tuators at the Department of Microsystems Engi- neering (IMTEK). “Areas like the stomach, for in- stance, can be represented with ease. The head is more difficult, because it’s farther from the coil.” On the other hand, the stomach and the head are not relevant for fundamental research in biology. The objects that biologists want in the tube for this purpose are quite a bit smaller: clus- ters of cells, individual cells, or tiny life forms. However, current MRI scanners are not suited to imaging such objects, explains Wallrabe: “When we place cells in the big tube with the enormous measuring coil, there is too much space between them and the coil. The entire signal is destroyed.” In order to scale down the technology to meet the size of the objects of research, Prof. Dr. Jür- gen Hennig, Head of the Department of X-Ray Diagnostics at the Freiburg University Medical Center, has launched a joint project with micro- systems engineers Ulrike Wallrabe and Prof. Dr. Jan Korvink. “The first step,” says Hennig, “is to reduce the size of the measuring coil, not that of the tube.” The size of the coil is reduced to meet that of the cell clusters, which are about 0.1 milli­ meters in diameter. The microsystems engineers have begun testing the new coils with large al- gae cells. “We want to find out how we can tease out signals that are as useful as possible from the probes,” says Wallrabe. The project on micro-­magnetic resonance (MICRO MR) re- ceives funding from the European Union. The advantage of MR over optical microscopy in bio- logical research lies in the fact that the cells do not have to be marked with fluorescent dyes and placed in a petri dish or on a piece of glass but can be analyzed in a healthy three-dimensional environment while still living. The three-dimensional test arrangement in- cludes approximately 25 tiny golden coils per High-tech sewing machine: Gold wire is coiled around the cylinders with the help of a wire ­bonding technique. 29uni'wissen 03