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MRI
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MRI

Magnetic Resonance Imagery

Principle

An MRI experiment consists in detecting the precession of the magnetization of the atom nuclei that constitute matter. First, this magnetization has to be lined up in a particular direction thanks to a magnetic field with a strong intensity, B0, called “main field”. Then the magnetization must be rotated thanks to a second magnetic field (B1) that is alternating and perpendicular to the main field, and that is applied for a short amount of time thanks to an emission antenna. Finally, we detect the signal made by magnetization when it returns to its original position in a precession movement, thanks to a reception antenna.

Depending on the nature of the environment of the nucleus, magnetization does not goes back to equilibrium at the same speed. This is how we make an MRI map of the body.

Superconductors in MRIs

Inside a MRI coil; SiemensThe stronger the main field, the stronger the signal when magnetization reaches its balance again and the better the image quality. This is why the progress in MRIs is strongly linked to the creation of new devices with always-stronger fields. The main magnetic field is generated by a large superconducting electromagnet in which an electric current flows. The weak resistance of superconductors allows very strong currents to flow with no heating in the material, and hence enables to get very high field values of several teslas. Such field intensities could not be obtained with a copper electromagnet because the high resistance of the conductor would cause, when strong currents flow, Joule effect losses (thermal dissipation) so strong the coil would melt.

Outside a MRI coil; CNRS Photothèque / Benoît RAJAUOnce the current flows in the coil and the magnetic field is established, the only thing to do is close the coil and hence “trap” the current, as in the permanent currents experiment. The current can flow and will never dissipate since there is no resistance. The electric supply that initiated the current can then be unplugged: the current and the inducted magnetic field will stay the same as long as the temperature is cold enough. The helium tanks in MRIs must hence be filled regularly in order to compensate the evaporation of liquid helium.

Superconductors have hence been at the origin of the emergence of new generation of MRI coils for the last ten years and have helped enhancing the quality of the images and the comfort of the patients.

Prospects

CNRS Photothèque / Christophe LEBEDINSKYIn some configurations, the image quality can be limited by the reception of the copper reception antenna that detects the signal emitted by magnetization when it reaches its balance back. The use of a superconductor to build the reception antenna is hence a very efficient means to strongly decrease the resistance and hence significantly increase the quality of the images. This solution is still being studied and has to go along with the specific development of cryogenic tools in order to expend on a larger scale.

The increase of the value of magnetic fields is also being actively studied, especially in France by the Neurospin laboratory.

NMR

An NMR experiment in physics; CNRS Photothèque / Benoît RAJAU , LPSPhysicists and chemists use a technique similar to MRI called NMR (Nuclear Magnetic Resonance). NMR experiments in chemistry; CNRS Photothèque / Christophe LEBEDINSKYIn this case, the idea does not consist in getting an image of the material thanks to the magnets of the nuclei but rather to understand what characterizes the electrons surrounding these nuclei. In chemistry, this enables to determine the structure of complicated molecules. In physics, NMR enables to measure the static and dynamic magnetic properties of the electrons in matter. It is used to study superconductors, for instance.

 

 

CNRSSociété Française de PhysiqueTriangle de la physique
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CNRSSociété Française de PhysiqueTriangle de la physique