LIN: Forschungsabteilungen > Akkustik, Lernen, Sprache > Unterpunkt Ebene 3 > Unterpunkt Ebene 4
Titel: LIN Layout
Ute Bommerich, Jörg Stadler, Frank Angenstein
Although MRI is an indispensable and widely applied method in medical diagnosis and research this technique still suffers from a relative low sensitivity. This restriction is caused by the direct correlation of signal intensities to population differences of the involved spin states. This polarization of nuclear spins in a magnetic field at thermal equilibrium is determined by Boltzmann distribution which is affected by the magnetic field strength, temperature, and the gyromagnetic ratio of the observed nucleus. Even in high magnetic fields the polarization of proton spin levels, for example, is only about 10-5. A promising concept for sensitivity enhancement in NMR and MRI is to circumvent the described Boltzmann distribution by using hyperpolarization methods. These methods generate polarizations that overcome the just described population differences by selective population of special spin states.
One of these methods, called PHIP (ParaHydrogen Induced Polarization), has already been successfully applied in NMR spectroscopy since 1987. This technique increases the nuclear polarization by transfer of a special spin isomer of hydrogen on certain substrates by means of a chemical reaction. This spin isomer, namely parahydrogen, in which the magnetic moments of both nuclei in the molecule cancel because of their antiparallel orientation, can be transferred on unsaturated substrates via a homogeneous hydrogenation reaction using an appropriate catalyst.
The population of the resulting spin levels in the product molecule follows strict symmetry criteria. As a result only spin states with appropriate symmetry are occupied leading to an effective overpopulation of those levels as it is illustrated in the scheme below.
Because only the ab and ba states of the product system correlate with the nuclear spin function of parahydrogen, these states are overpopulated and give rise to strong characteristic antiphase signals.
The above shown pictures illustrate the hyperpolarization effect of the just labelled substance (left picture) in comparison to the non-polarized form of the same substrate (right picture). The MR images of phantoms containing hyperpolarized diethyl ether were measured on a Bruker 4.7T BioSpec (200 MHz) small animal MR scanner using an optimized 2D gradient echo (FLASH) sequence.