Relaxation Rate Dispersion of Blood due to Paramagnetic Ions: An Implications for Magnetic Resonance Relaxation Dispersion Imaging

The proton relaxation rate dispersion of paramagnetic ions in blood samples was studied in the frequency interval from 10 kHz to 10 MHz. with the Field cycling method. We applied, the samples were first magnetized by a relatively high magnetic field (0.5 T). By electronic means this field was rapidly (1 ms) reduced to a lower value in the interval 0.1 mT - 0.5 T, where the excited proton spin may relax during a time interval of about 3×T1max. Then the magnetic field is again quickly raised up to the higher level for the detection of the NMR-signal. The relaxation characteristics were analyzed by applying a model with three compartments of water proton-spin exchange. For each compartment we estimated a correlation frequency by fitting the dispersion curves to a sum of Lorentz distributions. We found that low concentrations of paramagnetic ions have a large influence on the relaxation rate dispersion in the low frequency region <10 MHz. This effect is suggested to be used for mapping of inorganic paramagnetic or organic free-radical compounds in medical applications as contrast agents and for tracing cellular activity by subtracting MR images recorded at high (>100 mT) and low (<10 mT) relaxation fields. Such sequences of images could be used to study the oxygen status and metabolism of the brain as well as the generation and distribution of reactive oxygen species (ROS). This technique called “Magnetic resonance Relaxation Dispersion Imaging” (MARDI) would be particularly suitable to analyze various neurological diseases such as stroke, MS, Alzheimer´s disease as well as brain tumor progression. It might also be suitable for imaging of tumour oxygenation and in vivo ROS distributions in radiation therapy, which could be used in dose planning and for analyzing and optimizing the effect of various radiation therapy regimes. An Overhauser-enhanced pre-polarized MRI system has been demonstrated for imaging studies of tumor hypoxia and red-ox status to be used as radiotherapy prognostic factors.