k-space Models in MRI Using the Concept of Partitions, Applications with special reference to Overhauser-enhanced techniques

Abstract: A new model describing the image formation in magnetic resonance imaging (MRI) has been developed. The k-space description of MRI pulse sequences is expanded by introducing new dimensions to describe the phase dispersion due to flow, acceleration, jerk, diffusion and magnetic field inhomogeneities. When the multi-dimensional k-space is combined with a partition model it becomes possible to model the contrast behavior of pulse sequences used in clinical routine. The magnetization created through the interaction between rf pulses and the spin system is handled as separated units, i.e. partitions, and in the present model, a partition is visualized as a set of parameters rather than a vector sum taken over a collection of spins. A computer application based on the model has been developed, and calculated contrast data show excellent agreement with experimentally measured data generated using a clinical scanner. Furthermore calculated images and 3-D data sets, showing effects due to flow, movement, diffusion and magnetic field inhomogeneities are in close agreement with results presented in the literature. Also signal calculations of the enhancement induced by the Overhauser effect show good agreement with data from in vitro measurements. Virtually any pulse sequence may be used as input to the calculation procedure and thus it may serve as a tool for evaluating new pulse sequences and for educational purposes as well. The possibility of performing Overhauser-enhanced MR imaging (OMRI) has been evaluated by using a new paramagnetic substance based on the trityl class of molecules. Both in vitro and in vivo animal experiments were performed. The in vivo animal images, generated using a prototype scanner system, demonstrate a SNR superior to those generated using contrast media based on nitroxides. Application of the model above indicate that the results may be improved even further by using field-cycling techniques and/or more sophisticated multi-echo pulse sequences.