Jos Rufino Solera Urea1, Salvador Olmos Gass1, Valerij G. Kiselev2
1Aragon Institute of Engineering Research, Universidad de Zaragoza, Zaragoza, Spain; 2Dept. of Diagnostic Radiology, Medical Physics, University Hospital Freiburg, Freiburg, Germany
The signal attenuation observed in DSCMRI measurements is considered largely to obey to susceptibility-induced magnetic inhomogeneities at the mesoscopic scale. Another mesoscopic process contributing to increased spin dephasing is the diffusion of tissue water carrying a transverse magnetisation M into the blood pool, where it then experiences faster relaxation due to the presence of paramagnetic contrast agent. To quantify this effect, an effective extravascular dephased volume is defined. Analytical expressions are given for various exchange regimes and numerical estimates are compared with the vascular volume. Results indicate that in the brain the exchange of tissue magnetisation across the bloodbrain barrier is permeability limited and does not contribute significantly to the signal dephasing. However, the contribution of magnetisation exchange may be important in organs with increased capillary permeability and/or blood volume. The method is applicable to other problems in quantitative perfusion MRI.