Arterial spin tagging perfusion imaging of rat brain - Dependency on magnetic field strength

Perfusion-weighted imaging (PWI), using the method of arterial spin tagging , is strongly T-1-dependent. This translates into a high field dependency o f the perfusion signal intensity. In order to determine the expected signal improvement at higher magnetic fields we compared perfusion-weighted image s in rat brain at 4.7 T and 7 T. Application of PWI to focal ischemia and f unctional activation of the brain and the use of two different anesthetics allowed the observation of a wide range of flow values. For all these (path o-)physiological conditions switching from 4.7 T to 7 T resulted in a signi ficant increase of mean perfusion signal intensity by a factor of 2.96. The ratio of signal intensities of homotopic regions in the ipsi- and contrala teral hemisphere was field-independent. The relative contribution of a) T-1 relaxation time, b) net magnetization, c) the Q-value of the receiver coil s and d) the degree of adiabatic inversion to the signal improvement at hig her field strength were discussed. It was shown that the main parameters co ntributing to the higher signal intensity are the lengthening of T-1 and th e higher magnetization at the higher magnetic field.