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 of the perfusion signal intensity. In order to determine the expected signal improvement at higher magnetic fields we compared perfusion-weighted images in rat brain at 4.7 T and 7 T. Application of PWI to focal ischemia and functional activation of the brain and the use of two different anesthetics allowed the observation of a wide range of flow values. For all these (patho-)physiological conditions switching from 4.7 T to 7 T resulted in a significant increase of mean perfusion signal intensity by a factor of 2.96. The ratio of signal intensities of homotopic regions in the ipsi- and contralateral hemisphere was field-independent. The relative contribution of a) T(1) relaxation time, b) net magnetization, c) the Q-value of the receiver coils and d) the degree of adiabatic inversion to the signal improvement at higher field strength were discussed. It was shown that the main parameters contributing to the higher signal intensity are the lengthening of T(1) and the higher magnetization at the higher magnetic field.