Strong BOLD-effect with TurboCRAZED MRI following hyperoxia in the rat brain at 16.4 T

Introduction: MR imaging techniques based on the indirect detection of intermolecular multiple quantum (iMQC) coherences provide pronounced BOLD signal. The aim of this study was to asses the percent signal change observable via the iMQC BOLD-effect at 16.4T in the rat brain using the accelerated method TurboCRAZED (1). Subjects and Methods: Experiments were performed on a 16.4T/26cm horizontal bore Magnex magnet interfaced to a Bruker spectrometer. The imaging gradient system had an inner diameter of 12cm. MR images were acquired with homebuilt 14mm and 22mm loop coils. Healthy adult Wistar rats (255g and 603g) were anesthetized by inhalation of Isoflurane. Breathing was monitored and body temperature was kept constant using electric heating pads. Results: High resolution TurboCRAZED images were acquired in the rat brain with a nominal inplane resolution of (156μm)2 and a slice thickness of 500μm within one hour measuring time (Fig.1). Experiments with different iMQC correlation length (dc) provided different contrast, because the spatial origin of the local CRAZED signal is in a fair approximation a spherical shell, which can be shifted along the sphere radius by simply changing the area under the correlation gradient in the pulse sequence. TurboCRAZED images were also sensitive to the BOLD-effect (Fig.2). A 30 minutes time series of four images was acquired, where the animal was inhaling air in the initial and pure oxygen in the final 15 minutes. The maximum of the difference image was 8.8 from the maximum value in the same brain region of the summed air-images. Conclusion: High resolution TurboCRAZED images of the rat brain can be obtained in experimental times that allow for fMRI experiments. In combination with the pronounced sensitivity for the BOLD-effect, the tunable iMQC contrast (2) may provide additional information in small animal fMRI, which is not available with conventional methods