Modulation of Susceptibility-Weighted Contrast in Volunteers by Using Caffeine

Purpose: Changes in cerebral blood flow can be induced by exogenous agents, such as carbogen or caffeine. One consequence of these physiological challenges is a change of the oxygenation level of the venous blood, which influences the BOLD signal or the SWI contrast. The aim of this work was to investigate the influence of caffeine on the contrast observed with high-resolution susceptibility-weighted imaging (SWI) in volunteers. Materials and Methods: High resolution T₂*-weighted images were acquired in four volunteers on a 1.5 T system using a velocity compensated 3D gradient echo imaging sequence. Sequence parameters were: TR/TE/FA=67 ms/50 ms/25°, in-plane resolution=0.5×0.75mm², slice thickness=1.8mm, scan time 13min. A native scan was performed first, then the subjects drank two cups of coffee (equivalent to 300mg caffeine) through a plastic tube directly in the scanner without changing position. The 3D scan was then repeated 4 times to cover a period of nearly one hour after drinking coffee. Phase and magnitude images were reconstructed from the raw data. The phase images were unwrapped using homodyne filtering. The phase and magnitude images were combined to produce susceptibility-weighted images (SWI). The SWI and magnitude images were spatially realigned and maps of relative signal changes between the different scans were computed. Results and Discussion: Maps of relative signal change clearly visualized the caffeine-induced response of venous vessels. Larger veins, such as the thalamostriate vein or the internal cerebral veins, showed a distinct signal decrease of up to 20%. Signal intensities in gray and white matter, however, changed only marginally during the post-caffeine scans. These signal changes have to be compared with the results obtained during breathing of carbogen where signal increases and decreases of 8% and –5% in gray and white matter, respectively, were observed. The signal in larger veins increased by even more than 300%. Conclusion: BOLD imaging is able to visualize the cerebral vascular response to caffeine. Signal changes induced by caffeine were distinctly weaker compared to changes observed with carbogen. However, further studies are required to investigate the signal response and to optimize the applied dose. One advantage of caffeine is that it is better tolerated compared to the inhalation of carbogen.