Macroscopic susceptibility in ultra high field MRI

Purpose: Magnetic susceptibility provides the basis for functional studies and image artifacts in MRI. In this work, magnetic susceptibility and the a ssociated artifacts were analyzed at 8 T in phantoms and in the human head. Method: A mineral oil phantom was constructed in which three cylindrical ai r-filled tubes were inserted. This phantom was analyzed with gradient-recal led echo and SE imaging techniques acquired using varying TEs and receiver bandwidths. To visualize the presence of magnetic susceptibility artifacts in the bead at 8 T, near axial, coronal, and sagittal GE images were also a cquired from human volunteers. Results: The use of gradient-recalled echo imaging resulted in the producti on of significant magnetic susceptibility artifacts. These artifacts could be readily visualized in phantom samples containing air-filled cylindrical tubes. In the human head, susceptibility artifacts produced significant ima ge distortion in the skull base region. Tn this area, susceptibility artifa cts often resulted in the complete loss of MR signal. Magnetic susceptibili ty artifacts were manifested as bands of varying signal intensity in the fr ontal lobe and temporal bone region. In addition, they produced clear disto rtions in the: appearance of brain vasculature and seemed to accentuate the relative size of venous structures within the brain. Conclusion: When using gradient-recalled echo imaging in combination with r elatively long TE values, magnetic susceptibility artifacts can be severe a t 8 T. These artifacts could be reduced by increasing receiver bandwidths a nd by lowering effective TEs. As ultra high field MRI provides a fertile gr ound for the study of susceptibility artifacts in MRI, improvements obtaine d at this Weld strength will have a direct impact on studies performed at l ower field strengths.