MR SUSCEPTIBILITY MISREGISTRATION CORRECTION

We present a new in vivo method to correct the nonlinear, object-shape -dependent and material-dependent spatial distortion in MR images caus ed by magnetic susceptibility variations. This distortion across the a ir/tissue interface before and after the correction is quantified usin g a phantom. The results are compared to the ''distortion-free'' CT im ages of the same phantom by fusing CT and MR images using fiducials, w ith a registration accuracy of better than a millimeter. The distortio n at the bone/tissue boundary is negligible compared to the typical MR I resolution of 1 mm, while that at the air/tissue boundary creates di splacements of about 2 mm (for B(o) = 1.5 T and G(x) = 3.13 mT/m). Thi s is a significant value if MRI is to provide highly accurate geometri c measurements, as in the case of target localization for stereotaxic surgery. Our correction scheme provides MR images with accuracy simila r to that of CT: 1 mm. We also present a new method to estimate the ma gnetic susceptibility of materials from MR images. The magnetic suscep tibility of cortical bone is measured using a SQUED magnetometer, and is found to be -8.86 ppm (with respect to air), which is quite similar to that of tissue (-9 ppm).