MR susceptibility distortion quantification and correction for stereotaxy

We demonstrate a new in-vivo correction scheme for the non-linear, shape dependent spatial distortion in MR images due to magnetic susceptibility variations. Geometric distortion at the air/tissue and tissue/bone boundaries before and after the correction is quantified using a phantom. Assuming CT is distortion-free, MR images were compared to CT images of the phantom. Edges of the images were detected. Using fiducials, transformation from CT to MR was determined. CT edges were projected onto MR planes. Corrected and uncorrected MR edges were compared with CT edges. Magnetic susceptibility of cortical bone was measured using a Superconducting Quantum Interference Device (SQUID) magnetometer and found to be -8.86 ppm which is quite similar to that of tissue (-9 ppm). As expected, the distortion at the bone/tissue interface was negligible while that at the air/tissue interface created displacements of about 2.0 mm with a 1.5 T main magnetic field and a 3.13 mT/m gradient field. This is a significant value if MR images are used to localize targets with the degree of accuracy expected for stereotaxic surgery. Our correction scheme diminished the errors to the same level of accuracy as CT.