LAUSR

Purpose MR images inherently suffer from spatial distortions. Spatial distortions play an important role particularly in quantitative MRI, where MRI parameters are employed as quantitative biomarkers, and in MRI-based treatment planning for advanced radiotherapy applications, which require high spatial accuracy in target localization and definition. Methods and results Spatial distortions stem from sources which either relate (sequence dependent distortions) or do not relate (non-sequence dependent distortions) to the measurement conditions (field strength, pulse sequence, scanning parameters, etc). Non-sequence dependent distortions arise from gradient field non-linearity and, thus, are related to the MRI system. Although MRI scanners utilize post-imaging correction algorithms which reduce gradient field non-linearity distortions, residual distortions may still be considerable at areas distant from the isocenter. These can be estimated and corrected with the aid of specially designed phantoms. Sequence dependent distortions arise from static magnetic field inhomogeneity (ΔBo), magnetic susceptibility (ΔBx) and chemical shift (ΔBcs) phenomena, collectively referred to as field inhomogeneities, which may be machine-related (ΔBo), or patient-induced ((ΔBx) and (ΔBcs)). Sequence dependent spatial distortions scale linearly with B0 and can be partly controlled through the optimization of the scanning parameters and conditions that affect them. However, patient-induced relevant phenomena cannot be predicted and may not be constant in time (e.g., susceptibility effects related to the presence of gadolinium-based contrast agents). Therefore, patient-specific distortion characterization and/or correction has drawn considerable attention. Relevant studies rely on either the field mapping technique or the read gradient polarity reversal method. Both methodologies not only account for susceptibility and chemical shift distortions but also for ΔBo related distortion. It is worth noting that the total distortion magnitude measured is mostly affected by the location dependent relative signs of distortion components stemming from different sources, such as B0 inhomogeneity and susceptibility effects. Spatial distortions also result in signal intensity distortions, since the signal intensity of a homogeneous voxel is either compressed or extended in a voxel of different size, shape and position. Conclusions Overall, it is essential to characterize, reduce and correct spatial distortions, so as not to adversely affect MRI quantitative results or introduce inaccuracies in MRI-based treatment planning applications.