Deformation needed to recover reference CT scans from MRI data.

<p>The maps show the in-plane absolute Euclidean displacement required to recover the true shape of the phantom in axial (<i>x-y</i>) and coronal (<i>x-z</i>) MRI data from the between-scanner variability assessment. Phase encoding is in the horizontal direction (<i>x</i>) in both planes. It is evident that scanning using an identical (standard) sequence ensures better correspondence between images from scanners “B”-“F” in terms of distortion effects. Correspondence declined when stroke sequences “b”-“f” were used, particularly in the coronal plane. Furthermore, overall distortion was characterized by two differing patterns; images from system “A” were uniformly stretched in both directions, whereas minor non-linearities were present in images from systems “B”-“F”. It should be emphasized that while these deformation maps successfully demonstrate the overall distortion of the MRI data compared to reference images, they do not represent the true geometric distortion in the MRI systems; a phantom with a large number of equidistant control points (grid structure) is often required for this purpose. Corresponding MRI images are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0162545#pone.0162545.s003" target="_blank">S2 Fig</a>.