The accuracy of image registration for the brain and the nasopharynx usingexternal anatomical landmarks

We investigated the accuracy of 3D image registration using markers that ar e repeatedly applied to external anatomical landmarks on the head. The purp ose of this study is to establish a lower limit of the errors that would oc cur in, for instance, MRI-SPECT matching, which in some situations can only be achieved using external landmarks. Marker matching was compared with (s ingle-modality) volume matching for 20 MRI scans. The results were compared with a published expression for the target registration error (TRE) which gives the 3D distribution of the mismatch between both scans. It was found that the main error source is reapplying the external markers on the anatom ical landmarks. The published expression describes the relative distributio n of the TRE in space well, hut tends to underestimate the actual registrat ion error. This deviation is due to anisotropy in the error distribution of the marker position (errors in the direction perpendicular to the skin sur face are in general much smaller than errors in other directions). A simula tion of marker matching with anisotropy in the errors confirmed this findin g. With four reapplied markers, the TRE is 6 mm or smaller in most regions of the head.