External reference markers for the correction of head rotation in brain single-photon emission tomography

Accurate reorientation of brain single-photon emission tomography (SPET) is required for quantitative procedures and for correlation with other imaging modalities. Traditionally, brain SPET has utilized reoriented slices parallel to the orbitomeatal line (OML). Reorientation using internal landmarks would be more convenient but has not been systematically compared with the use of external landmarks. We compared the interobserver reproducibility for defining the sagittal and coronal angular deviations using internal landmarks, a visual method based upon external reference markers, and an automated method based upon external reference markers. Internal landmarks were inaccurate for defining the OML whether this was based upon the frontal-occipital or frontal-cerebellar plane. External reference markers resulted in significantly lower interobserver differences for both sagittal and coronal reorientation. An operator-independent implementation proved to be feasible and provided an objective measure of marker coplanarity. In summary, external reference markers should be used when reproducible reorientation and ROI placement are required.