A New Subtraction and 3D Display Program for the Assessment of Epilepsy with Brain SPECT
It is well established that localisation of the seizure focus using Ictal brain SPECT is enhanced by computerised subtraction of the Inter-ictal scan. We describe a new locally written program with a graphical user interface that integrates coregistration of the Ictal to the Inter-ictal scan, count normalisation to the cerebellum or whole brain, image subtraction, and mouse-driven 3D display of the results. Rigid coregistration of the whole images was usually sufficient, but options for pre-masking of extra-cerebral activity and/or rigid+zoom coregistration are also offered. Post-coregistration count normalization started with user-definition of an ellipsoidal region over the cerebellum or whole-brain with optional ellipsoidal excisions. This was followed by two iterations of: computing the mean value in the top 20% of region voxels and elimination of region voxels with values below a threshold relative to this mean. The mean for each scan of the top 20% of the voxels remaining in the region was used for count normalization. The Ictal and Inter-ictal scans and their positive and negative difference images were displayed together in 4 linked panels with mouse-driven roam through their transaxial, sagittal and coronal sections. Dragging the cursor horizontally across a sagittal image, for example, caused the corresponding coronal sections to change in all 4 panels, and so on. Scaling of the individual panels was also mouse-driven. This flexible multi-panel 3D display was well accepted by the physicians and the difference images improved localisation of the seizure focus (separate abstract). The program is freely available.