Intersubject Coregistration

An emerging application is intersubject image registration to generate mean images of a control population. In most clinical applications the mean image acts as an atlas and reference against which abnormalities in a patient scan can be objectively and quantitatively assessed. Intersubject registration also allows comparison of pathologic imaging abnormalities within and across patient populations of interest.

Assessing significance of abnormalities in SPECT and PET images is the most common reported clinical application of intersubject coregistration. Richardson and Koepp et al. [22, 23] used intersubject coregistration to investigate abnormalities of central benzodiazepine binding with [11C] flumazenil PET in two partial epilepsy patient groups (epilepsy of mesial temporal lobe origin and neocortical origin, the latter specifically from focal cortical dysgenesis). They took advantage of control subject normative data mapped into the same stereotactic space as the patient image volumes. Then they used statistical parametric mapping [24] to objectively measure the distribution of normal and abnormal benzodiazepine binding in each patient. The coregistered MRI allowed important correction for partial volume effects with the use of convolving gray matter from segmented MRI.

Evaluation of SPECT regional CBF disturbances in dementia has also been demonstrated to benefit from anatomic standardization allowed by intersubject coregistration [25-27]. In each study, this approach enabled more accurate observer-independent assessment of patterns and severity of regional CBF change in Alzheimer's disease patients. Bartenstein et al. [25] showed an increase in sensitivity in the detection of significant abnormalities in early or mild dementia, an area of great difficulty in the diagnostic value of functional imaging. Houston et al. [27] employed a further level of analysis in using a principal components analysis. This procedure produces a variant "normal equivalent image'' to help account for normal deviants within the control image. Such images form additional building blocks for an atlas that allows a more realistic normative comparison. With or without principal

FIGURE 7 Intracranial electrode CT-MRI coregistration. (Top left) Original CT transaxial slice including two hippocampal depth electrode contacts (right and left). (Top right) Coregistered CT resliced in the plane of a coronal MRI slice. (Bottom left) Coregistered CT level and windowing adjustment to allow only visualization of electrode contacts. (Bottom right) Overlay of electrode contacts from resliced CT to corresponding MRI slice — MRI image contains visualization of electrodes without artifact and without need for special MRI-compatible electrodes.

components analysis, anatomic standardization using image coregistration and a mean control dataset is a definite advance in the objective interpretation of functional brain images that will undoubtedly spread with respect to routine use in clinical or diagnostic imaging.

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