Subjects rested comfortably on the imaging table with eyes closed and covered and environmental noise kept to a minimum, while the tracer injection was administered over a 30 s period. The subject’s head was then positioned in a moulded head-holder and aligned with the help of two crossed positioning lights. During the scan, the head was fixed with pressure pads over the zygomatic arches. Slices were acquired parallel to the orbito-meatal plane, starting at a level approximately 2 cm above the orbito-meatal line and at 1 cm intervals above this level; further details of the method have been described previously (Ebmeier et al, 1995).
Images were reconstructed using software supplied by Strichmann Medical Equipment (SME) for the Apple Macintosh. The SME reconstruction algorithm selects an enveloping ellipse, derived from an oversmoothed image of the brain. This, together with the absorption length parameter (95 mm), determines the Chang-like attention correction which was done with one iteration. Count distributions were deconvoluted into the radio-isotope concentrations responsible, using a Wiener filter with a correlation length of 6 mm.
Image analysis Two transverse slices were chosen for the ROI analysis, approximately 4 and 6 cm above the orbito-meatal line. A standard template was prepared by drawing regions of interest over corresponding brain atlas slices (Talairach et al, 1988). Although this atlas is oriented to an internal anatomical reference (the line between anterior and posterior commissures), the orbito-meatal line is almost parallel with it (Szikla et al, 1977). The ROIs included, in the lower slice, frontal, anterior and posterior cingulate, anterior temporal, posterior temporal, calcarine and occipital cortex, as well as caudate, putamen and thalamus.
The corresponding template for the higher slice contained frontal, anterior and posterior cingulate, parietal and occipital cortex. The templates are linearly and symmetrically deformed to fit different brain sizes and shapes, using the 20% isocontour line to define the cortical edge. ROI were thus preserved in their relative position to each other, and no additional adjustments were made for single regions that appeared to be out of position (Ebmeier et al, 1991). Regional count densities were normalised by proportional scaling to whole brain blood flow (derived from the two slices examined).