Introduction
Midbrain nuclei have many projections to other brain regions and pivotal influence on basic brain functions, such as attention and consciousness. They are often affected in neurodegenerative disease causing severe clinical symptoms. Improvement of spatial resolution with latest generation of PET brain scanners allows measurement of their functional activity.
Methods
Resting cerebral glucose metabolism (with eyes closed) was measured in seven young male normal volunteers (age 31.7 ± 5.0 yrs) on the high-resolution research tomography (HRRT; CTI, Knoxville; FWHM 2.2 mm) 1 . Data used for reconstruction were recorded from 30 to 60 min after i.v. injection of 370 MBq FDG. Reconstruction using OSEM-3D (span 3, 3 iterations) with correction for attenuation, scatter, and decay yielded data sets of 256×256×207 isotropic voxels (1.22 mm size). They were registered to the ICBM brain template 2 using a new hierarchical free-form block matching algorithm for adjustment of individual anatomical variation. The ICBM volumes of interest (VOI) template was modified to distinguish the midbrain regions listed in the table 1. Quality of VOI matching was checked by visual inspection of coregistered MRI and metabolic homogeneity of VOIs. For each VOI, significance of correlation was accepted at Results Glucose metabolism differed significantly among midbrain regions (p<0.001, see table 1) without significant asymmetry. All midbrain regions (except left inferior colliculus) showed significant positive bilateral correlations with cerebellum, other midbrain regions, globus pallidum, lateral geniculate body, pons, and some significant negative correlations with basal and lateral occipital cortex. Positive correlations with hippocampal and parahippocampal structures were closest and most extensive in dorsal midbrain, possibly mediated by reticular formation activity. Correlations with other cortical structures and basal ganglia were generally absent or negative.
Glucose metabolism (relative to global brain average)
Conclusion
This study demonstrates that functional analysis of major human midbrain nuclei is possible with HRRT PET. Their glucose metabolism in resting waking state is positively correlated with cerebellum and pons but only with few forebrain regions (pallidum, geniculate body, and hippocampal structures).