Senile plaques do not induce susceptibility effects in T2*-weighted MR microscopic images

Senile plaques are the hallmarks of Alzheimer's disease. They typically range from 16 to 150 microm in size with most less than 25 microm. Mechanisms by which they might affect MR contrast and thus be made visible in this imaging modality are still unknown. Plausibility studies suggested that they might have a different magnetic susceptibility than surrounding tissue. A large difference would cause the plaque and a relatively large volume of adjacent tissue to be hypo-intense in T2*-weighted MRI scans, thus causing them to appear larger than their actual sizes and perhaps visible even when their size is below the nominal resolution limit of the imaging experiment. To evaluate this hypothesis, formalin-fixed superior temporal gyrus samples obtained from two Alzheimer's disease and two control subjects were imaged using magnetic resonance microscopy at 11.7 T. Three dimensional T2*-weighted gradient echo images were recorded with an isotropic resolution of 23.4 microm. The imaging protocol was especially sensitive to susceptibility effects. Samples were then stained for amyloid and/or iron deposits. Hypo- and hyper-intense structures were clearly visible in MR images from all samples. Hyper-intense structures reflected fixative penetration within the vascular system. Almost all the hypo-intense structures were blood vessels. Their hypo-intense profile was probably caused by iron deposits associated with the cell aggregates that they contained. Only one hypo-intense spot could be matched with a plaque and this was one of the largest plaques in our samples. The remainder of the several hundred plaques were not visible in MR images. In histological slices the senile plaques were often larger than small blood vessels that were visible in the MR images. This suggests that susceptibility effects are not associated with senile plaques and do not provide a mechanism to differentiate them from surrounding tissue.