148 Chapter 6 be observed downstream of amyloid. Our observation that high baseline amyloid burden predisposes for decline over time in rCBF is in line with this hypothesis (1). They are also in line with studies suggesting low CBF to reflect decreases in metabolism and synaptic failure, because this places a decreasing CBF towards the end of the pathophysiology of AD (6, 7). Alternatively, the two-hit vascular hypothesis poses that damage to the microcirculation of the brain resulting from vascular risk factors (hit one) would lead to reduced CBF (5, 40). The compromised vessels would than lead to suboptimal breakdown and clearance of amyloid, which would result in amyloid accumulation (hit two). In this scenario, amyloid accumulation would be a downstream effect of reduced CBF. Our second observation, that low baseline rCBF also predisposes for an increase in amyloid burden over time, is in line with this second hypothesis. Therefore, our results provide evidence for both theories. In contrast to our longitudinal findings, we did not find cross-sectional associations between rCBF and amyloid burden, except in the occipital region, in which a higher amyloid burden was subtly associated with a higher rCBF. This might be interpreted as overcompensation, yet we feel that this result should not be overinterpreted, since the association did not survive correction for multiple testing. Previous studies in cognitively normal individuals found contradictory results. Some found lower CBF in amyloid positive individuals (17), others a higher CBF (14), or no amyloid related differences in CBF at all (19, 20). The lack of cross-sectional associations in our sample of cognitively normal individuals, in combination with the inconsistencies in literature, highlight the importance of longitudinal data to study how different measures of brain pathology contribute to cognitive decline and dementia. Pathology accumulates in the course of many years, and this further stresses the relevance of well-phenotyped clinical cohorts with sufficiently long duration of follow-up. Our study is among the first to investigate the longitudinal relationships between amyloid burden and rCBF. Only one other study in 28 nondemented elderly investigated change in CBF using [15O]H 2O PET in relation to amyloid status, and found both increases and decreases in CBF in amyloid positive individuals compared to amyloid negative individuals (15), This study concluded that the decrease in CBF in amyloid positive individuals represented a response to neuronal insult as a result of amyloid deposition, providing evidence for the theory that amyloid accumulation predisposes for a reduction in CBF. They interpreted the increases as a representation of a compensational effect by attempting to preserve neuronal function. The latter could be viewed in line with our cross-sectional finding of a high amyloid burden related to a high rCBF in the occipital region. However, we feel that this result should not be overinterpreted. Furthermore, there are a number of differences with our study.