Claiming the MRI-identified brain tissue change as a result of neurogenesis is now being challenged because neurogenesis in adult human brain has only been evidenced in very limited regions [1, 2]. One confounding factor that may contribute to the apparent tissue change is cerebral blood flow (CBF), which can change both MR parameters of tissue and tissue volume. While previous research has reported concurrent CBF and tissue volume changes, no one has explicitly assessed this “coincidence” by manipulating CBF in the healthy brain in a short time while keeping other conditions the same. In this study, we used caffeine to directly alter CBF and subsequently examined the CBF-tissue density interactions. We chose caffeine because it is well known to have robust effects of reducing CBF [3, 4]. All experiment procedures were approved by local IRB. 50 caffeine naïve healthy subjects (25 males and 25 females, age: 22.76±2.22 years, no medications, no more than 1 cup of coffee and 1 cup of tea in the week prior to the experiment) were recruited with written signed consent forms. All participants attended two imaging scan sessions before and after taking a 200 mg caffeine pill in two days with exactly 24 hours apart. The order of being caffeine free or taking caffeine prior to scan was counter-balanced. For each session, a high-resolution T1-weighted structural MRI and a six-minute resting brain perfusion MRI (using a GE product 3D BS-PCASL sequence (6 shots, 40 axial slices, TR=4690ms, TE=10.9ms, FOV=220x220 mm2, slice thickness=3.4mm, labeling time=1.5s, post label delay=1525ms, bandwidth=62.50Khz, matrix=512x512)) were acquired. Pre-post caffeine grey matter difference was examined using the state-of-art longitudinal voxel-based morphometry (VBM) analysis method [5] implemented in SPM12. CBF maps were spatially normalized into the MNI space using the registration information obtained from the mid-term structural image (created during the longitudinal VBM). Pre-post CBF difference was assessed using paired-t test. Correlation between tissue difference and CBF difference was calculated at each voxel in the standard brain space.Fig. 1 shows the observed effects of caffeine on CBF and tissue volume. Caffeine intake caused a global CBF reduction (Fig. 1A. voxel-wise P<0.05 with FWE correction). The whole brain mean CBF reduction was 15.3 ml/100g/min. Grey matter volume reduction after taking caffeine was observed in hippocampus, orbito-frontal cortex, ventral striatum, anterior cingulate cortex (ACC), insula, and superior temporal cortex (Fig. 1B). After regressing out the pre-post caffeine CBF difference from the tissue density difference, no significant caffeine induced tissue density change was found (Fig. 1C). Fig. 2 shows the correlation between the pre-post CBF difference and tissue density difference. Significant correlation was demonstrated in hippocampus, fusiform, visual cortex, putamen, prefrontal cortex, and lateral parietal cortex.We proved that CBF reduction attribute short-term apparent brain tissue density reduction. Although linear correlations between CBF reduction and tissue density reduction were only observed in several brain regions, our data showed that after regressing out CBF effects, the apparent tissue changes went away. Altogether, our data should raise cautions of reporting short-term brain tissue density changes as a result of neurogenesis, rather the observed apparent tissue “changes” should be corrected for CBF alterations.