Participants
Twenty-five normal older adult volunteers (13 males and 12 females; age, 60–81 years) participated in this follow-up study and were evaluated eventually (Figure 1). Participant recruitment was completed between September 2015 and March 2017. All participants were free of any significant medical conditions, such as chronic heart, kidney, and pulmonary diseases, and were not taking any prescribed, over the counter, or herbal medications. The volunteers were assessed to have no functional or cognitive impairment (Mini-Mental State Examination [MMSE] score ≥ 28) and had no neurological disease. In compliance with the guidelines of the Institutional Review Board of the University of Niigata, the study protocol was explained in detail to all potential participants or their proxies where appropriate, and written informed consent was obtained from all participants. All participants were provided with contact names and telephone numbers in the event of any adverse event related to the study, and were followed up within one month of the study to further confirm the absence or occurrence of study-related adverse events that were not otherwise self-reported by the study participant or proxy to the study coordinator. This follow-up project was conducted according to the human research guidelines of the Institutional Review Board of the University of Niigata, under the approval of the research ethics committee (approval numbers: 2015-2589), and the [15O]H2O PET study was registered at the UMIN Clinical Trials Registry as UMIN000011939. Each participant underwent both [15O]H2O and [18F]flutemetamol PET imaging procedures two weeks apart, and then underwent the same PET imaging procedures in a follow-up assessment performed two years later.
PET imaging
PET imaging was performed using a combined PET/CT scanner (Discovery ST Elite, GE Healthcare, Schenectady NY, USA) with a 15-cm field of view (FOV) positioned in the region of the cerebrum. For correcting photon attenuation, a low-dose CT scan was acquired in helical mode with the following parameters: 120 kV, 50 mA, 0.8 s per tube rotation, slice thickness of 3.75 mm with intervals of 3.27 mm, pitch of 0.875, and a table speed of 17.5 mm/rotation. During the scanning procedure, the participant’s head was rested in a foam-cushioned headrest, and a head strap was used to minimize head movement.
All PET emission scans were normalized for detector inhomogeneity and corrected for random coincidences, dead time, scattered radiation, and photon attenuation. These scans were reconstructed using three-dimensional ordered-subset expectation maximization (3D-OSEM) with two iterations and 28 subsets to obtain superior visual quality images, allowing manual definition of regions of interest (ROIs). For the reconstruction algorithms, the data were collected in a 128 × 128 × 47 matrix with a voxel size of 2.0 × 2.0 × 3.27 mm.
[15O]H2O PET
A 1000-MBq [15O]H2O synthesized online was injected intravenously using an automatic water injection system, followed immediately by a 10-mL saline flush at the speed of 1 mL/s (AM WR01; JFE Technos, Yokohama, Japan). After starting the injection, PET emission data were promptly acquired over 20 min in three-dimensional list mode with a 25.6-cm axial FOV and sorted into 47-time frames (18 × 10 s, 24 × 30 s, 5 × 60 s).
The CT and PET image data were transferred to a Xeleris 3.1 workstation (GE Healthcare) for PET data analysis. Manually defined ROIs (lateral and third ventricles, cortex of the frontal and occipital lobes) were drawn using volumetrix MI on a Xeleris 3.1 workstation. The tissue activity concentration in each ROI was expressed as the standardized uptake value (SUV, g/mL), corrected for the participant’s body weight and administered dose of radioactivity. Each tissue time activity concentration was fitted from the peak point in the cortex and from the start point in the ventricle to the following exponential curve by using SigmaPlot version 14.5 (Systat Software Inc, Illinois Chicago, USA).
y(t) = y0 + ae-bt
where y0 and b in each tissue were the implied tissue baseline SUV and in/out-flow pace, respectively (Figure 2(A)). To assess interstitial flow, we focused on two indices, namely, the ventricle and cortical y0 ratio as the indicator of water influx into the CSF space from the cortex and the cortical b as the speed of drainage flow from cortex. As shown in Figure 2(B), the former was defined as the influx ratio (IR) = y0(ventricle) / y0(cortex, representing the ratio of water flowing into the CSF space from the cortex, with higher values indicating a larger water flow. The latter was defined as the drain rate (DR) = b(cortex) × 103, representing the pace of water drainage into the CSF space from the cortex, with higher values indicating a faster flow speed.
As a control, we used previously reported data from 10 young individuals (21-30 years old) and 10 AD patients (59-84 years old). (15) Statistical analyses, t-tests, and paired-sample t-tests were performed using SPSS version 19.0 (IBM, Illinois Chicago, USA), and p-values less than 0.01 were regarded as statistically significant.
[18F]flutemetamol (Vizamyl®) PET
An intravenous bolus injection of 162–221 MBq [18F]flutemetamol (3.0-3.3 MBq/kg), produced using an automated synthesizer (FASTlab; GE Healthcare, Schenectady NY, USA) was administered, and a 20-min PET scan in 3-dimensional statistic mode was started after 90 min in accordance with the imaging acquisition guidelines.
The PET images scaled to 90% of the pons were visually assessed as either amyloid-positive or amyloid-negative on the basis of the training program instructions provided by GE Healthcare for the interpretation of [18F]flutemetamol images. A negative scan shows more radioactivity in the white matter than in the gray matter. Conversely, a positive scan shows gray matter radioactivity as intense as or exceeding that in the adjacent white matter in at least one of the five key regions (the posterior cingulate gyrus and precuneus, frontal cortex, lateral temporal cortex, parietal cortex, and striatum) (Figure 3).