Participants
Participants were recruited from the Neurology Department of Ningde Municipal Hospital and Ningde Normal University between November 2022 and November 2023. Following the inclusion/exclusion criteria, 35 patients (16 AD, 19 MCI) and 25 healthy elderly controls (NC) were included. Informed consent was obtained from all participants and from the families of those with cognitive impairment. This study was performed in line with the principles of the Declaration of Helsinki. Approval was granted by the Ethics Committee of Ningde Municipal Hospital (Approval No: 20200901).
Inclusion criteria were age between 50 and 85 years, willingness to undergo MRI and provide informed consent, and ability to complete imaging with family assistance or across multiple sessions if needed. Exclusion criteria included the presence of brain tumors; severe cardiovascular disease; history of psychiatric illness; previous traumatic brain injury; major neurological disorders such as central nervous system infections, Parkinson‘s disease, Huntington’s disease, epilepsy, or multiple sclerosis; severe systemic diseases; non-removable metallic implants; severe sensory impairments that prevented cognitive assessment; image artifacts; and left-handedness.
Diagnoses of Alzheimer’s disease (AD), mild cognitive impairment (MCI), and normal cognition (NC) were established by two neurologists using the National Institute on Aging–Alzheimer’s Association (NIA-AA) criteria. Probable AD required progressive cognitive and functional decline; MCI required objective cognitive impairment with preserved daily function; NC required no cognitive or neurological abnormalities. A consensus diagnosis was reached for all participants. Biomarker confirmation was not uniformly available, which constitutes a study limitation. All the tools and materials used in this study are listed in the Table of Materials.
Clinical data
Demographic and clinical data were collected, including age, sex, education, medical history (hypertension, diabetes, hyperlipidemia), and physical examination findings. Cognitive function was assessed using the mini-mental state examination (MMSE) and Montreal cognitive assessment (MoCA).
Image acquisition
Imaging was performed on a 3.0T MRI scanner using a dedicated head-neck coil. The imaging protocol included T2-weighted fluid-attenuated inversion recovery (T2-FLAIR), diffusion-weighted imaging (DWI), sagittal T1-weighted imaging, and phase-contrast MRI (comprising 2D cine phase-contrast and 4D flow sequences). All participants were positioned supine, head-first, with the head immobilized using foam pads and ear protection applied to reduce motion artifacts.
Two-dimensional cine phase-contrast MRI
A 2D cine phase-contrast sequence (Fast PC Cine Slice) was acquired perpendicular to the mid-cerebral aqueduct, as defined on the sagittal T1-weighted localizer images. The imaging parameters were as follows: field of view, 24 × 24 cm; slice thickness, 5.0 mm; repetition time/echo time, 20.2/6.1 ms; matrix, 288 × 256; pixel size, 0.83 × 0.94 mm; bandwidth, 31.25 kHz; number of excitations, 1.0; flip angle, 20°; encoding velocity, 12 cm/s; peripheral gating; and 20–30 phases per cardiac cycle. The encoding velocity was selected to adequately capture aqueductal CSF flow without aliasing. The acquisition duration ranged from 1 to 3 min, depending on the participant‘s heart rate.
Four-dimensional flow MRI
A 4D flow sequence (Sag 4D Flow) was performed to cover the cervical carotid region. The imaging parameters were as follows: field of view, 24 × 24 cm; slice thickness, 1.0 mm; slice spacing, 2.0 mm; 40–50 slices; repetition time/echo time, 6.6/2.8 ms; matrix, 180 × 180; pixel size, 1.33 × 1.33 mm; bandwidth, 62.50 kHz; number of excitations, 1.0; flip angle, 15°; encoding velocity, 50 cm/s; and 20–30 phases per cardiac cycle. The encoding velocity of 50 cm/s was chosen to adequately capture carotid arterial flow while minimizing aliasing. Acquisition duration ranged from 1 to 3 minutes, depending on heart rate. All images were analyzed using specialized post-processing software (Table of Materials).
Image processing and analysis
Analysis of cerebrospinal fluid-related parameters
Two-dimensional cine PC magnitude and phase images were analyzed using specialized post-processing software (Figure 1A–D). Two radiologists (each with three years of neuroimaging experience) independently drew regions of interest (ROIs) along the outer edge of the cerebral aqueduct at its maximum cross-sectional area, identified on the sagittal T1-weighted localizer and confirmed on the magnitude image. Each radiologist delineated the ROI twice, with an interval of at least two weeks between sessions. Background phase correction was performed using the built-in stationary tissue correction algorithm in the software, which subtracts the mean phase of a static tissue region (placed in the midbrain parenchyma adjacent to the aqueduct) from the velocity-encoded phase images to minimize eddy current and Maxwell term effects. The following parameters were generated for each participant: maximum systolic velocity (peak systolic velocity, PSV), maximum diastolic velocity (peak diastolic velocity, PDV), net stroke volume, and flow curves. The aqueduct cross-sectional area was recorded (Figure 2). Inter-rater and intra-rater reliability were assessed using two-way random-effects intraclass correlation coefficients (ICC) for absolute agreement; both exceeded 0.90 for all parameters.

Figure 1: Cerebrospinal fluid flow imaging acquisition and processing. (A) Sagittal T1-weighted localizer image showing the positioning of the 2D cine phase-contrast slice, set perpendicular to the mid-cerebral aqueduct (dashed line). (B) Magnitude image of the cine 2D sequence showing the anatomical structure of the cerebral aqueduct. (C) Phase image of the cine 2D sequence showing CSF flow signal intensity. Abbreviations: CA = cerebral aqueduct; CSF = cerebrospinal fluid. Please click here to view a larger version of this figure.

Figure 2: Representative cerebrospinal fluid flow velocity curve. Time–flow velocity curve showing the biphasic, quasi-sinusoidal CSF flow pattern over a single cardiac cycle. The curve illustrates changes in velocity during systole and diastole. Abbreviations: CSF = cerebrospinal fluid. Please click here to view a larger version of this figure.
Analysis of parameters related to internal carotid artery flow
Four-dimensional flow data were reconstructed and analyzed using the 4D Flow module in specialized post-processing software. Phase-offset correction was applied using a first-order polynomial fit to static tissue regions. The left internal carotid artery (LICA) was isolated using a semi-automatic threshold-based segmentation tool with manual refinement, which generated a 3D angiographic volume. A vessel centerline was automatically computed, and three cross-sectional planes orthogonal to the centerline were placed at the C1 segment (approximately 2–3 cm above the carotid bifurcation) with 5-mm spacing between adjacent planes. The left internal carotid artery was selected for analysis based on its consistent anatomical course and flow characteristics; the right internal carotid artery was not assessed to minimize scanning duration and participant burden. While a unilateral approach was adopted for feasibility in this exploratory study, bilateral assessment is warranted in future investigations. For each plane, the vessel lumen was contoured using a semi-automatic edge-detection algorithm with manual adjustment at each cardiac time frame (approximately 20 frames per cycle) (Figure 3A–D).
From the time-resolved lumen contours, the following parameters were computed: maximum flow velocity, flow volume per cardiac cycle, and mean wall shear stress (WSS). WSS was derived using the formula:
WSS = μ × (∂v/∂r)|_wall (1)
where µ = 0.004 Pa·s is the assumed blood viscosity, and ∂v/∂r is the velocity gradient normal to the vessel wall, obtained by fitting a parabolic profile to the near-wall velocity data at each circumferential position and averaging over the lumen circumference. Flow velocity, vector, and WSS maps were generated, and time-flow and time-WSS curves were plotted. Values from the three planes were averaged to obtain a single representative value per participant. All image processing was performed by the same two radiologists independently, and the mean of their measurements was used for statistical analysis (Figure 4).

Figure 3: Three-dimensional reconstruction and hemodynamic analysis of the left internal carotid artery. (A) Three-dimensional volume-rendered reconstruction of the left carotid artery following vessel segmentation. (B) Positioning of the three cross-sectional analysis planes (Flow 1–3) along the C1 segment. (C) Flow velocity maps. (D) Vector diagrams showing flow direction. (E) Wall shear stress maps. Abbreviations: LICA = left internal carotid artery; WSS = wall shear stress. Please click here to view a larger version of this figure.

Figure 4: Left internal carotid artery post-processing curves. Time–flow curves (left panel) and time–wall shear stress curves (right panel) across one cardiac cycle. Different colors represent the three sampling planes (red, yellow, green). Abbreviations: LICA = left internal carotid artery; WSS = wall shear stress. Please click here to view a larger version of this figure.
Statistical analysis
Data were analyzed using standard statistical software (Table of Materials). Normally distributed variables, assessed by the Shapiro–Wilk test, were expressed as mean ± SD and compared by one-way ANOVA. Non-normally distributed variables were expressed as median (IQR) and compared by the Kruskal–Wallis test. Categorical variables were compared by the chi-square test. ANCOVA with age and education as covariates was used for intergroup comparisons, with Bonferroni correction for multiple comparisons. Spearman correlation and multiple linear regression assessed relationships between imaging and cognitive measures, adjusting for vascular risk factors (hypertension, diabetes, hyperlipidemia). Receiver operating characteristic (ROC) analysis was conducted to evaluate the diagnostic performance of each imaging parameter. The area under the curve (AUC), sensitivity, and specificity were calculated. ROC analyses were performed without covariate adjustment to evaluate the intrinsic discriminative performance of each parameter. However, given the observed group differences in age and education, supplemental analyses adjusting for these covariates yielded consistent findings. A two-tailed p < 0.05 was considered statistically significant. A complete list of materials and equipment used in this study is provided in the Table of Materials.