Aging, prevalence and risk factors of MRI-visible enlarged perivascular spaces

Study sample

The Framingham Heart Study is a prospective, multigenerational, population-based cohort study that began with enrollment of the Original cohort of 5,209 participants in 1948. These participants have been examined approximately once every 2 years. The Offspring Cohort of 5,124 participants recruited in 1971 included offspring of the Original Cohort and their spouses and has been examined approximately every 4 years. The Third Generation (Gen 3, n=4,095) and the New Offspring (NOS, n=103) cohorts were recruited in 2002 and has been examined three times. Since the first three generations of the FHS participants were predominantly of white European descent, the Heart Study enrolled an OMNI cohort in 1994 that includes 506 individuals of African American, Hispanic, Asian, Indian, Pacific Islander and Native American descent.

Since 1999 participants have been invited to undergo brain MRI. The present study included participants from all cohorts with available brain MRI acquired between 2000 and 2015. Participants who underwent brain MRI attended a baseline clinical examination between 1997 and 2014 (25^th-31^st examination cycle for Original cohort participants, 7^th-9^th examination cycle for Offspring participants, 1^st-2^nd exam cycles for NOS/Gen 3 participants, and 2^nd-4^th exams for OMNI 1 participants).

The flow chart of the sample selection is described in Figure 1. In total, there are 10,589 MRI scans from 5,594 FHS participants, of which 4,658 MRI scans from 3,998 participants had ePVS ratings. Exclusion criteria for the present study included refusal or contraindication for MRI (pacemaker or other implantable devices, metallic foreign body, claustrophobia), scans with significant artifacts precluding ePVS assessment, scans without a corresponding clinic examination, and history of clinical stroke, dementia, and neurological conditions that could affect brain MRI measurements (such as head trauma, multiple sclerosis, brain tumor). Following these exclusions the sample included 4,199 scans from 3,710 participants.

3,377 participants only had one MRI scan. For the 822 participants who had multiple scans, we selected one scan corresponding to a unique primary clinic exam. If no primary clinic exam was assigned, we selected the latest scan within the exam cycle such that our final sample included one scan from each available exam cycle.

The final sample included 4,101 scans (270 Original cohort, 1814 Offspring, 19 NOS, 1848 Third Gen, 149 OMNI 1) from 3,710 participants. The Institutional Review Board of Boston University Medical Center approved the study protocol and informed consent was obtained from all subjects.

Brain MRI

Acquisition and analysis

Brain MRI acquisition measures and image processing methods have been described in detail [8, 9]. Brain MRI were acquired using a 1T (1999–2005) or 1.5T (after 2005) Magnetom scanner (Siemens Medical, Erlangen, Germany).

Enlarged perivascular spaces (ePVS)

The MRI characteristics of ePVS were based on prior consensus criteria by the Standards for Reporting Vascular Changes on Neuroimaging Criteria (STRIVE consortium) [10]. Briefly, ePVS met the following criteria: signal intensity similar to CSF on all sequences, adherence to the course of penetrating vessels, linear (parallel to the penetrating vessel) or round/ovoid (perpendicular to the penetrating vessel), and a diameter smaller than 3mm.

ePVS rating measures

Scans were rated by three investigators (JRR, PP, AS) blinded to the subjects’ demographic and clinical information. T2-weighted axial MRI sequences were the main sequence used for rating ePVS following a validated method [11]. Brain topography of ePVS was classified as centrum semiovale (CSO) and basal ganglia (BG). We also analyzed a mixed group including ePVS in both regions. The burden of ePVS in each region was categorized into grades based on ePVS counts: grade I (1-10), II (11-20), III (20-40) and IV (>40). In a subset of legacy scans from an older dataset, coronal acquisitions were of higher resolution than axial views and were used for ratings (N=1122). For these ratings using coronal views to approximate ratings using axial views we assessed the entire BG region in all slices above the anterior commissure and below the level of the roof of the lateral ventricles. Ratings were based on ePVS burden in the BG bilaterally or in case of presence of large incidental lesions (such as large covert infarcts – without corresponding clinical stroke or TIA event) in the contralesional side. Ratings of the CSO region were performed evaluating the corona radiata, above the level of the lateral ventricles, and subcortical white matter. We categorized burden of ePVS using the same categories as in axial views (Figure 2). In a subset of scans with available high-resolution axial and coronal reconstructions, we compared ePVS ratings in BG and CSO regions based on axial versus coronal sequences. ePVS ratings in BG and CSO were done on 20 scans rated in random order in two separate occasions using axial sequences first, and repeating ratings with the coronal sequence after changing scan order randomly and blinded to the axial ratings. Ratings in axial and coronal sequences were highly correlated with an intraclass correlation (ICC)=0.91.

ePVS ratings reliability measures

Intra-rater reproducibility was assessed for each rater, using 200 scans (JRR, PP) or 20 scans (AS), on two separate occasions 2 to 4 weeks apart, and changing randomly the order of scans between the two reading sessions. Inter-rater reproducibility measures were compared between the primary rater (JRR) and secondary raters (PP, AS) using 200 scans (JRR vs PP), and 20 scans (JRR vs AS). The intra-rater reliability was good to excellent (ICC basal ganglia JRR and PP=0.76, AS=0.74; centrum semiovale JRR= 0.81, PP=0.76, AS=0.81). Inter-rater reliability comparing two independent readers was excellent (ICC basal ganglia JRR vs PP = 0.86, JRR vs. AS =0.83; centrum semiovale JRR vs. PP= 0.8, JRR vs. AS= 0.90). For EPVS ratings using coronal sections we observed excellent intra-rater reproducibility (ICC=0.9).

Additional brain MRI markers of cerebral small vessel disease

We also used existent ratings of other CSVD markers based on brain MRI to evaluate the relation of ePVS and other CSVD markers. White matter hyperintensities volume (WMHV), total cranial and brain volumes were rated on FLAIR and T1 weighted MRI sequences respectively, using quantitative analyses as previously reported [12, 13]. We used the total cranial to brain volume (TCBV) ratio to account for head size differences. Presence of cerebral microbleeds (CMB) were rated on T2*weighted sequences using previously published guidelines [14, 15]. Covert brain infarcts (CBI) were determined according to STRIVE criteria [10, 16].

Vascular risk factors

Systolic (SBP) and diastolic (DBP) blood pressures (mmHg) were each taken as the average of the Framingham clinic physician’s two measurements. Hypertension status was evaluated using the JNC-7 criteria (SBP ≥140 mmHg or DBP≥90 mmHg or use of antihypertensive medications). In addition, hypertension categories were assessed using SBP and DBP measurements, regardless of anti-hypertensive use: normal (SBP <120 mmHg and DBP <80 mmHg), pre-hypertensive (SBP 120-139 mmHg or DBP 80-89mmHg), and hypertensive (SBP ≥140 mmHg or DBP ≥90 mm Hg).

Diabetes was defined as a random blood glucose ≥200 mg/dl (≥11.1 mmol/L) for the Original cohort, fasting glucose ≥126mg/dl (≥7 mmol/L) for the Offspring, Third Generation, NOS, and OMNI 1 cohorts, or use of insulin or oral hypoglycemic medications (for all cohorts).

Total cholesterol, high-density lipoprotein (HDL) cholesterol, and triglycerides (mg/dL) were measured on fasting specimens in the Offspring cohort, and random samples in the Original cohort. LDL-cholesterol concentrations (mg/dL) were calculated according to the Friedewald equation: LDL= (TC – HDL + (TG /5), where LDL = low density lipoprotein, TC = total cholesterol, HDL=high density lipoprotein, TG= triglycerides. Because the formula is unreliable for individuals with TG levels above 400mg/dL, participants whose triglyceride levels were above 400mg/dL had missing LDL values.

Body Mass Index (BMI) was measured by finding the ratio of the participants’ weight and squared height (kg/m²). Participants were classified as underweight (BMI < 18.5 kg/m²), normal (BMI 18.5-24.9 kg/m²), overweight (BMI 25.0-29.9 kg/m²), and obese (BMI > 30.0 kg/m²), as defined by the National Institutes of Health.

Statistical analysis

The main outcomes of interest in our analyses were CSO ePVS, BG ePVS, and a mixed CSO-BG score reflecting high burden ePVS in either region or both. High ePVS burden was defined as counts greater than 20 (i.e. grades III or IV) within each region. The CSO-BG mixed score denoted the number of regions with high ePVS burden (0=none, 1=one region, or 2=both regions). CSO and BG ePVS (grades I-IV) and the CSO-BG mixed score (0-2) were each treated as ordinal outcomes.

Descriptive statistics of demographic variables and risk factors are described for the overall sample by brain region using means and standard deviations for continuous variables and frequencies and relative frequencies for categorical variables. Prevalence of ePVS in men and women by 10-year age intervals are presented in bar charts for each region. Continuous vascular risk factors were standardized to mean zero and standard deviation of one and described in bar charts for the overall sample by 10-year age intervals. We also provide descriptive statistics for the relation of ePVS burden and other brain MRI CSVD markers (WMHV, TCBV ratio, CMB, and CBI).

Ordinal logistic regression analysis was used to calculate odds ratios and 95% confidence intervals for the association between each individual vascular risk factor and each of the following outcomes: CSO only ePVS, BG only ePVS, and mixed CSO-BG ePVS. Odds ratio estimates for SBP, DBP, total cholesterol, LDL, HDL, and BMI represent a one-standard deviation change in the proportional odds of ePVS. We used empirical (robust) standard error estimates to account for dependence, but potentially because of the relatively small number of repeated scans, the results were similar to those obtained using classical standard errors.

Models were adjusted for age at MRI, sex, FHS cohort, and time interval between risk factor measurement and MRI acquisition. Since the NOS and Third Generation cohorts had the same enrollment periods, nineteen participants in the NOS cohort were combined into the Third Generation cohort for analysis. We also adjusted for the MRI sequence used for ratings (axial vs. coronal) but no significant differences were noted.

All analyses were performed using Statistical Analyses System (SAS) software version 9.4 (SAS Institute, Cary, NC). A two-sided p-value <0.05 was considered statistically significant.

Age and sex specific prevalence of ePVS and vascular risk factors

Baseline demographic characteristics of our sample by ePVS grade in each brain region are shown in Table 1. The prevalence of high ePVS burden increased in both brain regions as average age increased. High burden ePVS (grades III and IV) was low in participants younger than 60 years old, less than 10% in the BG and CSO (Figure 3). In contrast, most participants over age 80 had high ePVS burden in both the BG and CSO regions, with predominance in the CSO. No substantial differences were observed in the prevalence of high ePVS burden across brain regions between men and women throughout the life span, although in the overall sample a smaller proportion of males was noted in participants with grade IV ePVS in the BG (29% men, 61% women).

In our sample, the age at which high burden ePVS in either brain region was first detected was between 50 and 60 years (Figure 3). While ePVS were visible in all scans, the prevalence of severe ePVS in the BG, CSO and mixed brain regions increased steadily by age group for both men and women (Figures 3, 4). Overall, inspection of the data did not show meaningful differences between men and women. Among scans of participants 80 years of age and over, 50% had high ePVS burden (grade III-IV) in the CSO region and 36% in the BG region.

Vascular risk factors including SBP, hypertension, fasting blood glucose, and triglycerides increased with age. On the other hand, other measures including DBP, total cholesterol, and LDL also increased in age until age 50-59 and then progressively decreased (Figures 5, 6). Of note, treatment use for vascular risk factors increased with age and seemed to parallel the decrease in the factors noted above (Figures 5, 6).

ePVS and other CSVD markers

Although the sample size for some of the MRI markers differed (WMHV n=3362, CMB n=2291), we observed that the burden of CSVD markers increased as the grades of ePVS increased in both brain regions (Table 2). We observed that mean WMH volume and prevalence of CMB and CBI increased while mean total brain volume decreased in both the CSO and BG as the burden of ePVS increased. The mixed score of CSO and BG showed similar relation to other CSVD markers where marker severity increased with higher burden of ePVS.

Multivariable analysis for the association of vascular risk factors and ePVS burden

Systolic and diastolic blood pressure were strongly associated with high ePVS burden in the CSO and BG, and the mixed CSO-BG score (Table 3). Similarly, hypertension was significantly associated with high ePVS burden in the CSO (OR: 1.66; 95% CI: 1.37, 2.01), BG (OR: 1.60; 95% CI: 1.32, 1.95) and in the mixed regions (OR: 1.49; 95% CI: 1.17, 1.90) (Table 3). The relation of hypertension category seemed to have a dose effect relation with ePVS burden in the CSO: we observed significant associations of prehypertension with increased ePVS burden (OR: 1.21; 95% CI: 1.05, 1.40) and stronger effect size with stage I hypertension or higher (OR: 1.49; 95% CI: 1.29, 1.72) (Table 3). Hypertension treatment use was also associated with high ePVS burden in the CSO and BG, but not the mixed region. Current smoking was associated with ePVS in the CSO (OR: 1.36; 95% CI: 1.07, 1.72), BG (OR:1.29; 95% CI: 1.03, 1.62) and mixed CSO-BG score (OR:1.42; 95% CI: 1.03, 1.98). A modest borderline association was seen between BMI and high ePVS burden in the CSO and BG.