Age-related normative changes in cerebral perfusion: Data from The Irish Longitudinal Study on Ageing (TILDA).

OBJECTIVE: To establish normative reference values for total grey matter cerebral blood flow (CBFGM) measured using pseudo-continuous arterial spin labelling (pCASL) MRI in a large cohort of community-dwelling adults aged 54 years and older. BACKGROUND: Quantitative assessment of CBFGM may provide an imaging biomarker for the early detection of those at risk of neurodegenerative diseases, such as Alzheimer's and dementia. However, the use of this method to differentiate normal age-related decline in CBFGM from pathological reduction has been hampered by the lack of reference values for cerebral perfusion. METHODS: The study cohort comprised a subset of wave 3 (2014-2015) participants from The Irish Longitudinal Study on Ageing (TILDA), a large-scale prospective cohort study of individuals aged 50 and over. Of 4309 participants attending for health centre assessment, 578 individuals returned for 3T multi-parametric MRI brain examinations. In total, CBFGM data acquired from 468 subjects using pCASL-MRI were included in this analysis. Normative values were estimated using Generalised Additive Models for Location Shape and Scale (GAMLSS) and are presented as percentiles, means and standard deviations. RESULTS: The mean age of the cohort was 68.2 ± 6.9 years and 51.7% were female. Mean CBFGM for the cohort was 36.5 ± 8.2 ml/100 g/min. CBFGM decreased by 0.2 ml/100 g/min for each year increase in age (95% CI = -0.3, -0.1; p ≤ 0.001) and was 3.1 ml/100 g/min higher in females (95% CI = 1.6, 4.5; p ≤ 0.001). CONCLUSIONS: This study is by far the largest single-site study focused on an elderly community-dwelling cohort to present normative reference values for CBFGM measured at 3T using pCASL-MRI. Significant age- and sex-related differences exist in CBFGM.

Abundance of P-Glycoprotein and Other Drug Transporters at the Human Blood-Brain Barrier in Alzheimer's Disease: A Quantitative Targeted Proteomic Study.

The human blood-brain barrier (BBB) transporter P-gp can efflux amyloid-ß (Aß) out of the central nervous system (CNS). Aß is thought to be the causative agent for Alzheimer's disease (AD). Using positron emission tomography imaging, we have shown that BBB P-gp activity is reduced in AD, as quantified by the in vivo brain distribution of the P-gp probe [11 C]-verapamil. Therefore, the aim of this study was to determine whether this reduced BBB P-gp activity in AD was due to decreased P-gp abundance at the BBB. Using targeted proteomics, we quantified the abundance of P-gp and other drug transporters in gray matter brain microvessels isolated from 43 subjects with AD and 38 age-matched controls (AMCs) from regions affected by AD (hippocampus and the parietal lobe of the brain cortex) and not affected by AD (cerebellum). First, P-gp abundance was decreased in the BBB of the hippocampus vs. the cerebellum in both subjects with AD and AMCs, and therefore was not AD-related. In addition, gray matter BBB abundance of P-gp (and of other transporters) in the hippocampus and the parietal lobe was not different between AD and AMC. The gray matter BBB abundance of all drug transporters decreased with age, likely due to age-dependent decrease in the density of brain microvessels. Collectively, the observed reduced in vivo cerebral BBB P-gp activity in AD cannot be explained by reduced abundance of P-gp at the BBB. Nevertheless, the drug transporter abundance at the human gray matter BBB data provided here can be used to predict brain distribution of drugs targeted to treat CNS diseases, including AD.

Effect of intravoxel incoherent motion on diffusion parameters in normal brain.

At very low diffusion weighting the diffusion MRI signal is affected by intravoxel incoherent motion (IVIM) caused by dephasing of magnetization due to incoherent blood flow in capillaries or other sources of microcirculation. While IVIM measurements at low diffusion weightings have been frequently used to investigate perfusion in the body as well as in malignant tissue, the effect and origin of IVIM in normal brain tissue is not completely established. We investigated the IVIM effect on the brain diffusion MRI signal in a cohort of 137 radiologically-normal patients (62 male; mean age = 50.2 ±â€¯17.8, range = 18 to 94). We compared the diffusion tensor parameters estimated from a mono-exponential fit at b = 0 and 1000 s/mm2 versus at b = 250 and 1000 s/mm2. The asymptotic fitting method allowed for quantitative assessment of the IVIM signal fraction f* in specific brain tissue and regions. Our results show a mean (median) percent difference in the mean diffusivity of about 4.5 (4.9)% in white matter (WM), about 7.8 (8.7)% in cortical gray matter (GM), and 4.3 (4.2)% in thalamus. Corresponding perfusion fraction f* was estimated to be 0.033 (0.032) in WM, 0.066 (0.065) in cortical GM, and 0.033 (0.030) in the thalamus. The effect of f* with respect to age was found to be significant in cortical GM (Pearson correlation ρ â€‹= â€‹0.35, p â€‹= â€‹3*10-5) and the thalamus (Pearson correlation ρ = 0.20, p = 0.022) with an average increase in f* of 5.17*10-4/year and 3.61*10-4/year, respectively. Significant correlations between f* and age were not observed for WM, and corollary analysis revealed no effect of gender on f*. Possible origins of the IVIM effect in normal brain tissue are discussed.

Observer variability of reference tissue selection for relativecerebral blood volume measurements in glioma patients.

OBJECTIVES: To assess observer variability of different reference tissues used for relative CBV (rCBV) measurements in DSC-MRI of glioma patients. METHODS: In this retrospective study, three observers measured rCBV in DSC-MR images of 44 glioma patients on two occasions. rCBV is calculated by the CBV in the tumour hotspot/the CBV of a reference tissue at the contralateral side for normalization. One observer annotated the tumour hotspot that was kept constant for all measurements. All observers annotated eight reference tissues of normal white and grey matter. Observer variability was evaluated using the intraclass correlation coefficient (ICC), coefficient of variation (CV) and Bland-Altman analyses. RESULTS: For intra-observer, the ICC ranged from 0.50-0.97 (fair-excellent) for all reference tissues. The CV ranged from 5.1-22.1 % for all reference tissues and observers. For inter-observer, the ICC for all pairwise observer combinations ranged from 0.44-0.92 (poor-excellent). The CV ranged from 8.1-31.1 %. Centrum semiovale was the only reference tissue that showed excellent intra- and inter-observer agreement (ICC>0.85) and lowest CVs (<12.5 %). Bland-Altman analyses showed that mean differences for centrum semiovale were close to zero. CONCLUSION: Selecting contralateral centrum semiovale as reference tissue for rCBV provides the lowest observer variability. KEY POINTS: • Reference tissue selection for rCBV measurements adds variability to rCBV measurements. • rCBV measurements vary depending on the choice of reference tissue. • Observer variability of reference tissue selection varies between poor and excellent. • Centrum semiovale as reference tissue for rCBV provides the lowest observer variability.

Gas-free calibrated fMRI with a correction for vessel-size sensitivity.

Calibrated functional magnetic resonance imaging (fMRI) is a method to independently measure the metabolic and hemodynamic contributions to the blood oxygenation level dependent (BOLD) signal. This technique typically requires the use of a respiratory challenge, such as hypercapnia or hyperoxia, to estimate the calibration constant, M. There has been a recent push to eliminate the gas challenge from the calibration procedure using asymmetric spin echo (ASE) based techniques. This study uses simulations to better understand spin echo (SE) and ASE signals, analytical modelling to characterize the signal evolution, and in vivo imaging to validate the modelling. Using simulations, it is shown how ASE imaging generally underestimates M and how this depends on several parameters of the acquisition, including echo time and ASE offset, as well as the vessel size. This underestimation is the result of imperfect SE refocusing due to diffusion of water through the extravascular environment surrounding the microvasculature. By empirically characterizing this SE attenuation as an exponential decay that increases with echo time, we have proposed a quadratic ASE biophysical signal model. This model allows for the characterization and compensation of the SE attenuation if SE and ASE signals are acquired at multiple echo times. This was tested in healthy subjects and was found to significantly increase the estimates of M across grey matter. These findings show promise for improved gas-free calibration and can be extended to other relaxation-based imaging studies of brain physiology.

An automated method measures variability in P-glycoprotein and ABCG2 densities across brain regions and brain matter.

Changes in P-glycoprotein and ABCG2 densities may play a role in amyloid-beta accumulation in Alzheimer's disease. However, previous studies report conflicting results from different brain regions, without correcting for changes in vessel density. We developed an automated method to measure transporter density exclusively within the vascular space, thereby correcting for vessel density. We then examined variability in transporter density across brain regions, matter, and disease using two cohorts of post-mortem brains from Alzheimer's disease patients and age-matched controls. Changes in transporter density were also investigated in capillaries near plaques and on the mRNA level. P-glycoprotein density varied with brain region and matter, whereas ABCG2 density varied with brain matter. In temporal cortex, P-glycoprotein density was 53% lower in Alzheimer's disease samples than in controls, and was reduced by 35% in capillaries near plaque deposits within Alzheimer's disease samples. ABCG2 density was unaffected in Alzheimer's disease. No differences were detected at the transcript level. Our study indicates that region-specific changes in transporter densities can occur globally and locally near amyloid-beta deposits in Alzheimer's disease, providing an explanation for conflicting results in the literature. When differences in region and matter are accounted for, changes in density can be reproducibly measured using our automated method.

Mean arterial pressure change associated with cerebral blood flow in healthy older adults.

We investigate over a 12-year period the association between regional cerebral blood flow (CBF) and cardiovascular risk factors in a prospective cohort of healthy older adults (81.96 ± 3.82 year-old) from the Cognitive REServe and Clinical ENDOphenotype (CRESCENDO) study. Cardiovascular risk factors were measured over 12 years, and gray matter CBF was measured at the end of the study from high-resolution magnetic resonance imaging using arterial spin labeling. The association between cardiovascular risk factors, their long-term change, and CBF was assessed using multivariate linear regression models. Women were observed to have higher CBF than men (p < 0.05). Increased mean arterial pressure (MAP) over the 12-year period was correlated with a low cerebral blood flow (p < 0.05, R(2) = 0.21), whereas no association was detected between CBF and MAP at the time of imaging. High levels of glycemia tended to be associated with low cerebral blood flow values (p < 0.05). Age, alcohol consumption, smoking status, body mass index, history of cardiovascular disease, and hypertension were not associated with CBF. Our main result suggests that change in MAP is the most significant predictor of future CBF in older adults.

Immediate remote ischemic postconditioning after hypoxia ischemia in piglets protects cerebral white matter but not grey matter.

Remote ischemic postconditioning (RIPostC) is a promising therapeutic intervention whereby brief episodes of ischemia/reperfusion of one organ (limb) mitigate damage in another organ (brain) that has experienced severe hypoxia-ischemia. Our aim was to assess whether RIPostC is protective following cerebral hypoxia-ischemia in a piglet model of neonatal encephalopathy (NE) using magnetic resonance spectroscopy (MRS) biomarkers and immunohistochemistry. After hypoxia-ischemia (HI), 16 Large White female newborn piglets were randomized to: (i) no intervention (n = 8); (ii) RIPostC - with four, 10-min cycles of bilateral lower limb ischemia/reperfusion immediately after HI (n = 8). RIPostC reduced the hypoxic-ischemic-induced increase in white matter proton MRS lactate/N acetyl aspartate (p = 0.005) and increased whole brain phosphorus-31 MRS ATP (p = 0.039) over the 48 h after HI. Cell death was reduced with RIPostC in the periventricular white matter (p = 0.03), internal capsule (p = 0.002) and corpus callosum (p = 0.021); there was reduced microglial activation in corpus callosum (p = 0.001) and more surviving oligodendrocytes in corpus callosum (p = 0.029) and periventricular white matter (p = 0.001). Changes in gene expression were detected in the white matter at 48 h, including KATP channel and endothelin A receptor. Immediate RIPostC is a potentially safe and promising brain protective therapy for babies with NE with protection in white but not grey matter.

Perfusion shift from white to gray matter may account for processing speed deficits in schizophrenia.

Reduced speed of cerebral information processing is a cognitive deficit associated with schizophrenia. Normal information processing speed (PS) requires intact white matter (WM) physiology to support information transfer. In a cohort of 107 subjects (47/60 patients/controls), we demonstrate that PS deficits in schizophrenia patients are explained by reduced WM integrity, which is measured using diffusion tensor imaging, mediated by the mismatch in WM/gray matter blood perfusion, and measured using arterial spin labeling. Our findings are specific to PS, and testing this hypothesis for patient-control differences in working memory produces no explanation. We demonstrate that PS deficits in schizophrenia can be explained by neurophysiological alterations in cerebral WM. Whether the disproportionately low WM integrity in schizophrenia is due to illness or secondary due to this disorder deserves further examination.

Cerebral oxygenation and optimal vascular brain organization.

The cerebral vascular network has evolved in such a way so as to minimize transport time and energy expenditure. This is accomplished by a subtle combination of the optimal arrangement of arteries, arterioles and capillaries and the transport mechanisms of convection and diffusion. Elucidating the interaction between cerebral vascular architectonics and the latter physical mechanisms can catalyse progress in treating cerebral pathologies such as stroke, brain tumours, dementia and targeted drug delivery. Here, we show that brain microvascular organization is predicated on commensurate intracapillary oxygen convection and parenchymal diffusion times. Cross-species grey matter results for the rat, cat, rabbit and human reveal very good correlation between the cerebral capillary and tissue mean axial oxygen convective and diffusion time intervals. These findings agree with the constructal principle.

Transport of treosulfan and temozolomide across an in-vitro blood-brain barrier model.

In vitro, treosulfan (TREO) has shown high effectiveness against malignant gliomas. However, a first clinical trial for newly diagnosed glioblastoma did not show any positive effect. Even though dosing and timing might have been the reasons for this failure, it might also be that TREO does not reach the brain in sufficient amount. Surprisingly, there are no published data on TREO uptake into the brain of patients, despite extensive research on this compound. An in-vitro blood-brain barrier (BBB) model consisting of primary porcine brain capillary endothelial cells was used to determine the transport of TREO across the cell monolayer. Temozolomide (TMZ), the most widely used cytotoxic drug for malignant gliomas, served as a reference. An HPLC-ESI-MS/MS procedure was developed to detect TREO and TMZ in cell culture medium. Parallel to the experimental approach, the permeability of TREO and the reference substance across the in-vitro BBB was estimated on the basis of their physicochemical properties. The detection limit was 30 nmol/l for TREO and 10 nmol/l for TMZ. Drug transport was measured in two directions: influx, apical-to-basolateral (A-to-B), and efflux, basolateral-to-apical (B-to-A). For TREO, the A-to-B permeability was lower (1.6%) than the B-to-A permeability (3.0%). This was in contrast to TMZ, which had higher A-to-B (13.1%) than B-to-A (7.2%) permeability values. The in-vitro BBB model applied simulated the human BBB properly for TMZ. It is, therefore, reasonable to assume that the values for TREO are also meaningful. Considering the lack of noninvasive, significant alternative methods to study transport across the BBB, the porcine brain capillary endothelial cell model was efficient to collect first data for TREO that explain the disappointing clinical results for this drug against cerebral tumors.

Radiation-induced endothelial cell loss and reduction of the relative magnitude of the blood flow in the rat spinal cord.

The main purpose of the present study was to examine the time-dependent alterations in the endothelial cell density that occur in the first 180 days after irradiation of the spinal cord and the functional role of these alterations in the spinal cord blood flow. An irradiated cervical spinal cord rat model (C2-T2 segment) was generated using a (60)Co irradiator to deliver 30 Gy. A significant loss of forelimb motor function was observed 180 days post-irradiation. The number of neurons in the anterior horn of the spinal cord began to decrease significantly 3 days post-irradiation compared with normal controls, reaching the lowest number at 90 days post-irradiation. A significant reduction in the endothelial cell density was observed from 14 days post-irradiation in the white matter and from 3 days post-irradiation in the gray matter. The lowest endothelial cell density was reached at 30 days post-irradiation in the white matter and at 60 days post-irradiation in the gray matter. A significant reduction in the microvessel density was observed from 3 days post-irradiation in both the white matter and the gray matter. The lowest microvessel density was reached at 90 days post-irradiation in both the white matter and the gray matter. A significant reduction in the relative magnitude of spinal cord blood flow was observed from 21 days post-irradiation. The lowest relative magnitude of spinal cord blood flow was reached at 90 days post-irradiation. We did not find any evidence of demyelination. The results revealed that a single 30-Gy irradiation dose resulted in impaired forelimb motor function, a decreased number of neurons, and reduced endothelial cell density, microvessel density and relative magnitude of spinal cord blood flow. However, a 30-Gy single-dose irradiation was not sufficient to induce demyelination in the rat spinal cord.

Intravascular contrast agent T2* relaxivity in brain tissue.

Dynamic susceptibility-weighted contrast-enhanced (DSC) MRI perfusion measurements depend on estimating intravascular contrast agent (CA) concentrations (C) from signal intensity changes in T2*-weighted images after bolus injection. Generally, linearity is assumed between relaxation and C, but previous studies have shown that compartmentalization of CA and secondary magnetic field perturbations generate deviations from linearity. Physical phantoms using bulk blood have been used to empirically determine the relationship between relaxation rate and C in large vessels. However, the relaxivity of CA in the microvasculature is not easily measured since constructing appropriate phantoms is difficult. Instead, theoretical relaxivity models have been developed. In this study, we empirically tested a non-linear expression based on static dephasing regime (SDR) and linear approximation. Signal-time curves in white (WM) and grey matter (GM) were converted to concentration time curves (CTCs) using both expressions. Parameters for both linear and non-linear formulations were adjusted to give a best agreement between cerebral blood volumes (CBV) calculated from WM and arterial CTCs in a group of normal subjects scanned at 3T. Optimized parameters were used to calculate blood volume in WM and GM in healthy subjects scanned at 3T and in meningioma patients scanned at 1.5T. Results from this study showed that a non-linear SDR formulation gave an acceptable functional form for tissue relaxivity, giving reliable CBV estimates at different field strengths and echo times.