Diagnostic yield of computed tomography angiography in patients presenting with spontaneous intracerebral hemorrhage.

BACKGROUND: Hypertension and cerebral amyloid angiopathy are the most common causes of spontaneous intracerebral hemorrhage (ICH); however, these conditions do not imply macrovascular pathology. Still, computed tomography (CT) angiography (CTA) is often performed in the acute phase in patients with ICH. PURPOSE: To assess the diagnostic yield of CTA in the detection of secondary etiology in consecutive patients with spontaneous ICH. MATERIAL AND METHODS: We performed a retrospective analysis of data from a prospective single-center cohort study of 203 patients presenting with spontaneous ICH admitted to a comprehensive stroke center over a two-year period (15 October 2016 to 15 October 2018). The underlying vascular pathology was assessed using CTA. RESULTS: CTA was performed in addition to non-contrast CT and/or magnetic resonance imaging (MRI). Vascular pathology was found in 11 of 203 (5.4%) patients and included arteriovenous malformations (n=4), aneurysms (n=4), vasospasms (n=1), cerebral venous thrombosis (n=1), and other vascular malformations (n=1). In eight cases, the finding was deemed symptomatic. Patients with vascular pathology on CTA more often had lobar located hemorrhages (63.6% vs. 36.4%, P = 0.049). Numerically, patients with vascular pathology were younger, had smaller hematoma volumes, and lower mortality. CONCLUSION: Underlying macrovascular pathology was detected on CTA in only approximately 1 of 20 consecutive patients with ICH. The patients with vascular pathology more often had a hemorrhage with a lobar location and young age and the present study is supportive of a risk-based stratification approach in performing CTA.

Exploring vascular contributions to cognitive impairment and dementia (ENIGMA): protocol for a prospective observational study.

BACKGROUND: Post-stroke cognitive impairment (PSCI) is common. However, the underlying pathophysiology remains largely unknown. Understanding the role of microvascular changes and finding markers that can predict PSCI, could be a first step towards better screening and management of PSCI. Capillary dysfunction is a pathological feature of cerebral small vessel disease and may play a role in the mechanisms underlying PSCI. Extracellular vesicles (EVs) are secreted from cells and may act as disease biomarkers. We aim to investigate the role of capillary dysfunction in PSCI and the associations between EV characteristics and cognitive function one year after acute ischemic stroke (AIS) and transient ischemic attack (TIA). METHODS: The ENIGMA study is a single-centre prospective clinical observational study conducted at Aarhus University Hospital, Denmark. Consecutive patients with AIS and TIA are included and followed for one year with follow-up visits at three and 12 months. An MRI is performed at 24 h and 12 months follow-up. EV characteristics will be characterised from blood samples drawn at 24 h and three months follow-up. Cognitive function is assessed three and 12 months after AIS and TIA using the Repeatable Battery for the Assessment of Neuropsychological Status. DISCUSSION: Using novel imaging and molecular biological techniques the ENIGMA study will provide new knowledge about the vascular contributions to cognitive decline and dementia. TRIAL REGISTRATION: The study is retrospectively registered as an ongoing observational study at ClinicalTrials.gov with the identifier NCT06257823.

Circulating DNA and frequency of colorectal cancer brain metastases in a presumed high-risk group.

This explorative prospective observational pilot study investigated if suggested risk factors, rectal cancer and lung metastases, could add to a relevant detection rate of asymptomatic brain metastases (BM) from colorectal cancer (CRC). Secondary, prognostic biological aspects were investigated by translational analysis of plasma samples. The study enrolled patients with rectal cancer and lung metastases. At inclusion, patients underwent a standard MRI scan of the brain. Cell-free DNA (cfDNA) level was measured by a direct fluorescence assay (DFA), and circulating tumor DNA (ctDNA) by ddPCR. BM was detected in one of twenty-nine included patients. Patients had higher cfDNA levels than healthy subjects (p < 0.01). Patients with the primary tumor in situ had higher cfDNA levels than those with resected primary tumor (p < 0.01). Patients with liver involvement had higher cfDNA levels (p = 0.12) and circulating tumor DNA levels (p = 0.01) than those without liver involvement. In conclusion, the modest incidence of BM does not justify routine MRI of the brain in this selected population. cfDNA by DFA could be a valuable tool when planning treatment and follow-up for CRC patients. Future studies should focus on identifying further characteristics and biomarkers associated with a high risk of BM, enhancing the possibility for early intervention.

Asymmetric amyloid deposition in preclinical Alzheimer's disease: A PET study.

Introduction: The typical spatial pattern of amyloid-ß (Aß) in diagnosed Alzheimer's disease (AD) is that of a symmetrical hemispheric distribution. However, Aß may be asymmetrically distributed in early stages of AD. Aß distribution on PET has previously been explored in MCI and AD, but it has yet to be directly investigated in preclinical AD (pAD). We examined how Aß was distributed in individuals with pAD and MCI using 11C-Pittsburgh Compound B (PiB) PET. Methods: In this PET study, 79 subjects were retrospectively enrolled, including 34 controls, 24 pAD, and 21 MCI. All subjects underwent APOE genotyping, 11C-PiB PET, MRI, and cognitive testing. We explored differences in Aß load, Aß lateralisation, and Aß distribution, as well as associations between Aß distribution and cognition. Results: The Aß asymmetry index (AI) differed between groups, with pAD having the highest Aß AI as compared to both controls and MCI. There was no clear Aß lateralisation in pAD, but there was a non-significant trend towards Aß being more left-lateralised in MCI. There were no correlations between the cognitive scores and Aß AI or Aß lateralisation in pAD or MCI. Conclusion: The distribution of Aß is most asymmetrical in pAD, as Aß first starts accumulating, and it then becomes less asymmetrical in MCI, when Aß has spread further, suggesting that more pronounced asymmetrical Aß distribution may be a distinguishing factor in pAD. Longitudinal studies examining the distribution of Aß across the AD continuum are needed.

Diffusional kurtosis imaging as a possible prognostic marker of cervical incomplete spinal cord injury outcome: a prospective pilot study.

BACKGROUND: Spinal cord injury (SCI) is associated with substantial chronic morbidity and mortality. Routine imaging techniques such as T1- and T2-weighted magnetic resonance imaging (MRI) are not effective in predicting neurological deficiency grade or outcome. Diffusional kurtosis imaging (DKI) is an MR imaging technique that provides microstructural information about biological tissue. There are no longitudinal prospective studies assessing DKI metrics in acute traumatic SCI. Therefore, the purpose of this study was to establish a DKI protocol for acute SCI and correlate the DKI metrics to the functional neurological outcome of the patients. METHODS: Eight consecutive SCI patients referred to our institution with cervical SCI were included in the study. An acute diagnostic MRI scan was supplemented with a novel fast, mean kurtosis DKI protocol, which describes the average deviation from Gaussian diffusional along nine different directions. Mean kurtosis values were measured at the injury site and normalized to the mean kurtosis values of a non-injured site. At discharge form specialized rehabilitation, patients were evaluated using the Spinal Cord Independence Measure-III (SCIM-III). The DKI metrics and SCIM-III were analysed using Spearman's rank correlation. RESULTS: This pilot study found a significant correlation between decreasing mean kurtosis values at the injury site of the spinal cord and higher grade of disability measured by the SCIM-III (p = 0.002). CONCLUSION: This pilot study found that DKI may be a valuable tool as a prognostic marker in the acute phase of SCI.

Telehealth in Multidisciplinary Target Delineation for Radiotherapy During the COVID-19 Pandemic. A Review and a Case.

Like all other medical specialties, radiotherapy has been deeply influenced by the COVID-19 pandemic. The pandemic has had severe influence on the entire patient trajectory in oncology, from diagnosis to treatment and follow-up. Many examples of how to deal with patient and staff safety, shortness of staff and other resources and the quest to continue high-quality, evidence-based treatment have been presented. The use of telemedicine and telehealth is frequently presented as a part of the solution to overcome these challenges. Some of the available presented solutions will only apply in an acute, local setting, whereas others might inspire the community to improve quality and cost-effectiveness of radiotherapy as well as knowledge sharing in the future. Some of the unresolved issues in many of the available technical solutions are related to data security and public regulation, for example, GDPR (General Data Protection Regulation) in the EU and HIPAA compliance (Health Insurance Portability and Accountability Act) in the USA. Using a solution that involves a supplier's server in a non-EU country is problematic within the EU. In this paper we shortly review the influence of COVID-19 on radiotherapy. We describe some of the possible solutions for telehealth in target delineation - a crucial part of high-quality radiotherapy, which often requires multidisciplinary effort, hands-on corporation, and high-quality multimodal imaging. Hereafter, our own technical solution will be presented as a case.

Cortical Atrophy, White Matter Lesions, and Bulb Configuration in Patients with Idiopathic Olfactory Loss and Other Causes of Olfactory Loss.

INTRODUCTION: While magnetic resonance imaging (MRI) is not included in the current guidelines for diagnosing olfactory disorders in the most recent position paper on olfactory dysfunction, both 1.5T and 3T MRI are commonly used in the diagnostic workup of many patients with olfactory loss. Often, MRI is used to rule out intracranial tumours, but other useful information may be obtained from MRI scans in these patients. The potential of MRI in olfactory loss depends on sufficient knowledge of structural changes in different aetiologies of olfactory loss. We present common clinical MRI findings in olfactory loss and evaluate the usefulness of structural integrity scores in differentiating between aetiologies. METHODS: In this study, we investigated if white matter hyperintensities (WMHs, measured by Fazekas score), global cortical atrophy (GCA), and medial temporal lobe atrophy (MTA) are more common in patients with idiopathic olfactory loss than in patients with acquired olfactory loss due to other aetiologies. Furthermore, we compared olfactory bulb (OB) configurations in different olfactory loss aetiologies. RESULTS: In 88 patients with olfactory loss, WMHs, GCA, and MTA were not more significant findings on MRI in idiopathic olfactory loss (n = 51) compared with other causes of acquired olfactory loss (Fazekas score p = 0.2977; GCA score p = 0.6748; MTA score p = 0.7851). Bulb configurations differed in patients suffering from post-traumatic olfactory loss and may aid in identifying the underlying aetiology in patients where trauma is among the suspected causes of olfactory loss. CONCLUSION: We recommend that structural MRI with an OB sequence is included in the diagnostic evaluation of olfactory loss with suspected congenital and post-traumatic aetiology and should be considered in idiopathic olfactory loss with suspected central aetiology (e.g., tumour).

Cerebral hemodynamics and capillary dysfunction in late-onset major depressive disorder.

In major depressive disorder (MDD), perfusion changes in cortico-limbic pathways are interpreted as altered neuronal activity, but they could also signify changes in neurovascular coupling due to altered capillary function. To examine capillary function in late-onset MDD, 22 patients and 22 age- and gender-matched controls underwent perfusion MRI. We measured normalized cerebral blood flow (nCBF), cerebral blood volume (nCBV), and relative transit-time heterogeneity (RTH). Resulting brain oxygenation was estimated in terms of oxygen tension and normalized metabolic rate of oxygen (nCMRO2). Patients revealed signs of capillary dysfunction (elevated RTH) in the anterior prefrontal cortex and ventral anterior cingulate cortex bilaterally and in the left insulate cortex compared to controls, bilateral hypometabolism (parallel reductions of nCBV, nCBF, and CMRO2) but preserved capillary function in the subthalamic nucleus and globus pallidus bilaterally, and hyperactivity with preserved capillary function (increased nCBF) in the cerebellum and brainstem. Our data support that perfusion changes in deep nuclei and cerebellum reflect abnormally low and high activity, respectively, in MDD patients, but suggest that microvascular pathology affects neurovascular coupling in ventral circuits. We speculate that microvascular pathology is important for our understanding of etiology of late-onset MDD as well as infererences about resulting brain activity changes.

Help-seeking behaviour and subsequent patient and system delays in stroke.

OBJECTIVES: Prehospital delay is the main reason why only a limited number of stroke patients receive reperfusion therapy. We aimed to investigate help-seeking behaviour in patients and bystanders after onset of stroke and subsequent patient and system delay. MATERIALS & METHODS: We conducted a cross-sectional study of 332 patients with stroke. We performed structured interviews and used data from the medical records and the Danish Stroke Registry. Primary outcomes were patient delay and system delay. RESULTS: The median patient delay was 280 min, and the median system delay was 97 min. For a patient delay of <3 h, an additional non-significant system delay of median 30 min was seen for a first contact to a general practitioner (GP), and an additional significant delay of median 490 min was seen for the small group of patients with a first contact to 'other' healthcare professionals compared to the Emergency Medical Services (EMS). For a patient delay of more than 3 h, an additional system delay of median 78 min was found when the first contact was directed to the out-of-hours primary care (OOH-PC). A total of 17% of patients were admitted to another hospital or department before arrival at the stroke centre; this resulted in a substantially prolonged system delay of a median of 431 min. CONCLUSIONS: Patient delay remains the main reason for delayed arrival at the stroke centre. Appropriate help-seeking behaviour and efficient pre-hospital triage are essential for reducing the prehospital delay and increasing the proportion of patients receiving reperfusion therapy.

[Screening for cerebral arteriovenous malformations in patients with hereditary haemorrhagic telangiectasia].

Patients with hereditary haemorrhagic telangiectasia (HHT) are known to suffer from cerebral arteriovenous malformations (CAVMs). In this review, we explore existing literature for bleeding risk, interventional therapy and neuroradiological features in HHT-related CAVMs. Studies estimate the annual intracerebral haemorrhage rate of CAVMs in HHT patients to be 0.667-1.014%. The clinician must balance bleeding risk and the non-negligible procedural risks of interventional therapy. We recommend, in agreement with European guidelines, that screening of asymptomatic HHT patients should only be carried out after careful information.

Long-term outcomes of endovascular simple coiling versus neurosurgical clipping of unruptured intracranial aneurysms: A systematic review and meta-analysis.

BACKGROUND: Previous studies on ruptured intracranial aneurysms have shown favourable long-term outcomes of patients undergoing endovascular coiling compared to neurosurgical clipping. We aimed to evaluate if these results also apply to patients with unruptured intracranial aneurysms (UIAs). METHODS: Embase, PubMed, and Cochrane Library were systematically searched for all studies reporting long-term (≥3 years) follow-up after coiling or clipping of UIAs. Thirteen studies involving 16,622 coiled patients and 13,606 clipped patients were included. Short-term outcome was defined as death ≤30 days after treatment. Long-term outcomes (>3 years) included all-cause mortality, morbidity (defined as modified Rankin Score 3-5 or Glasgow Outcome Score 2-3), cerebrovascular accident, intracerebral haemorrhage, additional repairs, and lost to follow-up. We calculated relative risk (RR), incidence and mortality rates (IR and MR), together with incidence and mortality rate ratio (IRR and MRR). RESULTS: Patients treated with simple coiling had lower short-term mortality than clipped patients (RR = 0.62 (95%CI 0.42-0.91)), but this difference disappeared after long-term follow-up ((MRR) = 0.89 (95%CI: 0.78-1.02). Coiled patients had higher retreatment rates than clipped patients (IRR = 1.70 (95%CI 1.50-1.93)). CONCLUSIONS: This systematic review and meta-analysis reports benefits and drawbacks of simple coiling versus neurosurgical clipping of UIAs. Future studies with longer follow-up time should account for differences in coiling techniques and confounding factors such as size and location of UIAs.

Impaired perfusion and capillary dysfunction in prodromal Alzheimer's disease.

INTRODUCTION: Cardiovascular disease increases the risk of developing Alzheimer's disease (AD), and growing evidence suggests an involvement of cerebrovascular pathology in AD. Capillary dysfunction, a condition in which capillary flow disturbances rather than arterial blood supply limit brain oxygen extraction, could represent an overlooked vascular contributor to neurodegeneration. We examined whether cortical capillary transit-time heterogeneity (CTH), an index of capillary dysfunction, is elevated in amyloid-positive patients with mild cognitive impairment (prodromal AD [pAD]). METHODS: We performed structural and perfusion weighted MRI in 22 pAD patients and 21 healthy controls. RESULTS: We found hypoperfusion, reduced blood volume, and elevated CTH in the parietal and frontal cortices of pAD-patients compared to controls, while only the precuneus showed focal cortical atrophy. DISCUSSION: We propose that microvascular flow disturbances antedate cortical atrophy and may limit local tissue oxygenation in pAD. We speculate that capillary dysfunction contributes to the development of neurodegeneration in AD.

Neuropsychological Status and Structural Brain Imaging in Adults With Simple Congenital Heart Defects Closed in Childhood.

Background Neurodevelopmental impairments are common in survivors of complex congenital heart defects (CHD). We report neuropsychological and brain imaging assessments in adults operated for isolated septal defects. Methods and Results Patients (mean age 25.6 yrs) who underwent childhood surgery for isolated atrial septal defect (n=34) or ventricular septal defect (n=32), and healthy matched peers (n=40), underwent a standard battery of neuropsychological tests and a 3.0T brain magnetic resonance imaging scan. Patient intelligence was affected with lower scores on Full-Scale intelligence quotient (P<0.001), Verbal Comprehension (P<0.001), Perceptual Reasoning (P=0.007), and Working Memory (P<0.001) compared with controls. Also, the CHD group had poorer visuospatial abilities (Immediate Recall, P=0.033; Delayed Recall, P=0.018), verbal memory (Trial 1, P=0.015; Total Learning, P<0.001; Delayed Recall, P=0.007), executive function (Executive Composite Score, P<0.001), and social recognition (Reading the Mind in the Eyes Test, P=0.002) compared with controls. Self-reported levels of executive dysfunction, attention deficits and hyperactivity behavior, and social cognition dysfunction were higher in the CHD group compared with population means and controls. We found similar global and regional morphometric brain volumes and a similar frequency of brain magnetic resonance imaging abnormalities in the 2 groups. The CHD group had a high occurrence of psychiatric disease and a larger need for special teaching during school age. Conclusions Children operated for simple CHD demonstrate poorer neurodevelopmental outcomes in adulthood when compared with healthy controls and expected population means. REGISTRATION URL: https://www.clini​caltr​ials.gov. Unique identifier: NCT03871881.

Abnormal Amyloid Load in Mild Cognitive Impairment: The Effect of Reducing the PiB-PET Threshold.

BACKGROUND AND PURPOSE: In vivo detection of ß-amyloid (Aß) plaques in Alzheimer's disease (AD) is now possible with 11 C-PiB positron emission tomography (PET). Conventionally, a cortical:cerebellar PiB uptake ratio threshold of 1.4-1.5 has been used to categorize at-risk subjects as "amyloid-positive" and "amyloid-negative." It has been suggested that this threshold is too conservative and may miss early amyloid pathology. We investigated the relationship between conventional and lower baseline 11 C-PiB PET thresholds for raised amyloid load and the subsequent clinical and radiological progression of mild cognitive impairment (MCI) cases longitudinally. METHODS: We serially determined the cortical amyloid load with 11 C-PiB PET of 44 MCI subjects over 2 years and compared findings with those for 12 healthy controls (HC) and 5 AD cases. RESULTS: Twenty-four subjects were classified as normal at baseline with mean cortical PiB standard uptake value ratios (SUVR) between 1.2 and 1.5. Their cognitive status remained stable over time. Three of these cases increased their amyloid load above a threshold of 1.5 over 2 years. Twenty-seven "raised amyloid" MCI cases with baseline cortical SUVRs above 1.5, showed deteriorating cognition. Note that 50% of these cases converted clinically to AD during the follow-up period. CONCLUSION: Use of a PiB SUVR threshold of >1.5 for raised amyloid missed 14.3% of MCI cases who likely had Thal stage 1 or 2 pathology and showed a progressive amyloid increase over 2 years. Lowering the threshold for abnormality to 1.3 abolished all false negatives but resulted in 75% of HCs being falsely diagnosed as raised amyloid subjects.

Oxygenation differs among white matter hyperintensities, intersected fiber tracts and unaffected white matter.

White matter hyperintensities of presumed vascular origin are frequently observed on magnetic resonance imaging in normal aging. They are typically found in cerebral small vessel disease and suspected culprits in the etiology of complex age- and small vessel disease-related conditions, such as late-onset depression. White matter hyperintensities may interfere with surrounding white matter metabolic demands by disrupting fiber tract integrity. Meanwhile, risk factors for small vessel disease are thought to reduce tissue oxygenation, not only by reducing regional blood supply, but also by impairing capillary function. To address white matter oxygen supply-demand balance, we estimated voxel-wise capillary density as an index of resting white matter metabolism, and combined estimates of blood supply and capillary function to calculate white matter oxygen availability. We conducted a cross-sectional study with structural, perfusion- and diffusion-weighted magnetic resonance imaging in 21 patients with late-onset depression and 21 controls. We outlined white matter hyperintensities and used tractography to identify the tracts they intersect. Perfusion data comprised cerebral blood flow, blood volume, mean transit time and relative transit time heterogeneity-the latter a marker of capillary dysfunction. Based on these, white matter oxygenation was calculated as the steady state cerebral metabolic rate of oxygen under the assumption of normal tissue oxygen tension and vice versa. The number, volume and perfusion characteristics of white matter hyperintensities did not differ significantly between groups. Hemodynamic data showed white matter hyperintensities to have lower blood flow and blood volume, but higher relative transit time heterogeneity, than normal-appearing white matter, resulting in either reduced capillary metabolic rate of oxygen or oxygen tension. Intersected tracts showed significantly lower blood flow, blood volume and capillary metabolic rate of oxygen than normal-appearing white matter. Across groups, lower lesion oxygen tension was associated with higher lesion number and volume. Compared with normal-appearing white matter, tissue oxygenation is significantly reduced in white matter hyperintensities as well as the fiber tracts they intersect, independent of parallel late-onset depression. In white matter hyperintensities, reduced microvascular blood volume and concomitant capillary dysfunction indicate a severe oxygen supply-demand imbalance with hypoxic tissue injury. In intersected fiber tracts, parallel reductions in oxygenation and microvascular blood volume are consistent with adaptations to reduced metabolic demands. We speculate, that aging and vascular risk factors impair white matter hyperintensity perfusion and capillary function to create hypoxic tissue injury, which in turn affect the function and metabolic demands of the white matter tracts they disrupt.

Increased cortical capillary transit time heterogeneity in Alzheimer's disease: a DSC-MRI perfusion study.

Alzheimer's disease (AD) is characterized by the accumulation of hyperphosphorylated tau and neurotoxic Aß in the brain parenchyma. Hypoxia caused by microvascular changes and disturbed capillary flows could stimulate this build-up of AD-specific proteins in the brain. In this study, we compared cerebral microcirculation in a cohort of AD and mild cognitive impairment (MCI) patients with that of age-matched controls, all without a history of diabetes or of hypertension for more than 2 years, using dynamic susceptibility contrast magnetic resonance imaging (DSC-MRI). Vascular flow disturbances were quantified using a parametric model and mapped to the mid-cortical surface for group-wise statistical analysis. We found widespread hypoperfusion in patients compared with controls and identified areas of increased relative capillary transit time heterogeneity (RTH), consistent with low tissue oxygen tension. Notably, RTH was positively correlated with white matter hyperintensities and positively correlated with symptom severity in the patient cohort. These correlations extended over large parts of the temporal, parietal, and frontal cortices. The results support the hypothesis of disturbed capillary flow patterns in AD and suggest that DSC-MRI may provide imaging biomarkers of impaired cerebral microcirculation in AD.

Endovascular Treatment of Basilar Artery Thrombosis Secondary to Bilateral Vertebral Artery Dissection with Symptom Onset Following Cervical Spine Manipulation Therapy.

BACKGROUND: Vertebral artery (VA) dissection (VAD) has been described following neck injury and can be associated with stroke, but the causal association with cervical spine manipulation therapy (cSMT) is controversial. The standard treatment for VAD is antithrombotic medical therapy. To highlight the considerations of an endovascular approach to VAD, we present a critical case of bilateral VAD causing embolic occlusion of the basilar artery (BA) in a patient with symptom debut following cSMT. CASE REPORT: A 37-year-old woman presented with acute onset of neurological symptoms immediately following cSMT in a chiropractic facility. Acute magnetic resonance imaging (MRI) showed ischemic lesions in the right cerebellar hemisphere and occlusion of the cranial part of the BA. Angiography depicted bilateral VAD. Symptoms remitted after endovascular therapy, which included dilatation of the left VA and extraction of thrombus from the BA. After 6 months, the patient had minor sensory and cognitive deficits. CONCLUSIONS: In severe cases, VAD may be complicated by BA thrombosis, and this case highlights the importance of a fast diagnostic approach and advanced intravascular procedure to obtain good long-term neurological outcome. Furthermore, this case underlines the need to suspect VAD in patients presenting with neurological symptoms following cSMT.

The capillary dysfunction hypothesis of Alzheimer's disease.

It is widely accepted that hypoperfusion and changes in capillary morphology are involved in the etiopathogenesis of Alzheimer's disease (AD). This is difficult to reconcile with the hyperperfusion observed in young high-risk subjects. Differences in the way cerebral blood flow (CBF) is coupled with the local metabolic needs during different phases of the disease can explain this apparent paradox. This review describes this coupling in terms of a model of cerebral oxygen availability that takes into consideration the heterogeneity of capillary blood flow patterns. The model predicts that moderate increases in heterogeneity requires elevated CBF in order to maintain adequate oxygenation. However, with progressive increases in heterogeneity, the resulting low tissue oxygen tension will require a suppression of CBF in order to maintain tissue metabolism. The observed biphasic nature of CBF responses in preclinical AD and AD is therefore consistent with progressive disturbances of capillary flow patterns. Salient features of the model are discussed in the context of AD pathology along with potential sources of increased capillary flow heterogeneity.