Increased oxygen extraction fraction is associated with prior ischemic events in patients with carotid occlusion

Transient susceptibility imaging as a measure of hemodynamic compromise: A pilot study

BACKGROUND AND PURPOSE

The purpose of this study was to test the hypothesis that hemodynamically compromised brains exhibit transient changes in magnetic susceptibility throughout the cardiac cycle, and to model these changes using Linear System Theory to extract an index that reflects cerebrovascular reserve.

MATERIALS AND METHODS

Eleven patients with angiographically-confirmed intracranial atherosclerotic disease with >50% stenosis were imaged with susceptibility weighted, cardiac-gated single shot images of cerebral Oxygen Extraction Fraction (OEF) at different timepoints of the cardiac cycle. Cardiac gating of the OEF acquisition allowed interrogation of oxygenated blood and the detection of changes throughout the cardiac cycle. Independent component analysis (ICA) of raw k-space data across the cardiac phase allowed MRI signal decomposition into dynamic and static components for image reconstruction. An asymmetry index score of the resultant parametric images were compared to test the hypothesis that variation in hemoglobin-induced susceptibility across the cardiac cycle indeed reflects pathophysiology of cerebrovascular disease. A mathematical model was derived to parameterize physiologic changes induced by the presence of a hemodynamically significant stenosis in the brain as a tissue impulse response parameter (β).

RESULTS

OEF was elevated in the affected hemisphere (50.34 ± 12.13% vs 46.93 ± 12.34%), but failed to reach statistical significance (p < .0796). Transient changes in the OEF signal showed significant distinction between healthy and compromised tissue (0.56 ± 0.067 vs 0.44 ± 0.067, p < .019)). The derived tissue impulse response function was found to be significant as well (10.72 ± 3.48 10-3 ms-1, 9.69 ± 3.51 10-3 ms-1; p < .037).

CONCLUSION

In this pilot study, we found transient OEF and β to be significant predictors of hemispheric compromise.

Measurement of Cerebral Oxygen Extraction Fraction Using Quantitative BOLD Approach: A Review

Quantification of brain oxygenation and metabolism, both of which are indicators of the level of brain activity, plays a vital role in understanding the cerebral perfusion and the pathophysiology of brain disorders. Magnetic resonance imaging (MRI), a widely used clinical imaging technique, which is very sensitive to magnetic susceptibility, has the possibility of substituting positron emission tomography (PET) in measuring oxygen metabolism. This review mainly focuses on the quantitative blood oxygenation level-dependent (qBOLD) method for the evaluation of oxygen extraction fraction (OEF) in the brain. Here, we review the theoretic basis of qBOLD, as well as existing acquisition and quantification methods. Some published clinical studies are also presented, and the pros and cons of qBOLD method are discussed as well.

Application of Cluster Analysis of Time Evolution for Magnetic Resonance Imaging -Derived Oxygen Extraction Fraction Mapping: A Promising Strategy for the Genetic Profile Prediction and Grading of Glioma

Background: The intratumoral heterogeneity of oxygen metabolism and angiogenesis are core hallmarks of glioma, unveiling that genetic aberrations associated with magnetic resonance imaging (MRI) phenotypes may aid in the diagnosis and treatment of glioma.

Objective: To explore the predictability of MRI-based oxygen extraction fraction (OEF) mapping using cluster analysis of time evolution (CAT) for genetic profiling and glioma grading.

Methods: Ninety-one patients with histopathologically confirmed glioma were examined with CAT for quantitative susceptibility mapping and quantitative blood oxygen level–dependent magnitude-based OEF mapping and dynamic contrast-enhanced (DCE) MRI. Imaging biomarkers, including oxygen metabolism (OEF) and angiogenesis [volume transfer constant, cerebral blood volume (CBV), and cerebral blood flow], were investigated to predict IDH mutation, O⁶-methylguanine-DNA-methyltransferase (MGMT) promoter methylation status, receptor tyrosine kinase (RTK) subgroup, and differentiation of glioblastoma (GBM) vs. lower-grade glioma (LGG). The corresponding DNA sequencing was also obtained. Results were compared with DCE-MRI using receiver operating characteristic (ROC) analysis.

Results: IDH1-mutated LGGs exhibited significantly lower OEF and hypoperfusion than IDH wild-type tumors (all p < 0.01). OEF and perfusion metrics showed a tendency toward higher values in MGMT unmethylated GBM, but only OEF retained significance (p = 0.01). Relative prevalence of RTK alterations was associated with increased OEF (p = 0.003) and perfusion values (p < 0.05). ROC analysis suggested OEF achieved best performance for IDH mutation detection [area under the curve (AUC) = 0.828]. None of the investigated parameters enabled prediction of MGMT status except OEF with a moderate AUC of 0.784. Predictive value for RTK subgroup was acceptable by using OEF (AUC = 0.764) and CBV (AUC = 0.754). OEF and perfusion metrics demonstrated excellent performance in glioma grading. Moreover, mutational landscape revealed hypoxia or angiogenesis-relevant gene signatures were associated with specific imaging phenotypes.

Conclusion: CAT for MRI-based OEF mapping is a promising technology for oxygen measurement and along with perfusion MRI can predict genetic profiles and tumor grade in a non-invasive and clinically relevant manner.

Clinical Impact: Physiological imaging provides an in vivo portrait of genetic alterations in glioma and offers a potential strategy for non-invasively selecting patients for individualized therapies.

Cerebral oxygen extraction fraction (OEF): Comparison of challenge-free gradient echo QSM+qBOLD (QQ) with 15O PET in healthy adults

We aimed to validate oxygen extraction fraction (OEF) estimations by quantitative susceptibility mapping plus quantitative blood oxygen-level dependence (QSM+qBOLD, or QQ) using ¹⁵O-PET. In ten healthy adult brains, PET and MRI were acquired simultaneously on a PET/MR scanner. PET was acquired using C[¹⁵O], O[¹⁵O], and H₂[¹⁵O]. Image-derived arterial input functions and standard models of oxygen metabolism provided quantification of PET. MRI included T1-weighted imaging, time-of-flight angiography, and multi-echo gradient-echo imaging that was processed for QQ. Region of interest (ROI) analyses compared PET OEF and QQ OEF. In ROI analyses, the averaged OEF differences between PET and QQ were generally small and statistically insignificant. For whole brains, the average and standard deviation of OEF was 32.8 ± 6.7% for PET; OEF was 34.2 ± 2.6% for QQ. Bland-Altman plots quantified agreement between PET OEF and QQ OEF. The interval between the 95% limits of agreement was 16.9 ± 4.0% for whole brains. Our validation study suggests that respiratory challenge-free QQ-OEF mapping may be useful for non-invasive clinical assessment of regional OEF impairment.

Magnetic resonance markers of bilateral neuronal metabolic dysfunction in patients with unilateral internal carotid artery occlusion

OBJECTIVES

To evaluate cerebral hemodynamic, metabolic and anatomic changes occurring in patients with unilateral occlusion of the internal carotid artery (ICA).

MATERIALS AND METHODS

Twenty-two patients with unilateral occlusion of ICA and twenty age and sex matched healthy subjects were included in the study. Single voxel proton magnetic resonance spectroscopy (¹H-MRS) of the centrum semiovale, semi-automated hippocampal volumetry in T1-weighted scans and transcranial Doppler examination (TCD) with calculation of Breath Holding Index (BHI) were performed in both groups. Metabolic, anatomic, and hemodynamic features were compared between the two groups.

RESULTS

The N-acetylaspartate (NAA)/choline (Cho) ratio was significantly lower in both hemispheres of enrolled patients compared to controls (p = 0.005 for the side with occlusion, p = 0.04 for the side without occlusion). The hippocampus volume was significantly reduced bilaterally in patients compared to healthy subjects (p = 0.049). A statistically significant difference in BHI values was observed between the side with occlusion and without occlusion (p = 0.037) of the patients, as well as between BHI values of the side with occlusion and healthy volunteers (p = 0.014).

DISCUSSION

Patients with unilateral ICA occlusion have reduced NAA/Cho ratio in the white matter of both hemispheres and have bilateral atrophy of hippocampus. The alteration of hemodynamics alone cannot explain these changes.

The Value of Susceptibility-Weighted Imaging (SWI) in Evaluating the Ischemic Penumbra of Patients with Acute Cerebral Ischemic Stroke

BACKGROUND

The aim of this study was to discuss the value of susceptibility-weighted imaging (SWI) in evaluating the ischemic penumbra of patients with acute cerebral ischemic stroke.

METHODS

Data were collected from 52 patients with acute cerebral ischemic stroke upon clinical diagnosis and routine examinations of magnetic resonance imaging (MRI), including SWI, diffusion-weighted imaging (DWI), and perfusion-weighted imaging (PWI) within 72 hours after onset in this retrospective study. The methods also included fusing the DWI and SWI images and calculating the volume of anomaly extension of DWI and PWI-MTT (mean transit time) using semi-automatic analysis software. The SWI-DWI and PWI-DWI mismatches were interpreted, and the statistical analysis was completed.

RESULTS

The two physicians found that the ischemic penumbra consistency is high throughout the SWI-DWI and PWI-DWI mismatches, without a significant difference (P > 0.05).

CONCLUSION

SWI-DWI mismatch can prevent the injection of contrast agents and make an accurate diagnosis of acute stroke ischemic penumbra, which helps guide the selection of the clinical therapeutic plan.

Combined Endovascular and Surgical Treatment of Chronic Carotid Artery Occlusion: Hybrid Operation

Objectives

The optimal treatment choice of chronic carotid artery occlusion (CAO) remains inconclusive. This study was aimed at exploring the safety and effectiveness of hybrid surgery in the treatment of CAO and at determining predictors for successful recanalization.

Methods

In this study, we enrolled 37 patients with CAO who underwent hybrid surgical treatment during the period 2016–2018. We extracted and analyzed patients' demographic data, disease characteristics, surgical success rates, perioperative complications, and prognosis.

Results

A total of 37 patients with symptomatic CAO underwent hybrid surgical treatment. Thirty cases (81.1%) were successfully recanalized, while seven were not. Blood reflux after carotid endarterectomy occurred in 18 patients (60%) of the success group and 1 (14.3%) of the failure group (OR, 9.0; 95% CI, 0.95-54.5; P = 0.042). The rate of distal ICA reconstruction below the clinoid segment was 20 (66.7%) in the success group and 1 (14.3%) in the failure group (OR, 12.0; 95% CI, 1.3-113.7; P = 0.029). In patients with successful recanalization, no ischemic events occurred after surgery and during follow-up, but restenosis of >50% was found in one case. In the failure group, two patients experienced recurrent ischemic events during follow-up. Perfusion imaging in successful recanalization cases is significantly improved, preoperative I/C ratio was 1.44 (IQR 1.27-1.55), and postoperative 1.12 (IQR 1.05-1.23). National Institutes of Health Stroke Scale (NIHSS) score of successful recanalization cases was 5.35 (2.26) before surgery and 2.03 (1.40) at 6 months (P < 0.01).

Conclusion

Hybrid surgery might be a safe and effective way to treat CAO. Distal internal carotid artery reconstruction to below the clinoid segment and blood reflux after carotid endarterectomy are predictors of successful recanalization.

Cerebral oxygen extraction fraction: Comparison of dual-gas challenge calibrated BOLD with CBF and challenge-free gradient echo QSM+qBOLD

PURPOSE

To compare cortical gray matter oxygen extraction fraction (OEF) estimated from 2 MRI methods: (1) the quantitative susceptibility mapping (QSM) plus quantitative blood oxygen level dependent imaging (qBOLD) (QSM+qBOLD or QQ), and (2) the dual-gas calibrated-BOLD (DGCB) in healthy subjects; and to investigate the validity of iso-cerebral metabolic rate of oxygen consumption assumption during hypercapnia using QQ.

METHODS

In 10 healthy subjects, 3 tesla MRI including a multi-echo gradient echo sequence at baseline and hypercapnia for QQ, as well as an EPI dual-echo pseudo-continuous arterial spin labeling for DGCB, were performed under a hypercapnic and a hyperoxic condition. OEFs from QQ and DGCB were compared using region of interest analysis and paired t test. For QQ, cerebral metabolic rate of oxygen consumption = cerebral blood flow*OEF*arterial oxygen content was generated for both baseline and hypercapnia, which were compared.

RESULTS

Average OEF in cortical gray matter across 10 subjects from QQ versus DGCB was 35.5 ± 6.7% versus 38.0 ± 9.1% (P = .49) at baseline and 20.7 ± 4.4% versus 28.4 ± 7.6% (P = .02) in hypercapnia: OEF in cortical gray matter was significantly reduced as measured in QQ (P < .01) and in DGCB (P < .01). Cerebral metabolic rate of oxygen consumption (in μmol O₂ /min/100 g) was 168.2 ± 54.1 at baseline from DGCB and was 153.1 ± 33.8 at baseline and 126.4 ± 34.2 (P < .01) in hypercapnia from QQ.

CONCLUSION

The differences in OEF obtained from QQ and DGCB are small and nonsignificant at baseline but are statistically significant during hypercapnia. In addition, QQ shows a cerebral metabolic rate of oxygen consumption decrease (17.4%) during hypercapnia.

Cerebral revascularization after the Carotid Occlusion Surgery Study: what candidates remain, and can we do better?

OBJECTIVE

Cerebral revascularization for carotid occlusion was previously a mainstay procedure for the cerebrovascular neurosurgeon. However, the 1985 extracranial-intracranial bypass trial and subsequently the Carotid Occlusion Surgery Study (COSS) provided level 1 evidence via randomized controlled trials against bypass for symptomatic atherosclerotic carotid occlusion disease. However, in a small number of patients optimal medical therapy fails, and some patients with flow-limiting stenosis develop a perfusion-dependent neurological examination. Therefore it is necessary to further stratify patients by risk to determine who may most benefit from this intervention as well as to determine perioperative morbidity in this high-risk patient population.

METHODS

A retrospective review was performed of all revascularization procedures done for symptomatic atherosclerotic cerebrovascular steno-occlusive disease. All patients undergoing revascularization after the publication of the COSS in 2011 were included. Perioperative morbidity and mortality were assessed as the primary outcome to determine safety of revascularization in this high-risk population. All patients had documented hypoperfusion on hemodynamic imaging.

RESULTS

At total of 35 revascularization procedures were included in this review. The most common indication was for patients with recurrent strokes, who were receiving optimal medical therapy and who suffered from cerebrovascular steno-occlusion. At 30 days only 3 perioperative ischemic events were observed, 2 of which led to no long-term neurological deficit. Immediate graft patency was good, at 94%. Long term, no further strokes or ischemic events were observed, and graft patency remained high at 95%. There were no factors associated with perioperative ischemic events in the variables that were recorded.

CONCLUSIONS

Cerebral revascularization may be done safely at high-volume cerebrovascular centers in high-risk patients in whom optimal medical therapy has failed. Further research must be done to develop an improved methodology of risk stratification for patients with symptomatic atherosclerotic cerebrovascular steno-occlusive disease to determine which patients may benefit from intervention. Given the high risk of recurrent stroke in certain patients, and the fact that patients fail medical therapy, surgical revascularization may provide the best method to ensure good long-term outcomes with manageable up-front risks.

Long-term Outcomes After Endovascular Recanalization in Patients with Chronic Carotid Artery Occlusion

Successful carotid artery stenting may correct ipsilateral hemisphere hypoperfusion and improve neurocognitive function in patients with chronic internal carotid artery occlusion (ICAO). Its effect on long-term outcomes, however, has never been studied. From May 2004 to April 2015, endovascular recanalization for chronic ICAO was attempted in 118 consecutive patients (119 lesions; 98 men; 67 ± 10 years old) with either recurrent neurologic events or objectively impaired ipsilateral hemisphere perfusion. Technical success in recanalization was achieved in 70 lesions (59%, 70/119). 3-months cumulative any stroke or death rate was 5% (6/119; 4 in recanalized group, 2 in failure group), including 2 periprocedural ischemic stroke, 2 intracranial hemorrhage, and 2 subarachnoid hemorrhage. In recanalized patients without periprocedural complication, 1-year reocclusion rate was 15% (10/65). Up to 7 years after procedure, cumulative events of transient ischemic attack (TIA), or any stroke, or death were 17 in recanalized group, compared with 23 in failure group (hazard ratio 0.51, 95% confidence interval 0.27 to 0.97; p = 0.04). The difference became more significant after excluding patients with periprocedure events (hazard ratio 0.41, 95% confidence interval 0.20 to 0.84, p = 0.015). In conclusions, the technical success and periprocedural complication rates of endovascular recanalization for chronic ICAO were acceptable. The cumulative event rates of any stroke or death up to 7 years were more favorable in patients after successful recanalization, compared to those in patients after failed procedure.

Hemodynamic mechanisms underlying elevated oxygen extraction fraction (OEF) in moyamoya and sickle cell anemia patients

Moyamoya is a bilateral, complex cerebrovascular condition characterized by progressive non-atherosclerotic intracranial stenosis and collateral vessel formation. Moyamoya treatment focuses on restoring cerebral blood flow (CBF) through surgical revascularization, however stratifying patients for revascularization requires abilities to quantify how well parenchyma is compensating for arterial steno-occlusion. Globally elevated oxygen extraction fraction (OEF) secondary to CBF reduction may serve as a biomarker for tissue health in moyamoya patients, as suggested in patients with sickle cell anemia (SCA) and reduced oxygen carrying capacity. Here, OEF was measured (TRUST-MRI) to test the hypothesis that OEF is globally elevated in patients with moyamoya (n = 18) and SCA (n = 18) relative to age-matched controls (n = 43). Mechanisms underlying the hypothesized OEF increases were evaluated by performing sequential CBF-weighted, cerebrovascular reactivity (CVR)-weighted, and structural MRI. Patients were stratified by treatment and non-parametric tests applied to compare study variables (significance: two-sided P < 0.05). OEF was significantly elevated in moyamoya participants (interquartile range = 0.38-0.45) compared to controls (interquartile range = 0.29-0.38), similar to participants with SCA (interquartile range = 0.37-0.45). CBF was inversely correlated with OEF in moyamoya participants. Elevated OEF was only weakly related to reductions in CVR, consistent with basal CBF level, rather than vascular reserve capacity, being most closely associated with OEF.

Hemodynamics and oxygen extraction in chronic large artery steno-occlusive disease: Clinical applications for predicting stroke risk

Depending on the adequacy of collateral sources of blood flow, arterial stenosis or occlusion may lead to reduced perfusion pressure and ultimately reduced blood flow in the distal territory supplied by that vessel. There are two well-defined compensatory mechanisms to reduced pressure or flow - autoregulatory vasodilation and increased oxygen extraction fraction. Other changes, such as metabolic downregulation, are likely. The positive identification of autoregulatory vasodilation and increased oxygen extraction fraction in humans is an established risk factor for future ischemic stroke in some disease states such as atherosclerotic carotid stenosis and occlusion. The mechanisms by which ischemic stroke may occur are not clear, and may include an increased vulnerability to embolic events. The use of hemodynamic assessment to identify patients with occlusive vasculopathy at an increased risk for stroke is very appealing for several different patient populations, such as those with symptomatic intracranial atherosclerotic disease, moyamoya phenomenon, complete internal carotid artery occlusion, and asymptomatic cervical carotid artery stenosis. While there is very good data for stroke risk prediction in some of these groups, no intervention based on these tools has been proven effective yet. In this manuscript, we will review these topics above and identify areas for future research.

Susceptibility-weighted Imaging in Thrombolytic Therapy of Acute Ischemic Stroke

Objective:

To provide a comprehensive and latest overview of susceptibility-weighted imaging (SWI) in the application of thrombolysis in acute ischemic stroke, and to update the decision-making effect and clinical value of SWI on identifying stroke patients suitable for thrombolytic therapy and possible benefits and risks followed.

Data Sources:

Literatures referred to this review were collected from PubMed, Medline, and EMBASE published till May 2017, using the search terms including susceptibility-weighted imaging, gradient-echo, T2*, thrombolysis, recombinant tissue plasminogen activator (rt-PA), thrombolytic therapy, and stroke.

Study Selection:

Papers in English or with available English abstracts were considered, with no limitation of study design. References were also identified from the bibliographies of identified articles and the authors’ files.

Results:

SWI is of guiding significance for thrombolytic therapy in stroke patients, it can predict the location and length of thrombus and ischemic penumbra. It is worthy of noting that susceptibility vessel sign (SVS) on SWI can be used to predict recanalization after thrombolytic therapy and whether it is better to implement endovascular thrombolectomy in combination or alone. SWI is sensitive in detecting cerebral microbleed (CMB), and CMB might not be a contraindication for thrombolytic therapy, yet CMBs in multiple foci could possibly be related to intracranial hemorrhage (ICH) after thrombolysis. SVS and CMB on SWI sequence are of instructive value in performing antiplatelet therapy after thrombolytic therapy. Cerebral venous change on SWI is related to lower recanalization rate and poor outcome after thrombolysis.

Conclusions:

It seems that SWI can be applied to guide individualized thrombolytic therapies and assist clinicians in making better decisions by weighing benefits and risks. However, there still exist controversies about the relationship between signs on SWI and thrombolytic therapy.

Susceptibility-weighted imaging predicts infarct size and early-stage clinical prognosis in acute ischemic stroke

Susceptibility-weighted imaging (SWI) is a non-invasive technique that can reveal venous structures and iron in the brain. This retrospective study evaluated SWI, relative to other imaging techniques, for determining cerebral infarct size and early-stage clinical prognosis in patients with acute ischemic stroke. Within 3 days after onset, 22 patients with acute ischemic stroke underwent SWI, diffusion-weighted imaging (DWI), perfusion-weighted imaging (PWI), fluid-attenuated inversion recovery (FLAIR), and magnetic resonance angiography (MRA). At least 7 days after onset, the patients also underwent cranial FLAIR or computed tomography (CT). The severity of neurological damage was adjudged with NIHSS (National Institutes of Health Stroke Scale) scores. The imaged cranial lesions were evaluated according to ASPECTS (Alberta Stroke Program Early CT Score). The SWI-ASPECTS significantly correlated with mean transit time (MTT)-ASPECTS (Spearman's test, r = 0.662, P = 0.001) in evaluating ischemic penumbra and significantly correlated with the FLAIR and CT-ASPECTS (Spearman's test, r = 0.765, P < 0.001) in predicting infarct size. SWI is feasible for the early evaluation of cerebral infarct size and clinical prognosis of patients with acute cerebral infarction. SWI is a useful predictor of early infarct growth and early-stage outcome.

Association between leukoaraiosis and cerebral blood flow territory alteration in asymptomatic internal carotid artery stenosis

AIM

To test the hypothesis that leukoaraiosis (also known as white matter lesion) is associated with cerebral blood flow territory change as revealed by territorial arterial spin-labeling (TASL) magnetic resonance imaging (MRI) in patients with asymptomatic internal carotid artery stenosis (aICAS).

MATERIALS AND METHODS

The institutional review board approved this study. Thirty-three patients with aICAS were included prospectively and divided into high-grade (ultrasonographic stenosis ≥70%, n=17) and low-grade (n=16) groups; 16 healthy subjects were also included. Cerebral flow territory was delineated for left ICA, right ICA, and vertebral arteries using TASL MRI and fuzzy clustering. Two licensed neuroradiologists independently and dichotomously rated the hemispherical asymmetry of flow territories. Flow territories were finalised by consensus, and when asymmetry was present, these were divided into normal and abnormal areas where the raters separately assessed leukoaraiosis based on fluid-attenuated inversion recovery images and the Fazekas scale.

RESULTS

The inter-rater agreement in the evaluation of flow territory asymmetry with TASL imaging in conjunction with time-of-flight angiogram is substantial (Cohen's kappa=0.82). Multinomial logistic regression (reference group=healthy subjects) indicates that global leukoaraiosis is not a predictor of aICAS after controlling for age, whereas in high-grade patients, the deep white matter lesion is more severe in the area receiving collateral circulation than in the area with normal flow territory (Wilcoxon signed-rank test, p=0.03).

CONCLUSION

TASL MRI is clinically feasible in aICAS and shows that more severe deep white matter lesions are associated with collateral circulation in high-grade patients.

Structure of the Medullary Veins of the Cerebral Hemisphere and Related Disorders

Deep medullary veins drain into subependymal veins with four convergence zones and show parallel distribution patterns adjacent to the body or inferior horn and a radial pattern in the frontal horn or trigon of the lateral ventricle. As white matter imaging develops such as diffusion tensor imaging or susceptibility-weighted imaging, requirements for understanding of white matter structures are increasing, not only for understanding of neuronal tracts but also for that of other structures including the fine anatomy of white matter vessels. Some disorders are related to deep medullary veins and show characteristic distributions of the lesions indicating the relationship to the medullary veins. When lesions show a parallel or radial distribution pattern in the certebral deep white matter, disorders related to deep medullary veins should be considered for differential diagnosis. In this review, we discuss disorders related to deep medullary veins, including (a) anomalies of the medullary veins, (b) hemorrhagic disorders related to the medullary veins (diffuse vascular injury due to high-energy trauma, deep medullary vein engorgement/thrombosis in neonates), (c) inflammatory changes that spread along the medullary veins, (d) neoplasms within the medullary veins, and (e) metabolic changes that lead to altered visualization of medullary veins. Understanding the anatomic structure of medullary veins in the cerebral hemisphere and becoming familiar with disorders in which the medullary veins play a major role in disease development may be helpful in the interpretation of brain images. ^©RSNA, 2017.

Proof of concept non-invasive estimation of peripheral venous oxygen saturation

BACKGROUND

Pulse oximeters continuously monitor arterial oxygen saturation. Continuous monitoring of venous oxygen saturation (SvO₂) would enable real-time assessment of tissue oxygen extraction (O₂E) and perfusion changes leading to improved diagnosis of clinical conditions, such as sepsis.

METHODS

This study presents the proof of concept of a novel pulse oximeter method that utilises the compliance difference between arteries and veins to induce artificial respiration-like modulations to the peripheral vasculature. These modulations make the venous blood pulsatile, which are then detected by a pulse oximeter sensor. The resulting photoplethysmograph (PPG) signals from the pulse oximeter are processed and analysed to develop a calibration model to estimate regional venous oxygen saturation (SpvO₂), in parallel to arterial oxygen saturation estimation (SpaO₂). A clinical study with healthy adult volunteers (n = 8) was conducted to assess peripheral SvO₂ using this pulse oximeter method. A range of physiologically realistic SvO₂ values were induced using arm lift and vascular occlusion tests. Gold standard, arterial and venous blood gas measurements were used as reference measurements. Modulation ratios related to arterial and venous systems were determined using a frequency domain analysis of the PPG signals.

RESULTS

A strong, linear correlation (r ²  = 0.95) was found between estimated venous modulation ratio (R_Ven) and measured SvO₂, providing a calibration curve relating measured R_Ven to venous oxygen saturation. There is a significant difference in gradient between the SpvO₂ estimation model (SpvO₂ = 111 - 40.6*R) and the empirical SpaO₂ estimation model (SpaO₂ = 110 - 25*R), which yields the expected arterial-venous differences. Median venous and arterial oxygen saturation accuracies of paired measurements between pulse oximeter estimated and gold standard measurements were 0.29 and 0.65%, respectively, showing good accuracy of the pulse oximeter system.

CONCLUSIONS

The main outcome of this study is the proof of concept validation of a novel pulse oximeter sensor and calibration model to assess peripheral SvO₂, and thus O₂E, using the method used in this study. Further validation, improvement, and application of this model can aid in clinical diagnosis of microcirculation failures due to alterations in oxygen extraction.

Harnessing Neuroimaging Capability in Pediatric Stroke: Proceedings of the Stroke Imaging Laboratory for Children Workshop

On June 5, 2015 the International Pediatric Stroke Study and the Stroke Imaging Laboratory for Children cohosted a unique workshop focused on developing neuroimaging research in pediatric stroke. Pediatric neurologists, neuroradiologists, interventional neuroradiologists, physicists, nurse practitioners, neuropsychologists, and imaging research scientists from around the world attended this one-day meeting. Our objectives were to (1) establish a group of experts to collaborate in advancing pediatric neuroimaging for stroke, (2) develop consensus clinical and research magnetic resonance imaging protocols for pediatric stroke patients, and (3) develop imaging-based research strategies in pediatric ischemic stroke. This article provides a summary of the meeting proceedings focusing on identified challenges and solutions and outcomes from the meeting. Further details on the workshop contents and outcomes are provided in three additional articles in the current issue of Pediatric Neurology.

Does applying resolution recovery to normal databases confer an advantage over conventional 3D-stereotactic surface projection techniques?

We evaluated a novel normal database (NDB) generated using single photon emission computed tomography (SPECT) data obtained from healthy brains by using a SPECT/CT system, analyzed using a resolution recovery (RR) technique applied to the three-dimensional stereotactic surface projection (3D-SSP) technique. We used a three-dimensional ordered subset expectation maximization method (3D-OSEM) with applied scatter correction (SC), attenuation correction, and RR to reconstruct the data. We verified the accuracy of the novel NDB's values (Z, extent, and error scores), and compared the novel NDB to the 3D-SSP technique by using simulated misery perfusion-related patient data from a conventional NDB. In addition, Z, extent, and error scores at the precuneus, cuneus, and posterior cingulate were compared under different reconstruction conditions by using the patient data. In the simulation, Z scores decreased when using the novel NDB corrected using computed tomography-based attenuation correction (CTAC), SC, and RR. The extent scores of the posterior cingulate increased using the novel NDB, relative to the other NDBs. The error score with the novel NDB without RR decreased by 15% compared to that of the conventional NDB. Z scores generated from patient data decreased in the novel NDB with RR. The extent scores tended to decrease in the novel NDB with RR. The extent scores in the novel NDB with RR improved at the posterior cingulate, compared to the scores with the other NDBs. However, applying RR to the novel NDB conferred no advantage because the cut-off of the current Z score must be reconsidered when using the additive RR technique.

Assessment of Hemodynamic Compromise Using Computed Tomography Perfusion in Combination with 123I-IMP Single-Photon Emission Computed Tomography without Acetazolamide Challenge Test

OBJECTIVES

The acetazolamide challenge test in conjunction with ¹²³I-IMP single-photon emission computed tomography (SPECT) is a known method of assessing cerebrovascular reserve capacity. In this study, we investigated whether CT perfusion in combination with resting state ¹²³I-IMP SPECT could be used instead of the acetazolamide challenge test to evaluate hemodynamic compromise in patients with atherosclerotic occlusive disease.

METHODS

Twenty consecutive patients with unilateral internal carotid artery or middle cerebral artery steno-occlusive disease were enrolled. ¹²³I-IMP SPECT was performed with and without the acetazolamide challenge test, and with CT perfusion. Cerebral blood flow (CBF), cerebral blood volume, and mean transit time (MTT) obtained by CT perfusion were compared with CBF and cerebrovascular reactivity (CVR) obtained by ¹²³I-IMP SPECT.

RESULTS

The asymmetry ratio of MTT as measured by CT perfusion showed a strong correlation with the CVR to acetazolamide as measured by ¹²³I-IMP SPECT (ρ = -.780, P <.001). Based on the CBF obtained through ¹²³I-IMP SPECT and the MTT obtained through CT perfusion, hemodynamic compromise was detected with high sensitivity (1.000) and specificity (.929), and a cutoff value of 30% was found to be suitable for the asymmetry ratio of MTT. MTT prolongation was significantly improved after revascularization surgery in hemodynamic compromise (P = .028).

CONCLUSION

MTT as measured by CT perfusion in combination with CBF as measured by resting state ¹²³I-IMP SPECT may be useful for evaluating hemodynamic compromise as an alternative to the acetazolamide challenge test.

Bypass surgery for the prevention of ischemic stroke: current indications and techniques

INTRODUCTION

Although most ischemic strokes are thromboembolic in origin and their management is endovascular or medical, some are haemodynamic in origin and their management may be surgical. We reviewed bypass indications, patient selection and surgical techniques used in our current practice.

METHODS

Extracranial-intracranial (EC-iC) bypass with superior temporal artery-to-middle cerebral artery (STA-MCA) bypass, high-flow interposition grafts and reconstructive techniques were used to treat patients with symptomatic ischemia.

RESULTS

During a 13-year period, 152 bypasses were performed for ischemia in 129 patients. Specific diagnoses included: (1) internal carotid artery (iCA) occlusion (58 bypasses); (2) MCA occlusion and, rarely, high-grade MCA stenosis (22 bypasses); (3) vertebrobasilar atherosclerotic steno-occlusive disease (2 bypasses); (4) moyamoya disease (65 bypasses); and (5) ischemic complications after aneurysm treatment (5 bypasses). of the 152 bypasses, 137 were conventional STA-MCA bypasses. fourteen patients had high-flow bypasses that included 4 "double-barrel" STA-MCA bypasses, 6 bypasses with interposition grafts to the cervical carotid artery, 2 subclavian artery-to-MCA bypasses, 1 MCA-to-posterior cerebral artery (PCA) bypass and 1 aorto-carotid bypass. The bypass patency rate was 96.1%.

CONCLUSIONS

Bypass surgery for the prevention of ischemic stroke is safe and elegant techniques have been developed. Patients with athero-occlusive disease, ischemic symptoms and haemodynamic insufficiency have significant risk of stroke if managed medically or left untreated. However, surgical intervention lacks supporting evidence from the recent Carotid occlusion Surgery Study (CoSS). Patients will be caught in a difficult position between a dismal natural history and an unproven surgical intervention. Clinicians must individualise their management until additional data are published or further consensus develops.

Cortical Cerebral Blood Flow, Oxygen Extraction Fraction, and Metabolic Rate in Patients with Middle Cerebral Artery Stenosis or Acute Stroke

BACKGROUND AND PURPOSE

With the advances of magnetic resonance technology, the CBF, oxygen extraction fraction, and cerebral metabolic rate of oxygen can be measured in MRI. Our aim was to measure the CBF, oxygen extraction fraction, and cerebral metabolic rate of oxygen use in patients with different severities of middle cerebral artery stenosis or acute stroke by using the arterial spin-labeling and susceptibility-weighted imaging techniques.

MATERIALS AND METHODS

Fifty-seven patients with MCA stenosis or acute stroke were recruited and classified into 4 groups: mild MCA stenosis (group 1), severe MCA stenosis (group 2), occluded MCA (group 3), and acute stroke (group 4). Arterial spin-labeling and SWI sequences were used to acquire CBF, oxygen extraction fraction, and cerebral metabolic rate of oxygen.

RESULTS

The oxygen extraction fraction in hemispheres with mild MCA stenosis (group 1) was remarkably higher than that in the contralateral hemisphere. In addition, hemispheres with severe MCA stenosis (group 2) had significantly lower CBF and a significantly higher oxygen extraction fraction than the contralateral hemisphere. Hemispheres with occluded MCA (group 3) or acute stroke (group 4) had a significantly lower CBF and cerebral metabolic rate of oxygen and a significantly higher oxygen extraction fraction than the contralateral hemisphere.

CONCLUSIONS

The oxygen extraction fraction gradually increased in groups 1-3. When this offset a decrease in CBF, the cerebral metabolic rate of oxygen remained at a normal level. An occluded MCA led to reduction in both the CBF and cerebral metabolic rate of oxygen. Moreover, the oxygen extraction fraction and cerebral metabolic rate of oxygen significantly increased and decreased, respectively, in the occluded MCA region during acute stroke.

Practical clinical use of dynamic susceptibility contrast magnetic resonance imaging for the surgical treatment of moyamoya disease

BACKGROUND

Precise evaluation of hemodynamic stress is important for the treatment of moyamoya disease (MMD).

OBJECTIVE

To explore whether dynamic susceptibility contrast magnetic resonance imaging could predict the effects and risk of indirect bypass surgery on MMD.

METHODS

Clinical data of patients with MMD who were evaluated preoperatively and postoperatively with dynamic susceptibility contrast magnetic resonance imaging and digital subtraction angiography were evaluated retrospectively. Indirect bypass surgery was performed on 115 hemispheres of 69 patients (mean age, 15 years; range, 3-54 years). We examined the correlations of ischemic events and revascularization with the mean transit time (MTT) delay to cerebellum.

RESULTS

The hemispheres that caused the ischemic events (responsible hemisphere) had a significantly longer preoperative MTT delay than the nonresponsible hemispheres (2.66 ± 1.34 vs 1.57 ± 1.09 seconds). The postoperative MTT delay fell significantly in the patients whose symptoms disappeared (preoperative, 2.61 ± 1.35 seconds; postoperative, 1.35 ± 0.96 seconds). Perioperative infarction occurred in 4 hemispheres (3.5%), and the MTT delay was significantly longer in those hemispheres than in the others (3.97 ± 1.20 vs 2.38 ± 1.34 seconds). The MTT delay was significantly longer in patients with higher angiographic stages. Indirect bypass surgery ameliorated the MTT delay to the same degree in adults and children. Digital subtraction angiography revealed that the induced revascularization was far superior in areas with longer MTT delays.

CONCLUSION

Dynamic susceptibility contrast magnetic resonance imaging proved to be a useful clinical imaging method for patients with MMD. It may be helpful for selecting candidates for MMD intervention and for predicting the effects and risks of surgery.

ABBREVIATIONS

DSC-MRI, dynamic susceptibility contrast magnetic resonance imagingMMD, moyamoya diseaseMTT, mean transit timeROI, region of interest.

The effect of regular exercise on antioxidant enzyme activities and lipid peroxidation levels in both hippocampi after occluding one carotid in rat

Regular exercise has beneficial effects on cerebrovascular diseases; however, its biochemical mechanisms are not fully known. The purpose of this study was to determine antioxidant enzyme activities and lipid peroxidation of both hippocampi after applying exercise followed by occluding one common carotid. Wistar rats were divided into four groups of control, exercise, hypoperfusion and exercise-hypoperfusion (exe-hypo). In the exercise and exe-hypo groups, the rats were forced to run on a treadmill for 1 h a day for 2 months. The right common carotid of the animals in the (exe-hypo) group was occluded after the cessation of exercise. Surgery without occlusion of the carotid was applied on the control (without exercise) and exercise groups. All animals were sacrificed 1 and 24 h after surgery. The levels of malondialdehyde (MDA) and antioxidant enzyme activities in the hippocampi were measured. A significant interaction was observed between the exercise and hypoperfusion in both hippocampi (p<0.05). In comparison with the control group, there was significant elevation of catalase activity in the right and left hippocampus of the hypo group at 24 h (p<0.0001). Regarding the differences between the hemispheres, there was a significant increase in MDA and decrease in catalase activity in the left hippocampus in hypoperfusion group, but the exercise in the exe-hypo group succeeded in abolishing these alterations which were caused by hypoperfusion, This study shows that exercise pre-conditioning prevents some alterations in brain oxidant-antioxidant status which are induced by cerebral hypoperfusion. Further studies are needed in order to clarify the mechanism of exercise.

Snapshot MR technique to measure OEF using rapid frequency mapping

Magnetic resonance (MR)-based oxygen extraction fraction (OEF) measurement techniques that use blood oxygen level-dependent (BOLD)-based approaches require the measurement of the R2' decay rate and deoxygenated blood volume to derive the local oxygen saturation in vivo. We describe here a novel approach to measure OEF using rapid local frequency mapping. By modeling the MR decay process in the static dephasing regime as two separate dissipative and oscillatory effects, we calculate the OEF from local frequencies measured across the brain by assuming that the biophysical mechanisms causing OEF-related frequency changes can be determined from the oscillatory effects. The Parameter Assessment by Retrieval from Signal Encoding (PARSE) technique was used to acquire the local frequency change maps. The PARSE images were taken on 11 normal volunteers, and 1 patient exhibiting hemodynamic stress. The mean MR-OEF in 11 normal subjects was 36.66±7.82%, in agreement with positron emission tomography (PET) literature. In regions of hemodynamic stress induced by vascular steal, OEF exhibits the predicted focal increases. These preliminary results show that it is possible to measure OEF using a rapid frequency mapping technique. Such a technique has numerous advantages including speed of acquisition, is noninvasive, and has sufficient spatial and temporal resolution.

Clinical significance of STA-MCA double anastomosis for hemodynamic compromise in post-JET/COSS era

BACKGROUND

Even after the recent randomized clinical trials JET and COSS, it is still unclear that impaired cerebrovascular reactivity (CVR) to acetazolamide and oxygen extraction fraction (OEF) can identify the candidates for superficial temporal artery to middle cerebral artery (STA-MCA) anastomosis. This prospective study was aimed to evaluate the benefits of STA-MCA "double" anastomosis on long-term outcome in patients with reduced cerebral blood flow (CBF) and CVR (Type 3 ischemia) and elevated OEF attributable to occlusive carotid diseases.

METHODS

This study included 49 patients with reduced CBF and CVR on SPECT in the ipsilateral MCA area. Using (15)O-gas PET, OEF was also measured in all patients. STA-MCA double anastomosis was recommended to the patients with Type 3 and elevated OEF. Those with Type 3 but normal OEF were medically treated.

RESULTS

Of 36 patients with Type 3 and elevated OEF, 25 consented to surgery. No perioperative morbidity or mortality were noted. The other 11 patients with Type 3 and elevated OEF were medically treated. Annual incidence of ipsilateral stroke was 0.7% and 6.5% in surgically and medically treated patients with Type 3 and elevated OEF, respectively (P = 0.0188). None of patients with Type 3 but normal OEF developed ipsilateral stroke during follow-up periods. STA-MCA "double" anastomosis significantly decreased OEF.

CONCLUSIONS

STA-MCA "double" anastomosis may still have the potential to reduce the risk of recurrent ipsilateral stroke in hemodynamically compromised patients. Further studies would be essential to advance diagnosis, surgical procedures, and perioperative managements to bring out maximal effects of bypass surgery.

Change in cortical vessel signs on susceptibility-weighted images after full recanalization in hyperacute ischemic stroke

BACKGROUND

The cortical vessel signs (CVSs) on susceptibility-weighted images (SWIs) have been reported in patients with hyperacute ischemic stroke. We evaluated the change of this susceptibility sign on the immediate SWI after full recanalization and its clinical implications.

METHODS

Nineteen hyperacute ischemic stroke patients who had acute large artery occlusion and underwent post-recanalization SWI were enrolled in this study. The patients had ICA (internal carotid artery, 2 cases), M1 (M1 segment of middle cerebral artery, 7 cases), M2 (M2 segment of middle cerebral artery, 1 cases), T (intracranial ICA bifurcation, 2 cases), ICA/M1 (4 cases) and basilar artery (3 cases) occlusion on imaging studies before thrombolysis and they underwent immediate magnetic resonance imaging, including the SWI, after full recanalization. The recanalization status was evaluated using the thrombolysis in cerebral infarction (TICI) score before and after thrombolysis. The SWI images were evaluated for the presence of asymmetry of veins over the ischemic territory and this was correlated with the site of stenosis or occlusion. The veins in the ischemic territory were classified as 'prominent' if there were more numerous veins and/or large veins with a greater signal loss observed compared with the opposite normal hemisphere, 'equal' if there were no significant difference in appearance in both the cerebral hemispheres, and 'less' if the veins were decreased in the affected area as compared with that of the normal cortex. Baseline clinical parameters and clinical outcomes were reviewed.

RESULTS

The initial TICI grades were 0 in all cases. After thrombolysis, TICI grades were 3 in all cases. The pre-recanalization SWIs were obtained in 10 of 19 patients and all 10 showed prominent CVSs over the affected side, which disappeared on the post-recanalization SWI. On the post-recanalization SWI, the observed veins in the affected area were equal (10/19), less (5/19), and both equal and less (4/19). Patients with equal cortical veins in the affected area had small lesions on diffusion-weighted image (DWI) (10/19), while patients with less cortical veins had medium to large lesions on DWI (9/19).

CONCLUSION

The prominent CVSs on SWI can be indicative of acute thromboembolic occlusion and its change immediately after recanalization can be used to reflect the metabolic status. After recanalization, the appearance of the equal CVS (return to normal) on SWI was associated with a favorable clinical outcome and infarction was avoided in our small series study.

Predicting stroke evolution: comparison of susceptibility-weighted MR imaging with MR perfusion

OBJECTIVES

To investigate the ability of susceptibility-weighted imaging (SWI) to predict stroke evolution in comparison with perfusion-weighted imaging (PWI).

METHODS

In a retrospective analysis of 15 patients with non-lacunar ischaemic stroke studied no later than 24 h after symptom onset, we used the Alberta Stroke Program Early CT Score (ASPECTS) to compare lesions on initial diffusion-weighted images (DWI), SWI, PWI and follow-up studies obtained at least 5 days after symptom onset. The National Institutes of Health Stroke Scale scores at entry and stroke risk factors were documented. The clinical-DWI, SWI-DWI and PWI-DWI mismatches were calculated.

RESULTS

SWI-DWI and mean transit time (MTT)-DWI mismatches were significantly associated with higher incidence of infarct growth (P = 0.007 and 0.028) and had similar ability to predict stroke evolution (P = 1.0). ASPECTS values on initial DWI, SWI and PWI were significantly correlated with those on follow-up studies (P ≤ 0.026) but not associated with infarct growth. The SWI ASPECTS values were best correlated with MTT ones (ρ = 0.8, P < 0.001).

CONCLUSIONS

SWI is an alternative to PWI to assess penumbra and predict stroke evolution. Further prospective studies are needed to evaluate the role of SWI in guiding thrombolytic therapy. Key Points • SWI can provide perfusion information comparable to MTT • SWI-DWI mismatch can indicate ischaemic penumbra • SWI-DWI mismatch can be a predictor for stroke evolution.

Bypass for the prevention of ischemic stroke

OBJECTIVE

Although most ischemic strokes are thromboembolic in origin and their management is endovascular or medical, some are hemodynamic in origin and their management may be surgical. Extracranial-intracranial bypass with superficial temporal artery-to-middle cerebral artery (MCA) bypass, high-flow interposition grafts, and reconstructive techniques have been developed. Clinical indications and efficacy are controversial, and this review examines current practices.

METHODS

Bypass surgery is indicated for patients with athero-occlusive disease that results in chronic, low cerebral blood flow accompanied by episodes of ischemic symptoms. Specific diagnoses include: (1) internal carotid artery occlusion; (2) MCA occlusion and, rarely, high-grade MCA stenosis; (3) vertebrobasilar atherosclerotic steno-occlusive disease; (4) vasculitis resulting in severe occlusive disease; and (5) moyamoya disease.

RESULTS

Discouraging results from the Extracranial-Intracranial Bypass Trial demonstrated the importance of selecting surgical patients based on objective measures of hemodynamic insufficiency. Two such tests are xenon-enhanced computed tomography with acetazolamide challenge and positron emission tomography with measurement of oxygen extraction fraction. Perfusion computed tomography may be another, more practical test. Surgical series, systematic reviews of the literature, and two new randomized clinical trials that use these diagnostic techniques reveal contradictory results. Although they demonstrate that bypass surgery has a morbidity rate of less than 5% and a patency rate of more than 95%, they have not proven a clear benefit.

CONCLUSIONS

Patients with athero-occlusive disease and symptoms of hemodynamic insufficiency have significant risk of stroke if left untreated or managed medically. On the other hand, surgical intervention lacks supporting evidence. Clinicians must individualize their management recommendations until additional data are published or further consensus develops.

Arterial spin-labeling MRI can identify the presence and intensity of collateral perfusion in patients with moyamoya disease

BACKGROUND AND PURPOSE

Determining the presence and adequacy of collateral blood flow is important in cerebrovascular disease. Therefore, we explored whether a noninvasive imaging modality, arterial spin labeling (ASL) MRI, could be used to detect the presence and intensity of collateral flow using digital subtraction angiography (DSA) and stable xenon CT cerebral blood flow as gold standards for collaterals and cerebral blood flow, respectively.

METHODS

ASL and DSA were obtained within 4 days of each other in 18 patients with Moyamoya disease. Two neurointerventionalists scored DSA images using a collateral grading scale in regions of interest corresponding to ASPECTS methodology. Two neuroradiologists similarly scored ASL images based on the presence of arterial transit artifact. Agreement of ASL and DSA consensus scores was determined, including kappa statistics. In 15 patients, additional quantitative xenon CT cerebral blood flow measurements were performed and compared with collateral grades.

RESULTS

The agreement between ASL and DSA consensus readings was moderate to strong, with a weighted kappa value of 0.58 (95% confidence interval, 0.52-0.64), but there was better agreement between readers for ASL compared with DSA. Sensitivity and specificity for identifying collaterals with ASL were 0.83 (95% confidence interval, 0.77-0.88) and 0.82 (95% confidence interval, 0.76-0.87), respectively. Xenon CT cerebral blood flow increased with increasing DSA and ASL collateral grade (P<0.05).

CONCLUSIONS

ASL can noninvasively predict the presence and intensity of collateral flow in patients with Moyamoya disease using DSA as a gold standard. Further study of other cerebrovascular diseases, including acute ischemic stroke, is warranted.

Silent cortical neuronal damage in atherosclerotic disease of the major cerebral arteries

In atherosclerotic internal carotid artery (ICA) or middle cerebral artery (MCA) disease, hemodynamic compromise may cause selective neuronal damage manifested as loss of central benzodiazepine receptors (BZRs) in the normal-appearing cerebral cortex, without overt episode of stroke. To investigate the association of decreases in cortical BZRs with hemodynamic compromise and the effect of angiotensin receptor blockers (ARBs) on these receptors in patients whose atherosclerotic ICA or MCA disease is asymptomatic, we measured BZRs using positron emission tomography and (11)C-flumazenil in 79 patients with asymptomatic atherosclerotic ICA or MCA disease and no cortical infarction. Three-dimensional stereotactic surface projections were used to calculate the BZR index, a measure of abnormally decreased BZRs in the cerebral cortex within the MCA distribution. Multiple regression analysis showed this index to be positively correlated with the value of oxygen extraction fraction, with the presence of silent subcortical infarcts, and with the presence of ischemic heart disease, whereas it was negatively correlated with the treatment of hypertension with ARBs. In asymptomatic atherosclerotic ICA or MCA disease, hemodynamic compromise is associated with selective neuronal damage manifested as decreases in cortical BZRs in the noninfarcted cerebral cortex, whereas ARBs are associated with preservation of cortical BZRs.

Cerebellum as the normal reference for the detection of increased cerebral oxygen extraction

Hemispheric ratios of oxygen extraction fraction (OEF), a proven methodology for the detection of severe hemodynamic impairment and stroke risk, are not sensitive for detecting bilateral hemispheric increases in OEF. The aim of this study was to investigate the use of cerebellum as the reference normal. We analyzed positron emission tomographic (PET) measurements of count-based OEF and clinical data from 57 patients with unilateral atherosclerotic carotid occlusion and 13 controls enrolled in a prospective study of stroke risk. The ipsilateral, contralateral, and total cerebellum were each evaluated as possible reference regions, and the ratios of the middle cerebral artery (MCA) hemispheric OEF counts against those in each reference region were determined. A statistically significant correlation (P<0.0001) was observed with all three MCA-to-cerebellar ratios when compared with the gold standard of ipsilateral-to-contralateral MCA hemispheric ratio. Kaplan-Meier analyses showed all MCA-to-cerebellar ratios to be predictive of stroke. By using the total cerebellum method, 7 strokes were found to have occurred in 20 patients with increased OEF (P=0.0007), compared with 7 strokes out of 16 patients with elevated OEF using the ipsilateral or contralateral cerebellum methods (P<0.0001). These methods may be useful for categorizing the hemodynamic status of patients with bilateral cerebral occlusive diseases, including atherosclerosis and moyamoya, to determine the association with the risk of subsequent stroke.

Susceptibility weighted imaging in cerebral hypoperfusion-can we predict increased oxygen extraction fraction?

INTRODUCTION

Increased concentrations of deoxyhemoglobin within veins can induce susceptibility changes resulting in increased conspicuity in susceptibility weighted imaging (SWI). Compensatory mechanisms following reduced cerebral perfusion due to carotid occlusive disease may not be sufficient to meet demands of ischemic tissue and increased tissue oxygen extraction ratio results in relative increase in deoxyhemoglobin levels in the venous blood draining affected hemisphere. We assessed whether patients with carotid disease display prominence of veins over affected cerebral hemisphere.

METHODS

Eighteen patients with unilateral carotid occlusion or critical carotid stenosis proven by magnetic resonance angiography (MRA) were selected. The medical records and MRI findings including SWI and MRA were reviewed. The SWI images were studied for the presence of asymmetry of veins over the cerebral hemispheres and were correlated with the site and severity of stenosis or occlusion. The veins were assumed to be conspicuous and asymmetric if there were more numerous veins and/or large veins with greater signal loss observed compared with opposite normal hemisphere.

RESULTS

In about half of patients, prominence of veins was noted in the cerebral hemisphere ipsilateral to side of occlusion. This was not observed in patients with significant extracranial carotid stenosis. The SWI abnormalities were seen extending beyond the boundaries of occluded vascular territory. There was good agreement between two observers in all the patients who showed positive finding. Also there was no interobserver variation in patients with negative findings.

CONCLUSION

The increased susceptibility arising out of increased deoxyhemoglobin to oxyhemoglobin ratio leads to visualization of prominent veins over the affected cerebral hemisphere on SWI.

Red blood cell transfusion increases cerebral oxygen delivery in anemic patients with subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Anemia is common after subarachnoid hemorrhage and may exacerbate the reduction in oxygen delivery (DO(2)) underlying delayed cerebral ischemia. The association between lower hemoglobin and worse outcome, including more cerebral infarcts, supports a role for red blood cell transfusion to correct anemia. However, the cerebral response to transfusion remains uncertain, because higher hemoglobin may increase viscosity and further impair cerebral blood flow (CBF) in the setting of vasospasm.

METHODS

Eight patients with aneurysmal subarachnoid hemorrhage and hemoglobin <10 g/dL were studied with (15)O-positron emission tomography before and after transfusion of 1 U red blood cells. Paired t tests were used to analyze the change in global and regional CBF, oxygen extraction fraction, and oxygen metabolism after transfusion. DO(2) was calculated from CBF and arterial oxygen content. CBF, oxygen metabolism, and DO(2) are reported in mL/100 g/min.

RESULTS

Transfusion resulted in a 15% rise in hemoglobin (8.7+/-0.8 to 10.0+/-1.0 g/dL) and arterial oxygen content (11.8+/-1.0 to 13.6+/-1.1 mL/dL; both P<0.001). Global CBF remained stable (40.5+/-8.1 to 41.6+/-9.9), resulting in an 18% rise in DO(2) from 4.8+/-1.1 to 5.7+/-1.4 (P=0.017). This was associated with a fall in oxygen extraction fraction from 0.49+/-0.11 to 0.41+/-0.11 (P=0.11) and stable oxygen metabolism. Rise in DO(2) was greater (28%) in regions with oligemia (low DO(2) and oxygen extraction fraction > or =0.5) at baseline but was attenuated (10%) within territories exhibiting angiographic vasospasm, where CBF fell 7%.

CONCLUSIONS

Transfusion of red blood cells to anemic patients with subarachnoid hemorrhage resulted in a significant rise in cerebral DO(2) without lowering global CBF. This was associated with reduced oxygen extraction fraction, which may improve tolerance of vulnerable brain regions to further impairments of CBF. Further studies are needed to confirm the benefit of transfusion on delayed cerebral ischemia and balance this against potential systemic and cerebral risks.

The evolution of cerebral revascularization surgery

Among the relatively few surgeons to be awarded the Nobel Prize was Alexis Carrel, a French surgeon and pioneer in revascularization surgery at the turn of the 20th century. The authors trace the humble beginnings of cerebral revascularization surgery through to the major developments that helped shape the modern practice of cerebral bypass surgery. They discuss the cornerstone studies in the development of this technique, including the Extracranial/Intracranial Bypass Study initiated in 1977. Recent innovations, including modern techniques to monitor cerebral blood flow, microanastomosis techniques, and ongoing trials that play an important role in the evolution of this field are also evaluated.

Superficial temporal artery to middle cerebral artery bypass: past, present, and future

OBJECT

The aim of this study was to review the historical developments and current status of superficial temporal artery (STA) to middle cerebral artery (MCA) bypass.

METHOD

A literature review was performed to review the origins and current uses of the STA bypass procedure in neurosurgery.

RESULTS

The idea of providing additional blood supply to the brain to prevent stroke and maintain neurological function has been present in the mind of neurosurgeons for many decades. In 1967 the first STA-MCA bypass was done by M. G. Yaşargil, and an enormous step was made into the field of microneurosurgery and cerebral revascularization. During the decades that followed, this technique was used as an adjuvant or a definitive surgical treatment for occlusive disease of the extracranial and intracranial cerebral vessels, skull base tumors, aneurysms, carotid-cavernous fistulas, cerebral vasospasm, acute cerebral ischemia, and moyamoya disease. With the results of the first randomized extracranial-intracranial (EC-IC) bypass trial and the development of endovascular techniques such as angioplasty for intracranial atherosclerotic disease and cerebral vasospasm, the indications for STA-MCA bypass became limited. Neurosurgeons continued to perform EC-IC bypasses as an adjuvant to clipping of aneurysms and in the treatment of skull base tumors and moyamoya disease; the procedure is less commonly used for atherosclerotic carotid artery occlusion (CAO) with definite evidence of hemodynamic insufficiency. The evidence that patients with symptomatic CAO and "misery perfusion" have an increased stroke risk has prompted a second trial for evaluating EC-IC bypass for stroke prevention. The Carotid Occlusion Surgery Study is a new trial designed to determine whether STA-MCA bypass can reduce the incidence of stroke in these patients. New trials will also reveal the role of the STA-MCA bypass in the prevention of hemorrhages in moyamoya disease.

CONCLUSIONS

The role of STA-MCA bypass in the management of cerebrovascular disease continues to be refined and evaluated using advanced imaging techniques and by performing randomized trials for specific purposes, including symptomatic CAO.

Flow-assisted surgical cerebral revascularization

✓Extracranial–intracranial bypass surgery has advanced from a mere technical feat to a procedure requiring careful patient selection and a justifiable decision-making paradigm. Currently available technologies for flow measurement in the perioperative and intraoperative setting allow a more structured and analytical approach to decision making. The purpose of this report is to review the use of flow measurement in cerebral revascularization, presenting algorithms for flow-assisted surgical planning, technique, and surveillance.

Mechanisms of ischemic stroke secondary to large artery atherosclerotic disease

Atherosclerotic occlusive disease of the cervical and intracranial arteries leads to ischemic stroke through two separate, but interrelated, mechanisms: local thrombosis or embolism from atherosclerotic plaque, and hemodynamic failure (low flow). In this article, the author discusses the evidence linking these two mechanisms with cerebral ischemia, and the evidence for the synergistic effects of thromboembolism and impaired hemodynamics. An understanding of these two mechanisms is important because these mechanisms provide the rationale for revascularization for patients who have atherosclerotic stenosis or occlusion. In addition, the biologic imaging of atherosclerotic plaques and hemodynamic assessment eventually will play an important role in stratifying patient risk and guiding physiologically based patient selection for intervention.

No effect of low-dose statins treatment on cerebral blood flow in humans with atherosclerotic cerebrovascular disease

Animal studies have suggested that the reduction in stroke risk observed with 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) therapy is owing to an increase in basal cerebral blood flow (CBF). The purpose of the study was to determine if statin therapy was associated with increased CBF in humans with cerebrovascular atherosclerotic disease. Quantitative measurements of CBF were obtained on study entry in 97 patients with carotid artery occlusion enrolled in a prospective study of cerebral hemodynamics and stroke risk. This study represents a post hoc analysis of CBF measurements based on whether patients were receiving statin therapy at the time of CBF measurement. Global and regional CBF (including hemispheric, basal ganglia, and arterial borderzones), and baseline clinical, epidemiologic, and laboratory stroke risk factors were compared between the two groups. Nineteen of the 97 patients were on a statin agent on study entry. The statin group was younger, had significantly lower LDL levels and included more women. Statin therapy was not associated with higher baseline values of CBF in global or regional analyses. Mean middle cerebral artery territory CBF (+/-s.d.) ipsilateral to the occluded carotid artery was 37.6+/-12.7 mL/100 g min for the statin group (n=19) compared with 38.6+/-12.7 mL/100 g min for the nonstatin group (n=78). Contralateral values were 42.9+/-13.5 and 44.2+/-13.3 mL/100 g min for the statin and nonstatin groups, respectively. We conclude that the stroke risk reduction observed with statin therapy in humans likely involves mechanisms other than an increased basal CBF.

The Management of Blood Pressure After Stroke

BACKGROUND

Control of hypertension is a well-established goal of primary prevention of stroke, but management of blood pressure in patients with a previous stroke or in the setting of acute stroke is complicated by the effect blood pressure changes may have on cerebral perfusion.

REVIEW SUMMARY

For patients with previous transient ischemic attack or chronic stroke, blood pressure reduction appears to be a safe and important facet of the secondary prevention of recurrent stroke. Less information is available concerning blood pressure management in acute stroke. Current protocols require strict blood pressure control in patients who are treated with thrombolytic therapy, to reduce the risk of hemorrhagic complications. In patients presenting with acute intracerebral hemorrhage, blood pressure reduction does not appear to cause significant reduction of cerebral blood flow, but at this time there are no studies to determine if there is a clinical benefit of acute blood pressure reduction in these patients. Finally, blood pressure reduction is not routinely recommended in patients with acute ischemic stroke, as it may precipitate further cerebral ischemia. Preliminary studies suggest, in fact, that there may be a role in the future for blood pressure elevation in the treatment of patients with acute ischemic stroke.

CONCLUSIONS

Current data support the use of blood pressure reduction in the secondary prevention of stroke in patients with cerebrovascular disease. In the setting of acute stroke, however, data are limited and blood pressure management must be tailored to the specific clinical situation.

Patterns of Infarction in Hemodynamic Failure

BACKGROUND AND PURPOSE

The mechanism of stroke in patients with atherosclerotic occlusive disease and hemodynamic failure may be primarily hemodynamic or a combination of hemodynamic and embolic factors. The purpose of this study was to investigate the clinical and imaging features of stroke in these patients.

METHODS

Eleven patients with complete atherosclerotic carotid artery occlusion and increased oxygen extraction fraction measured in the hemisphere distal to the occlusion developed an ischemic stroke during the observation phase of a prospective study of cerebral hemodynamics and stroke risk. The medical and study records related to the endpoint event for these 11 patients were reviewed. Records were reviewed for evidence of associated hypotension and for specific details of the neurological deficit. Infarct location was characterized, based on review of imaging and clinical features, as: (1) middle cerebral artery (MCA) core; (2) possible cortical border zone, or (3) internal border zone.

RESULTS

One patient had a retinal infarction; the remaining 10 had MCA territory strokes. Six of the 10 infarctions occurred in the MCA core territory. Two of these 6 were fatal hemispheric events. One of the 10 infarctions occurred in the cortical border zone region. Two of the remaining 3 infarctions were localized to the internal border zone. One was indeterminate.

CONCLUSIONS

The clinical features and radiological patterns of stroke in many patients with hemodynamic impairment failure and carotid occlusion are most consistent with large artery thromboembolic stroke. These data suggest a synergistic effect between embolic and hemodynamic mechanisms for large artery thromboembolic stroke.

Asymptomatic Carotid Artery Stenosis: Past, Present and Future

BACKGROUND

The role of carotid endarterectomy (CEA) for asymptomatic carotid artery stenosis (aCAS) remains a matter of debate. It seems that not only the degree of stenosis, but also other factors have to be taken in account to improve patient selection and increase the benefit of CEA for aCAS.

METHODS AND RESULTS

The literature pertaining aCAS was reviewed in order to describe the natural history, risk of stroke and benefit of CEA for patients with aCAS in regard to several factors.

CONCLUSION

The benefit of CEA for aCAS is low. Current factors influencing the indication for CEA are severity of stenosis, age, contralateral disease, stenosis progression to >80%, gender, concomitant operations and life expectancy. To improve patient selection investigations will concentrate on plaque characteristics and instability and cerebral hemodynamics and metabolism.