Vasodilatory capacity of the cerebral vasculature in patients with carotid artery stenosis

Correlation between Morphological and Hemodynamic Parameters of Carotid Arteries and Cerebral Vasomotor Reactivity

The function of cerebral small vessels can be assessed using cerebral vasomotor reactivity (VMR). Our aim in this retrospective cross-sectional study was to investigate a correlation between carotid artery stenosis measured through ultrasonographic morphological and hemodynamic parameters and cerebral VMR. A total of 285 patients (125 males; mean age 54) were included. The breath-holding index (BHI) was used to evaluate cerebral VMR. Ultrasonographic carotid artery parameters were collected: the presence and characteristics of carotid plaques, the degree of carotid diameter stenosis, intima-media thickness (IMT), peak systolic velocity (PSV), and end diastolic velocity (EDV). Additionally, hemodynamic parameters of the middle cerebral artery (MCA) were evaluated, including the mean flow velocity (MFV) and pulsatility index (PI). The following was collected from patients' medical histories: age, gender, and vascular risk factors. A negative correlation between the BHI and age (r = -0.242, p < 0.01), BHI and the presence of carotid plaques, BHI and IMT (r = -0.203, p < 0.01), and BHI and the PI of MCA on both sides (r = -0.268, p < 0.01) was found. We found a positive correlation between the BHI in the left MCA and EDV in the left internal carotid artery (r = 0.121, p < 0.05). This study shows the correlation between cerebral VMR and carotid stenosis but indicates a higher influence of morphological parameters on VMR values.

NO Deficiency Compromises Inter- and Intrahemispheric Blood Flow Adaptation to Unilateral Carotid Artery Occlusion

Carotid artery stenosis (CAS) affects approximately 5-7.5% of older adults and is recognized as a significant risk factor for vascular cognitive impairment (VCI). The impact of CAS on cerebral blood flow (CBF) within the ipsilateral hemisphere relies on the adaptive capabilities of the cerebral microcirculation. In this study, we aimed to test the hypothesis that the impaired availability of nitric oxide (NO) compromises CBF homeostasis after unilateral carotid artery occlusion (CAO). To investigate this, three mouse models exhibiting compromised production of NO were tested: NOS1 knockout, NOS1/3 double knockout, and mice treated with the NO synthesis inhibitor L-NAME. Regional CBF changes following CAO were evaluated using laser-speckle contrast imaging (LSCI). Our findings demonstrated that NOS1 knockout, NOS1/3 double knockout, and L-NAME-treated mice exhibited impaired CBF adaptation to CAO. Furthermore, genetic deficiency of one or two NO synthase isoforms increased the tortuosity of pial collaterals connecting the frontoparietal and temporal regions. In conclusion, our study highlights the significant contribution of NO production to the functional adaptation of cerebrocortical microcirculation to unilateral CAO. We propose that impaired bioavailability of NO contributes to the impaired CBF homeostasis by altering inter- and intrahemispheric blood flow redistribution after unilateral disruption of carotid artery flow.

Assessment of cerebral autoregulatory function and inter-hemispheric blood flow in older adults with internal carotid artery stenosis using transcranial Doppler sonography-based measurement of transient hyperemic response after carotid artery compression

Unhealthy vascular aging promotes atherogenesis, which may lead to significant internal carotid artery stenosis (CAS) in 5 to 7.5% of older adults. The pathogenic factors that promote accelerated vascular aging and CAS also affect the downstream portion of the cerebral microcirculation in these patients. Primary treatments of significant CAS are eversion endarterectomy or endarterectomy with patch plasty. Factors that determine adequate hemodynamic compensation and thereby the clinical consequences of CAS as well as medical and surgical complications of carotid reconstruction surgery likely involve the anatomy of the circle of Willis (CoW), the magnitude of compensatory inter-hemispheric blood flow, and the effectiveness of cerebral microcirculatory blood flow autoregulation. This study aimed to test two hypotheses based on this theory. First, we hypothesized that patients with symptomatic and asymptomatic CAS would exhibit differences in autoregulatory function and inter-hemispheric blood flow. Second, we predicted that anatomically compromised CoW would associate with impaired inter-hemispheric blood flow compensation. We enrolled older adults with symptomatic or asymptomatic internal CAS (>70% NASCET criteria; n = 46) and assessed CoW integrity by CT angiography. We evaluated transient hyperemic responses in the middle cerebral arteries (MCA) after common carotid artery compression (CCC; 10 s) by transcranial Doppler sonography (TCD). We compared parameters reflecting autoregulatory function (e.g., transient hyperemic response ratio [THRR], return to baseline time [RTB], changes of vascular resistance) and inter-hemispheric blood flow (residual blood flow velocity). Our findings revealed that CAS was associated with impaired cerebral vascular reactivity. However, we did not observe significant differences in autoregulatory function or inter-hemispheric blood flow between patients with symptomatic and asymptomatic CAS. Moreover, anatomically compromised CoW did not significantly affect these parameters. Notably, we observed an inverse correlation between RTB and THRR, and 49% of CAS patients exhibited a delayed THRR, which associated with decreased inter-hemispheric blood flow. Future studies should investigate how TCD-based evaluation of autoregulatory function and inter-hemispheric blood flow can be used to optimize surgical techniques and patient selection for internal carotid artery revascularization.

Early Diagnosis of Intracranial Internal Carotid Artery Stenosis Using Extracranial Hemodynamic Indices from Carotid Doppler Ultrasound

Atherosclerotic intracranial internal carotid artery stenosis (IICAS) is a leading cause of strokes. Due to the limitations of major cerebral imaging techniques, the early diagnosis of IICAS remains challenging. Clinical studies have revealed that arterial stenosis may have complicated effects on the blood flow’s velocity from a distance. Therefore, based on a patient-specific one-dimensional hemodynamic model, we quantitatively investigated the effects of IICAS on extracranial internal carotid artery (ICA) flow velocity waveforms to identify sensitive hemodynamic indices for IICAS diagnoses. Classical hemodynamic indices, including the peak systolic velocity (PSV), end-diastolic velocity (EDV), and resistive index (RI), were calculated on the basis of simulations with and without IICAS. In addition, the first harmonic ratio (FHR), which is defined as the ratio between the first harmonic amplitude and the sum of the amplitudes of the 1st−20th order harmonics, was proposed to evaluate flow waveform patterns. To investigate the diagnostic performance of the indices, we included 52 patients with mild-to-moderate IICAS (<70%) in a case−control study and considered 24 patients without stenosis as controls. The simulation analyses revealed that the existence of IICAS dramatically increased the FHR and decreased the PSV and EDV in the same patient. Statistical analyses showed that the average PSV, EDV, and RI were lower in the stenosis group than in the control group; however, there were no significant differences (p > 0.05) between the two groups, except for the PSV of the right ICA (p = 0.011). The FHR was significantly higher in the stenosis group than in the control group (p < 0.001), with superior diagnostic performance. Taken together, the FHR is a promising index for the early diagnosis of IICAS using carotid Doppler ultrasound methods.

Unsupervised arterial spin labeling image super-resolution via multi-scale generative adversarial network

PURPOSE

Arterial spin labeling (ASL) magnetic resonance imaging (MRI) is an advanced non-invasive imaging technology that can measure cerebral blood flow (CBF) quantitatively without a contrast agent injection or radiation exposure. However, because of the weak labeling, conventional ASL images usually suffer from low signal-to-noise ratio (SNR), poor spatial resolution, and long acquisition time. Therefore, a method that can simultaneously improve the spatial resolution and SNR is needed.

METHODS

In this work, we proposed an unsupervised super-resolution (SR) method to improve ASL image resolution based on a pyramid of generative adversarial networks (GAN). Through layer-by-layer training, the generators can learn features from the coarsest to the finest. The last layer's generator which contains fine details and textures was used to generate the final SR ASL images. In our proposed framework, the corresponding T1-weighted MR image was supplied as a second-channel input of the generators to provide high-resolution prior information. In addition, a low-pass-filter loss term was included to suppress the noise of the original ASL images. To evaluate the performance of the proposed framework, a simulation study and two real-patient experiments based on the in vivo datasets obtained from 3 healthy subjects on a 3T MR scanner were conducted, regarding the low-resolution (LR) to normal-resolution (NR) and the NR-to-SR tasks. The proposed method was compared to the nearest neighbor interpolation, trilinear interpolation, 3rd order B-splines interpolation methods, and deep image prior (DIP) with the peak signal-to-noise ratio (PSNR) and structural similarity index (SSIM) as the quantification metrics. The averaged ASL images acquired with 44 min acquisition time were used as the ground truth for real-patient LR-to-NR study. The ablation studies of low-pass-filter loss term and T1-weighted MR image were performed based on simulation data.

RESULTS

For the simulation study, results show that the proposed method achieved significantly higher PSNR (p-value < 0.05) and SSIM (p-value < 0.05) than the nearest neighbor interpolation, trilinear interpolation, 3rd order B-splines interpolation, and DIP methods. For the real-patient LR-to-NR experiment, results show that the proposed method can generate high-quality SR ASL images with clearer structure boundaries and low noise levels, and has the highest mean PSNR and SSIM. For real-patient NR-to-SR tasks, the structure of the results using the proposed method is sharper and clearer, which are the most similar to the structure of the reference 44 min acquisition image than other methods. The proposed method also shows the ability to remove artifacts in the NR image while super-resolution. The ablation study verified that the low-pass-filter loss term and T1-weighted MR image are necessary for the proposed method.

CONCLUSIONS

The proposed unsupervised multi-scale GAN framework can simultaneously improve spatial resolution and reduce image noise. Experiment results from simulation data and 3 healthy subjects show that the proposed method achieves better performance than the nearest neighbor interpolation, the trilinear interpolation, the 3rd order B-splines interpolation, and DIP methods. This article is protected by copyright. All rights reserved.

Assessment of Cerebral Vasodilatory Capacity as Part of Catheter-Based Cerebral Angiography

BACKGROUND AND PURPOSE

Cerebral vasodilatory capacity assessment for risk stratification in patients with extracranial arterial stenosis or occlusion may be useful. We describe a new method that assesses cerebral vasodilatory capacity as part of catheter-based cerebral angiography.

METHODS

We prospectively assessed regional cerebral blood volume (rCBV) in the arterial distribution of interest using a controlled contrast injection in the common carotid or the subclavian arteries. rCBV maps were created using a predefined algorithm based on contrast distribution in the venous phase (voxel size: .466 mm³ ). rCBV maps were acquired again after selective administration of intra-arterial nicardipine (2.0 mg) distal to the stenosis. Two independent observers graded the change in rCBV in 10 predefined anatomical regions within the tributaries of the artery of interest (0 = reduction, 1 = no change, 2 = increase) and total rCBV change scores were summated.

RESULTS

Twenty-five patients with internal carotid artery stenosis (n = 18; 0-90% in severity) or extracranial vertebral artery stenosis (n = 7; 0-100% in severity) were assessed. There was an increase in rCBV in a tributary of the artery of interest in 18 of 25 after intra-arterial nicardipine (mean score: 11.98; range 0-19.5). There was no change or decrease in rCBV in 7 of 25 patients. The mean rCBV change score was similar in patients with an assessment of internal carotid artery or vertebral artery distributions (12.2 ± 5.3; 11.4 ± 2.5; P = .68).

CONCLUSION

Selective vasodilatory response to intra-arterial nicardipine in the affected arterial distribution during catheter-based cerebral angiography may provide new data for risk stratification.

A New Technique for the Assessment of Cerebral Vasodilatory Capacity as Part of Catheter-Based Cerebral Angiography

BACKGROUND

Previous studies have demonstrated the value of cerebral vasodilatory capacity assessment for risk stratification in patients with extracranial arterial stenosis or occlusion. We describe a new method that assesses cerebral vasodilatory capacity as part of catheter-based cerebral angiography.

METHODS

We assessed regional cerebral blood volume (rCBV) in the arterial distribution of interest using a controlled contrast injection through a diagnostic catheter placed in the common carotid or the subclavian artery. rCBV maps were created using predefined algorithm based on contrast distribution in the venous phase (voxel size 0.466 mm³) into high, intermediate, low, and no detectable rCBV regions. rCBV maps were acquired again after the administration of intra-arterial nicardipine (1.5-2.5 mg), and percentage increases of the area of various grades of rCBV were calculated.

RESULTS

Three patients with internal carotid artery stenosis (32% - 64% in severity) and 1 patient with extracranial vertebral artery stenosis (46% in severity) were assessed. There was a variable but consistent increase in the area of high rCBV in the ipsilateral hemisphere in 3 patients with internal carotid artery flow (5.5%-24.5%) and the cerebellum (9.6%) in 1 patient with vertebral artery flow assessments. The increase in high rCBV was most prominent in the patient who received 2.5 mg (24.5%) and least prominent in a patient who received 1.5 mg (5.5%) of intra-arterial nicardipine. There was a concurrent reduction in areas of intermediate and low rCBV (shift) in 3 patients, and there was an increase in all areas of rCBV grades (addition) in 1 patient.

CONCLUSIONS

Selective assessment of cerebral vasodilatory response in the affected arterial distribution is feasible during catheter-based cerebral angiography.

Stimulus-evoked changes in cerebral vessel diameter: A study in healthy humans

The high metabolic demand of neuronal tissue, coupled with its relatively low energy storage capacity, requires that increases in neuronal activation are quickly matched with increased blood flow to ensure efficient supply of oxygen and nutrients to the tissue. For this to occur, dilation of nearby arterioles must be coordinated with the dilation of larger upstream feeding arteries. As it stands, the exact spatial extent of such dilation in humans is unknown. Using non-invasive time-of-flight magnetic resonance angiography in healthy participants, we developed an automatic methodology for reconstructing cerebral arterial vessels and quantifying their diameter on a voxel-by-voxel basis. Specifically, we isolated the posterior cerebral artery (PCA) supplying each occipital lobe and quantified its vasodilation induced by visual stimulation. Stimulus-induced changes were strongest (∼30%) near primary visual cortex and progressively decreased in a non-linear fashion as a function of distance. Surprisingly, weak - albeit significant - changes (∼2%) were observed ∼70 mm from the visual cortex. This demonstrates that visual stimulation modulates vascular tone along the bulk of the PCA segment, and thus may have important implications for our understanding of functional hyperemia in healthy and diseased states.

Pulmonary and circulatory parameter guided anesthesia in patients with ischemic stroke undergoing endovascular recanalization

Background and purpose

Endovascular recanalization in ischemic stroke is often performed under general anesthesia. Some studies have shown a detrimental effect of general anesthesia. The reasons are unknown.

Methods

This was an observational study with retrospective and prospective phases. From 2008 to 2010, 60 patients treated by endovascular recanalization due to proximal vessel occlusion were analyzed with regard to ventilation parameters, blood gas values, blood pressure, and clinical parameters (pre-protocol phase). Subsequently, a protocol with target values for end-tidal CO2 (Petco2) and systolic blood pressure (SBP) was introduced and prospectively analyzed in 64 patients in 2012 (protocol phase).

Results

In the pre-protocol phase, significant hypocapnia (<30 mm Hg), a decrease in SBP after intervention (p<0.001), and an increase in SBP after extubation (p<0.001) were observed. After implementing the protocol in 2012, 63% of Petco2 values and 55% of SBP values (median) of the duration of intervention were within the predefined range. Severe hypocapnia and hypotension (SBP <100 mm Hg) after the intervention were significantly reduced. Longer duration of Petco2 values within 40–45 mm Hg, intracerebral hemorrhage, longer door to needle time, older age, unsuccessful recanalization, longer duration of endovascular treatment, and higher cumulative dose of norepinephrine were associated with an unfavorable outcome (modified Rankin Scale score >2). Intracerebral hemorrhage (OR 0.028, p=0.001), age (OR 0.9, p=0.013), and cumulative dose of norepinephrine (OR 0.142, p=0.003) were independent predictors of an unfavorable outcome.

Conclusions

In patients receiving endovascular stroke treatment under general anesthesia, the cumulative dose of norepinephrine was an independent predictor of an unfavorable outcome. Further studies are needed to evaluate the optimal management of blood pressure in these patients, and whether avoidance of catecholamines could partly explain the improved outcomes for patients treated under conscious sedation in retrospective studies.

Assessment of middle cerebral artery diameter during hypocapnia and hypercapnia in humans using ultra-high-field MRI

In the evaluation of cerebrovascular CO2 reactivity measurements, it is often assumed that the diameter of the large intracranial arteries insonated by transcranial Doppler remains unaffected by changes in arterial CO2 partial pressure. However, the strong cerebral vasodilatory capacity of CO2 challenges this assumption, suggesting that there should be some changes in diameter, even if very small. Data from previous studies on effects of CO2 on cerebral artery diameter [middle cerebral artery (MCA)] have been inconsistent. In this study, we examined 10 healthy subjects (5 women, 5 men, age 21-30 yr). High-resolution (0.2 mm in-plane) MRI scans at 7 Tesla were used for direct observation of the MCA diameter during hypocapnia, -1 kPa (-7.5 mmHg), normocapnia, 0 kPa (0 mmHg), and two levels of hypercapnia, +1 and +2 kPa (7.5 and 15 mmHg), with respect to baseline. The vessel lumen was manually delineated by two independent observers. The results showed that the MCA diameter increased by 6.8 ± 2.9% in response to 2 kPa end-tidal P(CO2) (PET(CO2)) above baseline. However, no significant changes in diameter were observed at the -1 kPa (-1.2 ± 2.4%), and +1 kPa (+1.4 ± 3.2%) levels relative to normocapnia. The nonlinear response of the MCA diameter to CO2 was fitted as a continuous calibration curve. Cerebral blood flow changes measured by transcranial Doppler could be corrected by this calibration curve using concomitant PET(CO2) measurements. In conclusion, the MCA diameter remains constant during small deviations of the PET(CO2) from normocapnia, but increases at higher PET(CO2) values.

Hyperventilation, cerebral perfusion, and syncope

This review summarizes evidence in humans for an association between hyperventilation (HV)-induced hypocapnia and a reduction in cerebral perfusion leading to syncope defined as transient loss of consciousness (TLOC). The cerebral vasculature is sensitive to changes in both the arterial carbon dioxide (PaCO2) and oxygen (PaO2) partial pressures so that hypercapnia/hypoxia increases and hypocapnia/hyperoxia reduces global cerebral blood flow. Cerebral hypoperfusion and TLOC have been associated with hypocapnia related to HV. Notwithstanding pronounced cerebrovascular effects of PaCO2 the contribution of a low PaCO2 to the early postural reduction in middle cerebral artery blood velocity is transient. HV together with postural stress does not reduce cerebral perfusion to such an extent that TLOC develops. However when HV is combined with cardiovascular stressors like cold immersion or reduced cardiac output brain perfusion becomes jeopardized. Whether, in patients with cardiovascular disease and/or defect, cerebral blood flow cerebral control HV-induced hypocapnia elicits cerebral hypoperfusion, leading to TLOC, remains to be established.

Functional imaging of cerebral perfusion

The functional imaging of perfusion enables the study of its properties such as the vasoreactivity to circulating gases, the autoregulation and the neurovascular coupling. Downstream from arterial stenosis, this imaging can estimate the vascular reserve and the risk of ischemia in order to adapt the therapeutic strategy. This method reveals the hemodynamic disorders in patients suffering from Alzheimer's disease or with arteriovenous malformations revealed by epilepsy. Functional MRI of the vasoreactivity also helps to better interpret the functional MRI activation in practice and in clinical research.

Near-infrared transillumination back scattering sounding--new method to assess brain microcirculation in patients with chronic carotid artery stenosis

Purpose

The purpose of the study was to assess the responses of pial artery pulsation (cc-TQ) and subarachnoid width (sas-TQ) to acetazolamide challenge in patients with chronic carotid artery stenosis and relate these responses to changes in peak systolic velocity (PSV), cerebral blood flow (CBF), cerebral blood volume (CBV), mean transit time (MTT) and time to peak response (TTP).

Methods

Fifteen patients with carotid artery stenosis ≥90% on the ipsilateral side and <50% on the contralateral side were enrolled into the study. PSV was assessed using colour-coded duplex sonography, CBF, CBV, MTT and TTP with perfusion computed tomography, cc-TQ and sas-TQ with near-infrared transillumination/backscattering sounding (NIR-T/BSS).

Results

Based on the ipsilateral/contralateral cc-TQ ratio after acetazolamide challenge two groups of patients were distinguished: the first group with a ratio ≥1 and the second with a ratio <1. In the second group increases in CBF and CBV after the acetazolamide test were significantly higher in both hemispheres (ipsilateral: +33.0%±8.1% vs. +15.3%±4.4% and +26.3%±6.6% vs. +14.3%±5.1%; contralateral: +26.8%±7.0% vs. +17.6%±5.6% and +20.0%±7.3% vs. +10.0%±3.7%, respectively), cc-TQ was significantly higher only on the ipsilateral side (+37.3%±9.3% vs. +26.6%±8.6%) and the decrease in sas-TQ was less pronounced on the ipsilateral side (−0.7%±1.5% vs. −10.2%±1.5%), in comparison with the first group. The changes in sas-TQ following the acetazolamide test were consistent with the changes in TTP.

Conclusions

The ipsilateral/contralateral cc-TQ ratio following acetazolamide challenge may be used to distinguish patient groups characterized by different haemodynamic parameters. Further research on a larger group of patients is warranted.

Arterial Spin Labeling (ASL) fMRI: advantages, theoretical constrains, and experimental challenges in neurosciences

Cerebral blood flow (CBF) is a well-established correlate of brain function and therefore an essential parameter for studying the brain at both normal and diseased states. Arterial spin labeling (ASL) is a noninvasive fMRI technique that uses arterial water as an endogenous tracer to measure CBF. ASL provides reliable absolute quantification of CBF with higher spatial and temporal resolution than other techniques. And yet, the routine application of ASL has been somewhat limited. In this review, we start by highlighting theoretical complexities and technical challenges of ASL fMRI for basic and clinical research. While underscoring the main advantages of ASL versus other techniques such as BOLD, we also expound on inherent challenges and confounds in ASL perfusion imaging. In closing, we expound on several exciting developments in the field that we believe will make ASL reach its full potential in neuroscience research.

Novel MRI approaches for assessing cerebral hemodynamics in ischemic cerebrovascular disease

Changes in cerebral hemodynamics underlie a broad spectrum of ischemic cerebrovascular disorders. An ability to accurately and quantitatively measure hemodynamic (cerebral blood flow and cerebral blood volume) and related metabolic (cerebral metabolic rate of oxygen) parameters is important for understanding healthy brain function and comparative dysfunction in ischemia. Although positron emission tomography, single-photon emission tomography, and gadolinium-MRI approaches are common, more recently MRI approaches that do not require exogenous contrast have been introduced with variable sensitivity for hemodynamic parameters. The ability to obtain hemodynamic measurements with these new approaches is particularly appealing in clinical and research scenarios in which follow-up and longitudinal studies are necessary. The purpose of this review is to outline current state-of-the-art MRI methods for measuring cerebral blood flow, cerebral blood volume, and cerebral metabolic rate of oxygen and provide practical tips to avoid imaging pitfalls. MRI studies of cerebrovascular disease performed without exogenous contrast are synopsized in the context of clinical relevance and methodological strengths and limitations.

Time course of vascular reactivity using repeated phase-contrast MR angiography in patients with carotid artery stenosis

BACKGROUND AND PURPOSE

Cerebral vascular reactivity assessment is typically performed with 2 perfusion measurements before and after a vasodilatory challenge. The aim of this study was to assess the time course of the vasodilatory effect in the brain-feeding arteries after a challenge with acetazolamide in patients with a stenosis of the internal carotid artery (ICA).

METHODS

Twenty-one patients with a symptomatic ICA stenosis and 18 healthy control subjects underwent 2-dimensional phase-contrast MR angiography to repeatedly measure the blood flow (mL/min) in both ICAs at baseline and in 5-minute intervals for 30 minutes after intravenous administration of acetazolamide.

RESULTS

At baseline, the blood flow was significantly lower in the stenosed ICAs of patients (155 ± 17 mL/min) than in the contralateral ICAs (237 ± 21 mL/min, P<0.05) and the ICAs of healthy control subjects (249 ± 15 mL/min, P<0.05) and remained lower throughout the time course. The maximum vasodilatory effect in the stenosed ICAs was observed after 15.3 ± 0.9 minutes, which was significantly later than in the contralateral ICAs (within 12.9 ± 0.7 minutes, P<0.05) and healthy ICAs (within 12.8 ± 0.8 minutes, P<0.05).

CONCLUSIONS

The onset of the maximum vasodilatory effect after administration of acetazolamide is delayed in patients with a symptomatic ICA stenosis.