Laleh Zarrinkoob

Selective antegrade cerebral perfusion (SACP) maintains cerebral perfusion during aortic arch surgery and is mainly achieved by cannulating unilaterally (uSACP) or bilaterally (bSACP). The choice of applying uSACP or bSACP is often made according to institutional practice, whereas neurological complications may be reduced if the approaches instead were applied according to the patient’s individual cerebral collateral circulation. However, no method for predicting intraoperative cerebral collateral capacity is currently available. This study evaluates a computational method for predicting the cerebral perfusion pressure (CPP) in patients undergoing aortic arch surgery with SACP. Twenty-two patients underwent elective aortic arch surgery with bSACP, including a five-minute intervention of right-sided uSACP. Patients preoperatively underwent computed tomography angiography and 4D flow magnetic resonance imaging to assess patient-specific arterial anatomy and blood flows. Computational fluid dynamics modelling estimated the vascular resistance of the vascular beds distal of the segmented arterial anatomy. Based on these resistances, a subsequent model predicted CPP during uSACP and bSACP given intraoperatively measured input. There was no difference between predicted and measured left side CPP for uSACP (-3.0 ± 11.2 mmHg, p=0.25; r=0.75, p<0.001) or bSACP (-0.4 ± 5.7 mmHg, p=0.78; r=0.94, p<0.001). There was good group level agreement (90% CI, uSACP: [-7.5, 1.4]; bSACP: [-2.6, 1.9]) whereas limits of agreement showed substantial spread (uSACP: [-24.9, 18.9], bSACP: [-11.5, 10.8]). The new method predicted CPP on group level, while improved individual prediction accuracy may be achievable through refined arterial segmentation and more accurate estimation of intraoperative vascular resistances.

Modelling of hemodynamics in the circle of Willis (CoW) depends on vascular segmentation, which may vary based on imaging modality. Computed tomography angiography (CTA) is commonly used in clinic but involves radiation and injection of contrast agents, whereas magnetic resonance angiography (MRA) offers a non-invasive alternative. This study aims to compare CoW morphology and modelled cerebral perfusion pressure of CTA and MRA segmentations, validating if MRA can replace CTA in modelling workflows. CTA and time-of-flight MRA (TOF-MRA) of the CoW was performed in 19 patients undergoing elective aortic arch surgery (67±7 years, 8 women). The CoW was semi-automatically segmented based on signal intensity thresholding. A TOF-MRA threshold was optimized against the CTA segmentation, using the CTA as reference standard. Computational fluid dynamics (CFD) modelling with boundary conditions based on subject-specific flow rates from 4D flow MRI simulated cerebral perfusion pressure in the segmented geometries. A baseline simulation and a unilateral brain inflow simulation, i.e., occlusion of a carotid, were carried out. Linear mixed models indicated there was no effect of choice of modality on either average arterial lumen area (CTA − TOF-MRA: -0.2±1.3 mm ² ; p=0.762) or baseline pressure drops (0.2±1.9 mmHg; p=0.257). In the unilateral inflow simulation, we found no difference in pressure laterality (-6.6±18.4 mmHg; p=0.185) or collateral flow rate (10±46 ml/min; p=0.421). TOF-MRA geometries can with signal intensity thresholding be matched to produce similar morphology and modelled cerebral perfusion pressure to CTA geometries. The modelled pressure drops over the collateral arteries were sensitive to the segmentation regardless of modality.

Background: Induced hypertension is used clinically to increase cerebral blood flow (CBF) in conditions such as vasospasm after subarachnoid hemorrhage. However, increased blood pressure also raises pulsatile force. Cerebrovascular compliance plays a key role in buffering flow dynamics and protecting the microcirculation, but whether it adapts to elevated pressure remains unclear. This study assessed the response of compliant cerebral arteries to induced hypertension in healthy adults using phase-contrast magnetic resonance imaging (PCMRI) and two compliance models: a two-element Windkessel (C_WK) and a simplified model (C_VP), representing the extremes of pulsatility transmission at the capillary level. Methods: Eighteen healthy adults (median age: 34 years; 9 females) underwent PCMRI at baseline and after increasing mean arterial pressure by 20% using norepinephrine (NE) infusion. PCMRI quantified CBF and cardiac output, while cerebrovascular resistance and systemic vascular resistance were derived. Flow waveforms were combined with blood pressure to assess C_WK and C_VP in CBF, ascending/descending aorta, and external carotid arteries, while corresponding regions of interest were used to calculate cross-sectional flow areas. Data are reported as median (interquartile range). Results: NE increased cerebrovascular compliance significantly; C_WK by 110% (56% to 163%; P=0.001) and C_VP by 11% (−2% to 26%; P=0.018). C_WK increased in the external carotid artery by 12% (1% to 32%; P=0.037) but did not change in the ascending or descending aorta. C_VP decreased in the descending aorta by 5% (-11% to 2%; P=0.028), with no changes in the ascending aorta or external carotid artery. Cross-sectional area of cerebral arteries contributing to CBF decreased by 5% (-17% to -3%; P=0.033), while the ascending and descending aorta areas increased by 7% (4% to 11%; P=0.012) and 8% (6% to 11%; P<0.001), respectively. Conclusion: Cerebral arteries enhanced their compliance during NE-induced hypertension, unlike systemic arteries, regardless of the assumed degree of pulsatility transmission.

Background: Compromised cerebral blood flow can contribute to future ischemic events in patients with symptomatic carotid artery disease. However, there is limited knowledge of the effects on cerebral hemodynamics resulting from a reduced internal carotid artery (ICA) blood flow rate (BFR).Purpose: Investigate how reduced ICA‐BFR, relates to BFR in the cerebral arteries.Study Type: Prospective.Subjects: Thirty‐eight patients, age 72 ± 6 years (11 female).Field Strength/Sequence: 3‐Tesla, four‐dimensional phase‐contrast magnetic resonance imaging (4D‐PCMRI).Assessment: Patients with ischemic stroke or transient ischemic attack were evaluated regarding the degree of stenosis. 4D‐PCMRI was used to measure cerebral BFR in 38 patients with symptomatic carotid stenosis (≥50%). BFR in the cerebral arteries was assessed in two subgroups based on symptomatic ICA‐BFR: reduced ICA‐flow (t‐test/paired t‐test) were used to compare groups and hemispheres. Significance was determined at P < 0.05.Results: The degree of stenosis was not significantly different, 80% (95% confidence interval [CI] = 73%–87%) in the reduced ICA‐flow vs. 72% (CI = 66%–76%) in the preserved ICA‐flow; P = 0.09. In the reduced ICA‐flow group, a significantly reduced BFR was found in the ipsilateral middle cerebral artery and anterior cerebral artery (A1), while significantly increased in the contralateral A1. Retrograde BFR was found in the posterior communicating artery and ophthalmic artery. Significant BFR laterality was present in all paired arteries in the reduced ICA‐flow group, contrasting the preserved ICA‐flow group (P = 0.14–0.93).Data Conclusions: 4D‐PCMRI revealed compromised cerebral BFR due to carotid stenosis, not possible to detect by solely analyzing the degree of stenosis. In patients with reduced ICA‐flow, collaterals were not sufficient to maintain symmetrical BFR distribution to the two hemispheres.Evidence Level: 2Technical Efficacy: Stage 3

Background: Adequate cerebral perfusion is central during general anesthesia. However, perfusion is not readily measured bedside. Clinicians currently rely mainly on mean arterial pressure (MAP) as a surrogate, even though the relationship between blood pressure and cerebral blood flow is not well understood. The aim of this study was to apply phase-contrast magnetic resonance imaging to characterize blood flow responses in healthy volunteers to commonly used pharmacologic agents that increase or decrease arterial blood pressure. Methods: Eighteen healthy volunteers aged 30 to 50 yr were investigated with phase-contrast magnetic resonance imaging. Intra-arterial blood pressure monitoring was used. First, intravenous noradrenaline was administered to a target MAP of 20% above baseline. After a wash-out period, intravenous labetalol was given to a target MAP of 15% below baseline. Cerebral blood flow was measured using phase-contrast magnetic resonance imaging and defined as the sum of flow in the internal carotid arteries and vertebral arteries. Cardiac output (CO) was defined as the flow in the ascending aorta. Results: Baseline median cerebral blood flow was 772 ml/min (interquartile range, 674 to 871), and CO was 5,874 ml/min (5,199 to 6,355). The median dose of noradrenaline was 0.17 µg · kg⁻¹ · h⁻¹ (0.14 to 0.22). During noradrenaline infusion, cerebral blood flow decreased to 705 ml/min (606 to 748; P = 0.001), and CO decreased to 4,995 ml/min (4,705 to 5,635; P = 0.01). A median dose of labetalol was 120 mg (118 to 150). After labetalol boluses, cerebral blood flow was unchanged at 769 ml/min (734 to 900; P = 0.68). CO increased to 6,413 ml/min (6,056 to 7,464; P = 0.03). Conclusions: In healthy, awake subjects, increasing MAP using intravenous noradrenaline decreased cerebral blood flow and CO. These data do not support inducing hypertension with noradrenaline to increase cerebral blood flow. Cerebral blood flow was unchanged when decreasing MAP using labetalol. Editor’s Perspective: What We Already Know about This Topic: What This Article Tells Us That Is New:

Background: Compromised cerebral blood flow has been identified as a contributing risk factor for future ischemic events in patients with symptomatic carotid artery disease. Nevertheless, the hemodynamic impact of carotid stenosis is rarely evaluated. The aim of this observational cross-section study was to investigate how a reduced internal carotid artery (ICA) blood flow rate (BFR), rather than stenosis degree, relates to BFR in the cerebral arteries. Methods: Four-dimensional phase-contrast magnetic resonance imaging (4D-PCMRI) was used to measure cerebral blood flow in 38 patients with symptomatic carotid stenosis (≥50%), being considered for carotid endarterectomy. The BFR in the cerebral arteries was compared between two subgroups that were based on the BFR in the symptomatic (ipsilateral) ICA: I. reduced ICA flow (P =0.09). In the reduced group compared to the preserved ICA flow group, a reduced BFR was found in the ipsilateral middle cerebral artery (MCA) (108±32 vs. 136±24 mL/min; P P P P 0.05). Conclusions: 4D-PCMRI revealed compromised cerebral blood flow in patients with carotid stenosis, not possible to detect by solely analyzing the stenosis degree. In patients with reduced ICA flow, collateral recruitment was not sufficient to maintain symmetrical BFR distribution to the two hemispheres.

SUMMARY: Carotid near-occlusion is a frequently overlooked diagnosis when CTA examinations are assessed in routine practice. To evaluate the potential value of phase-contrast MR imaging in identifying near-occlusion, we examined 9 carotid near-occlusions and 20 cases of conventional ≥50% carotid stenosis (mean stenosis degree, 65%) with phase-contrast MR imaging. Mean ICA flow was lower in near-occlusions (52 mL/min) than in conventional ≥50% stenosis (198 mL/min, P < .001). ICA flow of ≤110 mL/min was 100% sensitive and specific for near-occlusion. Phase-contrast MR imaging is a promising tool for diagnosing carotid near-occlusion.

Background and Purpose—: Four-dimensional phase-contrast magnetic resonance imaging enables quantification of blood flow rate (BFR; mL/min) in multiple cerebral arteries simultaneously, making it a promising technique for hemodynamic investigation in patients with stroke. The aim of this study was to quantify the hemodynamic disturbance and the compensatory pattern of collateral flow in patients with symptomatic carotid stenosis. Methods—: Thirty-eight patients (mean, 72 years; 27 men) with symptomatic carotid stenosis (≥50%) or occlusion were investigated using 4-dimensional phase-contrast magnetic resonance imaging. For each patient, BFR was measured in 19 arteries/locations. The ipsilateral side to the symptomatic carotid stenosis was compared with the contralateral side. Results—: Internal carotid artery BFR was lower on the ipsilateral side (134±87 versus 261±95 mL/min; P P P =0.001, and −2±12 versus 6±6 mL/min, P =0.03, respectively). Despite these compensations, BFR in MCA was lower on the ipsilateral side, and this laterality was more pronounced in patients with severe carotid stenosis (≥70%). Although comparing ipsilateral MCA BFR between stenosis groups (P =0.95). Conclusions—: With a novel approach using 4-dimensional phase-contrast magnetic resonance imaging, we could simultaneously quantify and rank the importance of collateral routes in patients with carotid stenosis. An important observation was that contralateral internal carotid artery mainly secured the bilateral anterior cerebral artery territory. Because of the collateral recruitment, compromised BFR in MCA is not necessarily related to the degree of carotid stenosis. These findings highlight the importance of simultaneous investigation of the hemodynamics of the entire cerebral arterial tree.

Excessive pulsatile flow caused by aortic stiffness is thought to be a contributing factor for several cerebrovascular diseases. The main purpose of this study was to describe the dampening of the pulsatile flow from the proximal to the distal cerebral arteries, the effect of aging and sex, and its correlation to aortic stiffness. Forty-five healthy elderly (mean age 71 years) and 49 healthy young (mean age 25 years) were included. Phase-contrast magnetic resonance imaging was used for measuring blood flow pulsatility index and dampening factor (proximal artery pulsatility index/distal artery pulsatility index) in 21 cerebral and extra-cerebral arteries. Aortic stiffness was measured as aortic pulse wave velocity. Cerebral arterial pulsatility index increased due to aging and this was more pronounced in distal segments of cerebral arteries. There was no difference in pulsatility index between women and men. Dampening of pulsatility index was observed in all cerebral arteries in both age groups but was significantly higher in young subjects than in elderly. Pulse wave velocity was not correlated with cerebral arterial pulsatility index. The increased pulsatile flow in elderly together with reduced dampening supports the pulse wave encephalopathy theory, since it implies that a higher pulsatile flow is reaching distal arterial segments in older subjects.