Unilateral fetal-type circle of Willis anatomy causes right-left asymmetry in cerebral blood flow with pseudo-continuous arterial spin labeling: A limitation of arterial spin labeling-based cerebral blood flow measurements?

Real-time model-based cerebral perfusion calculation for ischemic stroke

BACKGROUND AND OBJECTIVES

Clinical diagnosis of ischemic stroke commonly relies on examining cerebral perfusion changes by using computed tomography perfusion (CTP) techniques. However, the radiation dose in CTP is quite higher in comparison to computed tomography angiography (CTA), with associated costs and time.

METHODS

Hence, this study established a lumped-parameter model (LPM) of brain tissue microcirculation (BTM) based on CTA, aiming to achieve real-time calculation of cerebral perfusion. After validation of calculated flow results with clinical data, the BTM-LPM model was used to examine the changes in cerebral perfusion following ischemic stroke, in which the effects of nine anatomical structures of the Circle of Willis (CoW) together with various distribution patterns of stenosis in the feeding arteries were considered.

RESULTS

When compared the calculated flow results from BTM-LPM with the clinically measured data of literature, the mean squared error (MSE) value for the feeding arteries was 3.9 % and its total value for microcirculatory flow in each region was 0.1 %. Notably, the calculation time was 35.6 s. In the case of the CoW missing the left and right posterior communicating artery, a 60 % stenosis of the basilar artery is likely to cause ischemic damage to some temporal and occipital lobes of the right and left hemispheres. While in the case of the CoW missing the anterior communicating artery and the left posterior communicating artery, ischemic damage to the entire frontal lobe and parts of the temporal and parietal lobes of the left hemisphere was found when 80 % stenosis occurred in the left internal carotid artery.

CONCLUSIONS

The BTM-LPM proposed in this study could accurately calculate cerebral perfusion in real time and demonstrated the importance of CoW anatomy in different ischemic injuries to cerebral tissue. The calculated cerebral perfusion would be a reference value for early diagnosis and preoperative planning of different ischemic injuries to cerebral tissue, thereby the BTM-LPM holds great promising for replacing CTP examination.

Clinical applications of arterial spin labeling of the intracranial compartment in vascular anomalies-A case-based review

Arterial spin labeling (ASL) is a magnetic resonance perfusion technique that allows for quantification of cerebral blood flow (CBF) without the use of contrast or radiation. Several applications of ASL have been described in diagnosis of strokes and stroke mimics, intracranial tumors, and other conditions. Various vascular anomalies exhibit specific CBF patterns that correlate with different signal intensities on ASL. In this case-based review, we demonstrate the utility of ASL in diagnosis and surveillance of vascular anomalies in the intracranial compartment.

Current state and guidance on arterial spin labeling perfusion MRI in clinical neuroimaging

This article focuses on clinical applications of arterial spin labeling (ASL) and is part of a wider effort from the International Society for Magnetic Resonance in Medicine (ISMRM) Perfusion Study Group to update and expand on the recommendations provided in the 2015 ASL consensus paper. Although the 2015 consensus paper provided general guidelines for clinical applications of ASL MRI, there was a lack of guidance on disease-specific parameters. Since that time, the clinical availability and clinical demand for ASL MRI has increased. This position paper provides guidance on using ASL in specific clinical scenarios, including acute ischemic stroke and steno-occlusive disease, arteriovenous malformations and fistulas, brain tumors, neurodegenerative disease, seizures/epilepsy, and pediatric neuroradiology applications, focusing on disease-specific considerations for sequence optimization and interpretation. We present several neuroradiological applications in which ASL provides unique information essential for making the diagnosis. This guidance is intended for anyone interested in using ASL in a routine clinical setting (i.e., on a single-subject basis rather than in cohort studies) building on the previous ASL consensus review.

Asymmetry in cerebral perfusion from circle of Willis arterial variations in normal population

BACKGROUND

Angiographic and cadaveric studies have evidenced variations in the circle of Willis (CoW). Age-related changes in cerebral hemodynamics may be attributable to vascular variations.

OBJECTIVES

The objective is to assess interdependence of completeness of CoW with age using non-invasive MRA and cerebral perfusion using arterial spin labeling (ASL).

METHODS

This single-center, prospective study segregated 189 subjects into three groups: ≤5, 5 to 18, and >18 years. Angiographic (complete CoW and vascular asymmetry index) using TOF and contrast-enhanced- (CE-) MRA, and perfusion (perfusion asymmetry index) data using ASL were obtained.

RESULTS

One hundred and six (56.08%) subjects showed complete CoW on TOF and 100 (52.91%) on CE-MRA. Anterior and posterior collateral pathways were more prevalent in the younger population. Completeness of CoW decreased with increasing age, group 1 (54/60, 90% TOF; 51/60, 85% CE), group 2 (39/64, 60% TOF; 37/64, 56.92% CE), and group 3 (13/65, 20.31% TOF; 12/65, 18.75% CE); p-value < .0001. A statistically significant decrease in cerebral and cerebellar perfusion with increasing age was seen. Cerebellar to frontal perfusion change was higher in group 1. Fetal posterior cerebral artery (PCA) led to ipsilateral low and contralateral hyperperfusion flow asymmetries between occipital lobes.

CONCLUSIONS

This study shows that a complete CoW is commoner in pediatrics than adults and with increasing age, the completeness of CoW decreases paralleled by decrease in cerebral and cerebellar perfusion. There is age-related shift of perfusion from hindbrain to forebrain and the regression of PCoA occurs with increasing age leading to alterations in cerebral perfusion and hemodynamics.

Clinical application of brain perfusion imaging in detecting stroke mimics: A review

Stroke mimics constitute a significant proportion of patients with suspected acute ischemic stroke. These conditions may resemble acute ischemic stroke and demonstrate abnormalities on perfusion imaging sequences. The most common stroke mimics include seizure/epilepsy, migraine with aura, brain tumors, functional disorders, infectious encephalopathies, Wernicke's encephalopathy, and metabolic abnormalities. Brain perfusion imaging techniques, particularly computed tomography perfusion and magnetic resonance perfusion, are being widely used in routine clinical practice for treatment selection in patients presenting with large vessel occlusion. At the same time, the utilization of these imaging modalities enables the opportunity to better diagnose patients with stroke mimics in a time-sensitive setting, leading to appropriate management, decision-making, and resource allocation. In this review, we describe patterns of perfusion abnormalities that could discriminate patients with stroke mimics from those with acute ischemic stroke and provide specific case examples to illustrate these perfusion abnormalities. In addition, we discuss the challenges associated with interpretation of perfusion images in stroke-related pathologies. In general, perfusion imaging can provide additional information in some cases-when used in combination with conventional magnetic resonance imaging and computed tomography-and might help in detecting stroke mimics among patients who present with acute onset focal neurological symptoms.

Arterial Spin Labeling technique and clinical applications of the intracranial compartment in stroke and stroke mimics - A case-based review

Magnetic resonance imaging perfusion (MRP) techniques can improve the selection of acute ischemic stroke patients for treatment by estimating the salvageable area of decreased perfusion, that is, penumbra. Arterial spin labeling (ASL) is a noncontrast MRP technique that is used to assess cerebral blood flow without the use of intravenous gadolinium contrast. Thus, ASL is of particular interest in stroke imaging. This article will review clinical applications of ASL in stroke such as assessment of the core infarct and penumbra, localization of the vascular occlusion, and collateral status. Given the nonspecific symptoms that patients can present with, differentiating between stroke and a stroke mimic is a diagnostic dilemma. ASL not only helps in differentiating stroke from stroke mimic but also can be used to specify the exact mimic when used in conjunction with the symptomatology and structural imaging. In addition to a case-based overview of clinical applications of the ASL in stroke and stroke mimics in this article, the more commonly used ASL labeling techniques as well as emerging ASL techniques, future developments, and limitations will be reviewed.

Apparent posterior cerebral artery territory perfusion asymmetry on arterial spin labeling MRI is a common non-pathologic finding in patients with a unilateral fetal posterior cerebral artery

Purpose

To determine the frequency of apparent posterior cerebral artery (PCA) territory asymmetry seen on arterial spin labeling (ASL) imaging in patients with a unilateral fetal PCA, but without underlying clinical or imaging pathology to suggest true hypoperfusion.

Methods

A search of radiology reports from 1/2017 through 6/2020 was performed with the inclusion term "fetal PCA.” Eighty patients were included with unilateral fetal PCA confirmed on MRA or CTA, with brain MRI including ASL imaging, and without conventional imaging abnormality or clinical symptoms referable to the PCA territories. Cases were evaluated by two independent readers for visually apparent PCA perfusion asymmetries. ASL imaging consisted of pseudocontinuous ASL with 1.5 s labeling duration and 2 s post-labeling delay adapted from white paper recommendations.

Results

Thirteen of 80 cases (16.2%) had apparent hypoperfusion in the PCA territory contralateral to the side of the fetal PCA. Agreement between readers was near perfect (97.5%, κ = 0.91). This finding was more common in patients who were older, scanned on a 3.0 T magnet, and who had non-visualization of the posterior communicating artery contralateral to the fetal PCA.

Conclusion

Apparent PCA hypoperfusion on ASL is not uncommon in patients with a contralateral fetal PCA who have no clinical or conventional imaging findings to suggest true hypoperfusion. This phenomenon is likely due to differential blood velocities between the carotid and vertebral arteries that result in differential arterial transit times and labeling efficiency. It is important for radiologists to know that apparent hypoperfusion may arise from variant circle of Willis anatomy.

Normative distribution of posterior circulation tissue time-to-maximum: Effects of anatomic variation, tracer kinetics, and implications for patient selection in posterior circulation ischemic stroke

The generalization of perfusion-based, anterior circulation large vessel occlusion selection criteria to posterior circulation stroke is not straightforward due to physiologic delay, which we posit produces physiologic prolongation of the posterior circulation perfusion time-to-maximum (Tmax). To assess normative Tmax distributions, patients undergoing CTA/CTP for suspected ischemic stroke between 1/2018-3/2019 were retrospectively identified. Subjects with any cerebrovascular stenoses, or with follow-up MRI or final clinical diagnosis of stroke were excluded. Posterior circulation anatomic variations were identified. CTP were processed in RAPID and segmented in a custom pipeline permitting manually-enforced arterial input function (AIF) and perfusion estimations constrained to pre-specified vascular territories. Seventy-one subjects (mean 64 ± 19 years) met inclusion. Median Tmax was significantly greater in the cerebellar hemispheres (right: 3.0 s, left: 2.9 s) and PCA territories (right: 2.9 s; left: 3.3 s) than in the anterior circulation (right: 2.4 s; left: 2.3 s, p < 0.001). Fetal PCA disposition eliminated ipsilateral PCA Tmax delays (p = 0.012). Median territorial Tmax was significantly lower with basilar versus any anterior circulation AIF for all vascular territories (p < 0.001). Significant baseline delays in posterior circulation Tmax are observed even without steno-occlusive disease and vary with anatomic variation and AIF selection. The potential for overestimation of at-risk volumes in the posterior circulation merits caution in future trials.

Variations in the Circle of Willis in a large population sample using 3D TOF angiography: The Tromsø Study

The main arteries that supply blood to the brain originate from the Circle of Willis (CoW). The CoW exhibits considerable anatomical variations which may have clinical importance, but the variability is insufficiently characterised in the general population. We assessed the anatomical variability of CoW variants in a community-dwelling sample (N = 1,864, 874 men, mean age = 65.4, range 40–87 years), and independent and conditional frequencies of the CoW’s artery segments. CoW segments were classified as present or missing/hypoplastic (w/1mm diameter threshold) on 3T time-of-flight magnetic resonance angiography images. We also examined whether age and sex were associated with CoW variants. We identified 47 unique CoW variants, of which five variants constituted 68.5% of the sample. The complete variant was found in 11.9% of the subjects, and the most common variant (27.8%) was missing both posterior communicating arteries. Conditional frequencies showed patterns of interdependence across most missing segments in the CoW. CoW variants were associated with mean-split age (P = .0147), and there was a trend showing more missing segments with increasing age. We found no association with sex (P = .0526). Our population study demonstrated age as associated with CoW variants, suggesting reduced collateral capacity with older age.

Spatial coefficient of variation of arterial spin labeling MRI as a cerebrovascular correlate of carotid occlusive disease

Clinical interpretation of arterial spin labeling (ASL) perfusion MRI in cerebrovascular disease remains challenging mainly because of the method's sensitivity to concomitant contributions from both intravascular and tissue compartments. While acquisition of multi-delay images can differentiate between the two contributions, the prolonged acquisition is prone to artifacts and not practical for clinical applications. Here, the utility of the spatial coefficient of variation (sCoV) of a single-delay ASL image as a marker of the intravascular contribution was evaluated by testing the hypothesis that sCoV can detect the effects of differences in label arrival times between ipsi- and contra-lateral hemispheres even in the absence of a hemispheric difference in CBF. Hemispheric lateralization values for sCoV and CBF were computed from ASL images acquired on 28 patients (age 73.9 ± 10.2 years, 8 women) with asymptomatic unilateral carotid occlusion. The results showed that sCoV lateralization predicted the occluded side with 96.4% sensitivity, missing only 1 patient. In contrast, the sensitivity of the CBF lateralization was 71.4%, with 8 patients showing no difference in CBF between hemispheres. The findings demonstrate the potential clinical utility of sCoV as a cerebrovascular correlate of large vessel disease. Using sCoV in tandem with CBF, vascular information can be obtained in image processing without the need for additional scan-time.

Association between Circle of Willis Configuration and Rupture of Cerebral Aneurysms

Background and Objectives: Intracranial hemorrhage caused by the rupture of brain aneurysms occurs in almost 10 per 100,000 people whereas the incidence of such aneurysms is significantly higher, accounting for 4–9%. Linking certain factors to cerebral aneurysm rupture could help in explaining the significantly lower incidence of their rupture compared to their presence. The aim of this study is to determine the association between the corresponding circle of Willis configurations and rupture of cerebral aneurysms. Materials and Methods: A group of 114 patients treated operatively for aruptured cerebral aneurysm and a group of 56 autopsied subjects were involved in the study. Four basic types of the circle of Willis configurations were formed—two symmetric types A and C, and two asymmetric types B and D. Results: A statistically significantly higher presence of asymmetry of the circle of Willis was determined in the group of surgically-treated subjects (p = 0.001),witha significant presence of asymmetric Type B in this group (p < 0.001). The changeson the A1 segment in the group of surgically-treated subjects showed a statistically significant presence compared to the group of autopsied subjects (p = 0.001). Analyzing the presence of symmetry of the circle of Willis between the two groups, that is, the total presence of symmetric types A and C, indicated their statistically significant presence in the group of autopsied patients (p < 0.001). Conclusions: Changes such as hypoplasia or aplasia of A1 and the resulting asymmetry of the circle of Willis directly affect the possibility of the rupture of cerebral aneurysms. Detection of the corresponding types of the circle of Willis after diagnostic examination can be the basis for the development of a protocol for monitoring such patients.

Uncommon Association of Two Anatomical Variants of Cerebral Circulation: A Fetal-Type Posterior Cerebral Artery and Inferred Artery of Percheron, Complicated with Paramedian Thalamomesencephalic Stroke-Case Presentation and Literature Review

Background

The unilateral fetal variant of the posterior cerebral artery (FPCA) is characterized by the congenital absence of the P1 arterial segment. The artery of Percheron (AOP) is an uncommon vascular variant, in which a single dominant thalamoperforating arterial trunk arises from one P1 segment, bifurcates, and provides bilateral supply to the paramedian thalami and rostral midbrain.

Case Presentation

This is a retrospective case study of a 37-year-old man with multiple lifestyle risk factors (chronic marijuana and tobacco abuse), who suffered a thalamomesencephalic stroke, rapidly worsening to comatose state. After restoration of consciousness, he clinically manifested with left paramedian midbrain syndrome. Imaging demonstrated an asymmetric paramedian thalamic infarction with mesencephalon extension, patency of the basilar, vertebral arteries, and left PCA and right-sided FPCA, respectively. Left-sided thalamoperforating arterioles were not differentiated; AOP was inferred. Neither evident clinical source of embolus nor prothrombotic states were found. Mobile cardiac telemetry and transesophageal echocardiography were not available. The diagnosis was established too late for thrombolytic treatment. Anticoagulation was indicated during the acute and subacute stages, followed by low dose of antiplatelet.

Discussion

This uncommon cerebrovascular configuration (FPCA+AOP) might be the fourth case described in the literature. Sustained rehabilitation and abstinence from tobacco and cannabis led to favorable outcomes.

Pseudo-asymmetry of cerebral blood flow in arterial spin labeling caused by unilateral fetal-type circle of Willis: Technical limitation or a way to better understanding physiological variations of cerebral perfusion and improving arterial spin labeling acquisition?

In the recently published article, "Unilateral fetal-type circle of Willis anatomy causes right-left asymmetry in cerebral blood flow with pseudo-continuous arterial spin labeling: A limitation of arterial spin labeling-based cerebral blood flow measurements?", it was shown by the method of arterial spin labeling (ASL) that unilateral fetal-type circle of Willis could induce variation of blood flow in cerebellar and posterior cerebral artery territory. We believe that the reported observation, rather than being a limitation, gives several interesting cues for understanding the ASL sequence. In this commentary, we formulate some suggestions regarding the use of ASL in clinical practice, discuss the potential causes of the above-mentioned pseudo-asymmetry and consider future improvements of the ASL technique.