Diagnostic separation of conventional ⩾50% carotid stenosis and near-occlusion with phase-contrast MRI

Abstract

Introduction: The aim of this study was to assess sensitivity, specificity and interrater reliability of phase-contrast MRI (PC-MRI) for diagnosing carotid near-occlusion. Patients and methods: Prospective cross-sectional study conducted between 2018 and 2021. We included participants with suspected 50%–100% carotid stenosis on at least one side, all were examined with CT angiography (CTA) and PC-MRI and both ICAs were analyzed. Degree of stenosis on CTA was the reference test. PC-MRI-based blood flow rates in extracranial ICA and intracranial cerebral arteries were assessed. ICA-cerebral blood flow (CBF) ratio was defined as ICA divided by sum of both ICAs and Basilar artery. Results: We included 136 participants. The ICAs were 102 < 50% stenosis, 88 conventional ⩾50% stenosis (31 with ⩾70%), 49 near-occlusion, 12 occlusions, 20 unclear cause of small distal ICA on CTA and one excluded. For separation of near-occlusion and conventional stenoses, ICA flow rate and ICA-CBF ratio had the highest area under the curve (AUC; 0.98–0.99) for near-occlusion. ICA-CBF ratio ⩽0.225 was 90% (45/49) sensitive and 99% (188/190) specific for near-occlusion. Inter-rater reliability for this threshold was excellent (kappa 0.98). Specificity was 94% (29/31) for cases with ⩾70% stenosis. PC-MRI had modest performance for separating <50% and conventional ⩾50% stenosis (highest AUC 0.74), and eight (16%) of near-occlusions were not distinguishable from occlusion (no visible flow). Conclusion: ICA-CBF ratio ⩽0.225 on PC-MRI is an accurate and reliable method to separate conventional ⩾50% stenosis and near-occlusion that is feasible for routine use. PC-MRI should be considered further as a potential standard method for near-occlusion detection, to be used side-by-side with established modalities as PC-MRI cannot separate other degrees of stenosis.

Quantifying Carotid Stenosis: History, Current Applications, Limitations, and Potential: How Imaging Is Changing the Scenario

“…The flow reduction caused by severe stenoses results in a smaller distal ICA (the collapse). It is fundamental to differentiate this condition from carotid "pseudoocclusion", which is typically caused by terminal intracranial ICA occlusion due to thromboembolism [50,51]. "String sign" [49] describes a very small distal ICA but is inappropriate as a synonym for near-occlusion because it also includes dissections but excludes nearocclusions with moderate distal ICA collapse (without full collapse).…”

“…CTA has been the first-line modality for diagnosing carotid near-occlusion in recent years. Key features include a small extracranial ICA caliber compared to the contralateral ICA and external carotid artery and focal severe stenosis with minimal to no luminal contrast opacification [51,54].…”

“…The diagnosis is set by systematic feature interpretation, assessing whether the distal ICA is small and whether proximal severe stenosis is the most reasonable cause [49,51]. While measurement criteria have been proposed, they have not been successfully validated [55,56].…”

“…A recent innovation is the use of phase-contrast MRI, which distinguishes conventional stenosis and near-occlusion well [51]. Given that near-occlusions are difficult to diagnose, it is not surprising that many think that the condition is rare, but it actually represents nearly a third of cases with ≥ 50% stenoses [64].…”