Redistribution of cerebropetal blood flow in patients with carotid artery stenosis measured non-invasively with fast cine phase contrast MR angiography

Phase-contrast magnetic resonance imaging of intracranial and extracranial blood flow in carotid near-occlusion

PURPOSE

Compare extracranial internal carotid artery flow rates and intracranial collateral use between conventional ≥ 50% carotid stenosis and carotid near-occlusion, and between symptomatic and asymptomatic carotid near-occlusion.

METHODS

We included patients with ≥ 50% carotid stenosis. Degree of stenosis was diagnosed on CTA. Mean blood flow rates were assessed with four-dimensional phase-contrast MRI.

RESULTS

We included 110 patients of which 83% were symptomatic, and 38% had near-occlusion. Near-occlusions had lower mean internal carotid artery flow (70 ml/min) than conventional ≥ 50% stenoses (203 ml/min, P < .001). Definite use of ≥ 1 collateral was found in 83% (35/42) of near-occlusions and 10% (7/68) of conventional stenoses (P < .001). However, there were no differences in total cerebral blood flow (514 ml/min vs. 519 ml/min, P = .78) or ipsilateral hemispheric blood flow (234 vs. 227 ml/min, P = .52), between near-occlusions and conventional ≥ 50% stenoses, based on phase-contrast MRI flow rates. There were no differences in total cerebral or hemispheric blood flow, or collateral use, between symptomatic and asymptomatic near-occlusions.

CONCLUSION

Near-occlusions have lower internal carotid artery flow rates and more collateral use, but similar total cerebral blood flow and hemispheric blood flow, compared to conventional ≥ 50% carotid stenosis.

Measurement of cerebral perfusion with arterial spin labeling: Part 2. Applications

Arterial spin labeling (ASL) uses magnetic resonance imaging methods to measure cerebral blood flow (CBF) non-invasively. ASL CBF validly localizes brain function and may be especially useful for studies where the time frame of behavioral change is more than a few minutes, such as in longitudinal and treatment studies. ASL measures of cerebral perfusion are highly accurate in detecting lesion laterality in temporal lobe epilepsy, stenotic-occlusive disease, and brain tumors. Among lesioned patients, ASL CBF has excellent concurrent validity when correlated with CBF measured by Positron Emission Tomography or with dynamic susceptibility-weighted magnetic resonance. ASL CBF can predict tumor grading in vivo and can predict six-month response to the surgical treatment of brain tumors. ASL's capability to selectively and non-invasively tag flow in major vessels may refine the monitoring of treatment of cerebrovascular disease and brain tumors. Conclusions about the utility of ASL are limited by the small sample sizes of the studies currently in the literature and by the uncertainty caused by the effect of brain disease on transit times of the magnetic tag. As the method evolves, ASL techniques will likely become more widely used in clinical research and practice.

Structural MRI of carotid artery atherosclerotic lesion burden and characterization of hemispheric cerebral blood flow before and after carotid endarterectomy

Collateral circulation plays a major role in maintaining cerebral blood flow (CBF) in patients with internal carotid artery (ICA) stenosis. CBF can remain normal despite severe ICA stenosis, making the benefit of carotid endarterectomy (CEA) or stenting difficult to assess. Before and after surgery, we assessed CBF supplied through the ipsilateral (stenotic) or contralateral ICA individually with a novel hemisphere-selective arterial spin-labeling (ASL) perfusion MR technique. We further explored the relationship between CBF and ICA obstruction ratio (OR) acquired with a multislice black-blood imaging sequence. For patients with unilateral ICA stenosis (n = 19), conventional bilateral labeling did not reveal interhemispheric differences. With unilateral labeling, CBF in the middle cerebral artery (MCA) territory on the surgical side from the ipsilateral supply (53.7 +/- 3.3 ml/100 g/min) was lower than CBF in the contralateral MCA territory from the contralateral supply (58.5 +/- 2.7 ml/100 g/min), although not statistically significant (p = 0.09). The ipsilateral MCA territory received significant (p = 0.02) contralateral supply (7.0 +/- 2.7 ml/100 g/min), while ipsilateral supply to the contralateral side was not reciprocated. After surgery (n = 11), ipsilateral supply to the MCA territory increased from 57.3 +/- 5.7 to 67.3 +/- 5.4 ml/100 g/min (p = 0.03), and contralateral supply to the ipsilateral MCA territory decreased. The best predictor of increased CBF on the side of surgery was normalized presurgical ipsilateral supply (r(2) = 0.62, p = 0.004). OR was less predictive of change, although the change in normalized contralateral supply was negatively correlated with OR(excess) (=OR(ipsilateral) - OR(contralateral)) (r(2) = 0.58, p = 0.006). The results demonstrate the effect of carotid artery stenosis on blood supply to the cerebral hemispheres, as well as the relative role of collateral pathways before surgery and redistribution of blood flow through these pathways after surgery. Unilateral ASL may better predict hemodynamic surgical outcome (measured by improved perfusion) than ICA OR.

Analysis of haemodynamic disturbance in the atherosclerotic carotid artery using computational fluid dynamics

Computational fluid dynamics (CFD) provides a means for the quantitative analysis of haemodynamic disturbances in vivo, but most work has used phantoms or idealised geometry. Our purpose was to use CFD to analyse flow in carotid atherosclerosis using patient-specific geometry and flow data. Eight atherosclerotic carotid arteries and one healthy control artery were imaged with magnetic resonance angiography (MRA) and duplex ultrasound, and the data used to construct patient-specific computational models used for CFD and wall shear stress (WSS) analysis. There is a progressive change in three-dimensional (3-D) velocity profile and WSS profile with increasing severity of stenosis, characterised by increasing restriction of areas of low WSS, change in oscillation patterns, and progressive rise in WSS within stenoses and downstream jets. Areas of turbulent, retrograde flow and of low WSS are demonstrated in the lee of the stenoses. This study presents the largest CFD analysis of abnormal haemodynamics at the atheromatous carotid bifurcation using patient-specific data and provides the basis for further investigation of causal links between haemodynamic variables and atherogenesis and formation of unstable plaque. We propose that this provides a means for the prospective assessment of relative stroke risk in patients with carotid atherosclerosis.

Pulsatile ocular blood flow in patients with asymmetric internal carotid artery stenosis

BACKGROUND

The pulsatile ocular blood flow (POBF) of patients with asymmetric internal carotid artery (ICA) stenosis was measured bilaterally. The purpose of this study was to investigate the sensitivity of POBF assessments in screening for ICA stenosis.

METHODS

Seventeen subjects with asymmetric ICA stenosis confirmed with carotid duplex ultrasound were recruited. The diagnoses of ICA stenosis were categorised into less than 50 per cent, 50 to 69 per cent and 70 per cent or greater. Their POBF and ocular dimensions were measured after local anaesthesia. Twenty-three normal healthy subjects were recruited as the control. All of the subjects had an inter-ocular axial length difference of less than one millimetre.

RESULTS

There were 14 males and three females in the patient group with a median age of 74 years. Fourteen patients had reduced POBF to the eye ipsilateral on the side with greater ICA stenosis. The reduction in POBF ranged from 3.1 to 49.2 per cent. Two patients had reduced POBF contralateral to the side with greater ICA stenosis. One patient had similar POBF between the eyes. The control group included six males and 17 females with a median age of 65 years. The difference in their POBF between the eyes ranged from 0.3 to 13.0 per cent.

CONCLUSIONS

Stenosis of the internal carotid artery was found to reduce the POBF ipsilateral to the side with greater stenosis. The overlap in the difference in inter-ocular POBF between the patient and control groups led to a low sensitivity (calculated as 41.2 per cent) for screening ICA stenosis from an assessment of POBF. Therefore, POBF assessment is not a good diagnostic tool for screening for ICA stenosis.