MR perfusion studies of brain for patients with unilateral carotid stenosis or occlusion: evaluation of maps of "time to peak" and "percentage of baseline at peak"

Cortical Thinning in High-Grade Asymptomatic Carotid Stenosis

BACKGROUND AND PURPOSE

High-grade carotid artery stenosis may alter hemodynamics in the ipsilateral hemisphere, but consequences of this effect are poorly understood. Cortical thinning is associated with cognitive impairment in dementia, head trauma, demyelination, and stroke. We hypothesized that hemodynamic impairment, as represented by a relative time-to-peak (TTP) delay on MRI in the hemisphere ipsilateral to the stenosis, would be associated with relative cortical thinning in that hemisphere.

METHODS

We used baseline MRI data from the NINDS-funded Carotid Revascularization and Medical Management for Asymptomatic Carotid Stenosis-Hemodynamics (CREST-H) study. Dynamic contrast susceptibility MR perfusion-weighted images were post-processed with quantitative perfusion maps using deconvolution of tissue and arterial signals. The protocol derived a hemispheric TTP delay, calculated by subtraction of voxel values in the hemisphere ipsilateral minus those contralateral to the stenosis.

RESULTS

Among 110 consecutive patients enrolled in CREST-H to date, 45 (41%) had TTP delay of at least 0.5 seconds and 9 (8.3%) subjects had TTP delay of at least 2.0 seconds, the maximum delay measured. For every 0.25-second increase in TTP delay above 0.5 seconds, there was a 0.006-mm (6 micron) increase in cortical thickness asymmetry. Across the range of hemodynamic impairment, TTP delay independently predicted relative cortical thinning on the side of stenosis, adjusting for age, sex, hypertension, hemisphere, smoking history, low-density lipoprotein cholesterol, and preexisting infarction (P=0.032).

CONCLUSIONS

Our findings suggest that hemodynamic impairment from high-grade asymptomatic carotid stenosis may structurally alter the cortex supplied by the stenotic carotid artery.

Different Grades of Collateral Circulation for Evaluating Cerebral Hemodynamic Status in Carotid Artery Stenosis

Normally, ipsilateral hemodynamic compromise of patients with carotid stenosis (CS) is subjectively identified by collateral circulation through cerebral angiography in the clinical process. It is unclear whether collaterals would linearly determine cerebral perfusion in CS patients. This study aimed to investigate the independent role of collateral circulation on cerebral perfusion in CS patients and the underlying interrelations among them. From 2017 to 2020, 124 CS patients who underwent carotid endarterectomy (CEA) with both preoperative CTP and digital substruction angiography (DSA) images were enrolled. Division of subgroups was based on degree of CS (50-70%, 70-90%, and near-occlusion (NO)) and grades of collateral circulation by DSA. Differences in CTP parameters between CS patients with different collateral circulation were analyzed. Among 124 CS patients, grades 2 and 3 were highly associated with carotid NO (n = 22, 32.35% and n = 22, 32.35%) compared with others (P < 0.0001). The collateral circulation was found to have poor relation with cerebral perfusion parameters in all enrolled patients but significantly improved ipsilateral cerebral perfusion in patients with carotid NO (P < 0.05). Linear hemodynamic compromise was barely related to degree of CS in lobes supplied by middle cerebral artery (MCA) except the frontal lobe (P < 0.05). The grades of collateral circulation are positively associated with degree of CS while having nonsignificant effect on cerebral perfusion. Overall, severity of CS is poorly related to hemodynamic status while the perfectibility of compensation defined by grades of collateral circulation effectively alleviates ipsilateral cerebral perfusion deficit in carotid NO.

Asymptomatic carotid artery stenosis is associated with cerebral hypoperfusion

OBJECTIVE

We have shown that almost 50% of patients with asymptomatic carotid stenosis (ACS) will demonstrate cognitive impairment. Recent evidence has suggested that cerebral hypoperfusion is an important cause of cognitive impairment. Carotid stenosis can restrict blood flow to the brain, with consequent cerebral hypoperfusion. In contrast, cross-hemispheric collateral compensation through the Circle of Willis, and cerebrovascular vasodilation can also mitigate the effects of flow restriction. It is, therefore, critical to develop a clinically relevant measure of net brain perfusion in patients with ACS that could help in risk stratification and in determining the appropriate treatment. To determine whether ACS results in cerebral hypoperfusion, we developed a novel approach to quantify interhemispheric cerebral perfusion differences, measured as the time to peak (TTP) and mean transit time (MTT) delays using perfusion-weighted magnetic resonance imaging (PWI) of the whole brain. To evaluate the utility of using clinical duplex ultrasonography (DUS) to infer brain perfusion, we also assessed the relationship between the PWI findings and ultrasound-based peak systolic velocity (PSV).

METHODS

Structural and PWI of the brain and magnetic resonance angiography of the carotid arteries were performed in 20 patients with ≥70% ACS. DUS provided the PSV, and magnetic resonance angiography provided plaque geometric measures at the stenosis. Volumetric perfusion maps of the entire brain from PWI were analyzed to obtain the mean interhemispheric differences for the TTP and MTT delays. In addition, the proportion of brain volume that demonstrated a delay in TTP and MTT was also measured. These proportions were measured for increasing severity of perfusion delays (0.5, 1.0, and 2.0 seconds). Finally, perfusion asymmetries on PWI were correlated with the PSV and stenosis features on DUS using Pearson's correlation coefficients.

RESULTS

Of the 20 patients, 18 had unilateral stenosis (8 right and 10 left) and 2 had bilateral stenoses. The interhemispheric (left-right) TTP delays measured for the whole brain volume identified impaired perfusion in the hemisphere ipsilateral to the stenosis in 16 of the 18 patients. More than 45% of the patients had had ischemia in at least one half of their brain volume, with a TTP delay >0.5 second. The TTP and MTT delays showed strong correlations with PSV. In contrast, the correlations with the percentage of stenosis were weaker. The correlations for the PSV were strongest with the perfusion deficits (TTP and MTT delays) measured for the whole brain using our proposed algorithm (r = 0.80 and r = 0.74, respectively) rather than when measured on a single magnetic resonance angiography slice as performed in current clinical protocols (r = 0.31 and r = 0.58, respectively).

CONCLUSIONS

Interhemispheric TTP and MTT delay measured for the whole brain using PWI has provided a new tool for assessing cerebral perfusion deficits in patients with ACS. Carotid stenosis was associated with a detectable reduction in ipsilateral brain perfusion compared with the opposite hemisphere in >80% of patients. The PSV measured at the carotid stenosis using ultrasonography correlated with TTP and MTT delays and might serve as a clinically useful surrogate to brain hypoperfusion in these patients.

Dissociation among hemodynamic measures in asymptomatic high grade carotid artery stenosis

BACKGROUND

Cerebral blood flow (CBF) regulation is a critical element in cerebrovascular pathophysiology, particularly in large vessel disease, but the best method to use for hemodynamic assessment is not clear. We examined 4 different blood-flow related measures in patients with unilateral high-grade carotid artery disease, assessing asymmetry between the occluded vs non-occluded side, and the correlations among the measures.

METHODS

Thirty-three patients (age 50-93, 19 M) with unilateral 80-100% ICA occlusion but no stroke underwent: 1) mean flow velocity (MFV) in both middle cerebral arteries by transcranial Doppler (TCD), 2) quantitative resting CBF using pseudo-continuous arterial spin labeling (pCASL) MRI, 3) vasomotor reactivity (VMR) in response to 5% CO2 inhalation, and 4) dynamic cerebral autoregulation (DCA) assessing the counter-regulation of blood flow to spontaneous changes in blood pressure using TCD monitoring and finger photoplethysmography. Paired t-tests and Pearson correlations assessed side-to-side differences within each measure, and correlations between measures.

RESULTS

CBF (p=0.001), MFV (p<0.001), VMR (p=0.008), and DCA (p=0.047) all showed significantly lower values on the occluded side. The 4 measures were independent of each other on correlation analysis, even when controlling for age and anterior circle of Willis collateral (all partial correlations <0.233 and p-values >0.468).

CONCLUSIONS

These 4 measures showed high sensitivity to the occluded carotid artery, but their dissociation suggests that any given measure only partially characterizes the hemodynamic state. Additional research is needed to explore the multifaceted biology of cerebral blood flow regulation.

Quantitative Magnetic Resonance Angiography in Internal Carotid Artery Occlusion with Primary Collateral Pathway

Background and Purpose

Quantitative magnetic resonance angiography (Q-MRA) enables direct measurement of volume flow rate (VFR) of intracranial arteries. We aimed to evaluate the collateral flows in internal carotid artery (ICA) occlusion with primary collateral pathway via circle of Willis using Q-MRA, and to compare them between patients who recently developed ipsilateral symptomatic ischemia and those who did not.

Methods

Between 2012 and 2014, 505 patients underwent Q-MRA in our institution. Among these, 33 patients who had unilateral ICA occlusion with primary collateral pathway were identified, and grouped into asymptomatic patients, stable patients with chronic infarction, and symptomatic patients with acute/subacute infarction. Mean VFR (mVFR) in intracranial arteries was measured and compared between the patients’ groups. Kruskal-Wallis test was used for statistical analysis.

Results

Six patients were asymptomatic, fifteen with chronic infarction were stable, and twelve with acute/subacute infarction were symptomatic. The mVFR of ipsilateral middle cerebral artery in symptomatic patients was significantly lower than those in stable or asymptomatic patients (73.7±45.6 mL/min vs. 119.9±36.1 mL/min vs. 121.8±42.0 mL/min; P = 0.04). Total sum of the mVFR of ipsilateral anterior, middle, and posterior cerebral arteries was significantly lower in symptomatic patients than those in other groups (229.3 ± 51.3 mL/min vs. 282.0±68.6 mL/min vs. 314.0±44.4 mL/min; P = 0.02).

Conclusions

Q-MRA could be helpful to demonstrate the difference in the degree of primary collateral flow in ICA occlusion between the patients with recent symptomatic ischemia and those without.

Use of perfusion imaging and other imaging techniques to assess risks/benefits of acute stroke interventions

The advent of multimodal neuroimaging has provided acute stroke care providers with an armamentarium of sophisticated imaging options to utilize for guidance in clinical decision-making and management of acute ischemic stroke patients. Here, we propose a framework and potential algorithm-based methodology for imaging modality selection and utilization for the purpose of achieving optimal stroke clinical care. We first review imaging options that may best inform decision-making regarding revascularization eligibility, with a focus on the imaging modalities that best identify critical inclusion and exclusion criteria. Next, we review imaging methods that may guide the successful achievement of revascularization once it has been deemed desirable and feasible. Further, we review imaging modalities that may best assist in both the noninterventional care of acute stroke as well as the identification of stroke-mimics. Finally, we review imaging techniques under current investigation that show promise to improve future acute stroke management.

Progressive changes in cerebral perfusion after carotid stenting: a dynamic susceptibility contrast perfusion weighted imaging study

BACKGROUND

We aimed to assess baseline inter-hemispheric perfusion differences, before carotid artery stenting (CAS), of severe internal carotid artery (ICA) stenosis, and to evaluate perfusion changes over time after CAS by means of perfusion weighted imaging (PWI).

METHODS

Dynamic susceptibility contrast PWI was performed in 33 patients with severe ICA stenosis 1 day before and 1 day after CAS, and repeated in 23 patients 30 days after CAS. Cerebral blood volume, cerebral blood flow, arrival time (T0), mean transit time, and time to peak (TTP) relative values comparing symptomatic and asymptomatic hemispheres before CAS were obtained. Pre- and post-CAS values were also compared in the treated hemisphere and middle cerebral artery (MCA) territories. The influence of collateral circulation and contralateral ICA stenosis were evaluated.

RESULTS

Before CAS, TTP was significantly increased in the affected hemisphere and MCA territory in all patients (p=0.007 and p=0.021, respectively). After treatment, normalization was observed for TTP and T0 30 days after CAS in all patients, mainly in patients with any grade of contralateral stenosis and with one or both functioning communicating arteries (42.4% of patients had both, and 42.4% showed only one-anterior communicating artery=11 patients (78.6%) and posterior communicating artery=3 patients (21.4%)).

CONCLUSIONS

Inter-hemispheric perfusion differences in patients with severe ICA stenosis normalize after CAS in the long term, up to 30 days after the procedure.

Imaging of cerebral perfusion in acute stroke

Rapid imaging of cerebral perfusion in acute stroke is needed for timely triage of patients for thrombolytic therapy. Accurate quantitative perfusion imaging is required for proper assessment of penumbral brain parenchyma truly at risk for extension of infarction from the irreversible core infarction. CT and MRI techniques offer rapidity and availability for acute stroke imaging, including that of cerebral perfusion. CT perfusion techniques are readily available, but suffer from limited brain coverage of present multislice scanners. MRI offers whole brain coverage, but suffers from less availability and higher cost than CT. Presently, development is directed towards increasing the quantitative accuracy of cerebral perfusion imaging and validation of surrogate parameters, such as time to peak (TTP). In the future, the need for rapid and frequent assessment of cerebral perfusion and its metabolic correlates, with minimal or no radiation, will probably be met by MRI.

Changes in perfusion-weighted magnetic resonance imaging after carotid angioplasty with stent

Carotid artery stenosis due to arteriosclerosis increases the risk of cerebral ischemia via embolic phenomena or reduced blood flow. The changes in cerebral perfusion that may occur after treatment are not clearly understood. This study evaluated the changes in cerebral microcirculation following carotid angioplasty with stenting (CAS) under cerebral protection with filters using ultrafast gradient echo (GRE) perfusion weighted imaging (PWI) with magnetic resonance imaging (MRI). Prospectively, 21 cervical carotid stenosis patients, mean age 69.95 years, underwent MRI 12 h before and 72 h after CAS. PWI parameters were collected for statistical analysis: cerebral blood volume (CBV), mean transit time (MTT) and time to peak (TTP). Statistical analysis was applied to absolute parameters and to values normalized against those from the contralateral parenchyma. The main finding of this study was improved hemodynamics for the normalized data after CAS, shown by reduced MTT (p<0.001) and TTP (p=0.019) in the territory fed by the middle cerebral artery ipsilateral to the CAS. Absolute data showed increased blood volume in the cerebral hemispheres after CAS, which was more accentuated on the stent side (p=0.016) than the contralateral side (p=0.029). Early improvements in cerebral perfusion, mainly seen in the normalized data, were clearly demonstrated in the timing parameters - TTP & MTT - after CAS.

Do acute diffusion- and perfusion-weighted MRI lesions identify final infarct volume in ischemic stroke?

BACKGROUND AND PURPOSE

An acute mismatch on diffusion-weighted MRI (DWI) and perfusion-weighted MRI (PWI) may represent the "tissue-at-risk." It is unclear which "semiquantitative" perfusion parameter most closely identifies final infarct volume.

METHODS

Acute stroke patients underwent DWI and PWI (dynamic-susceptibility contrast imaging) on admission (baseline), and T2-weighted imaging (T2WI) at 1 or 3 months after stroke. "Semiquantitative" mean transit time (MTTsq=first moment of concentration/time curve), cerebral blood volume (CBVsq=area under concentration/time curve), and cerebral blood flow (CBFsq=CBVsq/MTTsq) were calculated. DWI and PWI lesions were measured at baseline and final infarct volume on T2WI acquired > or =1 month after stroke. Baseline DWI, CBFsq, and MTTsq lesion volumes were compared with final T2WI lesion volume.

RESULTS

Among 46 patients, baseline DWI and CBFsq lesions were not significantly different from final T2WI lesion volume, but baseline MTTsq lesions were significantly larger. The correlation with final T2WI lesion volume was strongest for DWI (Spearman rank correlation coefficient rho=0.68), intermediate for CBFsq (rho=0.55), and weakest for MTTsq (rho=0.49) baseline lesion volumes. Neither DWI/CBFsq nor DWI/MTTsq mismatch predicted lesion growth; lesion growth was equally common in those with and without mismatch.

CONCLUSIONS

Of the 2 PWI parameters, CBFsq lesions most closely identifies, and MTTsq overestimates, final T2WI lesion volume. "DWI/PWI mismatch" does not identify lesion growth. Patients without "DWI/PWI mismatch" are equally likely to have lesion growth as those with mismatch and should not be excluded from acute stroke treatment.

Dynamic susceptibility contrast perfusion imaging of cerebral ischemia in nonhuman primates: Comparison of Gd-DTPA and NMS60

PURPOSE

To study a new gadolinium (Gd) contrast agent-NMS60-for MR perfusion-weighted imaging (PWI) of brain tissue.

MATERIALS AND METHODS

NMS60 is a Gd3+ trimer with a molecular weight of 2158 Daltons, and a T2 relaxivity almost three times higher than that of Gd-DTPA. Middle cerebral artery (MCA) occlusion was induced in nine nonhuman primates. The animals were scanned acutely and for up to six follow-up time points. PWI peak, and time-to-peak maps were generated, and perfusion deficit volumes were measured from these maps. The values of peak, time-to-peak, and perfusion deficit volume were compared between NMS60 and GD-DTPA.

RESULTS

These results demonstrate that there was no significant difference in our calculated perfusion parameters between the two contrast agents.

CONCLUSION

The two agents were found to be equally effective for PWI for acute and chronic stroke in primates. Along with its previously demonstrated advantage for T1-enhanced imaging, the current results show that NMS60 is a viable contrast agent for use in stroke patients.

Early carotid endarterectomy after ischemic stroke improves diffusion/perfusion mismatch on magnetic resonance imaging: report of two cases

OBJECTIVE AND IMPORTANCE

The functional magnetic resonance imaging techniques of diffusion-weighted imaging and perfusion-weighted imaging allow for ultra-early detection of brain infarction and concomitant identification of blood flow abnormalities in surrounding regions, which may represent brain "at risk."

CLINICAL PRESENTATION

We report two patients with acute ischemic stroke associated with ipsilateral high-grade carotid stenosis. The first patient, a 64-year-old woman with a remote history of ischemic stroke and a vertebral artery aneurysm, presented with worsening of her preexisting right hemiparesis. The second patient, another 64-year-old woman with known multiple intracranial aneurysms and bilateral high-grade internal carotid artery stenosis, was admitted for the elective microsurgical clipping of an enlarging giant left carotid-ophthalmic artery aneurysm. Postoperatively, she developed right hemiparesis and mild aphasia. Both patients showed progressive worsening of their neurological deficits in the setting of small or undetected diffusion-weighted imaging abnormalities and large perfusion-weighted imaging defects.

INTERVENTION

After prompt carotid endarterectomy, symptoms in both patients resolved or improved. Follow-up magnetic resonance imaging scans demonstrated resolution or significant improvement in the perfusion abnormalities in both patients.

CONCLUSION

Carotid endarterectomy in the setting of diffusion-weighted/perfusion-weighted imaging mismatch can lead to improvement in cerebral perfusion as evidenced by resolution of the perfusion-weighted imaging lesion. Diffusion/perfusion magnetic resonance imaging may be useful in identifying patients with severe neurological deficits but without large territories of infarction who may safely undergo early surgical revascularization.

Which MR-derived perfusion parameters are the best predictors of infarct growth in hyperacute stroke? Comparative study between relative and quantitative measurements

PURPOSE

To compare predictors of infarct growth in hyperacute stroke from a retrospective review of various relative and quantitative parameters calculated at perfusion-weighted magnetic resonance (MR) imaging performed within 6 hours after ictus.

MATERIALS AND METHODS

Fluid-attenuated inversion recovery and diffusion- and perfusion-weighted images were obtained in 66 patients. The initial infarct was delineated on diffusion-weighted images; the hemodynamic disturbance, on apparent mean transit time (MTT) maps; and the final infarct, on follow-up fluid-attenuated inversion recovery images. Relative (without and with deconvolution) and quantitative values of the bolus arrival time, time to peak (TTP), apparent MTT or MTT, cerebral blood volume (CBV), peak height, and cerebral blood flow (CBF) index or CBF were calculated for initial infarct, infarct growth (final minus initial infarct contour), viable hemodynamic disturbance (apparent MTT minus final infarct contour), and contralateral mirror regions. Univariate and multivariate analyses (receiver operating characteristic curves and discriminant analysis) were performed to compare the diagnostic performance of these parameters for predicting infarct growth.

RESULTS

At univariate analysis, relative peak height and quantitative CBF were the best predictors of infarct growth; at multivariate analysis, a function of peak height and TTP for relative measurements and CBF alone for quantitative measurements. Quantitative and relative measurements (without or with deconvolution) worked equally well. A combined relative peak height or TTP threshold (<54% or >5.2 seconds, respectively) had a sensitivity of 71% and a specificity of 98%. A quantitative CBF threshold (<35 mL/min/100 g) had a sensitivity of 69% and a specificity of 85%.

CONCLUSION

A combination of relative peak height and TTP measurements allowed the best prediction of infarct growth, which obviates more complex quantitative calculation.