Wall shear stress in major cerebral arteries as a function of age and gender--a study of 301 healthy volunteers.

BACKGROUND: The hemodynamic force of wall shear stress (WSS) has demonstrated a critical role in atherogenesis. PURPOSE: To study the effect of age and gender on mean WSS (MWSS) values in major cerebral arteries. METHOD: Thirteen cerebral arterial location sites in 301 healthy (157 M, 144 F; mean 47 ± 15 years; range 18-84 years old) were studied. Quantitative magnetic resonance angiography was used to obtain volume flow and diameter, and subsequently to calculate MWSS via the Hagen-Poiseuille equation. RESULTS: MWSS decreased significantly with age in all vessels, declining from 9.5 to 5.7 dynes/cm(2) in the neck vessels and from 22.9 to 16.2 dynes/cm2 in the intracranial vessels. MWSS is significantly higher in females than in males in all six neck vessels. The most significant drop in MWSS occurred between the age groups 48-57 and 58-67 (P < .05 for 12 vessels). CONCLUSION: The overall decline in MWSS observed with age may be due to a decrease in flow. However, the marked drop in MWSS between the 48-57 and 58-67 age groups corresponded with an increase in diameter and systolic blood pressure rather than a significant drop in flow.

Leptomeningeal collateral volume flow assessed by quantitative magnetic resonance angiography in large-vessel cerebrovascular disease.

BACKGROUND/PURPOSE: Leptomeningeal collateral volume flow has not been previously quantified. Quantitative magnetic resonance angiography (QMRA) can determine flow in the large vessels of the intracranial circulation. METHODOLOGY: We reviewed consecutive QMRA studies performed between December 1, 2004 and August 30, 2005, for cases showing asymmetrically higher flow in a posterior cerebral artery (PCA) just distal to the origin of the posterior communicating artery ipsilateral to a hemodynamic middle cerebral artery (MCA) or internal carotid artery lesion. The mean, range, and standard deviation (SD) of the flow rate in the PCAs, MCAs, and PCA ipsilateral-contralateral difference were calculated. Ipsilateral and contralateral PCA flow rates were compared using the Student's t-test. RESULT: Sixteen studies met selection criteria. Mean age was 52 years (range 21-79) and 9 were female. MCA flow was below QMRA detection limits in 6 studies. Mean measurable ipsilateral MCA flow reduction was 84 mL/min (range 9-147, SD 51.4). Mean ipsilateral PCA flow was 118 mL/min (range 72-206, SD 38.5) and mean contralateral PCA flow was 68 mL/min (range 35-144, SD 30.5, P < .001); mean difference was 50 mL/min (range 10-93, SD 24.3). CONCLUSION: Leptomeningeal collateral flow can be assessed with QMRA and may be substantial.

Evaluation of extracranial-intracranial bypass using quantitative magnetic resonance angiography.

OBJECT: To date, angiography has been the primary modality for assessing graft patency following extracranial-intracranial bypass. The utility of a noninvasive and quantitative method of assessing bypass function postoperatively was evaluated using quantitative magnetic resonance (MR) angiography. METHODS: One hundred one cases of bypass surgery performed over a 5.5-year period at a single institution were reviewed. In 62 cases, both angiographic and quantitative MR angiographic data were available. Intraoperative flow measurements were available in 13 cases in which quantitative MR angiography was performed during the early postoperative period (within 48 hours after surgery). There was excellent correlation between quantitative MR angiographic flow and angiographic findings over the mean 10 months of imaging follow up. Occluded bypasses were consistently absent on quantitative MR angiograms (four cases). The flow rates were significantly lower in those bypasses that became stenotic or reduced in diameter as demonstrated by follow-up angiography (nine cases) than in those bypasses that remained fully patent (mean +/- standard error of the mean, 37 +/- 13 ml/minute compared with 105 +/- 7 ml/minute, p = 0.001). Flows were appreciably lower in poorly functioning bypasses for both vein and in situ arterial grafts. All angiographically poor bypasses (nine cases) were identifiable by absolute flows of less than 20 ml/minute or a reduction in flow greater than 30% within 3 months. Good correlation was seen between intraoperative flow measurements and early postoperative quantitative MR angiographic flow measurements (13 cases, Pearson correlation coefficient = 0.70, p = 0.02). CONCLUSIONS: Bypass grafts can be assessed in a noninvasive fashion by using quantitative MR angiography. This imaging modality provides not only information regarding patency as shown by conventional angiography, but also a quantitative assessment of bypass function. In this study, a low or rapidly decreasing flow was indicative of a shrunken or stenotic graft. Quantitative MR angiography may provide an alternative to standard angiography for serial follow up of bypass grafts.

Use of quantitative magnetic resonance angiography to stratify stroke risk in symptomatic vertebrobasilar disease.

BACKGROUND AND PURPOSE: Symptomatic vertebrobasilar disease (VBD) carries a high risk of recurrent stroke. We sought to determine whether a management algorithm consisting of quantitative hemodynamic assessment could stratify stroke risk and guide the need for intervention. METHODS: All patients with symptomatic VBD at our institution are evaluated by a standard protocol including quantitative magnetic resonance angiography (QMRA). Patients are stratified on the basis of the presence or absence of distal flow compromise. Those with low distal flow are offered intervention (surgical or endovascular); all patients receive standard medical therapy. We reviewed the clinical outcome of patients managed with this protocol from 1998 to 2003. RESULTS: Follow-up was available for 47 of 50 patients over a mean interval of 28 months. Stroke and combined stroke/transient ischemic attack free survival at 2 years was calculated using the Kaplan-Meier curve. Patients with normal distal flow (n=31) had an event-free survival of 100% and 96%, respectively. Comparatively, patients with low distal flow (n=16) experienced a 71% and 53% event-free survival, demonstrating a significantly higher risk of recurrent ischemia (P=0.003). Patients with low flow who subsequently underwent treatment (n=12) had an 82% event-free survival. Cox proportional hazards analysis demonstrated that flow status affected event-free survival regardless of covariates. CONCLUSIONS: Patients with symptomatic VBD demonstrating low distal flow on QMRA appear to have a high risk of stroke; conversely, those with normal flow seem to have a benign course and may be optimally managed with medical therapy alone.

The cut flow index: an intraoperative predictor of the success of extracranial-intracranial bypass for occlusive cerebrovascular disease.

OBJECTIVE: There has been a resurgence of interest in selective extracranial-intracranial bypass for revascularization of cerebrovascular occlusive disease. We evaluated the usefulness of intraoperative blood flow measurements in predicting graft success after extracranial-intracranial bypass. METHODS: A retrospective review of 51 cases of extracranial-intracranial bypass for purposes of flow augmentation in the setting of cerebrovascular occlusive disease was performed. In all cases, free flow from the cut end of the donor vessel, termed cut flow, was measured. The cut flow index (CFI) (bypass flow [ml/min] / cut flow [ml/min]) was derived and correlated with bypass patency, postoperative bypass flow, cerebrovascular reserve testing, and clinical outcome. RESULTS: The CFI was a significant predictor of bypass patency (P = 0.002). Using a CFI of 0.5 as a threshold, the bypass patency rate was 92% in cases with a CFI greater than 0.5 compared with 50% in cases with a CFI less than 0.5. Intraoperative bypass flow correlated well with postoperative measurements obtained from quantitative phase-contrast magnetic resonance imaging. An analysis of cases with a poor CFI indicated that a logical interpretation of bypass function can be performed during surgery. CONCLUSION: A poor CFI can alert surgeons to potential difficulties with the donor vessel, anastomosis, or recipient vessel during surgery. Furthermore, a CFI closely approximating 1.0 provides physiological confirmation of impaired cerebrovascular reserve in the recipient bed.

A patient-specific computer model to predict outcomes of the balloon occlusion test.

OBJECT: Balloon occlusion tests (BOTs) are performed to identify patients who are at risk for ischemia and stroke following permanent internal carotid artery (ICA) occlusion. The object of this work was to determine whether patient-specific blood flow modeling can be used to identify patients in whom the BOT would not be tolerated. METHODS: The test was performed in 16 patients who underwent BOT with continuous neurological and electroencephalographic monitoring, followed by a hypotensive challenge. During hypotension a tracer was injected so that single-photon emission tomography (SPECT) scans could be obtained. Each individual brain circulation was modeled using information gained from phase-contrast magnetic resonance (MR) angiography and digital subtraction (DS) angiography, and the predicted effect of the BOT was evaluated. Six patients did not tolerate the BOT; in these patients, decreases in middle cerebral artery (M1 segment) blood flow of 41 +/- 27% (mean +/- standard deviation), anterior cerebral artery (A3 segment) flow of 56 +/- 33%, and posterior cerebral artery (P2 segment) flow of 4 +/- 13% ipsilateral to the site of occlusion were found with modeling; these changes were significantly greater than the percentage of changes measured in the contralateral hemisphere (p < 0.05). Ten patients who tolerated the BOT well had calculated decreases in ipsilateral flows of only 9 +/- 6% for the M1 segment, 12 +/- 40% for the A3 segment, and 17 +/- 21% for the P2 segment during BOT modeling. CONCLUSIONS: A decrease in blood flow in both the ipsilateral M1 and A3 segments that was greater than 20%, calculated by flow modeling of the BOT, was 100% sensitive and 100% specific in identifying patients who could not tolerate the BOT. Blood flow modeling, coupled with DS angiography and noninvasive phase-contrast MR angiography measurements to make calculations patient specific, can be used to identify patients who have an elevated risk of ischemia during the BOT.

Hemodynamic evaluation of basilar and vertebral artery angioplasty.

OBJECTIVE: The postangioplasty evaluation of a stenotic vessel is often conducted by studying serial angiograms to determine the anatomic reduction in stenosis. In flow-limiting stenosis, the hemodynamic change that accompanies these anatomic changes is of great importance in evaluating the success of the angioplasty. The purpose of this article is to demonstrate the usefulness of phase contrast magnetic resonance angiography (PCMRA) in evaluating the hemodynamic changes that occur after angioplasty of the basilar and vertebral arteries. METHODS: Between January 1998 and February 2000, PCMRA was performed for the hemodynamic evaluation of 130 patients who presented at our institution. Twenty-six patients were evaluated for vertebrobasilar insufficiency, and flow rates of their vertebral and basilar arteries were determined. In five patients, angioplasty was done on the basilar or vertebral arteries, and PCMRA was performed to determine flow rates before and after the procedure. RESULTS: Of the five patients undergoing angioplasty, the average percentage of stenosis was 81%. The average increase in basilar artery flow rate was 46 ml/min (P < 0.05) after angioplasty. Two of these patients are described. One patient demonstrated the comparison of flow rates in the vertebral and basilar arteries after angioplasty. The second patient showed follow-up flow rates measured after angioplasty and up to 4 months later to predict restenosis. CONCLUSION: The use of flow rate data before and after angioplasty is helpful not only to evaluate the treatment immediately after the procedure but also to evaluate the effectiveness of the treatment during a long period. PCMRA provides a noninvasive method for measuring arterial flow rates with far-reaching implications in neurosurgery.