Meide Zhao

Improved phase-contrast flow quantification by three-dimensional vessel localization

In this paper, a method of three-dimensional (3D) vessel localization is presented to allow the identification of a vessel of interest, the selection of a vessel segment, and the determination of a slice orientation to improve the accuracy of phase-contrast magnetic resonance (PCMR) angiography. A marching-cube surface-rendering algorithm was used to reconstruct the 3D vasculature. Surface-rendering was obtained using an iso-surface value determined from a maximum intensity projection (MIP) image. This 3D vasculature was used to find a vessel of interest, select a vessel segment, and to determine the slice orientation perpendicular to the vessel axis. Volumetric flow rate (VFR) was obtained in a phantom model and in vivo using 3D localization with double oblique cine PCMR scanning. PCMR flow measurements in the phantom showed 5. 2% maximum error and a standard deviation of 9 mL/min during steady flow, 7.9% maximum error and a standard deviation of 13 mL/min during pulsatile flow compared with measurements using an ultrasonic transit-time flowmeter. PCMR VFR measurement error increased with misalignment at 10, 20, and 30 degrees oblique to the perpendicular slice in vitro and in vivo. The 3D localization technique allowed precise localization of the vessel of interest and optimal placement of the slice orientation for minimum error in flow measurements.

Regional Cerebral Blood Flow Using Quantitative MR Angiography

BACKGROUND AND PURPOSE: We sought to derive regional cerebral blood flow using vessel flows from quantitative MR angiography (qMRA). MATERIALS AND METHODS: Flow rates in the 15 major cerebral arteries were measured on retrospectively gated fast 2D phase-contrast MR angiography obtained in 83 healthy adult volunteers (age range, 24–74 years; mean, 42 years). The arterial network of the brain was partitioned into 12 different regions, in which flows were calculated from the measured flows of the 15 cerebral arteries. RESULTS: The mean flows of the 15 arteries and the 12 regions were calculated. The mean total cranial flow and the mean total cerebral blood flow were 949 ± 158 mL/min and 695 ± 113 mL/min, respectively. The mean regional flows for the anterior and posterior circulation were 483 ± 87 mL/min and 212 ± 34 mL/min, respectively. The relative contributions of the flows in the 11 regions to their parent regions were obtained. The mean flows in the individual arteries and the regions with age were also calculated. The mean flows for the female group were significantly lower than those for the male group (P < .001) for the 2 common carotids and the cranial circulation and left/right extracranial circulation. However, the intracranial circulation was not different between sexes. CONCLUSIONS: The 12 regions in the cerebral circulation were identified and formed into a partition tree, and the mean regional flow for each region was determined using vessel flows from qMRA.

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.

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.

RIEVL: recursive induction learning in hand gesture recognition

Presents a recursive inductive learning scheme that is able to acquire hand pose models in the form of disjunctive normal form expressions involving multivalued features. Based on an extended variable-valued logic, our rule-based induction system is able to abstract compact rule sets from any set of feature vectors describing a set of classifications. The rule bases which satisfy the completeness and consistency conditions are induced and refined through five heuristic strategies. A recursive induction learning scheme in the RIEVL algorithm is designed to escape local minima in the solution space. A performance comparison of RIEVL with other inductive algorithms, ID3, NewID, C4.5, CN2, and HCV, is given in the paper. In the experiments with hand gestures, the system produced the disjunctive normal form descriptions of each pose and identified the different hand poses based on the classification rules obtained by the RIEVL algorithm. RIEVL classified 94.4 percent of the gesture images in our testing set correctly, outperforming all other inductive algorithms.

In Vivo Evaluation of Quantitative MR Angiography in a Canine Carotid Artery Stenosis Model

BACKGROUND AND PURPOSE: Large-vessel cerebral blood flow quantification has emerged as a potential predictor of stroke risk. QMRA uses phase-contrast techniques to noninvasively measure vessel flows. To evaluate the in vivo accuracy of QMRA for measuring the effects of progressive arterial stenosis, we compared this technique with invasive flow measurements from a sonographic transit-time flow probe in a canine model. MATERIALS AND METHODS: A sonographic flow probe was implanted around the CCA of hound dogs (n = 4) under general anesthesia. Pulsatile blood flow and arterial pressure were continuously recorded during CCA flow measurements with QMRA. A vascular tourniquet was applied around the CCA to produce progressive stenosis and varying flow rates. Statistical comparisons were made by using the Pearson product moment correlation coefficient. RESULTS: A total of 60 paired CCA flow measurements were compared. Mean blood flows ranged between 21 and 691 mL/min during QMRA acquisition as measured by the flow probe. The correlation coefficients between flow probe and QMRA measurements for mean, maximum, and minimum volume flow rates were 0.99 (P < .0001), 0.98 (P < .0001), and 0.96 (P < .0001), respectively. The overall proportional difference between the 2 techniques was 7.8 ± 1%. Measurements at higher flow rates and in the absence of arterial stenosis had the lowest PD. CONCLUSIONS: Noninvasive CCA flow measurements by using QMRA are accurate compared with invasive flow-probe measurements in a canine arterial flow model with stenosis and may be useful for the evaluation of the hemodynamic effects of stenosis caused by cerebrovascular atherosclerosis.

Inductive learning in hand pose recognition

Hand pose recognition for gestural systems must be able to handle variations in user hand anatomy, perspective effects, and the idiosyncrasies of individual gesture presentation. We present an inductive learning system that is able to derive a rulebase of disjunctive normal form formulae in which each DNF describes a hand pose, and each conjunct within the DNF constitutes a single rule. The system applies a suite of feature detectors during learning and is able to determine a salient subset of features for a given set of hand poses. Only the reduced feature set needs to be computed at recognition time. The rule-based induction system is capable of both exact and flexible matching. The latter produces a list of hand poses ordered by goodness of match when presented with a new hand pose image. We present the results of our experiments in which we trained our system with 931 instances of 20 different hand poses. Our system produced compact rule sets and had a recognition rate of 94%.

Leptomeningeal Collateral Volume Flow Assessed by Quantitative Magnetic Resonance Angiography in Large-Vessel Cerebrovascular Disease

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.

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

The hemodynamic force of wall shear stress (WSS) has demonstrated a critical role in atherogenesis.