Andreas Harloff

Time-resolved 3D MR velocity mapping at 3T: Improved navigator-gated assessment of vascular anatomy and blood flow

To evaluate an improved image acquisition and data‐processing strategy for assessing aortic vascular geometry and 3D blood flow at 3T.

Three-dimensional analysis of segmental wall shear stress in the aorta by flow-sensitive four-dimensional-MRI

To assess the distribution and regional differences of flow and vessel wall parameters such as wall shear stress (WSS) and oscillatory shear index (OSI) in the entire thoracic aorta.

In vivo wall shear stress distribution in the carotid artery: effect of bifurcation geometry, internal carotid artery stenosis, and recanalization therapy.

Background— The purpose of this study was to analyze the in vivo distribution of absolute wall shear stress (WSSabs) and oscillatory shear index (OSI) in the carotid bifurcation and to evaluate its dependence on bifurcation geometry, the presence of internal carotid artery (ICA) stenosis, and recanalization therapy. Methods and Results— Time-resolved 3D blood flow was acquired with flow-sensitive 4D MRI in 64 normal carotid bifurcations and 17 carotid arteries with moderate ICA stenosis (48±6%) or after surgical recanalization. Among 64 normal arteries, atherogenic wall parameters were consistently concentrated in proximal bulb regions of the common (CCA) and internal (ICA) carotid arteries. The fraction of the carotid bulb exposed to atherosclerosis-prone wall parameters (low WSSabs below and high OSI above group-defined 20% and 10% thresholds) was correlated with the individual bifurcation geometry. Multiple regressions revealed significant ( P <0.01) relationships (β, 0.44 to 0.48) between the areas with atherosclerosis-prone wall parameters and the dICA/dCCA diameter ratio. The size of regions exposed to high OSI demonstrated highly significant ( P ≤0.01) relationships with all analyzed geometry parameters (dICA/dCCA β, 0.48; tortuosity β, ≤−0.56; bifurcation angle β, ≥0.47). Moderate ICA stenosis altered the distribution of wall parameters (45%/61% reduction of individually low WSSabs/high OSI in the proximal ICA), which were relocated to segments distal to the arterial stenosis. WSSabs/OSI topology after recanalization was similar compared with the normal wall parameter distribution. Conclusions— Flow-sensitive 4D MRI identified alterations in the segmental in vivo WSS distribution associated with atherosclerotic disease, surgical therapy, and individual bifurcation geometry and could be a valuable technique to assess the individual risk of flow-mediated atherosclerosis and carotid plaque progression.Background—The purpose of this study was to analyze the in vivo distribution of absolute wall shear stress (WSSabs) and oscillatory shear index (OSI) in the carotid bifurcation and to evaluate its dependence on bifurcation geometry, the presence of internal carotid artery (ICA) stenosis, and recanalization therapy. Methods and Results—Time-resolved 3D blood flow was acquired with flow-sensitive 4D MRI in 64 normal carotid bifurcations and 17 carotid arteries with moderate ICA stenosis (48±6%) or after surgical recanalization. Among 64 normal arteries, atherogenic wall parameters were consistently concentrated in proximal bulb regions of the common (CCA) and internal (ICA) carotid arteries. The fraction of the carotid bulb exposed to atherosclerosis-prone wall parameters (low WSSabs below and high OSI above group-defined 20% and 10% thresholds) was correlated with the individual bifurcation geometry. Multiple regressions revealed significant (P<0.01) relationships (&bgr;, 0.44 to 0.48) between the areas with atherosclerosis-prone wall parameters and the dICA/dCCA diameter ratio. The size of regions exposed to high OSI demonstrated highly significant (P⩽0.01) relationships with all analyzed geometry parameters (dICA/dCCA &bgr;, 0.48; tortuosity &bgr;, ⩽−0.56; bifurcation angle &bgr;, ≥0.47). Moderate ICA stenosis altered the distribution of wall parameters (45%/61% reduction of individually low WSSabs/high OSI in the proximal ICA), which were relocated to segments distal to the arterial stenosis. WSSabs/OSI topology after recanalization was similar compared with the normal wall parameter distribution. Conclusions—Flow-sensitive 4D MRI identified alterations in the segmental in vivo WSS distribution associated with atherosclerotic disease, surgical therapy, and individual bifurcation geometry and could be a valuable technique to assess the individual risk of flow-mediated atherosclerosis and carotid plaque progression.

3D blood flow characteristics in the carotid artery bifurcation assessed by flow-sensitive 4D MRI at 3T

To determine three‐dimensional (3D) blood flow patterns in the carotid bifurcation, 10 healthy volunteers and nine patients with internal carotid artery (ICA) stenosis ≥50% were examined by flow‐sensitive 4D MRI at 3T. Absolute and mean blood velocities, pulsatility index (PI), and resistance index (RI) were measured in the common carotid arteries (CCAs) by duplex sonography (DS) and compared with flow‐sensitive 4D MRI. Furthermore, 3D MRI blood flow patterns in the carotid bifurcation of volunteers and patients before and after recanalization were graded by two independent readers. Blood flow velocities measured by MRI were 31–39% lower than in DS. However, PI and RI differed by only 13–16%. Rating of 3D flow characteristics in the ICA revealed consistent patterns for filling and helical flow in volunteers. In patients with ICA stenosis, 3D blood flow visualization was successfully employed to detect markedly altered filling and helical flow patterns (forward‐moving spiral flow) in the ICA bulb and to evaluate the effect of revascularization, which restored filling and helical flow. Our results demonstrate the feasibility of flow‐sensitive 4D MRI for the quantification and 3D visualization of physiological and pathological flow patterns in the carotid artery bifurcation. Magn Reson Med 61:65–74, 2009.

Therapeutic Strategies After Examination by Transesophageal Echocardiography in 503 Patients With Ischemic Stroke

Background and Purpose— Transesophageal echocardiography (TEE) is the gold standard in detecting high-risk (ie, aortic thrombi) and potential sources (ie, patent foramen ovale [PFO]) of cerebral embolism. We sought to evaluate the additional information and therapeutic impact provided by TEE in stroke patients and to characterize patients in whom TEE is indispensable. Methods— We included 503 consecutive patients (mean age 62.2 years) with acute brain ischemia. Each patient received TEE and the following routine diagnostics: ultrasound of brain supplying arteries, ECG or Holter-ECG, transthoracic echocardiography, and brain imaging (computed tomography or MRI). Stroke etiology was classified according to the Trial of Org 10172 in Acute Stroke Treatment (TOAST) criteria. High-risk sources in TEE were: aortic thrombi or plaques ≥4 mm, thrombi in left atrial cavity/left atrial appendage, spontaneous echo contrast, and left atrial flow velocity <30 cm/s. Potential sources in TEE were PFO, atrial septal aneurysm, and aortic plaques <4 mm. Results— Stroke etiology was determined by routine diagnostics in 276 of 503 patients (54.9%). Of the remaining 227 patients (undetermined etiology), 212 (93.4%) were candidates for oral anticoagulation (OA). TEE revealed a high-risk source, with indication for OA in 17 of them (8.0%). A potential source leading to OA was found in an additional 48 patients (22.6%). The remaining 147 patients (69.3%) were treated by platelet inhibitors or statins. Conclusions— TEE strongly influenced secondary prevention and led to OA in one third of our patients with stroke of undetermined etiology. TEE is indispensable in all patients being candidates for OA when routine diagnostics cannot clarify stroke etiology.

Dynamic Cerebral Autoregulation in Acute Ischemic Stroke Assessed From Spontaneous Blood Pressure Fluctuations

Background and Purpose— This study investigates dynamic cerebral autoregulation assessed from spontaneous blood pressure (ABP) and cerebral blood flow velocity (CBFV) fluctuations and its time course in acute ischemic stroke. Methods— Forty patients admitted with acute ischemic stroke in the territory of the middle cerebral artery (MCA) were enrolled. Admission National Institutes of Health Stroke score was 6±4. Study 1 was performed within 22 (±11) hours and study 2 was performed within 134 (±25) hours of ictus. The final analysis comprised 33 and 29 patients for study 1 and study 2, respectively. Twenty-five age- and sex-matched controls were studied. ABP (Finapres method) and CBFV in both MCAs (transcranial Doppler) were recorded over 10 minutes. Correlations between diastolic and mean ABP and CBFV fluctuations were averaged, yielding the correlation coefficient indices (Dx, Mx). Transfer function analysis was applied to obtain phase shift and gain between ABP and CBFV oscillations. Results— No disturbance of autoregulation was indicated by all parameters at study 1. Separate analyses for clinical severity, stroke side, and size did not reveal significant differences for the various autoregulatory indices at study 1 and 2. At study 2, MCA flow velocity was significantly increased on both sides, the autoregulation index Mx was slightly but significantly (P<0.05) worse on both sides in comparison to study 1, and phase showed a trend toward poorer values on affected sides. No significant differences to controls occurred. Clinical outcome in patients completing both studies was good in all but one patient. Conclusions— Dynamic cerebral autoregulation assessed from spontaneous blood pressure fluctuations does not seem to be relevantly disturbed in early minor MCA stroke. At the subacute stage, slight autoregulatory disturbance may be present.

Complex Plaques in the Proximal Descending Aorta. An Underestimated Embolic Source of Stroke

Background and Purpose— To investigate the incidence of retrograde flow from complex plaques (≥4-mm-thick, ulcerated, or superimposed thrombi) of the descending aorta (DAo) and its potential role in embolic stroke. Methods— Ninety-four consecutive acute stroke patients with aortic plaques ≥3-mm-thick in transesophageal echocardiography were prospectively included. MRI was performed to localize complex plaques and to measure time-resolved 3-dimensional blood flow within the aorta. Three-dimensional visualization was used to evaluate if diastolic retrograde flow connected plaque location with the outlet of the left subclavian artery, left common carotid artery, or brachiocephalic trunk. Complex DAo plaques were considered an embolic source if retrograde flow reached a supra-aortic vessel that supplied the territory of visible acute and embolic retinal or cerebral infarction. Results— Only decreasing heart rate was correlated (P<0.02) with increasing flow reversal to the aortic arch. Retrograde flow from complex DAo plaques reached the left subclavian artery in 55 (58.5%), the left common carotid artery in 23 (24.5%), and the brachiocephalic trunk in 13 patients (13.8%). Based on routine diagnostics and MRI of the ascending aorta/aortic arch, stroke etiology was determined in 57 and cryptogenic in 37 patients. Potential embolization from DAo plaques was then identified in 19 of 57 patients (33.3%) with determined and in 9 of 37 patients (24.3%) with cryptogenic stroke. Conclusions— Retrograde flow from complex DAo plaques was frequent in both determined and cryptogenic stroke and could explain embolism to all brain territories. These findings suggest that complex DAo plaques should be considered a new source of stroke.

Time-resolved, 3-dimensional magnetic resonance flow analysis at 3 T: visualization of normal and pathological aortic vascular hemodynamics.

Flow-sensitive 3-dimensional magnetic resonance imaging at 3 T and advanced 3-dimensional visualization were used to visualize local and global vascular hemodynamics in the thoracic aorta. In patients with pathological geometric alterations of the thoracic aorta, this technique revealed considerable changes in local blood flow characteristics, compared with normal volunteers. Specifically, relatively small geometric changes, such as a partially thrombosed aortic arch or a mild aneurysm of the ascending aorta, resulted in major disturbances of local blood flow patterns within and even further downstream to the pathology.

Reproducibility of flow and wall shear stress analysis using flow-sensitive four-dimensional MRI.

To systematically investigate the scan–rescan reproducibility and observer variability of flow‐sensitive four‐dimensional (4D) MRI in the aorta for the assessment of blood flow and global and segmental wall shear stress.

In vivo assessment of wall shear stress in the atherosclerotic aorta using flow-sensitive 4D MRI

Our purpose was to correlate atherogenic low wall shear stress (WSS) and high oscillatory shear index (OSI) with the localization of aortic plaques. Flow‐sensitive four‐dimensional MRI was used to acquire three‐dimensional blood flow in the aorta of 62 patients with proven aortic atherosclerosis and 31 healthy volunteers. Multiplanar data analysis of WSS magnitude and OSI in 12 wall segments was performed in analysis planes distributed along the aorta. Disturbed WSS and OSI were defined as areas exposed to low WSS magnitude and high OSI beyond individual 15% thresholds. Planewise analysis revealed a good correlation (r = 0.85) of individual low WSS magnitude but not of high OSI with plaque distribution. Although plaques occurred only rarely in the ascending aorta, the incidence of low WSS magnitude and high OSI was similar to findings in other aortic segments where plaques occurred more frequently. Case‐by‐case comparisons of plaque location and critical wall parameters revealed a shift of atherogenic WSS magnitude (78% of all cases) and OSI (91%) to wall segments adjacent to the atheroma. Our results indicate that the predictive value of WSS for plaque existence depends on the aortic segment and that locations of critical wall parameters move to neighboring segments of regions affected by atherosclerosis. Magn Reson Med, 2010.