Support vector machine learning‐based cerebral blood flow quantification for arterial spin labeling MRI

Purpose : To develop a multivariate machine learning classification‐based cerebral blood flow (CBF) quantification method for arterial spin labeling (ASL) perfusion MRI. Methods : The label and control images of ASL MRI were separated using a machine‐learning algorithm, the support vector machine (SVM). The perfusion‐weighted image was subsequently extracted from the multivariate (all voxels) SVM classifier. Using the same pre‐processing steps, the proposed method was compared with standard ASL CBF quantification method using synthetic data and in‐vivo ASL images. Results : As compared with the conventional univariate approach, the proposed ASL CBF quantification method significantly improved spatial signal‐to‐noise‐ratio (SNR) and image appearance of ASL CBF images. Conclusion : the multivariate machine learning‐based classification is useful for ASL CBF quantification. Hum Brain Mapp 35:2869–2875, 2014. © 2013 Wiley Periodicals, Inc.     

Changes in cerebral blood flow during steady-state cycling exercise: a study using oxygen-15-labeled water with PET

Cerebral blood flow (CBF) during dynamic exercise has never been examined quantitatively using positron emission tomography (PET). This study investigated changes in CBF that occur over the course of a moderate, steady-state cycling exercise. Global and regional CBF (gCBF and rCBF, respectively) were measured using oxygen-15-labeled water (H₂¹⁵O) and PET in 10 healthy human subjects at rest (Rest), at the onset of exercise (Ex1) and at a later phase in the exercise (Ex2). At Ex1, gCBF was significantly (P<0.01) higher (27.9%) than at Rest, and rCBF was significantly higher than at Rest in the sensorimotor cortex for the bilateral legs (M1_Leg and S1_Leg), supplementary motor area (SMA), cerebellar vermis, cerebellar hemispheres, and left insular cortex, with relative increases ranging from 37.6% to 70.5%. At Ex2, gCBF did not differ from Rest, and rCBF was significantly higher (25.9% to 39.7%) than at Rest in only the M1_Leg, S1_Leg, and vermis. The areas showing increased rCBF at Ex1 were consistent with the central command network and the anatomic pathway for interoceptive stimuli. Our results suggest that CBF increases at Ex1 in parallel with cardiovascular responses then recovers to the resting level as the steady-state exercise continues.     

Cerebral blood flow is an earlier indicator of perfusion abnormalities than cerebral blood volume in Alzheimer's disease

The purpose of this study was to elucidate whether cerebral blood flow (CBF) can better characterize perfusion abnormalities in predementia stages of Alzheimer''s disease (AD) than cerebral blood volume (CBV) and whether cortical atrophy is more associated with decreased CBV or with decreased CBF. We compared measurements of CBV, CBF, and mean cortical thickness obtained from magnetic resonance images in a group of healthy controls, patients with mild cognitive impairment (MCI) who converted to AD after 2 years of clinical follow-up (MCI-c), and patients with mild AD. A significant decrease in perfusion was detected in the parietal lobes of the MCI-c patients with CBF parametric maps but not with CBV maps. In the MCI-c group, a negative correlation between CBF values and cortical thickness in the right parahippocampal gyrus suggests an increase in CBF that depends on cortical atrophy in predementia stages of AD. Our study also suggests that CBF deficits appear before CBV deficits in the progression of AD, as CBV abnormalities were only detected at the AD stage, whereas CBF changes were already detected in the MCI stage. These results confirm the hypothesis that CBF is a more sensitive parameter than CBV for perfusion abnormalities in MCI-c patients.     

The vascular steal phenomenon is an incomplete contributor to negative cerebrovascular reactivity in patients with symptomatic intracranial stenosis

‘Vascular steal'' has been proposed as a compensatory mechanism in hemodynamically compromised ischemic parenchyma. Here, independent measures of cerebral blood flow (CBF) and blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) responses to a vascular stimulus in patients with ischemic cerebrovascular disease are recorded. Symptomatic intracranial stenosis patients (n=40) underwent a multimodal 3.0T MRI protocol including structural (T₁-weighted and T₂-weighted fluid-attenuated inversion recovery) and hemodynamic (BOLD and CBF-weighted arterial spin labeling) functional MRI during room air and hypercarbic gas administration. CBF changes in regions demonstrating negative BOLD reactivity were recorded, as well as clinical correlates including symptomatic hemisphere by infarct and lateralizing symptoms. Fifteen out of forty participants exhibited negative BOLD reactivity. Of these, a positive relationship was found between BOLD and CBF reactivity in unaffected (stenosis degree<50%) cortex. In negative BOLD cerebrovascular reactivity regions, three patients exhibited significant (P<0.01) reductions in CBF consistent with vascular steal; six exhibited increases in CBF; and the remaining exhibited no statistical change in CBF. Secondary findings were that negative BOLD reactivity correlated with symptomatic hemisphere by lateralizing clinical symptoms and prior infarcts(s). These data support the conclusion that negative hypercarbia-induced BOLD responses, frequently assigned to vascular steal, are heterogeneous in origin with possible contributions from autoregulation and/or metabolism.     

Regional neurovascular coupling and cognitive performance in those with low blood pressure secondary to high-level spinal cord injury: improved by alpha-1 agonist midodrine hydrochloride

Individuals with high-level spinal cord injury (SCI) experience low blood pressure (BP) and cognitive impairments. Such dysfunction may be mediated in part by impaired neurovascular coupling (NVC) (i.e., cerebral blood flow responses to neurologic demand). Ten individuals with SCI >T6 spinal segment, and 10 age- and sex-matched controls were assessed for beat-by-beat BP, as well as middle and posterior cerebral artery blood flow velocity (MCAv, PCAv) in response to a NVC test. Tests were repeated in SCI after 10 mg midodrine (alpha₁-agonist). Verbal fluency was measured before and after midodrine in SCI, and in the control group as an index of cognitive function. At rest, mean BP was lower in SCI (70±10 versus 92±14 mm Hg; P<0.05); however, PCAv conductance was higher (0.56±0.13 versus 0.39±0.15 cm/second/mm Hg; P<0.05). Controls exhibited a 20% increase in PCAv during cognition; however, the response in SCI was completely absent (P<0.01). When BP was increased with midodrine, NVC was improved 70% in SCI, which was reflected by a 13% improved cognitive function (P<0.05). Improvements in BP were related to improved cognitive function in those with SCI (r²=0.52; P<0.05). Impaired NVC, secondary to low BP, may partially mediate reduced cognitive function in individuals with high-level SCI.     

Capillary transit time heterogeneity and flow-metabolism coupling after traumatic brain injury

Most patients who die after traumatic brain injury (TBI) show evidence of ischemic brain damage. Nevertheless, it has proven difficult to demonstrate cerebral ischemia in TBI patients. After TBI, both global and localized changes in cerebral blood flow (CBF) are observed, depending on the extent of diffuse brain swelling and the size and location of contusions and hematoma. These changes vary considerably over time, with most TBI patients showing reduced CBF during the first 12 hours after injury, then hyperperfusion, and in some patients vasospasms before CBF eventually normalizes. This apparent neurovascular uncoupling has been ascribed to mitochondrial dysfunction, hindered oxygen diffusion into tissue, or microthrombosis. Capillary compression by astrocytic endfeet swelling is observed in biopsies acquired from TBI patients. In animal models, elevated intracranial pressure compresses capillaries, causing redistribution of capillary flows into patterns argued to cause functional shunting of oxygenated blood through the capillary bed. We used a biophysical model of oxygen transport in tissue to examine how capillary flow disturbances may contribute to the profound changes in CBF after TBI. The analysis suggests that elevated capillary transit time heterogeneity can cause critical reductions in oxygen availability in the absence of ‘classic'' ischemia. We discuss diagnostic and therapeutic consequences of these predictions.     

三维超声造影在诊断宫腔粘连中的应用价值

目的探讨三维超声造影(3D-CEUS)在诊断宫腔粘中的应用价值。方法对71例经阴道三维超声(3DTVS)检查提示怀疑宫腔粘连的患者行宫腔镜检查,以宫腔镜检查为金标准,比较两者诊断宫腔粘连的准确率;在月经20~23d对3D-TVS诊断疑似宫腔粘连的患者行声诺维(sonovue)3D-CEUS检查,获得内膜厚度、容积(V)、回声强度(MG)、血管指数(VI)、血流指数(FI)、血管血流综合指数(VFI)等相关指标;对经sonovue 3D-CEUS疑似宫腔粘连的的患者,于月经第3~7天行宫腔镜检查,回顾性分析不同程度宫腔粘连的患者与3D-CEUS获得的V、MG、VI、FI、VFI值的关系。结果 3DTVS与宫腔镜比较,3D-TVS诊断宫腔粘连的符合率是90.14%;宫腔镜检查轻、中、重度3组,各组间子宫内膜厚度和容积比较均有显著性差异(P0.05);轻度和重度组子宫内膜MG分别(39.75±7.46)、(45.64±6.10),其差异具有统计学意义(P0.05);轻度和重度组组间子宫内膜VI、FI、VFI相比较,差异均具有统计学意义(P0.05),而中度组与其它两组比较均无显著性差异(P0.05)。经静脉推注30mg sonovue造影剂后,3D-CEUS检查结果显示:3组内膜血流VI值分别较造影前显著增加(P0.001),轻度和重度、中度和重度组的内膜VI、FI、VFI相比较,差异均具有统计学意义(P0.05)。结论 3DCEUS检查不仅可以较准确的诊断宫腔粘连,而且通过计算机辅助分析技术对宫腔粘连患者子宫内膜V、MG、VI、FI、VFI进行定量测量,对宫腔粘连的程度有更客观的评价。