Reactivity of the middle cerebral artery to carbon dioxide

Transcranial Doppler ultrasound (TCD) is used for the assessment of cerebral blood flow velocity (CBFV) at the middle cerebral artery (MCA) with the assumption that diameter of the artery does not change. Thus, CBFV is equivalent to cerebral blood flow (CBF). The purpose of this thesis was determine if the MCA dilates during hypercapnia (HC) and/or constricts during hypocapnia (HO) in healthy young and older adults using 3T magnetic resonance imaging (MRI). We also determined how these changes in MCA cross-sectional area (CSA) influence estimates of CBF and cerebrovascular reactivity (CVR) from TCD in young and older adults. Lastly, we compared whether changes in MCA CSA mimic those at the internal carotid artery (ICA) as assessed with duplex ultrasound during HC and HO. For all studies, HC was induced with 6% carbon dioxide and HO with hyperventilation at 30 breaths per minute, each for five minutes. T2-weighted sagittal images of the MCA were performed with MRI and collection of an image took approximately one minute. When assessing the peak response there was a significant increase in MCA CSA during HC and a decrease during HO. Using these MCA CSA values to calculate CBF resulted in a greater percent change during each protocol compared to CBFV. Changes in MCA CSA were also examined every minute over the five minute periods of HC and HO and significant increases were seen within the first minute of HC while decreases during HO were not evident until minute four. No changes in ICA CSA occurred during HC or HO. Using CBF rather than CBFV to calculate CVR resulted in a greater CVR for each protocol. Finally, when the response to HC was compared between young and older adults the increase in MCA CSA was reduced in older adults compared to young. Cerebrovascular conductance was also reduced in older adults compared to young during HC, while CVR was not different. In summary, the diameter of the MCA changes during manipulations of carbon dioxideand CBFV underestimates CBF and CVR. Also, CVR may not be the best metric to compare the vasodilatory response to HC between groups

The effect of cholinergic loss on the structure and function of the neurovascular unit: implications for cerebral amyloid angiopathy

Innervation of cerebral blood vessels is important for the regulation of vascular tone and adequate cerebral perfusion. Alzheimer’s disease (AD) is characterised by a loss of cholinergic innervation of the neurovascular unit (NVU). This loss may contribute not only to inefficient cerebrovascular perfusion, but also to the failure of removal of amyloid-β (Aβ), leading to its accumulation as cerebral amyloid angiopathy (CAA). This hypothesis was tested by mimicking loss of cholinergic innervation of the cortex and hippocampus in adult male C57BL/6 mice using the immunospecific toxin mu-p75-saporin. Using quadruple labelling immunohistochemistry and 3D reconstructions of the NVU, loss of perivascular cholinergic innervation was observed at the smooth muscle cells and basement membrane in the cortex and hippocampus, with additional perivascular loss at the astrocyte endfeet in the cortex. Arterial spin labelling fMRI revealed no differences in resting cerebral blood flow between control and saporin-treated mice in either the cortex or hippocampus. However, denervated vessels in the cortex, but not the hippocampus, failed to respond to pharmacological stimulation of endothelial nitric oxide synthase (eNOS). Further studies revealed a decrease in eNOS expression in the cortex, but an increase in eNOS expression and activity in the hippocampus following loss of cholinergic input. No differences were noted between control and saporin-treated mice in the efficiency of removal of Aβ along perivascular basement membranes in either the cortex or hippocampus. Treatment of TetO-APPSweInd mice with saporin resulted in a trend towards higher CAA load. These data suggest that there are innate differences between NVUs of the cortex and the hippocampus and a difference in their functional response to loss of cholinergic input. Loss of cholinergic input at the NVU may contribute to accumulation of Aβ; however, this likely requires additional pathological factors for CAA to develop

Monitoring Alzheimer's disease in transgenic mice with ultra high field magnetic resonance imaging

While aging remains one of the most significant risk factors for development of Alzheimer disease (AD), increasing evidence strongly points to the potential roles of cerebrovascular and white matter abnormalities in the disease development. A better understanding of the manner in which these abnormalities contribute to disease progression can be achieved by in vivo characterization of AD related pathologies. To this end, MR based techniques serve as effective non-invasive tools to longitudinally monitor changes in AD brain. In this thesis, a variety of MR based techniques were optimized and employed to longitudinally monitor the AD progression in transgenic mouse models of the disease at 9.4T and 17.6T. In Chapter 2, age-dependent blood flow alterations were examined in a Tg2576 mouse model of Alzheimer's disease using MR angiography at 17.6T. AD is linked to abnormalities in the vascular system. In Chapter 3, in vivo T2 changes were longitudinally monitored in the corpus callosum, of the Tg2576 mice. In Chapter 4, age-dependent regional brain T1 and T2 changes in healty mice were established at 17.6T. In vivo imaging of these mouse models at ultra-high magnetic field strengths can permit a better understanding of the underlying cellular mechanism of AD.The Centre for Medical Systems Biology (CMSB), Internationale Stichting Alzheimer Onderzoek and Alzheimer NederlandSolid state NMR/Biophysical Organic Chemistr

High-Field Functional MRI from the Perspective of Single Vessels in Rats and Humans

Functional MRI (fMRI) has been employed to map brain activity and connectivity based on the neurovascular coupled hemodynamic signal. However, in most cases of fMRI studies, the cerebral vascular hemodynamic signal has been imaged in a spatially smoothed manner due to the limit of spatial resolution. There is a need to improve the spatiotemporal resolution of fMRI to map dynamic signal from individual venule or individual arteriole directly. Here, the thesis aims to provide a vascular-specific view of hemodynamic response during active state or resting state. To better characterize the temporal features of task-related fMRI signal from different vascular compartments, we implemented a line-scanning method to acquire vessel-specific blood-oxygen-level-dependent (BOLD) / cerebral-blood-volume (CBV) fMRI signal at 100-ms temporal resolution with sensory or optogenetic stimulation. Furthermore, we extended the line-scanning method with multi-echo scheme to provide vessel-specific fMRI with the higher contrast-to-noise ratio (CNR), which allowed us to directly map the distinct evoked hemodynamic signal from arterioles and venules at different echo time (TE) from 3 ms to 30 ms. The line-scanning fMRI methods acquire single k-space line per TR under a reshuffled k space acquisition scheme which has the limitation of sampling the fMRI signal in real-time for resting-state fMRI studies. To overcome this, we implemented a balanced Steady-state free precession (SSFP) to map task-related and resting-state fMRI (rsfMRI) with high spatial resolution in anesthetized rats. We reveal venule-dominated functional connectivity for BOLD fMRI and arteriole-dominated functional connectivity for CBV fMRI. The BOLD signal from individual venules and CBV signal from individual arterioles show correlations at an ultra-slow frequency (< 0.1 Hz), which are correlated with the intracellular calcium signal measured in neighboring neurons. In complementary data from awake human subjects, the BOLD signal is spatially correlated among sulcus veins and specified intracortical veins of the visual cortex at similar ultra-slow rhythms. This work provides a high-resolution fMRI approach to resolve brain activation and functional connectivity at the level of single vessels, which opened a new avenue to investigate brian functional connectivity at the scale of vessels

EVALUATION OF EARLY TUMOR ANGIOGENESIS USING ULTRASOUND ACOUSTIC ANGIOGRAPHY

Cancer angiogenesis is a feature of tumor growth that produces disorganized and dysfunctional vascular networks. Acoustic angiography is a unique implementation of contrast-enhanced ultrasound that allows us to visualize microvasculature with high resolution and contrast, including blood vessels as small as 100 to 150 micrometers. These angiography images can be analyzed to evaluate the morphology of the blood vessels for the purpose of detecting and diagnosing tumors. This thesis describes the implementation, advantages, and disadvantages of acoustic angiography and evaluates tumor vasculature in a pre-clinical cancer model. Measurements of tortuosity and vascular density in tumor regions were significantly higher than those of control regions, including in the smallest palpable tumors (2-3 mm). Additionally, abnormal tortuosity extended beyond the margin of tumors, as distal tissue separated from the tumor by at least 4 mm exhibited higher tortuosity than healthy individuals. Vascular tortuosity was negatively correlated to distance from the tumor margin using linear regression. Analysis of full images to detect tumors was performed using a reader study approach to assess visual interpretations, and quantitative analysis combined tortuosity with spatial relationships between vessels using a density-based clustering approach. Visual assessment using a reader study design resulted in an area under the receiver operating characteristic (ROC) curve of approximately 0.8, and the ROC curve was significantly correlated with tumor diameter, indicating that larger tumors were detected more accurately using this approach. Quantitative analysis of the same images used a density-based clustering algorithm to combine vessels in an image into clusters based on their tortuosity (using 2 metrics), radius, and proximity to one another. In tumors, highly tortuous vessels were closely packed, forming large clusters in the analysis, while control images lacked such patterns and formed much smaller clusters. Therefore, maximum cluster size was used to detect tumors, achieving an area under the ROC curve of 0.96. Finally, superharmonic molecular imaging was used to image targeted microbubbles with higher contrast to tissue ratios than conventional molecular imaging. These molecular images were combined with vascular acoustic angiography images to begin to relate the expression of endothelial markers of angiogenesis with vascular features such as tortuosity.Doctor of Philosoph