Functional MRI of rat kidney using BOLD & ASL techniques

Noninvasive functional magnetic resonance imaging (fMRI) methods may provide new ways to detect and track renal hemodynamic changes in-vivo. In this thesis, two fMRI methods were correlated with simultaneous invasive hemodynamic measurements. A particular goal of this thesis was to measure the relative contributions of oxygenation and perfusion changes to changes in the relaxation rate, T2*. Also, an MRI compatible motion detector was built and along with the invasive probes was interfaced to a data acquisition system for use during scanning. Drug-induced changes in renal oxygenation and blood flow were measured by BOLD- & ASL-MRI noninvasively, while a dual oxygenation/perfusion optical-probe was used for the invasive measurements. Six sets of results were obtained. Values of T2*, LDF and pO2 correlated in four of the data sets while the other two were discrepant. BOLD images were of high quality while ASL perfusion maps were of inadequate spatial resolution and poor quality

Validation of diffuse correlation spectroscopy measurements of rodent cerebral blood flow with simultaneous arterial spin labeling MRI; towards MRI-optical continuous cerebral metabolic monitoring.

Cerebral blood flow (CBF) during stepped hypercapnia was measured simultaneously in the rat brain using near-infrared diffuse correlation spectroscopy (DCS) and arterial spin labeling MRI (ASL). DCS and ASL CBF values agree very well, with high correlation (R=0.86, p< 10(-9)), even when physiological instability perturbed the vascular response. A partial volume effect was evident in the smaller magnitude of the optical CBF response compared to the MRI values (averaged over the cortical area), primarily due to the inclusion of white matter in the optically sampled volume. The 8.2 and 11.7 mm mid-separation channels of the multi-distance optical probe had the lowest partial volume impact, reflecting ~75 % of the MR signal change. Using a multiplicative correction factor, the ASL CBF could be predicted with no more than 10% relative error, affording an opportunity for real-time relative cerebral metabolism monitoring in conjunction with MR measurement of cerebral blood volume using super paramagnetic contrast agents.R01 EB006385 - NIBIB NIH HHS; R01 EB001954 - NIBIB NIH HHS; R01 NS057476 - NINDS NIH HHS; P41 RR014075 - NCRR NIH HHS; R01 HD042908-07 - NICHD NIH HHS; R01 EB002066 - NIBIB NIH HHS; R01 HD042908-06 - NICHD NIH HHS; R01 HD042908 - NICHD NIH HHSPublished versio

Calibration of diffuse correlation spectroscopy with a time-resolved near-infrared technique to yield absolute cerebral blood flow measurements

A primary focus of neurointensive care is the prevention of secondary brain injury, mainly caused by ischemia. A noninvasive bedside technique for continuous monitoring of cerebral blood flow (CBF) could improve patient management by detecting ischemia before brain injury occurs. A promising technique for this purpose is diffuse correlation spectroscopy (DCS) since it can continuously monitor relative perfusion changes in deep tissue. In this study, DCS was combined with a time-resolved near-infrared technique (TR-NIR) that can directly measure CBF using indocyanine green as a flow tracer. With this combination, the TR-NIR technique can be used to convert DCS data into absolute CBF measurements. The agreement between the two techniques was assessed by concurrent measurements of CBF changes in piglets. A strong correlation between CBF changes measured by TR-NIR and changes in the scaled diffusion coefficient measured by DCS was observed (R2 = 0.93) with a slope of 1.05 ± 0.06 and an intercept of 6.4 ± 4.3% (mean ± standard error)

신생 백서를 이용한 저산소성 허혈성 뇌손상 모델에서 동맥스핀표지 관류 자기공명영상법을 통한 뇌혈류 변화 평가

학위논문(박사)--서울대학교 대학원 :의과대학 의학과,2020. 2. 천정은.Purpose The purpose of this study was to evaluate cerebral blood flow (CBF) changes over time in the animal model of neonatal hypoxic-ischemic brain injury using arterial spin labeling (ASL) and correlate CBF changes with development of diffusion restriction on diffusion-weighted imaging (DWI). Materials and methods Six 7-day-old neonatal rats with unilateral carotid artery ligation underwent multiple ASL and DWI MRI scans at 9.4 T before and during hypoxia (8% O2). One 7-day-old rat underwent ASL MRI without surgical ligation or hypoxia as a normal CBF control. Delayed T2-weighted MR imaging and histological examination were performed on day 3 post-hypoxia. CBF values on ASL were measured in four brain areas (i.e., ipsilateral and contralateral cortical and deep areas). The development of diffusion restriction was also evaluated in each of the four areas (i.e., DWI-positive[+] vs. DWI-negative[-] areas). Regional CBF changes over time were evaluated. Regional CBF values and their changes over time were compared between the DWI(+) and DWI(-) areas. Results Regional CBF values before hypoxia were significantly lower than those of the normal control (CBF in normal control vs. CBF before hypoxia: 147.8 vs. 39.2 ± 19.7 in ipsilateral cortex, p < 0.01; 151.5 vs. 49.2 ± 21.2 in ipsilateral deep area, p < 0.01; 150.0 vs. 108.2 ± 22.2 in contralateral cortex, p = 0.014; and 165.0 vs. 104.22 ± 26.0 in contralateral deep area, p < 0.01). After exposure to hypoxia, CBF values decreased in all areas (mean CBF difference: -25.5 in ipsilateral cortex, p = 0.057; -21.5 in ipsilateral deep area, p = 0.012; -52.2 in contralateral cortex, p<0.01; -36.4 in contralateral deep area, p<0.01). Eleven areas with diffusion restriction were included in the DWI(+) area group, whereas 13 areas showing no diffusion restriction were included in the DWI(-) area group. The regional CBF values in the DWI(+) area were estimated to be 34.6 ml/100 g/min lower than those in the DWI(-) area. On delayed T2-weighted MRI, the diffusion-restricted areas presented as areas of bright signal intensity or heterogeneous mixed signal intensity with volume loss, which correlated to areas of infarction or ischemia on histology. Conclusion The ASL perfusion MRI technique made it possible to evaluate regional CBF changes over time during exposure to hypoxia in neonatal rats with unilateral carotid artery ligation. Damaged brain areas that matched well with the diffusion restricted areas had significantly lower CBF values at all time points, compared to preserved areas without diffusion restriction. CBFs measured with ASL may be utilized as a useful imaging indicator of subsequent hypoxic ischemic brain damage.목적 신생 백서를 이용한 저산소성 허혈성 뇌손상 모델에서 동맥스핀표지(ASL) 관류자기공명영상을 이용하여 시간 경과에 따른 대뇌혈류(CBF)의 변화를 평가하고, 뇌혈류 변화와 확산강조영상(DWI) 및 조직학적 소견을 비교 평가하고자 한다. 방법 생후 7일된 신생아 쥐 6마리에서 일측 경동맥을 결찰한 뒤 저산소(8% O2)를 가하면서 9.4T MRI에서 ASL 및 DWI MRI 검사를 시행하였다. 생후 7일된 쥐 한 마리를 외과적 결찰이나 저산소증없이 ASL MRI를 시행하여 정상 뇌혈류 대조군으로 사용하였다. 3일 뒤 T2강조영상과 조직검사를 시행하였다. ASL CBF 값을 4개의 뇌영역(즉, 동측과 반대측 피질 및 심부 영역)에서 측정하였다. 각 영역에서 DWI상 제한확산의 발생 여부를 평가하였다 (즉, DWI-양성 대 DWI-음성군). 시간 경과에 따른 CBF 변화를 평가하였다. 시간 경과에 따른 CBF 변화 양상을 DWI양성군과 음성군간에 비교하였다. 결과 일측 경동맥 결찰 후 CBF 값은 정상 대조군에 비해 유의하게 낮아졌다 (정상 대조군 CBF vs.대 저산소증 전 CBF: 동측 대뇌피질, 147.8 vs. 39.2 ± 19.7, p <0.01; 동측 심부영역, 151.5 vs. 49.2 ± 21.2, p <0.01; 반대측 피질, 150.0 vs. 108.2 ± 22.2, p = 0.014; 반대측 심부영역, 165.0 vs 104.22 ± 26.0, p <0.01). 저산소에 노출 된 후 CBF 값은 모든 영역에서 감소했다 (평균 CBF 차이, 동측 피질에서 -25.5, p = 0.057; 동측 심부영역에서 -21.5, p = 0.012, 반대쪽 피질에서 -52.2, p <0.01, 반대측 심부영역에서 -36.4, p <0.01). DWI양성군에는 확산제한이 있는 11개의 영역이 포함되었고 DWI음성군에는 확산제한이 없었던 13개의 영역이 포함되었다. DWI-양성군의 CBF 값은 DWI-음성군보다 평균 34.6 ml/100g/min 낮았다. 지연 T2강조 MRI에서 확산 제한을 보였던 영역은 밝은 신호 강도 또는 부피 감소를 동반한 혼합 신호 강도 영역으로 보였으며 조직학상의 경색이나 허혈 영역과 일치하였다. 결론 ASL 관류자기공명영상법을 이용하여 신생백서에서 일측 경동맥 동맥 결찰 후 저산소증에 노출되는 동안, 시간에 따른 CBF 변화를 평가할 수 있었다. 확산 제한 영역과 잘 일치하는 손상 뇌영역은 확산 제한이 없었던 보존 뇌영역에 비해 모든 시점에서 CBF 값이 현저히 낮았다. ASL 기법으로 측정된 CBF값은 향후 저산소성 허혈성 뇌 손상 발생을 예측하는데 유용한 영상지표로서 이용될 수 있다.Introduction 9 Materials & Methods 12 Results 18 Discussion 29 Conclusion 35 Reference 36 국문초록 39 Figure 1 15 Figure 2 20 Figure 3 24 Figure 4 25 Figure 5 26 Figure 6 28 Table 1 19 Table 2 22 Table 3 23Docto

Development of an Awake Behaving model for Laser Doppler Flowmetry in Mice

Bien que le cerveau ne constitue que 2% de la masse du corps chez les humains, il présente l’activité métabolique la plus élevée dans le corps, et en conséquence, constitue un organe hautement vascularisé. En fait, l’approvisionnement en sang dans le cerveau est strictement modulé au niveau régional par un mécanisme fondamental nommé couplage neurovasculaire (CNV), qui associe les besoins métaboliques locaux au flux sanguin cérébral [1, 2]. Notre compréhension du CNV sous des conditions physiologiques et pathologiques a été améliorée par un large éventail d’études menées chez les rongeurs. Néanmoins, ces études ont été réalisées soit sous anesthésie, soit chez la souris éveillée et immobilisée, afin d’éviter le mouvement de la tête pendant l'acquisition de l'image. Les anesthésiques, ainsi que le stress induit par la contention, peuvent altérer l'hémodynamique cérébrale, ce qui pourrait entraver les résultats obtenus. Par conséquent, il est essentiel de contrôler ces facteurs lors de recherches futures menées au sujet de la réponse neurovasculaire. Au cours de l’étude présente, nous avons développé un nouveau dispositif pour l'imagerie optique éveillée, où la tête de la souris est immobilisée, mais son corps est libre de marcher, courir ou se reposer sur une roue inclinée. En outre, nous avons testé plusieurs protocoles d'habituation, selon lesquels la souris a été progressivement entraînée pour tolérer l’immobilisation de tête, afin de minimiser le stress ressenti lors des sessions d'imagerie. Enfin, nous avons, pour la première fois, cherché à valider l'efficacité de ces protocoles d'habituation dans la réduction du stress, en mesurant l'évolution des taux plasmatiques de corticostérone tout au long de notre étude. Nous avons noté que les souris s'étaient rapidement adaptées à la course sur la roue et que les signes visibles de stress (luttes, vocalisations et urination) étaient nettement réduits suite à deux sessions d'habituation. Néanmoins, les taux de corticostérone n'ont pas été significativement réduits chez les souris habituées, par rapport aux souris naïves qui ont été retenues sur la roue sans entraînement préalable (p> 0,05). Ce projet met en évidence la nécessité d'une mesure quantitative du stress, car une réduction des comportements observables tels que l'agitation ou la lutte peut être indicative non pas d'un niveau de stress plus faible, mais plutôt d'un désespoir comportemental. Des recherches supplémentaires sont nécessaires pour déterminer si la fixation de la tête lors de l'imagerie optique chez la souris peut être obtenue avec des niveaux de stress plus faibles, et si le stress induit par la contrainte effectuée avec notre dispositif est associé à des changements de la réponse hémodynamique.Whilst the brain only constitutes 2% of total body weight in humans, it exhibits the highest metabolic activity in the body, and as such is a highly vascularized organ. In fact, regional blood supply within the brain is strictly modulated through a fundamental process termed neurovascular coupling (NVC), which couples local metabolic needs with cerebral blood flow [1, 2]. A wide array of optical imaging studies in rodents has enhanced our understanding of NVC under physiological and pathological conditions. Nevertheless, these studies have been performed either under anesthesia, or in the awake mouse using restraint to prevent head-motion during image acquisition. Both anesthetics and restraint-induced stress have been clearly shown to alter cerebral hemodynamics, thereby potentially interfering with the obtained results [3, 4]. Hence, it is essential to control for these factors during future research which investigates the neurovascular response. In the present study, we have developed a new apparatus for awake optical imaging, where the mouse is head-restraint whilst allowed to walk, run or rest on an inclined wheel. In addition, we have tested several habituation protocols, according to which the mouse was gradually trained to tolerate head-restraint, in order to minimize the stress experienced during imaging sessions. Lastly, we have, for the first time, sought to validate the efficiency of these habituation protocols in reducing stress, by measuring the evolution of plasma corticosterone levels throughout the study. We noted that the mice had quickly adapted to running on the wheel, and that the overt signs of stress (struggling, vocalizations and urination) were clearly reduced within two habituation sessions. Nevertheless, corticosterone levels were not significantly reduced in habituated mice, relative to naïve mice that were restrained on the wheel without prior training (p > 0.05). This project highlights the necessity for a quantitative measure of stress, as a reduction in observable behaviors such as agitation or struggling may be indicative not of lower stress, but rather, of behavioral despair. Further research is needed to determine whether head-fixation during optical imaging in mice can be achieved with lower stress levels, and if restraint-induced stress using our apparatus is associated with changes in the hemodynamic response

Digging Deeper with Diffuse Correlation Spectroscopy

Patients with neurological diseases are vulnerable to cerebral ischemia, which can lead to brain injury. In the intensive care unit (ICU), neuromonitoring techniques that can detect flow reductions would enable timely administration of therapies aimed at restoring adequate cerebral perfusion, thereby avoiding damage to the brain. However, suitable bedside neuromonitoring methods sensitive to changes of blood flow and/or oxygen metabolism have yet to be established. Near-infrared spectroscopy (NIRS) is a promising technique capable of non-invasively monitoring flow and oxygenation. Specifically, diffuse correlation spectroscopy (DCS) and time-resolved (TR) NIRS can be used to monitor blood flow and tissue oxygenation, respectively, and combined to measuring oxidative metabolism. The work presented in this thesis focused on advancing a DCS/TR-NIRS hybrid system for acquiring these physiological measurements at the bedside. The application of NIRS for neuromonitoring is favourable in the neonatal ICU since the relatively thin scalp and skull of infants has minimal effect on the detected optical signal. Considering this application, the validation of a combined DCS/NIRS method for measuring the cerebral metabolic rate of oxygen (CMRO2) was investigated in Chapter 2. Although perfusion changes measured by DCS have been confirmed by various flow modalities, characterization of photon scattering in the brain is not clearly understood. Chapter 3 presents the first DCS study conducted directly on exposed cortex to confirm that the Brownian motion model is the best flow model for characterizing the DCS signal. Furthermore, a primary limitation of DCS is signal contamination from extracerebral tissues in the adult head, causing CBF to be underestimated. In Chapter 4, a multi-layered model was implemented to separate signal contributions from scalp and brain; derived CBF changes were compared to computed tomography perfusion. Overall, this thesis advances DCS techniques by (i) quantifying cerebral oxygen metabolism, (ii) confirming the more appropriate flow model for analyzing DCS data and (iii) demonstrating the ability of DCS to measure CBF accurately despite the presence of a thick (1-cm) extracerebral layer. Ultimately, the work completed in this thesis should help with the development of a hybrid DCS/NIRS system suitable for monitoring cerebral hemodynamics and energy metabolism in critical-ill patients

Preclinical models of middle cerebral artery occlusion: new imaging approaches to a classic technique

Stroke remains a major burden on patients, families, and healthcare professionals, despite major advances in prevention, acute treatment, and rehabilitation. Preclinical basic research can help to better define mechanisms contributing to stroke pathology, and identify therapeutic interventions that can decrease ischemic injury and improve outcomes. Animal models play an essential role in this process, and mouse models are particularly well-suited due to their genetic accessibility and relatively low cost. Here, we review the focal cerebral ischemia models with an emphasis on the middle cerebral artery occlusion technique, a “gold standard” in surgical ischemic stroke models. Also, we highlight several histologic, genetic, and in vivo imaging approaches, including mouse stroke MRI techniques, that have the potential to enhance the rigor of preclinical stroke evaluation. Together, these efforts will pave the way for clinical interventions that can mitigate the negative impact of this devastating disease

Development of a novel diffuse correlation spectroscopy platform for monitoring cerebral blood flow and oxygen metabolism: from novel concepts and devices to preclinical live animal studies

New optical technologies were developed to continuously measure cerebral blood flow (CBF) and oxygen metabolism (CMRO2) non-invasively through the skull. Methods and devices were created to improve the performance of near-infrared spectroscopy (NIRS) and diffuse correlation spectroscopy (DCS) for use in experimental animals and humans. These were employed to investigate cerebral metabolism and cerebrovascular reactivity under different states of anesthesia and during models of pathological states. Burst suppression is a brain state arising naturally in pathological conditions or under deep general anesthesia, but its mechanism and consequences are not well understood. Electroencephalography (EEG) and cortical hemodynamics were simultaneously measured in rats to evaluate the coupling between cerebral oxygen metabolism and neuronal activity in the burst suppressed state. EEG bursts were used to deconvolve NIRS and DCS signals into the hemodynamic and metabolic response function for an individual burst. This response was found to be similar to the stereotypical functional hyperemia evoked by normal brain activation. Thus, spontaneous burst activity does not cause metabolic or hemodynamic dysfunction in the cortex. Furthermore, cortical metabolic activity was not associated with the initiation or termination of a burst. A novel technique, time-domain DCS (TD-DCS), was introduced to significantly increase the sensitivity of transcranial CBF measurements to the brain. A new time-correlated single photon counting (TCSPC) instrument with a custom high coherence pulsed laser source was engineered for the first-ever simultaneous measurement of photon time of flight and DCS autocorrelation decays. In this new approach, photon time tags are exploited to determine path-length-dependent autocorrelation functions. By correlating photons according to time of flight, CBF is distinguished from superficial blood flow. Experiments in phantoms and animals demonstrate TD-DCS has significantly greater sensitivity to the brain than existing transcranial techniques. Intracranial pressure (ICP) modulates both steady-state and pulsatile CBF, making CBF a potential marker for ICP. In particular, the critical closing pressure (CrCP) has been proposed as a surrogate measure of ICP. A new DCS device was developed to measure pulsatile CBF non-invasively. A novel method for estimating CrCP and ICP from DCS measurement of pulsatile microvascular blood flow in the cerebral cortex was demonstrated in rats.2018-03-08T00:00:00

Functional imaging of the brain vasculature in pre-clinical models of amyloidosis

One of the pathological hallmarks of Alzheimer's disease is amyloid‑β accumulation in the parenchymal brain tissue. Amyloid‑β is also found in the vessel wall of patients with cerebral amyloid angiopathy (CAA). These pathological accumulations of the amyloid‑β peptide are referred to as amyloidosis. Both patients with AD and CAA also commonly show cerebrovascular dysfunction. The aim of this thesis was to improve our understanding of the relation between cerebrovascular dysfunction and amyloidosis. To that end, cerebrovascular function measurements were designed and carried out in mouse models of cerebral amyloidosis. Chapter 2 and 3 show improvements of the current techniques to measure cerebrovascular function in mice. Surprisingly however, no difference was found in cerebrovascular function in two different models of amyloidosis, as shown in chapter 4 and 5. Possible explanations of the negative findings are further discussed in chapter 6. Despite the negative connotation of the outcome this thesis, this work is another small step towards a better understanding of the exact relationship between cerebrovascular dysfunction and amyloid‑β deposition in AD and CAA patients. Ultimately, this will help in the design of highly needed novel therapies for AD and CAA. Netherlands Organization for Scientific Research (NWO) supported the research, under research program VIDI, project ‘Amyloid and Vessels’ (864.13.014); Alzheimer Nederland supported printing of the thesisLUMC / Geneeskund