Quantifying cerebellum grey matter and white matter perfusion using pulsed arterial spin labeling

β1 integrins play a critical role maintaining vascular integrity in the hypoxic spinal cord, particularly in white matter

Interactions between extracellular matrix (ECM) proteins and β1 integrins play an essential role maintaining vascular integrity in the brain, particularly under vascular remodeling conditions. As blood vessels in the spinal cord are reported to have distinct properties from those in the brain, here we examined the impact of β1 integrin inhibition on spinal cord vascular integrity, both under normoxic conditions, when blood vessels are stable, and during exposure to chronic mild hypoxia (CMH), when extensive vascular remodeling occurs. We found that a function-blocking β1 integrin antibody triggered a small degree of vascular disruption in the spinal cord under normoxic conditions, but under hypoxic conditions, it greatly enhanced (20-fold) vascular disruption, preferentially in spinal cord white matter (WM). This resulted in elevated microglial activation as well as marked loss of myelin integrity and reduced density of oligodendroglial cells. To understand why vascular breakdown is localized to WM, we compared expression levels of major BBB components of WM and grey matter (GM) blood vessels, but this revealed no obvious differences. Interestingly however, hypoxyprobe staining demonstrated that the most severe levels of spinal cord hypoxia induced by CMH occurred in the WM. Analysis of brain tissue revealed a similar preferential vulnerability of WM tracts to show vascular disruption under these conditions. Taken together, these findings demonstrate an essential role for β1 integrins in maintaining vascular integrity in the spinal cord, and unexpectedly, reveal a novel and fundamental difference between WM and GM blood vessels in their dependence on β1 integrin function during hypoxic exposure. Our data support the concept that the preferential WM vulnerability described may be less a result of intrinsic differences in vascular barrier properties between WM and GM, and more a consequence of differences in vascular density and architecture.

Regional contribution of vascular dysfunction in white matter dementia: clinical and neuropathological insights

The maintenance of adequate blood supply and vascular integrity is fundamental to ensure cerebral function. A wide range of studies report vascular dysfunction in white matter dementias, a group of cerebral disorders characterized by substantial white matter damage in the brain leading to cognitive impairment. Despite recent advances in imaging, the contribution of vascular-specific regional alterations in white matter dementia has been not extensively reviewed. First, we present an overview of the main components of the vascular system involved in the maintenance of brain function, modulation of cerebral blood flow and integrity of the blood-brain barrier in the healthy brain and during aging. Second, we review the regional contribution of cerebral blood flow and blood-brain barrier disturbances in the pathogenesis of three distinct conditions: the archetypal white matter predominant neurocognitive dementia that is vascular dementia, a neuroinflammatory predominant disease (multiple sclerosis) and a neurodegenerative predominant disease (Alzheimer's). Finally, we then examine the shared landscape of vascular dysfunction in white matter dementia. By emphasizing the involvement of vascular dysfunction in the white matter, we put forward a hypothetical map of vascular dysfunction during disease-specific progression to guide future research aimed to improve diagnostics and facilitate the development of tailored therapies.

Dynamic 13 C MR spectroscopy as an alternative to imaging for assessing cerebral metabolism using hyperpolarized pyruvate in humans

PURPOSE

This study is to investigate time-resolved ¹³ C MR spectroscopy (MRS) as an alternative to imaging for assessing pyruvate metabolism using hyperpolarized (HP) [1-¹³ C]pyruvate in the human brain.

METHODS

Time-resolved ¹³ C spectra were acquired from four axial brain slices of healthy human participants (n = 4) after a bolus injection of HP [1-¹³ C]pyruvate. ¹³ C MRS with low flip-angle excitations and a multichannel ¹³ C/¹ H dual-frequency radiofrequency (RF) coil were exploited for reliable and unperturbed assessment of HP pyruvate metabolism. Slice-wise areas under the curve (AUCs) of ¹³ C-metabolites were measured and kinetic analysis was performed to estimate the production rates of lactate and HCO3- . Linear regression analysis between brain volumes and HP signals was performed. Region-focused pyruvate metabolism was estimated using coil-wise ¹³ C reconstruction. Reproducibility of HP pyruvate exams was presented by performing two consecutive injections with a 45-minutes interval.

RESULTS

[1-¹³ C]Lactate relative to the total ¹³ C signal (tC) was 0.21-0.24 in all slices. [¹³ C] HCO3- /tC was 0.065-0.091. Apparent conversion rate constants from pyruvate to lactate and HCO3- were calculated as 0.014-0.018 s^-1 and 0.0043-0.0056 s^-1 , respectively. Pyruvate/tC and lactate/tC were in moderate linear relationships with fractional gray matter volume within each slice. White matter presented poor linear regression fit with HP signals, and moderate correlations of the fractional cerebrospinal fluid volume with pyruvate/tC and lactate/tC were measured. Measured HP signals were comparable between two consecutive exams with HP [1-¹³ C]pyruvate.

CONCLUSIONS

Dynamic MRS in combination with multichannel RF coils is an affordable and reliable alternative to imaging methods in investigating cerebral metabolism using HP [1-¹³ C]pyruvate.

Giant central nervous system tuberculoma in pediatric patients: surgical case series

Central nervous system tuberculoma is rare and challenging situation. Clinical records of patients with pathologically proven tuberculoma were retrospectively reviewed. Clinical presentation, lesion location, radiological characteristics, perioperative and surgical management, and outcome is summarized and analyzed. Eight patients were included and there was one girl. Age ranged from 3 to 14 years with mean age 9.8 years. Clinical duration ranged from 20 days to 2 years, and 3 patients had previous lung tuberculosis with anti-TB treatment. The lesion was in cerebellum in 6 cases, including 1 involving basal ganglia and 1 involving thalamus. The lesion was in basal ganglia, thalamus, and third ventricle in 1 case, and in T12-L1 spinal cord in another. Cerebellar lesion was resected via paramedian suboccipital approach in 5 patients, basal ganglia lesion via trans-cortical frontal horn approach in 2 patients, and intra-spinal lesion via trans-laminar approach in 1 patient. Follow-up ranged from 10 to 24 months. Of the 8 patients, 6 returned to normal life. One patient had cerebellar lesion resected and the thalamic lesion reduced in size after anti-TB treatment. One patient died from TB spreading. Our data showed that most patients can be successfully treated by resection of the lesion. Low T2 signal, ring shaped enhancement and peripheral edema strongly suggest tuberculoma. Empirical anti-TB treatment should be initiated perioperatively.

A new framework for assessing subject-specific whole brain circulation and perfusion using MRI-based measurements and a multi-scale continuous flow model

A large variety of severe medical conditions involve alterations in microvascular circulation. Hence, measurements or simulation of circulation and perfusion has considerable clinical value and can be used for diagnostics, evaluation of treatment efficacy, and for surgical planning. However, the accuracy of traditional tracer kinetic one-compartment models is limited due to scale dependency. As a remedy, we propose a scale invariant mathematical framework for simulating whole brain perfusion. The suggested framework is based on a segmentation of anatomical geometry down to imaging voxel resolution. Large vessels in the arterial and venous network are identified from time-of-flight (ToF) and quantitative susceptibility mapping (QSM). Macro-scale flow in the large-vessel-network is accurately modelled using the Hagen-Poiseuille equation, whereas capillary flow is treated as two-compartment porous media flow. Macro-scale flow is coupled with micro-scale flow by a spatially distributing support function in the terminal endings. Perfusion is defined as the transition of fluid from the arterial to the venous compartment. We demonstrate a whole brain simulation of tracer propagation on a realistic geometric model of the human brain, where the model comprises distinct areas of grey and white matter, as well as large vessels in the arterial and venous vascular network. Our proposed framework is an accurate and viable alternative to traditional compartment models, with high relevance for simulation of brain perfusion and also for restoration of field parameters in clinical brain perfusion applications.

An accurate simulation of blood-flow in the human brain can be used for improved diagnostics and assignment of personalized treatment regimes. However, current algorithms are limited to simulation of blood flow within tumours only, and in terms of parameter estimation, traditional compartment models have limited accuracy due to lack of spatial connectivity within the models. As a remedy, we propose a data-driven computational fluid dynamics model where the geometric domains for simulation are defined from state-of-the art MR acquisitions enabling a segmentation of large arteries and veins. In the capillary tissue we apply a two-compartment porous media model, where the perfusion is pressure-driven and is defined as the transition of blood from arterial to venous side. In addition, we propose a model for dealing with the intermediate scale problem where the vessels are undetectable and the flow does not adhere to requirements of porous media flow. For this scale, we propose a support function distributing the fluid in a nearby region around the vessel terminals. Combining these elements, we have developed a novel full human brain blood-flow simulator.

Quantifying normal human brain metabolism using hyperpolarized [1-13C]pyruvate and magnetic resonance imaging

Hyperpolarized 13C Magnetic Resonance Imaging (13C-MRI) provides a highly sensitive tool to probe tissue metabolism in vivo and has recently been translated into clinical studies. We report the cerebral metabolism of intravenously injected hyperpolarized [1-13C]pyruvate in the brain of healthy human volunteers for the first time. Dynamic acquisition of 13C images demonstrated 13C-labeling of both lactate and bicarbonate, catalyzed by cytosolic lactate dehydrogenase and mitochondrial pyruvate dehydrogenase respectively. This demonstrates that both enzymes can be probed in vivo in the presence of an intact blood-brain barrier: the measured apparent exchange rate constant (kPL) for exchange of the hyperpolarized 13C label between [1-13C]pyruvate and the endogenous lactate pool was 0.012 ± 0.006 s-1 and the apparent rate constant (kPB) for the irreversible flux of [1-13C]pyruvate to [13C]bicarbonate was 0.002 ± 0.002 s-1. Imaging also revealed that [1-13C]pyruvate, [1-13C]lactate and [13C]bicarbonate were significantly higher in gray matter compared to white matter. Imaging normal brain metabolism with hyperpolarized [1-13C]pyruvate and subsequent quantification, have important implications for interpreting pathological cerebral metabolism in future studies.

Reduced perfusion in normal-appearing white matter in mild to moderate hypertension as revealed by 3D pseudocontinuous arterial spin labeling

PURPOSE

To investigate the hemodynamic changes of normal-appearing white matter (NAWM) in hypertension using the 3D pseudocontinuous arterial spin labeling (pCASL) technique.

MATERIALS AND METHODS

Seventy-three subjects, including a patient group (n = 41; 30 males; age = 47.7 ± 8.3 years; test-time blood pressure [BP] = 155 ± 23/98 ± 11 mmHg) and an age-matched control group (n = 32; 14 males; age = 46 ± 8.3 years; test-time BP = 117 ± 8/76 ± 10 mmHg), were recruited and scanned on a 3.0T magnetic resonance imaging (MRI) system using routine MRI sequences and 3D pCASL sequence. The routine MRI sequences were used to further define the NAWM. The cerebral blood flow (CBF) values in various regions of interest (ROIs) were extracted. One-way analysis of variance (ANOVA) and unpaired t-test were performed to evaluate the significance of the intergroup difference in CBF modifications.

RESULTS

Compared to healthy volunteers, CBF values in global gray matter (GM) and various NAWM regions were found to be lower (P < 0.05) in hypertensive patients, except for genu of corpus callosum (CC), cingulate gyrus, amygdala, pallidum, putamen, and thalamus (P > 0.05). Furthermore, compared to the control group, mild hypertension showed significantly reduced CBF in various ROIs (P < 0.05), but no intergroup differences in GM, R anterior horn of periventricular WM, and genu of CC (P > 0.05), while moderate hypertension showed reduced CBF in all ROIs (P < 0.05). However, it was observed that, between mild and moderate hypertensive patients, there were no statistically significant difference in CBF values except for genu of CC (P < 0.05).

CONCLUSION

3D pCASL has the ability to detect subtle hemodynamic abnormalities in NAWM regions at relatively early stages of hypertension. The observed decreases in CBF in these regions may suggest an increased risk of cerebral small vessel diseases.