Capillary transit time heterogeneity inhibits cerebral oxygen metabolism in patients with reduced cerebrovascular reserve capacity from steno-occlusive disease

MR Imaging Techniques for Microenvironment Mapping of the Glioma Tumors: A Systematic Review

RATIONALE AND OBJECTIVES

The tumor microenvironment (TME) is a critical regulator of cancer progression, metastasis, and treatment response. Currently, various imaging approaches exist to assess the pathophysiological features of the TME. This systematic review provides an overview of magnetic resonance imaging (MRI) methods used in clinical practice to characterize the pathophysiological features of the gliomas TME.

METHODS

This review involved a systematic comprehensive search of original open-access articles reporting the clinical use of MR imaging in glioma patients of all ages in the PubMed, Scopus, and Web of Science databases between January 2010 and December 2023. We restricted our research to papers published in the English language.

RESULTS

A total of 1137 studies were preliminarily identified through electronic database searches. After duplicate studies were removed, 44 studies met the eligibility criteria. The glioma TME was accompanied by alterations in metabolism, pH, vascularity, oxygenation, and extracellular matrix components, including tumor-associated macrophages, and sodium concentration.

CONCLUSION

Multiparametric MRI is capable of noninvasively assessing the pathophysiological features and tumor-supportive niches of the TME, which is in line with its application in personalized medicine.

The cerebral blood flow response to neuroactivation is reduced in cognitively normal men with β-amyloid accumulation

BACKGROUND

Accumulation of β-amyloid (Aβ) in the brain is a hallmark of Alzheimer's Disease (AD). Cerebral deposition of Aβ initiates deteriorating pathways which eventually can lead to AD. However, the exact mechanisms are not known. A possible pathway could be that Aβ affects the cerebral vessels, causing inadequate cerebrovascular function. In the present study, we examined if Aβ accumulation is associated with a reduced cerebral blood flow response (CBF) to neuronal activation by visual stimulation (ΔCBFVis.Act.) in cognitively normal subjects from the Metropolit Danish Male Birth Cohort.

METHODS

64 subjects participated in the present study. ΔCBFVis.Act. was measured using arterial spin labelling (ASL) combined with blood-oxygen-level-dependent (BOLD) MRI. Neuronal activation was obtained by visual stimulation by a flickering checkerboard presented on a screen in the MRI-scanner. Brain Aβ accumulation and cerebral glucose metabolism were assessed by PET imaging using the radiotracers [11C]Pittsburgh Compound-B (PiB) and [18F]Fluorodeoxyglucose (FDG), respectively. Cortical thickness was measured from structural MRI.

RESULTS

ΔCBFVis.Act. correlated negatively ( β = -32.1 [95% confidence interval (CI): -60.2; -4.1], r = -0.30, p = 0.025) with PiB standardized uptake value ratio (SUVr) in the brain regions activated by visual stimulation. ΔCBFVis.Act. did not correlate with FDG SUVr ( β = 1.9 [CI: -23.8; 27.6], r = 0.02, p = 0.88) or cortical thickness ( β = 10.3 [CI: -8.4; 29.0], r = 0.15, p = 0.27) in the activated brain regions. Resting CBF did not correlate with PiB SUVr neither in the regions activated by visual stimulation ( β = -17.8 [CI:-71.9; 36.2], r =- 0.09, p = 0.51) nor in the remaining cortex ( β = 5.2 [CI:-3.9; 14.2], r = 0.15, p = 0.26).

CONCLUSION

We found a correlation between high PiB SUVr and reduced CBF response to neuronal activation, indicating a link between Aβ accumulation and impaired cerebrovascular function. The impairment was not associated with cortical thinning or hypometabolism, suggesting that Aβ accumulation affecting brain vessel function could be a very early pathology leading to neurodegenerative disease.

Macro- and microvascular contributions to cerebral structural alterations in patients with asymptomatic carotid artery stenosis

Atherosclerosis can underly internal carotid artery stenosis (ICAS), a major risk factor for ischemic stroke, as well as small vessel disease (SVD). This study aimed to investigate hemodynamics and structural alterations associated with SVD in ICAS patients. 28 patients with unilateral asymptomatic ICAS and 30 age-matched controls underwent structural (T1-/T2-weighted and diffusion tensor imaging [DTI]) and hemodynamic (pseudo-continuous arterial spin labeling and dynamic susceptibility contrast) magnetic resonance imaging. SVD-related alterations were assessed using free water (FW), FW-corrected DTI, and peak-width of skeletonized mean diffusivity (PSMD). Furthermore, cortical thickness, cerebral blood flow (CBF), and capillary transit time heterogeneity (CTH) were analyzed. Ipsilateral to the stenosis, cortical thickness was significantly decreased in the posterior dorsal cingulate cortex (p = 0.024) and temporal pole (p = 0.028). ICAS patients exhibited elevated PSMD (p = 0.005), FW (p < 0.001), and contralateral alterations in FW-corrected DTI metrics. We found significantly lateralized CBF (p = 0.011) and a tendency for lateralized CTH (p = 0.067) in the white matter (WM) related to ICAS. Elevated PSMD and FW may indicate a link between SVD and WM changes. Contralateral alterations were seen in FW-corrected DTI, whereas hemodynamic and cortical changes were mainly ipsilateral, suggesting SVD might influence global brain changes concurrent with ICAS-related hemodynamic alterations.

Brain perfusion estimation by Tikhonov model-free deconvolution in a long axial field of view PET/CT scanner exploring five different PET tracers

PURPOSE

New total-body PET scanners with a long axial field of view (LAFOV) allow for higher temporal resolution due to higher sensitivity, which facilitates perfusion estimation by model-free deconvolution. Fundamental tracer kinetic theory predicts that perfusion can be estimated for all tracers despite their different fates given sufficiently high temporal resolution of 1 s or better, bypassing the need for compartment modelling. The aim of this study was to investigate whether brain perfusion could be estimated using model-free Tikhonov generalized deconvolution for five different PET tracers, [15O]H2O, [11C]PIB, [18F]FE-PE2I, [18F]FDG and [18F]FET. To our knowledge, this is the first example of a general model-free approach to estimate cerebral blood flow (CBF) from PET data.

METHODS

Twenty-five patients underwent dynamic LAFOV PET scanning (Siemens, Quadra). PET images were reconstructed with an isotropic voxel resolution of 1.65 mm3. Time framing was 40 × 1 s during bolus passage followed by increasing framing up to 60 min. AIF was obtained from the descending aorta. Both voxel- and region-based calculations of perfusion in the thalamus were performed using the Tikhonov method. The residue impulse response function was used to estimate the extraction fraction of tracer leakage across the blood-brain barrier.

RESULTS

CBF ranged from 37 to 69 mL blood min-1 100 mL of tissue-1 in the thalamus. Voxelwise calculation of CBF resulted in CBF maps in the physiologically normal range. The extraction fractions of [15O]H2O, [18F]FE-PE2I, [11C]PIB, [18F]FDG and [18F]FET in the thalamus were 0.95, 0.78, 0.62, 0.19 and 0.03, respectively.

CONCLUSION

The high temporal resolution and sensitivity associated with LAFOV PET scanners allow for noninvasive perfusion estimation of multiple tracers. The method provides an estimation of the residue impulse response function, from which the fate of the tracer can be studied, including the extraction fraction, influx constant, volume of distribution and transit time distribution, providing detailed physiological insight into normal and pathologic tissue.

Cortical oxygen extraction fraction using quantitative BOLD MRI and cerebral blood flow during vasodilation

Introduction: We aimed to demonstrate non-invasive measurements of regional oxygen extraction fraction (OEF) from quantitative BOLD MRI modeling at baseline and after pharmacological vasodilation. We hypothesized that OEF decreases in response to vasodilation with acetazolamide (ACZ) in healthy conditions, reflecting compensation in regions with increased cerebral blood flow (CBF), while cerebral metabolic rate of oxygen (CMRO2) remained unchanged. We also aimed to assess the relationship between OEF and perfusion in the default mode network (DMN) regions that have shown associations with vascular risk factors and cerebrovascular reactivity in different neurological conditions. Material and methods: Eight healthy subjects (47 ± 13 years, 6 female) were scanned on a 3 T scanner with a 32-channel head coil before and after administration of 15 mg/kg ACZ as a pharmacological vasodilator. The MR imaging acquisition protocols included: 1) A Gradient Echo Slice Excitation Profile Imaging Asymmetric Spin Echo scan to quantify OEF, deoxygenated blood volume, and reversible transverse relaxation rate (R2 ') and 2) a multi-post labeling delay arterial spin labeling scan to measure CBF. To assess changes in each parameter due to vasodilation, two-way t-tests were performed for all pairs (baseline versus vasodilation) in the DMN brain regions with Bonferroni correction for multiple comparisons. The relationships between CBF versus OEF and CBF versus R2' were analyzed and compared across DMN regions using linear, mixed-effect models. Results: During vasodilation, CBF significantly increased in the medial frontal cortex (P = 0.004 ), posterior cingulate gyrus (pCG) (P = 0.004 ), precuneus cortex (PCun) (P = 0.004 ), and occipital pole (P = 0.001 ). Concurrently, a significant decrease in OEF was observed only in the pCG (8.8%, P = 0.003 ) and PCun (8.7 % , P = 0.001 ). CMRO2 showed a trend of increased values after vasodilation, but these differences were not significant after correction for multiple comparisons. Although R2' showed a slightly decreasing trend, no statistically significant changes were found in any regions in response to ACZ. The CBF response to ACZ exhibited a stronger negative correlation with OEF (β = - 0.104 ± 0.027 ; t = - 3.852 , P < 0.001 ), than with R2' (β = - 0.016 ± 0.006 ; t = - 2.692 , P = 0.008 ). Conclusion: Quantitative BOLD modeling can reliably measure OEF across multiple physiological conditions and captures vascular changes with higher sensitivity than R2' values. The inverse correlation between OEF and CBF across regions in DMN, suggests that these two measurements, in response to ACZ vasodilation, are reliable indicators of tissue health in this healthy cohort.