MRI reveals that early changes in cerebral blood volume precede blood-brain barrier breakdown and overt pathology in MS-like lesions in rat brain

Blood-brain barrier water exchange measurements using FEXI: Impact of modeling paradigm and relaxation time effects

PURPOSE

To evaluate potential modeling paradigms and the impact of relaxation time effects on human blood-brain barrier (BBB) water exchange measurements using FEXI (BBB-FEXI), and to quantify the accuracy, precision, and repeatability of BBB-FEXI exchange rate estimates at 3 T $$ \mathrm{T} $$ .

METHODS

Three modeling paradigms were evaluated: (i) the apparent exchange rate (AXR) model; (ii) a two-compartment model (2 CM $$ 2\mathrm{CM} $$ ) explicitly representing intra- and extravascular signal components, and (iii) a two-compartment model additionally accounting for finite compartmentalT 1 $$ {\mathrm{T}}_1 $$ andT 2 $$ {\mathrm{T}}_2 $$ relaxation times (2 CM r $$ 2{\mathrm{CM}}_r $$ ). Each model had three free parameters. Simulations quantified biases introduced by the assumption of infinite relaxation times in the AXR and2 CM $$ 2\mathrm{CM} $$ models, as well as the accuracy and precision of all three models. The scan-rescan repeatability of all paradigms was quantified for the first time in vivo in 10 healthy volunteers (age range 23-52 years; five female).

RESULTS

The assumption of infinite relaxation times yielded exchange rate errors in simulations up to 42%/14% in the AXR/2 CM $$ 2\mathrm{CM} $$ models, respectively. Accuracy was highest in the compartmental models; precision was best in the AXR model. Scan-rescan repeatability in vivo was good for all models, with negligible bias and repeatability coefficients in grey matter ofRC AXR = 0 . 43 $$ {\mathrm{RC}}_{\mathrm{AXR}}=0.43 $$  s - 1 $$ {\mathrm{s}}^{-1} $$ ,RC 2 CM = 0 . 51 $$ {\mathrm{RC}}_{2\mathrm{CM}}=0.51 $$  s - 1 $$ {\mathrm{s}}^{-1} $$ , andRC 2 CM r = 0 . 61 $$ {\mathrm{RC}}_{2{\mathrm{CM}}_r}=0.61 $$  s - 1 $$ {\mathrm{s}}^{-1} $$ .

CONCLUSION

Compartmental modelling of BBB-FEXI signals can provide accurate and repeatable measurements of BBB water exchange; however, relaxation time and partial volume effects may cause model-dependent biases.

Developing a metabolic clearance rate framework as a translational analysis approach for hyperpolarized 13C magnetic resonance imaging

Hyperpolarized carbon-13 magnetic resonance imaging is a promising technique for in vivo metabolic interrogation of alterations between health and disease. This study introduces a formalism for quantifying the metabolic information in hyperpolarized imaging. This study investigated a novel perfusion formalism and metabolic clearance rate (MCR) model in pre-clinical stroke and in the healthy human brain. Simulations showed that the proposed model was robust to perturbations in T₁, transmit B₁, and k_PL. A significant difference in ipsilateral vs contralateral pyruvate derived cerebral blood flow (CBF) was detected in rats (140 ± 2 vs 89 ± 6 mL/100 g/min, p < 0.01, respectively) and pigs (139 ± 12 vs 95 ± 5 mL/100 g/min, p = 0.04, respectively), along with an increase in fractional metabolism (26 ± 5 vs 4 ± 2%, p < 0.01, respectively) in the rodent brain. In addition, a significant increase in ipsilateral vs contralateral MCR (0.034 ± 0.007 vs 0.017 ± 0.02/s, p = 0.03, respectively) and a decrease in mean transit time (31 ± 8 vs 60 ± 2 s, p = 0.04, respectively) was observed in the porcine brain. In conclusion, MCR mapping is a simple and robust approach to the post-processing of hyperpolarized magnetic resonance imaging.

Metabolomics detects clinically silent neuroinflammatory lesions earlier than neurofilament-light chain in a focal multiple sclerosis animal model

Background

Despite widespread searches, there are currently no validated biofluid markers for the detection of subclinical neuroinflammation in multiple sclerosis (MS). The dynamic nature of human metabolism in response to changes in homeostasis, as measured by metabolomics, may allow early identification of clinically silent neuroinflammation. Using the delayed-type hypersensitivity (DTH) MS rat model, we investigated the serum and cerebrospinal fluid (CSF) metabolomics profiles and neurofilament-light chain (NfL) levels, as a putative marker of neuroaxonal damage, arising from focal, clinically silent neuroinflammatory brain lesions and their discriminatory abilities to distinguish DTH animals from controls.

Methods

¹H nuclear magnetic resonance (NMR) spectroscopy metabolomics and NfL measurements were performed on serum and CSF at days 12, 28 and 60 after DTH lesion initiation. Supervised multivariate analyses were used to determine metabolomics differences between DTH animals and controls. Immunohistochemistry was used to assess the extent of neuroinflammation and tissue damage.

Results

Serum and CSF metabolomics perturbations were detectable in DTH animals (vs. controls) at all time points, with the greatest change occurring at the earliest time point (day 12) when the neuroinflammatory response was most intense (mean predictive accuracy [SD]—serum: 80.6 [10.7]%, p < 0.0001; CSF: 69.3 [13.5]%, p < 0.0001). The top discriminatory metabolites at day 12 (serum: allantoin, cytidine; CSF: glutamine, glucose) were all reduced in DTH animals compared to controls, and correlated with histological markers of neuroinflammation, particularly astrogliosis (Pearson coefficient, r—allantoin: r = − 0.562, p = 0.004; glutamine: r = − 0.528, p = 0.008). Serum and CSF NfL levels did not distinguish DTH animals from controls at day 12, rather, significant differences were observed at day 28 (mean [SEM]—serum: 38.5 [4.8] vs. 17.4 [2.6] pg/mL, p = 0.002; CSF: 1312.0 [379.1] vs. 475.8 [74.7] pg/mL, p = 0.027). Neither serum nor CSF NfL levels correlated with markers of neuroinflammation; serum NfL did, however, correlate strongly with axonal loss (r = 0.641, p = 0.001), but CSF NfL did not (p = 0.137).

Conclusions

While NfL levels were elevated later in the pathogenesis of the DTH lesion, serum and CSF metabolomics were able to detect early, clinically silent neuroinflammation and are likely to present sensitive biomarkers for the assessment of subclinical disease activity in patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02614-8.

Assessing the effect of anesthetic gas mixtures on hyperpolarized 13 C pyruvate metabolism in the rat brain

PURPOSE

To determine the effect of altering anesthetic oxygen protocols on measurements of cerebral perfusion and metabolism in the rodent brain.

METHODS

Seven rats were anesthetized and underwent serial MRI scans with hyperpolarized [1-¹³ C]pyruvate and perfusion weighted imaging. The anesthetic carrier gas protocol used varied from 100:0% to 90:10% to 60:40% O₂ :N₂ O. Spectra were quantified with AMARES and perfusion imaging was processed using model-free deconvolution. A 1-way ANOVA was used to compare results across groups, with pairwise t tests performed with correction for multiple comparisons. Spearman's correlation analysis was performed between O₂ % and MR measurements.

RESULTS

There was a significant increase in bicarbonate:total ¹³ C carbon and bicarbonate:¹³ C pyruvate when moving between 100:0 to 90:10 and 100:0 to 60:40 O₂ :N₂ O % (0.02 ± 0.01 vs. 0.019 ± 0.005 and 0.02 ± 0.01 vs. 0.05 ± 0.02, respectively) and (0.04 ± 0.01 vs. 0.03 ± 0.01 and 0.04 ± 0.01 vs. 0.08 ± 0.02, respectively). There was a significant difference in ¹³ C pyruvate time to peak when moving between 100:0 to 90:10 and 100:0 to 60:40 O₂ :N₂ O % (13 ± 2 vs. 10 ± 1 and 13 ± 2 vs. 7.5 ± 0.5 s, respectively) as well as significant differences in cerebral blood flow (CBF) between gas protocols. Significant correlations between bicarbonate:¹³ C pyruvate and gas protocol (ρ = -0.47), mean transit time and gas protocol (ρ = 0.41) and ¹³ C pyruvate time-to-peak and cerebral blood flow (ρ = -0.54) were also observed.

CONCLUSIONS

These results demonstrate that the detection and quantification of cerebral metabolism and perfusion is dependent on the oxygen protocol used in the anesthetized rodent brain.

Cessation of anti-VLA-4 therapy in a focal rat model of multiple sclerosis causes an increase in neuroinflammation

Background

Positron emission tomography (PET) can be used for in vivo evaluation of the pathology associated with multiple sclerosis. We investigated the use of longitudinal PET imaging and the 18-kDa translocator protein (TSPO) binding radioligand [¹⁸F]GE-180 to detect changes in a chronic multiple sclerosis-like focal delayed-type hypersensitivity experimental autoimmune encephalomyelitis (fDTH-EAE) rat model during and after anti-VLA-4 monoclonal antibody (mAb) treatment. Thirty days after lesion activation, fDTH-EAE rats were treated with the anti-VLA-4 mAb (n = 4) or a control mAb (n = 4; 5 mg/kg, every third day, subcutaneously) for 31 days. Animals were imaged with [¹⁸F]GE-180 on days 30, 44, 65, 86 and 142. Another group of animals (n = 4) was used for visualisation the microglia with Iba-1 at day 44 after a 2-week treatment period.

Results

After a 2-week treatment period on day 44, there was a declining trend (p = 0.067) in [¹⁸F]GE-180-binding in the anti-VLA-4 mAb-treated animals versus controls. However, cessation of treatment for 4 days after a 31-day treatment period increased [¹⁸F]GE-180 binding in animals treated with anti-VLA-4 mAb compared to the control group (p = 0.0003). There was no difference between the groups in TSPO binding by day 142.

Conclusions

These results demonstrated that cessation of anti-VLA-4 mAb treatment for 4 days caused a transient rebound increase in neuroinflammation. This highlights the usefulness of serial TSPO imaging in the fDTH-EAE model to better understand the rebound phenomenon.

Gadolinium and Multiple Sclerosis: Vessels, Barriers of the Brain, and Glymphatics

The pathogenesis of multiple sclerosis is characterized by a cascade of pathobiologic events, ranging from focal lymphocytic infiltration and microglia activation to demyelination and axonal degeneration. MS has several of the hallmarks of an inflammatory autoimmune disorder, including breakdown of the BBB. Gadolinium-enhanced MR imaging is currently the reference standard to detect active inflammatory lesions in MS. Knowledge of the patterns and mechanisms of contrast enhancement is vital to limit the radiologic differential diagnosis in the staging and evaluation of MS lesion activity. The aim of this review was the following: 1) to outline the pathophysiology of the effect of lymphocyte-driven inflammation in MS, 2) to describe the effects of gadolinium on the BBB and glymphatic system, and 3) to describe gadolinium enhancement patterns and artifacts that can mimic lesions in MS.

Multiparametric measurement of cerebral physiology using calibrated fMRI

The ultimate goal of calibrated fMRI is the quantitative imaging of oxygen metabolism (CMRO₂), and this has been the focus of numerous methods and approaches. However, one underappreciated aspect of this quest is that in the drive to measure CMRO₂, many other physiological parameters of interest are often acquired along the way. This can significantly increase the value of the dataset, providing greater information that is clinically relevant, or detail that can disambiguate the cause of signal variations. This can also be somewhat of a double-edged sword: calibrated fMRI experiments combine multiple parameters into a physiological model that requires multiple steps, thereby providing more opportunity for error propagation and increasing the noise and error of the final derived values. As with all measurements, there is a trade-off between imaging time, spatial resolution, coverage, and accuracy. In this review, we provide a brief overview of the benefits and pitfalls of extracting multiparametric measurements of cerebral physiology through calibrated fMRI experiments.

ABCD1 dysfunction alters white matter microvascular perfusion

Cerebral X-linked adrenoleukodystrophy is a devastating neurodegenerative disorder caused by mutations in the ABCD1 gene, which lead to a rapidly progressive cerebral inflammatory demyelination in up to 60% of affected males. Selective brain endothelial dysfunction and increased permeability of the blood–brain barrier suggest that white matter microvascular dysfunction contributes to the conversion to cerebral disease. Applying a vascular model to conventional dynamic susceptibility contrast magnetic resonance perfusion imaging, we demonstrate that lack of ABCD1 function causes increased capillary flow heterogeneity in asymptomatic hemizygotes predominantly in the white matter regions and developmental stages with the highest probability for conversion to cerebral disease. In subjects with ongoing inflammatory demyelination we observed a sequence of increased capillary flow heterogeneity followed by blood–brain barrier permeability changes in the perilesional white matter, which predicts lesion progression. These white matter microvascular alterations normalize within 1 year after treatment with haematopoietic stem cell transplantation. For the first time in vivo, our studies unveil a model to assess how ABCD1 alters white matter microvascular function and explores its potential as an earlier biomarker for monitoring disease progression and response to treatment.

Perilesional Inflammation in Neurocysticercosis - Relationship Between Contrast-Enhanced Magnetic Resonance Imaging, Evans Blue Staining and Histopathology in the Pig Model

Background

Disease manifestations in neurocysticercosis (NCC) are frequently due to inflammation of degenerating Taenia solium brain cysts. Exacerbated inflammation post anthelmintic treatment is associated with leakage of the blood brain barrier (BBB) using Evans blue (EB) staining. How well EB extravasation into the brain correlates with magnetic resonance imaging (MRI) using gadolinium (Gd) enhancement as a contrast agent and pericystic inflammation was analyzed in pigs harboring brain cysts of Taenia solium.

Methodology/Principal Findings

Three groups of 4 naturally infected pigs were assessed. The first and second groups were treated with both praziquantel plus albendazole and sacrificed two and five days post treatment, respectively. A third untreated group remained untreated. Pigs were injected with EB two hours prior to evaluation by Gd-enhanced T1-MRI, and euthanized. The EB staining for each cyst capsule was scored (EB grades were 0: 0%; 1: up to 50%; 2: over 50% but less than 100%; 3: 100%). Similarly, the Gd enhancement around each cyst was qualitatively and quantitatively scored from the MRI. The extent of pericystic inflammation on histology was scored in increasing severity as IS1, IS2, IS3 and IS4. Grade 3 EB staining and enhancement was only seen in treated capsules. Also, treated groups had higher Gd intensity than the untreated group. Grades of enhancement correlated significantly with Gd enhancement intensity. EB staining was correlated with Gd enhancement intensity and with IS4 in the treated groups. These correlations were stronger in internally located cysts compared to superficial cysts in treated groups.

Significance

EB staining and Gd enhancement strongly correlate. The intensity of enhancement determined by MRI is a good indication of the degree of inflammation. Similarly, EB staining highly correlates with the degree of inflammation and may be applied to study inflammation in the pig model of NCC.

Neurocysticercosis (NCC) is a frequent parasitic infection of the human brain in developing countries. The symptomatology of human NCC after antiparasitic treatment is generally related to inflammation. The presence and degree of enhancement after intravascular injection of the contrast agent gadolinium in magnetic resonance imaging (MRI) is commonly considered an evidence of blood brain barrier (BBB) leakage. Experimentally, the presence and degree of extravasation of Evans blue (EB) after intravascular injection into the tissues of the brain is a direct measure of blood brain barrier leakage. The BBB leakage of gadolinium in neurocysticercosis is commonly used as an indirect measure of inflammation but has never been experimentally proven. Here we evaluated the relationship between contrast T1-MRI, EB staining and histology findings in naturally infected pigs. There was a strong correlation between EB staining, contrast MRI and histopathology findings after antiparasitic treatment. This correlation was stronger when cysts were internally located in the brain than in superficial cysts partly located in the subarachnoid space (meninges). Contrast-enhanced MRI is a non invasive tool used in diagnosis and follow up of NCC patients. This study shows that the use of EB staining allows for the same conclusions as when using MRI post-treatment, and that both techniques correlate with histopathology findings. These results support the use of EB staining to study NCC using the porcine model as well as validate MRI enhancement to assess brain inflammation in patients.

Cognitive impairment is associated with subcortical magnetic resonance imaging grey matter T2 hypointensity in multiple sclerosis

Grey matter hypointensity on T2-weighted magnetic resonance imaging (MRI) scans, suggesting iron deposition, has been described in multiple sclerosis (MS) and is related to physical disability, disease course and brain atrophy. We tested the hypothesis that subcortical grey matter T2 hypointensity is related to cognitive impairment after adjusting for the effect of MRI lesion and atrophy measures. We studied 33 patients with MS and 14 healthy controls. Normalized T2 signal intensity in the caudate, putamen, globus pallidus and thalamus, total brain T1-hypointense lesion volume (T1LV), fluid-attenuated inversion-recovery-hyperintense lesion volume (FLLV) and brain parenchymal fraction (BPF) were obtained quantitatively. A neuropsychological composite score (NCS) encompassed new learning, attention, working memory, spatial processing and executive function. In each of the regions of interest, the normalized T2 intensity was lower in the MS versus control group (all P <0.001). Regression modelling tested the relative association between all MRI variables and NCS. Globus pallidus T2 hypointensity was the only variable selected in the final model ( R2 = 0.301, P = 0.007). Pearson correlations between MRI and NCS were T1LV: r = -0.319; FLLV: r = -0.347; BPF: r = 0.374; T2 hypointensity of the caudate: r = 0.305; globus pallidus: r = 0.395; putamen: r = 0.321; and thalamus: r = 0.265. Basal ganglia T2 hypointensity and BPF demonstrated the strongest associations with cognitive impairment on individual cognitive subtests. Subcortical grey matter T2 hypointensity is related to cognitive impairment in MS, supporting the clinical relevance of T2 hypointensity as a biological marker of MS tissue damage. These data implicate a role for basal ganglia iron deposition in neuropsychological dysfunction.

Diffusion fMRI detects white-matter dysfunction in mice with acute optic neuritis

Optic neuritis is a frequent and early symptom of multiple sclerosis (MS). Conventional magnetic resonance (MR) techniques provide means to assess multiple MS-related pathologies, including axonal injury, demyelination, and inflammation. A method to directly and non-invasively probe white-matter function could further elucidate the interplay of underlying pathologies and functional impairments. Previously, we demonstrated a significant 27% activation-associated decrease in the apparent diffusion coefficient of water perpendicular to the axonal fibers (ADC⊥) in normal C57BL/6 mouse optic nerve with visual stimulation using diffusion fMRI. Here we apply this approach to explore the relationship between visual acuity, optic nerve pathology, and diffusion fMRI in the experimental autoimmune encephalomyelitis (EAE) mouse model of optic neuritis. Visual stimulation produced a significant 25% (vs. baseline) ADC⊥ decrease in sham EAE optic nerves, while only a 7% (vs. baseline) ADC⊥ decrease was seen in EAE mice with acute optic neuritis. The reduced activation-associated ADC⊥ response correlated with post-MRI immunohistochemistry determined pathologies (including inflammation, demyelination, and axonal injury). The negative correlation between activation-associated ADC⊥ response and visual acuity was also found when pooling EAE-affected and sham groups under our experimental criteria. Results suggest that reduction in diffusion fMRI directly reflects impaired axonal-activation in EAE mice with optic neuritis. Diffusion fMRI holds promise for directly gauging in vivo white-matter dysfunction or therapeutic responses in MS patients.

T₂-weighted MRI detects presymptomatic pathology in the SOD1 mouse model of ALS

Neuroinflammation has been identified as a potential therapeutic target in amyotrophic lateral sclerosis (ALS), but relevant biomarkers are needed. The superoxide dismutase (SOD1)(G93A) transgenic mouse model of ALS offers a unique opportunity to study and potentially manipulate presymptomatic pathology. While T₂-weighted magnetic resonance imaging (MRI) has been shown to be sensitive to pathologic changes at symptom onset, no earlier biomarkers were previously identified and the underlying histopathologic correlates remain uncertain. To address these issues, we used a multimodal MRI approach targeting structural (T₂, T₁, apparent diffusion coefficient (ADC), magnetization transfer ratio (MTR)), vascular (gadolinium diethylene triamine pentaacetic acid), and endothelial (vascular cell adhesion molecule-microparticles of iron oxide) changes, together with histopathologic analysis from presymptomatic to symptomatic stages of disease. Presymptomatic changes in brainstem nuclei were evident on T₂-weighted images from as early as 60 days (P<0.05). Histologic indices of vacuolation, astro- and microglial activation all correlated with T₂-weighted changes. Significant reductions in ADC (P<0.01) and MTR (P<0.05) were found at 120 days in the same brainstem nuclei. No changes in T₁ relaxation, vascular permeability, or endothelial activation were found at any stage of disease. These findings suggest that T₂-weighted MRI offers the strongest biomarker potential in this model, and that MRI has unique potential for noninvasive and longitudinal assessment of presymptomatically applied therapeutic and neuroprotective agents.

Investigation of immune and CNS-mediated effects of fingolimod in the focal delayed-type hypersensitivity multiple sclerosis model

We examined the effect of fingolimod (0.1 and 0.3 mg/kg/day orally) on blood-brain barrier (BBB) function, demyelination and leukocyte recruitment at different stages of the focal delayed-type hypersensitivity (DTH) multiple sclerosis model in Lewis rats using immunohistochemistry and gadolinium (Gd)-enhancing magnetic resonance imaging (MRI). During DTH lesion formation, fingolimod reduced BBB breakdown (52%; p = 0.05), and lymphocyte (53%; p = 0.016) and macrophage/activated microglia (49%; p = 0.002) recruitment to the DTH lesion compared with vehicle-treated controls. Following DTH lesion establishment, fingolimod reduced the area of BBB breakdown (75%; p = 0.04), lymphocyte recruitment to the DTH lesion (41%; p = 0.01) and activated microglia outside of the lesion core (p = 0.01), but did not reduce recruitment of macrophages/activated microglia within the DTH lesion. During the chronic disease phase, when the BBB was resealed, fingolimod reduced the area of demyelination by 43% (p = 0.019) compared with vehicle-treated controls, while not affecting lymphocyte recruitment within the lesion. Fingolimod had different beneficial effects during different stages of DTH, reducing BBB breakdown and lesion development/brain tissue damage whilst reducing lymphocyte recruitment when BBB breakdown was apparent, but reducing demyelination independent of lymphocyte infiltration behind an intact BBB. These results suggest a direct CNS effect of fingolimod in this model.

Structural and functional effects of metastases in rat brain determined by multimodal MRI

Metastasis to the brain results in significant impairment of brain function and poor patient survival. Currently, magnetic resonance imaging (MRI) is under-utilised in monitoring brain metastases and their effects on brain function. Here, we sought to establish a model of focal brain metastasis in the rat that enables serial multimodal structural and functional MRI studies, and to assess the sensitivity of these approaches to metastatic growth. Female Berlin-Druckrey-IX rats were injected intracerebrally with metastatic ENU1564 cells in the ventroposterior medial nucleus (VPM) of the thalamus, a relay node of the whisker-to-barrel cortex pathway. Animals underwent multimodal structural and vascular MRI, as well as functional MRI of the cortical blood oxygenation level dependent (BOLD) responses to whisker pad stimulation. T2 , diffusion, magnetisation transfer and perfusion weighted MRI enabled differentiation between a central area of more advanced metastatic growth and penumbral regions of co-optive perivascular micrometastatic growth, with magnetisation transfer MRI being the most sensitive to micrometastatic growth. Areas of cortical BOLD activation in response to whisker pad stimulation were significantly reduced in the hemisphere containing metastases in the VPM. The reduction in BOLD response correlated with metastatic burden in the thalamus, and was sensitive to the presence of smaller metastases than currently detectable clinically. Our findings suggest that multimodal MRI provides greater sensitivity to tumour heterogeneity and micrometastatic growth than single modality contrast-enhanced MRI. Understanding the relationships between these MRI parameters and the underlying pathology may greatly enhance the utility of MRI in diagnosis, staging and monitoring of brain metastasis.

Cerebral arterial bolus arrival time is prolonged in multiple sclerosis and associated with disability

Alterations in the overall cerebral hemodynamics have been reported in multiple sclerosis (MS); however, their cause and significance is unknown. While potential venous causes have been examined, arterial causes have not. In this study, a multiple delay time arterial spin labeling magnetic resonance imaging sequence at 3T was used to quantify the arterial hemodynamic parameter bolus arrival time (BAT) and cerebral blood flow (CBF) in normal-appearing white matter (NAWM) and deep gray matter in 33 controls and 35 patients with relapsing-remitting MS. Bolus arrival time was prolonged in MS in NAWM (1.0±0.2 versus 0.9±0.2 seconds, P=0.031) and deep gray matter (0.90±0.18 versus 0.80±0.14 seconds, P=0.001) and CBF was increased in NAWM (14±4 versus 10±2 mL/100 g/min, P=0.001). Prolonged BAT in NAWM (P=0.042) and deep gray matter (P=0.01) were associated with higher expanded disability status score. This study demonstrates alteration in cerebral arterial hemodynamics in MS. One possible cause may be widespread inflammation. Bolus arrival time was longer in patients with greater disability independent of atrophy and T2 lesion load, suggesting alterations in cerebral arterial hemodynamics may be a marker of clinically relevant pathology.

Monitoring ferumoxide-labelled neural progenitor cells and lesion evolution by magnetic resonance imaging in a model of cell transplantation in cerebral ischaemia

Efficacy of neural stem/progenitor cell (NPC) therapies after cerebral ischaemia could be better evaluated by monitoring in vivo migration and distribution of cells post-engraftment in parallel with analysis of lesion volume and functional recovery. Magnetic resonance imaging (MRI) is ideally placed to achieve this, but still poses several challenges. We show that combining the ferumoxide MRI contrast agent Endorem with protamine sulphate (FePro) improves iron oxide uptake in cells compared to Endorem alone and is non-toxic. Hence FePro complex is a better contrast agent than Endorem for monitoring NPCs. FePro complex-labelled NPCs proliferated and differentiated normally in vitro, and upon grafting into the brain 48 hours post-ischaemia they were detected in vivo by MRI. Imaging over four weeks showed the development of a confounding endogenous hypointense contrast evolution at later timepoints within the lesioned tissue. This was at least partly due to accumulation within the lesion of macrophages and endogenous iron. Neither significant NPC migration, assessed by MRI and histologically, nor a reduction in the ischaemic lesion volume was observed in NPC-grafted brains.  Crucially, while MRI provides reliable information on engrafted cell location early after an ischaemic insult, pathophysiological changes to ischaemic lesions can interfere with cellular imaging at later timepoints.

Cerebral blood volume affects blood-brain barrier integrity in an acute transient stroke model

Insufficient vascular reserve after an ischemic stroke may induce biochemical cascades that subsequently deteriorate the blood-brain barrier (BBB) function. However, the direct relationship between poor cerebral blood volume (CBV) restoration and BBB disruption has not been examined in acute stroke. To quantify BBB integrity at acute stages of transient stroke, in particular for cases in which extravasation of the standard contrast agent (Gd-DTPA) is not observed, we adopted the water exchange index (WEI), a novel magnetic resonance image-derived parameter to estimate the water permeability across the BBB. The apparent diffusion coefficient (ADC) and R2 relaxation rate constant were also measured for outlining the tissue abnormality, while fractional CBV and WEI were quantified for assessing vascular alterations. The significantly decreased ADC and R2 in the ischemic cortices did not correlate with the changes in CBV or WEI. In contrast, a strong negative correlation between the ipsilesional WEI and CBV was found, in which stroke mice were clustered into two groups: (1) high WEI and low CBV and (2) normal WEI and CBV. The low CBV observed for mice with a disrupted BBB, characterized by a high WEI, indicates the importance of CBV restoration for maintaining BBB stability in acute stroke.

Magnetic resonance imaging reveals therapeutic effects of interferon-beta on cytokine-induced reactivation of rat model of multiple sclerosis

Interferon-β (IFN-β) drugs are considered to derive their beneficial effects on multiple sclerosis (MS) progression via their antiinflammatory properties, but the precise mechanism of action remains unclear. Here, we sought to discover how IFN-β impacts on inflammation-associated aggravation of MS-like lesions in rat. Animals with dormant focal experimental allergic encephalomyelitis (EAE) lesions were challenged intravenously with a replication-deficient adenovirus vector carrying interleukin (IL)-1β cDNA (AdIL-1β). Aggravation of inflammation and demyelination within the focal EAE lesion was observed after AdIL-1β injection with associated changes in tissue structure detected by diffusion and magnetization transfer imaging. Postgadolinium-DTPA T1-weighted images revealed contrast enhancement in the ipsilateral meninges, indicating breakdown of the blood-cerebrospinal fluid barrier, and increased left/right regional cerebral blood volume ratio was also observed after AdIL-1β injection. To determine the role of IFN-β on reactivation of the EAE lesion, rats were treated with therapeutic doses of IFN-β and focal EAE lesions showed significantly reduced reactivation in response to systemic AdIL-1β injection. In conclusion, these findings indicate a central role for peripheral IL-1β expression in the mechanism of MS lesion reactivation and that the therapeutic effects of IFN-β may, at least in part, reflect suppression of the effects of peripheral inflammation on MS lesion pathogenesis.

Phase contrast MRI is an early marker of micrometastatic breast cancer development in the rat brain

The early growth of micrometastatic breast cancer in the brain often occurs through vessel co-option and is independent of angiogenesis. Remodeling of the existing vasculature is an important step in the evolution of co-opting micrometastases into angiogenesis-dependent solid tumor masses. The purpose of this study was to determine whether phase contrast MRI, an intrinsic source of contrast exquisitely sensitive to the magnetic susceptibility properties of deoxygenated hemoglobin, could detect vascular changes occurring independent of angiogenesis in a rat model of breast cancer metastases to the brain. Twelve nude rats were administered 10(6) MDA-MB-231BRL 'brain-seeking' breast cancer cells through intracardiac injection. Serial, multiparametric MRI of the brain was performed weekly until metastatic disease was detected. The results demonstrated that images of the signal phase (area under the receiver operating characteristic curve, 0.97) were more sensitive than T(2)* gradient echo magnitude images (area under the receiver operating characteristic curve, 0.73) to metastatic brain lesions. The difference between the two techniques was probably the result of the confounding effects of edema on the magnitude of the signal. A region of interest analysis revealed that vascular abnormalities detected with phase contrast MRI preceded tumor permeability measured with contrast-enhanced MRI by 1-2  weeks. Tumor size was correlated with permeability (R(2)= 0.23, p < 0.01), but phase contrast was independent of tumor size (R(2)= 0.03). Histopathologic analysis demonstrated that capillary endothelial cells co-opted by tumor cells were significantly enlarged, but less dense, relative to the normal brain vasculature. Although co-opted vessels were vascular endothelial growth factor-negative, vessels within larger tumor masses were vascular endothelial growth factor-positive. In conclusion, phase contrast MRI is believed to be sensitive to vascular remodeling in co-opting brain tumor metastases independent of sprouting angiogenesis, and may therefore aid in preclinical studies of angiogenic-independent tumors or in the monitoring of continued tumor growth following anti-angiogenic therapy. Published 2011. This article is a US Government work and is in the public domain in the USA.

Characterizing brain oxygen metabolism in patients with multiple sclerosis with T2-relaxation-under-spin-tagging MRI

In this study, venous oxygen saturation and oxygen metabolic changes in multiple sclerosis (MS) patients were assessed using a recently developed T2-relaxation-under-spin-tagging (TRUST) magnetic resonance imaging (MRI), which measures the superior sagittal venous sinus blood oxygenation (Yv) and cerebral metabolic rate of oxygen (CMRO(2)), an index of global oxygen consumption. Thirty patients with relapsing-remitting MS and 30 age-matched healthy controls were studied using TRUST at 3 T MR. The mean expanded disability status scale (EDSS) of the patients was 2.3 (range, 0 to 5.5). We found significantly increased Yv (P<0.0001) and decreased CMRO(2) (P=0.003) in MS patients (mean±s.d.: 65.9%±5.1% and 138.8±35.4 μmol per 100 g per minute) as compared with healthy control subjects (60.2%±4.0% and 180.2±24.8 μmol per 100 g per minute, respectively), implying decrease of oxygen consumption in MS. There was a significant positive correlation between Yv and EDSS and between Yv and lesion load in MS patients (n=30); on the contrary, there was a significant negative correlation between CMRO(2) and EDSS and between CMRO(2) and lesion load (n=12). There was no correlation between Yv and brain atrophy measures. This study showed preliminary evidence of the potential utility of TRUST in global oxygen metabolism. Our results of significant underutilization of oxygen in MS raise important questions regarding mitochondrial respiratory dysfunction and neurodegeneration of the disease.

MRI signature in a novel mouse model of genetically induced adult oligodendrocyte cell death

Two general pathological processes contribute to multiple sclerosis (MS): acute inflammation and degeneration. While magnetic resonance imaging (MRI) is highly sensitive in detecting abnormalities related to acute inflammation both clinically and in animal models of experimental autoimmune encephalomyelitis (EAE), the correlation of these readouts with acute and future disabilities has been found rather weak. This illustrates the need for imaging techniques addressing neurodegenerative processes associated with MS. In the present work we evaluated the sensitivity of different MRI techniques (T(2) mapping, macrophage tracking based on labeling cells in vivo by ultrasmall particles of iron oxide (USPIO), diffusion tensor imaging (DTI) and magnetization transfer imaging (MTI)) to detect histopathological changes in a novel animal model making use of intrinsic, temporally and spatially controlled triggering of oligodendrocyte cell death. This mouse model allows studying the MRI signature associated to neurodegenerative processes of MS in the absence of adaptive inflammatory components that appear to be foremost in the EAE models. Our results revealed pronounced T(2) hyperintensities in brain stem and cerebellar structures, which we attribute to structural alteration of white matter by pronounced vacuolation. Brain areas were found devoid of significant macrophage infiltration in line with the absence of a peripheral inflammatory response. The significant decrease in diffusion anisotropy derived from DTI measures in these structures is mainly caused by a pronounced decrease in diffusivity parallel to the fiber indicative of axonal damage. Triggering of oligodendrocyte ablation did not translate into a significant increase in radial diffusivity. Only minor decreases in MT ratio have been observed, which is attributed to inefficient removal of myelin debris.

The blood-brain barrier and microvascular water exchange in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia in the elderly. Although traditionally considered a disease of neurofibrillary tangles and amyloid plaques, structural and functional changes in the microvessels may contribute directly to the pathogenesis of the disease. Since vascular dysfunction often precedes cognitive impairment, understanding the role of the blood-brain barrier (BBB) in AD may be key to rational treatment of the disease. We propose that water regulation, a critical function of the BBB, is disturbed in AD and results in abnormal permeability and rates of water exchange across the vessel walls. In this paper, we describe some of the pathological events that may disturb microvascular water exchange in AD and examine the potential of a relatively new imaging technique, dynamic contrast-enhanced MRI, to quantify water exchange on a cellular level and thus serve as a probe of BBB integrity in AD.

Neuroprotection by dimethyloxalylglycine following permanent and transient focal cerebral ischemia in rats

Dimethyloxalylglycine (DMOG) is an inhibitor of prolyl-4-hydroxylase domain (PHD) enzymes that regulate the stability of hypoxia-inducible factor (HIF). We investigated the effect of DMOG on the outcome after permanent and transient middle cerebral artery occlusion (p/tMCAO) in the rat. Before and after pMCAO, rats were treated with 40 mg/kg, 200 mg/kg DMOG, or vehicle, and with 40 mg/kg or vehicle after tMCAO. Serial magnetic resonance imaging (MRI) was performed to assess infarct evolution and regional cerebral blood flow (rCBF). Both doses significantly reduced infarct volumes, but only 40 mg/kg improved the behavior after 24 hours of pMCAO. Animals receiving 40 mg/kg were more likely to maintain rCBF values above 30% from the contralateral hemisphere within 24 hours of pMCAO. DMOG after tMCAO significantly reduced the infarct volumes and improved behavior at 24 hours and 8 days and also improved the rCBF after 24 hours. A consistent and significant upregulation of both mRNA and protein levels of vascular endothelial growth factor (VEGF) and endothelial nitric oxide synthase (eNOS) was associated with the observed neuroprotection, although this was not consistently related to HIF-1α levels at 24 hours and 8 days. Thus, DMOG afforded neuroprotection both at 24 hours after pMCAO and at 24 hours and 8 days after tMCAO. This effect was associated with an increase of VEGF and eNOS and was mediated by improved rCBF after DMOG treatment.

Comparison of MRI signatures in pattern I and II multiple sclerosis models

The majority of individuals with multiple sclerosis (MS) exhibit T-cell- and macrophage-dominated lesions (patterns I and II; as opposed to III and IV). These lesions, in turn, may be distinguished on the basis of whether or not there are immunoglobulin and complement depositions at the sites of active myelin destruction; such depositions are found exclusively in pattern II lesions. The main aim of this study was to determine whether pattern I and pattern II MS lesions exhibit distinct MRI signatures. We have used a recently described focal MOG-induced EAE model of the rat brain, which recapitulates many of the hallmarks of pattern II MS; we compared this with our previous work in a delayed type hypersensitivity model of a pattern I type lesion in the rat brain. Demyelinating lesions with extensive inflammation were generated, in which the T2-weighted signal was increased. Magnetisation transfer ratio (MTR) maps revealed loss and subsequent incomplete recovery of the structure of the corpus callosum, together with changes in tissue water diffusion and an associated increase in ventricle size. Notably, the MTR changes preceeded histological demyelination and may report on the processes leading to demyelination, rather than demyelination per se. Immunohistochemically, these MRI-detectable signal changes correlated with both inflammatory cell infiltration and later loss of myelin. Breakdown of the blood-brain barrier and an increase in the regional cerebral blood volume were also evident in and around the lesion site at the early stage of the disease. Interestingly, however, the MRI signal changes in this pattern II type MS lesion were remarkably consistent with those previously observed in a pattern I lesion. These findings suggest that the observed signal changes reflect the convergent histopathology of the two models rather than the underlying mechanisms of the disease.

Systemic inflammatory response reactivates immune-mediated lesions in rat brain

The potential association between microbial infection and reactivation of a multiple sclerosis (MS) lesion is an important issue that remains unresolved, primarily because of the absence of suitable animal models and imaging techniques. Here, we have evaluated this question in an empirical manner using immunohistochemistry and magnetic resonance imaging (MRI), before and after the induction of a systemic inflammatory response in two distinct models of MS. In a pattern-II-type focal myelin oligodendrocyte glycoprotein-experimental autoimmune encephalomyelitis model, systemic endotoxin injection caused an increase in regional cerebral blood volume (rCBV) around the lesion site after 6 h, together with a reduction in the magnetization transfer ratio of the lesioned corpus callosum. These changes were followed by an increase in the diffusion of tissue water within the lesion 24 h after endotoxin challenge and new leukocyte recruitment as revealed both immunohistochemically and by MRI tracking of ultrasmall superparamagnetic iron oxide-labeled macrophages. Importantly, we detected in vivo expression of E- and P-selectin in quiescent lesions by MRI-detectable glyconanoparticles conjugated to sialyl Lewis(X). This finding may explain, at least in part, the ability of quiescent MS lesions to rapidly reinitiate the cell recruitment processes. In a pattern-I-type delayed-type hypersensitivity response model, a similar effect of endotoxin challenge on rCBV was observed, together with delayed breakdown of the blood-brain barrier, showing that systemic infection can alter the pathogenesis of MS-like lesions regardless of lesion etiology. These findings will have important implications for the management and monitoring of individuals with MS.

The role of dietary antioxidant insufficiency on the permeability of the blood-brain barrier

Our previous studies implicated vitamin E deficiency as a risk factor for equine motor neuron disease, a possible model of human amyotrophic lateral sclerosis, and showed direct effects of this deficiency on brain vascular endothelium. To gain better understanding of the pathogenesis of equine motor neuron disease, we determined the effects of dietary antioxidant insufficiency and the resultant brain tissue oxidative stress on blood-brain barrier permeability. Rats (n = 40) were maintained on a diet deficient of vitamin E for 36 to 43 weeks; 40 controls were fed a normal diet. Permeability of the blood-brain barrier in the cerebral cortex was investigated using rhodamine B, and lipid peroxidation was measured as a marker for oxidative stress. Animals on the vitamin E-deficient diet showed less weight gain and had higher brain lipid peroxidation compared with the controls. Fluorometric studies demonstrated greater rhodamine B in the perivascular compartment and central nervous system parenchyma in rats on the deficient diet compared with controls. These results suggest that a deficiency in vitamin E increases brain tissue oxidative stress and impairs the integrity of the blood-brain barrier. These observations may have relevance to the pathogenesis of amyotrophic lateral sclerosis and other neurologic diseases.

Molecular targets for disrupting leukocyte trafficking during multiple sclerosis

Autoimmune diseases of the central nervous system (CNS) involve the migration of abnormal numbers of self-directed leukocytes across the blood–brain barrier that normally separates the CNS from the immune system. The cardinal lesion associated with neuroinflammatory diseases is the perivascular infiltrate, which comprises leukocytes that have traversed the endothelium and have congregated in a subendothelial space between the endothelial-cell basement membrane and the glial limitans. The exit of mononuclear cells from this space can be beneficial, as when virus-specific lymphocytes enter the CNS for pathogen clearance, or might induce CNS damage, such as in the autoimmune disease multiple sclerosis when myelin-specific lymphocytes invade and induce demyelinating lesions. The molecular mechanisms involved in the movement of lymphocytes through these compartments involve multiple signalling pathways between these cells and the microvasculature. In this review, we discuss adhesion, costimulatory, cytokine, chemokine and signalling molecules involved in the dialogue between lymphocytes and endothelial cells that leads to inflammatory infiltrates within the CNS, and the targeting of these molecules as therapies for the treatment of multiple sclerosis.

Inflammation and dephosphorylation of the tight junction protein occludin in an experimental model of multiple sclerosis

Multiple sclerosis (MS) is a disease of the CNS in which inflammation, demyelination and neurodegeneration contribute to its initiation and progression. A frequently employed model of MS is experimental autoimmune encephalomyelitis (EAE). Here, to gain new insights into the disease process, an analysis of proteins in extracts of lumbar spinal cord from naïve and EAE rats was undertaken. The data mainly confirm that inflammation and blood-brain barrier (BBB) breakdown are the major hallmarks of disease in this model. Given their importance in the BBB, junctional proteins were further investigated. Occludin, a protein localizing to tight junctions in brain endothelial cells, showed strikingly increased migration in EAE when analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). This increased migration was mimicked by in vitro phosphatase treatment, implying its dephosphorylation in EAE. Occludin dephosphorylation coincided with the onset of inflammation, slightly preceding visible signs of disease, and was just prior to apparent changes in BBB permeability. These findings suggest occludin is a target for signaling processes in EAE, perhaps regulating the response of the BBB to the inflammatory environment as seen in MS.

Seeing is believing: in vivo evolution of multiple sclerosis pathology with magnetic resonance

Multiple sclerosis (MS) is considered a prototypical inflammatory autoimmune disease of the central nervous system that affects both myelin and axon. One of the most challenging aspects of MS is understanding the nature and mechanism of tissue injury because inflammation, demyelination, axonal degeneration, microvascular injury, and atrophy are all identified in histopathologic studies. Magnetic resonance (MR) imaging provides an in vivo examination of the brain that directly defines the extent of the pathology. In recent years, extensive MR studies have had a major impact on MS not only in making an early diagnosis but also in understanding of the disease. By exploiting the natural history and histopathologic correlation, conventional and various novel quantitative MR techniques have demonstrated the ability to image underlying pathological processes in MS. This review examines the role of different MR techniques in going beyond anatomical imaging and produces a more comprehensive overview of the pathophysiological changes which occur and evolve in MS.

Cerebral blood perfusion changes in multiple sclerosis

The proximity of immune cell aggregations to the vasculature is a hallmark of multiple sclerosis. Furthermore, it is widely accepted that inflammation is able to modulate the microcirculation. Until recently, the detection of cerebral blood perfusion changes was technically challenging, and perfusion studies in multiple sclerosis patients yielded contradictory results. However, new developments in fast magnetic resonance imaging have enabled us to image the cerebral hemodynamics based on the dynamic tracking of a bolus of paramagnetic contrast agents (dynamic susceptibility contrast). This review discusses the technical principles, possible pitfalls, and potential for absolute quantification of cerebral blood volume and flow in a clinical setting. It also outlines recent findings on inflammation associated perfusion changes, which are inseparable from pathological considerations in multiple sclerosis.

Diffusion tensor imaging of post mortem multiple sclerosis brain

Magnetic resonance imaging (MRI) is being used to probe the central nervous system (CNS) of patients with multiple sclerosis (MS), a chronic demyelinating disease. Conventional T₂-weighted MRI (cMRI) largely fails to predict the degree of patients' disability. This shortcoming may be due to poor specificity of cMRI for clinically relevant pathology. Diffusion tensor imaging (DTI) has shown promise to be more specific for MS pathology. In this study we investigated the association between histological indices of myelin content, axonal count and gliosis, and two measures of DTI (mean diffusivity [MD] and fractional anisotropy [FA]), in unfixed post mortem MS brain using a 1.5-T MR system. Both MD and FA were significantly lower in post mortem MS brain compared to published data acquired in vivo. However, the differences of MD and FA described in vivo between white matter lesions (WMLs) and normal-appearing white matter (NAWM) were retained in this study of post mortem brain: average MD in WMLs was 0.35 × 10^− 3 mm²/s (SD, 0.09) versus 0.22 (0.04) in NAWM; FA was 0.22 (0.06) in WMLs versus 0.38 (0.13) in NAWM. Correlations were detected between myelin content (Tr_myelin) and (i) FA (r = − 0.79, p < 0.001), (ii) MD (r = 0.68, p < 0.001), and (iii) axonal count (r = − 0.81, p < 0.001). Multiple regression suggested that these correlations largely explain the apparent association of axonal count with (i) FA (r = 0.70, p < 0.001) and (ii) MD (r = − 0.66, p < 0.001). In conclusion, this study suggests that FA and MD are affected by myelin content and – to a lesser degree – axonal count in post mortem MS brain.