Permanent tissue damage in multiple sclerosis lesions is associated with reduced pre-lesion myelin and axon volume fractions

BACKGROUND

The use of advanced magnetic resonance imaging (MRI) techniques in MS research has led to new insights in lesion evolution and disease outcomes. It has not yet been determined if, or how, pre-lesional abnormalities in normal-appearing white matter (NAWM) relate to the long-term evolution of new lesions.

OBJECTIVE

To investigate the relationship between abnormalities in MRI measures of axonal and myelin volume fractions (AVF and MVF) in NAWM preceding development of black-hole (BH) and non-BH lesions in people with MS.

METHODS

We obtained magnetization transfer and diffusion MRI at 6-month intervals in patients with MS to estimate MVF and AVF during lesion evolution. Lesions were classified as either BH or non-BH on the final imaging visit using T₁ maps.

RESULTS

Longitudinal data from 97 new T₂ lesions from 9 participants were analyzed; 25 lesions in 8 participants were classified as BH 6-12 months after initial appearance. Pre-lesion MVF, AVF, and MVF/AVF were significantly lower, and T₁ was significantly higher, in the lesions that later became BHs (p < 0.001) compared to those that did not. No significant pre-lesion abnormalities were found in non-BH lesions (p > 0.05).

CONCLUSION

The present work demonstrated that pre-lesion abnormalities are associated with worse long-term lesion-level outcome.

DCE-MRI of Brain Fluid Barriers: In Vivo Water Cycling at the Human Choroid Plexus

Metabolic deficits at brain-fluid barriers are an increasingly recognized feature of cognitive decline in older adults. At the blood-cerebrospinal fluid barrier, water is transported across the choroid plexus (CP) epithelium against large osmotic gradients via processes tightly coupled to activity of the sodium/potassium pump. Here, we quantify CP homeostatic water exchange using dynamic contrast-enhanced MRI and investigate the association of the water efflux rate constant (k_co) with cognitive dysfunction in older individuals. Temporal changes in the longitudinal relaxation rate constant (R₁) after contrast agent bolus injection were measured in a CP region of interest in 11 participants with mild cognitive dysfunction [CI; 73 ± 6 years] and 28 healthy controls [CN; 72 ± 7 years]. k_co was determined from a modified two-site pharmacokinetic exchange analysis of the R₁ time-course. K^trans, a measure of contrast agent extravasation to the interstitial space was also determined. Cognitive function was assessed by neuropsychological test performance. k_co averages 5.8 ± 2.7 s^-1 in CN individuals and is reduced by 2.4 s^-1 [ca. 40%] in CI subjects. Significant associations of k_co with global cognition and multiple cognitive domains are observed. K^trans averages 0.13 ± 0.07 min^-1 and declines with age [-0.006 ± 0.002 min^-1 yr^-1], but shows no difference between CI and CN individuals or association with cognitive performance. Our findings suggest that the CP water efflux rate constant is associated with cognitive dysfunction and shows an age-related decline in later life, consistent with the metabolic disturbances that characterize brain aging.

Gray matter blood-brain barrier water exchange dynamics are reduced in progressive multiple sclerosis

Background and Purpose

To compare transcapillary wall water exchange, a putative marker of cerebral metabolic health, in brain T₂ white matter (WM) lesions and normal appearing white and gray matter (NAWM and NAGM, respectively) in individuals with progressive multiple sclerosis (PMS) and healthy controls (HC).

Methods

Dynamic‐contrast‐enhanced 7T MRI data were obtained from 19 HC and 23 PMS participants. High‐resolution pharmacokinetic parametric maps representing tissue microvascular and microstructural properties were created by shutter‐speed (SS) paradigm modeling to obtain estimates of blood volume fraction (v_b), water molecule capillary efflux rate constant (k_po), and the water capillary wall permeability surface area product (P_wS ≡ v_b*k_po). Linear regression models were used to investigate differences in (i) k_po and P_wS between groups in NAWM and NAGM, and (ii) between WM lesions and NAWM in PMS.

Results

High‐resolution parametric maps were produced to visualize tissue classes and resolve individual WM lesions. Normal‐appearing gray matter k_po and P_wS were significantly decreased in PMS compared to HC (p ≤ .01). Twenty‐one T₂ WM lesions were analyzed in 10 participants with PMS. k_po was significantly decreased in WM lesions compared to PMS NAWM (p < .0001).

Conclusions

Transcapillary water exchange is reduced in PMS NAGM compared to HC and is further reduced in PMS WM lesions, suggesting pathologically impaired brain metabolism. k_po provides a sensitive measure of cerebral metabolic activity and/or coupling, and can be mapped at higher spatial resolution than conventional imaging techniques assessing metabolic activity.

MRI characteristics of Japanese macaque encephalomyelitis: Comparison to human diseases

BACKGROUND AND PURPOSE

To describe MRI findings in Japanese macaque encephalomyelitis (JME) with emphasis on lesion characteristics, lesion evolution, normal-appearing brain tissue, and similarities to human demyelinating disease.

METHODS

MRI data were obtained from 114 Japanese macaques, 30 presenting neurological signs of JME. All animals were screened for presence of T2 -weighted white matter signal hyperintensities; animals with behavioral signs of JME were additionally screened for contrast-enhancing lesions. Whole-brain quantitative T1 maps were collected, and histogram analysis was performed with regression across age to evaluate microstructural changes in normal appearing brain tissue in JME and neurologically normal animals. Quantitative estimates of blood-brain-barrier (BBB) permeability to gadolinium-based-contrast agent (GBCA) were obtained in acute, GBCA-enhancing lesions. Longitudinal imaging data were acquired for 15 JME animals.

RESULTS

One hundred and seventy-three focal GBCA-enhancing lesions were identified in 30 animals demonstrating behavioral signs of neurological dysfunction. JME GBCA-enhancing lesions were typically focal and ovoid, demonstrating highest BBB GBCA permeability in the lesion core, similar to acute, focal multiple sclerosis lesions. New GBCA-enhancing lesions arose rapidly from normal-appearing tissue, and BBB permeability remained elevated for weeks. T1 values in normal-appearing tissue were significantly associated with age, but not with sex or disease.

CONCLUSIONS

Intense, focal neuroinflammation is a key MRI finding in JME. Several features of JME compare directly to human inflammatory demyelinating diseases. Investigation of JME combined with the development and validation of noninvasive imaging biomarkers offers substantial potential to improve diagnostic specificity and contribute to the understanding of human demyelinating diseases.

Observation of Reduced Homeostatic Metabolic Activity and/or Coupling in White Matter Aging

BACKGROUND AND PURPOSE

Transvascular water exchange plays a key role in the functional integrity of the blood-brain barrier (BBB). In white matter (WM), a variety of imaging modalities have demonstrated age-related changes in structure and metabolism, but the extent to which water exchange is altered remains unclear. Here, we investigated the cumulative effects of healthy aging on WM capillary water exchange.

METHODS

A total of 38 healthy adults (aged 36-80 years) were studied using 7T dynamic contrast enhanced MRI. Blood volume fraction (v_b ) and capillary water efflux rate constant (k_po ) were determined by fitting changes in the ¹ H₂ O longitudinal relaxation rate constant (R₁ ) during contrast agent bolus passage to a two-compartment exchange model. WM volume was determined by morphometric analysis of structural images.

RESULTS

R₁ values and WM volume showed similar trajectories of age-related decline. Among all subjects, v_b and k_po averaged 1.7 (±0.5) mL/100 g of tissue and 2.1 (±1.1) s^-1 , respectively. While v_b showed minimal changes over the 40-year-age span of participants, k_po declined 0.06 s^-1 (ca. 3%) per year (r = -.66; P < .0005), from near 4 s^-1 at age 30 to ca. 2 s^-1 at age 70. The association remained significant after controlling for WM volume.

CONCLUSIONS

Previous studies have shown that k_po tracks Na⁺ , K⁺ -ATPase activity-dependent water exchange at the BBB and likely reflects neurogliovascular unit (NGVU) coupled metabolic activity. The age-related decline in k_po observed here is consistent with compromised NGVU metabolism in older individuals and the dysregulated cellular bioenergetics that accompany normal brain aging.

Myelin repair stimulated by CNS-selective thyroid hormone action

Oligodendrocyte processes wrap axons to form neuroprotective myelin sheaths, and damage to myelin in disorders, such as multiple sclerosis (MS), leads to neurodegeneration and disability. There are currently no approved treatments for MS that stimulate myelin repair. During development, thyroid hormone (TH) promotes myelination through enhancing oligodendrocyte differentiation; however, TH itself is unsuitable as a remyelination therapy due to adverse systemic effects. This problem is overcome with selective TH agonists, sobetirome and a CNS-selective prodrug of sobetirome called Sob-AM2. We show here that TH and sobetirome stimulated remyelination in standard gliotoxin models of demyelination. We then utilized a genetic mouse model of demyelination and remyelination, in which we employed motor function tests, histology, and MRI to demonstrate that chronic treatment with sobetirome or Sob-AM2 leads to significant improvement in both clinical signs and remyelination. In contrast, chronic treatment with TH in this model inhibited the endogenous myelin repair and exacerbated disease. These results support the clinical investigation of selective CNS-penetrating TH agonists, but not TH, for myelin repair.

Discovery of a CLN7 model of Batten disease in non-human primates

We have identified a natural Japanese macaque model of the childhood neurodegenerative disorder neuronal ceroid lipofuscinosis, commonly known as Batten Disease, caused by a homozygous frameshift mutation in the CLN7 gene (CLN7^−/−). Affected macaques display progressive neurological deficits including visual impairment, tremor, incoordination, ataxia and impaired balance. Imaging, functional and pathological studies revealed that CLN7^−/− macaques have reduced retinal thickness and retinal function early in disease, followed by profound cerebral and cerebellar atrophy that progresses over a five to six-year disease course. Histological analyses showed an accumulation of cerebral, cerebellar and cardiac storage material as well as degeneration of neurons, white matter fragmentation and reactive gliosis throughout the brain of affected animals. This novel CLN7^−/− macaque model recapitulates key behavioral and neuropathological features of human Batten Disease and provides novel insights into the pathophysiology linked to CLN7 mutations. These animals will be invaluable for evaluating promising therapeutic strategies for this devastating disease.

Measurement science in the circulatory system

The dynamics of the cellular and molecular constituents of the circulatory system are regulated by the biophysical properties of the heart, vasculature and blood cells and proteins. In this review, we discuss measurement techniques that have been developed to characterize the physical and mechanical parameters of the circulatory system across length scales ranging from the tissue scale (centimeter) to the molecular scale (nanometer) and time scales of years to milliseconds. We compare the utility of measurement techniques as a function of spatial resolution and penetration depth from both a diagnostic and research perspective. Together, this review provides an overview of the utility of measurement science techniques to study the spatial systems of the circulatory system in health and disease.

Feasibility of shutter-speed DCE-MRI for improved prostate cancer detection

The feasibility of shutter-speed model dynamic-contrast-enhanced MRI pharmacokinetic analyses for prostate cancer detection was investigated in a prebiopsy patient cohort. Differences of results from the fast-exchange-regime-allowed (FXR-a) shutter-speed model version and the fast-exchange-limit-constrained (FXL-c) standard model are demonstrated. Although the spatial information is more limited, postdynamic-contrast-enhanced MRI biopsy specimens were also examined. The MRI results were correlated with the biopsy pathology findings. Of all the model parameters, region-of-interest-averaged K(trans) difference [ΔK(trans) ≡ K(trans)(FXR-a) - K(trans)(FXL-c)] or two-dimensional K(trans)(FXR-a) vs. k(ep)(FXR-a) values were found to provide the most useful biomarkers for malignant/benign prostate tissue discrimination (at 100% sensitivity for a population of 13, the specificity is 88%) and disease burden determination. (The best specificity for the fast-exchange-limit-constrained analysis is 63%, with the two-dimensional plot.) K(trans) and k(ep) are each measures of passive transcapillary contrast reagent transfer rate constants. Parameter value increases with shutter-speed model (relative to standard model) analysis are larger in malignant foci than in normal-appearing glandular tissue. Pathology analyses verify the shutter-speed model (FXR-a) promise for prostate cancer detection. Parametric mapping may further improve pharmacokinetic biomarker performance.

Discrimination of benign and malignant breast lesions by using shutter-speed dynamic contrast-enhanced MR imaging

PURPOSE

To assess the accuracy of the shutter-speed approach compared with standard approach dynamic contrast material-enhanced magnetic resonance (MR) imaging pharmacokinetic analysis for breast cancer diagnosis.

MATERIALS AND METHODS

This study was approved by the institutional review board and was HIPAA compliant. Informed consent was obtained from 89 high-risk women (age range, 28-83 years) who had 92 suspicious lesions with negative findings at mammography (but visible at MR imaging). Each underwent a research dynamic contrast-enhanced MR imaging examination just prior to a clinical MR imaging-guided interventional procedure. Tumor region of interest (ROI) averaged and (for some) pixel-by-pixel dynamic contrast-enhanced time-course data, together with mean arterial input function, were subjected to serial standard and shutter-speed approach analyses to extract pharmacokinetic parameters, including rate constant for passive contrast reagent transfer between plasma and interstitium (K(trans)) and interstitial space volume fraction, or v(e). Pathologic findings were used as reference standards. Diagnostic accuracy was assessed with receiver operating characteristic analyses.

RESULTS

The pathologic analyses revealed 20 malignant and 72 benign lesions. Positive predictive value of the institutional clinical breast MR imaging protocol was 22%. At 100% sensitivity, ROI-averaged shutter-speed approach K(trans) had significantly (P = .008) higher diagnostic specificity than standard approach K(trans): 86.1% versus 77.8%. The difference in the ROI-averaged K(trans) parameter value, or ΔK(trans) (≡ K(trans) [shutter-speed approach] - K(trans) [standard approach]), had even higher specificity (88.9%). Combined use of ROI analysis and pixel-by-pixel mapping of ΔK(trans) achieved 98.6% specificity at 100% sensitivity.

CONCLUSION

The use of the shutter-speed dynamic contrast-enhanced MR imaging method has the potential to improve breast cancer diagnostic accuracy and reduce putatively unnecessary biopsy procedures that yield benign pathologic findings.

SUPPLEMENTAL MATERIAL

http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.11102413/-/DC1.

Dynamic-contrast-enhanced-MRI with extravasating contrast reagent: rat cerebral glioma blood volume determination

The accurate mapping of the tumor blood volume (TBV) fraction (vb) is a highly desired imaging biometric goal. It is commonly thought that achieving this is difficult, if not impossible, when small molecule contrast reagents (CRs) are used for the T1-weighted (Dynamic-Contrast-Enhanced) DCE-MRI technique. This is because angiogenic malignant tumor vessels allow facile CR extravasation. Here, a three-site equilibrium water exchange model is applied to DCE-MRI data from the cerebrally-implanted rat brain U87 glioma, a tumor exhibiting rapid CR extravasation. Analyses of segments of the (and the entire) DCE data time-course with this "shutter-speed" pharmacokinetic model, which admits finite water exchange kinetics, allow TBV estimation from the first-pass segment. Pairwise parameter determinances were tested with grid searches of 2D parametric error surfaces. Tumor blood volume (vb), as well as ve (the extracellular, extravascular space volume fraction), and Ktrans (a CR extravasation rate measure) parametric maps are presented. The role of the Patlak Plot in DCE-MRI is also considered.