Evaluation of the visual system in a rat model of chronic glaucoma using manganese-enhanced magnetic resonance imaging

Applications of Manganese-Enhanced Magnetic Resonance Imaging in Ophthalmology and Visual Neuroscience

Understanding the mechanisms of vision in health and disease requires knowledge of the anatomy and physiology of the eye and the neural pathways relevant to visual perception. As such, development of imaging techniques for the visual system is crucial for unveiling the neural basis of visual function or impairment. Magnetic resonance imaging (MRI) offers non-invasive probing of the structure and function of the neural circuits without depth limitation, and can help identify abnormalities in brain tissues in vivo. Among the advanced MRI techniques, manganese-enhanced MRI (MEMRI) involves the use of active manganese contrast agents that positively enhance brain tissue signals in T1-weighted imaging with respect to the levels of connectivity and activity. Depending on the routes of administration, accumulation of manganese ions in the eye and the visual pathways can be attributed to systemic distribution or their local transport across axons in an anterograde fashion, entering the neurons through voltage-gated calcium channels. The use of the paramagnetic manganese contrast in MRI has a wide range of applications in the visual system from imaging neurodevelopment to assessing and monitoring neurodegeneration, neuroplasticity, neuroprotection, and neuroregeneration. In this review, we present four major domains of scientific inquiry where MEMRI can be put to imperative use — deciphering neuroarchitecture, tracing neuronal tracts, detecting neuronal activity, and identifying or differentiating glial activity. We deliberate upon each category studies that have successfully employed MEMRI to examine the visual system, including the delivery protocols, spatiotemporal characteristics, and biophysical interpretation. Based on this literature, we have identified some critical challenges in the field in terms of toxicity, and sensitivity and specificity of manganese enhancement. We also discuss the pitfalls and alternatives of MEMRI which will provide new avenues to explore in the future.

Manganese-Enhanced Magnetic Resonance Imaging: Overview and Central Nervous System Applications With a Focus on Neurodegeneration

Manganese-enhanced magnetic resonance imaging (MEMRI) rose to prominence in the 1990s as a sensitive approach to high contrast imaging. Following the discovery of manganese conductance through calcium-permeable channels, MEMRI applications expanded to include functional imaging in the central nervous system (CNS) and other body systems. MEMRI has since been employed in the investigation of physiology in many animal models and in humans. Here, we review historical perspectives that follow the evolution of applied MRI research into MEMRI with particular focus on its potential toxicity. Furthermore, we discuss the more current in vivo investigative uses of MEMRI in CNS investigations and the brief but decorated clinical usage of chelated manganese compound mangafodipir in humans.

In vivo visuotopic brain mapping with manganese-enhanced MRI and resting-state functional connectivity MRI

The rodents are an increasingly important model for understanding the mechanisms of development, plasticity, functional specialization and disease in the visual system. However, limited tools have been available for assessing the structural and functional connectivity of the visual brain network globally, in vivo and longitudinally. There are also ongoing debates on whether functional brain connectivity directly reflects structural brain connectivity. In this study, we explored the feasibility of manganese-enhanced MRI (MEMRI) via 3 different routes of Mn(2+) administration for visuotopic brain mapping and understanding of physiological transport in normal and visually deprived adult rats. In addition, resting-state functional connectivity MRI (RSfcMRI) was performed to evaluate the intrinsic functional network and structural-functional relationships in the corresponding anatomical visual brain connections traced by MEMRI. Upon intravitreal, subcortical, and intracortical Mn(2+) injection, different topographic and layer-specific Mn enhancement patterns could be revealed in the visual cortex and subcortical visual nuclei along retinal, callosal, cortico-subcortical, transsynaptic and intracortical horizontal connections. Loss of visual input upon monocular enucleation to adult rats appeared to reduce interhemispheric polysynaptic Mn(2+) transfer but not intra- or inter-hemispheric monosynaptic Mn(2+) transport after Mn(2+) injection into visual cortex. In normal adults, both structural and functional connectivity by MEMRI and RSfcMRI was stronger interhemispherically between bilateral primary/secondary visual cortex (V1/V2) transition zones (TZ) than between V1/V2 TZ and other cortical nuclei. Intrahemispherically, structural and functional connectivity was stronger between visual cortex and subcortical visual nuclei than between visual cortex and other subcortical nuclei. The current results demonstrated the sensitivity of MEMRI and RSfcMRI for assessing the neuroarchitecture, neurophysiology and structural-functional relationships of the visual brains in vivo. These may possess great potentials for effective monitoring and understanding of the basic anatomical and functional connections in the visual system during development, plasticity, disease, pharmacological interventions and genetic modifications in future studies.

In vivo manganese-enhanced MRI for visuotopic brain mapping

This study explored the feasibility of localized manganese-enhanced MRI (MEMRI) via 3 different routes of Mn(2+) administrations for visuotopic brain mapping of retinal, callosal, cortico-subcortical, transsynaptic and horizontal connections in normal adult rats. Upon fractionated intravitreal Mn(2+) injection, Mn enhancements were observed in the contralateral superior colliculus (SC) and lateral geniculate nucleus (LGN) by 45-60% at 1-3 days after initial Mn(2+) injection and in the contralateral primary visual cortex (V1) by about 10% at 2-3 days after initial Mn(2+) injection. Direct, single-dose Mn(2+) injection to the LGN resulted in Mn enhancement by 13-21% in V1 and 8-11% in SC of the ipsilateral hemisphere at 8 to 24 hours after Mn(2+) administration. Intracortical, single-dose Mn(2+) injection to the visual cortex resulted in Mn enhancement by 53-65% in ipsilateral LGN, 15-26% in ipsilateral SC, 32-34% in the splenium of corpus callosum and 17-25% in contralateral V1/V2 transition zone at 8 to 24 hours after Mn(2+) administration. Notably, some patchy patterns were apparent near the V1/V2 border of the contralateral hemisphere. Laminar-specific horizontal cortical connections were also observed in the ipsilateral hemisphere. The current results demonstrated the sensitivity of MEMRI for assessing the neuroarchitecture of the visual brains in vivo without depth-limitation, and may possess great potentials for studying the basic neural components and connections in the visual system longitudinally during development, plasticity, pharmacological interventions and genetic modifications.

Impact of repeated topical-loaded manganese-enhanced MRI on the mouse visual system

PURPOSE

Optic nerve degeneration in diseases such as glaucoma and multiple sclerosis evolves in months to years. The use of Mn(2+)-Enhanced Magnetic Resonance Imaging (MEMRI) in a time-course study may provide new insights into the disease progression. Previously, we demonstrated the feasibility of using a topical administration for Mn(2+) delivery to the visual system. This study is to evaluate the impact of biweekly or monthly repeated Mn(2+) topical administration and the pH levels of the Mn(2+) solutions for MEMRI on the mouse visual pathway.

METHODS

Using groups of mice, the MEMRI with an acidic or pH neutralized 1 M MnCl(2) solution was performed. To evaluate the feasibility of repeated MEMRIs, topical-loaded MEMRI was conducted biweekly seven times or monthly three times. The enhancement of MEMRI in the visual system was quantified. After repeated MEMRIs, the corneas were examined by optical coherence tomography. The retinal ganglion cells (RGCs) and optic nerves were examined by histology.

RESULTS

All mice exhibited consistent enhancements along the visual system following repeated MEMRIs. The acidic Mn(2+) solution induced a greater MEMRI enhancement as compared with a neutral pH Mn(2+) solution. Significant 20% RGC loss was found after three biweekly Mn(2+) inductions, but no RGC loss was found after three monthly Mn(2+) treatments. The corneal thickness was found increased after seven biweekly topical-loaded MEMRI.

CONCLUSIONS

Acidic Mn(2+) solutions enhanced the uptake of Mn(2+) observed on the MEMRI. Increasing the time intervals of repeated Mn(2+) topical administration reduced the adverse effects caused by MEMRI.

Possibilities and limitations for high resolution small animal MRI on a clinical whole-body 3T scanner

OBJECT

To investigate the potential of a clinical 3 T scanner to perform MRI of small rodents.

MATERIALS AND METHODS

Different dedicated small animal coils and several imaging sequences were evaluated to optimize image quality with respect to SNR, contrast and spatial resolution. As an application, optimal grey-white-matter contrast and resolution were investigated for rats. Furthermore, manganese-enhanced MRI was applied in mice with unilateral crush injury of the optic nerve to investigate coil performance on topographic mapping of the visual projection.

RESULTS

Differences in SNR and CNR up to factor 3 and more were observed between the investigated coils. The best grey-white matter contrast was achieved with a high resolution 3D T (2)-weighted TSE (SPACE) sequence. Delineation of the retino-tectal projection and detection of defined visual pathway damage on the level of the optic nerve could be achieved by using a T (1)-weighted, 3D gradient echo sequence with isotropic resolution of (0.2 mm)(3).

CONCLUSIONS

Experimental studies in small rodents requiring high spatial resolution can be performed by using a clinical 3 T scanner with appropriate dedicated coils.

Manganese-enhanced MRI of hypoxic-ischemic brain injuries using Mn-DPDP

In this study, Mn-dipyridoxaldiphosphate (MnDPDP), a clinically approved manganese contrast agent for hepatic and pancreatic imaging, was demonstrated for the first time for manganese-enhanced MRI (MEMRI) in brains of normal young rats (n = 4) and rats with hypoxic-ischemic (H-I) insult at postnatal day 7 (n = 8). After a single intraperitoneal injection of low dosage with 0.1micromol/g in postnatal 14 days, 2D T1-weighted image (T1WIs), T1 maps, T2-weighted images (T2WIs) and T2 maps were acquired at 7 Tesla 1 day before, 1 day and 7 days after MnDPDP injection. The image contrast changes induced by MnDPDP appeared as the hyperintensity in T1WIs and the hypointensity in T2WIs. T1 and T2 values decreased in the regions of Mn enhancement. Such enhancement presented as a delayed pattern that was more pronounced in 7 day after MnDPDP injection, suggesting the sustained Mn accumulation due to MnDPDP. Moreover, the MnDPDP enhancement in H-I brains was more pronounced in the lesion sites and was easily detectable in T1WI, T1 map, T2WI and T2 map. The results demonstrated here support the possibility of using MnDPDP as a 'slow release' Mn(2+) for clinical diagnosis of various neuropathologies.

Manganese-enhanced MRI of the DBA/2J mouse model of hereditary glaucoma

PURPOSE

To test the hypothesis that manganese-enhanced magnetic resonance imaging (MEMRI) is a sensitive approach for measuring of age-related ocular changes in experimental pigmentary glaucoma.

METHODS

Four groups of light-adapted mice were studied using MEMRI: young (2-3 months), C57BL/6 (negative controls), and DBA/2J mice and aged (10-11 months) C57BL/6 and DBA/2J mice. In all mice, eye perimeter, optic nerve head width, iridocorneal angle, ciliary body area, and total and inner retinal thickness, and a surrogate of retinal ion regulation (intraretinal uptake of manganese) were assessed from MEMRI data and compared. Axon counts were obtained from optic nerves harvested from MEMRI-assessed eyes.

RESULTS

As the C57BL/6 and DBA/2J mice aged, differential and significant changes in ocular perimeter, retinal thickness, iridocorneal angle, ciliary body area, and optic nerve head width were readily measured from MEMRI data (P < 0.05). In C57BL/6 mice, only inner retinal thickness and perimeter were correlated. In DBA/2J mice, ocular perimeter was correlated with total and inner retinal thickness, ciliary body area, optic nerve head width, and iridocorneal angle. Comparison of young and aged mice revealed a subnormal intraretinal manganese uptake (P < 0.05) in aged DBA/2J mice, but not in aged C57BL/6 mice. Manganese uptake did not correlate with the ocular perimeter. Axon density in the optic nerve correlated with MEMRI-measured optic nerve head width (P < 0.05).

CONCLUSIONS

These studies provide a baseline of noninvasive MEMRI-detectable changes associated with age in a common animal model of hereditary glaucoma that may be useful in the longitudinal evaluation of therapeutic success.