Improved neuronal tract tracing using manganese enhanced magnetic resonance imaging with fast T(1) mapping

Temporal changes in the T1 and T2 relaxation rates (DeltaR1 and DeltaR2) in the rat brain are consistent with the tissue-clearance rates of elemental manganese

Temporal changes in the T(1) and T(2) relaxation rates (DeltaR(1) and DeltaR(2)) in rat olfactory bulb (OB) and cortex were compared with the absolute manganese (Mn) concentrations from the corresponding excised tissue samples. In vivo T(1) and T(2) relaxation times were measured before, and at 1, 7, 28, and 35 d after intravenous infusion of 176 mg/kg MnCl(2). The values of DeltaR(1), DeltaR(2), and absolute Mn concentration peaked at day 1 and then declined to near control levels after 28 to 35 d. The Mn bioelimination rate from the rat brain was significantly faster than that reported using radioisotope techniques. The R(1) and R(2) relaxation rates were linearly proportional to the underlying tissue Mn concentration and reflect the total absolute amount of Mn present in the tissue. The in vivo Mn r(1) and r(2) tissue relaxivities were comparable to the in vitro values for aqueous Mn(2+). These results demonstrate that loss of manganese-enhanced MRI (MEMRI) contrast after systemic Mn(2+) administration is due to elimination of Mn(2+) from the brain.

Accounting for nonspecific enhancement in neuronal tract tracing using manganese enhanced magnetic resonance imaging

Manganese enhanced MRI (MEMRI) is an emerging technique for tracing neuronal pathways in vivo. However, manganese may leak into blood vessels or cerebrospinal fluid (CSF) after local injection and can be circulated to and taken up by brain regions that may not have connections to the targeted pathways. Comparing enhancement time courses after intranasal injection with intravenous infusion of MnCl(2) in rats, the early enhancements in the pituitary gland (Pit) and hippocampus indicate the contrasts in those regions in the olfactory tract-tracing experiment were caused by such systemic effects. Since the Pit has easy access to manganese from the blood and its signal is proportional to other brain regions after intravenous infusion, it was used as an internal reference for the systemic effects. Applying intensity normalization by the Pit signal to tract-tracing data from the olfactory bulb led to reduced contrast in the hippocampus. These results demonstrate that nonspecific enhancements in MEMRI tract-tracing studies may have to be taken into account and that normalization by the Pit signal can compensate these effects.

Combination of Mn(2+)-enhanced and diffusion tensor MR imaging gives complementary information about injury and regeneration in the adult rat optic nerve

PURPOSE

To evaluate manganese (Mn(2+))-enhanced MRI (MEMRI) and diffusion tensor imaging (DTI) as tools for detection of axonal injury and regeneration after intravitreal peripheral nerve graft (PNG) implantation in the rat optic nerve (ON).

MATERIALS AND METHODS

In adult Fischer rats, retinal ganglion cell (RGC) survival was evaluated in Flurogold (FG) back-filled retinal whole mounts after ON crush (ONC), intravitreal PNG, and intravitreal MnCl(2) injection (150 nmol) at 0 and 20 days post lesion (dpl). MEMRI and echo-planar DTI (DTI-EPI) was obtained of noninjured ON one day after intravitreal MnCl(2) injection, and at 1 and 21 dpl after ONC, intravitreal PNG, and intravitreal MnCl(2) injections given at 0 and 20 dpl. GAP-43 immunohistochemistry was performed after the last MRI.

RESULTS

ONC reduced RGC density in retina by 94% at 21 dpl compared to noninjured ON without MnCl(2) injections. Both intravitreal PNG and intravitreal MnCl(2) injections improved RGC survival in retina, which was reduced by 90% (ONC+MnCl(2)), 82% (ONC+PNG), and 74% (ONC+PNG+MnCl(2)) compared to noninjured ON. DTI-derived parameters (fractional anisotropy [FA], mean diffusivity, axial diffusivity lambda( parallel), and radial diffusivity lambda( perpendicular)) were unaffected by the presence of Mn(2+) in the ON. At 1 dpl, CNR(MEMRI) and lambda( parallel) were reduced at the injury site, while at 21 dpl they were increased at the injury site compared to values measured at 1 dpl. GAP-43 immunoreactive axons were present in the ON distal to the ONC injury site.

CONCLUSION

MEMRI and DTI enabled detection of functional and structural degradation after rat ON injury, and there was correlation between the MRI-derived and immunohistochemical measures of axon regeneration.

Layer specific tracing of corticocortical and thalamocortical connectivity in the rodent using manganese enhanced MRI

Information about layer specific connections in the brain comes mainly from classical neuronal tracers that rely on histology. Manganese Enhanced MRI (MEMRI) has mapped connectivity along a number of brain pathways in several animal models. It is not clear at what level of specificity neuronal connectivity measured using MEMRI tracing can resolve. The goal of this work was to determine if neural tracing using MEMRI could distinguish layer inputs of major pathways of the cortex. To accomplish this, tracing was performed between hemispheres of the somatosensory (S1) cortex and between the thalamus and S1 cortex. T(1) mapping and T(1) weighted pulse sequences detected layer specific tracing after local injection of MnCl(2). Approximately 12 h following injections into S1 cortex, maximal T(1) reductions were observed at 0.6+/-0.07 and 1.1+/-0.12 mm from the brain surface in the contralateral S1. These distances correspond to the positions of layer 3 and 5 consistent with the known callosal inputs along this pathway. Four to six hours following injection of MnCl(2) into the thalamus there were maximal T(1) reductions between 0.7+/-0.08 and 0.8+/-0.08 mm from the surface of the brain, which corresponds to layer 4. This is consistent with terminations of the known thalamocortical projections. In order to observe the first synapse projection, it was critical to perform MRI at the right time after injections to detect layer specificity with MEMRI. At later time points, tracing through the cortical network led to more uniform contrast throughout the cortex due to its complex neuronal connections. These results are consistent with well established neuronal pathways within the somatosensory cortex and demonstrate that layer specific somatosensory connections can be detected in vivo using MEMRI.

Monitoring dynamic alterations in calcium homeostasis by T (1)-weighted and T (1)-mapping cardiac manganese-enhanced MRI in a murine myocardial infarction model

Manganese has been used as a T(1)-weighted MRI contrast agent in a variety of applications. Because manganese ions (Mn(2+)) enter viable myocardial cells via voltage-gated Ca(2+) channels, manganese-enhanced MRI is sensitive to the viability and inotropic state of the heart. In spite of the established importance of Ca(2+) regulation in the heart both before and after myocardial injury, monitoring strategies to assess Ca(2+) homeostasis in affected cardiac tissues are limited. This study implements a T(1)-mapping method to obtain quantitative information both dynamically and over a range of MnCl(2) infusion doses. To optimize the current Mn(2+) infusion protocols, we performed both dose-dependent and temporal washout studies. A non-linear relationship between infused MnCl(2) solution dose and increase in left ventricular wall relaxation rate (DeltaR(1)) was observed. Control mice also exhibited significant Mn(2+) clearance over time, with a decrease in DeltaR(1) of approximately 50% occurring in just 2.5 h. The complicated efflux time dependence possibly suggests multiple efflux mechanisms. With the use of the measured relationship between infused Mn(2+) dose, DeltaR(1), and inductively coupled plasma mass spectrometry data analysis provided a means of estimating the absolute heart Mn concentration in vivo. We show that this technique has the sensitivity to observe or monitor potential alterations in Ca(2+) handling in vivo because of the physiological remodeling after myocardial infarction. Left ventricular free wall DeltaR(1) values were significantly lower (P = 0.005) in the adjacent zone, surrounding the injured myocardial tissue, than in healthy tissue. This inferred reduction in Mn concentration can be used to estimate potentially salvageable myocardium in vivo for future treatment or evaluation of disease progression.

Quantitative manganese tract tracing: dose-dependent and activity-independent terminal labelling in the mouse visual system

At concentrations sufficient for visualisation using MRI, manganese (Mn) is believed to behave as a calcium analogue. This study examines different concentrations of Mn for enhanced MR tract tracing. The premise of activity-dependent axonal transport was also examined by partial or complete blockade of retinal ganglion cell activity. Quantitative T(1) maps and semi-quantitative normalised signal intensities in the superior colliculi facilitated assessment of applied intraocular concentrations and activity dependence, respectively. Varying the concentration of applied Mn revealed a non-monotonic profile, with optimal, unfavourable and undesirable effects noted: 25 mM proved optimal, showing a maximal decrease in T(1), whereas 400 mM was associated with no terminal-field enhancement. The estimated vitreal concentration for optimal transport of Mn (2 mM) is substantially lower than that used in previous studies of the mouse. Both the partial blockade of inputs to 50% of retinal ganglion cells by a mGluR6 glutamate agonist and the complete blockade of all retinal ganglion cell activity with tetrodotoxin failed to decrease the relative enhancement in the superior colliculus. The failure to prevent axonal transport of Mn by blocking activity (and therefore theoretically the intracellular influx) appeared to be paradoxical. The optimal vitreal concentration of Mn has previously been shown to facilitate massive intracellular uptake of Mn, competitively blocking calcium, and 1 mM Mn blocks neurotransmission pre-synaptically. These results suggest that, at concentrations required for optimal Mn-enhanced MRI tract tracing in the visual system of the mouse, the uptake and transport of Mn may be dominated by passive mechanisms, which may also block neurotransmission.

Manganese enhanced MRI reveals functional circuitry in response to odorant stimuli

To investigate the circuitry involved in detecting odorants in the rodent brain, we developed a method using manganese-enhanced MRI (MEMRI) to map the flow of neural information from the olfactory sensory neurons (OSNs) to the central layers of the olfactory bulb. Studies have shown that Mn(2+) enters active neurons and is transported anterogradely to axon terminals where it can cross synapses to functionally trace neural networks. Thus, by delivering MnCl(2) directly into the nasal cavity of mice and then exposing them to defined odorants, Mn(2+) is preferentially taken up by activated OSNs. Using the time course of the MRI signal, we generated maps of Mn(2+) accumulation in the olfactory bulb for both glomerular and mitral cell layers. Results demonstrated that overlapping yet distinct enhancement patterns were produced by exposure to either octanal, acetophenone, or carvone. Notably, areas of Mn(2+) accumulation in the mitral cell layer were similar to those in the glomerular layer consistent with neural information that passes from specific OSNs to specific mitral cells. Finally, by correlating specific Mn(2+) signal peaks to genetically labeled glomeruli that are known to be activated by the odorant octanal, we show that MEMRI maps can be resolved at the level of individual glomeruli.

Mapping prefrontal circuits in vivo with manganese-enhanced magnetic resonance imaging in monkeys

Manganese-enhanced magnetic resonance imaging (MEMRI) provides a powerful tool to study multisynaptic circuits in vivo and thereby to link information about neural structure and function within individual subjects. Making the best use of MEMRI in monkeys requires minimizing manganese-associated neurotoxicity, maintaining sensitivity to manganese-dependent signal changes and mapping transport throughout the brain without a priori anatomical hypotheses. Here, we performed intracortical injections of isotonic MnCl(2), comparisons of preinjection and postinjection scans, and voxelwise statistical mapping. Isotonic MnCl(2) did not cause cell death at the injection site, damage to downstream targets of manganese transport, behavioral deficits, or changes in neuronal responsiveness. We detected and mapped manganese transport throughout cortical-subcortical circuits by using voxelwise statistical comparisons of at least 10 preinjection and two postinjection scans. We were able to differentiate between focal and diffuse projection fields and to distinguish between the topography of striatal projections from orbitofrontal and anterior cingulate cortex in a single animal. This MEMRI approach provides a basis for combining circuit-based anatomical analyses with simultaneous single-unit recordings and/or functional magnetic resonance imaging in individual monkeys. Such studies will enhance our interpretations of functional data and our understanding of how neuronal activity is transformed as it propagates through a circuit.

Convertible manganese contrast for molecular and cellular MRI

We describe here the use of inorganic manganese based particles as convertible MRI agents. As has been demonstrated with iron oxide particles, manganese oxide and manganese carbonate particles can be internalized within phagocytotic cells, being subsequently shuttled to endosomes and/or lysosomes. As intact particles, only susceptibility-induced MRI contrast is exhibited, most often seen as dark contrast in susceptibility-weighted images. Modulation of MRI contrast is accomplished by the selective degradation of these particles within the endosomal and lysosomal compartments of cells. Upon particle deconstruction in the endosomes and lysosomes, the dissolved Mn(2+) acts as a T(1) agent, eliciting bright contrast in T(1)-weighted images. This modulation of MRI contrast is demonstrated both in vitro in cells in culture, and also in vivo, in rat brain. These particles are the potential building blocks for an entire class of new environmentally responsive MRI contrast agents.

Manganese-enhanced MRI: an exceptional tool in translational neuroimaging

The metal manganese is a potent magnetic resonance imaging (MRI) contrast agent that is essential in cell biology. Manganese-enhanced magnetic resonance imaging (MEMRI) is providing unique information in an ever-growing number of applications aimed at understanding the anatomy, the integration, and the function of neural circuits both in normal brain physiology as well as in translational models of brain disease. A major drawback to the use of manganese as a contrast agent, however, is its cellular toxicity. Therefore, paramount to the successful application of MEMRI is the ability to deliver Mn2+ to the site of interest using as low a dose as possible while preserving detectability by MRI. In the present work, the different approaches to MEMRI in translational neuroimaging are reviewed and challenges for future identified from a practical standpoint.

Fractionated manganese-enhanced MRI

We investigated the use of manganese-enhanced MRI (MEMRI) with fractionated doses as a way to retain the unique properties of manganese as a neuronal contrast agent while lessening its toxic effects in animals. First, we followed the signal enhancement on T1-weighted images of the brains of rats receiving 30 mg/kg fractions of MnCl2 . 4H2O every 48 h and found that the signal increased in regions with consecutive fractionated doses and ultimately saturated. Second, we used T1 mapping to test whether the amount of MRI-visible manganese that accumulated depended on the concentration of manganese in the fractions. For a fixed cumulative dose of 180 mg/kg MnCl2 . 4H2O, increasing fraction doses of 6 x 30 mg/kg, 3 x 60 mg/kg, 2 x 90 mg/kg and 1 x 180 mg/kg produced progressively shorter T1 values. The adverse systemic health effects, including complications at the injection site and poor animal well-being, also rose with the fraction dose. Thus, fractionated MEMRI can be used to balance the properties of manganese as a contrast agent in animals against its toxic effects.

Brain redox imaging using blood-brain barrier-permeable nitroxide MRI contrast agent

Reactive oxygen species (ROS) and compromised antioxidant defense may contribute to brain disorders such as stroke, amyotrophic lateral sclerosis, etc. Nitroxides are redox-sensitive paramagnetic contrast agents and antioxidants. The ability of a blood-brain barrier (BBB)-permeable nitroxide, methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (MC-P), as a magnetic resonance-imaging (MRI) contrast agent for brain tissue redox imaging was tested. MC-P relaxation in rodent brain was quantified by MRI using a fast Look-Locker T(1)-mapping sequence. In the cerebral cortex and thalamus, the MRI signal intensity increased up to 50% after MC-P injection, but increased only by 2.7% when a BBB-impermeable nitroxide, 3CxP (3-carboxy-2,2,5,5,5-tetramethylpyrrolidine-1-oxyl) was used. The maximum concentrations in the thalamus and cerebral cortex after MC-P injection were calculated to be 1.9+/-0.35 and 3.0+/-0.50 mmol/L, respectively. These values were consistent with the ex vivo data of brain tissue and blood concentration obtained by electron paramagnetic resonance (EPR) spectroscopy. Also, reduction rates of MC-P were significantly decreased after reperfusion following transient MCAO (middle cerebral artery occlusion), a condition associated with changes in redox status resulting from oxidative damage. These results show the use of BBB-permeable nitroxides as MRI contrast agents and antioxidants to evaluate the role of ROS in neurologic diseases.

Axon tracing in the adult rat optic nerve and tract after intravitreal injection of MnDPDP using a semiautomatic segmentation technique

PURPOSE

To develop and validate an objective technique for 3D segmentation of manganese-enhanced MR images of the optic nerve/tract (ON) in adult rats to improve contrast-to-noise (CNR) calculations and use the technique to ascertain if manganese dipyridoxyl diphosphate (MnDPDP) gives sufficient Mn(2+) enhancement compared to MnCl(2) when used for functional imaging of the visual pathway.

MATERIALS AND METHODS

Intravitreous injection of the manganese-releasing MR contrast agent MnDPDP (30 nmol Mn(2+)) was performed to trace the ON in adult rats (n = 4). A positive control group of rats (n = 5) received a standard preparation of MnCl(2) (200 nmol Mn(2+)), while gadodiamide (1500 nmol Gd(3+)) was administered in negative control rats (n = 2). An objective technique for 3D segmentation of the enhanced ON was developed. CNR profiles along the ON were calculated by resampling the 3D image-volume in 2D planes perpendicular to the Mn(2+) enhanced ON in 0.2 mm steps, 4 mm along the segmented ON measured from the lamina cribrosa.

RESULTS

The ON was successfully segmented and CNR calculated accurately within 2 minutes in a representative 3D MR image volume. Intravitreal MnDPDP injection resulted in significant MRI contrast enhancement of the retina and ON after 12-24 hours similar to that of MnCl(2) injection.

CONCLUSION

3D semiautomated image segmentation and the use of MnDPDP can improve in vivo axon tracing based on MRI. Mn(2+) was found to be released from MnDPDP after intravitreal injection in sufficient amounts to obtain functional tracing of the adult rat primary visual pathway.

Cerebrospinal fluid to brain transport of manganese in a non-human primate revealed by MRI

Manganese overexposure in non-human primates and humans causes a neurodegenerative disorder called manganism thought to be related to an accumulation of the metal in the basal ganglia. Here, we assess changes in the concentration of manganese in regions of the brain of a non-human primate (the common marmoset, Callithrix jacchus) following four systemic injections of 30 mg/kg MnCl2 H2O in the tail vein using T1-weighted magnetic resonance imaging (MRI) and compare these to changes in the rat following the same exposure route and dose. The doses were spaced 48 h apart and we imaged the animals 48 h after the final dose. We find that marmosets have significantly larger T1-weighted image enhancements in regions of the brain compared to rats, notably in the basal ganglia and the visual cortex. To confirm this difference across species reflects actual differences in manganese concentrations and not variations in the MRI properties of manganese, we measured the longitudinal relaxivity of manganese (chi1) in the in vivo brain and found no significant species' difference. The high manganese uptake in the marmoset basal ganglia and visual cortex can be explained by CSF-brain transport from the large lateral ventricles and we confirm this route of uptake with time-course MRI during a tail-vein infusion of manganese. There is also high uptake in the substructures of the hippocampus that are adjacent to the ventricles. The large manganese accumulation in these structures on overexposure may be common to all primates, including humans.

Detection of cortical laminar architecture using manganese-enhanced MRI

Changes in manganese-enhanced MRI (MEMRI) contrast across the rodent somatosensory cortex were compared to the cortical laminae as identified by tissue histology and administration of an anatomical tracer to cortex and thalamus. Across the cortical thickness, MEMRI signal intensity was low in layer I, increased in layer II, decreased in layer III until mid-layer IV, and increased again, peaking in layer V, before decreasing through layer VI. The reeler mouse mutant was used to confirm that the cortical alternation in MEMRI contrast was related to laminar architecture. Unlike in wild-type mice, the reeler cortex showed no appreciable changes in MEMRI signal, consistent with absence of cortical laminae in histological slides. The tract tracing ability of MEMRI was used to further confirm assignments and demonstrate laminar specificity. Twelve to 16 h after stereotaxic injections of MnCl(2) to the ventroposterior thalamic nuclei, an overall increase in signal intensity was detected in primary somatosensory cortex compared to other brain regions. Maximum intensity projection images revealed a distinctly bright stripe located 600-700 microm below the pial surface, in layer IV. The data show that both systemic and tract tracing forms of MEMRI are useful for studying laminar architecture in the brain.

Central neural activity in rats with tinnitus evaluated with manganese-enhanced magnetic resonance imaging (MEMRI)

The pathophysiology of tinnitus, the perception of sound in the absence of acoustic stimulation, is largely unknown, although several lines of research implicate long-term neuroplastic loss of inhibition. The evidence to date suggests that the neuroplastic alterations are likely to be found in multiple brain structures. The present study used manganese-enhanced magnetic resonance imaging (MEMRI) to assess the pattern of neural activity in the central auditory pathway of rats with psychophysical evidence of chronic acoustic-exposure-induced tinnitus. Manganese, an activity-dependent paramagnetic contrast agent, accumulates in active neurons through voltage-gated calcium channels, primarily at synapses, and serves as both a structural and functional indicator. Comparison images were obtained from normal subjects exposed to external tinnitus-like sound, and from tinnitus subjects treated with vigabatrin, a GABA agonist shown to eliminate the psychophysical evidence of tinnitus in rats. MEMRI indicated: (1) In rats with evidence of tinnitus, activity was generally elevated in the auditory brainstem, with significant elevation in the cerebellar paraflocculus, the posterior ventral cochlear nucleus, and the inferior colliculus; in general forebrain structures showed decreased activity, although MEMRI may be a less sensitive indicator of forebrain activity than brainstem activity; (2) in normal rats exposed to a tinnitus-like sound, a similar pattern of elevated brainstem activity and decreased forebrain activity was evident, with the notable exception of the paraflocculus, where artificial tinnitus had no effect and (3) vigabatrin, decreased brainstem activity to control levels, in rats with prior evidence of tinnitus, and decreased forebrain activity to below control levels. It was concluded that chronic tinnitus in rats is associated with focal activity elevation in the auditory brainstem and increased activity in the paraflocculus that may be unique to tinnitus.

Manganese: a unique neuroimaging contrast agent

Manganese is a strong magnetic resonance imaging relaxation agent with unique biological properties that make it suitable for in vivo studies of neuroachitecture, neuronal tracts and neuronal function in animals. However, in humans large doses of manganese are neurotoxic and cause damage, primarily to the basal ganglia, resulting in a form of parkinsonism termed manganism. If low doses can be safely used and detected in the human brain, manganese will provide insight into neuroanatomy, connectivity, function and neuropathology.

In vivo axonal transport rates decrease in a mouse model of Alzheimer's disease

Axonopathy is a pronounced attribute of many neurodegenerative diseases. In Alzheimer's disease (AD), axonal swellings and degeneration are prevalent and may contribute to the symptoms of AD senile dementia. Current limitations in identifying the contribution of axonal damage to AD include the inability to detect when this damage occurs in relation to other identifiers of AD because of the invasiveness of existing methods. To overcome this, we further developed the MRI methodology Manganese Enhanced MRI (MEMRI) to assess in vivo axonal transport rates. Prior to amyloid-beta (Abeta) deposition, the axonal transport rates in the Tg2576 mouse model of AD were normal. As Abeta levels increased and before plaque formation, we observed a significant decrease in axonal transport rates of the Tg2576 mice compared to controls. After plaque formation, the decline in the transport rate in the Tg2576 mice became even more pronounced. These data indicate that in vivo axonal transport rates decrease prior to plaque formation in the Tg2576 mouse model of AD.

Manganese-enhanced MRI in a rat model of Parkinson's disease

PURPOSE

To measure intra- and inter-hemispheric connectivity within the basal ganglia (BG) nuclei in healthy and in unilateral 6-hydroxydopamine (6-OHDA) Parkinson disease rat model in order to test the BG interhemispheric connectivity hypothesis.

MATERIAL AND METHODS

The manganese-enhanced MRI (MEMRI) method with direct injection of manganese chloride into the entopeduncular (EP), substantia nigra (SN), and the Habenula nuclei in unilateral 6-OHDA (N = 22) and sham-operated (N = 16) rat groups was used. MEMRI measurements were applied before, 3, 24, and 48 hours post-manganese injection. Signal enhancements in T1-weighted images were compared between groups.

RESULTS

Manganese injection into the EP nucleus resulted with bihemispheric signal enhancements in the habenular complex (Hab) at both groups with stronger enhancements in the 6-OHDA group. It also exhibited lower sensorimotor cortex signal enhancement in the 6-OHDA rat group. SN manganese injection caused enhanced anteroventral thalamic and habenular nuclei signals in the 6-OHDA rat group. Manganese habenula injection revealed enhanced interpeduncular (IP) and raphe nuclei signals of the 6-OHDA rat group.

CONCLUSION

Modulations in the effective intra- and interhemispheric BG connectivity in unilateral 6-OHDA Parkinson's disease (PD) rat model support the BG interhemispheric connectivity hypothesis and suggest a linkage between the dopaminergic and serotonergic systems in PD, in line with clinical symptoms.