Experimental protocol for activation-induced manganese-enhanced MRI (AIM-MRI) based on quantitative determination of Mn content in rat brain by fast T1 mapping

Association of medullary reticular formation ventral part with spasticity in mice suffering from photothrombotic stroke

Strokes cause spasticity via stretch reflex hyperexcitability in the spinal cord, and spastic paralysis due to involuntary muscle contraction in the hands and fingers can severely restrict skilled hand movements. However, the underlying neurological mechanisms remain unknown. Using a mouse model of spasticity after stroke, we demonstrate changes in neuronal activity with and without electrostimulation of the afferent nerve to induce the stretch reflex, measured using quantitative activation-induced manganese-enhanced magnetic resonance imaging. Neuronal activity increased within the ventral medullary reticular formation (MdV) in the contralesional brainstem during the acute post-stroke phase, and this increase was characterised by activation of circuits involved in spasticity. Interestingly, ascending electrostimulation inhibited the MdV activity on the stimulation side in normal conditions. Moreover, immunohistochemical staining showed that, in the acute phase, the density of GluA1, one of the α-amino-3 hydroxy‑5 methyl -4 isoxazolepropionic acid receptor (AMPAR) subunits, at the synapses of MdV neurons was significantly increased. In addition, the GluA1/GluA2 ratio in these receptors was altered at 2 weeks post-stroke, confirming homeostatic plasticity as the underlying mechanisms of spasticity. These results provide new insights into the relationship between impaired skilled movements and spasticity at the acute post-stroke phase.

Neuroimaging uncovers neuronal and metabolic changes in pain modulatory brain areas in a rat model of chemotherapy-induced neuropathy - MEMRI and ex vivo spectroscopy studies

Chemotherapy-induced neuropathy (CIN) is one of the most common complications of cancer treatment with sensory dysfunctions which frequently include pain. The mechanisms underlying pain during CIN are starting to be uncovered. Neuroimaging allows the identification of brain circuitry involved in pain processing and modulation and has recently been used to unravel the disruptions of that circuitry by neuropathic pain. The present study evaluates the effects of paclitaxel, a cytostatic drug frequently used in cancer treatment, at the neuronal function in the anterior cingulate cortex (ACC), hypothalamus and periaqueductal gray (PAG) using manganese-enhanced magnetic resonance imaging (MEMRI). We also studied the metabolic profile at the prefrontal cortex (PFC) and hypothalamus using ex vivo spectroscopy. Wistar male rats were intraperitoneal injected with paclitaxel or vehicle solution (DMSO). The evaluation of mechanical sensitivity using von Frey test at baseline (BL), 21 (T21), 28 (T28), 49 (T49) and 56 days (T56) after CIN induction showed that paclitaxel-injected rats presented mechanical hypersensitivity from T21 until T56 after CIN induction. The evaluation of the locomotor activity and exploratory behaviors using open-field test at T28 and T56 after the first injection of paclitaxel revealed that paclitaxel-injected rats walked higher distance with higher velocity at late point of CIN accompanied with a sustained exhibition of anxiety-like behaviors. Imaging studies performed using MEMRI at T28 and T56 showed that paclitaxel treatment increased the neuronal activation in the hypothalamus and PAG at T56 in comparison with the control group. The analysis of data from ex vivo spectroscopy demonstrated that at T28 paclitaxel-injected rats presented an increase of N-acetyl aspartate (NAA) levels in the PFC and an increase of NAA and decrease of lactate (Lac) concentration in the hypothalamus compared to the control group. Furthermore, at T56 the paclitaxel-injected rats presented lower NAA and higher taurine (Tau) levels in the PFC. Together, MEMRI and metabolomic data indicate that CIN is associated with neuroplastic changes in brain areas involved in pain modulation and suggests that other events involving glial cells may be happening.

A companion to the preclinical common data elements and case report forms for in vivo rodent neuroimaging: A report of the TASK3-WG3 Neuroimaging Working Group of the ILAE/AES Joint Translational Task Force

The International League Against Epilepsy/American Epilepsy Society (ILAE/AES) Joint Translational Task Force established the TASK3 working groups to create common data elements (CDEs) for various aspects of preclinical epilepsy research studies, which could help improve the standardization of experimental designs. In this article, we discuss CDEs for neuroimaging data that are collected in rodent models of epilepsy, with a focus on adult rats and mice. We provide detailed CDE tables and case report forms (CRFs), and with this companion manuscript, we discuss the methodologies for several imaging modalities and the parameters that can be collected.

Manganese-Labeled Alginate Hydrogels for Image-Guided Cell Transplantation

Cell transplantation has been studied extensively as a therapeutic strategy for neurological disorders. However, to date, its effectiveness remains unsatisfactory due to low precision and efficacy of cell delivery; poor survival of transplanted cells; and inadequate monitoring of their fate in vivo. Fortunately, different bio-scaffolds have been proposed as cell carriers to improve the accuracy of cell delivery, survival, differentiation, and controlled release of embedded stem cells. The goal of our study was to establish hydrogel scaffolds suitable for stem cell delivery that also allow non-invasive magnetic resonance imaging (MRI). We focused on alginate-based hydrogels due to their natural origin, biocompatibility, resemblance to the extracellular matrix, and easy manipulation of gelation processes. We optimized the properties of alginate-based hydrogels, turning them into suitable carriers for transplanted cells. Human adipose-derived stem cells embedded in these hydrogels survived for at least 14 days in vitro. Alginate-based hydrogels were also modified successfully to allow their injectability via a needle. Finally, supplementing alginate hydrogels with Mn ions or Mn nanoparticles allowed for their visualization in vivo using manganese-enhanced MRI. We demonstrated that modified alginate-based hydrogels can support therapeutic cells as MRI-detectable matrices.

Manganese Dynamics in Mouse Brain After Systemic MnCl2 Administration for Activation-Induced Manganese-Enhanced MRI

Activation-induced manganese-enhanced MRI (AIM-MRI) is an attractive tool for non-invasively mapping whole brain activities. Manganese ions (Mn²⁺) enter and accumulate in active neurons via calcium channels. Mn²⁺ shortens the longitudinal relaxation time (T1) of H⁺, and the longitudinal relaxation rate R1 (1/T1) is proportional to Mn²⁺ concentration. Thus, AIM-MRI can map neural activities throughout the brain by assessing the R1 map. However, AIM-MRI is still not widely used, partially due to insufficient information regarding Mn²⁺ dynamics in the brain. To resolve this issue, we conducted a longitudinal study looking at manganese dynamics after systemic administration of MnCl₂ by AIM-MRI with quantitative analysis. In the ventricle, Mn²⁺ increased rapidly within 1 h, remained high for 3 h, and returned to near control levels by 24 h after administration. Microdialysis showed that extracellular Mn returned to control levels by 4 h after administration, indicating a high concentration of extracellular Mn²⁺ lasts at least about 3 h after administration. In the brain parenchyma, Mn²⁺ increased slowly, peaked 24–48 h after administration, and returned to control level by 5 days after a single administration and by 2 weeks after a double administration with a 24-h interval. These time courses suggest that AIM-MRI records neural activity 1–3 h after MnCl₂ administration, an appropriate timing of the MRI scan is in the range of 24–48 h following systemic administration, and at least an interval of 5 days or a couple of weeks for single or double administrations, respectively, is needed for a repeat AIM-MRI experiment.

Manganese threonine chelate-a new enteric contrast agent for MRI: a pilot study on rats

Enteric contrast agents are important in gastrointestinal MRI. However, no currently available agent is well established as the standard of care. In this study, in vitro relaxivities of manganese threonine chelate (Mn-Thr), a common nutritional food supplement, were measured at 1.5 T and 3 T with further investigation of its efficacy and safety in vivo as an enteric contrast agent. According to the calculated relaxivities, T₁ W and T₂ W TSE sequences of Mn-Thr solutions at different concentrations were acquired, and the optimal concentration for dark lumen imaging on both T₁ W and T₂ W images was determined in vitro. To validate the optimal concentration in vivo, eight Sprague-Dawley rats were randomly divided into two groups. Each group received rectal injection of either 2.00 g/L (about 3.80 mM) Mn-Thr or saline as an enteric contrast agent and underwent MRI. After a time interval of one week, the same procedures were repeated with the alternative contrast agent. Animals were sacrificed after the second MRI. Tissue manganese quantification and histopathological examination were obtained. Qualitative MR image quality assessments were performed and compared between Mn-Thr and saline. Measured T₁ and T₂ relaxivities of Mn-Thr were significantly higher than those of MnCl₂ in vitro (p < 0.05). At the concentration of 2.00 g/L (about 3.80 mM), Mn-Thr produced a dark lumen on T₁ W and T₂ W images both in vitro and in vivo. Compared with saline, Mn-Thr showed significantly more homogenous luminal signal and increased bowel wall conspicuity in image quality assessments. Tissue manganese concentrations were not significantly different between two groups. Histopathological examinations were normal in both groups. Our data suggest that Mn-Thr possesses favorable paramagnetic properties and can create a homogenous dark lumen on T₁ W and T₂ W images without obvious side effects in healthy rats. As a commercially available nutritional food supplement, Mn-Thr appears to be a promising enteric contrast agent for MRI.

Deciphering functional brain circuitry during morphine withdrawal with dynamic manganese-enhanced MRI

Withdrawal plays a key role in the development of addiction, and several brain regions, such as the extended amygdala, are functional during this stage. Manganese-enhanced magnetic resonance imaging (MEMRI) is a promising method for directly studying neural activity during morphine withdrawal, versus functional MRI, which is based on hemodynamic alterations. The functional brain circuitry associated with morphine withdrawal has not been thoroughly investigated, and there are very few longitudinal studies utilizing MEMRI to explore brain activity during this stage. In our experiments, we revealed essential brain regions involved in morphine withdrawal by application of a novel dynamic MEMRI approach, and demonstrated dynamic alterations of functional brain activities in these associated brain regions. Our results demonstrate that the dynamic MEMRI approach is an effective method that may be applied to reveal dynamic alterations in functional brain activity during morphine withdrawal.

Visualization of Brain Activity in a Neuropathic Pain Model Using Quantitative Activity-Dependent Manganese Magnetic Resonance Imaging

Human brain imaging studies have revealed several regions that are activated in patients with chronic pain. In rodent brains, functional changes due to chronic pain have not been fully elucidated, as brain imaging techniques such as functional magnetic resonance imaging and positron emission tomography (PET) require the use of anesthesia to suppress movement. Consequently, conclusions derived from existing imaging studies in rodents may not accurately reflect brain activity under awake conditions. In this study, we used quantitative activation-induced manganese-enhanced magnetic resonance imaging to directly capture the previous brain activity of awake mice. We also observed and quantified the brain activity of the spared nerve injury (SNI) neuropathic pain model during awake conditions. SNI-operated mice exhibited a robust decrease of mechanical nociceptive threshold 14 days after nerve injury. Imaging on SNI-operated mice revealed increased neural activity in the limbic system and secondary somatosensory, sensory-motor, piriform, and insular cortex. We present the first study demonstrating a direct measurement of awake neural activity in a neuropathic pain mouse model.

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.

In Vivo Visualization of Active Polysynaptic Circuits With Longitudinal Manganese-Enhanced MRI (MEMRI)

Manganese-enhanced magnetic resonance imaging (MEMRI) is a powerful tool for in vivo non-invasive whole-brain mapping of neuronal activity. Mn²⁺ enters active neurons via voltage-gated calcium channels and increases local contrast in T₁-weighted images. Given the property of Mn²⁺ of axonal transport, this technique can also be used for tract tracing after local administration of the contrast agent. However, MEMRI is still not widely employed in basic research due to the lack of a complete description of the Mn²⁺ dynamics in the brain. Here, we sought to investigate how the activity state of neurons modulates interneuronal Mn²⁺ transport. To this end, we injected mice with low dose MnCl₂ 2. (i.p., 20 mg/kg; repeatedly for 8 days) followed by two MEMRI scans at an interval of 1 week without further MnCl₂ injections. We assessed changes in T₁ contrast intensity before (scan 1) and after (scan 2) partial sensory deprivation (unilateral whisker trimming), while keeping the animals in a sensory enriched environment. After correcting for the general decay in Mn²⁺ content, whole brain analysis revealed a single cluster with higher signal in scan 1 compared to scan 2: the left barrel cortex corresponding to the right untrimmed whiskers. In the inverse contrast (scan 2 > scan 1), a number of brain structures, including many efferents of the left barrel cortex were observed. These results suggest that continuous neuronal activity elicited by ongoing sensory stimulation accelerates Mn²⁺ transport from the uptake site to its projection terminals, while the blockage of sensory-input and the resulting decrease in neuronal activity attenuates Mn²⁺ transport. The description of this critical property of Mn²⁺ dynamics in the brain allows a better understanding of MEMRI functional mechanisms, which will lead to more carefully designed experiments and clearer interpretation of the results.

Manganese concentration mapping in the rat brain with MRI, PET, and autoradiography

PURPOSE

Mn²⁺ is used as a contrast agent and marker for neuronal activity with magnetic resonance imaging (MRI) in rats and mice, but its accumulation is generally not assessed quantitatively. In this work, nonradioactive Mn and ⁵² Mn are injected simultaneously in rats, and imaged with MRI, positron emission tomography (PET) and autoradiography (AR). Mn distributions are compared between modalities, to assess the potential and limitations on quantification of Mn with MRI, and to investigate the potential of multimodal measurement of Mn accumulation.

METHODS

MRI (in vivo), PET (in vivo and post mortem), and AR (ex vivo) were acquired of rat brains, for which animals received simultaneous intraperitoneal (IP) or intracerebrovertricular (ICV)-targeted injections containing the positron-emitting radionuclide ⁵² Mn and additional nonradioactive MnCl₂ , which acts as an MRI contrast agent. Pre and postinjection MR images were fit for the longitudinal relaxation rate (R1), coregistered, and subtracted to generate R1 difference maps, which are expected to be proportional to change in Mn concentration in tissue. AR and PET images were coregistered to smoothed R1 difference maps.

RESULTS

Similar spatial distributions were seen across modalities, with Mn accumulation in the colliculus, near the injection site, and in the 4th ventricle. There was no ⁵² Mn accumulation measurable with PET in the brain after IP injection. In areas of very highly localized and concentrated ⁵² Mn accumulation in PET or AR, consistent increases of R1 were not seen with MRI. Scatter plots of corresponding voxel R1 difference and PET or AR signal intensity were generated and fit with least squares linear models within anatomical regions. Linear correlations were observed, particularly in regions away from very highly localized and concentrated Mn accumulation at the injection site and the 4th ventricle. Accounting for radioactive decay of ⁵² Mn, the MnCl₂ longitudinal relaxivity was between 4.0 and 5.1 s^-1 /mM, which is within 22% of the in vitro relaxivity.

CONCLUSIONS

This proof-of-concept study demonstrates that MR has strong potential for quantitative assessment of Mn accumulation in the brain, although local discrepancies from linear correlation suggest limitations to this use of MR in areas of inflammation or very high concentrations of Mn. These discrepancies also suggest that a combination of modalities may have additional utility for discriminating between different pools of Mn accumulation in tissue.

Redox dynamics of manganese as a mitochondrial life-death switch

Sten Orrenius, M.D., Ph.D., pioneered many areas of cellular and molecular toxicology and made seminal contributions to our knowledge of oxidative stress and glutathione (GSH) metabolism, organellar functions and Ca+2-dependent mechanisms of cell death, and mechanisms of apoptosis. On the occasion of his 80th birthday, we summarize current knowledge on redox biology of manganese (Mn) and its role in mechanisms of cell death. Mn is found in all organisms and has critical roles in cell survival and death mechanisms by regulating Mn-containing enzymes such as manganese superoxide dismutase (SOD2) or affecting expression and activity of caspases. Occupational exposures to Mn cause "manganism", a Parkinson's disease-like condition of neurotoxicity, and experimental studies show that Mn exposure leads to accumulation of Mn in the brain, especially in mitochondria, and neuronal cell death occurs with features of an apoptotic mechanism. Interesting questions are why a ubiquitous metal that is essential for mitochondrial function would accumulate to excessive levels, cause increased H2O2 production and lead to cell death. Is this due to the interactions of Mn with other essential metals, such as iron, or with toxic metals, such as cadmium? Why is the Mn loading in the human brain so variable, and why is there such a narrow window between dietary adequacy and toxicity? Are non-neuronal tissues similarly vulnerable to insufficiency and excess, yet not characterized? We conclude that Mn is an important component of the redox interface between an organism and its environment and warrants detailed studies to understand the role of Mn as a mitochondrial life-death switch.

Identifying non-toxic doses of manganese for manganese-enhanced magnetic resonance imaging to map brain areas activated by operant behavior in trained rats

Manganese-enhanced magnetic resonance imaging (MEMRI) offers unique advantages such as studying brain activation in freely moving rats, but its usefulness has not been previously evaluated during operant behavior training. Manganese in a form of MnCl₂, at a dose of 20mg/kg, was intraperitoneally infused. The administration was repeated and separated by 24h to reach the dose of 40mg/kg or 60mg/kg, respectively. Hepatotoxicity of the MnCl₂ was evaluated by determining serum aspartate aminotransferase, alanine aminotransferase, total bilirubin, albumin and protein levels. Neurological examination was also carried out. The animals were tested in visual cue discriminated operant task. Imaging was performed using a 3T clinical MR scanner. T1 values were determined before and after MnCl₂ administrations. Manganese-enhanced images of each animal were subtracted from their baseline images to calculate decrease in the T1 value (ΔT1) voxel by voxel. The subtracted T1 maps of trained animals performing visual cue discriminated operant task, and those of naive rats were compared. The dose of 60mg/kg MnCl₂ showed hepatotoxic effect, but even these animals did not exhibit neurological symptoms. The dose of 20 and 40mg/kg MnCl₂ increased the number of omissions and did not affect the accuracy of performing the visual cue discriminated operant task. Using the accumulated dose of 40mg/kg, voxels with a significant enhanced ΔT1 value were detected in the following brain areas of the visual cue discriminated operant behavior performed animals compared to those in the controls: the visual, somatosensory, motor and premotor cortices, the insula, cingulate, ectorhinal, entorhinal, perirhinal and piriform cortices, hippocampus, amygdala with amygdalohippocampal areas, dorsal striatum, nucleus accumbens core, substantia nigra, and retrorubral field. In conclusion, the MEMRI proved to be a reliable method to accomplish brain activity mapping in correlation with the operant behavior of freely moving rodents.

Ceftriaxone prevents the neurodegeneration and decreased neurogenesis seen in a Parkinson's disease rat model: An immunohistochemical and MRI study

Manganese-enhanced magnetic resonance imaging (MEMRI) is a widely used technique for detecting neuronal activity in the brain of a living animal. Ceftriaxone (CEF) has been shown to have neuroprotective effects in neurodegenerative diseases. The present study was aimed at clarifying whether, in an 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinson's disease (PD) rat model, the known CEF-induced neuronal protection was accompanied by neurogenesis and decreased loss of neuronal activity. After MPTP lesioning (day 0), the rats were treated with CEF (100mg/kg/day, i.p.) or saline for 15 days. They were then injected with MnCl2 (40mg/kg, i.p.) on day 13 and underwent a brain MRI scan on day 14, then the brain was taken for histological evaluation on day 15. The results showed that MPTP lesioning resulted in decreased neuronal activity and density in the nigrostriatal dopaminergic (DAergic) system and the hippocampal CA1, CA3, and dentate gyrus (DG) areas and reduced neurogenesis in the DG, but in hyperactivity in the subthalamic nucleus (STN). These neuronal changes were prevented by CEF treatment. Positive correlations between MEMRI R1 values and neuronal density in the hippocampus were evidenced. Neuronal densities in the hippocampus and SNc were positively correlated. In addition, the R1 value of the STN showed a positive correlation with its neuronal activity but showed a negative correlation with the density of DAergic neurons in the SNc. Therefore, MEMRI R1 value may serve as a good indicator for PD severity and the effect of treatment. To our knowledge, this is the first study showing that CEF prevents loss of neuronal activity and neurogenesis in the brain of PD rats. CEF may therefore have clinical potential in the treatment of PD.

Quantitative activation-induced manganese-enhanced MRI reveals severity of Parkinson's disease in mice

We demonstrate that activation-induced manganese-enhanced magnetic resonance imaging with quantitative determination of the longitudinal relaxation time (qAIM-MRI) reveals the severity of Parkinson's disease (PD) in mice. We first show that manganese ion-accumulation depends on neuronal activity. A highly active region was then observed by qAIM-MRI in the caudate-putamen in PD-model mice that was significantly correlated to the severity of PD, suggesting its involvement in the expression of PD symptoms.

Functional Magnetic Resonance Imaging of Rats with Experimental Autoimmune Encephalomyelitis Reveals Brain Cortex Remodeling

Cortical reorganization occurring in multiple sclerosis (MS) patients is thought to play a key role in limiting the effect of structural tissue damage. Conversely, its exhaustion may contribute to the irreversible disability that accumulates with disease progression. Several aspects of MS-related cortical reorganization, including the overall functional effect and likely modulation by therapies, still remain to be elucidated. The aim of this work was to assess the extent of functional cortical reorganization and its brain structural/pathological correlates in Dark Agouti rats with experimental autoimmune encephalomyelitis (EAE), a widely accepted preclinical model of chronic MS. Morphological and functional MRI (fMRI) were performed before disease induction and during the relapsing and chronic phases of EAE. During somatosensory stimulation of the right forepaw, fMRI demonstrated that cortical reorganization occurs in both relapsing and chronic phases of EAE with increased activated volume and decreased laterality index versus baseline values. Voxel-based morphometry demonstrated gray matter (GM) atrophy in the cerebral cortex, and both GM and white matter atrophy were assessed by ex vivo pathology of the sensorimotor cortex and corpus callosum. Neuroinflammation persisted in the relapsing and chronic phases, with dendritic spine density in the layer IV sensory neurons inversely correlating with the number of cluster of differentiation 45-positive inflammatory lesions. Our work provides an innovative experimental platform that may be pivotal for the comprehension of key mechanisms responsible for the accumulation of irreversible brain damage and for the development of innovative therapies to reduce disability in EAE/MS.

SIGNIFICANCE STATEMENT

Since the early 2000s, functional MRI (fMRI) has demonstrated profound modifications in the recruitment of cortical areas during motor, cognitive, and sensory tasks in multiple sclerosis (MS) patients. Experimental autoimmune encephalomyelitis (EAE) represents a reliable model of the chronic-progressive variant of MS. fMRI studies in EAE have not been performed extensively up to now. This paper reports fMRI studies in a rat model of MS with somatosensory stimulation of the forepaw. We demonstrated modifications in the recruitment of cortical areas consistent with data from MS patients. To the best of our knowledge, this is the first report of cortical remodeling in a preclinical in vivo model of MS.

Manganese-52 positron emission tomography tracer characterization and initial results in phantoms and in vivo

PURPOSE

Manganese(II) is employed as a contrast agent with magnetic resonance imaging (MRI) for study of neuronal activation in rats and mice. However, at the concentrations required for MRI, Mn may induce pharmacological or toxic effects. Positron emission tomography (PET) imaging of (52)MnCl2 at tracer doses has the potential to allow similar Mn studies as manganese-enhanced MRI while providing quantitative results and avoiding toxic effects. In this work, (52)MnCl2 is produced and characterized as a PET tracer in phantoms and in rats.

METHODS

(52)MnCl2 was produced by proton irradiation of natural Cr foil and separated by column chromatography. Images were acquired on a Siemens Focus 120 small animal PET scanner. Phantom images were acquired to assess uniformity, resolution, cascade background correction, and count rate linearity. Images of rats were also acquired after systemic and intracerebroventricular (ICV) administration of (52)MnCl2 to investigate Mn(II) distribution in vivo.

RESULTS

Irradiation yield was 74.6 ± 8.5 kBq/μA min (52)Mn at end of bombardment with initial specific activity of at least 3.5 MBq/nmol. (52)Mn PET images show similar uniformity and resolution to (18)F. (18)F based detector efficiency normalization is adequate for (52)Mn imaging. Subtraction of a rescaled random events distribution from sinogram data is effective for cascade correction of (52)Mn PET data. After systemic injection, (52)Mn appears in structures throughout the body of rats, including bones, liver, intestines, and the pituitary gland, but does not appear detectably throughout the brain. After ICV injection, (52)Mn remains in the brain and spinal cord.

CONCLUSIONS

(52)Mn is a promising tracer for small animal PET imaging, yielding image quality comparable to (18)F. Potential applications include studies similar to Mn-enhanced neuronal MRI, and in other organ systems including bones, spinal cord, and the digestive tract.

Manganese-enhanced MRI of rat brain based on slow cerebral delivery of manganese(II) with silica-encapsulated Mn x Fe(1-x) O nanoparticles

In this work, we report a monodisperse bifunctional nanoparticle system, MIO@SiO2 -RITC, as an MRI contrast agent [core, manganese iron oxide (MIO); shell, amorphous silica conjugated with rhodamine B isothiocyanate (RITC)]. It was prepared by thermal decomposition and modified microemulsion methods. The nanoparticles with varying iron to manganese ratios displayed different saturated magnetizations and relaxivities. In vivo MRI of rats injected intravenously with MIO@SiO2-RITC nanoparticles exhibited enhancement of the T1 contrast in brain tissue, in particular a time-delayed enhancement in the hippocampus, pituitary gland, striatum and cerebellum. This is attributable to the gradual degradation of MIO@SiO2-RITC nanoparticles in the liver, resulting in the slow release of manganese(II) [Mn(II)] into the blood pool and, subsequently, accumulation in the brain tissue. Thus, T1-weighted contrast enhancement was clearly detected in the anatomic structure of the brain as time progressed. In addition, T2*-weighted images of the liver showed a gradual darkening effect. Here, we demonstrate the concept of the slow release of Mn(II) for neuroimaging. This new nanoparticle-based manganese contrast agent allows one simple intravenous injection (rather than multiple infusions) of Mn(II) precursor, and results in delineation of the detailed anatomic neuroarchitecture in MRI; hence, this provides the advantage of the long-term study of neural function.

Three-dimensional reconstruction of brain structures of the rodent Octodon degus: a brain atlas constructed by combining histological and magnetic resonance images

Degus (Octodon degus) are rodents that are becoming more widely used in the neuroscience field. Degus display several more complex behaviors than rats and mice, including complicated social behaviors, vocal communications, and tool usage with superb manual dexterity. However, relatively little information is known about the anatomy of degu brains. Therefore, for these complex behaviors to be correlated with specific brain regions, a contemporary atlas of the degu brain is required. This manuscript describes the construction of a three-dimensional (3D) volume rendered model of the degu brain that combines histological and magnetic resonance images. This atlas provides several advantages, including the ability to visualize the surface of the brain from any angle. The atlas also permits virtual cutting of brain sections in any plane and provides stereotaxic coordinates for all sections, to be beneficial for both experimental surgeries and radiological studies. The reconstructed 3D atlas is freely available online at: http://brainatlas.brain.riken.jp/degu/modules/xoonips/listitem.php?index_id=24 .

3D printing of rat salivary glands: The submandibular-sublingual complex

The morphology and the functionality of the murid glandular complex, composed of the submandibular and sublingual salivary glands (SSC), were the object of several studies conducted mainly using magnetic resonance imaging (MRI). Using a 4.7 T scanner and a manganese-based contrast agent, we improved the signal-to-noise ratio of the SSC relating to the surrounding anatomical structures allowing to obtain high-contrast 3D images of the SSC. In the last few years, the large development in resin melting techniques opened the way for printing 3D objects starting from a 3D stack of images. Here, we demonstrate the feasibility of the 3D printing technique of soft tissues such as the SSC in the rat with the aim to improve the visualization of the organs. This approach is useful to preserve the real in vivo morphology of the SCC in living animals avoiding the anatomical shape changes due to the lack of relationships with the surrounding organs in case of extraction. It is also harmless, repeatable and can be applied to explore volumetric changes occurring during body growth, excretory duct obstruction, tumorigenesis and regeneration processes. 3D printing allows to obtain a solid object with the same shape of the organ of interest, which can be observed, freely rotated and manipulated. To increase the visibility of the details, it is possible to print the organs with a selected zoom factor, useful as in case of tiny organs in small mammalia. An immediate application of this technique is represented by educational classes.

Fast cardiac T1 mapping in mice using a model-based compressed sensing method

Direct measurement of the longitudinal relaxation time T1 provides objective and quantitative diagnostic information. However, current T1 mapping methods are generally time consuming without the aid of fast imaging. This study used a model-based compressed sensing method for fast cardiac T1 mapping in small animals. Based on the physics of magnetization recovery, the aliasing artifact associated with under-sampling was removed by exploiting the sparsity of the signals in the T1 recovery direction. Simulation study was performed to evaluate the reconstruction accuracy under various experimental conditions. Several approaches that accounted for phase variations were compared for optimized reconstruction in the phantom study. In vivo validation was performed on a cardiac manganese-enhanced MRI study using mice. Accurate reconstruction of the under-sampled images and the resulting T1 maps were achieved in both simulation and MRI studies on phantom and in vivo mice. These results suggest that the current compressed sensing method allows fast (<80 s) T1 mapping of the mouse heart at high spatial resolution (234×469 μm2).

Biocompatible and pH-sensitive PLGA encapsulated MnO nanocrystals for molecular and cellular MRI

Inorganic manganese-based particles are becoming attractive for molecular and cellular imaging, due to their ability to provide bright contrast on MRI, as opposed to the dark contrast generated from iron-based particles. Using a single emulsion technique, we have successfully fabricated pH-sensitive poly(lactic-co-glycolic acid) (PLGA)-encapsulated manganese oxide (MnO) nanocrystals. Two classes of particles were fabricated at ∼140 nm and 1.7 μm and incorporated 15 to 20 nm MnO nanocrystals with high encapsulation efficiencies. Intact particles at physiological pH cause little contrast in MRI, but following endocytosis into low pH compartments within the cells, the particles erode and MnO dissolves to release Mn(2+). This causes the cells to appear bright on MR images. The magnitude of the change in MRI properties is as high as 35-fold, making it the most dynamic "smart" MRI contrast agent yet reported. Possible applications of these MnO particles include slow release Mn(2+), tumor targeting, and confirmation of cell uptake.

Manganese-enhanced magnetic resonance imaging (MEMRI) of rat brain after systemic administration of MnCl₂: hippocampal signal enhancement without disruption of hippocampus-dependent behavior

Manganese (Mn(2+))-enhanced magnetic resonance (MR) imaging (MEMRI) in rodents offers unique opportunities for the longitudinal study of hippocampal structure and function in parallel with cognitive testing. However, Mn(2+) is a potent toxin and there is evidence that it can interfere with neuronal function. Thus, apart from causing adverse peripheral side effects, Mn(2+) may disrupt the function of brain areas where it accumulates to produce signal enhancement and, thereby, Mn(2+) administration may confound cognitive testing. Here, we examined in male adult Lister hooded rats if a moderate systemic dose of MnCl₂ (200 μmol/kg; two intraperitoneal injections of 100 μmol/kg separated by 1 h) that produces hippocampal MR signal enhancement would disrupt hippocampal function. To this end, we used a delayed-matching-to-place (DMP) watermaze task, which requires rapid allocentric place learning and is highly sensitive to interference with hippocampal function. Tested on the DMP task 1 h and 24 h after MnCl₂ injection, rats did not show any impairment in indices of memory performance (latencies, search preference) or any sensorimotor effects. However, MnCl₂ injection caused acute peripheral effects (severe ataxia and erythema, i.e. redness of paws, ears, and nose) which subsided over 30 min. Additionally, rats injected with MnCl₂ showed reduced weight 1 day after injection and failed to reach the normal weight-growth curve of control rats within the 16 days monitored. Our results indicate that 200 μmol/kg MnCl₂ produces hippocampal MR signal enhancement without disrupting hippocampus-dependent behavior on a rapid place learning task, even though attention must be paid to peripheral side effects.