Manganese-enhanced magnetic resonance imaging (MEMRI) of brain activity and applications to early detection of brain ischemia

A primer on in vivo cell tracking using MRI

Cell tracking by in vivo magnetic resonance imaging (MRI) offers a collection of multiple advantages over other imaging modalities, including high spatial resolution, unlimited depth penetration, 3D visualization, lack of ionizing radiation, and the potential for long-term cell monitoring. Three decades of innovation in both contrast agent chemistry and imaging physics have built an expansive array of probes and methods to track cells non-invasively across a diverse range of applications. In this review, we describe both established and emerging MRI cell tracking approaches and the variety of mechanisms available for contrast generation. Emphasis is given to the advantages, practical limitations, and persistent challenges of each approach, incorporating quantitative comparisons where possible. Toward the end of this review, we take a deeper dive into three key application areas - tracking cancer metastasis, immunotherapy for cancer, and stem cell regeneration - and discuss the cell tracking techniques most suitable to each.

Automatic method for individual parcellation of manganese-enhanced magnetic resonance imaging of rat brain

Aims

To construct an automatic method for individual parcellation of manganese-enhanced magnetic resonance imaging (MEMRI) of rat brain with high accuracy, which could preserve the inherent voxel intensity and Regions of interest (ROI) morphological characteristics simultaneously.

Methods and results

The transformation relationship from standardized space to individual space was obtained by firstly normalizing individual image to the Paxinos space and then inversely transformed. On the other hand, all the regions defined in the atlas image were separated and resaved as binary mask images. Then, transforming the mask images into individual space via the inverse transformations and reslicing using the 4th B-spline interpolation algorithm. The boundary of these transformed regions was further refined by image erosion and expansion operator, and finally combined together to generate the individual parcellations. Moreover, two groups of MEMRI images were used for evaluation. We found that the individual parcellations were satisfied, and the inherent image intensity was preserved. The statistical significance of case-control comparisons was further optimized.

Conclusions

We have constructed a new automatic method for individual parcellation of rat brain MEMRI images, which could preserve the inherent voxel intensity and further be beneficial in case-control statistical analyses. This method could also be extended to other imaging modalities, even other experiments species. It would facilitate the accuracy and significance of ROI-based imaging analyses.

Advanced high resolution three-dimensional imaging to visualize the cerebral neurovascular network in stroke

As an important method to accurately and timely diagnose stroke and study physiological characteristics and pathological mechanism in it, imaging technology has gone through more than a century of iteration. The interaction of cells densely packed in the brain is three-dimensional (3D), but the flat images brought by traditional visualization methods show only a few cells and ignore connections outside the slices. The increased resolution allows for a more microscopic and underlying view. Today's intuitive 3D imagings of micron or even nanometer scale are showing its essentiality in stroke. In recent years, 3D imaging technology has gained rapid development. With the overhaul of imaging mediums and the innovation of imaging mode, the resolution has been significantly improved, endowing researchers with the capability of holistic observation of a large volume, real-time monitoring of tiny voxels, and quantitative measurement of spatial parameters. In this review, we will summarize the current methods of high-resolution 3D imaging applied in stroke.

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-enhanced MRI depicts a reduction in brain responses to nociception upon mTOR inhibition in chronic pain rats

Neuropathic pain induced by a nerve injury can lead to chronic pain. Recent studies have reported hyperactive neural activities in the nociceptive-related area of the brain as a result of chronic pain. Although cerebral activities associated with hyperalgesia and allodynia in chronic pain models are difficult to represent with functional imaging techniques, advances in manganese (Mn)-enhanced magnetic resonance imaging (MEMRI) could facilitate the visualization of the activation of pain-specific neural responses in the cerebral cortex. In order to investigate the alleviation of pain nociception by mammalian target of rapamycin (mTOR) modulation, we observed cerebrocortical excitability changes and compared regional Mn²⁺ enhancement after mTOR inhibition. At day 7 after nerve injury, drugs were applied into the intracortical area, and drug (Vehicle, Torin1, and XL388) effects were compared within groups using MEMRI. Therein, signal intensities of the insular cortex (IC), primary somatosensory cortex of the hind limb region, motor cortex 1/2, and anterior cingulate cortex regions were significantly reduced after application of mTOR inhibitors (Torin1 and XL388). Furthermore, rostral-caudal analysis of the IC indicated that the rostral region of the IC was more strongly associated with pain perception than the caudal region. Our data suggest that MEMRI can depict pain-related signal changes in the brain and that mTOR inhibition is closely correlated with pain modulation in chronic pain rats.

Adult brain activation in response to pain is changed by neonatal painful stimulation according to sex: A manganese-enhanced MRI study

Although it is known that nociceptive stimulation in the first postnatal week in rats is useful to model preterm pain, resulting in activation of specific brain areas, as assessed in vivo using manganese-enhanced magnetic resonance imaging (MEMRI), little is known about its long-term effects and sex specificity. Here we aimed to investigate whether inflammatory pain induced in male and female adult rats modify the pattern of brain activation between animals subjected or not to neonatal pain. For this, Complete Freund's adjuvant (CFA) was injected into the left hind paw of rat pups on postnatal day 1 (P1) or P8 to induce inflammatory response. During adulthood, CFA-treated and control animals were injected with CFA 1 hr prior MRI. MEMRI has the ability to enhance the contrast of selective brain structures in response to a specific stimulus, as the pain. MEMRI responses were consistent with activation of nociceptive pathways and these responses were reduced in animals treated with CFA on P1, but increased in animals treated on P8, mainly in the female group. In agreement, P8 female group showed exacerbated responses in the thermal nociceptive test. Using MEMRI, we conclude that the natural ability of adult rats to recognize and react to pain exposition is modified by neonatal painful exposition, mainly among females.

Mechanism of Manganese Dysregulation of Dopamine Neuronal Activity

Manganese exposure produces Parkinson's-like neurologic symptoms, suggesting a selective dysregulation of dopamine transmission. It is unknown, however, how manganese accumulates in dopaminergic brain regions or how it regulates the activity of dopamine neurons. Our in vivo studies in male C57BLJ mice suggest that manganese accumulates in dopamine neurons of the VTA and substantia nigra via nifedipine-sensitive Ca²⁺ channels. Manganese produces a Ca²⁺ channel-mediated current, which increases neurotransmitter release and rhythmic firing activity of dopamine neurons. These increases are prevented by blockade of Ca²⁺ channels and depend on downstream recruitment of Ca²⁺-activated potassium channels to the plasma membrane. These findings demonstrate the mechanism of manganese-induced dysfunction of dopamine neurons, and reveal a potential therapeutic target to attenuate manganese-induced impairment of dopamine transmission.SIGNIFICANCE STATEMENT Manganese is a trace element critical to many physiological processes. Overexposure to manganese is an environmental risk factor for neurologic disorders, such as a Parkinson's disease-like syndrome known as manganism. We found that manganese concentration-dependently increased the excitability of dopamine neurons, decreased the amplitude of action potentials, and narrowed action potential width. Blockade of Ca²⁺ channels prevented these effects as well as manganese accumulation in the mouse midbrain in vivo Our data provide a potential mechanism for manganese regulation of dopaminergic neurons.

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.

Manganese-Enhanced MRI of the Brain in Healthy Volunteers

BACKGROUND AND PURPOSE

The manganese ion is used as an intracellular MR imaging contrast agent to study neuronal function in animal models, but it remains unclear whether manganese-enhanced MR imaging can be similarly useful in humans. Using mangafodipir (Teslascan, a chelated manganese-based contrast agent that is FDA-approved), we evaluated the dynamics of manganese enhancement of the brain and glandular structures in the rostral head and neck in healthy volunteers.

MATERIALS AND METHODS

We administered mangafodipir intravenously at a rate of 1 mL/minute for a total dose of 5 μmol/kg body weight. Nine healthy adult volunteers (6 men/3 women; median age, 43 years) completed baseline history and physical examination, 3T MR imaging, and blood work. MR imaging also followed mangafodipir administration at various time points from immediate to 7 days, with delayed scans at 1-3 months.

RESULTS

The choroid plexus and anterior pituitary gland enhanced within 10 minutes of infusion, with enhancement persisting up to 7 and 30 days, respectively. Exocrine (parotid, submandibular, sublingual, and lacrimal) glands also enhanced avidly as early as 1 hour postadministration, generally resolving by 1 month; 3 volunteers had residual exocrine gland enhancement, which resolved by 2 months in 1 and by 3 months in the other 2. Mangafodipir did not affect clinical parameters, laboratory values, or T1-weighted signal in the basal ganglia.

CONCLUSIONS

Manganese ions released from mangafodipir successfully enable noninvasive visualization of intra- and extracranial structures that lie outside the blood-brain barrier without adverse clinical effects, setting the stage for future neuroradiologic investigation in disease.

Activity-induced MEMRI cannot detect functional brain anomalies in the APPxPS1-Ki mouse model of Alzheimer's disease

Alzheimer’s disease (AD) is the most common cause of dementia. Aside neuropathological lesions, abnormal neuronal activity and brain metabolism are part of the core symptoms of the disease. Activity-induced Manganese-Enhanced Magnetic Resonance Imaging (MEMRI) has been proposed as a powerful approach to visualize evoked brain activity in rodents. Here, we evaluated the relevance of MEMRI in measuring neuronal (dys-)function in the APPxPS1 knocked-in (KI) mouse model of AD. Brain anomalies were firstly demonstrated in APPxPS1-Ki mice using cognitive testing (memory impairment) and histological mapping of immediate early gene products (decreased density of fos-positive neurons). Paradoxically, MEMRI analyses were not able to confirm the occurrence of neuronal hypoactivities in vivo. We then performed a neuropathological analysis that highlighted an abnormal increased permeability of the blood-brain barrier (BBB) in APPxPS1-Ki mice. We hypothesized that diffuse weakening of the BBB results in an uncontrolled diffusion of the MR contrast agent and a lack of correlation between manganese accumulation and neuronal activity. These results bring to light a limitation of the activity-induced MEMRI approach when applied to the APPxPS1-Ki mouse model as well as other mouse models harboring a compromised BBB.

Focal brain lesions induced with ultraviolet irradiation

Lesion and inactivation methods have played important roles in neuroscience studies. However, traditional techniques for creating a brain lesion are highly invasive, and control of lesion size and shape using these techniques is not easy. Here, we developed a novel method for creating a lesion on the cortical surface via 365 nm ultraviolet (UV) irradiation without breaking the dura mater. We demonstrated that 2.0 mWh UV irradiation, but not the same amount of non-UV light irradiation, induced an inverted bell-shaped lesion with neuronal loss and accumulation of glial cells. Moreover, the volume of the UV irradiation-induced lesion depended on the UV light exposure amount. We further succeeded in visualizing the lesioned site in a living animal using magnetic resonance imaging (MRI). Importantly, we also observed using an optical imaging technique that the spread of neural activation evoked by adjacent cortical stimulation disappeared only at the UV-irradiated site. In summary, UV irradiation can induce a focal brain lesion with a stable shape and size in a less invasive manner than traditional lesioning methods. This method is applicable to not only neuroscientific lesion experiments but also studies of the focal brain injury recovery process.

Development of manganese-enhanced magnetic resonance imaging of the rostral ventrolateral medulla of conscious rats: Importance of normalization and comparison with other regions of interest

Spinally projecting neurons in the rostral ventrolateral medulla (RVLM) are believed to contribute to pathophysiological alterations in sympathetic nerve activity and the development of cardiovascular disease. The ability to identify changes in the activity of RVLM neurons in conscious animals and humans, especially longitudinally, would represent a clinically important advancement in our understanding of the contribution of the RVLM to cardiovascular disease. To this end, we describe the initial development of manganese-enhanced magnetic resonance imaging (MEMRI) for the rat RVLM. Manganese (Mn²⁺ ) has been used to estimate in vivo neuronal activity in other brain regions because of both its paramagnetic properties and its entry into and accumulation in active neurons. In this initial study, our three goals were as follows: (1) to validate that Mn²⁺ enhancement occurs in functionally and anatomically localized images of the rat RVLM; (2) to quantify the dose and time course dependence of Mn²⁺ enhancement in the RVLM after one systemic injection in conscious rats (66 or 33 mg/kg, intraperitoneally); and (3) to compare Mn²⁺ enhancement in the RVLM with other regions to determine an appropriate method of normalization of T₁ -weighted images. In our proof-of-concept and proof-of-principle studies, Mn²⁺ was identified by MRI in the rat RVLM after direct microinjection or via retrograde transport following spinal cord injections, respectively. Systemic injections in conscious rats produced significant Mn²⁺ enhancement at 24 h (p < 0.05). Injections of 66 mg/kg produced greater enhancement than 33 mg/kg in the RVLM and paraventricular nucleus of the hypothalamus (p < 0.05 for both), but only when normalized to baseline scans without Mn²⁺ injection. Consistent with findings from our previous functional and anatomical studies demonstrating subregional neuroplasticity, Mn²⁺ enhancement was higher in the rostral regions of the RVLM (p < 0.05). Together with important technical considerations, our studies support the development of MEMRI as a potential method to examine RVLM activity over time in conscious animal subjects.

Transcranial manganese delivery for neuronal tract tracing using MEMRI

There has been a growing interest in the use of manganese-enhanced MRI (MEMRI) for neuronal tract tracing in mammals, especially in rodents. For this MEMRI application, manganese solutions are usually directly injected into specific brain regions. Recently it was reported that manganese ions can diffuse through intact rat skull. Here the local manganese concentrations in the brain tissue after transcranial manganese application were quantified and the effectiveness of tracing from the area under the skull where delivery occurred was determined. It was established that transcranially applied manganese yields brain tissue enhancement dependent on the location of application on the skull and that manganese that enters the brain transcranially can trace to deeper brain areas.

Modification in CSF specific gravity in acutely decompensated cirrhosis and acute on chronic liver failure independent of encephalopathy, evidences for an early blood-CSF barrier dysfunction in cirrhosis

Although hepatic encephalopathy (HE) on the background of acute on chronic liver failure (ACLF) is associated with high mortality rates, it is unknown whether this is due to increased blood-brain barrier permeability. Specific gravity of cerebrospinal fluid measured by CT is able to estimate blood-cerebrospinal fluid-barrier permeability. This study aimed to assess cerebrospinal fluid specific gravity in acutely decompensated cirrhosis and to compare it in patients with or without ACLF and with or without hepatic encephalopathy. We identified all the patients admitted for acute decompensation of cirrhosis who underwent a brain CT-scan. Those patients could present acute decompensation with or without ACLF. The presence of hepatic encephalopathy was noted. They were compared to a group of stable cirrhotic patients and healthy controls. Quantitative brain CT analysis used the Brainview software that gives the weight, the volume and the specific gravity of each determined brain regions. Results are given as median and interquartile ranges and as relative variation compared to the control/baseline group. 36 patients presented an acute decompensation of cirrhosis. Among them, 25 presented with ACLF and 11 without ACLF; 20 presented with hepatic encephalopathy grade ≥ 2. They were compared to 31 stable cirrhosis patients and 61 healthy controls. Cirrhotic patients had increased cerebrospinal fluid specific gravity (CSF-SG) compared to healthy controls (+0.4 %, p < 0.0001). Cirrhotic patients with ACLF have decreased CSF-SG as compared to cirrhotic patients without ACLF (-0.2 %, p = 0.0030) that remained higher than in healthy controls. The presence of hepatic encephalopathy did not modify CSF-SG (-0.09 %, p = 0.1757). Specific gravity did not differ between different brain regions according to the presence or absence of either ACLF or HE. In patients with acute decompensation of cirrhosis, and those with ACLF, CSF specific gravity is modified compared to both stable cirrhotic patients and healthy controls. This pattern is observed even in the absence of hepatic encephalopathy suggesting that blood-CSF barrier impairment is manifest even in absence of overt hepatic encephalopathy.

Multi-system state shifts and cognitive deficits induced by chronic morphine during abstinence

Chronic morphine administration induces neural plasticity followed by withdraw. And clinic observation indicates that obvious cognitive deficits are found during withdrawal. However, current neural substrates that regulate dysfunction in withdrawal are unknown. In our studies, chronic morphine administration was used to induce the spontaneous withdrawal model in rats. A series of cognitive abilities was tested to explore brain function. To further evaluate the neural substrates of dysfunction, Manganese-enhanced MRI(MEMRI) was used to map the dysfunctional regions in vivo.We observed that chronic morphine administration could induce obvious withdrawal behaviors in abstinence followed by cognitive impairments, such as impairments in working memory, reward, interaction and enhancement of anxiety. Our in-vivo MEMRI data using the voxel-wise comparisons showed that the manganese-enhanced signal intensity (VMI) within morphine withdrawal groups was increased in cingulate cortex (Cg), secondary motor cortex (M2), CA3 subfield of hippocampus, dorsal striatum (D-striatum), retrosplenial cortex (RS), shell subregion of NAc (AcbSh), core subregion of NAc (AcbC), central nucleus of amygdala (CeC), basolateral amygdaloid nucleus (BLA), central amygdaloid nucleus (CeM), anterior hypothalamic area, central (AHC), ventral tegmental area (VTA) and scaphoid thalamic nucleus (SC).However, decreasing of VMI was found in the ventrolateral striatum (V-striatum) and lateral posterior thalamic nucleus (LP) compared to the control group. These brain regions were beleived to be components of the memory, executive, limbic and regulatory systems. Therefore, our present studies indicate that withdrawal induced by chronic morphine adiministration could disturb brain function leading to multi-systems state shifts and cognitive deficits in abstinence.

Hollow manganese oxide nanoparticle-enhanced MRI of hypoxic-ischaemic brain injury in the neonatal rat

OBJECTIVE

To determine the utility of hollow manganese oxide nanoparticle (HMON)-enhanced MRI in depicting and monitoring apoptotic area following hypoxic-ischaemic injury in a neonatal rat brain and to evaluate the longitudinal evolution of hypoxic-ischaemic brain injury (HII) up to 21 days.

METHODS

The institutional animal care and use committee approval was obtained. The Rice-Vannucci model of HII was used in 7-day-old rat pups (n = 17). MRI was performed 1, 3, 7, 14 and 21 days after HII with intraperitoneal injection of HMON. Relative contrast values in the injured hemisphere and mean apparent diffusion coefficient values were calculated at each time point. Apoptosis and reactive astrogliosis were detected by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labelling (TUNEL) and glial fibrillary acidic protein staining, and the distribution and intensity of immunohistochemical staining were directly compared with those of HMON enhancement on MRI.

RESULTS

The dorsolateral thalamus, hippocampus and remaining cortex of the injured hemisphere showed HMON enhancement from 3 to 21 days after HII. The mean relative contrast values in the dorsolateral thalamus showed an increase from a negative value at 1 day to 16.5 ± 4.8% at 21 days. The apoptotic cells and reactive astrocytes were observed on immunohistochemical staining from 1 to 21 days after HII. The accumulation of apoptotic cells regionally matched with the areas of HMON enhancement, while that of reactive astrocytes did not.

CONCLUSION

The areas of HMON enhancement showed best spatial agreement with those of apoptosis on TUNEL staining. Both HMON enhancement and TUNEL-positive cells were observed up to 21 days after HII. Advances in knowledge: The strength of our study is the visualization of apoptotic area in vivo using HMON-enhanced MRI, and we also showed that HII has a prolonged evolution lasting for several weeks.

Combining systemic and stereotactic MEMRI to detect the correlation between gliosis and neuronal connective pathway at the chronic stage after stroke

BACKGROUND

The early dysfunction and subsequent recovery after stroke, characterized by the destruction and remodeling of connective pathways between cortex and subcortical regions, is associated with neuroinflammation. As major components of the inflammatory process, reactive astrocytes have double-edged effects on pathological progression. The temporal patterns of astrocyte and neuronal pathway activity can be revealed by systemic and stereotactic manganese-enhanced magnetic resonance imaging (MEMRI), respectively. In the present study, we aimed to detect an association between astrocyte activity and recovery of neuronal connective pathways by combining systemic with stereotactic MEMRI.

METHODS

Fifty adult rats, divided into two groups, underwent a 60-min occlusion of the middle cerebral artery. The groups were given either a systemic administration or stereotactic injection of MnCl2 at 1, 3, 7, and 14 days after stroke and underwent MRI 4 and 2 days later, respectively. Immunofluorescence (IF) of group 1 was conducted to corroborate the results. Repetitive behavioral testing was also performed with all rats at 1, 3, 7, and 14 days to obtain a functional score.

RESULTS

Ring- or crescent-shaped enhancements formed in the striatal peri-infarct regions (STR) at 11 and 18 days. This was concurrent with the activity of glial fibrillary acidic protein (GFAP)-positive astrocytes, which mainly localized at the peri-infarct region and significantly increased in number at 11 and 18 days after stroke. Microglia/macrophages, detected by IF, mainly localized in the lesion core, rather than in the region of enhancement. The ipsilateral substantia nigra (SN) revealed Mn-related signal enhancement reduction and subsequent signs of the recovery process at 3 to 5 days and 9 to 16 days, respectively. Behavioral testing showed that sensorimotor functions were initially disturbed, but subsequently recovered at 7 and 14 days.

CONCLUSIONS

We found a positive temporal correlation between astrogliosis and the recovery of neuronal connective pathways at the chronic stage by using the in vivo method of MEMRI. Our results highlighted the potential contribution of astrocytes to the neuronal recovery of these connective pathways.

A New Approach Using Manganese-Enhanced MRI to Diagnose Acute Mesenteric Ischemia in a Rabbit Model: Initial Experience

PURPOSE

Manganese-enhanced MRI (MEMRI) has been applied to a wide range of biological and disease research. The purpose of the study was to use MEMRI to diagnose the acute mesenteric ischemia (AMI).

METHODS

The institutional experimental animal ethics committee approved this study. To optimize the dose of Mn(2+) infusion, a dose-dependent curve was obtained using Mn(2+)-enhanced T 1 map MRI by an intravenous infusion 2.5-20 nmol/g body weight (BW) of 50 nmol/L MnCl2. The eighteen animals were divided into control, sham-operated, and AMI groups. AMI models were performed by ligating the superior mesenteric artery (SMA). T 1 values were measured on T 1 maps in regions of the small intestinal wall and relaxation rate (ΔR 1) was calculated.

RESULTS

A nonlinear relationship between infused MnCl2 solution dose and increase in small intestinal wall ΔR 1 was observed. Control animal exhibited significant Mn(2+) clearance over time at the dose of 15 nmol/g BW. In the AMI model, ΔR 1 values (0.95 ± 0.13) in the small intestinal wall were significantly lower than in control group (2.05 ± 0.19) after Mn(2+) infusion (P < 0.01).

CONCLUSION

The data suggest that MEMRI shows potential as a diagnostic technique that is directly sensitive to the poor or absent perfusion in AMI.

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.

Monitoring dynamic alterations in calcium homeostasis by T1-mapping manganese-enhanced MRI (MEMRI) in the early stage of small intestinal ischemia-reperfusion injury

Manganese-enhanced MRI studies have proven to be useful in monitoring physiological activities associated with calcium ions (Ca(2+)) due to the paramagnetic property of the manganese ion (Mn(2+)), which makes it an excellent probe of Ca(2+) . In this study, we developed a method in which a Mn(2+)-enhanced T1 -map MRI could enable the monitoring of Ca(2+) influx during the early stages of intestinal ischemia-reperfusion (I/R) injury. The Mn(2+) infusion protocol was optimized by obtaining dose-dependent and time-course wash-out curves using a Mn(2+)-enhanced T1-map MRI of rabbit abdomens following an intravenous infusion of 50 mmol/l MnCl2 (5-10 nmol/g body weight (BW)). In the rabbit model of intestinal I/R injury, T1 values were derived from the T1 maps in the intestinal wall region and revealed a relationship between the dose of the infused MnCl2 and the intestinal wall relaxation time. Significant Mn(2+) clearance was also observed over time in control animals after the infusion of Mn(2+) at a dose of 10 nmol/g BW. This technique was also shown to be sensitive enough to monitor variations in calcium ion homeostasis in vivo after small intestinal I/R injury. The T1 values of the intestinal I/R group were significantly lower (P < 0.05) than that of the control group at 5, 10, and 15 min after Mn(2+) infusion. Our data suggest that MnCl2 has the potential to be an MRI contrast agent that can be effectively used to monitor changes in intracellular Ca(2+) homeostasis during the early stages of intestinal I/R injury.

Manganese-Enhanced MRI: Biological Applications in Neuroscience

Magnetic resonance imaging (MRI) is an excellent non-invasive tool to investigate biological systems. The administration of the paramagnetic divalent ion manganese (Mn²⁺) enhances MRI contrast in vivo. Due to similarities between Mn²⁺ and calcium (Ca²⁺), the premise of manganese-enhanced MRI (MEMRI) is that the former may enter neurons and other excitable cells through voltage-gated Ca²⁺ channels. As such, MEMRI has been used to trace neuronal pathways, define morphological boundaries, and study connectivity in morphological and functional imaging studies. In this article, we provide a brief overview of MEMRI and discuss recently published data to illustrate the usefulness of this method, particularly in animal models.

Manganese-Enhanced Magnetic Resonance Imaging Reflects Brain Pathology During Progressive HIV-1 Infection of Humanized Mice

Progressive human immunodeficiency viral (HIV) infection commonly leads to a constellation of cognitive, motor, and behavioral impairments. These are collectively termed HIV-associated neurocognitive disorders (HAND). While antiretroviral therapy (ART) reduces HAND severity, it does not affect disease prevalence. Despite decades of research, there remain no biomarkers for HAND and all potential comorbid conditions must first be excluded for a diagnosis to be made. To this end, we now report that manganese (Mn(2+))-enhanced magnetic resonance imaging (MEMRI) can reflect brain region-specific HIV-1-induced neuropathology in chronically virus-infected NOD/scid-IL-2Rγc(null) humanized mice. MEMRI diagnostics mirrors the abilities of Mn(2+) to enter and accumulate in affected neurons during disease. T1 relaxivity and its weighted signal intensity are proportional to Mn(2+) activities in neurons. In 16-week virus-infected humanized mice, altered MEMRI signal enhancement was easily observed in affected brain regions. These included, but were not limited to, the hippocampus, amygdala, thalamus, globus pallidus, caudoputamen, substantia nigra, and cerebellum. MEMRI signal was coordinated with levels of HIV-1 infection, neuroinflammation (astro- and micro-gliosis), and neuronal injury. MEMRI accurately demonstrates the complexities of HIV-1-associated neuropathology in rodents that reflects, in measure, the clinical manifestations of neuroAIDS as it is seen in a human host.

Advanced magnetic resonance spectroscopy and imaging techniques applied to brain development and animal models of perinatal injury

Magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) are widely used in the field of brain development and perinatal brain injury. Due to technical progress the magnetic field strength (B0) of MR systems has continuously increased, favoring (1)H-MRS with quantification of up to 18 metabolites in the brain and short echo time (TE) MRI sequences including phase and susceptibility imaging. For longer TE techniques including diffusion imaging modalities, the benefits of higher B0 have not been clearly established. Nevertheless, progress has also been made in new advanced diffusion models that have been developed to enhance the accuracy and specificity of the derived diffusion parameters. In this review, we will describe the latest developments in MRS and MRI techniques, including high-field (1)H-MRS, phase and susceptibility imaging, and diffusion imaging, and discuss their application in the study of cerebral development and perinatal brain injury.

Voltage-gated calcium channels: Determinants of channel function and modulation by inorganic cations

Voltage-gated calcium channels (VGCCs) represent a key link between electrical signals and non-electrical processes, such as contraction, secretion and transcription. Evolved to achieve high rates of Ca(2+)-selective flux, they possess an elaborate mechanism for selection of Ca(2+) over foreign ions. It has been convincingly linked to competitive binding in the pore, but the fundamental question of how this is reconcilable with high rates of Ca(2+) transfer remains unanswered. By virtue of their similarity to Ca(2+), polyvalent cations can interfere with the function of VGCCs and have proven instrumental in probing the mechanisms underlying selective permeation. Recent emergence of crystallographic data on a set of Ca(2+)-selective model channels provides a structural framework for permeation in VGCCs, and warrants a reconsideration of their diverse modulation by polyvalent cations, which can be roughly separated into three general mechanisms: (I) long-range interactions with charged regions on the surface, affecting the local potential sensed by the channel or influencing voltage-sensor movement by repulsive forces (electrostatic effects), (II) short-range interactions with sites in the ion-conducting pathway, leading to physical obstruction of the channel (pore block), and in some cases (III) short-range interactions with extracellular binding sites, leading to non-electrostatic modifications of channel gating (allosteric effects). These effects, together with the underlying molecular modifications, provide valuable insights into the function of VGCCs, and have important physiological and pathophysiological implications. Allosteric suppression of some of the pore-forming Cavα1-subunits (Cav2.3, Cav3.2) by Zn(2+) and Cu(2+) may play a major role for the regulation of excitability by endogenous transition metal ions. The fact that these ions can often traverse VGCCs can contribute to the detrimental intracellular accumulation of metal ions following excessive release of endogenous Cu(2+) and Zn(2+) or exposure to non-physiological toxic metal ions.

Quantitative activity-induced manganese-dependent MRI for characterizing cortical layers in the primary somatosensory cortex of the rat

The ability of Mn(2+) to follow Ca(2+) pathways upon stimulation transform them into remarkable surrogate markers of neuronal activity using activity-induced manganese-dependent MRI (AIM-MRI). In the present study, a precise follow-up of physiological parameters during MnCl2 and mannitol infusions improved the reproducibility of AIM-MRI allowing in-depth evaluation of the technique. Pixel-by-pixel T1 data were investigated using histogram distributions in the barrel cortex (BC) and the thalamus before and after Mn(2+) infusion, after blood brain barrier opening and after BC activation. Mean BC T1 values dropped significantly upon trigeminal nerve (TGN) stimulation (-38 %, P = 0.02) in accordance with previous literature findings. T1 histogram distributions showed that 34 % of T1s in the range 600-1500 ms after Mn(2+ )+ mannitol infusions shifted to 50-350 ms after TGN stimulation corresponding to a twofold increase of the percentage of pixels with the lowest T1s in BC. Moreover, T1 changes in response to stimulation increased significantly from superficial cortical layers (I-III) to deeper layers (V-VI). Cortical cytoarchitecture detection during a functional paradigm was performed extending the potential of AIM-MRI. Quantitative AIM-MRI could thus offer a means to interpret local neural activity across cortical layers while identification of the role of calcium dynamics in vivo during brain activation could play a key role in resolving neurovascular coupling mechanisms.

Manganese enhanced magnetic resonance imaging (MEMRI): a powerful new imaging method to study tinnitus

Manganese enhanced magnetic resonance imaging (MEMRI) is a method used primarily in basic science experiments to advance the understanding of information processing in central nervous system pathways. With this mechanistic approach, manganese (Mn(2+)) acts as a calcium surrogate, whereby voltage-gated calcium channels allow for activity driven entry of Mn(2+) into neurons. The detection and quantification of neuronal activity via Mn(2+) accumulation is facilitated by "hemodynamic-independent contrast" using high resolution MRI scans. This review emphasizes initial efforts to-date in the development and application of MEMRI for evaluating tinnitus (the perception of sound in the absence of overt acoustic stimulation). Perspectives from leaders in the field highlight MEMRI related studies by comparing and contrasting this technique when tinnitus is induced by high-level noise exposure and salicylate administration. Together, these studies underscore the considerable potential of MEMRI for advancing the field of auditory neuroscience in general and tinnitus research in particular. Because of the technical and functional gaps that are filled by this method and the prospect that human studies are on the near horizon, MEMRI should be of considerable interest to the auditory research community. This article is part of a Special Issue entitled <Annual Reviews 2014>.

Manganese: a new contrast agent for lung imaging?

Lung parenchyma remains one of the most difficult tissues to be imaged by means of magnetic resonance imaging (MRI). Several MRI techniques are routinely used for lung imaging. However, manganese-enhancement MRI (MEMRI) technique has not been associated with pulmonary MRI. Here, we evaluated T(1) -enhancement in the rat lung after a manganese instillation, using a 4.7 T magnet with a radial ultrashort echo time sequence. Our data showed that the signal intensity was increased in lungs receiving a manganese solution compared with a control solution to the lungs. MR signal enhancements above 30% were measured in lung parenchyma following 200 µl instillation of a 1 mm manganese chloride solution. MEMRI, therefore, may be a useful novel tool for enhancing signal intensity and image contrast in lung tissue.

Determination of the detectable concentration of manganese used in neuronal MEMRI and its effect on cortical neurons in vitro

OBJECTIVE

Manganese (Mn(2+))-enhanced magnetic resonance imaging (MEMRI) has received increasing attention because of its functional and anatomic value in brain studies. However, the contrast agent, Mn(2+), will lead to neurotoxicity at high concentrations, which limits its use in biomedical research. This study was designed to determine whether Mn(2+) can significantly enhance the signal intensity (SI) of primary cultured cortical neurons at non-toxic levels.

METHOD

Neurons were incubated with different concentrations of Mn(2+) (control and 0.01, 0.05, 0.10, and 0.20 mM), then a cellular MRI was performed in vitro and the intracellular Mn(2+) concentrations were analyzed by ICP-MS. At the same time, the cell viability, LDH release assay, intracellular ROS level, and apoptosis were measured 24 h after treatment.

RESULTS

(1) After the neurons were treated with Mn(2+) at a low concentration (0.01 mM), there was no impact on cell viability and cytotoxicity, and no significant signal was enhanced on MEMRI. (2) When the neurons were exposed to higher concentrations of Mn(2+) (0.05, 0.1, and 0.2 mM), a significant increase in signal quality was achieved, but cell viability was significantly reduced and the intracellular ROS formation and percentage of TUNEL-positive cells were increased significantly.

CONCLUSION

At Mn(2+) concentrations > 0.05 mM, significant enhancement of MEMRI SI occurred, but with overt cytotoxicity.

Manganese-enhanced MRI reveals early-phase radiation-induced cell alterations in vivo

For tumor radiotherapy, the in vivo detection of early cellular responses is important for predicting therapeutic efficacy. Mn(2+) is used as a positive contrast agent in manganese-enhanced MRI (MEMRI) and is expected to behave as a mimic of Ca(2+) in many biologic systems. We conducted in vitro and in vivo MRI experiments with Mn(2+) to investigate whether MEMRI can be used to detect cell alterations as an early-phase tumor response after radiotherapy. Colon-26 cells or a subcutaneously grafted colon-26 tumor model were irradiated with 20 Gy of X-rays. One day after irradiation, a significant augmentation of G2-M-phase cells, indicating a cell-cycle arrest, was observed in the irradiated cells in comparison with the control cells, although both early and late apoptotic alterations were rarely observed. The MEMRI signal in radiation-exposed tumor cells (R1: 0.77 ± 0.01 s(-1)) was significantly lower than that in control cells (R1: 0.82 ± 0.01 s(-1)) in vitro. MEMRI signal reduction was also observed in the in vivo tumor model 24 hours after irradiation (R1 of radiation: 0.97 ± 0.02 s(-1), control: 1.10 ± 0.02 s(-1)), along with cell-cycle and proliferation alterations identified with immunostaining (cyclin D1 and Ki-67). Therefore, MEMRI after tumor radiotherapy was successfully used to detect cell alterations as an early-phase cellular response in vitro and in vivo.

Altered transition metal homeostasis in mice following manganese injections for manganese-enhanced magnetic resonance imaging

In manganese-enhanced magnetic resonance imaging (MEMRI), the paramagnetic divalent ion of manganese (Mn(2+)) is injected into animals to generate tissue contrast, typically at much higher exposures than have been previously used in studies of Mn toxicity. Here we investigate the effect of these injections on the homeostasis of the transition metals iron and copper in mice to see if there are disruptions which should be considered in MEMRI studies. Manganese shares transport proteins with other transition metals including iron and copper, so it is possible that changes in manganese levels in tissue following injections of the metal may affect other metal levels too. This in turn may affect MRI contrast or the investigation of disease processes in the animal models being imaged. In this study, we measured manganese, iron, and copper concentrations in the blood, kidney, liver and in brain regions in mice treated with four injections of 30 mg/kg MnCl(2) 4H(2)O (dry chemical weight/body weight)-a common dose used in MEMRI. In addition to the expected increases in manganese in tissues, we noted a statistically significant reduction in copper in the kidney and liver. Also, we noted a statistically significant decrease in concentration of iron in the thalamus of the brain. These findings suggest that the high doses of manganese injected in MEMRI studies can disrupt the homeostasis of other transition metals in mice.

Tumor enhancement effect of overexpressed manganese-superoxide dismutase in manganese-enhanced MR imaging

PURPOSE

Manganese-enhanced magnetic resonance imaging (MEMRI), used to trace neuronal connections and visualize brain activity, has recently been suggested useful for tumor detection, but the mechanism of tumor enhancement by manganese (Mn) is poorly understood. Our recent report of preferential enhancement of human mesothelioma cells with higher levels of manganese-superoxide dismutase (Mn-SOD) expression may suggest a correlation between Mn-SOD expression and enhancement. We investigate this possibility further using engineered human ovarian cancer cells overexpressing Mn-SOD.

METHODS

We subcutaneously implanted SK-OV-3 human ovarian cancer cells stably overexpressing Mn-SOD (SK-Mn-SOD) into athymic nude mice and SK-OV-3 cells with plasmid DNAs carrying neomycin-resistant genes (SK-neo) into the same mice for controls. We conducted MEMRI in the tumor-bearing mice and compared enhancement between the 2 tumors.

RESULTS

Subcutaneous SK-Mn-SOD tumors were preferentially enhanced in MEMRI compared to SK-neo tumors. After Mn enhancement, the T(1)-relaxation rate (R(1)=1/T(1)) increased significantly for SK-Mn-SOD but not SK-neo tumors.

CONCLUSION

In some tumors, high expression of Mn-SOD may be a biological factor responsible for enhanced signal in MEMRI.

Impaired associative taste learning and abnormal brain activation in kinase-defective eEF2K mice

Memory consolidation is defined temporally based on pharmacological interventions such as inhibitors of mRNA translation (molecular consolidation) or post-acquisition deactivation of specific brain regions (systems level consolidation). However, the relationship between molecular and systems consolidation are poorly understood. Molecular consolidation mechanisms involved in translation initiation and elongation have previously been studied in the cortex using taste-learning paradigms. For example, the levels of phosphorylation of eukaryotic elongation factor 2 (eEF2) were found to be correlated with taste learning in the gustatory cortex (GC), minutes following learning. In order to isolate the role of the eEF2 phosphorylation state at Thr-56 in both molecular and system consolidation, we analyzed cortical-dependent taste learning in eEF2K (the only known kinase for eEF2) ki mice, which exhibit reduced levels of eEF2 phosphorylation but normal levels of eEF2 and eEF2K. These mice exhibit clear attenuation of cortical-dependent associative, but not of incidental, taste learning. In order to gain a better understanding of the underlying mechanisms, we compared brain activity as measured by MEMRI (manganese-enhanced magnetic resonance imaging) between eEF2K ki mice and WT mice during conditioned taste aversion (CTA) learning and observed clear differences between the two but saw no differences under basal conditions. Our results demonstrate that adequate levels of phosphorylation of eEF2 are essential for cortical-dependent associative learning and suggest that malfunction of memory processing at the systems level underlies this associative memory impairment.

Dose dependence and temporal evolution of the T1 relaxation time and MRI contrast in the rat brain after subcutaneous injection of manganese chloride

Divalent manganese ion (Mn(2+)) is a widely used T(1) contrast agent in manganese-enhanced MRI studies to visualize functional neural tracts and anatomy in the brain in vivo. In animal studies, Mn(2+) is administered at a dose that will maximize the contrast, while minimizing its toxic effects. In rodents, systemic administration of Mn(2+) via intravenous injection has been shown to create unique MRI contrast in the brain at a maximum dose of 175 mg kg(-1). However, intravenous administration of Mn(2+) results in faster bioelimination of excess Mn(2+) from the plasma due to a steep concentration gradient between plasma and bile. By contrast, following subcutaneous injection (LD(50) value = 320 mg kg(-1)), Mn(2+) is released slowly into the bloodstream, thus avoiding immediate hepatic elimination resulting in prolonged accumulation of Mn(2+) in the brain via the choroid plexus than that obtained via intravenous administration. The goal of this study was to investigate MRI dose response of Mn(2+) in rat brain following subcutaneous administration of Mn(2+). Dose dependence and temporal dynamics of Mn(2+) after subcutaneous injection can prove useful for longitudinal in vivo studies that require brain enhancement to persist for a long period of time to visualize neuroarchitecture like in neurodegenerative disease studies.

Effect of manganese chloride on the neurochemical profile of the rat hypothalamus

Manganese (Mn(2+))-enhanced magnetic resonance imaging studies of the neuronal pathways of the hypothalamus showed that information about the regulation of food intake and energy balance circulate through specific hypothalamic nuclei. The dehydration-induced anorexia (DIA) model demonstrated to be appropriate for studying the hypothalamus with Mn(2+)-enhanced magnetic resonance imaging. Manganese is involved in the normal functioning of a variety of physiological processes and is associated with enzymes contributing to neurotransmitter synthesis and metabolism. It also induces psychiatric and motor disturbances. The molecular mechanisms by which Mn(2+) produces alterations of the hypothalamic physiological processes are not well understood. (1)H-magnetic resonance spectroscopy measurements of the rodent hypothalamus are challenging due to the distant location of the hypothalamus resulting in limited measurement sensitivity. The present study proposed to investigate the effects of Mn(2+) on the neurochemical profile of the hypothalamus in normal, DIA, and overnight fasted female rats at 14.1 T. Results provide evidence that γ-aminobutyric acid has an essential role in the maintenance of energy homeostasis in the hypothalamus but is not condition specific. On the contrary, glutamine, glutamate, and taurine appear to respond more accurately to Mn(2+) exposure. An increase in glutamine levels could also be a characteristic response of the hypothalamus to DIA.

Detection of neuronal activity and metabolism in a model of dehydration-induced anorexia in rats at 14.1 T using manganese-enhanced MRI and 1H MRS

In this study, hypothalamic activation was performed by dehydration-induced anorexia (DIA) and overnight food suppression (OFS) in female rats. The assessment of the hypothalamic response to these challenges by manganese-enhanced MRI showed increased neuronal activity in the paraventricular nuclei (PVN) and lateral hypothalamus (LH), both known to be areas involved in the regulation of food intake. The effects of DIA and OFS were compared by generating T-score maps. Increased neuronal activation was detected in the PVN and LH of DIA rats relative to OFS rats. In addition, the neurochemical profile of the PVN and LH were measured by (1) H MRS at 14.1T. Significant increases in metabolite levels were measured in DIA and OFS relative to control rats. Statistically significant increases in γ-aminobutyric acid were found in DIA (p=0.0007) and OFS (p<0.001) relative to control rats. Lactate increased significantly in DIA (p=0.03), but not in OFS, rats. This work shows that manganese-enhanced MRI coupled to (1) H MRS at high field is a promising noninvasive method for the investigation of the neural pathways and mechanisms involved in the control of food intake, in the autonomic and endocrine control of energy metabolism and in the regulation of body weight.

Manganese enhanced MRI (MEMRI): neurophysiological applications

Manganese ion (Mn(2+)) is a calcium (Ca(2+)) analog that can enter neurons and other excitable cells through voltage gated Ca(2+) channels. Mn(2+) is also a paramagnetic that shortens the spin-lattice relaxation time constant (T(1)) of tissues where it has accumulated, resulting in positive contrast enhancement. Mn(2+) was first investigated as a magnetic resonance imaging (MRI) contrast agent approximately 20 years ago to assess the toxicity of the metal in rats. In the late 1990s, Alan Koretsky and colleagues pioneered the use of manganese enhanced MRI (MEMRI) towards studying brain activity, tract tracing and enhancing anatomical detail. This review will describe the methodologies and applications of MEMRI in the following areas: monitoring brain activity in animal models, in vivo neuronal tract tracing and using MEMRI to assess in vivo axonal transport rates.

In vivo diffusion tensor MRI of the mouse retina: a noninvasive visualization of tissue organization

Diffusion tensor MRI (DTI) is a method for the noninvasive assessment of cellular organization and integrity in vivo. In this study, in vivo DTI was performed to demonstrate its ability to reflect photoreceptor cell alignment in adult C57BL/6 wild-type mice. Age-matched retinal degeneration 1 (rd1) mice were employed as a negative control, i.e. loss of the photoreceptor cell layer. In wild-type mice, DTI-estimated cell alignment suggests that the MR-detected outer retinal layer comprises cells aligning perpendicular to the retinal surface, consistent with the known organization of photoreceptor cells. The MR-detected outer retinal layer exhibits a lower apparent diffusion coefficient and higher fractional anisotropy than the other two MR-detected retinal layers (p < 0.05 for all comparisons). In rd1 mice, the remaining MR-detected retinal layer exhibits different cell alignment, apparent diffusion coefficient and fractional anisotropy from that of the MR-detected outer retinal layer in wild-type mice (p < 0.05 for all comparisons), reflecting the degeneration of photoreceptor cells in rd1 mouse retina. Overall, our findings suggest that in vivo DTI assessment of mouse retina with normal physiology or degenerative pathology is feasible.

Molecular imaging of mesothelioma by detection of manganese-superoxide dismutase activity using manganese-enhanced magnetic resonance imaging

Malignant mesothelioma (MM) is a fatal malignancy with a rapidly increasing incidence in industrialized countries because of the widespread use of asbestos in the past centuries. Early diagnosis of MM is critical for a better prognosis, but this is often difficult because of the lack of disease-specific diagnostic imaging. Here, we report that manganese-enhanced magnetic resonance imaging (MEMRI) represents a promising approach for a more selective mesothelioma imaging by monitoring a high-level expression of manganese-superoxide dismutase (Mn-SOD), which is observed in many MM. We found that most human MM cells overexpressed Mn-SOD protein compared with human mesothelial cells and that NCI-H226 human MM cells highly expressed Mn-SOD and augmented Mn accumulation when loaded with manganese chloride (MnCl(2)). The cells showed marked T(1)-signal enhancement on in vitro MRI after incubation with MnCl(2) because of the T(1) shortening effect of Mn(2+). H226 subcutaneous tumor was preferentially enhanced compared with a lung adenocarcinoma cell tumor and another human MM cell tumor in MnCl(2)-enhanced T(1)-weighted MR image (T(1)WI), correlating with their respective Mn-SOD expression levels. Moreover, in a more clinically relevant setting, H226 xenografted pleural tumor was markedly enhanced and readily detected by MEMRI using manganese dipyridoxyl diphosphate (MnDPDP), a clinically used contrast agent, as well as MnCl(2). Therefore, we propose that MEMRI can be a potentially powerful method for noninvasive detection of MM, with high spatial resolution and marked signal enhancement, by targeting Mn-SOD.

Development of a MR-visible compound for tracing neuroanatomical connections in vivo

Traditional studies of neuroanatomical connections require injection of tracer compounds into living brains, then histology of the postmortem tissue. Here, we describe and validate a compound that reveals neuronal connections in vivo, using MRI. The classic anatomical tracer CTB (cholera-toxin subunit-B) was conjugated with a gadolinium-chelate to form GdDOTA-CTB. GdDOTA-CTB was injected into the primary somatosensory cortex (S1) or the olfactory pathway of rats. High-resolution MR images were collected at a range of time points at 11.7T and 7T. The transported GdDOTA-CTB was visible for at least 1 month post-injection, clearing within 2 months. Control injections of non-conjugated GdDOTA into S1 were not transported and cleared within 1-2 days. Control injections of Gd-Albumin were not transported either, clearing within 7 days. These MR results were verified by classic immunohistochemical staining for CTB, in the same animals. The GdDOTA-CTB neuronal transport was target specific, monosynaptic, stable for several weeks, and reproducible.

Manganese-enhanced magnetic resonance imaging (MEMRI)

The use of manganese ions (Mn(2+)) as an MRI contrast agent was introduced over 20 years ago in studies of Mn(2+) toxicity in anesthetized rats (1). Manganese-enhanced MRI (MEMRI) evolved in the late nineties when Koretsky and associates pioneered the use of MEMRI for brain activity measurements (2) as well as neuronal tract tracing (3). Currently, MEMRI has three primary applications in biological systems: (1) contrast enhancement for anatomical detail, (2) activity-dependent assessment and (3) tracing of neuronal connections or tract tracing. MEMRI relies upon the following three main properties of Mn(2+): (1) it is a paramagnetic ion that shortens the spin lattice relaxation time constant (T(1)) of tissues, where it accumulates and hence functions as an excellent T(1) contrast agent; (2) it is a calcium (Ca(2+)) analog that can enter excitable cells, such as neurons and cardiac cells via voltage-gated Ca(2+) channels; and (3) once in the cells Mn(2+) can be transported along axons by microtubule-dependent axonal transport and can also cross synapses trans-synaptically to neighboring neurons. This chapter will emphasize the methodological approaches towards the use of MEMRI in biological systems.

Longitudinal manganese-enhanced magnetic resonance imaging of delayed brain damage after hypoxic-ischemic injury in the neonatal rat

BACKGROUND

Hypoxia-ischemia (HI) in the neonatal brain results in a prolonged injury process. Longitudinal studies using noninvasive methods can help elucidate the mechanisms behind this process. We have recently demonstrated that manganese-enhanced magnetic resonance imaging (MRI) can depict areas with activated microglia and astrogliosis 7 days after hypoxic-ischemic brain injury.

OBJECTIVE

The current study aimed to follow brain injury after HI in rats longitudinally and compare manganese enhancement of brain areas to the development of injury and presence of reactive astrocytes and microglia.

METHODS

The Vannucci model for hypoxic-ischemic injury in the neonatal rat was used. Pups were injected with either MnCl(2) or saline after 6 h and again on day 41 after HI. Longitudinal MRI (T(1) weighted) was performed 1, 3, 7 and 42 days after HI. The brains were prepared for immunohistochemistry after the final MRI.

RESULTS

There was severe loss of cerebral tissue from day 7 to day 42 after HI. Most manganese-enhanced areas in the hippocampus, thalamus and basal ganglia at day 7 were liquefied after 42 days. Manganese-enhancement on day 42 corresponded to areas of activated microglia and reactive astrocytes in the remaining cortex, hippocampus and amygdala. However, the main area of enhancement was in the remaining thalamus in a calcified area surrounded by activated microglia and reactive astrocytes.

CONCLUSION

Manganese-enhanced MRI can be a useful tool for in vivo identification of cerebral tissue undergoing delayed cell death and liquefaction after HI. Manganese enhancement at a late stage seems to be related to the accumulation of manganese in calcifications and gliotic tissue.

Experimental stroke research: the contributions of in vivo MRI

Stroke is a disease that develops from the very acute time point of first symptoms during the next several hours and further to a chronic time period of days or even weeks. During this evolution process, a whole series of pathophysiological events takes place. Therefore, the disease is characterized by a continuously changing pathophysiological pattern. In consequence, as the disease develops over time, different imaging modalities must be chosen to accurately describe the status of stroke. In the present chapter, we have divided the evolution of stroke into various dominant steps of the cascade of events, with corresponding time windows. Choice of MRI variables for depiction of the most important aspects during these time windows are presented and their information content is discussed for diagnosis and for investigations into a better understanding of the underlying mechanisms for the disease as well as the relevance of these imaging tools in success assessments for therapeutic strategies.

Manganese-enhanced magnetic resonance imaging

Manganese-enhanced magnetic resonance imaging (MEMRI) relies on contrasts that are due to the shortening of the T (1) relaxation time of tissue water protons that become exposed to paramagnetic manganese ions. In experimental animals, the technique combines the high spatial resolution achievable by MRI with the biological information gathered by tissue-specific or functionally induced accumulations of manganese. After in vivo administration, manganese ions may enter cells via voltage-gated calcium channels. In the nervous system, manganese ions are actively transported along the axon. Based on these properties, MEMRI is increasingly used to delineate neuroanatomical structures, assess differences in functional brain activity, and unravel neuronal connectivities in both healthy animals and models of neurological disorders. Because of the cellular toxicity of manganese, a major challenge for a successful MEMRI study is to achieve the lowest possible dose for a particular biological question. Moreover, the interpretation of MEMRI findings requires a profound knowledge of the behavior of manganese in complex organ systems under physiological and pathological conditions. Starting with an overview of manganese pharmacokinetics and mechanisms of toxicity, this chapter covers experimental methods and protocols for applications in neuroscience.