Manganese‐guided cellular MRI of human embryonic stem cell and human bone marrow stromal cell viability

This study investigated the ability of MnCl₂ as a cellular MRI contrast agent to determine the in vitro viability of human embryonic stem cells (hESC) and human bone marrow stromal cells (hBMSC). Basic MRI parameters including T₁ and T₂ values of MnCl₂‐labeled hESC and hBMSC were measured and viability signal of manganese (Mn²⁺)‐labeled cells was validated. Furthermore, the biological activity of Ca²⁺‐channels was modulated utilizing both Ca²⁺‐channel agonist and antagonist to evaluate concomitant signal changes. Metabolic effects of MnCl₂‐labeling were also assessed using assays for cell viability, proliferation, and apoptosis. Finally, in vivo Mn²⁺‐guided MRI of the transplanted hESC was successfully achieved and validated by bioluminescence imaging. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Mn‐bicine: A low affinity chelate for manganese ion enhanced MRI

The toxicity of free Mn²⁺ is a bottleneck for the in vivo application of manganese ion enhanced MRI. To reduce free Mn²⁺ concentration ([Mn²⁺]), a low affinity chelate reagent: N,N‐bis(2‐hydroxyethyl)glycine (bicine) was used. Considering the conditional association constant of Mn‐bicine at pH 7.4 (10^2.9 M^?1), (i) a 100 mM Mn‐bicine solution should contain about 10 mM of free manganese ion, but (ii) free manganese will make up 3/4 of the final plasma concentration (0.5 mM) with an intravenous infusion of 100 mM Mn‐bicine. The T₁ relaxivity of Mn‐bicine in a 5 mM Mn‐bicine solution was estimated as 5 mM^?1 sec^?1 at 24°C, 7 T in a pH range of 6.8–7.5. Mn‐bicine demonstrated a tendency for better contractility when employed with an isolated perfused frog heart, compared with MnCl₂. A venous infusion of 100 mM Mn‐bicine (8.3 μmol kg^?1 min^?1) showed a minimal decrease and maintained a constant heart rate level and arterial pressure in rats, while rats infused with 100 mM of MnCl₂ showed a significant suppression of the hemodynamic functions. Thus, Mn‐bicine appears to be a better choice for maintaining the vital conditions of experimental animals, and may improve the reproducibility of manganese ion enhanced MRI. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Temporal changes in the T1 and T2 relaxation rates (ΔR1 and ΔR2) in the rat brain are consistent with the tissue‐clearance rates of elemental manganese

Temporal changes in the T₁ and T₂ relaxation rates (ΔR₁ and ΔR₂) in rat olfactory bulb (OB) and cortex were compared with the absolute manganese (Mn) concentrations from the corresponding excised tissue samples. In vivo T₁ and T₂ relaxation times were measured before, and at 1, 7, 28, and 35 d after intravenous infusion of 176 mg/kg MnCl₂. The values of ΔR₁, ΔR₂, and absolute Mn concentration peaked at day 1 and then declined to near control levels after 28 to 35 d. The Mn bioelimination rate from the rat brain was significantly faster than that reported using radioisotope techniques. The R₁ and R₂ relaxation rates were linearly proportional to the underlying tissue Mn concentration and reflect the total absolute amount of Mn present in the tissue. The in vivo Mn r₁ and r₂ tissue relaxivities were comparable to the in vitro values for aqueous Mn²⁺. These results demonstrate that loss of manganese‐enhanced MRI (MEMRI) contrast after systemic Mn²⁺ administration is due to elimination of Mn²⁺ from the brain. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Manganese transport in the rat optic nerve evaluated with spatial‐ and time‐resolved magnetic resonance imaging

Purpose:

1) To evaluate a novel theoretical model for in vivo axonal Mn²⁺ transport with MRI data from the rat optic nerve (ON); and 2) to compare predictions from the new model with previously reported experimental data.

Materials and Methods:

Time‐resolved in vivo T₁‐weighted magnetic resonance imaging (MRI) of adult female Sprague–Dawley rat (n = 9) ON was obtained at different timepoints after intravitreal MnCl₂ injection. A concentration‐dependent and a rate‐dependent function for the Mn²⁺ retinal ganglion cell (RGC) axon entrance was convolved with three different transport functions and each model system was optimized to fit the ON data.

Results:

The rate‐limited input function gave a better fit to the data than the concentration‐limited input. Simulations showed that the rate‐limited input leads to a semilogarithmic relationship between injected dose and Mn²⁺ concentration in the ON, which is in agreement with previously reported in vivo experiments. A random walk transport model and an anterograde predominant slow model gave a similar fit to the data, both better than an anterograde predominant fast model.

Conclusion:

The results indicate that Mn²⁺ input into RGC axons is limited by a maximum entrance rate into the axons. Also, a wide range of apparent Mn²⁺ transport rates seems to be involved, different from synaptic vesicle transport rates, meaning that manganese does not depict synaptic vesicle transport rates directly. J. Magn. Reson. Imaging 2010;32:551–560. © 2010 Wiley‐Liss, Inc.     

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 (Mn²⁺) is a widely used T₁ contrast agent in manganese‐enhanced MRI studies to visualize functional neural tracts and anatomy in the brain in vivo. In animal studies, Mn²⁺ is administered at a dose that will maximize the contrast, while minimizing its toxic effects. In rodents, systemic administration of Mn²⁺ 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 Mn²⁺ results in faster bioelimination of excess Mn²⁺ from the plasma due to a steep concentration gradient between plasma and bile. By contrast, following subcutaneous injection (LD₅₀ value = 320 mg kg^?1), Mn²⁺ is released slowly into the bloodstream, thus avoiding immediate hepatic elimination resulting in prolonged accumulation of Mn²⁺ 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 Mn²⁺ in rat brain following subcutaneous administration of Mn²⁺. Dose dependence and temporal dynamics of Mn²⁺ 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. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Manganese‐enhanced MRI detection of neurodegeneration in neonatal hypoxic‐ischemic cerebral injury

In this study, we investigated the Mn‐enhanced MRI (MEMRI) for detecting neurodegenerative processes in neonatal hypoxic‐ischemic (H‐I) cerebral injury. Seven‐day‐old rats were induced with H‐I injury, and scanned for T₁‐weighted image (T₁WI) and T₂‐weighted image (T₂WI) with and without systemic MnCl₂ administration. Serial histological analysis was performed for Mn‐superoxide dismutase (Mn‐SOD) and glutamine synthetase (GS), which are Mn‐binding enzymes against the oxidative stress and glutamate excitotoxicity in neurodegeneration. In the acute phase (first 2 days), the ipsilateral lesion exhibited no Mn enhancement in T₁WIs, with histology showing no Mn‐SOD and GS production. In the mid‐phase (from day 3), Mn enhancement was found in the cortex, basal ganglia, and hippocampus, correlating with local Mn‐SOD and GS increase. In the late phase, the enhancement became more localized to the pericyst basal ganglia and cortex, and then gradually diminished. In T₂WIs, a signal decrease was observed from day 3 in the corresponding regions. Hypointense voids gradually formed in the late phase, correlating with the local iron accumulation. H‐I rats without Mn²⁺ administration exhibited similar but weak changes in T₁WIs and T₂WIs from days 14 and 7, respectively. These results indicate that Mn²⁺ may be a useful in vivo probe for monitoring Mn‐SOD and GS enzymatic activities. Magn Reson Med, 2008. © 2008 Wiley‐Liss, Inc.     

Rapid T1 mapping of mouse myocardium with saturation recovery look‐locker method

Dynamic contrast‐enhanced MRI using gadolinium or manganese provides unique characterization of myocardium and its pathology. In this study, an electrocardiography (ECG) triggered saturation recovery Look‐Locker method was developed and validated for fast cardiac T₁ mapping in small animal models. By sampling the initial portion of the longitudinal magnetization recovery curve, high temporal resolution (～3 min) can be achieved at a high spatial resolution (195 × 390 μm2) in mouse heart without the aid of parallel imaging or echo‐planar imaging. Validation studies were performed both in vitro on a phantom and in vivo on C57BL/6 mice (n = 6). Our results showed a strong agreement between T₁ measured by saturation recovery Look‐Locker and by the standard saturation recovery method in vitro or inversion recovery Look‐Locker in vivo. The utility of saturation recovery Look‐Locker in dynamic contrast‐enhanced MRI studies was demonstrated in manganese‐enhanced MRI experiments in mice. Our results suggest that saturation recovery Look‐Locker can provide rapid and accurate cardiac T₁ mapping for studies using small animal models. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Fat‐water separated delayed hyperenhanced myocardial infarct imaging

Fat deposition associated with myocardial infarction (MI) has been reported as a commonly occurring phenomenon. Magnetic resonance imaging (MRI) has the ability to efficiently detect MI using T₁‐sensitive contrast‐enhanced sequences and fat via its resonant frequency shift. In this work, the feasibility of fat‐water separation applied to the conventional delayed hyperenhanced (DHE) MI imaging technique is demonstrated. A three‐point Dixon acquisition and reconstruction was combined with an inversion recovery gradient‐echo pulse sequence. This allowed fat‐water separation along with T₁ sensitive imaging after injection of a gadolinium contrast agent. The technique is demonstrated in phantom experiments and three subjects with chronic MI. Areas of infarction were well defined as conventional hyperenhancement in water images. In two cases, fatty deposition was detected in fat images and confirmed by precontrast opposed‐phase imaging. Magn Reson Med 60:503–509, 2008. © 2008 Wiley‐Liss, Inc.     

Monitoring bone marrow‐originated mesenchymal stem cell traffic to myocardial infarction sites using magnetic resonance imaging

How stem cells promote myocardial repair in myocardial infarction (MI) is not well understood. The purpose of this study was to noninvasively monitor and quantify mesenchymal stem cells (MSC) from bone marrow to MI sites using magnetic resonance imaging (MRI). MSC were dual‐labeled with an enhanced green fluorescent protein and micrometer‐sized iron oxide particles prior to intra‐bone marrow transplantation into the tibial medullary space of C57Bl/6 mice. Micrometer‐sized iron oxide particles labeling caused signal attenuation in T₂*‐weighted MRI and thus allowed noninvasive cell tracking. Longitudinal MRI demonstrated MSC infiltration into MI sites over time. Fluorescence from both micrometer‐sized iron oxide particles and enhanced green fluorescent protein in histology validated the presence of dual‐labeled cells at MI sites. This study demonstrated that MSC traffic to MI sites can be noninvasively monitored in MRI by labeling cells with micrometer‐sized iron oxide particles. The dual‐labeled MSC at MI sites maintained their capability of proliferation and differentiation. The dual‐labeling, intra‐bone marrow transplantation, and MRI cell tracking provided a unique approach for investigating stem cells' roles in the post‐MI healing process. This technique can potentially be applied to monitor possible effects on stem cell mobilization caused by given treatment strategies. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Convertible manganese contrast for molecular and cellular MRI

We describe here the use of inorganic manganese based particles as convertible MRI agents. As has been demonstrated with iron oxide particles, manganese oxide and manganese carbonate particles can be internalized within phagocytotic cells, being subsequently shuttled to endosomes and/or lysosomes. As intact particles, only susceptibility‐induced MRI contrast is exhibited, most often seen as dark contrast in susceptibility‐weighted images. Modulation of MRI contrast is accomplished by the selective degradation of these particles within the endosomal and lysosomal compartments of cells. Upon particle deconstruction in the endosomes and lysosomes, the dissolved Mn²⁺ acts as a T₁ agent, eliciting bright contrast in T₁‐weighted images. This modulation of MRI contrast is demonstrated both in vitro in cells in culture, and also in vivo, in rat brain. These particles are the potential building blocks for an entire class of new environmentally responsive MRI contrast agents. Magn Reson Med 60:265–269, 2008. © 2008 Wiley‐Liss, Inc.     

T2‐weighted MRI of post‐infarct myocardial edema in mice

T₂‐weighted, cardiac magnetic resonance imaging (T₂w CMR) can be used to noninvasively detect and quantify the edematous region that corresponds to the area at risk (AAR) following myocardial infarction (MI). Previously, CMR has been used to examine structure and function in mice, expediting the study of genetic manipulations. To date, CMR has not been applied to imaging of post‐MI AAR in mice. We developed a whole‐heart, T₂w CMR sequence to quantify the AAR in mouse models of ischemia and infarction. The ΔB₀ and ΔB₁ environment around the mouse heart at 7 T were measured, and a T₂‐preparation sequence suitable for these conditions was developed. Both in vivo T₂w and late gadolinium enhanced CMR were performed in mice after 20‐min coronary occlusions, resulting in measurements of AAR size of 32.5 ± 3.1 (mean ± SEM)% left ventricular mass, and MI size of 50.1 ± 6.4% AAR size. Excellent interobserver agreement and agreement with histology were also found. This T₂w imaging method for mice may allow for future investigations of genetic manipulations and novel therapies affecting the AAR and salvaged myocardium following reperfused MI. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Manganese‐enhanced MRI of salivary glands and head and neck tumors in living subjects

Manganese‐enhanced MRI has previously been used for visualization of brain architecture and functional mapping of neural pathways. The present work investigated the potential of manganese‐enhanced MRI for noninvasive imaging of salivary glands in living subjects. Marked shortening of T₁ was observed in salivary glands of naïve mice (n = 5) 24–48 h after systemic administration of MnCl₂ (0.4 mmol/kg, intraperitoneally). Three‐dimensional MR microscopy confirmed selective contrast enhancement of salivary gland tissues post–MnCl₂ injection. Ectopic and orthotopic head and neck tumor xenografts also showed an increase in R₁ at 24 h following MnCl₂ injection (0.2 mmol/kg, intraperitoneally). However, tumor enhancement was minimal compared to salivary gland tissue. Salivary gland R₁ values were lower in mice bearing orthotopic head and neck tumors compared to naïve mice. These results demonstrate, for the first time, the usefulness of manganese‐enhanced MRI in the visualization of salivary glands and head and neck tumors in vivo. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Variable flip angle‐based fast three‐dimensional T1 mapping for delayed gadolinium‐enhanced MRI of cartilage of the knee: Need for B1 correction

Delayed gadolinium‐enhanced MRI of cartilage is a technique, which involves T₁ mapping to identify changes in the structural integrity of cartilage associated with osteoarthritis. Currently, the gold standard is 2D inversion recovery turbo spin echo, which suffers from long acquisition times and limited coverage. Three‐dimensional variable flip angle (VFA) is an alternate technique, which has been shown to be accurate when an estimate of T₁ is available a priori. This study validates the variable flip angle method for delayed gadolinium‐enhanced MRI of cartilage of the femoro‐tibial knee cartilage. When amplitude of (excitation) radiofrequency field inhomogeneities were minimized using nonselective pulses and amplitude of (excitation) radiofrequency field correction using an additional acquisition of a amplitude of (excitation) radiofrequency field map, the accuracy of T₁ measurements were improved, and slice‐to‐slice variations over the 3D volume were minimized. In conclusion, fast 3D T₁ mapping using the variable flip angle method with amplitude of (excitation) radiofrequency field correction appears to be an efficient and accurate method for delayed gadolinium‐enhanced MRI of cartilage of the knee. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Fast cardiac T(1) 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). Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

A B1‐insensitive high resolution 2D T1 mapping pulse sequence for dGEMRIC of the HIP at 3 Tesla

Early detection of cartilage degeneration in the hip may help prevent onset and progression of osteoarthritis in young patients with femoroacetabular impingement. Delayed gadolinium‐enhanced MRI of cartilage is sensitive to cartilage glycosaminoglycan loss and could serve as a diagnostic tool for early cartilage degeneration. We propose a new high resolution 2D T₁ mapping saturation–recovery pulse sequence with fast spin echo readout for delayed gadolinium‐enhanced magnetic resonance imaging of cartilage of the hip at 3 T. The proposed sequence was validated in a phantom and in 10 hips, using radial imaging planes, against a rigorous multipoint saturation–recovery pulse sequence with fast spin echo readout. T₁ measurements by the two pulse sequences were strongly correlated (R² > 0.95) and in excellent agreement (mean difference = ?8.7 ms; upper and lower 95% limits of agreement = 64.5 and ?81.9 ms, respectively). T₁ measurements were insensitive to B₁₊ variation as large as 20%, making the proposed T₁ mapping technique suitable for 3 T. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Temporal and noninvasive monitoring of inflammatory‐cell infiltration to myocardial infarction sites using micrometer‐sized iron oxide particles

Micrometer‐sized iron oxide particles (MPIO) are a more sensitive MRI contrast agent for tracking cell migration compared to ultrasmall iron oxide particles. This study investigated the temporal relationship between inflammation and tissue remodeling due to myocardial infarction (MI) using MPIO‐enhanced MRI. C57Bl/6 mice received an intravenous MPIO injection for cell labeling, followed by a surgically induced MI seven days later (n = 7). For controls, two groups underwent either sham‐operated surgery without inducing an MI post‐MPIO injection (n = 7) or MI surgery without MPIO injection (n = 6). The MRIs performed post‐MI showed significant signal attenuation around the MI site for the mice that received an intravenous MPIO injection for cell labeling, followed by a surgically induced MI seven days later, compared to the two control groups (P < 0.01). The findings suggested that the prelabeled inflammatory cells mobilized and infiltrated into the MI site. Furthermore, the linear regression of contrast‐to‐noise ratio at the MI site and left ventricular ejection function suggested a positive correlation between the labeled inflammatory cell infiltration and cardiac function attenuation during post‐MI remodeling (r² = 0.98). In conclusion, this study demonstrated an MRI technique for noninvasively and temporally monitoring inflammatory cell migration into the myocardium while potentially providing additional insight concerning the pathologic progression of a myocardial infarction. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Myocardial fat quantification in humans: Evaluation by two‐point water‐fat imaging and localized proton spectroscopy

Proton MR spectroscopy (¹H‐MRS) has been used for in vivo quantification of intracellular triglycerides within the sarcolemma. The purpose of this study was to assess whether breath‐hold dual‐echo in‐ and out‐of‐phase MRI at 3.0 T can quantify the fat content of the myocardium. Biases, including T₁, T*₂, and noise, that confound the calculation of the fat fraction were carefully corrected. Thirty‐four of 46 participants had both MRI and MRS data. The fat fractions from MRI showed a strong correlation with fat fractions from MRS (r = 0.78; P < 0.05). The mean myocardial fat fraction for all 34 subjects was 0.7 ± 0.5% (range: 0.11–3%) assessed with MRS and 1.04 ± 0.4% (range: 0.32–2.44%) assessed with in‐ and out‐of‐phase MRI (P < 0.05). Scanning times were less than 15 sec for Dixon imaging, plus an additional minute for the acquisition used for T*₂ calculation, and 15‐20 min for MRS. The average postprocessing time for MRS was 3 min and 5 min for MRI including T*₂ measurement. We conclude that the dual echo method provides a rapid means to detect and quantifying myocardial fat content in vivo. Correction/adjustment for field inhomogeneity using three or more echoes seems crucial for the dual echo approach. Magn Reson Med 63:892–901, 2010. © 2010 Wiley‐Liss, Inc.     

High‐resolution magnetic resonance colonography and dynamic contrast‐enhanced magnetic resonance imaging in a murine model of colitis

Inflammatory bowel disease, including ulcerative colitis, is characterized by persistent or recurrent inflammation and can progress to colon cancer. Colitis is difficult to detect and monitor noninvasively. The goal of this work was to develop a preclinical imaging method for evaluating colitis. Herein, we report improved MRI methods for detecting and characterizing colitis noninvasively in mice, using high‐resolution in vivo MR images and dynamic contrast‐enhanced MRI studies, which were confirmed by histologic studies in a murine model of colitis. C57Bl6/J male mice were treated with 2.5% dextran sulfate sodium in their drinking water for 5 days to induce colitis. MR images were acquired using a 9.4‐T Bruker scanner from 5–25 days following dextran sulfate sodium treatment. In dynamic contrast‐enhanced MRI studies, Gd uptake (K^trans) and its distribution (v_e) were measured in muscle and normal and inflamed colons after administering Gd‐diethyltriaminepentaacetic acid (Gd‐DTPA). T₂‐weighted MR images distinguished normal colon from diffusely thickened colonic wall occurring in colitis (P <0.0005) and correlated with histologic features. Values of K^trans and v_e obtained from dynamic contrast‐enhanced MRI were also significantly different in inflamed colons compared to normal colon (P < 0.0005). The results demonstrate that both T₂‐weighted anatomic imaging and quantitative analysis of dynamic contrast‐enhanced MRI data can successfully distinguish colitis from normal colon in mice. Magn Reson Med 63:922–929, 2010. © 2010 Wiley‐Liss, Inc.     

First‐pass contrast‐enhanced myocardial perfusion MRI in mice on a 3‐T clinical MR scanner

First‐pass contrast‐enhanced myocardial perfusion MRI in rodents has so far not been possible due to the temporal and spatial resolution requirements. We developed a new first‐pass perfusion MR method for rodent imaging on a clinical 3.0‐T scanner (Philips Healthcare, Best, The Netherlands) that employed 10‐fold k‐space and time domain undersampling with constrained image reconstruction, using temporal basis sets (k‐t principle component analysis) to achieve a spatial resolution of 0.2 × 0.2 × 1.5mm³ and an acquisition window of 43 msec. The method was successfully tested in five healthy and four infarcted mice (C57BL/6J) at heart rates of 495.1 ± 45.8 beats/min. Signal‐intensity‐time profiles showed a percentage myocardial signal increase of 141.3 ± 38.9% in normal mice, compared with 44.7 ± 32.4% in infarcted segments. Mean myocardial blood flow by Fermi function for constrained deconvolution in control mice was 7.3 ± 1.5 mL/g/min, comparable to published literature, with no significant differences between three myocardial segments. In infarcted segments, myocardial blood flow was significantly reduced to 1.2 ± 0.8 mL/g/min (P < 0.01). This is the first report of first‐pass myocardial perfusion MR in a mouse model on a clinical 3‐T MR scanner and using a k‐t undersampling method. Data were acquired on a 3‐T scanner, using an approach similar to clinical acquisition protocols, thus facilitating translation of imaging findings between rodent and human studies. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Axonal tracing of the normal and regenerating visual pathway of mouse,rat, frog,and fish using manganese‐enhanced MRI (MEMRI)

Purpose:

To assess optic nerve (ON) regeneration after injury by applying manganese‐enhanced MRI (MEMRI) in a study of comparative physiology between nonregenerating rat and mouse species and regenerating frog and fish species.

Materials and Methods:

The normal visual projections of rats, mice, frogs, and fish was visualized by intravitreal MnCl₂ injection followed by MRI. Rats and mice with ON crush (ONC) were divided into nonregenerating (ONC only), and regenerating animals with peripheral nerve graft (ONC+PNG; rats) or lens injury (ONC+LI; mice) and monitored by MEMRI at 1 and 20 days post‐lesion (dpl). Frog and fish with ON transection (ONT) were monitored by MEMRI up to 6 months postlesion (mpl).

Results:

Signal intensity profiles of the Mn²⁺‐enhanced ON were consistent with ON regeneration in the ONC+PNG and ONC+LI rat and mice groups, respectively, compared with the nonregenerating ONC groups. Furthermore, signal intensity profiles of the Mn²⁺‐enhanced ON obtained between 1 mpl and 6 mpl in the fish and frog groups, respectively, were consistent with spontaneous, complete ON regeneration.

Conclusion:

Taken together, these results demonstrate that MEMRI is a viable method for serial, in vivo monitoring of normal, induced, and spontaneously regenerating optic nerve axons in different species. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.