SPIRAL MRI for in vivo lithium-7 imaging: a feasibility study in mice after oral lithium treatment.

Lithium has been the frontline treatment for bipolar disorder for over 60 years. However, its mode of action and distribution in the brain is still incompletely understood. The primary isotope of lithium, lithium-7 (7Li), is a magnetic resonance (MR) active, spin-3/2 nucleus. However, its low MR sensitivity and the small brain size of mice make 7Li MR imaging (MRI) difficult in preclinical research. We tested four MRI sequences (FLASH, RARE, bSSFP, and SPIRAL) on lithium-containing phantoms, and bSSFP and SPIRAL on orally lithium-treated adult C57BL/6 mice. 7Li MR spectroscopy was acquired weekly at 9.4T to monitor the lithium uptake. The in vivo T1 relaxation time of 7Li was estimated in four mice. 4-h SPIRAL 7Li MRI was acquired in ten mice at a resolution of 2 × 2 × 3 mm3. SPIRAL MRI provided the highest signal-to-noise ratio (SNR) per unit acquisition time and the best image quality. We observed a non-homogeneous distribution of lithium in the mouse brain, with the highest concentrations in the cortex, ventricles, and basal brain regions. Almost no lithium signal was detected in the olfactory bulb and the cerebellum. We showed that in vivo 7Li MRI in mice is feasible, although with limited spatial resolution and SNR.

Radial compressed sensing imaging improves the velocity detection limit of single cell tracking time-lapse MRI.

PURPOSE: Time-lapse MRI enables tracking of single iron-labeled cells. Yet, due to temporal blurring, only slowly moving cells can be resolved. To study faster cells for example during inflammatory processes, accelerated acquisition is needed. METHODS: A rotating phantom system was developed to quantitatively measure the current maximum detectable speed of cells in time-lapse MRI. For accelerated cell tracking, an interleaved radial acquisition scheme was applied to phantom and murine brain in vivo time-lapse MRI experiments at 9.4 T. Detection of iron-labeled cells was evaluated in fully sampled and undersampled reconstructions with and without compressed sensing. RESULTS: The rotating phantom system enabled ultra-slow rotation of phantoms and a velocity detection limit of full-brain Cartesian time-lapse MRI of up to 172 µm/min was determined. Both phantom and in vivo measurements showed that single cells can be followed dynamically using radial time-lapse MRI. Higher temporal resolution of undersampled reconstructions reduced geometric distortion, the velocity detection limit was increased to 1.1 mm/min in vitro, and previously hidden fast-moving cells were recovered. In the mouse brain after in vivo labeling, a total of 42 ± 4 cells were counted in fully sampled, but only 7 ± 1 in undersampled images due to streaking artifacts. Using compressed sensing 33 ± 4 cells were detected. CONCLUSION: Interleaved radial time-lapse MRI permits retrospective reconstruction of both fully sampled and accelerated images, enables single cell tracking at higher temporal resolution and recovers cells hidden before due to blurring. The velocity detection limit as determined with the rotating phantom system increased two- to three-fold compared to previous results.

Functional Cardiovascular Characterization of the Common Marmoset (Callithrix jacchus).

Appropriate cardiovascular animal models are urgently needed to investigate genetic, molecular, and therapeutic approaches, yet the translation of results from the currently used species is difficult due to their genetic distance as well as their anatomical or physiological differences. Animal species that are closer to the human situation might help to bridge this translational gap. The common marmoset (Callithrix jacchus) is an interesting candidate to investigate certain heart diseases and cardiovascular comorbidities, yet a basic functional characterization of its hemodynamic system is still missing. Therefore, cardiac functional analyses were performed by utilizing the invasive intracardiac pressure-volume loops (PV loop) system in seven animals, magnetic resonance imaging (MRI) in six animals, and echocardiography in five young adult male common marmosets. For a direct comparison between the three methods, only data from animals for which all three datasets could be acquired were selected. All three modalities were suitable for characterizing cardiac function, though with some systemic variations. In addition, vena cava occlusions were performed to investigate the load-independent parameters collected with the PV loop system, which allowed for a deeper analysis of the cardiac function and for a more sensitive detection of the alterations in a disease state, such as heart failure or certain cardiovascular comorbidities.

Histological Findings and T2 Relaxation Time in Canine Menisci of Elderly Dogs-An Ex Vivo Study in Stifle Joints.

Osteoarthritis is a chronic disease that often affects the canine stifle joint. Due to their biomechanical function, the menisci in the canine stifle play an important role in osteoarthritis. They compensate for the incongruence in the joint and distribute and minimize compressive loads, protecting the hyaline articular cartilage from damage. Meniscal degeneration favors the development and progression of stifle joint osteoarthritis. Qualitative magnetic resonance imaging (MRI) is the current golden standard for detecting meniscal changes, but it has limitations in detecting early signs of meniscal degeneration. A quantitative MRI offers new options for detecting early structural changes. T2 mapping can especially visualize structural changes such as altered collagen structures and water content, as well as deviations in proteoglycan content. This study evaluated T2 mapping and performed a histological scoring of menisci in elderly dogs that had no or only low radiographic osteoarthritis grades. A total of 16 stifles from 8 older dogs of different sex and breed underwent ex vivo magnet resonance imaging, including a T2 mapping pulse sequence with multiple echoes. A histological analysis of corresponding menisci was performed using a modified scoring system. The mean T2 relaxation time was 18.2 ms and the mean histological score was 4.25. Descriptive statistics did not reveal a correlation between T2 relaxation time and histological score. Ex vivo T2 mapping of canine menisci did not demonstrate histological changes, suggesting that early meniscal degeneration can be present in the absence of radiological signs of osteoarthritis, including no significant changes in T2 relaxation time.

Inducing sterile pyramidal neuronal death in mice to model distinct aspects of gray matter encephalitis.

Up to one person in a population of 10,000 is diagnosed once in lifetime with an encephalitis, in 50-70% of unknown origin. Recognized causes amount to 20-50% viral infections. Approximately one third of affected subjects develops moderate and severe subsequent damage. Several neurotropic viruses can directly infect pyramidal neurons and induce neuronal death in cortex and hippocampus. The resulting encephalitic syndromes are frequently associated with cognitive deterioration and dementia, but involve numerous parallel and downstream cellular and molecular events that make the interpretation of direct consequences of sudden pyramidal neuronal loss difficult. This, however, would be pivotal for understanding how neuroinflammatory processes initiate the development of neurodegeneration, and thus for targeted prophylactic and therapeutic interventions. Here we utilized adult male NexCreERT2xRosa26-eGFP-DTA (= 'DTA') mice for the induction of a sterile encephalitis by diphtheria toxin-mediated ablation of cortical and hippocampal pyramidal neurons which also recruits immune cells into gray matter. We report multifaceted aftereffects of this defined process, including the expected pathology of classical hippocampal behaviors, evaluated in Morris water maze, but also of (pre)frontal circuit function, assessed by prepulse inhibition. Importantly, we modelled in encephalitis mice novel translationally relevant sequelae, namely altered social interaction/cognition, accompanied by compromised thermoreaction to social stimuli as convenient readout of parallel autonomic nervous system (dys)function. High resolution magnetic resonance imaging disclosed distinct abnormalities in brain dimensions, including cortical and hippocampal layering, as well as of cerebral blood flow and volume. Fluorescent tracer injection, immunohistochemistry and brain flow cytometry revealed persistent blood-brain-barrier perturbance and chronic brain inflammation. Surprisingly, blood flow cytometry showed no abnormalities in circulating major immune cell subsets and plasma high-mobility group box 1 (HMGB1) as proinflammatory marker remained unchanged. The present experimental work, analyzing multidimensional outcomes of direct pyramidal neuronal loss, will open new avenues for urgently needed encephalitis research.

Comparison of cine and real-time cardiac MRI in rhesus macaques.

Cardiac MRI in rhesus macaques, a species of major relevance for preclinical studies on biological therapies, requires artificial ventilation to realize breath holding. To overcome this limitation of standard cine MRI, the feasibility of Real-Time (RT) cardiac MRI has been tested in a cohort of ten adult rhesus macaques using a clinical MR-system. In spite of lower tissue contrast and sharpness of RT-MRI, cardiac functions were similarly well assessed by RT-MRI compared to cine MRI (similar intra-subject repeatability). However, systematic underestimation of the end-diastolic volume (31 ± 9%), end-systolic volume (20 ± 11%), stroke volume (40 ± 12%) and ejection fraction (13 ± 9%) hamper the comparability of RT-MRI results with those of other cardiac MRI methods. Yet, the underestimations were very consistent (< 5% variability) for repetitive measurements, making RT-MRI an appropriate alternative to cine MRI for longitudinal studies. In addition, RT-MRI enabled the analysis of cardio-respiratory coupling. All functional parameters showed lower values during expiration compared to inspiration, most likely due to the pressure-controlled artificial ventilation. In conclusion, despite systematic underestimation of the functional parameters, RT-MRI allowed the assessment of left ventricular function in macaques with significantly less experimental effort, measurement time, risk and burden for the animals compared to cine MRI.

Cardiac MRI in common marmosets revealing age-dependency of cardiac function.

The aim of this study was to establish a feasible and robust magnetic resonance imaging protocol for the quantitative assessment of cardiac function in marmosets and to present normal values of cardiac function across different ages from young adult, middle-aged, to very old clinically healthy animals. Cardiac MRI of 33 anesthetized marmosets at the age of 2-15 years was performed at 9.4 T using IntraGate-FLASH that operates without any ECG-triggering and breath holding. Normalized to post-mortem heart weight, the left ventricular end-diastolic volume (LV-EDV) was significantly reduced in older marmosets. The LV end-systolic volume (LV-ESV) and the LV stroke volume (LV-SV) showed a similar trend while the LV ejection fraction (LV-EF) and wall thickening remained unchanged. Similar observations were made for the right ventricle. Moreover, the total ventricular myocardial volume was lower in older monkeys while no significant difference in heart weight was found. In conclusion, IntraGate-FLASH allowed for quantification of left ventricular cardiac function but seems to underestimate the volumes of the right ventricle. Although less strong and without significant sex differences, the observed age related changes were similar to previously reported findings in humans supporting marmosets as a model system for age related cardiovascular human diseases.

Highly selective organ distribution and cellular uptake of inorganic-organic hybrid nanoparticles customized for the targeted delivery of glucocorticoids.

We previously reported that inorganic-organic hybrid nanoparticles (IOH-NPs) containing the synthetic glucocorticoid (GC) betamethasone show efficient anti-inflammatory activity in mice. Here, we employed IOH-NPs with the chemical composition Gd3+2[AMP]2-3 (AMP: adenosine monophosphate) to determine their in vivo distribution by magnetic resonance imaging after intraperitoneal injection. We show that IOH-NPs distribute throughout the peritoneal cavity from where they get rapidly cleared and then localize to abdominal organs. Our findings were confirmed by analyzing individual mouse organs ex vivo following injection of IOH-NPs with the chemical composition [ZrO]2+[(BMP)0.9(FMN)0.1]2- (BMP: betamethasone phosphate, FMN: flavin mononucleotide) or [ZrO]2+[(HPO4)0.9(FMN)0.1]2- using inductively coupled plasma mass spectrometry and flow cytometry. To characterize the mechanism of cellular uptake in vitro, we tested different cell lines for their ability to engulf IOH-NPs by flow cytometric analysis taking advantage of the incorporated fluorescent dye FMN. We found that IOH-NPs were efficiently taken up by macrophages, to a lesser extent by fibroblasts, epithelial cells, and myoblasts, and hardly at all by both T and B lymphocytes. Characterization of the endocytic pathway further suggested that IOH-NPs were internalized by macropinocytosis, and imaging flow cytometry revealed a strong colocalization of the engulfed IOH-NPs with the lysosomal compartment. Intracellular release of the functional anions from IOH-NPs was confirmed by the ability of the GC betamethasone to downregulate the expression of surface receptors on bone marrow-derived macrophages. Taken together, our findings unveil the mechanistic basis of an anti-inflammatory GC therapy with IOH-NPs, which may entail translational approaches in the future.

Non-contrast enhanced molecular characterization of C6 rat glioma tumor at 7 T.

PURPOSE: The aim of this study was to investigate, how to assess the relevant magnetization changes in the rat brain tissue due to the present of glioma tumor and its growth at a 7 T animal magnetic resonance imaging (MRI) system. MATERIAL AND METHODS: For this study, a custom-built two dimensional (2D) chemical exchange saturation transfer (CEST) pulse sequence was optimized for different tissue properties using fresh and cooked quail eggs. C6 tumor cells were investigated by in-vivo and post-mortem measurements in six Wistar rats using the optimized CEST sequence up to 5 weeks. Magnetization transfer ratio (MTR)- and asymmetric MTR (MTRasym)-maps of rat brains were created at different frequency offsets. In-vivo results were verified by 1H spectroscopic, histological and also in-vitro C6 cell culture examinations. RESULTS: The CEST module for the optimal visualization of magnetization effects consists of five RF-pulses, each with a duration of 20 ms and a flip angle of 180°. In-vivo and post-mortem z-spectra of the cerebrospinal fluid (CSF), cortex, myelinated/demyelinated, healthy and tumorous tissue and tumor rim were obtained. The magnetization level and shape of the z-spectra on the upfield and downfield from the water peak were not the same. The magnetization dips on the upfield and downfield from the water peak of the z-spectra disappeared due to the thermal denaturation in cooked quail eggs and due to formaldehyde-induced fixation in post-mortem rat brains. The z-spectra of the rat brain in a range of ±2 to ±4 ppm displayed valuable information about the differentiation of various brain regions from the tumor tissue. Histological examinations confirmed our results. The C6 cell culture examinations showed that the observed magnetization changes in the rat brain occurred only due to the interaction between glioma cells and their environment in the rat brain and not from the C6 tumor cells. CONCLUSIONS: Based on our in-vivo and post-mortem results, it is to be recommended to create the MTR-maps at a special offset frequency depending on the aim of research project instead of MTRasym-maps. Otherwise, the desired effect attenuates or vanishes.

Imperfect magnetic field gradients in radial k-space encoding-Quantification, correction, and parameter dependency.

PURPOSE: Sensitivity to imperfections of image-encoding gradient fields may significantly impair widespread use of radial MR data acquisition. Such imperfections can cause individual echo shifts for each spoke acquired in the k-space and may produce severe image artifacts. Therefore, fast and robust methods to quantify and correct for those echo shifts are required. THEORY AND METHODS: Echo shifts can be induced by inhomogeneities of the static magnetic field (δnB ) and by imbalances of the imaging gradients (δnG ) mainly caused by eddy currents. However, mismatch between data acquisition and gradient switching may additionally play a role. From the position of the echo maxima of 2 opposing spokes, δnG and δnB can be determined and corrected by adapting the read-dephasing gradient accordantly. This approach was implemented on MR-systems of different field strengths, gradient systems, and vendors, and the dependencies of echo shift and acquisition parameters were analyzed. Data sets of phantoms and of mice under in vivo conditions were obtained using RF-spoiled 2D radial-FLASH. RESULTS: The presented method allowed for echo-shift detection and correction of < 1 data point, significantly improving the image quality in vitro and in vivo. Moreover, the method separated the effect of imbalanced gradients from those of magnetic inhomogeneities. The observed echo shifts were MR system-specifically dependent on acquisition parameters such as gradient strengths and dwell time. CONCLUSIONS: By acquiring 12 spokes for a certain set of acquisition parameters, the proposed method successfully corrects echo shift-related image artifacts independently of the MR system.

Alterations of the Blood-Brain Barrier and Regional Perfusion in Tumor Development: MRI Insights from a Rat C6 Glioma Model.

OBJECTIVES: Angiogenesis and anti-angiogenetic medications play an important role in progression and therapy of glioblastoma. In this context, in vivo characterization of the blood-brain-barrier and tumor vascularization may be important for individual prognosis and therapy optimization. METHODS: We analyzed perfusion and capillary permeability of C6-gliomas in rats at different stages of tumor-growth by contrast enhanced MRI and dynamic susceptibility contrast (DSC) MRI at 7 Tesla. The analyses included maps of relative cerebral blood volume (CBV) and signal recovery derived from DSC data over a time period of up to 35 days after tumor cell injections. RESULTS: In all rats tumor progression was accompanied by temporal and spatial changes in CBV and capillary permeability. A leakage of the blood-brain barrier (slow contrast enhancement) was observed as soon as the tumor became detectable on T2-weighted images. Interestingly, areas of strong capillary permeability (fast signal enhancement) were predominantly localized in the center of the tumor. In contrast, the tumor rim was dominated by an increased CBV and showed the highest vessel density compared to the tumor center and the contralateral hemisphere as confirmed by histology. CONCLUSION: Substantial regional differences in the tumor highlight the importance of parameter maps in contrast or in addition to region-of-interest analyses. The data vividly illustrate how MRI including contrast-enhanced and DSC-MRI may contribute to a better understanding of tumor development.

Assessment of disintegrant efficacy with fractal dimensions from real-time MRI.

An efficient disintegrant is capable of breaking up a tablet in the smallest possible particles in the shortest time. Until now, comparative data on the efficacy of different disintegrants is based on dissolution studies or the disintegration time. Extending these approaches, this study introduces a method, which defines the evolution of fractal dimensions of tablets as surrogate parameter for the available surface area. Fractal dimensions are a measure for the tortuosity of a line, in this case the upper surface of a disintegrating tablet. High-resolution real-time MRI was used to record videos of disintegrating tablets. The acquired video images were processed to depict the upper surface of the tablets and a box-counting algorithm was used to estimate the fractal dimensions. The influence of six different disintegrants, of different relative tablet density, and increasing disintegrant concentration was investigated to evaluate the performance of the novel method. Changing relative densities hardly affect the progression of fractal dimensions, whereas an increase in disintegrant concentration causes increasing fractal dimensions during disintegration, which are also reached quicker. Different disintegrants display only minor differences in the maximal fractal dimension, yet the kinetic in which the maximum is reached allows a differentiation and classification of disintegrants.

Tablet disintegration studied by high-resolution real-time magnetic resonance imaging.

The present work employs recent advances in high-resolution real-time magnetic resonance imaging (MRI) to investigate the disintegration process of tablets containing disintegrants. A temporal resolution of 75 ms and a spatial resolution of 80 × 80 µm with a section thickness of only 600 µm were achieved. The histograms of MRI videos were quantitatively analyzed with MATLAB. The mechanisms of action of six commercially available disintegrants, the influence of relative tablet density, and the impact of disintegrant concentration were examined. Crospovidone seems to be the only disintegrant acting by a shape memory effect, whereas the others mainly swell. A higher relative density of tablets containing croscarmellose sodium leads to a more even distribution of water within the tablet matrix but hardly impacts the disintegration kinetics. Increasing the polacrilin potassium disintegrant concentration leads to a quicker and more thorough disintegration process. Real-time MRI emerges as valuable tool to visualize and investigate the process of tablet disintegration.

Correction of gradient-induced phase errors in radial MRI.

PURPOSE: To correct gradient-induced phase errors in radial MRI. METHODS: Gradient-induced eddy currents affect the MRI data acquisition by gradient delays and phase errors that may lead to severe image artifacts for non-Cartesian imaging scenarios such as radial trajectories. While gradient delays are dealt with by respective shifts of the acquisition window during radial image acquisition, this work introduces a simple method for quantifying and correcting phase errors from the actual data prior to image reconstruction. For a given gradient system, the approach yields a specific phase error per gradient that can be used for correcting the raw data. RESULTS: Phantom studies at 9.4 T demonstrated marked improvements in radial image quality. It could be shown that the phase correction is not compromised by data undersampling. Moreover, the selective correction of gradient-induced phase errors retained the phase information caused by different concentrations of a paramagnetic contrast agent. CONCLUSION: The proposed method does not require additional reference measurements and separately corrects for phase errors induced by eddy currents, while retaining the residual phase of the object which may carry physiologic information.