7T Magnetic Resonance Imaging Scanner Research Articles

7T MRI features in control human hippocampus and hippocampal sclerosis: an ex vivo study with histologic correlations.

Hippocampal sclerosis (HS) is the major structural brain lesion in patients with temporal lobe epilepsy (TLE). However, its internal anatomic structure remains difficult to recognize at 1.5 or 3 Tesla (T) magnetic resonance imaging (MRI), which allows neither identification of specific pathology patterns nor their proposed value to predict postsurgical outcome, cognitive impairment, or underlying etiologies. We aimed to identify specific HS subtypes in resected surgical TLE samples on 7T MRI by juxtaposition with corresponding histologic sections. Fifteen nonsclerotic and 18 sclerotic hippocampi were studied ex vivo using an experimental 7T MRI scanner. T2 -weighted images (T2wi) and diffusion tensor imaging (DTI) data were acquired and validated using a systematic histologic analysis of same specimens along the anterior-posterior axis of the hippocampus. In nonsclerotic hippocampi, differences in MR intensity could be assigned to seven clearly recognizable layers and anatomic boundaries as confirmed by histology. All hippocampal subfields could be visualized also in the hippocampal head with three-dimensional imaging and angulated coronal planes. Only four discernible layers were identified in specimens with histopathologically confirmed HS. All sclerotic hippocampi showed a significant atrophy and increased signal intensity along the pyramidal cell layer. Changes in DTI parameters such as an increased mean diffusivity, allowed to distinguish International League Against Epilepsy (ILAE) HS type 1 from type 2. Whereas the increase in T2wi signal intensities could not be attributed to a distinct specific histopathologic substrate, that is, decreased neuronal or increased glial cell densities, intrahippocampal projections and fiber tracts were distorted in HS specimens suggesting a complex disorganization of the cellular composition, fiber networks, as well as its extracellular matrix. Our data further advocate high-resolution MRI as a helpful and promising diagnostic tool for the investigation of hippocampal pathology along the anterior-posterior extent in TLE, as well as in other neurologic and neurodegenerative disorders.

Temporal analysis of Z-Gradient coil eddy currents in tungsten collimator with different resistivities for SPECT/MRI

Combining Single Photon Emission Computed Tomography (SPECT) with Magnetic Resonance Imaging (MRI) results in an interaction of the time-varying magnetic field gradients with the highly conducting tungsten collimator, which generates a secondary magnetic field causing spatial distortions in reconstructed MR images. Accurate simulations are important for the characterization of these eddy currents and to further optimize the gradient coils and the collimator design. This study investigated the temporal variations of eddy currents in a tungsten collimator for pre-clinical SPECT/MRI due to only the z-gradient coil for different resistivities of tungsten, which can be obtained through additive manufacturing. We modeled a z-gradient coil and a collimator using FEKO, a 3D electromagnetic simulation tool. A time analysis approach was used to generate the pulsed magnetic field gradient. The approach was validated with measurements using a 7T MRI scanner. Simulations show that when tungsten with high resistivity (ρ=370 nΩ.m) is used, eddy currents generate an added magnetic field representing 1.72 % of the nominal gradient field in a Field Of View (FOV) of 3 cm. This percentage increases rapidly for tungsten with lower resistivities, which was expected since a higher resistivity implies lower current densities. A higher density of tungsten is preferred leading to low resistivity but results in stronger induced eddy currents. A compromise needs to be made between the eddy currents strength and the design of the collimator. Using the insights gained from these simulations, the next step is to optimize the design of the gradient coils and the collimator to further reduce the eddy current effects.

Longitudinal monitoring of stem cell grafts in vivo using magnetic resonance imaging with inducible maga as a genetic reporter.

Purpose: The ability to longitudinally monitor cell grafts and assess their condition is critical for the clinical translation of stem cell therapy in regenerative medicine. Developing an inducible genetic magnetic resonance imaging (MRI) reporter will enable non-invasive and longitudinal monitoring of stem cell grafts in vivo. Methods: MagA, a bacterial gene involved in the formation of iron oxide nanocrystals, was genetically modified for in vivo monitoring of cell grafts by MRI. Inducible expression of MagA was regulated by a Tet-On (Tet) switch. A mouse embryonic stem cell-line carrying Tet-MagA (mESC-MagA) was established by lentivirus transduction. The impact of expressing MagA in mESCs was evaluated via proliferation assay, cytotoxicity assay, teratoma formation, MRI, and inductively coupled plasma atomic emission spectroscopy (ICP-OES). Mice were grafted with mESCs with and without MagA (mESC-MagA and mESC-WT). The condition of cell grafts with induced “ON” and non-induced “OFF” expression of MagA was longitudinally monitored in vivo using a 7T MRI scanner. After imaging, whole brain samples were harvested for histological assessment. Results: Expression of MagA in mESCs resulted in significant changes in the transverse relaxation rate (R2 or 1/T2) and susceptibility weighted MRI contrast. The pluripotency of mESCs carrying MagA was not affected in vitro or in vivo. Intracranial mESC-MagA grafts generated sufficient T2 and susceptibility weighted contrast at 7T. The mESC-MagA grafts can be monitored by MRI longitudinally upon induced expression of MagA by administering doxycycline (Dox) via diet. Conclusion: Our results demonstrate MagA could be used to monitor cell grafts noninvasively, longitudinally, and repetitively, enabling the assessment of cell graft conditions in vivo.

In vivo estimation of bone stiffness at the distal femur and proximal tibia using ultra-high-field 7-Tesla magnetic resonance imaging and micro-finite element analysis

The goal of this study was to demonstrate the feasibility of using 7-Tesla (7T) magnetic resonance imaging (MRI) and micro-finite element analysis (µFEA) to evaluate mechanical and structural properties of whole, cortical, and trabecular bone at the distal femur and proximal tibia in vivo. 14 healthy subjects were recruited (age40.7±15.7years). The right knee was scanned on a 7T MRI scanner using a 28 channel-receive knee coil and a three-dimensional fast low-angle shot sequence (TR/TE20ms/5.02ms, 0.234mm×0.234mm×1mm, 80 axial images, 7min 9s). Bone was analyzed at the distal femoral metaphysis, femoral condyles, and tibial plateau. Whole, cortical, and trabecular bone stiffness was computed using µFEA. Bone volume fraction (BVF), bone areas, and cortical thickness were measured. Trabecular bone stiffness (933.7±433.3MPa) was greater than cortical bone stiffness (216±152MPa) at all three locations (P<0.05). Across locations, there were no differences in bone stiffness (whole, cortical, or trabecular). Whole, cortical, and trabecular bone stiffness correlated with BVF (R≥0.69, P<0.05) and inversely correlated with corresponding whole, cortical, and trabecular areas (R≤-0.54, P<0.05), but not with cortical thickness (R<-0.11, P>0.05). Whole, cortical, and trabecular stiffness correlated with body mass index (R≥0.62, P<0.05). In conclusion, at the distal femur and proximal tibia, trabecular bone contributes 66-74% of whole bone stiffness. 7T MRI and µFEA may be used as a method to provide insight into how structural properties of cortical or trabecular bone affect bone mechanical competence in vivo.

Multimodal tumor imaging by iron oxides and quantum dots formulated in poly (lactic acid)-d-alpha-tocopheryl polyethylene glycol 1000 succinate nanoparticles

This work developed a multimodal imaging system by co-encapsulating superparamagnetic iron oxides (IOs) and quantum dots (QDs) in the nanoparticles of poly (lactic acid) - d-α-tocopheryl polyethylene glycol 1000 succinate (PLA-TPGS) for concurrent imaging of the magnetic resonance imaging (MRI) and the fluorescence imaging to combine their advantages and to overcome their disadvantages as well as to promote a sustained and controlled imaging with passive targeting effects to the diseased cells. The QDs and IOs-loaded PLA-TPGS NPs were prepared by a modified nanoprecipitation method, which were then characterized for their size and size distribution, zeta potential and the imaging agent encapsulation efficiency. The transmission electron microscopy (TEM) images showed direct evidence for the well-dispersed distribution of the QDs and IOs within the PLA-TPGS NPs. The cellular uptake and the cytotoxicity of the PLA-TPGS NPs formulation of QDs and IOs were investigated in vitro with MCF-7 breast cancer cells, which were conducted in close comparison with the free QDs and IOs at the same agent dose. The Xenograft model was also conducted for biodistribution of the QDs and IOs-loaded PLA-TPGS NPs among the various organs, which showed greatly enhanced tumor imaging due to the passively targeting effects of the NPs to the tumor. Images of tumors were acquired in vivo by a 7T MRI scanner. Further ex vivo images of the tumors were obtained by confocal laser scanning microscopy. Such a multimodal imaging system shows great advantages of both contrast agents making the resultant probe highly sensitive with good depth penetration, which confirms the diagnosis obtained from each individual imaging. With therapeutics co-encapsulation and ligand conjugation, such nanoparticles system can realize a multi-functional system for medical diagnosis and treatment.