Neuroradiology
Figure 4: Ultra-high-field Magnetic Resonance Imaging Spectroscopy
this structure is currently not feasible. However, successful imaging of the mesial temporal lobe region, including the parahippocampal gyri, has been reported.21
Magnetic Resonance Spectroscopy With 7 and 8 T UHF MRI,73
imaging biomarkers from a much smaller
volume within the lesion can be readily obtained to investigate the underlying pathophysiologies (see Figure 4). These biomarkers include:
• • •
biomolecular structures related to energy metabolism, such as glucose and creatine;
neurotransmitters, such as glutamate and gamma-aminobutyric acid (GABA);
compounds involved in cell growth, such as choline; compounds involved in axon growth, such as NAA; and
• compounds involved in osmoregulation, such as taurine and inositol; and molecules that are antioxidants, such as glutathione.61
Ultra-high-field 7 T chemical-shift imaging of the brain of a healthy human volunteer using simulation echo acquisition mode (STEAM) with a short echo time (18 ms) resolves additionally to the main resonances of N-acetylaspartate (NAA), creatine (Cr), and choline (Cho), as well as metabolites with smaller resonances in the spectra, such as myo-inositol (MI), scyllo-inositol (SI), and glutamine/glutamate (Glx).
Figure 5: Ultra-high-field 7 T Magnetic Resonance Imaging of the Knee
Diffusion-weighted Imaging and Diffusion Tensor Imaging The shortcomings of DWI and DTI, such as limited spatial resolution and low SNR, can be effectively mitigated by the transition to high magnetic field strengths. However, DTI with single-excitation protocols even at UHF strengths still faces serious challenges, including limited spatial resolution, susceptibility to magnetic field inhomogeneity, and low SNR. The parallel imaging technique sensitivity encoding (SENSE) helps to overcome these challenges and makes diagnostic DWI and fiber tracking possible at 7 and 8 T UHF MRI.
Cortical Activation
However, there are not many reports demonstrating the superiority of BOLD-related techniques using UHF MRI in the evaluation of CNS diseases. As substantial advances are made, particularly in overcoming B0 and B1 inhomogeneity and susceptibility artifacts, the efficacy of UHF MRI in brain activation studies will be better defined.
The increases in SNR afforded by UHF should substantially improve the assessment of cortical function using the blood-oxygenation-level-dependent (BOLD) technique.74
Musculoskeletal System
Top: Coronal image of the knee joint of a patient with a meniscal tear acquired at 1.5 T (left), 3 T (middle), and 7 T (right); Bottom: Enlarged views of the tear from the images above. Radial lines projected horizontally within the menisca are clearly visualized on the 7 T magnetic resonance image.
minimized to allow the orbital region and the hippocampus to be visualized.20,21
Specialized orbital surface coils will provide
Eight Tesla MRI has demonstrated excellent visualization of the microstructures of the orbit and good correlation with pathologic specimens in melanoma.20
further improved anatomic and functional imaging for the evaluation of the microenvironment of orbital diseases. High-quality images with a voxel size of 0.15 x 0.25 x 1 using 7 T UHF MRI and a dedicated surface coil have been optimized to provide high-spatial-resolution imaging of the human eye.20
Posterior Fossa and Medial Temporal Lobe
Compared with conventional MRI, much worse susceptibility artifacts are seen on UHF MRI at the skull base and posterior fossa. Unfortunately, the imaging quality of the posterior fossa is so poor that imaging of
20
Ultra-high-field Body Magnetic Resonance Imaging Body imaging including the cardiopulmonary, abdominal, and pelvic regions presents the greatest challenge in UHF MRI. The normal
US RADIOLOGY
For musculoskeletal imaging, UHF MRI provides improved SNR, imaging tissue contrast, spatial resolution, and functional imaging capability. With its close proximity to the targeted imaging area, extremity coil design may not be as challenging as in other imaging regions with larger fields of view, such as whole-body imaging. However, there are significant challenges in applying UHF MRI to musculoskeletal imaging owing to field inhomogeneity, susceptibility artifacts, chemical-shift artifacts, and pro longed T1 relaxation times. Despite these challenges, there are opportunities to improve musculoskeletal imaging by using UHF MRI. By modifying imaging pulse sequences and coil design, 7 T UHF MRI was reported to demonstrate feasibility and value in imaging and quantifying the microarchitecture of the cortical and trabecular bone, endplates, and cartilage of the wrist, tibia, ankle, and knee (see Figure 5).25,59,74
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