RESUMO
PURPOSE: We report a method based on the traveling-wave MRI approach, in order to acquire images of human lower limbs with an external waveguide at 3 T. METHODS: We use a parallel-plate waveguide and an RF surface coil for reception, while a whole-body birdcage is used for transmission. The waveguide and the surface coil are located right outside the magnet, in the magnetic resonance (MR) conditional devices zone. We ran numerical simulations to investigate the B 1 field generated by the surface coil located at one of the waveguides, as well as a saline-solution phantom positioned on the opposite side (150 cm away) inside the magnet. RESULTS: We obtained phantom images by varying the distance between the coil and the phantom, in order to investigate the signal-to-noise ratio and to validate our numerical simulations. Lower limb images of a healthy volunteer were also acquired, demonstrating the viability of this approach. Standard pulse sequences were used and no physical modifications were made to the MR imager. CONCLUSIONS: These numerical and experimental results show that traveling-wave MRI can produce high-quality images with only a simple waveguide and an RF coil located outside the magnet. This can be particularly favorable when acquiring images of lower limbs requiring a larger field of view.
Assuntos
Imageamento por Ressonância Magnética , Ondas de Rádio , Humanos , Extremidade Inferior/diagnóstico por imagem , Imagens de Fantasmas , Razão Sinal-RuídoRESUMO
We report a method for remote excitation of the RF signal for preclinical-equivalent ultra high field Magnetic Resonance Imaging (MRI). A parallel-plate waveguide together with a bio-inspired surface coil were used to perform remote excitation experiments to acquire images with a small-bore MR imager at 15.2 T. The imager bore size limits the RF coil transmitter dimensions, so the Gielis super-formula was used to design an RF coil with small dimensions. Electromagnetic simulations of the principal mode were run to study the waveguide filled with air and loaded with a saline solution-filled tube. Radiation patterns were also computed in a semi-anechoic chamber for the same scenarios as above. A saline solution-filled spherical phantom and a formaldehyde-fixed mouse phantom were used to acquire images. Radiation patterns showed an omnidirectional distribution with no side lobes, and a very smooth behaviour with almost no loss of information in the saline solution-filled tube and without. The theoretical wave impedance was calculated and compared with simulated results showing an excellent correspondence. Spherical phantom image data and simulation results of B1 were contrasted and showed an important correlation. Ex vivo mouse images were of high quality and exhibited clear delineation of anatomical structures. These imaging results are in very good agreement with the simulations. Numerical, theoretical and experimental results validate this approach, using a bio-inspired surface coil with a simple waveguide for preclinical small-bore MRI at ultra high field.