External waveguide magnetic resonance imaging for lower limbs at 3 T

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 B1 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.