Speaker
Description
The Brazilian Center for Research in Energy and Materials (CNPEM) has built a medium field Magnetic Resonance Image (MRI) prototype as part of an overall effort on the in-house development of fundamental MRI technology know-how. The system is comprised of a permanent-magnet-based dipole, a set of planar coils for gradient field generation and a radiofrequency system with a transmission-reception radiofrequency coil. The 0.4 T main-field system is based on NdFeB permanent-magnet disks and its magnetic structure (return yoke, poles and shimming pieces) made of low-carbon iron steel. The field homogeneity aimed at a 4 cm diameter spherical volume (DSV) was in the order of 10 ppm. To achieve these requirements, the pole surface and the magnet disks dimensions were optimized via finite elements magnetic simulations. A special procedure was devised allowing for the high magnetic force between the NdFeB disks at the assembly procedure. The magnet was characterized using a three-dimensional hall probe mapping system allowing for field shimming iterations. Two strategies were considered for the correction of homogeneity: movable ferromagnetic cylinders inside the disks, shimming the main-field direction; and lateral height shifts, adjusting angle inclination and correcting the transversal directions fields. The gradient subsystem consists of three planar coils on either side of the DSV, one pair per axis of the image. Each coil combination must generate a magnetic field linearly dependent on its corresponding axis, with high angular coefficient and low nonlinearities. This system must have small inductance and resistance, allowing for higher electrical current to pass through, thereby increasing the gradient field strength. Since there is a limit to how low these electrical properties can reach without compromising the field quality, an optimization algorithm is necessary. The open-source code pyCoilGen was used to find an optimal wire pattern and position of the coils. Parallel computing was employed to test hundreds of different configurations. Each generated coil pair had their electrical and field properties quantified to rank each individual design and choose the best suited result for fabrication and characterization. The magnetic subsystems were integrated into the final MRI prototype system allowing three-dimensional images with phantom samples to be acquired at the end of 2024.
