Purpose: Selection field, which contains a field-free line (FFL), is used in Magnetic Particle Imaging (MPI) to allow spatial selection of a specific region to achieve spatial encoding along a line. Since permanent magnets can generate a static magnetic field, there is no need to consider the issue of power consumption. Studies show that the shape and performance of permanent magnets affect the quantity of the magnetic field generated. In this study, FFL was formed by using dipole Halbach cylinders and the effects of magnet shapes on FFL performance were investigated.
Methods: Dipole Halbach rings consist of six different shapes of permanent magnets (square, hexagon, octagon, dodecagon, hexadecagon, circle) were used for generating FFL. NdFeB (neodymium-iron-boron) was chosen as the magnet material. For each model, magnetic flux density and gradient field strength were simulated using COMSOL Multiphysics 5.4.
Results: Magnetic flux density and gradient field strength increase on the y-axis from the rectangular magnet shape to the cylindrical shape. Gradient values obtained from the permanent magnets (square, hexagon, octagon, dodecagon, hexadecagon, circle) are 3.6 T/m, 4 T/m, 4.4 T/m, 4.6 T/m, 4.7 T/m, 4.8 T/m, respectively. The homogeneity of the gradient for each design was above 95% and did not change according to the shapes of the magnets used. Also, the gradient values were stable along 35 mm.
Conclusion: The outcomes show that Halbach magnet systems formed with different magnet shapes have a significant impact on FFL performance. Polygons were used to improve the square-shaped permanent magnets. Gradient has been increased by 25% using the polygonal magnet shapes. Future studies will be continued to increase the stability and bore size of the system.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the Technological Research Council of Turkey through TUBITAK Grants (115E776 & 115E777). No conflict of interest.