What are the details of the MRI scanner design?
The design of MRI scanner has to be very precise to allow for optimum performance with minimum interference. The design of the MRI system has to consider some trade-offs between customer needs, image quality and cost to the customer. 95% of full body MRI scanners are built in a cylindrical shape to provide a high quality magnetic field for a wide variety of procedures that satisfies patient requirements. To produce the high quality images required for the imaging techniques, the magnets have to be designed and oriented to give a spatial and temporal uniformity typically on the order of several parts per million over the whole volume being imaged. To achieve this high uniformity, a multiple coil design is implemented using between 6 to 10 coils positioned for tolerances on the order of micrometers. The increase in coils used results in a higher uniformity at the cost of an increase financially and in complexity. With this strong magnetic field in the scanner, the industry-standard safety limit for the maximum magnetic field outside the scanner has to be met at a maximum of 5 gausses. Eqn is used to calculate the magnet field outside of the scanning system.
Where B_cthe magnetic field at the dipole center, R is the radius of the magnetic field, θ is the angle between the magnet axis and the direction to the field point r.
Three types of shielding are implemented to reduce the field strength outside the system, active shielding; where large diameter superconducting coils suppress the magnetic field outside, passive shielding; iron attached to the outer.