There are three important or main components in an MRI scanner static magnetic field, radio frequency coils and gradient coils which majorly contribute towards creating of images, yet there are other components also...
Static magnetic field
it is an absolute necessity for MRI, providing the magnetic in magnetic resonance imaging.MRI scanner use strong magnetic field to align certain nuclei within the human body (commonly Hydrogen) to allow mapping of tissue properties. The magnetic field strength is in fact proportional to current strength ,so that by adjusting the current strength in a wire one could precisely control field intensity .these findings led to the development of electromagnets ,which generate their fields by passing current through tight coils of wire .Nearly all scanners today create static magnetic field through electromagnetism. There are in general two criteria for a suitable magnetic field in MRI ,the first is uniformity (homogeneity) and second is strength. Uniformity is necessary in what we want to create Images of the body that do not depend on which MRI scanner we are using or how the body is positioned in the field .If magnetic field were Inhomogeneous,the signal measured from a given part of the body would depend upon where it is located in the magnetic field
A simple design for generating a homogeneous magnetic field is the HELMHOLTZ PAIR
This is a pair of circular loops that carry identical current and are separated by a distance equal to the radius of the loops, an even more uniform field can be generated by a Solenoid, and the internal field near its center is highly homogeneous
Field Strength in contrast to uniformity requires force rather than finesse, to generate this field, we require enormous electric power and thus enormous expense.MRI scanners use Super conducting electromagnets whose wires are cooled by cryogens (liquid helium) to reduce their temperature to near absolute zero. Coil windings are typically made of metal alloys such as niobium-titanium, which when immersed in liquid helium reach temperatures of less than 12K (-261 deg).At this extremely low temperature ,the resistance in the wire disappears thereby enabling a strong and lasting electric current to be generated with no power requirements and minimal cost