The physical basis of image formation using MRI is quite different from CT, which is based on differential x-ray absorption coefficients. Even though both are tomographic imaging techniques, the meaning of bright and dark signals in MRI is relative to the pulse sequence eliciting them and has very little resemblance to the contrast in CT.
The core of an MRI system is the large magnet needed to generate the strong, constant, and uniform magnetic field, as well as the magnetic gradients. The magnet is composed of coils of a special superconducting wire that loses all resistance to electrical current when submerged in liquid helium. At this temperature, -450°F (-269°C), the coils can handle the relatively large currents of electricity required to produce the magnetic field. A specially designed, thermally insulated container encloses the magnet coils and the liquid helium. The liquid slowly boils away and must be replaced at regular intervals. The magnet is housed in a special room containing steel sheets and copper screen, which shield the system against interference by steel and radio waves on the outside and vice versa. The other components of the system, consisting of a radio transmitter, a sophisticated rf receiver, and a high-speed large-memory computer, are located near the operator's console just outside the room.
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