Edit from "MRI Indication for the referring physician"
http://www.gcnet.com/maven/aurora/mri/intro.html

Following is a quick overview of the main components of the MRI Hardware System. These include:
  1.The Main Magnet
  2.Gradient System
  3.RF Coils
  4.Transmitters
  5.Receivers
  6.Computer

THE MAIN MAGNET

The large donut-shaped structure in which the patient lies. This main magnetic field is kept constant. Its strength is measured in units called Tesla or T and ranges from 0.08 T to 2 T.

RECEIVER

The receiver picks up and amplifies signals from the body.

COMPUTER

All of the hardware except for the main magnet is under the control of a highly sophisticated computer and associated software.

GRADIENT SYSTEM

Usually comprised of three electromagnets that can be turned on and off rapidly, one each for three coordinates (X, Y, & Z directions). It is used to generate a small magnetic field that, when added to the main field, will select, slice, and encode position in the image.

RF COILS

Antennae that transmit and receive radio waves. There are two types:
  1.The larger volume coils which surround the body part being imaged.
  2.The surface coils which are placed directly under the area of interest and provide exquisite detail.
A great deal of effort is being made to improve surface coils.

TRANSMITTER

The MRI RF transmitter is similar to that used in radio stations.