Resistive magnets above the .03 tesla level are generally too expensive to operate. .

             The second kind is a permanent magnet. Permanent magnets must be very large, and can weigh many tons to generate a 0.4 tesla level strength. .

             Superconducting magnets are resistive magnets in a modified environment: they are surrounded by liquid helium at a temperature of -452.4F (Gould, DATE). The very low temperature reduces the resistance in the electrical wire, allowing for markedly more efficient use of the electricity used to create the magnetic field. While still expensive to operate, superconducting magnets can generate magnetic fields of up to 2.0 teslas (Gould, DATE). .

             MRI machines also use gradient magnets. These magnets are much weaker than the others, and their job is to cause the magnetic field to vary, and cause the MRI machine to focus on the selected part of the body. .

             Once the patient is in the MRI's magnetic field, nearly all of the billions of protons will line up with either the person's feet or head, canceling each other out. However, a few will not, and will spin in response to the magnetic pulse. The actual resonant frequency set for the machine will vary according to the type of tissue to be imaged, allowing the MRI to focus very precisely on specific spots on the body (Gould, DATE). The actual signal picked up by the MRI apparatus occurs when the magnets are turned off and the nonaligned hydrogen protons, which have been spinning, begin to slow down. That is the signal that is actually converted into a visual image (Gould, DATE). .

             Risks.

             Older MRI machines are solid tubes of relatively small diameter, with the patient lying on what amounts to a tray that slides in and out of the tube. In these enclosed tubes, some patients become severely claustrophobic (Nordenberg, 1999). More modern machines have open spaces on each side which help some people with that claustrophobic feeling. However, some people must take some kind of sedation in order to cope with the MRI environment, which in addition to being a very confining space, is quite noisy.