April 26, 1995

One Minute Papers - Questions and Answers

How do the "user-friendly" MRI machines work vs. the old catacomb type? (the opened vs. closed types)

The shape of the MRI machine is dictated primarily by the strong magnetic field it uses to record information about protons in a person's tissues. This field needs to be very uniform over a large region of space and the simplest way of producing such a uniform field is with a huge coil of current-carrying wire. The person would go inside the coil, in the uniform field and other parts of the MRI machine would record the information. While the coil could be dressed up to look more like a tubular hole than a coil of wire, it was still very confining. Newer MRI machines use two smaller coils of current carrying wire, one above the other, to create a uniform field for imaging. This arrangement is trickier because the two coils must be shaped very carefully to ensure that the field is appropriately uniform. Moreover, most MRI machines use superconducting wires in these coils to achieve very high magnetic fields. Since superconducting coils must be cooled to very low temperatures, they require liquid helium coolants and sophisticated thermal insulation. While the single coil magnets required only a single refrigerator and insulating chamber, those with two coil magnets required two refrigerators and insulating chambers. That increases the expense of the magnet and its operation, but produces a more open imaging region.

What exactly does the bone do with the X-rays that the skin doesn't?

The skin's atoms are too small to experience the photoelectric effect with X-rays. Most X-rays go right through skin and soft tissue. However calcium atoms are large enough to experience the photoelectric effect and thus absorb many of the X-rays. Bones cast a shadow on film, which is how an image of your bones is formed.

If bones "stop" electrons, then why do we see a skeletal image on an X-ray? Would we get a negative image?

The bones cast shadows on the film; wherever there is bone, few X-rays strike the film. When the film is developed, it turns black wherever X-rays hit it. Thus the areas which were shadowed by the bone appear white.

On an X-ray result picture, why is the film in the background blue? Is this the only way it will show up? If so why?

The X-ray image itself is formed by tiny black silver particles, just as in a normal black and white photographic negative. If those particles were supported by a clear plastic sheet, then the X-ray should appear either clear or black and have no color. The blue you are referring to must be caused either by a colored pigment in the plastic X-ray film sheet or by a colored light used to illuminate the X-ray. I suspect the later. Fluorescent lamps tend to be bluish and the ones used to view X-rays are probably particular blue. It probably increases the apparent contrast in the image so that small variations in density become visible.