Imaging and Diagnosis

For centuries, physicians relied on little more than their senses and their experience to make diagnoses. Not only were the inside workings of the body a mystery, but also the inside parts of the body. Physicians could feel hard bones, elastic muscles, and soft organs. But they had little knowledge of what lay beneath the skin. In 1543, the same year as the publication of Copernicus' heliocentric theory of the solar system, Andreas Vesalius produced the first essentially correct book of anatomy.

But dead bodies and living ones are two very different things. More than three centuries passed before a tool to look inside the living human body was developed. This tool was the X ray. It was first applied to studying the human body in 1895, four days after its discovery was announced.

X Rays—The First Imager

Wilhelm Konrad Roentgen, a German physicist, was studying a device called a Crookes tube. This tube contained a partial vacuum and had electric wires connected to two metallic electrodes inside the tube. When a high-voltage current was applied to the electrodes, the tube glowed with a phosphorescent light. In one experiment, Roentgen had the tube enclosed in a black paper cover. In his darkened laboratory, he noticed a greenish glow several feet away whenever he turned the tube on. Investigating, he found that the glow came from a paper screen coated with barium platinocyanide. Further experiment showed that the screen continued to glow even when it was shielded from the tube by several layers of cardboard and paper, and that a photographic plate near the tube was blackened. Roentgen came to the conclusion that some kind of invisible rays must be coming from the tube. He dubbed them X rays, the "x" being the common mathematical symbol for an unknown.

Further experimentation led Roentgen to discover that X rays travel through some, but not all, materials. In time, he experimented with his own body. In his first public lecture on the new rays, he called for a volunteer to be X-rayed. He placed the volunteer's hand between the Crookes tube and the photographic plate. When he developed the plate, he found the image of the hand. Although the X rays could pass through the hand, they were blocked in different amounts by different tissues. The dense bones appeared relatively light in color because they resisted the passage of X rays to the plate. Softer surrounding tissues appeared darker because they were less dense and offered less resistance to the passage of X rays. The image on the photographic plate was essentially a negative on the basis of tissue density. Medical personnel now read the negative image obtained directly on photographic film to evaluate the X-ray findings.

In a Crookes tube, electrons are propelled through a partial vacuum. Electrons are subatomic particles that carry a negative charge of electricity. The glow that Roentgen saw in the tube was light waves produced when the electrons bounced against the gas molecules remaining in the tube. The electrons that hit the glass walls produced the X rays.

Today we know that X rays and light are different wavelengths of the same kind of electromagnetic radiation. Electromagnetic radiation is produced whenever electrons are deflected from their paths. The high energy of the electrons in the Crookes tube and the sharp deflection on meeting molecules of glass produce X rays. The more moderate deflection of the electrons on meeting gas molecules in the tube produces light.

X rays gave physicians their first easy look inside the living body. The procedure required no surgery and caused no pain. The patient just held still the body part being photographed until the plate was exposed.

X rays provided a great deal of information not available before. Physicians could study the living skeletal system and organ placements and get a clear picture of the broken bones, which they could previously feel only through the skin. They could also identify small, hairline fractures that tactile examinations could not pick up, and they could detect tumors in some body parts and displacements of certain bones and organs.