Image Processing Reference
not lend themselves well to being focused by lenses or mirrors — they pass right
through the material of the optics without deflection. But lenses and focal-plane
arrays are not the only way to make an image from electromagnetic waves. As
seen in the last chapter, microwave imaging systems create images through precise
timing of return pulses of microwave energy, or, in the case of passive imaging, by
scanning a reflective parabolic antenna with a single detector at the focal point back
and forth over a scene. Most x-ray and gamma-ray images are made an entirely
different way, by a method that does not require a lens. The images we traditionally
associate with x rays are what are known as shadowgrams; that is, a recording of
an object's shadow projected onto a detector material, which could be a piece of
film or a sheet of glass coated with phosphor. Thus, the object under investigation
has to be placed between the x-ray source and the detector, as shown in Fig. 4.2.
It should be noted that there are techniques for focusing x rays—there will be an
astronomical example later.
The discovery of x rays was one of the last great scientific achievements of
the19 th century, and heralded a new age of physics. Wilhelm Roentgen was
awarded the first Nobel Prize in physics for a discovery he made in 1895: he
found that a Hittorf tube (a special evacuated tube with metal electrodes and a
high-voltage current) produced penetrating rays that fogged photographic film and
made fluorescent materials glow even when the tube was covered with black paper
to block the ultraviolet light that was also emitted by the tube. 3 Figure 4.3 shows a
schematic drawing of a Crookes tube, an x-ray tube similar to the Hittorf tube used
by Roentgen. The low-voltage power supply heats a filament called a cathode,
causing electrons to “boil off”—a process known as thermionic emission. The 100
kV supply accelerates the electrons towards the metal target, known as the anode.
These high-voltage electrons strike the target, causing electrons in the metal atoms
to emit x rays.
Roentgen's system produced x rays with sufficient energy (about 40 keV) to
image the bones in his hand using a fluorescent screen. He soon discovered that
photographic film is also sensitive to x-ray light. Figure 4.4 shows the first x-ray
image made with film; it is a picture of the hand of Roentgen's wife Bertha with
her wedding band clearly visible. X-ray images are also known as radiographs.
Doctors began using x rays as part of diagnosis and treatment of patients
almost immediately after Roentgen announced his discoveries. The exploitation
of x-ray imaging technology by the medical establishment raced far ahead of
any understanding of their effects, particularly on living tissue. The publicity
blitz that followed the initial discovery engendered a great deal of quackery and
misinformation, some of it with tragic consequences. Some writers in the early
1900s claimed that x rays could raise the dead, cure blindness, or treat skin
conditions, and many people were injured permanently with large, uncontrolled
doses of x rays. X rays (and gamma rays) are dangerous in high doses because
3 A translation of Roentgen's original paper can be found in E.C. Watson, “The Discovery of X Rays,”
American Journal of Physics13, 284 (1945).