Modern medical imaging began with an almost accidental discovery in the lab of Professor Wilhelm Roentgen in Germany on a November day in 1895.
Roentgen was experimenting with a Crooke’s Tube he had recently obtained from its inventor. This is a glass vessel from which air is withdrawn creating a near vacuum; at one end is an anode (positively charged) and at the other a cathode (negatively charged source of electrons); the tube is wired to be part of an electrical circuit. When a current is passed between these electrodes, the few particles within the tube are excited and fluoresce or glow (commonly blue or green); this results from the flow of high speed electrons (cathode rays) across the (voltage) potential difference imposed in the circuit. Roentgen had placed the Tube in a black box but to his amazement noted that a fluorescent screen nearby was glowing of its phosphors which he deduced to be excitement by radiation escaping the box. This unknown (X) radiation he simply labelled X-rays (they are also called Roentgen rays). As he studied their properties, he experimented by putting a hand on a fluorescent screen directly in the path of this radiation, getting this famous picture:
X-rays are produced when electrons are impelled against an anode metal target (tungsten; copper; molybdenum; platinum; others) as they pass through a vacuum tube at high speeds driven by voltages from 10 to 1000 kilovolts (kV). When incoming electrons interact with inner electrons in the metal, these latter are driven momentarily to higher energy levels (these orbital electrons are pushed into outer orbitals); when these excited electrons drop back to their initial orbits (a transition from a higher to a lower energy level), the energy they acquired is given off as radiation, including x-rays . Some of the scattered x-rays are collimated into beams (typically at conical angles up to 35°) that are directed towards targets (such as the human body). Soft body tissue absorbs less x-rays, i.e., passes more of the radiation, whereas bone and other solids prevent most of the x-rays from
Two classes of detectors record the x-ray-generated image: 1)Photographic film, in which the difference in gray levels or tones relates to varying absorption of the radiation in the beam impinging on the target (the convention is to use the exposed film [x-rays act on the silver halide {see page 12 of this Introduction} to reduce it to metal silver grains] in its negative form, such that bone will appear nearly white [thus, because bone absorbs efficiently, few x-rays strike the corresponding part of the film, leaving it largely unexposed; the soft tissue equivalents pass much more radiation and darken the film]); 2) fluorescent screens, that include phospors (elements or compounds that fluoresce or phosphoresce) coating a substrate; this occurs when electrons in the phosphors jump to higher level orbitals, with visible light given off either instantly when the electrons transition back to the lower state or with a time delay fractions of a second or seconds (afterglow), in a process similar to x-ray production; typical phosphors include Calcium tungstate or Barium Lead sulphate (many other compounds are available such as Lead oxide or those containing Gadolinium or Lanthanum; these screens in certain configurations allow realtime movements of the medical patient to be observed and the sreen images can be photographed or digitized.
Here is a typical hospital examining room that contains the setup used in x-ray radiology; the table on which the patient lies that can, in some instruments, be raised to a vertical position.
X-ray radiology is still the most commonly used medical instrument technique. Here are a sequence of images that illustrate typical uses and results. The first is a chest x-ray, (the skeletal bones are whitish since they absorb the radiation and thus the negative is not darkened and the lungs dark because more of the radiation has passed through them):
This next is a front and side view of the upper torso; the arrow points to a tuberculosis patch in the left lung:
Here is a negative x-ray film image of the pelvic area:
Compare this recent image of the human hand with that shown above as the first ever taken:
This next x-ray strikes a personal note: it shows the upper arm and shoulder area. There is a slanted break across the arm (humerus bone) near the shoulder socket. Fractured in several places, this image is that of the writer’s (NMS) wife taken on the night of a home accident in February, 2006.
This next picture is a mammogram (not my wife’s) showing a growth in the female breast:
The human skull is x-rayed mainly to spot signs of fracture. But, sometimes indications of tumors are present, as shown by the darker gray patch in the cranium of this individual’s skull:
The jaw and teeth are evident in this lateral view of the lower human skull:
Most of us gain our first experience and insight into x-rays when we have a small film inserted into our mouth and then the x-ray machine is placed against that part of our jaw. Here is a typical x-ray image of teeth, in which the whitest part of the negative corresponds to metal fillings:
An important variation in x-ray radiography is Fluoroscopy. In this method, either chemicals that react with x-rays are swallowed or inserted as an enema or chemicals/dyes are injected into the blood stream. These tend to increase the contrast between soft tissue response in the parts of the body receiving these fluids and surrounding bone and tissue. This pictorially highlights abnormalities.
Barium sulphate is a good example. When swallowed (either at once or commonly in gulps), the “Barium Cocktail” is especially useful in examining the digestive track. In this image, an obstruction in the esophagus carrying food and liquids into the stomach is made evident:
The large intestine or colon is strikingly emphasized in a patient who has just received a Barium enema
Still another variant is the Angiogram. This involves insertion of a catheter into an artery, accompanied by a dye that reacts to x-rays. It is commonly used to explore the areas in and around the heart. Here is a pair of views of the left ventricle of the heart when it is pumping and squeezing blood and thus contracting (systolic phase) and then expanding as blood is returned (diastolic phase):
This next image is an angiogram that has been colored to show blood vessels including the great trunk artery or aorta around the heart:
Using special methods, angiogram-like images can be made for the blood vessels in the human head:
