X-ray Imaging

Background of X-ray Imaging

X-rays were discovered accidentally by the Germany Physicist William Roentgen in 1895, while experimenting with electron beams in a gas discharge tube. He discovered that a screen in the laboratory started glowing when the electron tube was turned on, and continued to glow even after the tube was covered with a black cardboard. He placed his hand and saw a silhouette of his bones projected onto the fluorescent screen; indicating that the radiation had penetrated through the tissue except the bones. This remarkable discovery marked the beginning of advances in diagnostic techniques for examining broken bones, cavities, swallowed objects, etc. using X-rays.

The atoms in our body absorb visible light photons but X-ray photons pass through as they have high energy, however calcium ions in the bones absorb X-ray photos. An image of the bones can be obtained on a photographic plate placed behind the part to be viewed and opposite to the X-ray beam.

Properties of X-rays

• Like all electromagnetic waves, they travel in a straight line with speed of light in vacuum
• They expose photographic films
• They ionise gases
• They cause emission of electrons from metals
• They are not deviated by electric or magnetic fields
• They can reflect, refract and can superimposed to form interference patterns

Production of X-rays

X-rays are produced in thermionic tubes or valves (vacuum tubes).

The vacuum tube consists of a heated filament, an accelerating anode (positive terminal), and a heavy metal target. The heated filament emits electrons. The electrons are accelerated by accelerating anode. The high energy electrons bombard a heavy metal target to release electromagnetic waves (X-rays) of varying frequencies.

The kinetic energy ½ mv² on collision is converted to hf_max, highest energy is released with greatest loss of kinetic energy.

The spectrum shows spikes called characteristic lines generated from radiation due to electrons from lower energy levels being raised to higher energy levels in atoms of the target material. If the accelerating voltage is increased, the highest frequency is increased. These are hard X-rays. If the filament voltage is increased, the intensity of the X-rays increases without changing its frequency and hence increases its penetrating power. The high intensity spikes are characteristics of the target material. The anode is cooled to dissipate the heat produced.

Since X-rays are emitted in all directions, the tube is enclosed in a lead case with a small window to provide an outlet for the X-ray beam. A photograph of X-ray table is shown below:

From the figure above, you can see that, the patient is made to lie on the X-ray table. A photographic plate is placed beneath the area under examination. The X-ray beam is directed on the area. From the control panel, the technician or radiologist can control the timing and the frequency of the rays.

Applications of X-rays in Medicine

X-rays are used for:

1. Detecting fractures in bones or broken bones
2. To study organs in the digestive and endocrine systems with the help of a contrast medium which absorbs X-rays like a Barium Sulphate compound which is given to the patient to swallow and X-ray pictures are taken of the digestive system.
3. X-ray in conjunction with fluoroscopy; X-rays are made to pass through the body and impinge onto a fluorescent screen which glows. It can create a moving X-ray image.
4. A major application of X-rays is in angiography, to study blockages of blood vessels. An X-ray contrast medium is injected into a blood vessel and an X-ray called an Angiogram is taken.

You can also read: The Introduction to Medical Diagnostics Techniques

For study of blockage in coronary arteries, a catheter is guided to different coronary arteries through a large blood vessel (generally the inferior vena cava through the groin) and X-ray image is taken. Using digital image processing, the image is further enhanced.