- Definition
Radiography, commonly called "x-ray" is a type of imaging that uses x-rays to create 2D (flat plane) images of the exposed area of the body. An x-ray used here is high-energy light, and is part of the electromagnetic spectrum. It has so much energy (20-150keV) that it can go through some solid objects! The radiograph is used widely in health care to help us NON-INVASIVELY visualize internal organs.
So, how was this discovered, and how does it work?
- Discovery
Wilhelm Conrad Röntgen was a physicist working like any other day in his lab. He was studying what would happen if electricity passed through a gas in a very low pressure. That day, he covered the discharge tube with a thick black material so no light could escape, and shut off the room light. Dr. Röntgen noticed then that a paper plate placed nearby became fluorescent. He also found that if he placed objects with different thickness in the way, the resulting recording in the plate showed different levels of transparency. So when he returned with his wife Anna, and got her to put her hand in the way this time, the rays clearly showed an image of her bones. He named this mysterious rays "x-rays" because it was well.. mysterious. And the rest is history!
"I didn't think; I investigated."
Fun Fact:
Dr. Röntgen received the First Nobel Prize for Physics (1901) for the discovery of x-rays.
The element #111 roentgenium (Rg) is named after him.
- Mechanism
Now, x-rays are used commonly throughout the world, in all hospitals. An x-ray machine contains an x-ray tube, filter, collimator, and grid and screen-film cassette behind the patient. In the x-ray tube, electrons travel in high velocity from a negatively charged rod (cathode) to a positively charged one (anode; usually of tungsten or molybdenum). {this makes sense; opposites attract, and electrons are negatively charged!}
90% of the x-ray (bremsstrahlung) is produced when electrons change direction close to the anode and give off energy.
the other 10% comes from when the flying electrons knock off the inner-shell electrons relaxing in the anode. Electrons in the outer-shell then moves down to fill that void, releasing energy as x-ray in the process! All of this, mind you, is happening in a vacuum. These x-rays leave the tube, and pass through the collimator. At this level, the rays are refocused so that exposure is minimized for the patient. It will also make the image sharper (higher contrast) because more x-rays will be focused to the area of investigation.
Because x-rays have such high energy, it can go through solid objects. However, it cannot pass through higher-density material. In a patient, it can easily pass clothing and skin. Most of the x-rays can pass soft tissues, like fat and muscle. However, it cannot pass through the bone. This different in how much x-ray can travel through an object is how the radiographic image is created.
- Indications
Densities you can identify in a radiograph are: air (appears black), fat (dark gray), soft tissue (gray), bone (white), metal (very white).
Radiographs are very quick, have low radiation, and easily accessible. However, it has only one view, forcing us to see 3D structures in a 2D plane, and has low resolution. X-rays also mostly go through organs, so it is not appropriate for imaging liver, brain, details of heart, etc. For further investigation of a rare disease or a vague pathology, we might need stronger types of imaging like CT or MRI.
Common reasons you would use x-rays include:
- Fractures
- Lung disease (pneumonia, cancer, fibrosis)
- Spinal disease (abscesses, osseous cancer, scoliosis)
- Abdominal pathology (obstruction)