What is Radiation?

As a pediatric radiologist I have to be constantly aware of the radiation exposure I may be giving to my patients in my daily practice. Children are more sensitive to the effects of radiation, and they have a longer life to live to experience the potential sequelae of too much radiation. But radiation is also a part of our daily life. We are all exposed to radiation everyday outside of the hospital and outside of medical imaging.

Radiation exposure is part of our everyday life.

What is Radiation?

Radiation simply means the emission or transmission of energy in the form of waves or particles through space or through a material medium, such as human body tissues in the case of medical imaging. There are multiple different types of radiation, including:

• electromagnetic radiation, such as radio waves, microwaves, visible light, x-rays, and gamma radiation (γ)
• particle radiation, such as alpha radiation (α), beta radiation (β), and neutron radiation (particles of non-zero rest energy)
• acoustic radiation, such as ultrasound, sound, and seismic waves (dependent on a physical transmission medium)
• gravitational radiation, radiation that takes the form of gravitational waves, or ripples in the curvature of spacetime.

As you can see, medical imaging is only one type of radiation. Nuclear medicine studies involve mostly gamma radiation, beta radiation, and alpha radiation, while x-rays are photons emitted typically from a heated tungsten filament, and ultrasound involves a sound beam transmitted and reflected from human tissues.

How else is radiation classified? 

Radiation is often categorized as either ionizing or non-ionizing depending on the energy of the radiated particles. Ionizing radiation carries more than 10 eV, which is enough to ionize atoms and molecules, and break chemical bonds. This is an important distinction due to the large difference in harmfulness to patients. A common source of ionizing radiation is radioactive materials that emit α, β, or γ radiation, consisting of helium nuclei, electrons or positrons, and photons, respectively. Other sources include X-rays from medical radiography examinations and muons, mesons, positrons, neutrons and other particles that constitute the secondary cosmic rays that are produced after primary cosmic rays interact with Earth’s atmosphere.

Gamma rays, X-rays and the higher energy range of ultraviolet light constitute the ionizing part of the electromagnetic spectrum. The lower-energy, longer-wavelength part of the spectrum including visible light, infrared light, microwaves, and radio waves is non-ionizing; its main effect when interacting with tissue is heating. This type of radiation only damages cells if the intensity is high enough to cause excessive heating. Ultraviolet radiation has some features of both ionizing and non-ionizing radiation. While the part of the ultraviolet spectrum that penetrates the Earth’s atmosphere is non-ionizing, this radiation does far more damage to many molecules in biological systems than can be accounted for by heating effects, sunburn being a well-known example. These properties derive from ultraviolet’s power to alter chemical bonds, even without having quite enough energy to ionize atoms.

What percentage of our annual exposure to radiation does medical imaging constitute?

According to the United States Nuclear Regulatory Commission  since the beginning of time, all living creatures have been, and are still being, exposed to radiation. Nonetheless, most people are not aware of all the natural and man-made sources of radiation in our environment.

A chart of the public’s exposure to ionizing radiation (displayed below) shows that people generally receive a total annual dose of about 620 millirem. Of this total, natural sources of radiation account for about 50 percent, while man-made sources account for the remaining 50 percent.

Chart depicting sources of radiation exposure in the United States

What type of medical imaging study accounts for the most percentage of radiation exposure to patients?

This question is important and should probably be clarified. We’re talking only about ionizing radiation here. We’re not addressing non-ionizing radiation, which includes ultrasound and MRI. The chart below breaks down medical imaging procedures into the various types of radiology studies that expose patients to ionizing radiation.

Percentage of ionizing radiation exposure to patients due to various medical imaging examinations compared to non-medical sources.

As you can see, the largest proportion of radiation exposure to patients occurs due the use of computed tomography, most commonly referred to by lay people as CAT scans or CT scans. Importantly, please note this does not mean that CT scans are the highest dose examination in radiology. Certainly CT scans expose patients to more radiation than regular x-rays; however, there are plenty more examinations in radiology that expose patients to more radiation than CT, such as nuclear medicine cardiac stress tests, coronary angiograms, etc. The reason CT scans contribute the most overall radiation is that they are incredibly useful studies for deciphering a patient’s clinical problem and they are very commonly utilized in radiology.

What are you worried about when it comes to radiation exposure? Do you take any precautions to protect yourself from radiation.