Radiation Risk from Medical Diagnostic Imaging Equipment
By Augustus Douw August 19, 2017 @augustusdouw
Energy emitted from a source is generally referred to as radiation. Examples include heat or light from the sun, microwaves from an oven, X rays from an X-ray tube, and gamma rays from radioactive elements
The word radiation arises from the phenomenon of waves radiating (i.e., traveling outward in all directions) from a source. This aspect leads to a system of measurements and physical units that are applicable to all types of radiation.
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
- Electromagnetic radiation, such as radio waves, microwaves, visible light, x-rays, and gamma radiation
- 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.
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 living organisms. 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.
Naturally-Occurring “Background” Radiation Exposure
Radiation is permanently present throughout the environment, in the air, water, food, soil and in all living organisms. That means we are exposed to radiation from different sources all the time. Large proportion of the average annual radiation dose received by people results from natural environmental sources.
Each member of the world population is exposed, on average, to 2.4 mSv/yr of ionizing radiation from natural sources. In some areas (in different countries of the world) the natural radiation dose may be 5 to 10-times higher to large number of people.
What is Ionizing Radiation?
Ionizing radiation is radiation with enough energy so that during an interaction with an atom, it can remove tightly bound electrons from the orbit of an atom, causing the atom to become charged or ionized.
Here we are concerned with only one type of radiation, ionizing radiation, which occurs in two forms – waves or particles.
Non-ionizing radiation is the term given to radiation in the part of the electromagnetic spectrum where there is insufficient energy to cause ionization. It includes electric and magnetic fields, radio waves, microwaves, infrared, ultraviolet, and visible radiation.
Forms of electromagnetic radiation. These differ only in frequency and wave length.
- Heat waves
- Infrared light
- Visible light
- Ultraviolet light
- X rays
- Gamma rays
Longer wave length, lower frequency waves (heat and radio) have less energy than shorter wave length, higher frequency waves (X and gamma rays). Not all electromagnetic (EM) radiation is ionizing. Only the high frequency portion of the electromagnetic spectrum which includes X rays and gamma rays is ionizing.
Most of the more familiar types of electromagnetic radiation (e.g. visible light, radio waves) exhibit “wave-like” behavior in their interaction with matter (e.g. diffraction patterns, transmission and detection of radio signals). The best way to think of electromagnetic radiation is a wave packet called a photon. Photons are chargeless bundles of energy that travel in a vacuum at the velocity of light, which is 300,000 km/sec.
Specific forms of ionizing radiation:
Particulate radiation, consisting of atomic or subatomic particles (electrons, protons, etc.) which carry energy in the form of kinetic energy or mass in motion.
Electromagnetic radiation, in which energy is carried by oscillating electrical and magnetic fields traveling through space at the speed of light.
Alpha particles and beta particles are considered directly ionizing because they carry a charge and can, therefore, interact directly with atomic electrons through coulombic forces (i.e. like charges repel each other; opposite charges attract each other).
The neutron is an indirectly ionizing particle. It is indirectly ionizing because it does not carry an electrical charge. Ionization is caused by charged particles, which are produced during collisions with atomic nuclei.
The third type of ionizing radiation includes gamma and X rays, which are electromagnetic, indirectly ionizing radiation. These are indirectly ionizing because they are electrically neutral (as are all electromagnetic radiations) and do not interact with atomic electrons through coulombic forces.
What Are X-Rays in Medical Services?
X-rays are a form of ionizing radiation energy, like light or radio waves. Unlike light, x-rays can penetrate the body, which allows a radiologist to produce pictures of internal structures. The radiologist can view these on photographic film or on a TV or computer monitor.
X-ray examinations provide valuable information about your health and play an important role in helping your doctor make an accurate diagnosis. In some cases x-rays are used to assist with the placement of tubes or other devices in the body or with other therapeutic procedures.
When you mention the measurement of radiation, many people think of the classic Geiger counter used for measuring radioactivity intensity. But Geiger counters detect only the intensity of radioactive emissions. Measuring their impact on human tissues and health is more difficult. That’s where the sievert (Sv) and millisievert (mSv) come in. These units, the ones most commonly used in comparing imaging procedures, take into account the biological effect of radiation, which varies with the type of radiation and the vulnerability of the affected body tissue. Taking these into account, millisieverts describe what’s called the “equivalent dose.”
In simplified terms, one can compare the radiation exposure from one chest x-ray as equivalent to the amount of radiation exposure one experiences from natural surroundings in 10 days
About X-Ray Dosage and Source
- Only the amount of energy of any type of ionizing radiation that imparted to (or absorbed by) the human body can cause harm to health.
- To look at biological effects, we must know (estimate) how much energy is deposited per unit mass of the part (or whole) of our body with which the radiation is interacting.
- The international (SI) unit of measure for absorbed dose is the gray (Gy), which is defined as 1 joule of energy deposited in 1 kilogram of mass. The old unit of measure for this is the rad, which stands for “radiation absorbed dose.” – 1 Gy = 100 rad.
- Equivalent dose – the biological effect depends not only on the amount of the absorbed dose but also on the intensity of ionization in living cells caused by different type of radiations.
- Neutron, proton and alpha radiation can cause 5-20 times more harm than the same amount of the absorbed dose of beta or gamma radiation.
- The unit of equivalent dose is the Sievert (Sv). The old unit of measure is the rem. – 1 Sv = 100 rem.
Medical Applications of X-ray Technology
Medical imaging has led to improvements in the diagnosis and treatment of numerous medical conditions in humans and animals.
There are many types – or modalities – of medical imaging procedures, each of which uses different technologies and techniques. Computed tomography (CT), fluoroscopy, and radiography (“conventional X-ray” including mammography) all use ionizing radiation to generate images of the body. Ionizing radiation is a form of radiation that has enough energy to potentially cause damage to DNA and may elevate a person’s lifetime risk of developing cancer.
CT, radiography, and fluoroscopy all work on the same basic principle: an X-ray beam is passed through the body where a portion of the X-rays are either absorbed or scattered by the internal structures, and the remaining X-ray pattern is transmitted to a detector (e.g., film or a computer screen) for recording or further processing by a computer. These exams differ in their purpose:
- Radiography – a single image of any part of the body is recorded for later evaluation.
- Mammography is a special type of radiography to image the internal structures of breasts.
- Fluoroscopy – a continuous X-ray image is displayed on a monitor, allowing for real-time monitoring of a procedure or passage of a contrast agent (“dye”) through the body. Fluoroscopy can result in relatively high radiation doses, especially for complex interventional procedures (such as placing stents or other devices inside the body) which require fluoroscopy be administered for a long period of time.
- CT – many X-ray images are recorded as the detector moves around the patient’s body. A computer reconstructs all the individual images into cross-sectional images or “slices” of internal organs and tissues. A CT exam involves a higher radiation dose than conventional radiography because the CT image is reconstructed from many individual X-ray projections.
Ionizing Radiation and Cancer Risk
We’ve long known that children and teens who receive high doses of radiation to treat lymphoma or other cancers are more likely to develop additional cancers later in life. But we have no clinical trials to guide our thinking about cancer risk from medical radiation in healthy adults. Most of what we know about the risks of ionizing radiation comes from long-term studies of people who survived the 1945 atomic bomb blasts at Hiroshima and Nagasaki. These studies show a slightly but significantly increased risk of cancer in those exposed to the blasts, including a group of 25,000 Hiroshima survivors who received less than 50 mSv of radiation — an amount you might get from two or three CT scans. (See “Imaging procedures and their approximate effective radiation doses.”)
The atomic blast isn’t a perfect model for exposure to medical radiation, because the bomb released its radiation all at once, while the doses from medical imaging are smaller and spread over time. Still, most experts believe that can be almost as harmful as getting an equivalent dose all at once.
Ionizing radiation may cause damage to the cells in your body. This is usually very minor and does not cause any serious damage, however, large doses may cause the cells to become cancerous. A very low dose x-ray, such as a chest x-ray, has a tiny risk. CT scans, which use higher doses of x-rays, have a higher risk, although it is still a very small risk.
Your doctor is aware of the risks and benefits of x-rays, CT scans and nuclear medicine scans and should always balance the possible benefits of you having the test with the small risk. It is always appropriate for you to have the x-ray or scan if it benefits you. Finding out if you have something wrong with you and the best way to treat it outweighs the very small risk of the scan.
The amount of radioactive material used for nuclear medicine scans and PET scans is very small, however, the radiation can sometimes take as long as a few days to pass out of your body. The amount of radiation you receive from these scans is similar to what you receive from x-ray procedures.
Imaging procedures involving ionizing radiation are not usually recommended for pregnant women, but can be performed in an emergency.
If you are having an x-ray dye for your CT scan or a radioactive tracer for your nuclear medicine scan, there is a small risk of:
- An allergic reaction.
- Infection at the site of an injection
If you are concerned about the risks, talk to your doctor before the examination.
|Imaging Procedures And Their Approximate Effective Radiation Doses|
|Procedure||Average effective dose (mSv)||Range reported in the literature (mSv)|
|Bone density test+||0.001||0.00–0.035|
|X-ray, arm or leg||0.001||0.0002–0.1|
|X-ray, panoramic dental||0.01||0.007–0.09|
|X-ray, lumbar spine||1.5||0.5–1.8|
|CT, cardiac for calcium scoring||3||1.0–12|
|Nuclear imaging, bone scan||6.3|
|CT, whole body||variable||20 or more|
|Nuclear imaging, cardiac stress test||40.7|
Higher Radiation–Dose Imaging
Most of the increased exposure in the United States is due to CT scanning and nuclear imaging, which require larger radiation doses than traditional x-rays. A chest x-ray, for example, delivers 0.1 mSv, while a chest CT delivers 7 mSv (see the table) — 70 times as much. And that’s not counting the very common follow-up CT scans.
In a 2009 study from Brigham and Women’s Hospital in Boston, researchers estimated the potential risk of cancer from CT scans in 31,462 patients over 22 years. For the group as a whole, the increase in risk was slight — 0.7% above the overall lifetime risk of cancer in the United States, which is 42%. But for patients who had multiple CT scans, the increase in risk was higher, ranging from 2.7% to 12%. (In this group, 33% had received more than five CT scans; 5%, more than 22 scans; and 1%, more than 38.)
How to Minimize Medical Related Radiation Exposure
Unless you were exposed to high doses of radiation during cancer treatment in youth, any increase in your risk for cancer due to medical radiation appears to be slight. But we don’t really know for sure, since the effects of radiation damage typically take many years to appear, and the increase in high-dose imaging has occurred only since 1980.
So until more is known, you will want to keep your exposure to medical radiation as low as possible. You can do that in several ways, including these:
Discuss any high-dose diagnostic imaging with your clinician. If you need a CT or nuclear scan to treat or diagnose a medical condition, the benefits usually outweigh the risks. Still, if your clinician has ordered a CT, it’s reasonable to ask what difference the result will make in how your condition is managed; for example, will it save you an invasive procedure?
Keep track of your radiation exposure. The President’s Panel recommended that imaging device makers indicate the radiation dose for each x-ray, and that clinicians record radiation exposures in patients’ medical records. The FDA is considering both ideas. In the meantime, you can keep track of your own x-ray history. It won’t be completely accurate because different machines deliver different amounts of radiation, and because the dose you absorb depends on your size, your weight, and the part of the body targeted by the x-ray. But you and your clinician will get a ballpark estimate of your exposure.
Consider a lower-dose radiation test. If your clinician recommends a CT or nuclear medicine scan, ask if another technique would work, such as a lower-dose x-ray or a test that uses no radiation, such as ultrasound (which uses high-frequency sound waves) or MRI (which relies on magnetic energy). Neither ultrasound nor MRI appears to harm DNA or increase cancer risk.
Consider less-frequent testing. If you’re getting regular CT scans for a chronic condition, ask your clinician if it’s possible to increase the time between scans. And if you feel the CT scans aren’t helping, discuss whether you might take a different approach, such as lower-dose imaging or observation without imaging.
Don’t seek out scans. Don’t ask for a CT scan just because you want to feel assured that you’ve had a “thorough checkup.” CT scans rarely produce important findings in people without relevant symptoms. And there’s a chance the scan will find something incidental, spurring additional CT scans or x-rays that add to your radiation exposure.
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