Basics of Radiation

by Hetal Verma, MD

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    00:01 So let's discuss ionizing radiation a little bit.

    00:04 Whenever we discuss the field of radiology it's important to have at least a basic understanding of what ionizing radiation is and what the effects of radiation are.

    00:12 So radiation is the emission of energy in the form of waves.

    00:15 Electromagnetic radiation is the type that's used in radiography and CT scans.

    00:20 And it can actually be harmful when it's used in excess so it's important to remember that when you're ordering CT scans on a patient or even when you are the one performing them.

    00:30 Radiation can go in multiple different directions.

    00:34 There can be transmitted radiation which is the amount of radiation that actually goes through the detector and hits the detector to create an image.

    00:42 There is absorbed radiation which is the amount of radiation that interacts with the patient's tissues and that is measured in units which are called Gray units.

    00:51 And the scatter radiation, which is the amount of radiation that's deflected to a different direction and it's neither absorbed nor is it transmitted.

    01:01 So, if you're a bystander in a patient who is having a CT scan, you are susceptible to the scatter radiation which is bouncing off of the patient.

    01:10 There are multiple different sources of radiation; imaging is just one of them.

    01:17 Imaging constitutes about 50% of radiation these days, somewhat worldwide, but usually in countries that use CT scans more often.

    01:26 Other types of radiation include cosmic radiation which comes from outer space.

    01:32 Radiation can also come from radioactive material found within the soil and Radon is also an important source of radiation.

    01:41 So what are some biological effects of radiation? It can cause molecular damage and it can create free radicals within the body and this is one of the reasons why it can be hurtful.

    01:51 It also results in disruption of normal cellular metabolic function and mitosis.

    01:56 And because of these it can have multiple different effects on the human body.

    02:02 So there are deterministic effects and there are what are called stochastic effects.

    02:06 So deterministic effects are effects that occur at very high doses of radiation.

    02:10 It results in cell killing including skin erythema, cataracts, and sterility.

    02:17 And this really only occurs above a certain threshold, so with deterministic effects, you don't have any effect at all until you reach a certain threshold of radiation and then all of a sudden you have the effects of cell killing.

    02:29 Stochastic effects on the other hand are dose independent.

    02:32 They include carcinogenesis and genetic damage and if the dose increases the probability of a stochastic effect increases, so there is no threshold the way a deterministic effect has.

    02:45 So as you increase dose and as you do more and more CT scans, let's say, the probability of a stochastic effect will increase.

    02:53 Most susceptible to the effects of radiation are the bone marrow, colon, lung, and stomach.

    03:02 With moderate effects on the bladder, breast, liver, esophagus, and thyroid and the effects that these organs have are really induction of cancer.

    03:11 Children are obviously the most susceptible, they have the most stem cells and stem cells are very susceptible to radiation.

    03:20 So when imaging a child, it's always very, very, important to be careful as to the amount of radiation that you provide.

    03:26 So let's take a look at fetal risk of radiation.

    03:29 If a child is very susceptible to radiation, a fetus is actually even more so.

    03:34 So fetal risk of radiation actually depends on the days after conception that the fetus encounters the radiation.

    03:40 So within the first 1 to 10 days, the fetus has the highest risk of radiation and it really could result in fetal demise.

    03:47 And this is all when a fetus receives a dose of about 10mGy or more.

    03:52 About 20 to 40 days after conception, the fetus can have congenital anomalies which can present after birth.

    04:00 At about 50 to 70 days, the radiation can result in microcephaly.

    04:06 Further out, in about 70 to 150 days, it can lead to growth and mental retardation and again, these are all things that may or may not occur and you may not know until they're going to occur until years after the child is born.

    04:20 Greater than 150 days after conception, it can result in childhood malignancies.

    04:25 So it's very important to remember this chart and to know that in a pregnant female you really don't wanna be doing any kind of study that results in risk of radiation unless you absolutely have to.

    04:36 You really have to weigh the pros and cons and to see whether or not this patient really needs the imaging study.

    04:41 So again, risk of radiation really depends on the level of gestation.

    04:47 So how can we protect against radiation? There's something known as ALARA which stands for As Low As Reasonably Achievable.

    04:55 You wanna minimize the amount of imaging whenever possible and you wanna minimize imaging doses whenever possible.

    05:01 So CT scans can be performed in a variety of different ways.

    05:04 When you're performing a CT scan on a child, it's always very reasonable to lower the dose so that the child receives less radiation.

    05:11 You have to keep in mind however that when you lower the dose of an imaging exam, you're also lowering the sensitivity of that exam.

    05:17 You want exposed personnel to be monitored by a film badge so all radiologist and all technologist that work in the field always wear a film badge and that shows them how much radiation that person is receiving.

    05:30 Lead shielding is always used and you wanna increase your distance from the source.

    05:35 So as we know, scatter radiation is always present and you wanna be as far away from that source of scatter as possible especially when you're someone that works in the field and you're exposed to this day to day.

    05:45 Rooms are now designed with the shielding in place to help prevent radiation exposure.

    05:51 So let's take a look at the differences in radiography and CT.

    05:56 In terms of the mechanism of acquisition, both use ionizing radiation.

    06:00 CTs are actually a lot more expensive than a radiograph and they take a few seconds longer to perform.

    06:07 CTs are not portable, the patient has to go into the CT gantry while radiographs are portable.

    06:13 So in a patient that's not able to move around, radiographs are very good with imaging.

    06:18 Radiographs take just a few seconds, CTs take a little bit longer than that but really no longer than about a minute or so.

    06:24 And radiographs are performed without the administration of intravenous contrast.

    06:29 CT scans may or may not need intravenous contrast, so in a patient that has a contraindication to contrast this is also something to keep in mind.

    06:37 It's important to remember in terms of the radiation that CTs actually have a lot higher radiation than a radiograph does.

    About the Lecture

    The lecture Basics of Radiation by Hetal Verma, MD is from the course Introduction to Imaging. It contains the following chapters:

    • Ionizing Radiation
    • Effects, Risk and Protection of Radiation

    Included Quiz Questions

    1. If the leukemia is related to radiation exposure, this would be a stochastic effect.
    2. If the leukemia is related to radiation exposure, this would be a deterministic effect.
    3. The leukemia is linked to radiation exposure.
    4. The leukemia is not related to radiation exposure since the exposure occurred at a late gestational age.
    5. The mother should not have undergone any imaging due to the possibility of radiation effects on the fetus.
    1. Scatter
    2. Absorbed
    3. Transmitted
    4. Resorbed
    5. Deflected
    1. Ultrasound
    2. Cosmic radiation
    3. X-ray
    4. Soil
    5. Radon
    1. It results in skin erythema, cataract and sterility.
    2. It is dose independent.
    3. Increasing the dose increases the probability of its biological effect.
    4. There is no set threshold dose for the effect to occur.
    5. It can lead to the development of cancers.
    1. Fetal demise
    2. Microcephaly
    3. Mental retardation
    4. Childhood malignancies
    5. Urinary tract anomalies
    1. Use lead shielding for the family member of radiologists and technologists to prevent transmitting radiation.
    2. Follow ALARA - as low as reasonably achievable.
    3. All radiologists and technologists must be monitored by a film badge which shows the amount of radiation that the person is receiving.
    4. Use lead shields during the exposures.
    5. Design rooms with shields.

    Author of lecture Basics of Radiation

     Hetal Verma, MD

    Hetal Verma, MD

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