Free Radiography Practice Test

1. Which material should be used as the primary shielding around a beta-emitting radionuclide, such as phosphorus-32?

a. Lead, because high-density metals absorb most particle radiation
b. Plexiglas, because beta particles striking it are converted to harmless Bremsstrahlung radiation
c. Lead, because absorbers with larger atomic nuclei block radiation better than absorbers with smaller atomic nuclei
d. Plexiglas, because absorbers with larger atomic nuclei produce more Bremsstrahlung radiation than absorbers with smaller atomic nuclei

2. Which of the following pairings of radiation source and dosage of ionizing radiation received during one event is typical?

a. 1 millisievert (mSv) for an adult patient imaged with flat film chest x-rays
b. 40 millisieverts (mSv) for an adult patient imaged with full body computed tomography (CT) scanning
c. 50 microseiverts (µSv) for a patient given dental x-rays
d. 10 microseiverts (µSv) for a passenger flying from New York to San Francisco

3. The linear no-threshold model (LNT) of radiation biological effects is derived from all of the following ideas EXCEPT:

a. Cells have the capability of repairing radiation-induced damage to DNA
b. Any dose of ionizing radiation, even a tiny dose, increases the chance that a cell will undergo malignant change
c. The risk of undesirable biological effects increases as the cumulative radiation exposure of an individual increases
d. The risk of undesirable biological effects, such as malignancy, is proportional to the amount of ionizing radiation to which an individual is exposed

4. Which of the following statements is most accurate, regarding the control of the range of wavelengths in an x-ray beam?

a. The minimum wavelength depends on the voltage, whereas the maximum wavelength depends the filtration
b. The minimum wavelength depends on the filtration, whereas the maximum wavelength depends the voltage
c. Both the minimum wavelength and the maximum wavelength depend on the filtration
d. The maximum wavelength depends on the voltage, while the purpose of the filters is to reduce the amount of scattering

5. Which of the following statements is true regarding the PA orientation versus the AP orientation for chest radiography?

a. Silhouettes of both the heart and mediastinum appear larger in the AP view, compared with the PA view
b. Silhouettes of both the heart and mediastinum appear larger in the PA view, compared with the AP view
c. The heart appears larger in AP than PA orientation, while the mediastinum appears larger in PA than AP orientation
d. The heart appears smaller in AP than PA orientation, while the mediastinum appears smaller in PA than AP orientation

Answers & Explanations

1. D: Bremsstrahlung radiation consists of x-rays that are produced when high-velocity electrons interact with shielding materials. Electrons emitted from the nuclei of many radioactive isotopes (radionuclides) are known as beta particles. Beta particles are less harmful to biological tissue than other types of ionizing radiation, while Bremsstrahlung radiation is quite harmful. Nevertheless, high-energy beta emitters such as phosphorus-32 do require some type of shielding. Since the magnitude of Bremsstrahlung radiation produced when beta particles strike an absorber is proportional to the absorber's atomic number, the primary shielding around a beta emitter should consist of small atoms. Thus, polymethyl methacrylate (PMMA), known commonly by the trade names Plexiglas, Lucite, and Perspex, is used around beta emitters. Although lead cannot be used as primary shielding, it is appropriate as secondary protection, placed outside of the PMMA shield.

2. B: Compared with most other medical imaging modalities, computed tomography (CT) scanning provides relatively high exposure to ionizing radiation, with 40 millisieverts (mSv) falling in the normal range for a full-body CT. Flat film chest x-ray radiography (lateral and posterior-anterior) imparts ionizing radiation doses typically in the range of 10 to 100 microsieverts (µSv), but the dose should not be as high as 1 mSv. Dental x-rays typically are in the area of 5 µSv, with some variation, but should not provide exposures as high as 50 µSv. For a passenger flying from New York to San Francisco, an exposure of approximately 40 µSv is typical because of increased exposure to cosmic radiation as one ascends to higher altitudes.

3. A: In contrast to the threshold model, which posits that exposure to ionizing radiation must not be harmful below a certain threshold, the linear no-threshold model (LNT) assumes that any ionizing radiation can be harmful, even the background radiation dose that all people receive naturally from the environment. The LNT model is called "linear," because a line is graphed for low-dose exposures by extrapolating backwards from a line calculated from high-dose data. These high-dose data come from populations known to have suffered harmful effects from fairly high radiation exposures. If even very small radiation exposures can increase an individual's risk of ill effects, it follows that damage resulting from low-level exposure is cumulative for any given individual. Since it is known that cells actually do repair, not only radiation-induced DNA breakage, but damage to other cell structures, the LNT model has been challenged. If the LNT is wrong, the effects of chronic, low-level radiation exposure should not be cumulative at all. Nevertheless, in health care settings, a policy of "as low as reasonably achievable" (ALARA) is followed.

4. A: The energy of x-rays and all categories of electromagnetic radiation is given by the equation E = hc/λ, where E is the energy and λ is the wavelength. At the same time, energy (E) is proportional to voltage. Thus, v and λ are inversely proportional, which means that increasing the voltage shortens the wavelength of an x-ray beam. The value for λ, however, represents only the most energetic photons among all that are produced by the x-ray tube. Metals, such as aluminum, absorb the lower energy x-rays (those with a longer λ), while the higher energy x-rays pass through. Thus, sending the beam through filters sets a maximum on how long the waves reaching the patient can be. Filters are used because the lower energy x-rays are not useful in the generation of images but still can harm tissue, at least near the surface. Also, since the lower energy x-rays tend to scatter, if not filtered they would strike not only the target area on the patient, but other areas as well.

5. A: When compared with the PA view, the heart and mediastinum appear magnified when the AP orientation is used. This is because these structures are slightly further away from the photographic plate when the beam enters the chest and exits through the back than in the opposite scenario. To visualize why this is the case, think of a flashlight shining on your hand to project a shadow on the wall. Move the hand back from the wall and the image gets larger. Enlargement of either the heart or mediastinum can have very important clinical implications. This is one reason why it is important not to mislabel an AP image as a PA image, or vice versa.

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Last Updated: 03/01/2017


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