22 Radiation Physics, Biology & Protection Practice Questions & Answers

1. 1. A patient's gagging forces the operator to shorten the exposure time on the next image. Assuming the unit allows it, what change keeps the image density the same as before?

   • A.Move the position-indicating device closer, since a shorter distance restores lost density
   • B.Raise the milliamperage in proportion, since density depends on milliamperage times time✓ Answer
   • C.Raise the kilovoltage in proportion, since kilovoltage is the control that sets density
   • D.Change nothing, since exposure time affects only contrast and not the density of the image

   Density is set by the quantity of photons reaching the receptor, which is the product of milliamperage and exposure time (mAs). Cutting the time and raising the milliamperage proportionally keeps that product, and therefore the density, unchanged; kilovoltage governs beam energy and contrast, not density.

   Source: Iannucci & Howerton, Dental Radiography — exposure factors: milliamperage, exposure time and mAsReport a problem with this question

2. 2. Two photons leave the tubehead, and one has a shorter wavelength than the other. Compared with the longer-wavelength photon, what is true of the shorter-wavelength photon?

   • A.It has a lower frequency and less energy, so the filter removes it from the beam
   • B.It has a lower frequency but more energy, so it travels faster than the other one
   • C.It has a higher frequency but less energy, so superficial tissue absorbs it first
   • D.It has a higher frequency and more energy, so it penetrates tissue more readily✓ Answer

   Wavelength, frequency and energy are linked: as wavelength shortens, frequency and photon energy rise, and a higher-energy photon is more penetrating. Long-wavelength, low-energy photons would only be absorbed in surface tissue, which is why aluminum filtration removes them.

   Source: Iannucci & Howerton, Dental Radiography — properties of the x-ray beam: wavelength, frequency and penetrating powerReport a problem with this question

3. 3. Which statement about the physical characteristics of the x-radiation used in dental imaging is accurate?

   • A.X-rays follow curved paths that a magnetic field can bend toward the receptor surface
   • B.X-rays lose speed as they lose energy, so low-energy photons move more slowly
   • C.X-rays travel in straight lines at the speed of light and cannot be brought to a focus✓ Answer
   • D.X-rays carry a negative charge, which is what draws them toward mineralized tissue

   X-rays are weightless, chargeless packets of electromagnetic energy that travel in straight, diverging lines at the speed of light regardless of their energy, and they cannot be focused to a point. Because they diverge from a small source, beam intensity falls off with distance rather than being steered or concentrated.

   Source: Iannucci & Howerton, Dental Radiography — properties of x-radiationReport a problem with this question

4. 4. During a routine check, radiation is detected escaping from the sides of a tubehead housing rather than through the opening of the position-indicating device. How is that radiation classified, and what does it indicate?

   • A.Leakage radiation, which has no diagnostic use and points to a defective tubehead✓ Answer
   • B.Primary radiation, which is the useful beam and normally exits the housing at low levels
   • C.Characteristic radiation, which the housing produces routinely and which poses no hazard
   • D.Secondary radiation, which forms whenever the useful beam strikes the patient's tissues

   Radiation that escapes the tubehead housing in any direction other than through the collimated opening is leakage radiation: it contributes nothing to the image and adds dose to patient and staff. A properly shielded housing confines the beam to the opening, so detectable leakage means the unit is defective and must be taken out of service.

   Source: Iannucci & Howerton, Dental Radiography — primary, secondary, scatter and leakage radiationReport a problem with this question

5. 5. Why are the developing tissues of a child generally more sensitive to radiation than the same tissues in an adult?

   • A.They are highly differentiated and divide slowly, which delays the repair of damage
   • B.They hold less water, so fewer free radicals form when photons pass through the cells
   • C.They divide rapidly and are less differentiated, and such cells are more radiosensitive✓ Answer
   • D.They contain more mineral per gram, so they absorb a larger share of the primary beam

   The law of Bergonie and Tribondeau holds that cells are most radiosensitive when they are rapidly dividing, undifferentiated and have a high mitotic rate, which describes growing tissue in a child. That is why prescribing for children is especially individualized and exposure factors are reduced.

   Source: Iannucci & Howerton, Dental Radiography — law of Bergonie and Tribondeau, cell radiosensitivityReport a problem with this question

6. 6. Two people receive different radiation doses on the same day. How does the size of the dose affect the latent period that follows?

   • A.The dose has no effect on the latent period, which is fixed for each type of tissue
   • B.The larger dose shortens the latent period, since greater damage shows itself sooner✓ Answer
   • C.The larger dose lengthens the latent period, since repair runs longer at first
   • D.The smaller dose removes the latent period, since low doses cause no cell change

   The latent period is the interval between exposure and the first observable clinical signs, and it varies inversely with dose: the larger the dose, the sooner effects appear. The sequence of injury runs latent period, then period of injury, then recovery, and no dose is assumed to be entirely without effect.

   Source: Iannucci & Howerton, Dental Radiography — latent period, period of injury and recovery periodReport a problem with this question

7. 7. Radiation exposure produces a change in a cell of the exposed person's thyroid gland. How is that effect classified, and why?

   • A.A genetic effect, because the thyroid is linked hormonally to the reproductive organs
   • B.A genetic effect, because damage to the DNA of any cell can be transmitted to children
   • C.A somatic effect, because it arises in a body cell and is not passed to the offspring✓ Answer
   • D.A somatic effect, because only reproductive cells can be harmed in the exposed person

   Somatic effects occur in the body cells of the exposed individual and are not inherited, while genetic effects occur in reproductive cells and can be passed to offspring. The thyroid is a body tissue, so a change in one of its cells is somatic, however sensitive that gland happens to be.

   Source: Iannucci & Howerton, Dental Radiography — somatic versus genetic effects of radiationReport a problem with this question

8. 8. Which pairing correctly matches a radiation effect with the way it relates to dose?

   • A.Cancer is deterministic: the severity rises with dose once a threshold is exceeded
   • B.Skin reddening is deterministic: probability rises with dose but severity stays fixed
   • C.Skin reddening is stochastic: it may follow any dose, with probability rising by dose
   • D.Cancer is stochastic: the probability rises with dose and no threshold is assumed✓ Answer

   Stochastic effects such as cancer and genetic change are described as having no threshold, with the probability of occurrence rising as dose rises, while their severity does not depend on dose. Deterministic effects such as skin reddening appear only above a threshold and grow more severe as dose increases.

   Source: NCRP radiation protection recommendations; Iannucci & Howerton, Dental Radiography — stochastic and deterministic effectsReport a problem with this question

9. 9. An assistant reads that cells repair part of the damage radiation causes. Which view of that repair underlies radiation protection practice?

   • A.Repair is total below a threshold dose, and a small exposure leaves no damage behind
   • B.Repair is incomplete, so unrepaired damage accumulates across a working lifetime✓ Answer
   • C.Repair occurs in reproductive cells alone, so damage to body cells is always permanent
   • D.Repair is finished within days, so exposures a week apart never add to each other

   Cells repair some radiation damage during the recovery period, but the repair is incomplete, so a residue of unrepaired damage adds up over a lifetime of exposures. This cumulative effect, together with the linear non-threshold assumption used for dental doses, is why every exposure must be justified.

   Source: Iannucci & Howerton, Dental Radiography — cumulative effects and the recovery period; NCRP radiation protection recommendationsReport a problem with this question

10. 10. Which quantity does the gray (Gy) express?

    • A.The energy that radiation deposits in a given mass of tissue✓ Answer
    • B.The rate at which the tube emits photons during an exposure
    • C.The ionization that radiation produces in a given volume of air
    • D.The harm expected from a dose after weighting for the type of radiation

    The gray is the SI unit of absorbed dose, defined as the energy deposited by radiation per unit mass of the irradiated material, and its traditional counterpart is the rad. Biological weighting is not part of it; that step belongs to the sievert, and ionization in air belongs to the roentgen.

    Source: International System of Units definitions of radiation quantities; Iannucci & Howerton, Dental Radiography — units of radiation measurementReport a problem with this question

11. 11. A patient handout compares the risk of a dental imaging series with the risk from a year of natural background radiation. Which quantity makes that comparison meaningful, and why?

    • A.Effective dose in sieverts, since it weights dose for the sensitivity of each tissue✓ Answer
    • B.Exposure in coulombs per kilogram, since it measures ionization made in open air
    • C.Absorbed dose in grays, since the gray already accounts for the type of radiation
    • D.Absorbed dose in grays, since it reports deposited energy with no weighting applied

    Effective dose, expressed in sieverts, weights the dose delivered to each organ by that organ's sensitivity, which lets a partial-body exposure such as dental imaging be compared with whole-body background radiation. Absorbed dose in grays states deposited energy only, and exposure in coulombs per kilogram describes ionization in air.

    Source: International System of Units definitions of radiation quantities; NCRP recommendations on effective doseReport a problem with this question

12. 12. The roentgen and its SI counterpart, the coulomb per kilogram, are used to describe which of the following?

    • A.Energy the beam deposits in a measured mass of soft tissue
    • B.Biological damage the beam is expected to cause in an organ
    • C.Ionization the beam produces in a measured volume of air✓ Answer
    • D.Number of photons the tube produces at a chosen mA setting

    Exposure is the quantity of ionization a beam produces in air, measured traditionally in roentgens and in SI units as coulombs per kilogram. It is a measurement made in air rather than in tissue, so it says nothing directly about energy absorbed or biological harm.

    Source: International System of Units definitions of radiation quantities; Iannucci & Howerton, Dental Radiography — exposure, absorbed dose and dose equivalentReport a problem with this question

13. 13. A dental x-ray exposure switch is a dead-man type and is mounted so the operator can stay shielded. What is the radiation-safety reason for those two requirements?

    • A.The operator can steady the tubehead by hand while the exposure is being produced
    • B.The switch overrides the timer, so a large patient can be given a longer exposure
    • C.The exposure runs on after release, so the operator can reach the barrier during it
    • D.The exposure ends the instant the switch is released and is made from a shielded spot✓ Answer

    A dead-man switch terminates the exposure the moment pressure is released, so an exposure cannot continue without the operator's active control. Mounting it where the operator can remain behind a barrier or at the correct distance and angle keeps the operator out of the primary beam and away from scatter throughout the exposure.

    Source: Iannucci & Howerton, Dental Radiography — operator protection: barriers, distance and position; exposure switch requirementsReport a problem with this question

14. 14. An operator who usually stands 6 feet from the tubehead during an exposure moves to 3 feet, with nothing else changed. What happens to the radiation intensity at that position?

    • A.It stays at its former value, because the law applies to the primary beam alone
    • B.It rises to twice its former value, because intensity varies directly with the distance
    • C.It falls to half of its former value, because the closer spot lies in the beam's shadow
    • D.It rises to four times its former value, because intensity varies as distance squared✓ Answer

    By the inverse square law, intensity changes with the square of the change in distance, so halving the distance multiplies intensity by four while doubling it cuts intensity to one quarter. Distance is therefore one of the operator's most powerful protective measures, and the common error is to treat the relationship as linear.

    Source: Iannucci & Howerton, Dental Radiography — inverse square law and operator distanceReport a problem with this question

15. 15. A new team member asks what the ALARA principle requires in daily practice. Which explanation is correct?

    • A.Each dose limit applies to staff members, since patients gain a benefit from the images
    • B.Each exposure must be justified and held to the least dose that still gives a diagnosis✓ Answer
    • C.Each retake is acceptable because a digital receptor needs only a small amount of radiation
    • D.Each exposure is acceptable as long as the unit passes its scheduled equipment checks

    ALARA means as low as reasonably achievable: every exposure must be clinically justified and then delivered with the smallest dose that still yields a diagnostic image. Equipment inspections and the low dose of digital receptors reduce dose but never substitute for justifying the exposure in the first place.

    Source: ADA/FDA, The Selection of Patients for Dental Radiographic Examinations — ALARA principleReport a problem with this question

16. 16. An insurance carrier tells the office it wants a full-mouth series submitted with a claim, but the dentist's examination and history found no indication for imaging. What is the correct course of action?

    • A.The assistant exposes a smaller series, which satisfies the payer while limiting the dose
    • B.The office exposes the series once the patient signs a form accepting the payer's requirement
    • C.The office exposes the full series, since a payer's requirement establishes the need for imaging
    • D.The dentist decides from the clinical findings, and the office reports that no images are indicated✓ Answer

    Radiographs are prescribed by the dentist on the basis of the health history, the clinical examination and the patient's risk, never as a routine or as a condition imposed by a third party. Exposing images with no diagnostic indication delivers dose without benefit, which violates the justification requirement behind ALARA.

    Source: ADA/FDA, The Selection of Patients for Dental Radiographic Examinations — prescribing radiographs by clinical needReport a problem with this question

17. 17. Which description of informed consent for dental radiographs is correct?

    • A.The patient hears the purpose, benefits, risks and alternatives before any exposure✓ Answer
    • B.The patient signs one general treatment form that covers imaging for years afterward
    • C.The patient hears the number of images and the fee when the visit is being scheduled
    • D.The patient hears an explanation of the findings once the images have been reviewed

    Informed consent requires that the patient be told the purpose of the imaging, its benefits, its risks and the reasonable alternatives, and that this happen before the exposure is made so the patient can still decline. Consent obtained after the fact, or buried in a blanket treatment form, does not meet that standard.

    Source: ADA/FDA, The Selection of Patients for Dental Radiographic Examinations; Bird & Robinson, Modern Dental Assisting — informed consentReport a problem with this question

18. 18. A patient declines the bitewing images the dentist has prescribed. How should the assistant proceed?

    • A.Record the refusal in the chart, tell the dentist, and explain the diagnostic limits✓ Answer
    • B.Have the patient sign a waiver releasing the dentist from any missed disease findings
    • C.Enter the refusal as consent for treatment without images and continue with the visit
    • D.Expose one image as a compromise, note it in the chart, and let the dentist review it

    A patient may refuse radiographs, and the assistant's duty is to document that refusal in the record, inform the dentist and make sure the patient understands what cannot be diagnosed without the images. A waiver does not relieve the dentist of responsibility, and substituting a different image is a change to the prescription, which only the dentist can make.

    Source: Bird & Robinson, Modern Dental Assisting — patient refusal and documentation; ADA/FDA selection criteriaReport a problem with this question

19. 19. A patient asks how much radiation a set of dental images involves and whether it is dangerous. Which reply is best?

    • A.Assure the patient that dental imaging carries no risk at all, since the dose is small
    • B.Tell the patient the dose equals that of a chest scan but that the benefit is greater
    • C.Compare the dose honestly with everyday background radiation and name the safeguards✓ Answer
    • D.Say the dose cannot be put in ordinary terms and refer the question to a physicist

    An honest answer puts the dose in terms the patient can weigh, such as a comparison with the natural background radiation everyone receives, and describes the measures used to keep it low. Promising zero risk is inaccurate under the no-threshold assumption, and deflecting the question leaves a worried patient without the information needed to consent.

    Source: ADA/FDA, The Selection of Patients for Dental Radiographic Examinations; Bird & Robinson, Modern Dental Assisting — patient communication about radiationReport a problem with this question

20. 20. In one operatory several images each week must be repeated because the beam was not centered on the receptor. Which change reduces unnecessary patient exposure most directly?

    • A.Fit beam-alignment holders that link the receptor to the position-indicating device✓ Answer
    • B.Lower the exposure settings so that each repeated image delivers a smaller dose
    • C.Raise the exposure settings so the first image is dense enough to avoid a repeat
    • D.Add one image at the start of each series to serve as a guide for the rest of it

    A retake exposes the patient a second time for a single diagnostic image, so preventing retakes is the most direct dose reduction available. A beam-alignment device holds the receptor and aligns the position-indicating device with it, which prevents the off-centre beam that produces the unexposed corner known as a cone cut.

    Source: Iannucci & Howerton, Dental Radiography — retakes as a source of unnecessary exposure; beam-alignment devicesReport a problem with this question

21. 21. A 6-year-old patient is scheduled for the bitewing images the dentist prescribed. Which adjustment is appropriate for this patient?

    • A.Keep the adult exposure factors, since a receptor needs a set amount of radiation
    • B.Reduce the exposure factors, since the child is smaller and more radiosensitive✓ Answer
    • C.Keep adult factors but take fewer images, since dose depends on the image count
    • D.Raise the exposure factors, since children move and the image must be denser

    A child presents less tissue for the beam to penetrate and has rapidly dividing, radiosensitive growing tissues, so exposure factors are reduced from adult settings. Reducing the number of images is decided by the dentist on clinical need and does not replace adjusting the factors for the patient's size and age.

    Source: ADA/FDA, The Selection of Patients for Dental Radiographic Examinations; Iannucci & Howerton, Dental Radiography — patient size and age as protection factorsReport a problem with this question

22. 22. An assistant notices a crack in the housing of an x-ray tubehead, although the unit is still producing usable images. What is the correct course of action?

    • A.Cover the crack with a barrier and keep working, since barriers contain the leakage
    • B.Stop using the unit, tag it out of service, tell the dentist, and call a technician✓ Answer
    • C.Finish the day's patients on the unit, log the crack, and raise it at the next meeting
    • D.Keep using the unit from farther away, since added distance offsets leaked radiation

    A damaged housing can no longer be assumed to confine the beam, so the unit is a suspected source of leakage radiation and every further exposure adds dose that is not accounted for. The protocol for a suspected malfunction is to stop using the unit at once, remove it from service, report it to the dentist and have a qualified service technician evaluate it.

    Source: Iannucci & Howerton, Dental Radiography — leakage radiation and protocol for a suspected equipment malfunctionReport a problem with this question

Practice questions only — not real exam items, and not affiliated with or endorsed by the Dental Assisting National Board. Questions are written to the domains published in DANB's current RHS and ICE exam outlines, and cover digital radiography only, matching DANB's statement that film-based concepts are no longer tested. This bank does not cover the chairside assisting or dental anatomy component exams. What a dental assistant is legally allowed to do — including who may expose radiographs and what training a state requires first — is set by each state's dental board and is deliberately out of scope here. Exam format, eligibility, and scoring are set by DANB and change from time to time; confirm the current requirements with DANB and your state board before you register. Official DANB exam outlines →