Radiation physics, units and dosimetry

53 Gulf Prometric Radiographer (DHA, DOH, MOH, SCFHS, QCHP) practice questions on Radiation physics, units and dosimetry, part of Radiation Protection. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Hard

   An aluminium filter has an HVL of 3 mm for a given beam. A 9 mm layer of the same aluminium transmits what fraction of the beam?

   1. A6.25%
   2. B33%
   3. C25%
   4. D12.5%

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   Answer: D · 12.5%

   A thickness of 9 mm is three HVLs. Each HVL halves the intensity, so the transmitted fraction is 1/2 x 1/2 x 1/2 = 1/8, which is 12.5%. This simplification assumes a narrow beam and the same beam quality.

   • A: 6.25% is the transmission after four HVLs (12 mm).
   • B: 33% results from treating 9 mm as three equal thirds of attenuation.
   • C: 25% is the transmission after two HVLs (6 mm).

   Remember this

   n half-value layers transmit (1/2)^n of the beam.

2. Question 2 · Medium

   Barium sulphate is an effective radiographic contrast agent because barium:

   1. AHas the same atomic number as soft tissue, so it blends in
   2. BHas a very low density, which lets the x-rays pass through the lumen
   3. CEmits visible light after being irradiated by the beam
   4. DHas a K-edge near 37 keV, so photoelectric absorption is strong

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   Answer: D · Has a K-edge near 37 keV, so photoelectric absorption is strong

   Barium has a high atomic number (56) and its K-shell binding energy is 37.4 keV, within the energy range of the beam. Photons just above this energy are absorbed by the photoelectric effect much more than in soft tissue, so the gut lumen is outlined. Iodine (Z 53, K-edge 33 keV) works in the same way.

   • A: If barium matched the atomic number of soft tissue, it would give no contrast.
   • B: A low density would make barium transparent, not opaque.
   • C: Contrast agents attenuate x-rays and do not emit light.

   Remember this

   High atomic number with a K-edge in the beam energy range gives strong contrast.

3. Question 3 · Medium

   Coherent (classical) scattering differs from Compton scattering in that it:

   1. AProduces characteristic x-rays as the vacancy is filled
   2. BNeeds a photon energy of at least 1.02 MeV to occur
   3. CChanges only the photon's direction, with no ionisation
   4. DEjects an inner-shell electron from the atom it strikes

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   Answer: C · Changes only the photon's direction, with no ionisation

   In coherent scatter a low-energy photon, below about 10 keV, excites the whole atom and is re-emitted in a new direction with no energy loss. No electron is ejected, so there is no ionisation and no patient dose. It forms a small part of the interactions in diagnostic radiology.

   • A: Characteristic radiation follows photoelectric absorption and a vacancy being filled.
   • B: Pair production, not coherent scatter, needs 1.02 MeV.
   • D: Ejecting an inner-shell electron describes photoelectric absorption.

   Remember this

   Coherent scatter changes direction only, with no ionisation and no energy loss.

4. Question 4 · Medium

   Dose-area product (DAP) is measured in Gy cm2 and, for a given beam, is:

   1. ADetermined by the patient thickness alone, not by the beam
   2. BHighest at the image receptor, because the beam is widest there
   3. CConstant with distance, as dose falls while the area rises
   4. DThe same as the effective dose, expressed in sieverts per patient

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   Answer: C · Constant with distance, as dose falls while the area rises

   As the beam diverges, dose per unit area falls with the square of the distance while the area rises by the same factor, so their product stays constant. A DAP meter at the tube exit therefore reflects the whole beam leaving the tube and is used in fluoroscopy to track patient dose.

   • A: DAP depends on the kerma and the field size as well as on exposure factors.
   • B: DAP does not change along the beam, so no point has a higher value.
   • D: Effective dose is in sieverts and needs organ weighting; DAP is only an indicator.

   Remember this

   DAP is independent of distance from the focus and tracks total energy delivered.

5. Question 5 · Medium

   Why is a Geiger-Muller counter a poor choice for measuring the exposure rate in an intense pulsed x-ray beam?

   1. AIt reads in becquerels only and cannot be set for dose rate
   2. BDead time after each event makes it under-respond in pulsed beams
   3. CIt cannot detect photons of any energy range used in radiography
   4. DIt measures only alpha particles emitted by radioactive sources

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   Answer: B · Dead time after each event makes it under-respond in pulsed beams

   After each ionising event the tube needs a recovery period during which it cannot count. In a pulsed beam many photons arrive in a very short time, so most are missed and the meter can even read zero. Ionisation chambers do not suffer this problem and are the standard for x-ray survey.

   • A: A GM counter reads counts per second or dose rate, and its response depends on its calibration.
   • C: A GM counter detects photons well; that is not its limitation.
   • D: A GM counter detects beta and gamma radiation as well as alpha.

   Remember this

   Use an ionisation chamber, not a GM counter, to survey pulsed x-ray beams.

6. Question 6 · Medium

   Compared with a single-phase full-wave generator at the same set kVp and mAs, a three-phase 12-pulse generator gives:

   1. AHigher output but a lower maximum photon energy than single-phase
   2. BIdentical output, because the kVp and mAs settings are the same
   3. CHigher output and higher mean energy, because ripple is smaller
   4. DLower output and lower mean energy, because the ripple is larger

   Answer and explanation in LearnFree account

7. Question 7 · Hard

   A photon energy is doubled from 40 keV to 80 keV in a tissue. Photoelectric absorption probability changes by about a factor of:

   1. AOne eighth
   2. BOne sixteenth
   3. COne quarter
   4. DOne half

   Answer and explanation in LearnFree account

8. Question 8 · Hard

   After a series of examinations, three organs receive equivalent doses of: lung 2 mSv (weighting factor 0.12), stomach 1 mSv (0.12) and thyroid 4 mSv (0.04). Counting only these three, the effective dose is:

   1. A7 mSv
   2. B0.36 mSv
   3. C2.3 mSv
   4. D0.52 mSv

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9. Question 9 · Medium

   A declared-pregnant radiographer works in fluoroscopy and wears a lead apron. A second personal dosimeter is issued to estimate the dose to her fetus. It should be worn:

   1. AOn the finger as a ring
   2. BUnder the apron at abdomen level
   3. CAt collar level outside the apron
   4. DIn the pocket of the outer apron

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10. Question 10 · Medium

    The probability of Compton scattering in a tissue depends mainly on the:

    1. ACube of the photon energy of the incident x-ray beam
    2. BCube of the atomic number of the absorbing tissue in the beam path
    3. CThickness of the added filter in the tube housing
    4. DElectron density of the tissue, nearly independent of atomic number

    Answer and explanation in LearnFree account

11. Question 11 · Easy

    Thermoluminescent dosimeters used for personal and patient dose measurement most commonly contain:

    1. ALithium fluoride
    2. BAluminium oxide
    3. CArgon gas
    4. DSilver bromide

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12. Question 12 · Medium

    A mobile unit is moved from 100 cm to 200 cm source-to-image distance and the technique is not changed. The beam intensity reaching the receptor becomes:

    1. AOne quarter
    2. BOne eighth
    3. CFour times
    4. DOne half

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13. Question 13 · Medium

    An old record from a hospital in Muscat lists a cumulative annual whole-body dose of 2 rem for a radiographer. In SI units this equals:

    1. A20 mSv
    2. B2 mSv
    3. C0.2 mSv
    4. D200 mSv

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14. Question 14 · Hard

    An exposure is made at 90 kVp and 300 mA for 0.2 s on a six-pulse three-phase generator (heat unit factor 1.35). The heat load is:

    1. A3,600 heat units
    2. B7,290 heat units
    3. C5,400 heat units
    4. D7,614 heat units

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15. Question 15 · Hard

    At constant mAs and distance, a technologist raises the tube potential from 60 kVp to 120 kVp. Using the standard approximation, the x-ray intensity (output) changes by a factor of about:

    1. A4
    2. B8
    3. C16
    4. D2

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16. Question 16 · Medium

    An exposure of 10 mAs corresponds to roughly how many electrons crossing from cathode to anode?

    1. A6.25 x 10^13
    2. B6.25 x 10^18
    3. C6.25 x 10^16
    4. D6.25 x 10^15

    Answer and explanation in LearnFree account

17. Question 17 · Medium

    To raise the tube current in mA without changing the kVp, the generator increases the:

    1. AThickness of the added filtration in the tube housing
    2. BRotor speed of the anode during the exposure
    3. CFilament heating current (thermionic emission)
    4. DAnode angle chosen for the focal spot size

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18. Question 18 · Medium

    Air kerma, expressed in gray, represents the:

    1. ANumber of nuclear decays per second
    2. BEnergy absorbed per unit mass of the patient's tissue
    3. CBiological risk of the exposure
    4. DKinetic energy transferred to charged particles per unit mass of air

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19. Question 19 · Medium

    Pair production can occur only if the incident photon has an energy of at least:

    1. A511 keV
    2. B10 MeV
    3. C100 keV
    4. D1.02 MeV

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20. Question 20 · Hard

    A radiographer in Doha received 12, 18, 25 and 20 mSv in the first four years of a defined five-year period. Using the ICRP limits, the highest dose allowed in year five is:

    1. A20 mSv
    2. B5 mSv
    3. C50 mSv
    4. D25 mSv

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21. Question 21 · Medium

    Tungsten K-shell characteristic x-rays are produced only when the tube potential is above about:

    1. A120 kVp
    2. B70 kVp
    3. C100 kVp
    4. D20 kVp

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22. Question 22 · Easy

    The anode target of a general-purpose diagnostic x-ray tube is made of:

    1. AMolybdenum with a rhodium filter
    2. BTungsten, usually alloyed with rhenium
    3. CCopper plated with silver
    4. DAluminium with a lead backing

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23. Question 23 · Medium

    A department replaces film badges with OSL dosimeters. A key advantage of the OSL detector is that it:

    1. ACannot detect doses below 1 mSv
    2. BLoses its signal completely once it is read
    3. CCan be re-read and gives a stable, repeatable result
    4. DNeeds wet chemical processing like film

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24. Question 24 · Hard

    During a fluoroscopic examination the DAP meter reads 8 Gy cm2 and the beam at the skin entrance covers 20 cm x 10 cm. Ignoring backscatter, the average air kerma over the entrance field is:

    1. A160 mGy
    2. B400 mGy
    3. C4 mGy
    4. D40 mGy

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25. Question 25 · Medium

    A personal dosimeter is calibrated to report the quantity Hp(10). This is the:

    1. AEntrance skin dose at the patient surface
    2. BPersonal dose equivalent at 10 mm depth in soft tissue
    3. CDose-area product of the beam
    4. DCollective dose of the department

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