CT physics, protocols and image quality

56 Gulf Prometric Radiographer (DHA, DOH, MOH, SCFHS, QCHP) practice questions on CT physics, protocols and image quality, part of CT. 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

   A protocol gives acceptable image noise at 100 mAs. A radiologist asks for the noise to be halved without changing kVp, slice thickness or kernel. What mAs and relative dose does this need?

   1. A200 mAs, twice the dose
   2. B400 mAs, four times the dose
   3. C800 mAs, eight times the dose
   4. D141 mAs, 1.4 times the dose

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   Answer: B · 400 mAs, four times the dose

   Noise varies as the inverse square root of the number of photons, which is proportional to the mAs. To halve the noise, the mAs must be raised fourfold, which also gives four times the dose. Halving noise is therefore costly, which is why it is rarely chosen.

   • A: Doubling mAs only lowers noise by about 30%.
   • C: 800 mAs would reduce noise by a factor of 2.8.
   • D: 141 mAs lowers noise by about 16%.

   Remember this

   Halving noise needs four times the mAs and four times the dose.

2. Question 2 · Medium

   A routine chest CT in Abu Dhabi gives a DLP of 350 mGy·cm. Using a conversion factor (k) of 0.014 mSv per mGy·cm for the adult chest, as given in the European CT quality guidelines, the estimated effective dose is closest to:

   1. A49 mSv
   2. B0.49 mSv
   3. C25 mSv
   4. D4.9 mSv

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   Answer: D · 4.9 mSv

   Effective dose is estimated as DLP multiplied by a region-specific conversion factor: 350 times 0.014 equals 4.9 mSv. The k-factors come from the European guidelines and later reviews and differ slightly between sources. The result is an estimate for a reference adult and should not be used as a risk figure for an individual patient.

   • A: 49 mSv has a misplaced decimal point.
   • B: 0.49 mSv has a misplaced decimal point the other way.
   • C: 25 mSv would need a factor of about 0.07.

   Remember this

   Effective dose is estimated as DLP multiplied by a region-specific k-factor.

3. Question 3 · Medium

   On a neck CT, bright and dark streaks run horizontally across the shoulders, where the patient is widest. What is the best explanation?

   1. ACupping from the bowtie filter
   2. BPhoton starvation from high attenuation in the lateral direction
   3. CRing artefact from a faulty detector
   4. DPartial volume averaging in the lung apex

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   Answer: B · Photon starvation from high attenuation in the lateral direction

   Across the shoulders, too few photons reach the detector, so noise in those projections is very high and creates streaks. Raising the tube current for this region, using tube-current modulation, lowering the arms, or using iterative reconstruction reduces the effect.

   • A: Cupping is a centre-to-edge change in a round object.
   • C: Ring artefacts are circular, not horizontal.
   • D: Partial volume produces blurring at an interface.

   Remember this

   Photon starvation causes streaks across the shoulders; tube-current modulation and iterative reconstruction help.

4. Question 4 · Medium

   The CTDIvol value displayed for an adult head scan is calculated using a PMMA phantom of what diameter?

   1. A20 cm
   2. B8 cm
   3. C16 cm
   4. D32 cm

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   Answer: C · 16 cm

   Standard CT dose phantoms are PMMA cylinders: 16 cm in diameter for the head and 32 cm for the body. The same dose output looks different when expressed in each phantom, so the phantom used must be known when doses are compared.

   • A: 20 cm is a quality-control phantom size, not the dosimetry standard.
   • B: 8 cm is smaller than any standard dosimetry phantom.
   • D: 32 cm is the body phantom.

   Remember this

   CT dose phantoms are 16 cm for the head and 32 cm for the body.

5. Question 5 · Hard

   The display is set to level +40 HU and width 80 HU. A calcified plaque of +150 HU and a rib of +600 HU are both on the image. How do they appear?

   1. AThe plaque is white and the rib is grey
   2. BThe plaque is grey and the rib is white
   3. CBoth are pure white and cannot be told apart
   4. DBoth are black

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   Answer: C · Both are pure white and cannot be told apart

   This window displays 0 to +80 HU. Anything above +80 HU is displayed as the maximum white level, so +150 HU and +600 HU look identical. The window must be widened to separate dense structures.

   • A: The rib is higher than the plaque, so it cannot be darker.
   • B: +150 HU is above the window, so it is not grey.
   • D: The window ends at +80 HU, not below 0 HU.

   Remember this

   Any CT number above the top of the window displays as the same white.

6. Question 6 · Hard

   A CT abdomen and pelvis in Dubai has a CTDIvol of 10 mGy over a scanned length of 45 cm. With an effective dose conversion factor of 0.015 mSv per mGy·cm, what is the estimated effective dose?

   1. A67.5 mSv
   2. B6.75 mSv
   3. C4.5 mSv
   4. D0.68 mSv

   Answer and explanation in LearnFree account

7. Question 7 · Easy

   In helical CT, pitch is defined as table travel per gantry rotation divided by the:

   1. ATotal nominal beam collimation width
   2. BScan field of view
   3. CSlice thickness of one detector row only
   4. DGantry rotation time

   Answer and explanation in LearnFree account

8. Question 8 · Medium

   How does iterative reconstruction produce a lower-noise image than filtered back projection from the same raw data?

   1. AIt collects extra projections after the scan has finished, using stored raw data to fill gaps in the original acquisition
   2. BIt repeatedly compares simulated projections with measured data and updates the image using a noise model
   3. CIt increases the tube current retrospectively
   4. DIt applies a sharper kernel in one pass

   Answer and explanation in LearnFree account

9. Question 9 · Hard

   A CT abdomen scan has a CTDIvol of 12 mGy and a scan length of 40 cm. What is the dose-length product (DLP)?

   1. A12 mGy.cm
   2. B52 mGy.cm
   3. C30 mGy.cm
   4. D480 mGy.cm

   Answer and explanation in LearnFree account

10. Question 10 · Medium

    A dual-energy CT abdomen is acquired at two tube voltages after IV contrast. Which extra image can be produced from the data without a separate unenhanced scan?

    1. AA time-of-flight angiogram
    2. BA perfusion map of the liver
    3. CA delayed excretory phase
    4. DA virtual non-contrast image

    Answer and explanation in LearnFree account

11. Question 11 · Medium

    A posterior fossa image shows dark bands between the two petrous bones. What is the cause?

    1. ADetector element failure
    2. BPatient respiration
    3. CTruncation of the scan field of view
    4. DBeam hardening between two dense structures

    Answer and explanation in LearnFree account

12. Question 12 · Easy

    Which tissue typically has a CT number of about -100 HU?

    1. ALung parenchyma
    2. BFat
    3. CSkeletal muscle
    4. DClotted blood

    Answer and explanation in LearnFree account

13. Question 13 · Hard

    The centre reading of a body CT dose phantom is 20 mGy and the periphery reading is 30 mGy for one rotation at pitch 0.8. What is the CTDIvol?

    1. AAbout 21 mGy
    2. BAbout 27 mGy
    3. CAbout 25 mGy
    4. DAbout 33 mGy

    Answer and explanation in LearnFree account

14. Question 14 · Hard

    A paediatric abdominal CT shows CTDIvol 8 mGy. For the child's effective diameter, the AAPM Report 204 conversion factor is 1.8. What is the SSDE and what does it show?

    1. A14.4 mSv, the child's effective dose
    2. B14.4 mGy, a larger dose to the small body than the displayed CTDIvol
    3. C4.4 mGy, a smaller dose than the displayed CTDIvol
    4. D8 mGy, with no correction needed

    Answer and explanation in LearnFree account

15. Question 15 · Medium

    Which dose descriptor is a weighted average of the dose at the centre and the periphery of the CT dosimetry phantom?

    1. ACTDIw, calculated as one third centre plus two thirds periphery
    2. BDLP, calculated as the sum of the two measurements
    3. CSSDE, calculated as half the centre value
    4. DEffective dose, calculated as a simple mean

    Answer and explanation in LearnFree account

16. Question 16 · Easy

    The CT number at the middle of the displayed grey scale is called the:

    1. AMatrix size
    2. BField of view
    3. CWindow level
    4. DWindow width

    Answer and explanation in LearnFree account

17. Question 17 · Medium

    A protocol is changed from 5 mm to 2.5 mm slices at the same mAs and kVp. What is the expected effect on image noise?

    1. ANoise decreases, by about 40%
    2. BNoise increases, by about 40%
    3. CNoise is unchanged
    4. DNoise doubles

    Answer and explanation in LearnFree account

18. Question 18 · Medium

    A CT scan of an abdomen is reconstructed with thinner slices without changing the dose. Compared with thicker slices, the thinner slices will have:

    1. ALess partial volume averaging but more image noise
    2. BLess noise and less partial volume averaging
    3. CHigher noise and more partial volume averaging
    4. DMore partial volume averaging and less noise

    Answer and explanation in LearnFree account

19. Question 19 · Medium

    A patient with a spinal rod needs a CT of the lumbar spine at a Dubai trauma centre. Which approach reduces metal artefact most effectively?

    1. AMetal artefact reduction software, higher kVp and thin slices
    2. BSmooth soft-tissue kernel with a wider window
    3. CLower kVp and thicker slices
    4. DSwitch off tube-current modulation to reduce the noise

    Answer and explanation in LearnFree account

20. Question 20 · Easy

    In a modern multidetector CT scanner, how do the X-ray tube and the detector arc move during a scan?

    1. AOnly the table moves; both stay still
    2. BThe tube moves in a line across the patient
    3. CThe tube rotates inside a fixed detector ring
    4. DThey rotate together around the patient

    Answer and explanation in LearnFree account

21. Question 21 · Medium

    Which window setting suits routine viewing of brain parenchyma on an unenhanced head CT?

    1. AWidth about 2000, level about 400
    2. BWidth about 400, level about 40
    3. CWidth about 1500, level about -600
    4. DWidth about 80, level about 40

    Answer and explanation in LearnFree account

22. Question 22 · Easy

    A region of interest placed in the air-filled lumen of the trachea should read close to which CT number?

    1. A-100 HU
    2. B-1000 HU
    3. C0 HU
    4. D+100 HU

    Answer and explanation in LearnFree account

23. Question 23 · Medium

    A technologist wants to improve visibility of two soft-tissue structures that differ by only 10 HU on the workstation. What should be adjusted?

    1. AWiden the window width to include bone
    2. BRaise the window level to +500 HU
    3. CSwitch to a sharp bone kernel
    4. DNarrow the window width around their mean CT number

    Answer and explanation in LearnFree account

24. Question 24 · Medium

    A trauma head CT is reviewed for a subtle skull vault fracture. Which display window is the best choice?

    1. AWidth about 400, level about 40
    2. BWidth about 80, level about 40
    3. CWidth about 1500, level about -600
    4. DWidth about 2000, level about 500

    Answer and explanation in LearnFree account

25. Question 25 · Medium

    On a soft-tissue CT image, the window width is left at 400 HU and the window level is raised from 40 HU to 100 HU. How does the displayed liver, at about 60 HU, change?

    1. AIt looks brighter, as a higher level brightens tissue
    2. BIt shows more contrast, as the grey scale narrows
    3. CIt is unchanged, as its CT number is the same
    4. DIt looks darker, as it now falls below mid-grey

    Answer and explanation in LearnFree account