FRCR FRCR Physics and Radiation Safety 2 — Questions and Answers
Question 1: What is the annual occupational whole-body effective dose limit recommended by the NCRP for radiation workers in the US?
- 50 mSv (Correct answer)
- 20 mSv
- 100 mSv
- 5 mSv
Correct answer: 50 mSv
The NCRP (Report No. 116) recommends a 50 mSv annual effective dose limit for radiation workers, with a lifetime limit of 10 mSv × age.
Question 2: The photoelectric effect is enhanced by which factor in tissue?
- Atomic number cubed (Z³) (Correct answer)
- Photon energy squared
- Tissue density
- Atomic mass
Correct answer: Atomic number cubed (Z³)
The photoelectric effect varies approximately as Z³/E³, so higher atomic number materials absorb far more radiation, explaining contrast with iodine and barium.
Question 3: What is the purpose of added filtration in a diagnostic X-ray tube?
- Remove low-energy photons that add dose without contributing to image (Correct answer)
- Increase tube output
- Improve spatial resolution
- Reduce scattered radiation reaching detector
Correct answer: Remove low-energy photons that add dose without contributing to image
Added filtration (typically aluminum) removes low-energy photons that would be absorbed by the patient without contributing useful diagnostic information.
Question 4: Radiographic grids are used primarily to:
- Reduce scattered radiation reaching the detector (Correct answer)
- Increase the primary beam intensity
- Improve contrast by adding filtration
- Collimate the primary beam
Correct answer: Reduce scattered radiation reaching the detector
Grids selectively absorb off-axis scatter before it reaches the image receptor, improving image contrast at the cost of increased patient dose.
Question 5: Which phenomenon describes the increase in patient skin dose when the X-ray tube is operated close to the skin?
- Inverse square law effect (Correct answer)
- Backscatter enhancement
- Penumbra effect
- Beam hardening
Correct answer: Inverse square law effect
As the source-to-skin distance decreases, the inverse square law causes a rapid increase in dose rate at the skin surface.
Question 6: In fluoroscopy, which mode delivers the highest dose rate to the patient?
- High-dose-rate (boost/magnification) mode (Correct answer)
- Standard fluoroscopy mode
- Pulsed fluoroscopy mode
- Last-image-hold mode
Correct answer: High-dose-rate (boost/magnification) mode
High-dose-rate or magnification mode increases the dose rate significantly to improve image quality during complex interventional procedures.
FRCR FRCR Physics and Radiation Safety 2
What is the annual occupational whole-body effective dose limit recommended by the NCRP for radiation workers in the US?