FRCR FRCR Radiation Physics and Dosimetry 2 — Questions and Answers
Question 1: Which interaction predominates for diagnostic X-ray photons in soft tissue at energies around 30–100 keV?
- Photoelectric effect
- Compton scattering (Correct answer)
- Pair production
- Coherent scattering
Correct answer: Compton scattering
Compton scattering dominates in soft tissue across the diagnostic energy range (approximately 30–150 keV), producing scatter that degrades image contrast.
Question 2: What is the inverse square law relationship between X-ray intensity and distance from the source?
- Intensity is proportional to 1/distance² (Correct answer)
- Intensity is proportional to distance²
- Intensity is proportional to 1/distance
- Intensity is proportional to distance
Correct answer: Intensity is proportional to 1/distance²
The inverse square law states that radiation intensity decreases in proportion to the square of the distance from a point source, because the beam diverges over increasing area.
Question 3: In MRI physics, what does the T1 relaxation time represent?
- Time for longitudinal magnetization to recover to 63% of equilibrium (Correct answer)
- Time for transverse magnetization to decay to 37% of maximum
- Time between radiofrequency pulses
- Duration of the free induction decay signal
Correct answer: Time for longitudinal magnetization to recover to 63% of equilibrium
T1 (spin-lattice relaxation time) describes how quickly the longitudinal magnetization recovers toward its equilibrium value after an RF pulse by exchanging energy with the surrounding lattice.
Question 4: Which property of a CT scanner is measured by its modulation transfer function (MTF)?
- Spatial resolution performance (Correct answer)
- Noise characteristics
- Dose efficiency
- Contrast sensitivity
Correct answer: Spatial resolution performance
The MTF describes how faithfully the imaging system reproduces spatial detail across different spatial frequencies, quantifying the scanner's spatial resolution capability.
Question 5: What is the primary radiation hazard associated with radioactive iodine (I-131) therapy?
- Beta and gamma radiation emitted by the patient after treatment (Correct answer)
- Neutron activation of surrounding tissue
- Alpha particle emission causing local burns
- X-ray production from Bremsstrahlung only
Correct answer: Beta and gamma radiation emitted by the patient after treatment
I-131 emits both beta particles (causing local therapeutic effect in the thyroid) and gamma rays (which can irradiate staff, family members, and the public), requiring isolation precautions.
Question 6: Which principle governs the ALARA concept in radiation protection?
- Doses should be As Low As Reasonably Achievable given social and economic factors (Correct answer)
- All radiation exposure should be absolutely minimized regardless of cost
- Doses must be kept below annual legal dose limits at all times
- Radiation workers should avoid any measurable occupational dose
Correct answer: Doses should be As Low As Reasonably Achievable given social and economic factors
ALARA (As Low As Reasonably Achievable) requires that radiation doses be reduced as far as practicable, balancing protection benefit against the social and economic cost of further reduction.
FRCR FRCR Radiation Physics and Dosimetry 2
Which interaction predominates for diagnostic X-ray photons in soft tissue at energies around 30–100 keV?