FRCR Radiation Physics and Dosimetry Flashcards

Which interaction predominates for diagnostic X-ray photons in soft tissue at energies around 30–100 keV?

Answer: Compton scattering

Compton scattering dominates in soft tissue across the diagnostic energy range (approximately 30–150 keV), producing scatter that degrades image contrast.

What is the inverse square law relationship between X-ray intensity and distance from the source?

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.

In MRI physics, what does the T1 relaxation time represent?

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.

Which property of a CT scanner is measured by its modulation transfer function (MTF)?

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.

What is the primary radiation hazard associated with radioactive iodine (I-131) therapy?

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.

Which principle governs the ALARA concept in radiation protection?

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.