FRCR FRCR Physics and Radiation Safety

Question 1

What is the unit of absorbed radiation dose in the SI system?

Accepted Answer

Gray (Gy)

Answer

The Gray (Gy) is the SI unit of absorbed dose, defined as one joule of energy deposited per kilogram of tissue.

Question 2

Which interaction between X-rays and matter is most important for diagnostic imaging in soft tissue at typical diagnostic energies?

Accepted Answer

Compton scattering

Answer

Compton scattering dominates in soft tissue at diagnostic X-ray energies (above ~30 keV), producing scattered photons that degrade image contrast.

Question 3

What does the term 'half-value layer' (HVL) describe in radiology?

Accepted Answer

Thickness of material reducing beam intensity by 50%

Answer

The half-value layer is the thickness of a specified material required to reduce the intensity of an X-ray beam to half its original value.

Question 4

Which type of radiation has the highest linear energy transfer (LET)?

Accepted Answer

Alpha particles

Answer

Alpha particles are large, doubly charged, and deposit energy densely along a short track, giving them the highest LET.

Question 5

In CT dosimetry, what does CTDI (Computed Tomography Dose Index) measure?

Accepted Answer

Average dose delivered along the scan axis

Answer

CTDI represents the average dose profile along the z-axis for a standardized CT acquisition, measured in phantoms with ionization chambers.

FRCR - Fellow of the Royal College of Radiologists Practice Test