FRCR Radiation Physics and Dosimetry Flashcards

What does the term 'kVp' refer to in X-ray tube operation?

Answer: Peak kilovoltage applied across the tube

kVp (kilovoltage peak) is the maximum voltage applied between the anode and cathode of the X-ray tube, which determines the maximum energy of the emitted X-ray photons.

Which nuclear medicine principle underlies PET scanning?

Answer: Coincidence detection of two 511 keV annihilation photons emitted 180° apart

PET exploits positron-electron annihilation: when a positron is emitted by the radiotracer and meets an electron, two 511 keV gamma photons are emitted simultaneously in opposite directions, detected in coincidence.

What is the Larmor frequency in MRI?

Answer: The frequency at which protons precess in a given magnetic field

The Larmor frequency is the precession frequency of proton spins in a magnetic field, directly proportional to field strength (ω₀ = γB₀), and must match the RF pulse frequency for excitation.

In ultrasound physics, which tissue property mainly determines acoustic impedance?

Answer: Density and propagation speed of sound in the medium

Acoustic impedance (Z = ρv) is the product of tissue density (ρ) and the speed of sound (v) in that tissue, and differences in Z at interfaces determine how much sound is reflected.

Which type of radiation detector is most commonly used in modern digital radiography flat-panel detectors (indirect conversion)?

Answer: Amorphous silicon photodiode array with scintillator

Indirect conversion flat-panel detectors use a scintillator (typically CsI:Tl) to convert X-rays to visible light, then an amorphous silicon photodiode array converts light to an electronic signal.

What does the term 'dose-length product' (DLP) quantify in CT dosimetry?

Answer: Integrated dose over the scan length, expressed in mGy·cm

DLP is the product of the CT dose index volume (CTDIvol) and the scan length, giving a measure of the total radiation energy deposited during the entire CT examination.