STP Physical Sciences 5 — Questions and Answers
Question 1: What is the primary mechanism by which ultrasound causes tissue heating in therapeutic applications?
- Cavitation bubble collapse
- Absorption of acoustic energy converted to heat (Correct answer)
- Reflection at bone surfaces
- Doppler-induced friction
Correct answer: Absorption of acoustic energy converted to heat
Therapeutic ultrasound heats tissue primarily through absorption of acoustic energy, which is converted to thermal energy as the wave attenuates.
Question 2: Which of the following correctly describes the relationship between photon energy and wavelength?
- E = hλ
- E = hc/λ (Correct answer)
- E = λ/hc
- E = hc·λ
Correct answer: E = hc/λ
Photon energy E = hf = hc/λ, where h is Planck's constant, c is the speed of light, and λ is the wavelength.
Question 3: What does a quality factor (radiation weighting factor) account for in dose calculations?
- The energy of radiation absorbed per unit mass
- The biological effectiveness of different radiation types relative to X-rays (Correct answer)
- The activity of a radioactive source
- The penetrating power of radiation through shielding
Correct answer: The biological effectiveness of different radiation types relative to X-rays
The radiation weighting factor (w_R) accounts for the relative biological effectiveness of different radiation types, converting absorbed dose (Gy) to equivalent dose (Sv).
Question 4: In flow measurement using the Doppler effect in ultrasound, what does the Doppler shift frequency depend on?
- Only the ultrasound frequency used
- The velocity of blood, ultrasound frequency, and angle between beam and flow (Correct answer)
- Only the depth of the vessel
- The density of blood only
Correct answer: The velocity of blood, ultrasound frequency, and angle between beam and flow
The Doppler shift Δf = 2fv·cosθ/c, so it depends on the transmitted frequency, blood velocity, speed of sound in tissue, and the angle between the beam and flow direction.
Question 5: What does the term 'signal-to-noise ratio' (SNR) quantify in a measurement system?
- The ratio of peak signal amplitude to baseline amplitude
- The ratio of desired signal power to background noise power (Correct answer)
- The sensitivity of a detector to radiation
- The dynamic range of an analog-to-digital converter
Correct answer: The ratio of desired signal power to background noise power
SNR is the ratio of the power of the desired signal to the power of background noise, with a higher SNR indicating cleaner, more reliable measurements.
Question 6: Which property of laser light distinguishes it from ordinary white light?
- Higher photon energy
- Spatial and temporal coherence with monochromaticity (Correct answer)
- Faster propagation speed
- Ability to travel in a vacuum
Correct answer: Spatial and temporal coherence with monochromaticity
Laser light is highly monochromatic (single wavelength), spatially coherent (all wavefronts in phase), and temporally coherent, unlike incoherent broadband white light.
Question 7: In radiation dosimetry, what is the purpose of a TLD (thermoluminescent dosimeter)?
- To measure dose rate in real time using a Geiger tube
- To store absorbed dose as trapped electrons released as light upon heating (Correct answer)
- To image radiation distribution using a scintillation screen
- To amplify weak radiation signals using a photomultiplier
Correct answer: To store absorbed dose as trapped electrons released as light upon heating
TLDs contain crystals (e.g., LiF) in which ionising radiation traps electrons in metastable states; when heated, the electrons recombine and emit light proportional to the absorbed dose.
STP Physical Sciences 5
What is the primary mechanism by which ultrasound causes tissue heating in therapeutic applications?