STP Physical Sciences 4 — Questions and Answers
Question 1: In computed tomography (CT), what does the Hounsfield unit (HU) scale measure?
- The acoustic impedance of tissue
- The relative linear attenuation coefficient of tissue compared to water (Correct answer)
- The radioactivity concentration in tissue
- The magnetic susceptibility of tissue
Correct answer: The relative linear attenuation coefficient of tissue compared to water
Hounsfield units quantify X-ray attenuation relative to water (HU=0) and air (HU=−1000), enabling tissue differentiation in CT images.
Question 2: What property of a thermistor makes it useful as a temperature sensor?
- Its resistance increases linearly with temperature
- Its resistance changes significantly and predictably with temperature (Correct answer)
- Its capacitance varies with temperature
- Its inductance changes with temperature
Correct answer: Its resistance changes significantly and predictably with temperature
Thermistors exhibit a large, reproducible change in electrical resistance with temperature, making them sensitive and practical temperature sensors.
Question 3: What is the Larmor frequency in MRI physics?
- The frequency of gradient switching
- The resonant precession frequency of nuclei in a magnetic field (Correct answer)
- The rate of T1 relaxation
- The sampling rate of the analog-to-digital converter
Correct answer: The resonant precession frequency of nuclei in a magnetic field
The Larmor frequency ω₀ = γB₀ is the frequency at which nuclear spins precess around the applied magnetic field, and is used for RF excitation in MRI.
Question 4: Which radiation protection principle requires that doses be kept as low as reasonably practicable?
- Justification
- Optimisation (ALARP) (Correct answer)
- Limitation
- Graded approach
Correct answer: Optimisation (ALARP)
The ALARP (As Low As Reasonably Practicable) principle, also called optimisation, requires reducing radiation doses as far as reasonably achievable.
Question 5: A sinusoidal AC signal has a peak voltage of 340 V. What is its RMS voltage?
- 340 V
- 480 V
- 170 V
- 240 V (Correct answer)
Correct answer: 240 V
V_rms = V_peak / √2 = 340 / 1.414 ≈ 240 V, which is the standard UK mains supply RMS voltage.
Question 6: What is the main advantage of using a Faraday cage in electrophysiology measurements?
- It amplifies weak bioelectric signals
- It shields the measurement environment from external electromagnetic interference (Correct answer)
- It provides grounding for the patient
- It filters high-frequency noise from the signal
Correct answer: It shields the measurement environment from external electromagnetic interference
A Faraday cage is a conductive enclosure that attenuates external electric fields, preventing electromagnetic interference from corrupting sensitive bioelectric measurements.
Question 7: In positron emission tomography (PET), what energy are the annihilation photons that are detected?
- 140 keV
- 364 keV
- 511 keV (Correct answer)
- 1274 keV
Correct answer: 511 keV
When a positron annihilates with an electron, two photons each of 511 keV (equivalent to the rest mass energy of one electron, mc²) are emitted back-to-back.
STP Physical Sciences 4
In computed tomography (CT), what does the Hounsfield unit (HU) scale measure?