Physical Sciences Flashcards

In computed tomography (CT), what does the Hounsfield unit (HU) scale measure?

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.

What property of a thermistor makes it useful as a temperature sensor?

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.

What is the Larmor frequency in MRI physics?

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.

Which radiation protection principle requires that doses be kept as low as reasonably practicable?

Answer: Optimisation (ALARP)

The ALARP (As Low As Reasonably Practicable) principle, also called optimisation, requires reducing radiation doses as far as reasonably achievable.

A sinusoidal AC signal has a peak voltage of 340 V. What is its RMS voltage?

Answer: 240 V

V_rms = V_peak / √2 = 340 / 1.414 ≈ 240 V, which is the standard UK mains supply RMS voltage.

What is the main advantage of using a Faraday cage in electrophysiology measurements?

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.

In positron emission tomography (PET), what energy are the annihilation photons that are detected?

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.