CBET Applied Physical Science for Medical Devices (Fluid Dynamics, Acoustics, Optics) — Questions and Answers
Question 1: According to Poiseuille's Law, if the radius of an IV catheter is halved, how does the maximum flow rate through it change (assuming all other variables remain constant)?
- It decreases by half (50%)
- It decreases to one-quarter (25%)
- It decreases to one-sixteenth (6.25%) (Correct answer)
- It decreases to one-eighth (12.5%)
Correct answer: It decreases to one-sixteenth (6.25%)
Poiseuille's Law states that flow rate (Q) is proportional to the fourth power of the radius (r⁴). Halving the radius reduces r⁴ by (1/2)⁴ = 1/16. Therefore, the flow rate drops to 1/16th (6.25%) of original. This is why catheter gauge has a dramatic effect on rapid fluid resuscitation — going from a 14g to an 18g catheter causes a large drop in max flow.
Question 2: A pulse oximeter uses two wavelengths of light — typically 660 nm (red) and 940 nm (infrared). Why are these two specific wavelengths chosen?
- They penetrate skin to different depths, allowing tissue depth profiling
- Oxyhemoglobin and deoxyhemoglobin have opposite absorbance ratios at these wavelengths, enabling their proportional calculation (Correct answer)
- 660 nm activates the photodetector and 940 nm is the reference channel
- These wavelengths avoid interference from skin melanin, which absorbs all other visible light
Correct answer: Oxyhemoglobin and deoxyhemoglobin have opposite absorbance ratios at these wavelengths, enabling their proportional calculation
At 660 nm, deoxyhemoglobin (HbR) absorbs much more light than oxyhemoglobin (HbO₂). At 940 nm, the relationship reverses — HbO₂ absorbs more. By comparing the ratio of absorbances at both wavelengths, the SpO₂ algorithm can calculate the proportion of oxygenated to total hemoglobin. This isobestic-point straddling technique is the foundation of pulse oximetry.
Question 3: The Doppler effect in medical ultrasound is used to measure blood flow velocity. If blood is flowing TOWARD the transducer, how does the received frequency compare to the transmitted frequency?
- The received frequency is lower (negative Doppler shift)
- The received frequency is the same (Doppler shift only applies to moving transducers)
- The received frequency is higher (positive Doppler shift) (Correct answer)
- The frequencies alternate between higher and lower with each heartbeat
Correct answer: The received frequency is higher (positive Doppler shift)
The Doppler effect states that when a sound source and receiver are moving toward each other, the received frequency is higher than the transmitted frequency. When blood moves toward the transducer, the reflected ultrasound waves are compressed, increasing their frequency. The Doppler shift (Δf) is proportional to the blood velocity and the cosine of the angle of insonation.
Question 4: In fiber optic light delivery systems used in surgical endoscopes, what phenomenon allows light to travel through the fiber without escaping through the sides?
- Constructive interference between photons in the fiber core
- Total internal reflection at the core-cladding interface when the angle of incidence exceeds the critical angle (Correct answer)
- The fiber is coated with a reflective metallic layer that bounces light forward
- Polarization of light by the fiber material confines it to the central axis
Correct answer: Total internal reflection at the core-cladding interface when the angle of incidence exceeds the critical angle
Total internal reflection occurs when light traveling through a denser medium (the fiber core) strikes the interface with a less dense medium (the cladding) at an angle greater than the critical angle. All light is reflected back into the core — none escapes. This principle allows light to travel around bends through flexible fiber optic bundles used in laparoscopes and bronchoscopes.
Question 5: Medical air compressors used in hospitals must supply air at a regulated pressure to power pneumatic devices. According to Boyle's Law, if a compressed air tank at 2000 PSI is connected to a regulator and the output is set to 50 PSI, approximately what fraction of the original gas volume is delivered at the regulated pressure?
- 1/40th of the original compressed volume
- 40 times the original compressed volume (Correct answer)
- The volume is unchanged — only pressure changes
- 4 times the original compressed volume
Correct answer: 40 times the original compressed volume
Boyle's Law: P₁V₁ = P₂V₂ (at constant temperature). At 2000 PSI, the gas occupies a small volume V₁. At 50 PSI (40 times lower pressure), the same mass of gas expands to 40 × V₁. So the regulated output delivers 40 times the volume that was stored at high pressure. This principle governs cylinder duration calculations for medical gas cylinders.
Question 6: Ultrasound transducers used in diagnostic imaging are based on the piezoelectric effect. What does this effect describe?
- The generation of heat when a material is exposed to high-frequency sound
- The conversion between mechanical stress/deformation and electrical charge in certain crystals (Correct answer)
- The absorption of sound energy by dense biological tissue such as bone
- The refraction of sound waves at tissue boundaries with differing acoustic impedance
Correct answer: The conversion between mechanical stress/deformation and electrical charge in certain crystals
The piezoelectric effect (discovered by Pierre Curie) describes the ability of certain materials (quartz, PZT ceramics) to generate an electric charge when mechanically deformed (direct piezoelectric effect) and, conversely, to mechanically deform when an electric field is applied (converse piezoelectric effect). Ultrasound transducers exploit both: voltage pulses create sound waves (transmit), and returning echo waves create voltages (receive).
CBET Applied Physical Science for Medical Devices (Fluid Dynamics, Acoustics, Optics)
According to Poiseuille's Law, if the radius of an IV catheter is halved, how does the maximum flow rate through it change (assuming all other variables remain constant)?