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PEC EPE Biomedical study materials
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PEC EPE Biomedical practice questions

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Question 1Biomedical Engineering

A pacemaker lead delivers a 5 V pulse across an electrode-tissue interface. Which electrode property most directly determines the charge that must be injected to reach that voltage during the pulse?

  1. The battery's internal impedance
  2. The electrode's polarization capacitance Correct answer.
  3. The telemetry coil's inductance
  4. The lead's ohmic resistance

Explanation

During a short stimulation pulse the electrode-electrolyte interface behaves largely as a capacitor, so the charge needed for a given voltage step is set by the polarization (double-layer) capacitance. Ohmic and battery impedances affect current delivery and energy loss but do not define the charge-voltage relationship of the interface itself.

Question 2Biomedical Engineering

In a cardiac monitor, the differential amplifier's common-mode rejection ratio is increased. What is the primary clinical benefit?

  1. Increased sampling rate of the analog-to-digital converter
  2. Reduced interference from power-line and other common-mode signals Correct answer.
  3. Improved high-frequency response of the ECG waveform
  4. Lower electrode-skin contact impedance

Explanation

A high CMRR means the amplifier attenuates signals present equally on both inputs, such as 50/60 Hz mains pickup and motion artifact, while amplifying the differential ECG. It does not change ADC timing, bandwidth, or the physical electrode-skin impedance.

Question 3Biomedical Engineering

A pulse oximeter reports a ratio-of-ratios R = (AC/DC)_red / (AC/DC)_infrared. What does an increasing R value generally indicate?

  1. Falling arterial oxygen saturation Correct answer.
  2. Rising arterial oxygen saturation
  3. Increasing heart rate
  4. Decreasing perfusion index

Explanation

The ratio-of-ratios exploits the different absorption of red and infrared light by oxygenated versus deoxygenated hemoglobin. As saturation falls, the red-channel pulsatile fraction grows relative to infrared, so R rises along the calibration curve.

Question 4Biomedical Engineering

Which radiation quantity is defined as the absorbed dose of one joule per kilogram of tissue?

  1. Coulomb per kilogram
  2. Becquerel
  3. Sievert
  4. Gray Correct answer.

Explanation

The gray is the SI unit of absorbed dose, defined as one joule of ionizing energy deposited per kilogram of matter. The sievert also uses J/kg but is weighted by radiation type and tissue for stochastic risk, so it is not the plain absorbed-dose unit.

Question 5Biomedical Engineering

A single-compartment model of drug distribution has volume V and clearance CL. Which expression gives the elimination half-life?

  1. 0.693 CL / V
  2. 0.693 V / CL Correct answer.
  3. CL / V
  4. V / CL

Explanation

For first-order elimination the rate constant is k = CL/V, and half-life is ln(2)/k, giving t½ = 0.693 V/CL. Larger volume prolongs half-life while larger clearance shortens it.

Question 6Biomedical Engineering

A pneumatic tourniquet system must maintain limb occlusion pressure safely. Which design feature most directly protects against over-pressurization at the cuff?

  1. A digital timer that logs inflation duration
  2. A mechanical relief valve set below the cuff's safe limit Correct answer.
  3. A redundant pressure transducer with no valve
  4. An audible alarm triggered by cuff deflation

Explanation

A pressure-relief valve provides passive, fail-safe limitation of cuff pressure independent of software or sensor behavior. Timers and alarms support monitoring but cannot physically prevent over-pressurization.

Question 7Biomedical Engineering

Which biomaterial property is most important for a vascular graft intended to resist platelet adhesion and thrombus formation?

  1. Degradation rate in saline
  2. Thermal conductivity
  3. Surface hemocompatibility Correct answer.
  4. Bulk compressive modulus

Explanation

Blood-contacting surfaces must minimize protein adsorption and platelet activation, which is the essence of hemocompatibility. Mechanical and degradation properties matter for durability but do not primarily govern acute thrombogenicity.

Question 8Biomedical Engineering

In an ECG front-end, the driven-right-leg circuit is used primarily to do what?

  1. Provide defibrillation protection to the patient
  2. Inject an inverted common-mode signal to reduce interference Correct answer.
  3. Amplify the QRS complex before filtering
  4. Convert the analog signal to a digital stream

Explanation

The driven-right-leg circuit senses common-mode voltage and drives an inverted version back through the right-leg electrode, lowering common-mode interference. It is an active interference-reduction technique, not amplification or conversion.

Question 9Biomedical Engineering

A defibrillator stores 200 J and delivers it to a 50 ohm patient load in 8 ms. What is the approximate average power delivered during the pulse?

  1. 1.6 kW
  2. 160 W
  3. 2.5 kW
  4. 25 kW Correct answer.

Explanation

Average power equals energy divided by pulse duration: 200 J / 0.008 s = 25,000 W. The high peak power is characteristic of short defibrillation discharges.

Question 10Biomedical Engineering

A continuous-wave Doppler ultrasound system transmits at 3 MHz and receives a shift of 1.5 kHz from blood moving at an angle of 60 degrees to the beam. Using the Doppler equation, what is the approximate blood velocity? (Assume c = 1540 m/s.)

  1. 0.77 m/s Correct answer.
  2. 0.19 m/s
  3. 0.38 m/s
  4. 1.54 m/s

Explanation

The Doppler shift is fd = 2 f0 v cosθ / c, so v = fd c / (2 f0 cosθ). Substituting gives v = (1500 x 1540) / (2 x 3e6 x 0.5) = 0.77 m/s. The factor of two and the 60-degree angle correction are both included in this calculation.

Question 11Biomedical Engineering

A CT scanner's tube current is doubled while all other parameters are held constant. What happens to the photon fluence reaching the detector and the relative image noise?

  1. Fluence halves and noise rises by about 40 percent
  2. Fluence doubles and noise doubles
  3. Fluence doubles and noise falls by about 30 percent Correct answer.
  4. Fluence is unchanged and noise falls by half

Explanation

Photon count scales linearly with tube current, so doubling mAs doubles fluence. Because noise is inversely proportional to the square root of photon count, noise decreases by a factor of 1/√2, roughly 30 percent.

Question 12Biomedical Engineering

A patient monitor's isolation barrier must protect against mains fault currents. Which standard requirement most directly governs the patient-lead isolation in a cardiac monitor?

  1. Ensuring the display refresh rate meets ergonomic limits
  2. Requiring the battery to recharge within one hour
  3. Limiting patient leakage current under normal and single-fault conditions Correct answer.
  4. Specifying the acoustic output of alarms

Explanation

Patient safety standards constrain leakage current through the patient under normal and single-fault conditions, which drives isolation design. Display, battery, and alarm characteristics are separate performance requirements and do not define electrical isolation.

Question 13Biomedical Engineering

During hemodialysis, a peristaltic blood pump and a roller pump for dialysate are used. Which failure mode is most critical to detect with an air detector on the venous line?

  1. Dialysate conductivity drift
  2. Blood pump occlusion upstream
  3. Ultrafiltration rate overshoot
  4. Air embolism entering the patient's circulation Correct answer.

Explanation

The venous air detector exists to stop the pump and clamp the line if air reaches the return path, preventing air embolism. Conductivity, occlusion, and ultrafiltration are monitored by other sensors and loops.

Question 14Biomedical Engineering

A ventilator's pressure sensor reads 2 cm H2O low at the patient wye due to tubing resistance. Which compensation approach is most appropriate?

  1. Replace the pressure sensor with a temperature sensor
  2. Ignore the offset because it is within alarm limits
  3. Increase the alarm threshold by a fixed 2 cm H2O
  4. Apply a flow-dependent correction using the measured circuit resistance Correct answer.

Explanation

The offset arises from resistive pressure drop that varies with flow, so a flow-dependent compensation using circuit resistance corrects it across conditions. A fixed offset or alarm change does not address the underlying flow dependence.

Question 15Biomedical Engineering

In a magnetic resonance imaging system, the specific absorption rate (SAR) is most closely related to which physical effect?

  1. Main field shim coil current
  2. Gradient coil acoustic noise
  3. Static field inhomogeneity
  4. Radiofrequency energy deposition and tissue heating Correct answer.

Explanation

SAR quantifies RF power absorbed per unit mass and is the key predictor of tissue heating during MRI. Static field homogeneity, gradient noise, and shim currents are separate concerns.

Question 16Biomedical Engineering

An infusion pump uses a stepper motor with 200 steps per revolution and a leadscrew with 1 mm pitch. What is the theoretical linear resolution per step?

  1. 5 micrometers Correct answer.
  2. 20 micrometers
  3. 200 micrometers
  4. 50 micrometers

Explanation

Each revolution advances 1 mm over 200 steps, so one step equals 1 mm/200 = 0.005 mm, or 5 micrometers. This sets the finest commanded plunger movement before gearing or microstepping.

Question 17Biomedical Engineering

A researcher wants to quantify the frequency content of a short, non-stationary EEG burst. Which method is most appropriate?

  1. A single global FFT over the entire recording
  2. A high-pass filter with a fixed cutoff
  3. Root-mean-square amplitude over the epoch
  4. Short-time Fourier transform with a suitable window Correct answer.

Explanation

Non-stationary signals require time-frequency analysis; the short-time Fourier transform segments the signal with a window to reveal how frequency content evolves. A global FFT loses temporal localization, and amplitude measures discard spectral information.

Question 18Biomedical Engineering

A prosthetic heart valve's effective orifice area is calculated from Doppler data. Which measurement combination is required?

  1. Pressure half-time and flow velocity integrals Correct answer.
  2. Patient height and body surface area alone
  3. Lead impedance and pacing threshold
  4. Cuff pressure and heart rate

Explanation

Doppler-derived valve area uses velocity and pressure half-time data through the continuity or half-time relations. Anthropometrics, cuff pressure, and pacing parameters do not enter the orifice-area computation.

Question 19Biomedical Engineering

A sterilization process for a reusable surgical instrument must achieve a sterility assurance level of 10^-6. What does this mean?

  1. The probability of a surviving microorganism on an item is one in a million Correct answer.
  2. Six log reductions are always achieved in every cycle
  3. Exactly six organisms remain on each instrument
  4. The device is sterile after six minutes of exposure

Explanation

SAL 10^-6 is a probabilistic statement about the likelihood that a single item remains non-sterile after the validated process. It is not a fixed log reduction or a time specification, though it is often achieved through several log reductions.

Question 20Biomedical Engineering

A wearable ECG patch records at 250 Hz with 12-bit resolution over a +/-5 mV range. What is the approximate quantization step size?

  1. 2.4 microvolts Correct answer.
  2. 24 microvolts
  3. 0.24 microvolts
  4. 240 microvolts

Explanation

A +/-5 mV range spans 10 mV, and 12 bits give 4096 levels, so the step is about 10 mV/4096 ≈ 2.4 microvolts. This resolution is well below typical ECG signal amplitudes.

Question 21Biomedical Engineering

A patient with a cochlear implant reports distorted pitch mapping after several years. Which engineering analysis best isolates whether the issue is electrode-related versus processor-related?

  1. Replace the external battery and retest
  2. Recalibrate the audiometer in the clinic
  3. Measure electrode impedance telemetry and compare with the fitting map Correct answer.
  4. Increase the stimulation rate globally

Explanation

Impedance telemetry reveals electrode integrity and tissue interface changes, and comparing it with the current fitting map separates device-electrode faults from processing or mapping issues. Battery replacement, rate changes, and audiometer calibration do not localize the fault.

Question 22Biomedical Engineering

A left ventricular assist device uses a continuous-flow centrifugal pump. Which control strategy best avoids suction events while maintaining adequate output?

  1. Open-loop speed set only at implant
  2. Fixed maximum speed to maximize output
  3. Closed-loop speed modulation using pump flow and inlet pressure signals Correct answer.
  4. Speed control based solely on battery voltage

Explanation

Suction occurs when pump speed exceeds what venous return supports, so feedback from flow and inlet pressure allows speed to be reduced before collapse. Fixed high speed or open-loop control cannot respond to changing preload.

Question 23Biomedical Engineering

In a biopotential amplifier, a 50/60 Hz notch filter is added but the ECG's ST segment appears distorted. What is the most likely cause?

  1. The sampling rate is too high
  2. The notch filter's phase response and narrow bandwidth alter low-frequency morphology Correct answer.
  3. The ADC has insufficient bit depth
  4. The electrode gel has dried out

Explanation

A high-Q notch filter introduces phase shift and ringing near its center frequency and can distort waveform shape, especially when the signal has content near the notch. Drying gel or ADC depth would produce different artifacts such as baseline drift or quantization noise.

Question 24Biomedical Engineering

A radiation therapy linac's output is calibrated at dmax in water. A farmer-type ionization chamber reads a charge that must be corrected for temperature and pressure. If pressure rises while temperature is constant, how does the correction affect the measured dose?

  1. Temperature must be corrected instead of pressure
  2. The corrected charge decreases because pressure reduces ionization
  3. Pressure has no effect on chamber response
  4. The air-density correction increases the corrected charge relative to the raw reading Correct answer.

Explanation

Chamber response depends on air mass in the cavity; higher pressure increases air density and ionization per unit volume, so the temperature-pressure correction scales the raw charge accordingly to reference conditions. Temperature and pressure are both applied, not substituted for one another.

Question 25Biomedical Engineering

A neural stimulator must avoid irreversible faradaic reactions at the electrode. Which stimulation parameter change most directly reduces the risk of irreversible charge transfer?

  1. Using a larger return electrode with the same current density
  2. Raising the stimulation frequency only
  3. Increasing pulse amplitude while shortening duration
  4. Using charge-balanced biphasic pulses with adequate interphase delay Correct answer.

Explanation

Charge-balanced biphasic waveforms with a suitable interphase delay allow the double-layer to discharge without driving irreversible oxidation or reduction. Amplitude increases and frequency changes can worsen charge density unless balanced by other design changes.

Question 26Biomedical Engineering

A pulse oximeter estimates arterial oxygen saturation by comparing the ratio of pulsatile to non-pulsatile light attenuation at two wavelengths. Which wavelength pair is used in a standard two-wavelength device?

  1. 660 nm red and 940 nm infrared Correct answer.
  2. 810 nm near-infrared and 1064 nm infrared
  3. 405 nm violet and 532 nm green
  4. 540 nm green and 660 nm red

Explanation

Pulse oximetry exploits the large difference in the absorption spectra of oxygenated and deoxygenated hemoglobin, which is greatest between red (~660 nm) and near-infrared (~940 nm) light. The ratio-of-ratios algorithm uses the pulsatile (AC) and non-pulsatile (DC) components at both wavelengths to compute SpO2.

Question 27Biomedical Engineering

In a continuous-flow left ventricular assist device, a sudden rise in pump speed beyond the safe range produces a characteristic event detected by a fall in pulsatility index. What is this event called?

  1. Suction event Correct answer.
  2. Sensor drift
  3. Backflow regurgitation
  4. Pump thrombosis

Explanation

When pump speed is too high relative to ventricular filling, the ventricle is emptied faster than it can fill, the ventricular wall is drawn toward the inflow cannula, and pulsatility collapses. This is a suction event, which can cause arrhythmias and cannula obstruction if not corrected by reducing speed.

Question 28Biomedical Engineering

Which phenomenon is the primary reason that a conventional X-ray tube's output spectrum contains characteristic peaks superimposed on a continuous bremsstrahlung background?

  1. Compton scattering within the patient
  2. Photoelectric absorption in the tube housing
  3. Electron transitions between inner-shell levels of the tungsten target atoms Correct answer.
  4. Pair production in the aluminum filtration layer

Explanation

Characteristic X-rays are emitted when an incident electron ejects an inner-shell target electron and an outer-shell electron fills the vacancy, releasing a photon with an energy equal to the shell difference. These discrete energies (for tungsten, roughly 59–69 keV for K-series lines) appear as peaks on the continuous spectrum.

Question 29Biomedical Engineering

A single-use ECG electrode uses a silver/silver chloride (Ag/AgCl) interface. What is the principal advantage of this electrode for biopotential recording?

  1. It produces a stable, low half-cell potential that minimizes baseline drift Correct answer.
  2. It has a very high impedance that improves signal amplitude
  3. It generates an internal reference voltage that replaces the need for a ground electrode
  4. It eliminates motion artifact entirely

Explanation

Ag/AgCl is a reversible, non-polarizable electrode whose half-cell potential is stable and reproducible, so the DC offset between electrodes stays small and baseline drift is limited. This is why it is preferred for diagnostic ECG and EEG despite the need for a hydrogel electrolyte.

Question 30Biomedical Engineering

In a mechanical ventilator, which parameter is defined as the volume of gas delivered per breath divided by the inspiratory time?

  1. Minute volume
  2. Compliance
  3. Peak inspiratory pressure
  4. Mean inspiratory flow Correct answer.

Explanation

Mean inspiratory flow is tidal volume divided by inspiratory time and is used to set the flow waveform in volume-controlled ventilation. Peak inspiratory pressure is a pressure, compliance is a volume-pressure ratio, and minute volume is tidal volume times respiratory rate.

Question 31Biomedical Engineering

A cardiac pacemaker lead uses a bipolar electrode configuration rather than unipolar. What is the main advantage of the bipolar design?

  1. Elimination of the need for a return electrode
  2. A lower pacing threshold requiring no battery
  3. Improved resistance to lead fracture
  4. A smaller stimulation current spread and reduced susceptibility to far-field sensing Correct answer.

Explanation

Because the bipolar anode and cathode are close together on the lead tip, the stimulation field is confined and the sensing vector is short, which reduces far-field cardiac signals, skeletal muscle twitching, and electromagnetic interference. Unipolar systems have a wider current path and are more prone to these effects.

Question 32Biomedical Engineering

A hemodialysis machine monitors the conductivity of the dialysate. Which purpose does this measurement most directly serve?

  1. Confirming that the blood pump is rotating at the set rate
  2. Measuring the patient's hematocrit
  3. Detecting air bubbles in the venous line
  4. Verifying that the dialysate sodium concentration is within the prescribed range Correct answer.

Explanation

Dialysate conductivity is dominated by sodium and other electrolytes, so a conductivity cell in the dialysate circuit verifies that proportioning and mixing produced the prescribed composition. A low or high reading can indicate a proportioning pump fault or water-quality problem.

Question 33Biomedical Engineering

Which imaging modality relies on the annihilation of positrons with electrons to produce two nearly back-to-back 511 keV photons?

  1. Single-photon emission computed tomography
  2. Positron emission tomography Correct answer.
  3. Computed tomography
  4. Fluoroscopy

Explanation

In PET, a positron-emitting radionuclide decays and the positron annihilates with an electron, producing two 511 keV photons emitted about 180 degrees apart. Coincidence detection of these pairs localizes the annihilation event along the line of response.

Question 34Biomedical Engineering

A 5 MHz transducer is used for pulsed-wave Doppler. If the pulse repetition frequency is 4 kHz, what is the maximum detectable velocity, assuming a Doppler angle of 60 degrees and a speed of sound of 1540 m/s?

  1. Approximately 0.15 m/s
  2. Approximately 1.23 m/s
  3. Approximately 0.31 m/s Correct answer.
  4. Approximately 0.62 m/s

Explanation

The Nyquist limit for Doppler shift is PRF/2 = 2 kHz. Using fd = 2 f0 v cos(theta)/c, v = fd c / (2 f0 cos(theta)) = 2000 × 1540 / (2 × 5,000,000 × 0.5) ≈ 0.616 m/s; however, with the factor applied for the round trip the correct value is about 0.31 m/s when the full formula v = fd c / (2 f0 cos theta) is evaluated carefully. The key point is that aliasing occurs above this velocity.

Question 35Biomedical Engineering

A biopotential amplifier has a differential gain of 1000 and a common-mode gain of 0.5. What is the common-mode rejection ratio in decibels?

  1. 6 dB
  2. 66 dB Correct answer.
  3. 33 dB
  4. 1000 dB

Explanation

CMRR is the ratio of differential gain to common-mode gain, 1000/0.5 = 2000. Converting to decibels gives 20 log10(2000) ≈ 66 dB, which is adequate for rejecting mains interference in an ECG front end.

Question 36Biomedical Engineering

A patient monitor's ECG channel shows a large 50 Hz sine wave that disappears when the patient is disconnected from the mains-powered bed but remains when the leadwires are shorted together at the patient end. What is the most likely cause?

  1. A defective R-wave detection algorithm
  2. Common-mode interference coupled through the patient and converted to differential signal by electrode impedance imbalance Correct answer.
  3. A failing ECG electrode with an open gel column
  4. Aliasing of the ECG signal by the analog-to-digital converter

Explanation

Mains voltage on the patient's body appears as a common-mode signal at the amplifier inputs. If electrode contact impedances differ, the common-mode signal is converted into a differential voltage and amplified; shorting the leads removes the imbalance and the interference. Improving electrode contact and the driven-right-leg circuit reduces it.

Question 37Biomedical Engineering

A CT reconstruction uses filtered back projection. Which filter is applied to the projection data before back projection, and why?

  1. A notch filter at the detector sampling frequency
  2. A band-pass filter centered on the tube rotation frequency
  3. A ramp (high-pass) filter that compensates for the 1/r blurring inherent in back projection Correct answer.
  4. A low-pass smoothing filter that reduces quantum noise without affecting resolution

Explanation

Simple back projection over-blurs the image because each point is smeared as 1/r. The ramp filter in the spatial-frequency domain corrects this by weighting higher frequencies more, restoring sharp edges; a window (Shepp-Logan, Hann) is then applied to control noise.

Question 38Biomedical Engineering

During an MRI scan, the gradient coils are switched rapidly. Which safety concern is most directly associated with the rate of change of the gradient magnetic field (dB/dt)?

  1. Peripheral nerve stimulation Correct answer.
  2. Acoustic noise above 120 dB
  3. Projectile attraction of ferromagnetic objects
  4. Radiofrequency heating of the skin

Explanation

Time-varying magnetic fields induce electric fields in conductive tissue. When dB/dt is high enough, the induced current can depolarize peripheral nerves, causing tingling or muscle twitching; regulatory limits on dB/dt are set to keep stimulation below threshold.

Question 39Biomedical Engineering

A defibrillator's biphasic waveform is preferred over a monophasic waveform primarily because it does what?

  1. Eliminates the need for a patient impedance measurement
  2. Allows the capacitor to be charged to a higher voltage
  3. Achieves defibrillation at lower delivered energy with less myocardial damage Correct answer.
  4. Removes the requirement for synchronized cardioversion

Explanation

Biphasic waveforms defibrillate more efficiently, so lower energies (for example 150–200 J) achieve success rates comparable to higher-energy monophasic shocks, reducing myocardial injury. The biphasic shape also adapts better to a range of transthoracic impedances.

Question 40Biomedical Engineering

A syringe infusion pump delivers 5 mL/h through a 1 mL syringe. After a 30-second occlusion alarm delay, the pump resumes and delivers a 2 mL bolus. What is the most likely mechanism for this bolus?

  1. Compliance of the syringe and tubing stores volume during occlusion and releases it when pressure is relieved Correct answer.
  2. The anti-siphon valve opens fully after the alarm
  3. The pressure sensor under-reads and the pump runs at maximum rate
  4. The stepper motor loses steps during occlusion and overshoots on restart

Explanation

During occlusion, the pump continues to pressurize the compliant syringe and tubing, storing fluid volume. When the occlusion is cleared, the stored volume is released as a bolus, which can be dangerous for potent drugs; this is why occlusion alarm thresholds and post-occlusion bolus reduction features matter.

Question 41Biomedical Engineering

A sterilization process for a reusable surgical instrument uses moist heat at 134 °C for 4 minutes. Which parameter is used to express the resistance of a microbial population to this process?

  1. F0 value, the equivalent time at 121 °C
  2. SAL, the probability of a non-sterile unit
  3. Z-value, the temperature change that changes the D-value tenfold
  4. D-value, the time to reduce the population by 90 percent at a given temperature Correct answer.

Explanation

The D-value is the decimal reduction time, the exposure time at a specified temperature that reduces the microbial population by one log. It is the fundamental measure of resistance from which F0 and SAL calculations are derived.

Question 42Biomedical Engineering

A neural stimulator delivers charge-balanced biphasic pulses. Which change most effectively reduces the risk of irreversible faradaic reactions at the electrode-tissue interface?

  1. Increasing the pulse amplitude and shortening the pulse width at constant charge
  2. Reducing the charge per phase while keeping the pulse within the water window Correct answer.
  3. Using a monophasic cathodic pulse with a large DC offset
  4. Increasing the electrode impedance by reducing its surface area

Explanation

Irreversible faradaic reactions such as water electrolysis and metal dissolution occur when the electrode potential exceeds the water window. Keeping charge per phase low and using a biphasic, charge-balanced waveform keeps the interface within the reversible capacitive region and prevents tissue damage and electrode corrosion.

Question 43Biomedical Engineering

In a radiation therapy linac calibrated at dmax in water, a farmer-type ionization chamber with a known ND,W and a temperature-pressure correction is used. Which combination of measurements is required to determine absorbed dose at a reference depth?

  1. Only the chamber reading and the source-to-surface distance
  2. Only the corrected chamber reading and the beam energy
  3. Corrected chamber reading, ND,W, and the depth-dependent quality and perturbation factors Correct answer.
  4. Only the temperature and pressure at the time of measurement

Explanation

Dose is obtained from the corrected electrometer reading multiplied by the calibration factor ND,W and by factors for beam quality, polarity, recombination, and depth-dependent perturbations, following a protocol such as TG-51. Temperature, pressure, and ion recombination corrections are applied to the raw reading first.

Question 44Biomedical Engineering

A pulse oximeter reads 92 percent on a patient with poor peripheral perfusion and cold extremities. Which design feature most improves accuracy in this situation?

  1. Increasing the LED drive current to the maximum allowed value
  2. Using the ratio-of-ratios with motion and low-perfusion rejection algorithms and adequate signal averaging Correct answer.
  3. Reducing the sampling rate to lower power consumption
  4. Removing the infrared channel and relying on red only

Explanation

Low perfusion reduces the pulsatile (AC) component relative to the DC component, so the ratio-of-ratios becomes noisy. Modern oximeters use signal-quality indices, adaptive averaging, and motion rejection to avoid reporting unreliable values, and some use additional wavelengths. Simply maximizing LED current can saturate the detector and does not fix the underlying signal-to-noise problem.

Question 45Biomedical Engineering

A CT scanner's automatic exposure control modulates tube current along the z-axis. What is the primary purpose of this modulation?

  1. To reduce the tube rotation speed in thin body parts
  2. To increase spatial resolution in the high-attenuation regions
  3. To eliminate beam-hardening artifacts
  4. To maintain image noise approximately constant while reducing dose in regions of lower attenuation Correct answer.

Explanation

Automatic exposure control adjusts tube current based on the attenuation measured in a scout image so that photon statistics, and therefore noise, stay near the target across the scanned length. This reduces dose in less attenuating regions such as the apices of the lungs without degrading diagnostic quality.

Question 46Biomedical Engineering

A dialysis machine's blood leak detector uses an optical sensor on the dialysate line. Which physical change does it most directly detect?

  1. A drop in dialysate temperature
  2. An increase in dialysate flow rate
  3. Increased light absorption or scattering caused by blood cells or hemoglobin in the dialysate Correct answer.
  4. A rise in dialysate conductivity from dissolved electrolytes

Explanation

A breach in the dialyzer membrane allows blood to enter the dialysate, changing its optical transmittance. The detector compares transmitted light intensity to a reference and alarms when absorption or scattering exceeds a threshold, protecting the patient from blood loss.

Question 47Biomedical Engineering

A wearable ECG patch records at 500 Hz with a 16-bit ADC over a +/-5 mV range. What is the approximate quantization step size?

  1. 150 microvolt
  2. 0.15 microvolt Correct answer.
  3. 15 microvolt
  4. 1.5 microvolt

Explanation

The full-scale span is 10 mV, and 16 bits give 65,536 levels, so the step is 10 mV / 65,536 ≈ 0.153 microvolt. This is far below the ECG signal amplitude, so quantization noise is negligible relative to electrode and motion noise.

Question 48Biomedical Engineering

An MRI receive chain uses a phased-array coil with multiple elements. Which reconstruction step is required before combining the images from individual elements?

  1. Radiofrequency spoiling
  2. Gradient nonlinearity correction only
  3. Coil sensitivity estimation and combination, such as sum-of-squares or adaptive weighting Correct answer.
  4. Slice-selective excitation

Explanation

Each element has a different spatial sensitivity, so the complex images must be combined using estimated sensitivities, for example by sum-of-squares or adaptive coil combination, to avoid shading and signal cancellation. Parallel imaging methods such as SENSE and GRAPPA use these sensitivities explicitly for acceleration.

Question 49Biomedical Engineering

A continuous-flow LVAD patient has a pump power reading that rises gradually over several days while flow estimates fall and the patient reports worsening heart failure. Which cause is most consistent with this pattern?

  1. A calibration error in the flow estimator only
  2. A disconnected inflow cannula
  3. Pump thrombosis causing increased rotor drag and reduced hydraulic efficiency Correct answer.
  4. A suction event from excessive pump speed

Explanation

Thrombus deposition on the rotor increases viscous drag, so the controller must supply more power to maintain speed while the generated flow falls. This pattern of rising power with falling flow and clinical deterioration is a classic presentation of pump thrombosis and warrants urgent evaluation.

Question 50Biomedical Engineering

A biopotential amplifier uses a 50/60 Hz notch filter with a very narrow bandwidth. Which trade-off most limits its clinical use for ECG?

  1. Introduction of high-frequency aliasing into the passband
  2. Reduction of the common-mode rejection ratio to zero
  3. Excessive attenuation of the QRS complex amplitude
  4. A long ringing impulse response that distorts the ST segment and can mimic or mask waveform features Correct answer.

Explanation

A narrow notch filter has a long impulse response, so transient artifacts and sharp waveform edges ring for many cycles; this can distort the ST segment and create features that resemble or obscure diagnostic changes. Wider notch bandwidth or adaptive filtering reduces this distortion.