CBET Rehabilitation and Physical Therapy Equipment — Questions and Answers
Question 1: A therapeutic ultrasound unit used in physical therapy has a duty cycle setting. What does a 20% duty cycle at 1 MHz mean?
- The unit operates at 20% of its maximum output intensity
- The unit pulses on for 20% of each cycle period and is off for 80%, delivering pulsed (non-thermal) ultrasound (Correct answer)
- The unit runs for 20 minutes and automatically shuts off
- Only 20% of the transducer crystal surface is active
Correct answer: The unit pulses on for 20% of each cycle period and is off for 80%, delivering pulsed (non-thermal) ultrasound
Duty cycle in therapeutic ultrasound is the ratio of on-time to total cycle time, expressed as a percentage. A 20% duty cycle means the ultrasound energy is active for 2 ms and off for 8 ms of each 10 ms period (at 100 Hz pulse frequency). This pulsed delivery reduces thermal effects, making it appropriate for treating acute injuries where heat is contraindicated, while still providing mechanical (non-thermal) therapeutic effects.
Question 2: During PM testing of a TENS (Transcutaneous Electrical Nerve Stimulation) unit, a BMET measures the output current to verify it matches the set parameter. What is the primary reason TENS units use alternating or pulsed current rather than continuous DC?
- AC is cheaper to generate in battery-powered devices
- Continuous DC causes electrode polarization and tissue damage due to ion migration under the electrode (Correct answer)
- AC penetrates deeper into tissue than DC at the same voltage
- Pulsed current bypasses sensory nerve fibers and only stimulates motor fibers
Correct answer: Continuous DC causes electrode polarization and tissue damage due to ion migration under the electrode
Continuous direct current causes electrolytic effects at the electrode-skin interface — positive ions accumulate under the cathode and negative ions under the anode, causing chemical burns from pH changes and electrode polarization that impedes current flow. Pulsed or alternating current avoids net DC charge accumulation, preventing tissue damage while still providing therapeutic nerve stimulation.
Question 3: A shortwave diathermy (SWD) unit must be kept at minimum distances from other electronic equipment and implanted devices. At what RF frequency does most therapeutic SWD operate, and why is this frequency regulated?
- 1 MHz — regulated because it interferes with AM radio broadcasts
- 27.12 MHz — an ISM (Industrial, Scientific, Medical) band allocated by the FCC to minimize interference with communications (Correct answer)
- 2.45 GHz — the same frequency as microwave ovens, regulated for the same heating safety reasons
- 50 Hz — the power line frequency, regulated by NFPA 99
Correct answer: 27.12 MHz — an ISM (Industrial, Scientific, Medical) band allocated by the FCC to minimize interference with communications
Most shortwave diathermy units operate at 27.12 MHz, one of the ISM (Industrial, Scientific, Medical) radio-frequency bands internationally allocated for non-communication uses. Operating within the ISM band allows therapeutic devices to radiate RF energy without requiring individual communications licenses, while containing interference to a defined spectrum away from broadcast and emergency communication frequencies.
Question 4: A BMET is testing a phototherapy unit used to treat neonatal hyperbilirubinemia (jaundice). Which characteristic of the light output is MOST critical to measure during PM?
- Color temperature in Kelvins
- Irradiance (spectral power) in the 430–490 nm (blue) wavelength range at the treatment surface (Correct answer)
- Total luminous flux (lumens) across the visible spectrum
- Ultraviolet output above 380 nm to ensure bactericidal effect
Correct answer: Irradiance (spectral power) in the 430–490 nm (blue) wavelength range at the treatment surface
Phototherapy for neonatal jaundice works by photoisomerizing bilirubin in the skin into water-soluble isomers that can be excreted. This photo-reaction is driven specifically by blue light in the 430–490 nm range. The therapeutic dose is measured as irradiance (µW/cm²/nm) in this band at the skin surface. A lamp that produces sufficient total lumens but has degraded blue output will be ineffective, so spectral irradiance in the blue band is the critical PM measurement.
Question 5: A motorized cervical traction unit is set to apply 15 lbs of traction. During PM, the BMET's tension scale reads 11 lbs at the 15 lb setting. What is the percentage error, and does this require repair before returning to service?
- 26.7% error — yes, this exceeds the typical ±10% tolerance and requires repair (Correct answer)
- 26.7% error — no, traction devices have a ±30% tolerance by design
- 4 lbs error — only absolute lb errors matter; percentages are not used for traction PM
- 13.3% error — acceptable, as traction forces are inherently variable due to patient movement
Correct answer: 26.7% error — yes, this exceeds the typical ±10% tolerance and requires repair
Percentage error = (|measured − set| / set) × 100 = (|11 − 15| / 15) × 100 = (4/15) × 100 = 26.7%. Most traction device PM protocols specify a ±10% accuracy tolerance. A 26.7% error significantly exceeds this and requires the device to be taken out of service for repair or recalibration. Applying less force than intended could render therapy ineffective; more importantly, accuracy matters for patient safety.
Question 6: A NMES (Neuromuscular Electrical Stimulation) unit used for muscle rehabilitation produces biphasic waveforms. What is the clinical advantage of a balanced (charge-balanced) biphasic waveform over an unbalanced monophasic waveform for long-term stimulation?
- Balanced biphasic waveforms stimulate type II fast-twitch fibers more selectively
- The equal positive and negative charge phases cancel net DC charge accumulation, preventing electrode-skin interface damage during extended use (Correct answer)
- Biphasic waveforms deliver twice the energy per pulse, reducing treatment time by 50%
- Balanced waveforms prevent patient perception of the electrical stimulus, improving compliance
Correct answer: The equal positive and negative charge phases cancel net DC charge accumulation, preventing electrode-skin interface damage during extended use
A charge-balanced biphasic waveform has equal positive and negative charge per pulse (net charge = zero). This prevents the accumulation of net ionic charge at the electrode-tissue interface that would otherwise cause electrolytic reactions, pH changes, and tissue/electrode damage over long stimulation sessions. This is particularly important for implanted neurostimulators and surface electrodes used in extended NMES protocols.
CBET Rehabilitation and Physical Therapy Equipment
A therapeutic ultrasound unit used in physical therapy has a duty cycle setting.
What does a 20% duty cycle at 1 MHz mean?