CBET Rehabilitation and Physical Therapy Equipment

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?

Accepted Answer

The unit pulses on for 20% of each cycle period and is off for 80%, delivering pulsed (non-thermal) ultrasound

Answer

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?

Accepted Answer

Continuous DC causes electrode polarization and tissue damage due to ion migration under the electrode

Answer

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?

Accepted Answer

27.12 MHz — an ISM (Industrial, Scientific, Medical) band allocated by the FCC to minimize interference with communications

Answer

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?

Accepted Answer

Irradiance (spectral power) in the 430–490 nm (blue) wavelength range at the treatment surface

Answer

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?

Accepted Answer

26.7% error — yes, this exceeds the typical ±10% tolerance and requires repair

Answer

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.

CBET - Certified Biomedical Equipment Technician Practice Test

CBET - Certified Biomedical Equipment Technician Practice Test

CBET Rehabilitation and Physical Therapy Equipment