Flashcards in Module 1 practice exam Deck (11)
Define the terms “frequency” and “pulse repetition frequency” in relation to ultrasound, to clearly differentiate these two terms.
Ultrasound frequency is the frequency of the pressure variation cycle of the sound wave generated by the transducer. Pulse repetition frequency is the rate or frequency at which successive pulses of ultrasound are generated.
If the amplitude of an ultrasound wave decreases by a factor of 10, calculate the intensity ratio change, expressed in dB. Show your working. You may use log(10) = 1 and log(100) = 2, where these are logs to the base 10.
Intensity change is proportional to the square of the amplitude change, so the intensity change is 10 x 10 = 100. The dB intensity ratio measure is 10 log(100) = 10 x 2 = 20. You can also get this from the amplitude ratio as the dB change = 20 log(amplitude ratio) = 20 x log(10) = 20 x 1 = 20.
A sound wave has normal incidence on a soft tissue / muscle boundary. Given that the acoustic impedances of these two media are 1.63 x 106 kg m-2 s-1 and 1.70 x 106 kg m-2 s-1, calculate the intensity of the reflected wave compared to the intensity of the incident wave Remember to include your workings.
Reflection coefficient = reflected intensity / incident intensity = (difference in acoustic impedance)2 / (sum of acoustic impedance)2 = (1630000 – 1700000)2/(1630000 + 1700000)2 = 0.0004419
Define the term Pulse Repetition Period and estimate the Pulse Repetition Period when scanning to a depth of 12 cm.
Pulse Repetition Period is the time between transmitting successive pulses, and must be sufficient to allow the beam to travel to the maximum depth and back to the transducer. If the maximum depth is 12cm, and sound wave velocity = 1540000 cm/ s = 2 x depth / PRP, hence PRP = 2 x 12 / 154000 = 0.0001558 s = 0.0001558 x 1000 ms = 0.1558 ms
List the factors that contribute to attenuation of ultrasound travelling in soft tissue.
Attenuation results from absorption, reflection, scattering, beam divergence, and refraction in so much as refraction at a curved interface may produce beam divergence.
Using the expression for beam attenuation of 1 dB/cm/MHz, expressed in dB, what is the attenuation of a 2MHz ultrasound beam from a perfect reflector at a depth of 5cm from emission to reception at the transducer?
a. 0 dB
b. 5 dB
c. 10 dB
d. 20 dB
e. 40 dB
Attenuation = 1 x f (MHz) x 2 x 5 (cm) = 20 dB
The correct answer is: 20 dB
How do the loudness and intensity of an ultrasound wave compare?
a. The terms loudness and intensity are interchangeable, so they are essentially the same thing.
b. Increasing or decreasing the intensity produces a proportional increase or decrease in loudness.
c. Increasing or decreasing the intensity produces an increase or decrease in loudness, but there is no direct proportionality.
d. Increasing or decreasing the intensity produces no change in the loudness.
The correct answer is: Increasing or decreasing the intensity produces no change in the loudness.
The speed of an ultrasound wave is determined by:-
a. The sound wave frequency
b. The sound wave intensity
c. The sound wave amplitude
d. The propagating medium
The correct answer is: The propagating medium
An ultrasound wave in soft tissue has a frequency of 3 MHz. What is the approximate wavelength in mm?
a. 0.513 mm
b. 0.0513 mm
c. 5.13 mm
d. 1.95 mm
e. 0.195 mm
f. 0.0195 mm
The correct answer is: 0.513 mm
When a sound wave travelling in muscle crosses a soft tissue boundary, what angle of incidence is required to ensure the sound wave does not change direction?
a. Normal incidence (Zero degrees).
b. Ninety degree incidence (along the boundary).
c. Any angle of incidence.
d. None of the above as the direction will always change.
The correct answer is: Normal incidence (Zero degrees).