ULTRASOUND REVIEW
1a. Outline how ultrasound, in a medical context, is produced.
1b. Suggest the advantage in medical diagnosis of using ultrasound of frequency 1 MHz rather than 0.1
MHz.
1c. Ultrasound can be used to measure the dimensions of a blood vessel. Suggest why a B scan is preferable
to an A scan for this application.
2a. An ultrasound A-scan is performed on a patient. State one advantage and one disadvantage of using
ultrasound imaging in medicine compared to using x-ray imaging.
Advantage:
Disadvantage:
2b. Suggest why ultrasound gel is necessary during an ultrasound examination.
2c. Ultrasound of intensity 50 mW m-2 is incident on a muscle. The reflected intensity is 10 mW m-2.
Calculate the relative intensity level between the reflected and transmitted signals.
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2d. The graph shows a received signal incident upon a transducer to produce an A-scan. The density of the
soft tissue being examined is approximately 1090 kg m-3.
The acoustic impedance of soft tissue is 1.65 × 106 kg m-2 s-1. Show that the speed of sound in the soft tissue
is approximately 1500 m s–1.
2e. Estimate, using data from the graph, the depth of the organ represented by the dashed line.
2f. In the ultrasound scan the frequency is chosen so that the distance between the transducer and the
organ is at least 200 ultrasound wavelengths. Estimate, based on your response to (b)(ii), the minimum
ultrasound frequency that is used.
2g. A physician has a range of frequencies available for ultrasound. Comment on the use of higher
frequency sound waves in an ultrasound imaging study.
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ANSWERS/MARKSCHEME
1a. mention of AC voltage OR to piezo-electric crystal ✔
crystal vibrates «at its resonant frequency» ✔
1b. 1 MHz waves have shorter wavelength than 0.1 MHz ✔
can probe smaller size areas of organs/have higher resolution ✔
1c. a B scan is a computer generated combination of a large number of A scans ✔
allowing a measurement in different directions/two dimensional image ✔
2a. Advantage of ultrasound compared to X-rays:
no exposure to radiation
OR
relatively harmless
OR
can be performed in a doctor’s office
OR
can be used to measure blood flow rate
OR
Video image possible <<eg heart,
foetus>> ✔
Accept any reasonable advantage.
Disadvantage:
limited resolution
OR
difficulty imaging lungs or gastrointestinal system
OR
difficulty imaging any body part with a gas in it ✔
Accept any reasonable disadvantage.
Do not allow answers that contradict each other.
2b. [2 marks] gel has similar Z to skinOR
gel prevents acoustic mismatch ✔
without gel much ultrasound is reflected at skin OR gel increases ultrasound transmission ✔OWTTE
2c. [1 mark]
2d. [1 mark]
«ms–1» ✔ «≈1500ms–1»
Answer 1500 is given, check working or look for at least 3 significant figures.
2e. 4.5 × 10−2 «m»✔
2f.
«m» ✔
«Hz» ✔
2g. «compared to lower frequencies, higher frequencies»
have better resolution ✔
have greater attenuation ✔ used for superficial structures/organs ✔
have greater heating effect ✔ OWTTE Award [0] for contradictory comments or for any incorrect comment
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