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Question 1
Describe, with the aid of diagrams, a typical diagnostic ultrasound signal, including i) spectral content, ii) pressure amplitude and iii) wave shape. Explain what is meant by fractional bandwidth and discuss the advantages of large bandwidth ultrasound transducers.
What is meant by the terms acoustic impedance? Calculate the acoustic impedance of a tissue with density 1.1 gcm-1, if the speed of sound in this tissue is 1540 ms-1. Explain why the specific properties of bone and air can cause difficulties for ultrasound imaging and provide clinically relevant examples.
Discuss why there is such a wide variety of transducer designs on the market? What transducer specifications would you consider important for (i) paediatric cardiac imaging and (ii) ultrasound guided prostate biopsy, and explain your choices.
Question 2
Explain, with the aid of diagrams, the main signal processing steps involved in converting the reflected signals received by the ultrasound transducer into a real-time, B-mode image. If the dynamic range of the raw analog signal was 100dB, what would be the minimum number of bits required for analog-to-digital conversion?
Describe some of the common parameters that are user-selectable in the typical medical ultrasound scanner. Discuss some of the ways in which a user could improve the following image conditions : i) image looks too dark, ii) cannot see deeper structures, iii) cannot see fine detail, iv) image movement appears jerky.
Explain the relationship between imaging depth, pulse repetition rate (PRR) and frame rate (FR) for a real-time B-mode imaging system. An ultrasound scanner has been set up to image the kidneys. The final image has 200 lines. Calculate the best possible PRR and FR. Include references to any data sources used in your answer.
Question 3
Describe, with the aid of suitable equations and diagrams, the technique of pulsed wave Doppler velocimetry and colour flow Doppler velocimetry. Doppler signals can suffer from aliasing, what steps can be taken to reduce aliasing?
An ultrasound system is found to overestimate all angles by 3o. The scanner is used to interrogate a blood velocity of 1 ms-1 at an angle of 45o. How does this affect the resulting velocity measurement?
A pulsed Doppler ultrasonic probe insonates an artery at an angle of 45o. The average velocity in the range gate is 0.3 m/s and the depth is 6 cm from the transducer face. The frequency of the emitted signal is 6 MHz and the PRF is 5 kHz. Flow is directed toward the transducer, parallel to the artery.
(1) What is the average value of the Doppler shift in Hz?
(2) During the cardiac cycle, the max Doppler shift is 1.5kHz. What is the absolute value of the velocity giving rise to this frequency?
(3) What is the maximum velocity which can be detected accurately using the above parameters?
(4) What is the maximum velocity which can be detected if the Doppler angle is 300?
