Question 4

Describe the Principles of ultrasound imaging — including the Doppler Effect.

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College Answer

It was expected candidates would outline the underlying principles of ultrasound imaging (reflection, scattering, refraction, and attenuation) and discuss that the basic image is the result of reflection of the transmitted ultrasound wave. Most candidates appreciated that the amplitude of the reflected echo is a function of the acoustic mismatch of the tissues and the angle of incidence and many candidates provided details mathematical descriptions concerning these principles.

While high levels of technical details were not required the answer should include a mention of the use a piezoelectric transducer and that an ultrasound beam has 3 dimensions — Axial, Elevation and Lateral. Some comment of the modes of Display (A= Amplitude, M Time Motion, 21), etc) was expected.

Extra credit was given for answers that included details regarding limits of depth of penetration (longer wavelength penetrate deeper, but loose image quality with longer wavelengths) and the varying properties of human tissue regarding refraction and attenuation (little refraction (path deviation) in human tissue and air attenuates).

Specific comment on the Doppler Effect was required. It was expected candidates would described that it refers to the change in frequency of a sound wave reflected by a moving target and that the reflected frequency differs if moving toward or away. Correctly stating that the reflected Frequency is Higher Towards and Lower Away scored additional marks. Comments concerning obtaining the best Doppler images with lower frequencies (opposite to ultrasound) and colour Doppler attracted additional marks.