Describe the basic principles of ultrasound imaging including the
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 , 2D , 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 u/s ) and colour Doppler attracted additional marks.
References: Foundations of Anaesthesia – Basic Sciences for clinical practice,
Hemmings and Hopkins –Chapter 17