TE Enriched Sample (E)

Non-ionizing Medical Imaging 2 84 Summary Key Ideas Travelling of ultrasound in media • Acoustic impedance Z • How easily the waves travel through a medium • Formula: Z = ρc ( ρ : density of the medium; c : ultrasound speed in that medium) & A denser medium usually gives a higher wave speed and hence has a higher Z . • Intensity reflection coefficient α • When a wave meets a boundary, reflection occurs. If the intensity of the incident wave is 1 unit, the intensity of the reflected wave is α unit ( α < 1). • Formula: ( ) ( ) I I Z Z Z Z r 0 2 1 2 2 1 2 a = = + - I 0 I r Z 1 Z 2 • If the intensity of the incident wave is 1 unit, the transmitted wave is 1 − α unit. & Two materials with a large difference of Z (i.e. α is large) gives more reflected waves on the boundary. • Attenuation • Intensity drop of the waves when passing through a medium (due to energy loss and scattering) • Depends on material and frequency of waves • Stronger with higher frequency • An ultrasound scan and an endoscopy are non- ionizing imaging methods. Ultrasound scan • Principle: pulse-echo technique / detecting echoes from different boundaries s ct 2 = distance s distance s tissue boundary tissue boundary skin skin skin tissue boundary distance s tissue 1 tissue 2 time = t time = 0 time = t _ 2 • Larger difference in Z in two materials gives stronger echoes from the boundary. & Coupling medium is applied to patient’s skin to avoid loss on the air–skin boundary. • Echoes from deeper boundaries return at a later time. & Depth of the boundary (with wave speeds known) can be found. DSE Level-up Companion A revision exercise is provided for helping Ss to grip the key points. See Level-up Task 92A . Sample © United Prime Educational Publishing (HK) Limited, Pearson Education Asia Limited 2023 All rights reserved; no part of this publication may be reproduced, photocopied, recorded or otherwise, without the prior written permission of the Publishers.