Lý thuyết siêu âm chuẩn đoán trong y khoa

Lý thuyết siêu âm chuẩn đoán trong y khoa

[...]... amplitude will be reduced by one-half, and the intensity by a factor of four (−6 dB) after travelling about 4 cm, mostly due to viscous and relaxation absorption processes The remaining energy is scattered, effectively spreading the beam, and this scattered energy may undergo further scattering interactions An extremely small fraction of the energy returns to the transducer The majority of the transducer output... piezoelectrically active: it is desirable for this area to be as small as possible, since the voltage produced represents the spatial average of the acoustic pressure over this area The sensitivity of a membrane hydrophone increases slowly with frequency up to its resonance frequency (approximately 60 MHz for a 16 μm thick membrane) However, a more constant frequency response can be obtained by amplifying the hydrophone... National Physical Laboratory), its performance may be checked using a checksource (a transducer and drive unit that delivers a beam of known acoustic power) or, in some cases, by applying a known weight The use of a checksource is preferable to weights since it will also verify the acoustic performance of the target If the ultrasonic field is strongly convergent (e.g some strongly focused stationary beams),... reasons why there must be a capability within any healthcare system to make detailed independent measurements The first is a duty of care to those being scanned and the need to ensure that equipment is “fit for purpose”, meets all necessary standards and is properly maintained Measurements may be needed prior to acceptance on purchase, for routine QA in compliance with local or national quality systems,... skin-coupling the attenuation coefficient of the dermal and sub-dermal layers may also have a strong effect, since it may be high depending strongly on hydration, and fat and collagen content The acoustic pulse contains a broad spectrum of frequencies centred approximately at the resonant frequency of the piezoelectric source The amplitude and intensity of the wave reduces with distance at a rate of about 0.5 dB cm⁻1 MHz⁻1;... a sensitivity which varies more strongly with frequency, especially below 3 MHz However, they are cheaper to buy (from about £1200 in 2011), and their needle shape offers advantages in some situations Fibre-optic hydrophones are also available in which the pressure field modulates the amount of light reflected from the end of an optical fibre but these are more normally used in high intensity therapeutic... real-time if required The hydrophone calibration should be traceable to national standards (for instance at the National Physical Laboratory) Measurement uncertainty depends on the particular application but it is typically less than ±10% for acoustic pressure parameters, and ±21% for intensity parameters (95% confidence level) 3.3.1.2 Intensity Intensity is not measured directly It is calculated from... generated by the hydrophone without the need for a synchronizing trigger: this is proportional to the temporal-average intensity A digital equivalent to this is to continuously digitize the hydrophone signal With both the analogue and digital versions, the electrical noise power should be assessed and, if significant, corrected for 3.3.1.3 Hydrophone measurement systems Turnkey commercial hydrophone... membrane hydrophone described previously Multielement array hydrophones are available from Precision Acoustics Ltd Those preferring to build their own measurement system for lower cost could also look at the approach taken in Newcastle General Hospital (UK) They designed a lightweight and compact system, suitable for transportation by car between base and a hospital site, and by a small trolley within... in density and/or bulk compressibility, and hence in sound velocity In the low-megahertz range there is strong coherent (i.e in phase) forward scatter with generally weak scattering in all other directions Only the very low-level backscattered component contributes to pulse-echo imaging, and this constitutes a vanishingly small fraction of the incident energy The integrated backscattered energy from . appropriate property is the a enuation coeffi cient for intensity, 2α. The processes by which ultrasonic energy is absorbed by tissues are complex, and not fully understood. The frequency dependence. safety sessions at congresses are seldom well aended, and that the sneaky insertion of a lecture on a safety-related topic into a specialist session may be regarded by some as the opportunity. reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical or photocopying, recording, or otherwise for